Geothermal may beat batteries for energy storage | Hacker News Reader

Geothermal may beat batteries for energy storage

spectrum.ieee.org

196 pointsrbanffy287 comments

Many of the greenhouse Youtube channels I watch do a smaller DIY version of this. They have either dark barrels of water in the greenhouse with the windows facing south to the sun, or a thin metal wall filled with clay and pipes acting as a sun-heat-battery. They pump the water through the barrels or clay battery into pipes that are under ground to store the heat. In winter time they extract the heat from the ground keeping the greenhouse warm using a combination of solar and commercial power for the pumps. Heat is also extracted from compost bins at each end of the greenhouse. This works well in extremely cold climates in Canada and Alaska. A few of these folks do all of this without using any electricity at all and somehow manage to get water moving through the pipes using heat convection alone.

I've been thinking about doing something like this but connecting it to pipes under the foundation of my home so I can get rid of the wallboard heaters or just leave them off. Electricity is the only commercial utility near me.

This reminds me a lot of the Earthship community in the middle of the desert, which always focusses on smart ways of reusing water for different purposes, and by not wasting water as much as possible. The houses usually have a clean/grey/blackwater + a rainwormbox system to process fecals and reuse it for growing plants. Their air conditioning system is basically just a pipe in the ground where the hot air flows through, cools down, and automatically gets pushed through the house when they open the windows on the roof.

I was always wondering why there are no systems converting the unused electricity in potential energy by moving water to a higher ground. And more importantly: Why there are no water storages on the roof.

[1] https://www.youtube.com/watch?v=wVp5koAOu9M

> Why there are no water storages on the roof.

Quick math - 100 gallons of water 15 meters up off the ground has enough potential energy to run a microwave for 30 seconds.

Gravitational storage is pathetically weak. It only makes sense on massive, massive scales. In a residential setting, the storage will never outweigh the extra cost and risk of having so much water on your roof.

Secondary point: most residential structures are not designed to have 1000s or even 100s of gallons of water stored on the roof. Even 100 gallons is a huge amount of static load to add to a structure.

One minor but interesting exception - farmers have historically often put domestic water tanks on top of towers to ensure even water pressure. Direct pumping leads to cycles where the pressure in the system rises and falls as the pump cycles on and off, which is very unpleasant if you’re trying to take a shower!

I believe modern off-grid home water pumps use more sophisticated motor control to avoid the need for the raised tank.

Off-grid domestic water uses tanks fitted with rubber bladders that keep the water at a roughly constant pressure and reduce the frequency the pump has to cycle.

That's not an exception, that's an entirely different use for a raised tank. And not a very big one either.

I'd say it's a quite big one, "every city" has water towers to provide pressure to the people living in it, or they rely on the water source if it's higher up than the city.

There are probably exceptions, where this isn't true, but raised tanks make modern society possible.

https://youtu.be/yZwfcMSDBHs more on water towers, which are raised tanks of water.

We also pump water into fake lakes to later extract electricity from it. (https://youtu.be/66YRCjkxIcg)

Another good thing with water towers is that we can keep running the pumps at a lower RPM where efficiency than if we were trying to build a system with pumps to keep pressure.

Great channel if you're curious about everything related to humans relationship with water.

> I'd say it's a quite big one, "every city" has water towers

I meant not a big tank is needed for one house. I wasn't talking about the size of the use case.

> We also pump water into fake lakes to later extract electricity from it.

We do, because lakes are much much bigger than tanks on a house.

It depends what you're calling "not a very big one". On our farm we have a 5000 litre galvanised high tank for house-water.

Right, but do you need more than 50 of those for the specific purpose of evening out pressure?

I thought the point of water tanks is that one needs a simpler / weaker pump to maintain the water pressure.

i.e. if you want to maintain water pressure X, with a pump you need something that can push X, but with a tower, you just need a pump that can keep the tower full over the course of a more length period and just has to pump at the average of what it takes to keep it full over the course of that longer period (i.e. the average pressure needed to maintain the tower is less than the pressure you need to provide).

Here's some "slow math" in video form ;)

https://youtu.be/CMR9z9Xr8GM

I find him entertaining. Put a 55 gallon drum on his roof, pumping water up via solar and running lights at night. Closed loop system.

He also "does the math" and mentions exactly what you are saying in entertaining format. Just not worth it if you're not doing it on a massive scale, preferably way out of sight and danger.

Thanks for that dose of information. Basic arithmetic is so lacking in all these discussions. People throw all sort of fantastic pseudo-facts with little or no anchor to reality.

they do it a lot in NZ. Solar hot water heating.

Solar hot water is great, but that's not gravitational storage, e.g. pumping water up a hill with pumps and later using the flow of water back down to generate power.

You're right it isn't, but it is storing the water on top of the house.

Yes, but I'd expect it's much lesser amounts. 80 gallons is a decent sized water heater.

ahhh i understand what you were saying now.

NZ has a lot of hydro and a reasonable amount of geo thermal generation. Solar, not so much.

> I was always wondering why there are no systems converting the unused electricity in potential energy by moving water to a higher ground

There are a bunch of pumped storage facilities around [1]. But they work best at massive scale, so suitable locations are somewhat limited. Plus they are expensive to build and often face environmental protests (similar to building dams). Still, it's a solution I'm a fan of.

[1] https://en.wikipedia.org/wiki/List_of_pumped-storage_hydroel...

emphasis on massive scale.

Moving 500,000 kg (over 1 million pounds) 7.5 meters (~25 feet aka the height of a house) will give you about 10 kWh of energy. This is equivalent to running a 425W device all day, like a small air conditioner. The relationship is linear. Double the weight or the distance to double the energy. All of the metal at a scrap yard I know of amounts to less than half that weight, for reference.

I'm also a fan because pumped storage is a really interesting storage method, but it is beyond niche. It is very tough to move that kind of weight around efficiently for what you get back. Pumping water to great heights is not easy either. (see also: moving rail-carts up a mountain)

> All of the metal at a scrap yard I know of amounts to less than half that weight, for reference.

That's not a great reference point when you're trying to visualize to pumped storage, as water is 1t/m3 while steel is up around 7 or 8. Also, 500t of steel at a scrapyard seems very small - 70m3?

A better reference might be a back yard pool, which might be in the 30-40t range - so like lifting 15 back yard pools the height of your house to power a tiny AC.

good point, and yes its a small scrap yard. I was trying to emphasis that it's possible for a full-time commercial operation moving heavy metal to involve less weight than what was referenced. The backyard pool paints a better picture though

> Moving 500,000 kg (over 1 million pounds) 7.5 meters (~25 feet aka the height of a house) will give you about 10 kWh of energy.

In dollar terms, 10kWh is worth around $1. 1 million pounds is the weight of 2-5 residential homes, depending on size. Think about it: the cost to lift a couple of entire houses three stories up into the air is literally just one dollar. That’s why gravity energy storage only makes sense at a massive scale.

it's also why "storage" is a very loose term for gravity based energy storage. at a massive scale it is still only best at storing/discharging the difference between demand and supply - while still trying to keep actual energy production as close to demand as possible at all times. It really should never be used to power a city the way we would use a battery to power our phone. As in, spend significantly less time charging it than discharging it

They are building a lot of them in China

https://en.wikipedia.org/wiki/List_of_pumped-storage_hydroel...

Makes sense, it is definitely a useful tool. I just think it is insufficient to act as storage. It can be good at producing variable amounts of Watts on demand but not so good at storing enough Watt-hours to keep things running for very long. I can see a great appeal for it to help with load-balancing for a significant amount of choppiness between supply and demand on the hour timescale.

For something like solar, where we will want to store over half our daily energy production at peak storage (ideally 2-3 days worth I think) - I don't think it holds up. Additionally, it doesnt seem like a good bet as a primary mechanism for either storage or on-demand generation if energy consumption continues to increase due to the rather large coefficients involved for scaling it up.

"The United States generated 4,116 terawatt hours of electricity in 2021"[1]

4,116 TWh/year = 11.2 TWh/day

The storage capacities for the largest items listed on the wiki is on the magnitude of GWh. The scale goes kilo-, Mega-, Giga-, then Terra. So we are talking about a need on the order of a thousand pumped storage facilities per country. The US would need over 50 of them per state (on average) in order to keep everything running without production for 24 hours. Doesnt matter how many solar panels we have, if we get 1 dark day then we would run out of power. If we tried to rely on solar entirely, we'd also still need very roughly half that amount of storage just to get through the night.

lithium batteries are obviously much better suited for overnight storage, but I have no idea what the numbers are on how much lithium is physically available to use as such storage.

If we want to get on the order of monthly to yearly storage to allow, for example, solar panels in alaska to provide enough energy for a resident to get through months of darkness - I have no idea what the leading storage options are, probably lithium still

[1]https://www.statista.com/statistics/188521/total-us-electric...

Sodium ion is expected to sharply take over cost limited applications some time in the next couple of years. There are pilot mass production programs designed to avoid scarce materials that drop into existing processes. Natron have products on the market (at presumably high cost) targetting datacenters for high safety applications.

For longer scale storage it's a tossup between opportunistic pumped hydro, CAES where geology makes it easy, hydrogen in similar areaswith caverns, ammonia, synthetic hydrocarbons, sodium ion, and one of the emerging molten salt or redox flow battery technogies. Lithium isn't really in the running due to resource limits.

Wires also have a lot of value for decreasing the need for storage. Joining wind and solar 1000s of km apart can greatly reduce downtime. Replacing as much coal and oil with those, and maintaining the OCGT and CCGT fleet is the fastest and most economic way to target x grams of CO2e per kWh where x is some number much smaller than the 400 of pure fossil fuels but bigger than around 50. Surplus renewable power (as adding 3 net watts of solar is presently cheaper than the week of storage to get an isolated area through that one week where capacity is 1/3rd the average) will subsidize initial investments into better storage and electrolysis with no further interventions needed.

Awesome response. I've come across the molten salt option but havent researched in depth. I saw it referenced as something a lot of scientists are hyping up, but I am not sure what kind of engineering challenges exist for implementation and maintenance.

Second paragraph is a bit too information dense, I had trouble following some of it. Renewable energy deficiencies will be localized, so i understand how wires help here. A larger connected area produces more stability, makes sense. Agreed with the carbon reduction priority to tackle coal and oil first. Surplus renewable power acting as a subsidy checks out, but that is skirting around the energy storage problem imo. Sounds like you are saying "instead of storing renewable energy, get more than you need and sell it back to the grid and then use those funds to buy the energy back later". This would certainly work for local consumers, but doesnt do too much to help the power grid itself manage what to do with the surplus energy. Sell it to neighboring power grids? Ties in to the first point about connecting a larger area - but what are the limits here? Can we physically connect the sunny side of earth to the dark side? (ignoring that it seems logistically/legally prohibitive)

the question really comes down to what should we be spending money on to get "better storage"? What are the best solutions for long-term local storage?

> the question really comes down to what should we be spending money on to get "better storage"? What are the best solutions for long-term local storage?

The solution I'm proposing is basically 'the best place to spend your money on storage is to not spend it on storage yet'

If the goal is to reduce emissions asap, then focusing on the strategy that removes x% of 100% of the emissions rather than 100% of y% of the emissions makes sense unless there are enough resources/money that y% is more than x%. And storage is currently expensive enough that you need many times as much money for this to be true to 99.9% confidence.

Getting a wind + solar system that has at least y watts at least eg. 90% of the time is remarkably affordable already and still going down.

In excellent climates new solar costs less per MWh than fuel for a gas turbine (and is not far off fuel for a nuclear reactor). Wind is not much more. Distribution, dealing with less than ideal sites and oversupply increase the cost, but an ideal mix has very little storage (4-12 hours) which can be delivered by lithium batteries.

By relying on the existing fossil fuel/hydro/nuclear/whatever to pick up the last 10% for now, you can replace more coal/oil more quickly than other strategies. During this build all storage technologies where they make the most sense so that when that last 10% is needed, prices will have dropped. I'm fairly sure some mix of green hydrogen and green ammonia burning in those same turbines will be one of the winners (ammonia in particular has negligible marginal cost of capacity allowing for a strategic reserve, and will be needed to replace fossil fuel derived fertilizer anyway).

In the unlikely case that there's an overnight $2 trillion investment in new wind/solar/powerlines and production capacity to match in the US then choosing a dispatchable power source from some or all of: expensive green hydrogen, expensive abundant existing batteries, expensive pumped hydro, and expensive nuclear or immediately going all in on commercialising every vaguely promising electrolyser tech becomes the priority.

Completely agree with the hybrid approach wrt reducing emissions. I am talking more towards work that would be done concurrently with that.

> During this build all storage technologies where they make the most sense so that when that last 10% is needed, prices will have dropped

this is kind of the point of what I'm getting at. Without any investment, none of the storage technologies are going to make much progress. If not financial investment, then at least a time investment from research/science teams. then again, maybe opportunism/free market will take care of this and we can assume any progress that can be made will be made by people trying to make a name for themselves or be first to market. I'm still curious to size up what that progress might look like for discussion/entertainment purposes in any case

Good storage solutions would immediately pay dividends through arbitrage, which would keep electric prices stable, and then anywhere renewable energy generation is more than demand and storage is sufficient, that stable price point could come down below the cost of using coal/oil as well as any other continuous production method. We would be able to consolidate power generation over time, not just space, and realize gains from that. As in, use massive bursts of energy production to top off storage and use them to exactly meet demand. Maybe this opens the door for more alternative energy production methods as well (that are better suited for burst than steady)

In terms of promising technologies, they're broadly categorisable as thermal, kinetic, battery/fuel cell, and thermochemical. Most of the promising ones are far enough along the learning curve that other markets (such as green hydrogen/ammonia for fertiliser driving electrolysers and small scale/more efficient chemical reactors) will drive the learning curve.

Thermal storage concepts include:

Molten salt thermal. short/medium for high grade heat. Most high grade heat is dispatchable (fire) and so doesn't make sense to store, or expensive (solar thermal, nuclear) and so isn't worth pursuing.

Sand thermal batteries. Low grade heat for medium/long term. Only useful for heating and some industrial purposes. Has a minimum size (neighborhood). Literally dirt cheap.

Thermochemical. I guess this is kind of a fuel? Use case is for low grade heat so it can go here. Phase change materials like sodium acetate or reversible solution like NaOH seem really appealing for heating. Back of envelope says it's close to competitive with electric heating, so I'd expect more attention as it's cheaper than any technology that stores work. No idea why it isn't being rolled out. You could even charge it with heat pumps for extremely high efficiency if needed.

Kinetic:

Lifting stuff. Only really works for water without large subsidies and only if you already have at least one handy reservoir like a watershed or cavern. No reason to expect it would suddenly get cheaper as digging holes and moving big things is already something lots of industries try to do cheaply. Great addition to existing hydro.

Sinking stuff (using buoys to store energy). I can't comprehend how this can be viable. I have seen it espoused, but it doesn't pass back of the envelope test unless I did a dumb.

Squashing stuff. Compressed air energy storage. Tanks are just barely competitive with last gen batteries capacity-wise, efficiency isn't great. There are concepts for underwater bladders (let the watter do the holding) or cavern based storage that seem viable at current rates. Achievable with abundant materials so worst case scenario we nut up and spend$500/kWh. Key word CAES, cavern or underwater energy storage

Battery/fuel cell:

Lithium ion: One of the best options currently. Will be heavily subsidised by car buyers. Has hit limits of current mining production which puts a floor on price and is ecologically devistating.

X ion where x is probably sodium: Great slot in replacement. Barring large surprises will expect it to replace LiFePO4 very soon for most uses. Expect the learning rate of lithium ion manufacturing to continue resulting in a sharp jump to $60/kWh in 2021 dollars and eventual batteries around $30/kWh. Key word natron (have just brought their first product to market and are working with other parts of the supply chain to scale up)

Flow batteries, air batteries and fuel cells. These are almost the same concept. You have a chemical reaction that makes electricity with a circular resource like hydrogen, methane, ammonia, or electrolyte. Downside is most versions require a prohibitive amount of some metal like rutheneum or vanadium or something. Not a fundamental limit, but not sure it will be a great avenue as research goes back a fair ways. Aluminum-air batteries are one interesting concept. Essentially turning Al smelters into fuel production facilities. Keywords iron-air aluminum-air, redox-flow, direct methane fuel cell, ammonia fuel cell, ammonia cracking, nickel fuel cell.

Molten salt batteries. Incredibly simple, cheap and scalable concept that has no problems with dendrites (and so theoretically no cycle limit) with one limitation on portability (they must be hot, sloshing is bad) and one as yet insurmountable deal breaking flaw (incredibly corrosive material next to an airtight insulating seal). Look up Ambri for details of an attempt which has presumably failed by now. There is a more recent attempt using a much lower temperature salt and sodium sulfur which shows promise. Keywords ambri, sodium sulfur battery.

Thermochemical:

Any variation on burning stuff you didn't dig up.

Hydrogen is hard to store more than a few days worth, but underground caverns could help. I expect a massive scandal about fugitive hydrogen, toxicity and greenhouse effect in the 2030s sometime. It's borderline competitive to make now. Main limitation is cost of energy (solved by more wind and solar and more 4 hour storage) and cost of capital (platinum/palladium/rutheneum/nickel are usually required). Lots of work going on to reduce the latter and to increase power density and efficiency. If you were directing a billion dollars of public funds this would probably be the place to put it. Keywords $200/kw electrolyser, hysata 95% efficient.

Methane, ammonia, dimethyl ether, methanol, etc. These are all far easier to store than hydrogen. Production needs large scale but is borderline viable already if you have cheap hydrogen. Keywords ammonia energy storage, synthetic fuels, efuels, green ammonia, direct ammonia electrolysis.

Then there's virtual batteries.

Many loads like aluminum smelting can be much more variable than they are now. Rearranging workflows such that they can scale up or down by 50% and change worker tasks to suit has the same function as storage during any period where consumption isn't zero. EV's can kinda fit here too and kinda fit actual storage (especially if they power other things)

Biofuels. Not technically storage, more dispatchable, but it serves a similarfunction. Bagasse is an option for a few percent of power. Waste stream methane is a possibility for a couple % of power. Limited by the extremely low efficiency of photosynthesis so something PV based will likely be a better way of making hydrocarbons from air and sunlight. Most other 'biofuels' are either fossil fuels with extra steps or ways of getting paid green energy credits for burning native forests. Some grad student might surprise us by creating a super-algae that's 10% efficient and doesn't all get eaten if there's a single bacterium in the room. Detangling it all is hard, but I wouldn't be surprised if wind + solar + biofuels + reigning in the waste was enough -- it certainly works for some people doing off grid.

I'd expect a system based on sodium ion (or even lithium) batteries and synthetic fuels to render any fossil fuel mix unviable in the next decade or two. More scalable batteries or scalable fuel cells would hasten this somewhat.

CAES (Compressed Air Energy Storage)

"Compressed air storage vs. lead-acid batteries" (2022) https://www.pv-magazine.com/2022/07/21/compressed-air-storag... :

> Researchers in the United Arab Emirates have compared the performance of compressed air storage and lead-acid batteries in terms of energy stored per cubic meter, costs, and payback period. They found the former has a considerably lower CAPEX and a payback time of only two years.

FWIU China has the first 100MW CAES plant; and it uses some external energy - not a trompe or geothermal (?) - to help compress air on a FWIU currently ~one-floor facility.

Couldn't CAES tanks be filled with CO2/air to fight battery fires?

A local CO2 capture unit should be able to fill the tanks with extra CO2 if that's safe?

Should there be a poured concrete/hempcrete cask to set over burning batteries? Maybe a preassembled scaffold and "grid crane"?

How much CO2 is it safe to flood a battery farm with with and without oxygen tanks after the buzzer due to detected fire/leak? There could be infrared on posts and drones surrounding the facility.

Would it be cost-advisable to have many smaller tanks and compressors; each in a forkable, stackable, individually-maintainable IDK 40ft shipping container? Due to: pump curves for many smaller pumps, resilience to node failure?

If CAES is cheaper than the cheapest existing barriers, it can probably be made better with new-gen ultralight hydrogen tanks for aviation, but for air ballast instead?

Do submarines already generate electricity from releasing ballast?

(FWIW, like all modern locomotives - which are already diesel-electric generators - do not yet have regenerative braking.)

What a great read. Saving this for additional research later. I also saw something related to harvesting some energy from the tides, which I think would fall under lifting or sinking stuff and virtual batteries. I wasn't convinced on being able to scale it, or get too much of significance out of it, so i didnt read too much into what the exact technical implementation was.

If you're letting the ocean move it, it's generally known as tidal or wave power. It's appealing because it's very predictable and the variability is not correlated* with wind or solar. Afaik cost and placement of suitable sites is a dealbreaker.

One potential technogy https://www.abc.net.au/news/2022-07-31/wave-power-generator-...

New thing I just learned. The tidal stream industry seem confident they can beat nuclear at shockingly low investment. https://interregtiger.com/understanding-tidal-stream-energy/

If they're not lying, that's your high return investment for the future (if you're a policy maker...if you are a private investor then what happened to the solar industry when panels got cheap could happen again).

* uncorrellated variable sources need less storage when combined There are already regions that can work on wind, solar, and a small percentage of existing hydro with no storage for this reason. Tidal isn't completely uncorrelated -- roughly once a month your peaks will line up with solar peak production, and the trough will line up with peak demand for a few days -- but attaching two systems at distance will help with this and it reduces the load on hydro.

It's not "beyond niche", it accounts for 95%+ of worldwide stored energy and is the de-facto energy storage mechanism that all new battery storage technologies are compared against. It also has round trip efficiency comparable to the li-ion batteries (80-90%), which is incredibly hard to beat.

95% stored energy by what measurement? See my other comment. It is not accessible to everyone, nor can it be made accessible to everyone, and the current storage capacity is a marginal fraction of what we actually use. It's a short term load balancing tool that operates within a small energy window.

95%+ by total energy stored/provided.

There's little to no water use in the storage or discharge of pumped hydro, water goes from one reservoir into another. The limiting factor is how much water can be pumped/discharged, not how much water is available in storage (which tends to be significantly more than the amount pumped around). So there's little reason why they wouldn't currently be fully utilized.

It's true that it requires specific geography (water and a place to put water), but it turns out population centers tend to be developed near water sources, already store water for the sake of storing water, and water can feasibly be stored in large quantities underground as well. Which means there's practically many viable large capacity sites near the places that use electricity.

the 95% is misleading. it is barely storing or providing energy but it is a passthrough akin to plugging your phone into the charger 24/7 and saying your phone battery is providing 95% of the energy just because the wall outlet charges the battery first then the battery powers the phone (not an exact metaphor). If you unplug your phone and the phone dies 10 minutes later, you wouldnt say your phone has a good energy storage solution.

Pumped storage is great at what it does, no denying that. And what it does is allow energy production to remain near average while demand varies, and consequently allows energy production levels to be adjusted a bit slower. You aren't addressing the raw numbers though. It serves best as a compliment to a continuous energy production system. As an actual battery/storage solution, it is weak. So it will not be the solution used to store a massive amount of energy generated over a short period of time in order to be used over a longer period of time.

I agree they should be fully utilized, but I am trying to explain that if you fully utilize pumped storage you are still going to have an incomplete energy storage problem. Of course the water levels dont get near max or min capacity - it is designed to take out exactly what you put in as soon as possible or else there is too much risk. The raw storage capacity is small to medium sized - about 10 hours at max discharge (and max discharge might not be enough to keep up with demand entirely on its own).

Basically, the more energy you need to draw the faster you need to drain it and the more energy you want to store, the more massive your reservoir needs to be.

These things cannot be made 100 to 1000 times bigger, nor is there capacity to make 100 to 1000 times more of them. We are better off having them vs not having them but it isnt enough, and if we find a better solution it may become obsolete

The reservoirs we already have are rated in the thousands of MW. There is no storage problem, we already have large enough reservoirs where we can practically store years of energy indefinitely; especially now in drought conditions where reservoirs are regularly well below historical levels. So if there's ever a need to store solar energy from summer for use in winter, pumped hydro is the only energy storage solution that works.

They also don't have issue with storing energy quickly, they can all store energy at a significantly faster rate than they can discharge. We can run pumps as quickly as possible and install as many as you'd like, but the discharge has to be controlled (thus limited) because releasing massive amounts of water at once. So their main use case today is storing massive amounts of energy generated in a short amount of time and releasing slowly across a long period of time.

What the grid actually needs is faster discharge than charging, because that more accurately matches summer energy use patterns. This is what chemical batteries excel at which pumped hydro cannot easily do.

So it's unlikely we'll be able to make them 100-1000x bigger, but they're already 100-1000x bigger than other battery solutions. We should be able to make 100x more of them because the reservoirs already exist and very few of them currently are used as both power sources and energy storage, we simply need to add pumping capability to them in most cases.

Agreed on charge vs discharge comments.

We seem to disagree on the storage numbers. Genuinely curious if my math is wrong on this. I did research a bit more about recent advancements in pumped storage since my first comment and found that my original numbers were almost an order of magnitude smaller than what would likely be built today since I had referenced older tech. So admittedly, pumped storage is much more feasible than my original attitude suggested - which is great because id love for it to be all we need. However, I'm still not sold on it's ability to act as sufficient storage, and I do not see in any way how it could possibly keep things running for multiple days, let alone years of energy as you suggest.

There is a reason we only talk about pumped storage in terms of its discharge rate rather than its storage. We dont really use it for storage. We use it to store the difference between peak and average energy demand, not the total actual demand. You keep the generators running near average all the time, fill the reservoir during the demand valleys and drain the reservoir during demand peaks. Discharge effects ability to actually reach the peak demand, while storage effects how long you can sustain the demand. My point is even if we could discharge as fast as we need to, the reservoirs would empty in less than a day if we needed to rely upon them while energy production was down.

There is a new project (snowy 2.0) in Australia that will have a notable storage capacity of 350,000 MWh .

Current energy usage in the US is over 10 TWh per day. 350,000 MWh = 350 GWh = .35 TWh. So we would need 28 of this brand new top-end pumped hydro stations to hold 1 days worth of US energy demand in reserve. It's ballpark feasible, but lets keep in mind that this plant is costing Australia ~$5-10 billion and is working with two dams that already exist. Very much still in short-term load balancing territory.

This would also lock up 500,000 liters of water per 10kWh. 1 days worth of storage for US: 10TWh / 10kWh = 1 x 10^9; then x 500,000 liters = 5 x 10^14 liters of water = 100 cubic kilometers* (26 trillion gallons). Storing 1 years worth of energy would be 100 km^3 * 365 = 36,500 km^3; which is 3 times the size of Lake Superior (12,000 km^3). I still dont see this as an energy storage solution. MAYBE if use seawater and find a cost-effective way to build facilities into the coastline?

*(1 x 10^12 liter = 1 km^3)

Also to keep in mind that all of this is assuming CURRENT demand, which excludes the incoming energy demand increase for electric vehicle adoption. that's about 2-4 kWh per gallon of gasoline. US uses about 369 million gallons of gasoline on vehicles per day. We can add almost another 1 TWh for that, and then still whatever is necessary for increased usage in general.

It also doesn't consistently depreciate the way that batteries do.

That's around the weight of a fully loaded A380.

What if you go down instead of up? Drill like a kilometer or two and then build a huge cavern at that depth.

that is something people are doing. Also when you go down into rock, you are able to leverage pressure as energy storage as well - which is similar to what this article is about.

There was 1 design I saw where they have a large cylinder cut out of the ground but left in place (so it is loose). Pump water underneath it to raise the cylinder up, then flip the valve and the cylinder squeezes the water back out for power through gravity. I am not sure how the sealing works on that, probably similar to hydraulics

Old mine shafts have been used.

Also, any sort of hydro-based solution is susceptible to drought evaporating water below sufficient operation levels.

> I was always wondering why there are no systems converting the unused electricity in potential energy by moving water to a higher ground. And more importantly: Why there are no water storages on the roof.

Gravity is an incredibly feeble force, compared to electromagnetism (i.e. chemical bonds). Storing a practical amount of energy require a large mass × height.

energy storage density of heating water >> lifting water.

1m^2 of water heated by 50C (20C → 70C) = 58KWh

lifting 1m^2 of water by 10 meters (~ 3 stories!) = 0.03Kwh :-(

I think in the future we will see more homes with:

Gray water recovery

Thermal batteries (hot and cold)

- Excess solar energy can be use to "charge" the hot or cold reservoirs for later use depending on season.

Effluent heat recovery

- Home appliances generate lots of heat (clothes driers, ovens, refrigerators, ...) that is currently vented to the atmosphere. Capture and use it to "charge" the hot battery.

- I'd imagine a water (or other liquid) line that connects to each appliance's exhaust via a heat exchanger to make the above more efficient as a standard part of home plumbing.

Smart appliances that sync with the home's energy system

- eg: clothes dryer that has the option to turn only when there is excess electricity available. Same with EV charging, etc...

[Edited for typos]

> Lots of home appliances generate lots of heat (clothes driers, ovens, refrigerators, ...) that is currently vented to the atmosphere. it'd love to capture that and use it to "charge" the hot battery.

I've seen systems that use waste heat from central A/C to heat a swimming pool. In addition to the condenser they have a heat exchanger connected to the coolant line. Pool water is pumped through the heat exchanger and back out to the pool.

> Why there are no water storages on the roof.

Depends on the area, but sometimes collecting rain water is illegal.

The other issue is that when dealing with rainwater, you need to clean it. Untreated, rainwater will likely develop a bad case of algae.

It’s also really heavy - any amount likely to even light up a lightbulb for an hour or two would collapse a house without major reinforcement. Which would far outweigh any value of the storage.

There is a great youtube channel that tests out different energy storage and harvesting setups, a lot of them focused around water.

This is one where they use solar to pump water to the roof, and then use the potential energy overnight: https://www.youtube.com/watch?v=CMR9z9Xr8GM

A top comment on that video puts it into perspective well. That whole setup can store the energy of a single AA battery.

He would be so much better off just charging a cheap battery. All the money wasted on pumps, wood, barrel, pipes....

The efficiency of pumped hydro storage depends heavily on the height difference involved. There's not that many suitable sites, most have been developed already. It's not a technology that scales down well other than vanity projects.

Rooftop cisterns are very common globally, usually places where the water utility if it exists is unreliable. A huge number of humans get their water from a truck that comes by weekly or such to fill the cistern.

In more high income nations, this isn't necessary because the utility does the same thing at a larger scale with one or more large water towers interconnected.

Someone I know built an off grid air bnb on the beach in Mexico using Earth Ship concepts. I helped out with one of the buildings a little bit, doing the earth bag construction. The passive temperature regulation of those nice thick walls is impressive. He uses well water, and solves the water pressure problem by having a pump that sends it up hill to a pair of cisterns, for hot and cold water respectively. The hot one is the big black plastic ones you see on rooftops everywhere in Mexico. The cold one is made of reused plastic soda bottles and concrete underground. They both are surprisingly effective at what they do. The hot water is frankly way to hot to use without a mixer.

It'd be entirely pointless to use it as energy storage though. The pressure just isn't that high, and the total volume of water involved in the system isn't sufficient. He uses a battery array to buffer his solar.

As cool as all that is, you couldn't build an entire city that way. It's just not dense enough, and again ends up being mostly a vanity or tourist thing.

There are not many suitable sites on existing rivers, but the potential off river PHES resource is vast. This involves building two reservoirs, charging them with water brought in from elsewhere, then cycling the water back and forth. Additional water will be needed only to make up for evaporation and seepage, but this amount is at least an order of magnitude less than the water evaporated by a nuclear power plant of the same time-averaged power.

> This reminds me a lot of the Earthship community … I was always wondering why … there are no water storages on the roof.

All the standard Earthship designs I've studied do store their water on the roof, but they use batteries instead of water pumping to store unused electricity, because to run a conventional house's electrical systems at night you need to store tens of megajoules, and lifting water three meters only stores 0.029 MJ per cubic meter. A cubic meter of water weighs a tonne. So you'd need hundreds of tonnes of water to store the requisite amount of energy that way.

A 12-volt 24-amp-hour deep-cycle lead-acid battery goes for US$61 at retail, and that's nominally a megajoule; it replaces 33 tonnes of water at 3 meters of head, you can pick it up in one hand, and it doesn't require an electromechanical pump/turbine to convert the energy into a useful form. And if it shorts out, though it might cause a fire in your electrical room, it won't flood your house.

If you're building on a plain, you either need to support your upper water tank with earthworks (say, 2 tonnes per cubic meter of earth) or dig out a hole for a lower water tank for water to flow down into (also 2 tonnes per cubic meter of earth). If we want to store 700 cubic meters of water in a 2-meter-deep water tank whose bottom is 3 meters above grade, we need to pile up 2100 tonnes of dirt covering a water-tank-holding area of 350 m², a tank area with minimally a diameter of 21 meters. And you need a similarly sized tank down at grade level for it to drain into. You can cut this in half by putting the downhill tank in the hole you dug to get all that dirt, but it's still over a thousand tonnes of dirt. Aside from the 1400 square meters of water tank top and bottom surfaces, this would increase the earthmoving effort involved in building an Earthship by over an order of magnitude.

Then you need to pump the 700 tonnes of water out of a well, because that's four years' worth of rainfall on the area covered by your giant water tanks (assuming 250 mm rainfall per year). This is a feasible thing to do, and it's less water use than what cattle ranchers evaporate from their windmill-fed water tanks, but it would probably clash with the sensibilities of many Earthship types.

Alternatively you can put 20 deep-cycle batteries on shelves in a closet-sized concrete room. So that's what they do.

If your Earthship is situated at the foot of a 100-meter-tall mesa, the situation changes, because now you can store a megajoule per tonne of water. So you could use more reasonably sized tanks, like, 20 tonnes. But Earthships are mostly not designed for that situation, because it's rare.

> A 12-volt 24-amp-hour deep-cycle lead-acid battery goes for US$61 at retail, and that's nominally a megajoule;

Is lead acid still purchased by anyone as a new system? That's $400 per usable kwh (unless the 50% DoD rule of thumb no longer applies?) where lithium has a vastly higher cycle count at $300/kWh

Is there some upside (refurbishing lifecycle maybe)?

It's a good question! For years I've been hoping for the legendary Lithium-ion Crossover Event after which lead-acid batteries become obsolete for all purposes, but the auto parts stores in my neighborhood are still stubbornly stocking lead-acid starter batteries for some reason; so, too, the burglar alarm folks. Maybe the Crossover has already happened in the developed world, by which I mean China, but the news hasn't yet reached Argentina?

There's really no benefit to lithium batteries in a car. They perform much worse in the cold, and they are more expensive. You don't cycle your car battery 100-0-100 so the fact that the useable capacity of lead batteries is lower doesn't matter.

Yeah, but the question is whether they are more expensive. If they were cheaper (per watt in this case, rather than per joule) people in Brazil and northern Argentina would probably use them to start their motorcycles. I think they're closer to that crossover than to the cost-per-joule crossover.

I'm not sure if you're being facetious, but in thinking about it I think you've pointed out the remaining advantages. Current from lithium batteries has to go through a bms (and thus you have to build them to a current rating, even if your cells are 5C the cost of BMS may make lead acid cheaper) and they have low temperature issues so I can see them being useful in current limited situations and in situations where not thinking about them or the weather too much is more important than capacity.

I can't see this recommendingnlead acid in a capacity limited application unless it's a situation like Australia where half of the local shops are pricing them at 2012 rates.

I'm not being facetious, I'm just trying to understand the discrepancy. Your explanation isn't it: battery management systems do not account for a major part of the cost, we don't have low temperatures in Buenos Aires, and the auto parts stores are not selling the lead-acid batteries as specialty parts to people who drive in from Patagonia to replace their car batteries, similar to engine block heaters.

sbp looked into the issue and it seems that lead-acid is still cheaper.

https://justcatamarans.net/lithium-vs-lead-acid-batteries-co... claims that the Li-ion batteries they are trying to sell you are still more expensive.

https://news.energysage.com/lithium-ion-vs-lead-acid-batteri... says, "The one category in which lead acid batteries seemingly outperform lithium-ion options is in their cost. A lead acid battery system may cost hundreds or thousands of dollars less than a similarly-sized lithium-ion setup."

https://www.pv-magazine.com/2021/09/02/lithium-ion-vs-lead-a... says, "Citing previous studies, the researchers said that, for stationary energy storage, lead-acid batteries have an average energy capital cost of €253.50/kWh and lithium-ion batteries, €[1555]/kWh, and that their total average power cost is €333.50/kWh and €[2210]/kWh, respectively."

In the units I was using above, those costs are US$68.30/MJ, US$419.0/MJ, US$89.86/MJ, and US$595.5/MJ.

So it seems like the answer is that, yes, lead-acid is still purchased by some people as a new system, because it's still cheaper than lithium-ion. Probably what led to the error is that lead-acid in Argentina is more expensive than lithium-ion in China, or wherever Schroederingersat's at.

Are those prices up to date? There are several shops in Brisbane AU that will sell drop in 12V batteris retail for 350USD per nominal kWh (about $320 per real kWh at 0.1C) including taxes. The occasional special is in the 250-300 range. Ebike batteries with higher current are in the 300-500 range.

They're highly unsuitable for an engine starter at that price though (1C BMS) and have no built in low-temperature monitoring. There are bigger systems with better safety features for about $400US/kWh available in europe and asia.

Also note that nominal capacity of a lead acid battery is often not usable capacity. I was assuming 50% DoD as usable daily capacity which may be pessimistic.

It's possible they're not! But I'm suspicious of this notion that prices have dropped by a factor of 5 since last year, and have reached Australia but not Just Catamarans in Florida; maybe they're doing the calculation differently than you are. Or maybe there's a fraud going on.

FWIW your US$320/kWh works out to US$89/MJ. (I try to use SI units when I can; it saves a lot of hassle.)

I think it's fair to exclude taxes, but not to include "the occasional special", since the retailer is presumably taking a loss in that case and will be unwilling to sell you an arbitrarily large number of batteries at that price.

A thing I wonder about is how big a Li-ion battery you really need for an engine starter. 200 amps at 12 volts is only 2.4 kilowatts; a 15C battery with 0.6 MJ capacity could do that, which is about a dozen 20700 cells. You do need a 200-amp BMS, but I think the cost of the cells is still the issue.

Are you correcting for usable capacity in your prices? 50% DoD was typical for lead acid last I checked vs 85% for LiFePO4 (or 100% if you're happy with it lasting as long as lead acid at 50%)

As to fraud, I've seen at least four batteries perform as advertised. Also a 5x disparity in prices between different shops is entirely consistent. Prices have been dropping rapidly and there are enough people who go 'oh yeah, that was about right 3 years ago'. Plus retailers may have paid several times current retail for their stock if it's a year or two old and was bought at prices that hadn't been updated for a year.

Thanks, that's very interesting!

The price I gave in the comment you're replying to is just a literal unit conversion of the price you gave.

Water can freeze.

It is probably far more cost effective to buy a new air source heatpump. They’re so efficient and work well at low temps now. I have infloor radiant heat at a rural house in upper Midwest, have thought a lot about using solar heat pipes and adding a storage tank, and it’s all so fiddly, pipes and pumps and manifolds and glycol and maintenance, all custom, all expensive. I’m almost certainly better off with PV panels and off the shelf heat pumps.

Even a modern air source heat pump typically doesn't beat gas for $/kwh. Even in europe where gas prices are sky high, electricity still costs more when you look at the seasonal efficiency of heat pumps, which is typically advertised around 4.4, but various studies show that in most real world scenarios you typically won't reach the quoted lab numbers, so expect to get more like 2.5-3.0.

Study: https://www.sciencedirect.com/science/article/pii/S037877882...

Claims like that really aren’t helpful without numbers backing them up

the cited paper gives the efficiency of heat pump but we’re missing multiple crucial values to get from there to a comparison of heating costs

Edit: https://great-home.co.uk/air-source-heat-pump-running-costs-...

At current average British prices of 28p for electricity and 7p for gas, and using averages for both it looks like the commenter was mostly correct.

Not 100% if the spf Gives the realistic cop

In a new build that opted to forgo any gas connection, I’d be interested in seeing the costs from the substantial infra savings and hookup savings.

From that paper, the average COP measured was 3.06 (averaged between 2 and 7 degree outdoor temperatures, typical of the UK).

The UK electricity price is currently fixed at 34p/kwh for electricity and 10.3p for gas.

A new gas boiler has a COP(efficiency) of 1.054 (it can manage efficiency higher than 100% because gas is metered by the 'lower heating value', which assumes the exhaust gas escapes as steam, but the boilers actually condense most of that steam to water, getting additional energy out).

So. Total price is: Gas: 10.3/1.054 = 9.77p/kwh in your home.

Total ASHP price is: 34p/3.06 = 11.1p/kwh in your home.

And this analysis ignores the fact that ASHP's typically have much worse efficiency making hot shower water (which a gas boiler doesn't), and obviously also have considerably higher upfront costs too.

However, the ASHP can also be used to cool your home in the summer. In the past this has been a dubious benefit for most of northern Europe, but heat waves have been getting stronger, last longer, and happen more frequently as time goes on. This is turning into a serious consideration.

Also, you can theoretically power a ASHP with renewable energy, while there are few if any carbon neutral replacements for natural gas.

Most boilers purchased today allow use with a Hydrogen mix, and some under development allow 100% hydrogen. Before the widespread extraction of natural gas, towns were powered with 'town gas', which is ~50% Hydrogen, so this is very much proven tech.

There are a bunch of potential ways to make green hydrogen too.

So, there very much is a path to green with a gas boiler.

Green Hydrogen has not worked out so far. It is not clear that it even has a path forward. There are a lot of people hopeful that it will be a solution in the future, but as of today it is so uneconomical that even people willing to spend more to be green don't use it.

But making green hydrogen has pretty bad efficiency. It could never compete with a heat pump using the same electricity source, even if it had an efficiency of 100%, since the heat pump has an efficiency of 200-400%.

Yeah, these can make ASHP a good idea anyway, but the grandparent was nevertheless correct saying that they don’t beat gas for heat in terms of cost in normal (I.e. not current) circumstances.

I read somewhere there’s market failure too. Electricity price is largely proxy of gas price. While uk gov subsidises has they don’t electricity…

Thus the solar panels. So long as the installation proves to make financial sense, the PV production offsets the heat pump efficiency, even in winter months.

Exactly. Also, the top commenter noted that only electricity was available and currently was relying on massively inefficient baseboard heaters. Solar panels and heat pumps also have great rebate programs and eventually gas will be phased out. Some US cities are now banning gas on new construction in the near future.

The point is that custom solar/geothermal installs seem neat and efficient —- waste not, want not — but probably have a hard time competing with the economies of scale and low maintenance of PV panels and air source heat pumps.

This may work in mild climates, but the areas where winter heating is most needed will have the solar panels under a bed of snow during the winter months.

Additionally, winter months are usually the cloudy ones. It's not very uncommon to have just a couple of sunny days per month in European winters, driving down solar gains even if there's no snow cover.

It's not as bad as this.

In climates where you will have a bed of snow during the winter months, your optimum tilt angle for a fixed solar panel is something like 30° off of vertical or steeper. I find that on mine the snow falls off since it is a south facing, steep, dark surface.

You engineer to make sure you have enough energy captured each month to meet that months needs. That might lead you to a tilting mount, or just a fixed angle and having surplus energy most months.

In my case, panels were much more expensive when I designed my system, I initially roof mounted them, but when replacing the roof under them I moved them to a pole and went with a tilting mount and manually move the panels twice a year. My load is much higher in the summer, but I still need some power in the winter. And northern winters can be cloudy a lot and have limited sun even on a clear day. At my location there is about a factor of 5 difference in solar energy per square meter per month. So you design for each month and then pick a solution that is best. In my case the summer load is so high that even though I get 5 times the energy, it is still the driving force on sizing the system.

Interesting. Last winter my parents' rooftop solar had zero output until April, as it produces nothing until all the panels are clear of snow. Granted their installation is at a fixed angle that matches the roof, and this is in the Nordics.

During summer months the production mostly covers and partly exceeds their use, but the sell price is so much lower that it doesn't even begin to pay for the rest of the year. But that of course then relates to the installed capacity.

I wonder if it makes sense to build some simple resisting heating elements into the panels, to allow the snow to slide off. Shouldn’t use all that much energy to run it occasionally.

You can actually push a bit if current backwards through the cell to heat it up. Some people have tried this but not sure about what prevents this from being more wide spread

https://www.pv-magazine.com/2020/03/18/heating-solar-panels-...

So much of this discussion depends on where you are. In the US, a typical solar installation is on the pitched roof of a house. When the sun comes out, the dark roof and PV panels heat up, and the snow slides off. Rowhomes or apartment buildings in a city might have flat roofs with panels mounted on racks, but most US cities won't get enough snow often enough for this to be a problem.

My parents do have a traditional pitched roof. Winter sun is not warm enough to warm up anything, even if dark, until late winter. Plus the color of course is white if it's covered in snow :) Yeah very much location dependant.

They are on a pole on your roof or you moved them to the ground?

I moved them to the ground. The roof pitch is about 1:1 and I’m not feeling like trying to stick to it any more. The pole puts everything at a comfortable working height.

Selling PV electricity back to the grid is almost always many times worse, depending on state incentives, than consuming the electricity. For example, my utility sells electricity to me at 9 cents/KWh, but only buys from me at 2.5 cents, and charged me a fixed monthly fee for the meter to boot.

If the commenter’s house doesn’t have gas, then it doesn’t seem to make sense to install it, and the price differential in the US isn’t as great as in the UK (how’s that Brexit thing working out?).

Huh. We have a 1:1 credit system here in Washington.

Considering solar owners are all pushing electricity onto the grid at the same time, when it is needed the least, such a system doesn't make sense. There are real costs associated with getting rid of all that unwanted energy.

> when it is needed the least

I don’t think this is true in many areas. In our area, during the summer the neighborhood is running A/C when the sun is at its peak. During the winter, heating. Also the transport loss from a power station is nothing to sneeze at - my understanding is that locally produced power often just results in reduced demand on the larger grid.

I think solar makes sense for most who use air conditioning. That's a load matched pretty closely to the timing of PV generation.

Here in the Sierra foothills, it's been a blazing hot summer. About 90% of our PV generation powered our home air conditioning. We shipped very little energy to the grid on hot days. And it was awesome to have a comfortable environment without sucking grid power on the days when it hit 113°F.

This is an enormous subsidy to residential PV. Electricity wholesalers would love to be able to sell it to the grid at retail prices, like you are. Instead, they sell it for something like a quarter of retail price. This huge subsidy is only sustainable for so long as the PV penetration is low in residential market.

Yes. Even just being able to connect to the grid and only pay for the energy consumed is a subsidy. There is a large fixed cost to provide the connection and guarantee power will be available on it when demanded.

And Massachusetts.

It would make even more sense to sell that power back to the grid, and then spend the $$$ earned on gas, which would work out cheaper overall.

Obviously in many parts of the world, market distortion means the buy price and sell price for electricity is very different, and in that case a heat pump might make sense to combine with PV.

That's highly dependent on your local electricity and gas prices. A quick google search tells me residential electricity in Germany costs about 2-3x what it does where I live in the northwest US. We're on mostly government owned hydro power and electric prices have been stable for a long time, meanwhile gas keeps going up.

EDIT: Did some quick math using my last power bill, at current prices a heat pump just needs to be about 2.9 average COP to beat gas in cost for me, if gas keeps going up that'll keep dropping!

This seems like a (regulated) pricing error.

There should be no economic way that a high efficiency turbine produces and distributes electricity to heat homes at a greater than 1:1 ratio compared to storing, pressurizing, and delivering, then burning in irregularly maintained consumer homes.

Likely the error is that gas pipes to the house are subsidized (albeit the electrical likely is too, with heat pumps and induction stoves, the gas lines are unnecessarily redundant)

A furnace causes a very large second law loss, converting chemical energy to low grade heat. If that gas is used to drive turbines, and the work produced used to drive heat pumps, much of this entropy generation is avoided.

A home gas-driven heat pump could be a better option from an efficiency standpoint, but those are not widely available, probably for cost and reliability reasons.

"high efficiency" turbines are not all that efficient. Burning gas releases 100% of the available energy as heat. Converting that gas to electricity is << 50% efficient. You also lose ~5% just transmitting the electricity to homes.

Also, electricity from gas is relatively expensive. Other sources like coal or hydro are cheaper, which lowers the average cost of electricity.

If you combine gas turbines with district heating you can recover almost all of the heat energy. If you use the produced electricity for heat pumps you come out ahead. For climate change purposes it is way better since gas pipes to homes leak way more methane than those to central power stations

But gas turbines should be more efficient than a burner under a boiler or a burner in a furnace.

Yes, solar, wind, hydro, nuclear should be cheaper per unit (coal is not usually cheaper than gas in the US)

There's not really a "should be" in thermodynamics. All heat engine cycles have upper bounds of theoretical efficiency, and burning gas in a gas turbine to generate electricity to create resistive heat is never ever going to be more efficient than burning that gas at the point you need the heat. It's simply not possible. There's always going to be losses - the exhaust gas will contain energy, there will be mechanical and transmission losses. There is no physical way we can change that.

The best thermal power plants - combined cycle gas turbines - get about 60% efficiency. Most are closer to 45%.

What we can do is either use the excess heat from the gas turbine in district heating (which combined with resistive electrical heat probably approaches the same efficiency as a local gas boiler), or use that electricity to drive a heat pump, which gives a greater than 1x return on heat where you want it. An efficient combined cycle gas turbine driving a heat pump is going to give you more heating than the same gas being burned in a boiler. - 0.5 * 4 = 200% efficient.

You can also get (although they're much less common) gas powered heat pumps (eg propane fridges in RVs). They might have a "primary energy ratio" of 1.5-2, bringing the total system efficiency of a local gas powered heating system back up to pretty close to that of a remote generation + electrical heat pump system. The electrical system has the benefit that you can slot renewables into the mix as well.

> to create resistive heat

We're talking about heat pumps not resistive heat.

95% efficient gas furnaces are the norm for newer houses[1].

Getting electricity out of gas is fundamentally harder than getting heat out of it.

[1] https://iwae.com/resources/articles/95-afue-gas-furnace-work...

There is a large second law loss even in a "95% efficient" furnace. A gas-fired heat pump would have efficiency much higher than that.

That's kind of disappointing news. I had a skim through the article and was surprised to find that it's all based on models. There wasn't a single empirical measurement of an installation of either kind. Not that I don't believe in modeling and its usages, but I am going to radically discount the findings of this paper because there was no actual experiment performed here, just fiddling with models.

Give it a re-read. They collected data from actual houses (although granted only 6 boiler-years worth of hourly data), then fitted a best fit model to that data, then used the model for their conclusions.

They did that because they needed to compare the manufacturers datasheet lab figures to the real world figures, but there are 10+ variables that affect efficiency, and a direct comparison isn't possible unless all the variables match - hence using a model to act as the 'convertor'.

> They’re so efficient and work well at low temps now.

Can someone provide me model numbers for these heat pumps that “work well” at low temperatures? I’ve been through more than a few over the decades and currently have a Trane from last year and yep, still doesn’t hold up very well once the temperature hits freezing. Don’t get me wrong, I like heat pumps, but my eyes roll into the back of my skull when I hear over and over again that they’re “so efficient now” at low temperatures.

Fujitsu XLTH: https://www.fujitsugeneral.com/us/residential/technology/xlt...

Mitsubishi Hyper Heat and H2i Plus: https://www.mitsubishicomfort.com/articles/what-is-a-heat-pu...

Efficiency with these heat pumps severely decreases when temps go below 0F, but they still produce heat.

Their sites are admittedly not all that clear. GreenBuildingAdvisor.com has a ton of useful info on it, like https://www.greenbuildingadvisor.com/question/cold-weather-p....

If you actually live in an area where temperatures drop below -30C regularly, you might want to consider a ground source heat pump. They are a bit more expensive than the typical air source heat pumps since there's some digging involved, but for a new house they aren't that much more expensive since there's already digging for electricity, plumbing, etc. For example ground source heat pumps are getting quite popular in Finland.

Who’s talking about -30C? I’m talking about 32F, or 0C. I’ve yet to be impressed by a heat pump once the temperature gets at or especially below freezing.

Geothermal heat pumps are great but at least in the US they are wildly expensive compared to traditional ones. You either need a large lot for horizontal laying of the pipe or a DEEP hole if going vertical ($$$). Plus they’re uncommon enough that contractors are happy to charge a serious premium.

this was installed in my home to replace a oil furnace (air heating)

https://www.gree.ca/products/ducted-central-heat-pump?varian...

I am not sure if it's available outside of canada though.

interesting. i have a PV system. i want to use the half installed central heating system (only piping laid down) (no boiler or FCU/room unit) and i have this crazy idea. use solar water heater to heat water, that feeds into a water storage tank, that water gets heated MORE by an air source water heater and a small circulation pump uses that water storage to heat the house.

i am hoping, since we currently have 0 whole house heating, this would be better than nothing and the problem of not having consistent electricity to run the air source heat pump all times would be alleviated somehow by using solar water.

is that crazy?

I assume you’re on the grid?

Yeah, sadly, it’s crazy. Solar heaters are expensive and require a large area to get decent BTUs out of. If you can find free solar heaters that someone’s giving away, that can make the difference, but they’re often a few thousand dollars and that’s about equivalent to a Fujitsu or Mitsubishi heat pump base install, especially when you factor in all the extra plumbing and pumps to/from the solar thermal array.

WORSE, there’s maintenance. You’ve got the potential for leaks (probability rises to 1.0 over the years), and you have to worry about the water freezing. That can be mitigated by using a glycol mix, but then you’ve got glycol and that lowers the efficiency.

And then you have to think about climate. Is it reliably sunny in the winter? Or is it only sunny 1 out of 3 days, with long stretches of gray days? You are probably better off plowing that money into efficiency/insulation improvements to lower the overall heat load (and those improvements often have zero maintenance).

I’d suggest looking into an air source heatpump that can produce domestic hot water, paired with a large storage tank. Check out Artic Heat Pumps: https://www.arcticheatpumps.com/

I am currently looking to do something similar. I have zero heating in my house at the moment and have a 300L (65 gallon) thermosiphon solar water heater [1] (tank plus panels all in one system) that gives me plenty hot sanitary water.

I am looking to install an hydronic fan coil [2] radiator in my rooms and need water to heat them - I thought to use the excess heat from the solar heater to supply hot water to the fan coil units but I am not sure how do do that. If you have an idea, please share.

1. https://sol-aqua.com/thermosiphon-solar-water-heater/

2. https://www.arcticheatpumps.com/heat-pump-fan-coils.html

This is not so complicated, you just need tanks with heat exchanger loops. The solar water heater input comes into your tank at the bottom and goes through a copper coil, giving you domestic hot water.

_Some_ tanks will have a similar coil in the middle/top of the tank. What you can do is then run the cold return from your heating units through this coil. It will heat the radiator water before going to the fan coil unit. This is often done as a precursor "boost" before sending the water through a boiler/heater -- the solar heat is applied to cut the temperature differential and reduce the energy expended by the boiler/heater for the radiant heat, but you're not relying solely on solar heat.

You can get heat pumps that use a water storage tank with extra heat exchanger loops like this, precisely for providing domestic hot water. The DHW and radiator water never mix. There aren't any moving parts, so it's simple.

The only question is what the max temp is from the thermosiphon tank, and whether that exceeds either the max radiator water temp or, if you use some kind of modulating boiler/heater for the radiator, the max input temp.

The math for solar storage gets daunting, however. If your typical max temp for the thermosiphon tank is 120F on a sunny day and the cold return from the FCU comes back at 70F, then with 65 gallons you can store about 26K BTU in your tank. That sounds like a "2 ton"/24K heater, but it isn't because the BTU rating of heaters is _per hour_. So, if your house needs a ~24K BTU system for heating, a sunny day storing heat in your tank will get you about an hour's worth of free heating. 24K BTU/hour is a pretty small heat load for a typical house... (though plenty doable if you invest in insulation, windows, etc.)

So, the thermosiphon is maybe adequate to provide you with domestic hot water, but in a cold climate you'll need 3–5 more of them and a repurposed and well-insulated 500gal milk tank in a shed near your house with insulated below-ground pipes running to it.

Thanks for your input. So I understand that heating the house requires much more hot water than what my solar heater can probably provide. In that case, connecting it to the radiator system is not so beneficial, and probably better to rely on a heat pump for the radiator water (the solar heater will not provide much help to justify connecting to it)

Is that correct ?

Basically. There are all sorts of neat DIY examples of people using large solarthermal arrays and big storage tanks to provide adequate heat, but they’re major projects and custom.

For your system, what I’d potentially recommend is looking at a heat pump that can provide both domestic hot water -and- hot water for your radiators. They exist! Then, explain to your vendor/installer that you want a second heat exchanger in the tank for input from the thermosiphon. Then you would be providing solarthermal to both house heating and DHW— it won’t be enough, but it is theoretically better to apply that input across both loads, in a shared manner.

However, the question is cost. You’ve got a DHW system right now that works. To install a new system with a tank that can accommodate the thermosiphon input and provide DHW and radiant heat may be too expensive to be worth it.

A second thing for you to consider is the heating area of the radiators. Old radiators are designed for very hot water from boilers, ~180F. Heat pumps for radiant heating operate between about 95F-120F (lower is more efficient, but less responsive). Because of the lower temperature, they need a larger heating surface to deliver the same heat to the house. So, typically there’s a mix of larger European-style radiator panels and in-floor heat. In-floor heat is pretty great! A room at 65F feels like 72F when the floor is warm.

Thanks a lot for the detailed reply. Old radiators operate in high temperature but from what I understand, the fan coil heaters operate in lower temperature more equivalent to radiant floor heating.

For cost reasons, installing undefloor heating is not an option, that's why I opted for low temp radiators on top of floor. However, it sounds like the heat pump system will not be cheap either. According to BTU calculator I found online, seems like I need 28K BTUs to heat the home. I thought about pellet burners as well, but here in Europe, the cost of pellets has sky rocketed this year. I don't want to be dependent on gas either, also electricity is not cheap... So I am not sure exactly what i'll go with. I'll do the calculation, but your input on the effectiveness of the theromosiphon system is valuable, thank you.

Daikin makes air source heat pumps with water storage, and that can be paired with solarthermal: - https://www.daikin.com/products/ac/lineup/heat_pump - https://airconditionerlab.com/daikin-heat-pump/

A "variable speed compressor" is what you want in a heat pump, as these modulate their speed (and electricity usage) to produce a constant amount of heat. You might be okay with a 24K BTU system, especially when coupled with the thermosiphon and then some effort spent on insulation & air-sealing?

Here in the US the base system would be about $3K USD, but with installation costs on top of that.

this is interesting. i have been told about using 319 steel insulated tank to store hot water, i was also told that this is a "pretty big deal in terms of cost" but i am willing to make that investment.

here is my idea.

300Litre solar water heater+200Litre air source heat pump + storage + FCU.

I don’t have intuition for how large a 300L solar water heater would be — you mean the solar array on the roof, right?

If you have infloor radiant heat with concrete floors, then you might not need much water storage — the concrete provides heat mass. I would look more at deploying PV arrays for offsetting the electricity usage of the heat pump than adding more water storage and solarthermal. Solarthermal is technically more efficient in providing heat alone than equivalent electrical energy, but a heat pump uses that electricity to gather heat from outside, operating at 200-400% efficiency. Electricity is also more useful across the whole house, and of course is a great help in the summer for providing air conditioning.

yeah, the problem with infloor is that the floor is already laid down so i would have to redo the flooring for all rooms.

my use case is winter nov-feb which occasionally has snow, some year there is 0 snow, some year there is 12 inches, usually inbetween.

electricity from the grid is often down during this time period but when it does come, we could use the heat pump to heat up the "storage". 300L water heater in this case means 300L/day of water heater (yeah, rooftop solar water)

the storage can be anything, 500L, 1000L, 2000L, that is cost dependent only. if we feel more storage is needed, we just add another tank

heat pump is like 200L/hour so yeah

Ok, you're in Kashmir and don't have a reliable grid. I'm not sure it would make sense to spend the money on a heat pump if you didn't have reliable electricity to operate it (if you had sufficient PV panels, then it might be worth it).

Build It Solar has a ton of DIY projects for improving efficiency and using solar energy in various ways. You might find inspiration there: https://www.builditsolar.com/. In particular, they have plans for a solar collector that's a wood frame box with clear plastic sheeting and window screen material — extremely cheap and simple to build. It provides hot air, and it's best if you can fit a few cheap fans (like 120mm 5v PC fans) into it, powered by a small PV panel. I want to build one of these to heat my garage.

Some of the nice things about air-based solar collectors is that they're cheap, low maintenance, can't leak, and can't freeze. The downside is that storage is harder. If you have enough heat mass inside the house it might not matter, though.

Oh, also: an air source water heater would need an external heat source—otherwise you’ve just made a closed system and won’t be able to heat the house beyond what solar thermal provides, plus heat loss of the house.

I remember reading about the enertia house, which had an interesting system.

They used certain trees inside the house that would phase change at room temperature so would keep the temperature comfortable naturally.

I think using a phase change is much better than just straight heating or cooling

https://pogue.blogs.nytimes.com/2007/05/17/a-home-that-heats...

My uncle had a home like this built in the 70s. His whole home was solar heated.

AFAIK, it worked pretty well year round.

Solar heating is one of the most efficient usage of solar energy. The reason we don't generally do it more is because it's expensive and can be a headache to maintain (what happens when you spring a leak or your PVC pipes become brittle from sun exposure? Or when your heatbox ends up fogged or dirty).

In that case, it can be a lot simpler to rely on heat pumps and geothermal energy storage.

Forget geothermal, I'd love an air-exchange system for the summers where I live. It's not uncommon for the low to be in the 50s, with the high in the 80s. If I could just store some of that overnight cool in water and then run it through the central air system, I almost wouldn't need AC.

There's work being done on incorporating paraffins and other such materials into walls, so that they go through a phase change close to room temperature. That way they act like heat mass, but without all the weight: https://www.doityourself.com/stry/phasechangedrywall

So...I'll save on energy by turning my walls into candles. Interesting.

More thermal mass could help (e.g. lots of concrete or masonry might help)

I was reading someone's description of their greenhouse. They built a cistern under it to catch rainwater for watering plants and ran heat pipes into it for warmth in the winter. I've been dreaming of doing the same at some point.

Can you share some of these greenhouse channels? I’d love to follow them to learn about how to better take care of my greenhouse.

This reminds me of this article: https://www.lowtechmagazine.com/2015/12/reinventing-the-gree...

oh yeah the wall pipes would act as a thermo-siphon so that's neat.

I've seen bananas growing (and fruiting) on the north west coast of scotland using these techniques

There is a school close to where I live that does something like this.

During the summer they heat the bedrock using solar panels (thermal and electric via a heapump) to heat the bedrock far down using water pipes. In winter they extract this energy for heating, electricity and hot water making the school self sufficient for energy.

Here is a write-up from the municipality about it, but it's in Norwegian

https://www.drammen.kommune.no/om-kommunen/aktuelt/unikt-pro...

Edit: translated version

https://www-drammen-kommune-no.translate.goog/om-kommunen/ak...

In a smaller scale, this is also what the French solar panel manufacturer Dual Sun [^1] sell with their geothermal+hybrid panels solution for individual houses.

- In winter, heat is generated with a geothermal heat pump and an horizontal underground network of pipe.

- In summer, the thermal solar excess energy is used to warm up the soil that has been cool down during winter and also to avoid permafrost.

[1]: https://dualsun.com/applications/pompe-a-chaleur-solaire/

How effectively does the ground maintain this heat, and how much of it dissipates up and out?

As a side effect: using a heat pump to heat the ground during summer also cools the building on top! (When its at the other side of the heat pump loop)

Same idea in Alberta: http://dlsc.ca/borehole.htm

And at the Blavatnik School of Government building in Oxford: https://www.bsg.ox.ac.uk/about/our-building

That's just a ground heat pump no? I have the same on my house. It's pretty great. I get cooling in summer and heat in the winter.

If you're in an area that gets hot enough in summer for you to want cooling, then yes you'll be pumping heat into the ground during summer and extracting it in the winter. For those of us who don't get hot enough in summer to want cooling, we're always extracting heat from the ground, and so the ground we're extracting heat from gets cold, which reduces the efficiency of the system. Having a solar powered mechanism for putting some heat back into the ground in summer would help alleviate this problem.

Another approach would be to have a longer length of piping. The rate at which the soil temperature drops when you extract heat, and then 'recharges' naturally, will vary greatly if you have 100m vs 1000m of piping. This is why ground source heat pumps use pipes laid out in coils, as it greatly increases the surface area.

> the ground we're extracting heat from gets cold

Not if you’re using deep drilled holes (100m+). You can run a heat pump of the differential indefinitely

Apartment buildings in Finland have these 100m to 200m drill holes for the heat loops since you can't run those horizontally to the neighbor's yard. A bonus benefit is that the core of Earth has to basically cool down before they run out of heat to extract.

Wait how long does that heat last. I wouldn’t imagine more than a week since it’s not insulated?

You don't have to dig very deep for the soil on top to be decent insulation.

Interesting option. It's all about the cost and efficiencies in the end.

Converting electricity into heat and then back into electricity is a lossy process that is constrained by the second law of thermodynamics. Basically generating heat is easy and efficient. But converting it back to electricity is typically less than 50% efficient. Basically, it's several energy conversions in one process: you use electricity to produce heat, heat to produce mechanical energy, and then mechanical energy to produce electricity. All of those steps are lossy, some more than others. And it multiplies. It adds up to less than 50% efficiency, at best.

Lithium ion batteries are much better at over 90% efficiency. But they are relatively expensive for large amounts of storage. So they don't scale. But they have no moving parts and you can scale the capacity at which they absorb and release energy pretty much linearly. Which is why they are popular for balancing the grid.

Pumped hydro, which is a type of battery that has a relatively large amount of deployed capacity in some countries (many gwh of power stored) is both efficient and cost effective. But it's highly dependent on terrain. Yet parts of Canada run almost exclusively on hydro and pumped hydro.

Pumped thermal storage, where a reversible device is used to generate heat (and cold), then generate power from the heat (and the cold), can have efficiencies of up to 75%.

https://aip.scitation.org/doi/10.1063/1.4994054

"Insofar as the numbers I have presented in this paper are correct, they demonstrate that energy storage is a problem of 19th century science. No future laboratory breakthroughs or discoveries are required for solving it. All that is needed is fine engineering and assiduous attention to detail. Said poetically, this is 21st century rocket science."

Even resistive heat can give efficiencies > 50% if the storage temperature is sufficiently high. For example, the ENDURING system from NREL (now being commercialized by Babcock & Wilcox) has a projected round trip efficiency of 53%, using sand heated to 1200 C to store the heat. Heat transfer from sand to the working gas is via a fluidized bed heat exchanger, a nicely compact system with very large heat transfer area.

https://arpa-e.energy.gov/sites/default/files/2021-03/07%20D...

> But converting it back to electricity is typically less than 50% efficient.

It's generally much much worse than 50%. Say that you used a resistor to heat the water in a (perfectly insulated) boiler and want to know how much useful work you can extract from it, ideally. The answer is given by the exergy efficiency, which can be computed by attaching an imaginary heat engine to it, integrating the infinitesimal works δW = δQ⋅η_Carnot while the temperature drops from the initial T_boiler to T_ambient and dividing by the internal energy of the boiler.

The result is η_exergy = 1 - T_ambient/T_lmtd where

  T_lmtd = (T_boiler - T_ambient)/log(T_boiler/T_ambient)

is the logarithmic mean temperature difference. Even heating water to 90°C gives an efficiency of only 9%.

> Say that you used a resistor to heat the water

If you're looking to maximize round trip efficiency, this is probably not a good starting point. Better to use a heat pump, as mentioned elsewhere.

Yes, if you're counting the heat output as part of the useful energy, but parent was specifically talking about converting electricity into heat and back again.

So am I. If you're going from electricity to heat for storage and back to electricity, you'll get better efficiency by using a heat pump to generate that stored heat. In general, sticking to processes that are as close to reversible as possible will be the most efficient.

Ok, I see your point now.

Using batteries for the grid requires roughly the same amount of batteries as EV’s do. A 100kWh battery pack for a car * 1,000 charge cycles / 25 years = 4,000 kWh of battery per year * ~290 million cars. That’s enough to store almost 1/3 of all current electricity used in the US while ignoring busses and farm equipment etc.

Of course actual cost depends on how much storage we need and future battery prices but adding something like 20-50$/month on future electricity bills gets offset by both cheaper electricity from solar and wind and less excess generating capacity. In that context large scale batteries seem like a perfectly reasonable solution.

Which is supported by many current grid scale PV instillations including them so they can avoid a AC>DC step when charging the batteries and then sell during peak demand at a premium.

> Using batteries for the grid requires roughly the same amount of batteries as EV’s do

EVs have much harder constraints. The most obvious is that they need to move around (requiring high energy density, for both mass and volume); they also need to cope with sporadic charging times, and be reasonably fast to charge. It's very hard to compete with the leading Lithium-ion batteries in this space.

Grid storage isn't as constrained. Larger, heavier batteries are fine since they're just going to sit in one place. It's also easier to accomodate awkward/slow charging requirements, since they're always plugged in to the grid, and can be coordinated with other electricity sources/sinks if needed.

This allows different chemistries to compete, based on e.g. price, longevity, safety, etc.

Yep, it’s very possible that wildly different chemistry or even some other method wins. However, using the same battery chemistry in a cheaper form factor is the worst case. Aka if 2.3 Trillion on EV batteries works then the winner must cost less than 2.3 trillion.

Or have fewer externalities, since that’s never properly priced into anything.

This is never going to happen. As a car owner, you are in a market competing with large providers of grid scale storage, that have large purchasing power, control over the battery chemistry and finer specifications of their hardware which is made to order for their needs etc. Meanwhile, you have a mobility-optimized low weight battery that has a single, monopoly supplier that will very likely treat the spare parts market as a profit center 5 years down the road.

You will never be able to compete because you cannot differentiate, the product is fungible so mass always wins. It's essentially the bitcoin mining rig drama, the going was good until large scale mining operations were set up, with optimized ASICs etc. After that, good luck destroying your graphics card and car battery for pennies on the dollar.

Couldn't scheduling the EV charge when the demand is low while production is high help a lot balancing the grid, at basically no cost for the EV owner?

Let's say I have a EV with a smart charger that will keep the vehicle at least 60% charged, but charge up to 100% when the energy price is low (e.g. during the night).

> at basically no cost for the EV owner?

Only if you have a battery with unlimited charge cycles. This doesn't seem possible with current technology - and even if it were, manufacturers would still optimize for higher density and reasonable longevity after 100,000 miles. Most people would average a charging cycle a week, so they can't see the difference between a 2000 cycles battery and a 50,000 cycles one in the life of the car, but they can definitely feel the effects of range anxiety.

When you do daily or multiple times a day cycles, as is typical for grid applications, that's a completely different beast, for example a shallow charge cycle at 70% which increases the life 5x is much more profitable because it reduces overall battery replacement costs.

And when you factor in the much higher costs per Wh for car batteries, which are custom spare parts not mass produced commodity cells, you will find that the cost you incur in vehicle depreciation far exceeds the value you could earn from intra-day energy market speculation.

The idea is you wait to charge until prices drop, rather than charge as soon as possible which adds zero charge cycles or degradation. Discharging into the grid is unlikely to ever be profitable for the average consumer but it isn’t impossible for the economics to work out just look at how high Texes Grid prices have gotten during extreme events.

Delayed charging is already a common feature on many EV and could shift demand quite a bit in aggregate.

Also, battery degradation reduces range so there is an impetus to extend useful life well past expedited useable life. Aka if you want 95+% capacity at 100,000 miles that’s inherently increasing capacity at 1 million+ miles. Manufactures do care about resale value so useful capacity at 100k miles is likely to improve over time.

There is some talk about virtual energy providers that could agregate a large number of home users, receive an availability fee and only physically discharge during emergencies; that could work out economically and allow owners a positive revenue after depreciation, that could translate, for example, in lower prices for energy. That being said, I still think the whole fixed costs of the scheme (smart bidirectional meters, EV and charger support, coordination and administration costs etc.) would make it a money-losing proposal. A nice idea in theory, like say IP multicasting, but that cannot be made to work effectively in the real world.

The benefits of delayed charging are limited by the electric consumption of the transport sector. For an average family driving no more than 800 miles/month at 4 miles/kWh it works out less than 200kWh/month. If half of that charge is time flexible (with the rest being achieved at fastchargers or when the owner is in a hurry etc.), you get a 100 kWh/month dynamic load, a sizeable yet small fraction of the total household consumption. I would earmark that under smart-grid approaches, if correct pricing incentives are set at the meter it will happen automatically and not just for the EV charging loads.

A family only driving 800 miles a month is very low. The average driver is over 12k miles per year and 2 car families are extremely common.

Also, a relatively low percentage of charging is vis fast charging. Even a normal wall outlet can provide enough power do drive 15,000+ miles a year assuming normal habits, and level 2 home chargers are common.

I am not suggesting we use EV batteries for grid storage, just ~double the number of batteries created with half going to EV’s and half going to the grid.

Of course different constraints means different chemistry and form factors etc, but that’s only going to make grid batteries cheaper.

As someone that charges lithium batteries for both cars as well as for my house, I can promise you there is 0% of me that wants to cycle my vehicle battery from 100 to 0 every day to arbitrage $1.80 of electricity.

I am suggesting we build ~twice as many batteries as EV’s alone would need. With half being used for the grid not to directly use EV’s for grid storage.

Presumably if it gets anywhere close to similar scale grid batteries would end up with dramatically different form factors and chemistry as they don’t need to worry about collisions, charging time, etc.

But if you could cycle your battery from 50%-80% it’s possible you don’t get any meaningful wear on the battery.

You could also just charge in period of low usage of the grid (if there were special tariffs for that)

Depending on charging strategy, LFP batteries can do 4000 - 7000 cycles. 1000 is way too conservative, for any kind of EV battery.

Not used for electricity just cooling and heating with heat pumps

I wonder if it would be possible to scale down pumped hydro to something the size of a tall water tower. Probably not worth it due to the small height difference and volume, and you'd also lose efficiency due to the smaller turbine, but I still do wonder what the actual figures are like for something like that. Especially in cost per kWh stored compared to lithium.

I saw a video like a month ago on YouTube where somebody pumped something like 250 liters of water on his roof (7 meters elevation, if I remember correctly) and let it run through a homebuild generator.

The power capacity of that setup was like that of an AA battery, if I remember correctly.

You need huge amounts of water and lots of height difference.

Yeah, pumped hydro is one of those power sources that doesn't scale down at all.

However, home scale conventional hydro is a thing. If you live on a hill with a creek that runs down it you can install a generator that produces a usable amount of consistent power for the cost of a few hundred meters of PVC pipe, a barrel, some connectors, and a small electric turbine. Search for micro-hydro if you want to know more.

Like all hydro solutions it's heavily dependent on the geography and only suitable for well under .1% of the population, but where it is feasible it can be a huge step towards off-grid living. Add in some batteries, maybe supplement with some solar panels, and you've got all of the electricity you need year round at a somewhat affordable price.

> Search for micro-hydro if you want to know more.

I've probably seen everything YT has to offer on that. :-D

While I really enjoy those videos and the "let's just do it" vibe with old washing machines as generators, I'm also somewhat happy that Germany has rather strict regulations on how to use water from streams and rivers. Some installations are quite disturbing for local ecosystems. But being off-grid in remote areas these solutions seem like a probable compromise.

Most micro-hydro setups seem to divert only part of the stream and only do it for a few hundred meters or so. These streams are typically much too small to be salmon spawning grounds or the like so the ecological impact seems pretty minimal. It's not like they're drinking all of the water, it gets returned to the stream right below the turbine.

Based on some extremely back of napkin math (e.g. doing one of those activities where you cycle to power a light bulb) that seems pretty far off. Was the flow rate extremely low? Have a link?

250 litres x 7 metres is 1750 kilogram-force-metres.

Wolfram Alpha [0] says this is about 5 Watt-hours, and gives some other handy comparisons:

> ≈ 0.45 × metabolic energy of one gram of fat ( ≈ 38000 J )

> ≈ 0.63 × energy released by burning 1 gram of ethanol ( ≈ 27000 J )

> ≈ metabolic energy of one gram of sugar or protein ( ≈ 17000 J )

> ≈ (0.02 to 0.09) × typical kinetic energy of a car at highway speeds ( 200000 to 900000 J )

> ≈ 0.5 × typical battery energy content of an alkaline long-life C battery ( ≈ 9.6 W h )

> ≈ 0.55 × typical battery energy content of a nonrechargeable Lithium-Thionyl Chloride AA battery ( ≈ 8.6 W h )

> ≈ 0.92 × typical battery energy content of a carbon-zinc D battery ( ≈ 5.2 W h )

Gravitational potential energy is just really, really low energy density. Fortunately it's reasonable to have pumped-storage reservoirs that contain trillions of litres of water.

[0] https://www.wolframalpha.com/input?i=1750+kg+metres+in+watt-...

Would you be able to help me figure out the gravitational potential energy of the red hill tanks?

https://www.epa.gov/red-hill/what-red-hill-bulk-fuel-storage...

They were recently decommissioned due to leaks, but I've often wondered if they could be repurposed as gravity battery storage.

edit: if you want to check my math. I calculated the tanks at ~567 million liters, and am saying there is 120 meters of vertical distance.

They would be equivalent to a 0.1856 GW h (gigawatt hours) battery if repurposed in this way. https://www.wolframalpha.com/input?i=567810000*120+kg+metres...

I think a bit more than that, the article says 250 million gallons which is more like 960 million litres. That squares with the dimensions given: 15m x 15m x pi x 75m x 20 tanks = 1 million cubic metres = 1 billion litres.

Oahu uses about 225MW on average in residential electricity [0], so this could store an hour or so of excess capacity. That's not nothing, but you might want to 5x or 10x it to really smooth out demand and supply peaks for the island.

Note that you also need a set of tanks or a lake at the bottom of the hill. The tanks are 75m tall, so how are you accounting for that in your 120m of vertical distance?

[0] https://www.eia.gov/todayinenergy/detail.php?id=49036 gives 2,018 kWh per person in Hawaii

I subtracted 100 from their position on red hill at 220-ish meters to account for the fact that the tanks are buried in the ground (I don't know precisely where).

>Oahu uses about 225MW on average in residential electricity [0], so this could store an hour or so of excess capacity. That's not nothing, but you might want to 5x or 10x it to really smooth out demand and supply peaks for the island.

https://spectrumlocalnews.com/hi/hawaii/top-stories/2022/08/...

This solar plant & battery system was completed last month. The battery is able to store 144 MWh. This shows me that planners and people thinking about this are willing to invest & commit to power solutions at this scale. I don't know if you are aware or have read up on the red hill controversy. These tanks were used to store fuel by the Navy. They leaked earlier this year and poisoned the Red Hill aquifer and have caused quite a bit of local controversy.

I'm struggling with the idea of writing a letter to the editor outlining this concept, but also emphasizing how converting these tanks to a water storage/ energy storage facility would improve local resilience to drought, bring down water costs, improve environmental conditions etc.

I've seen some people talk about adding a heavy weight on top of the water in a pumped hydro system - something like a big concrete lid on top of a cylinder type reservoir.

Sounds like this one: https://www.youtube.com/watch?v=CMR9z9Xr8GM

They do say it is about equivalent to a single AA alkaline. It is not exactly an optimized efficient setup, but still, it gets the point across. Pumped hydro is a grid scale option, in places that have the ability to store a lot of water.

That's the one!

250L x 7m is 17kN, or about 5W/h, or about 2 AA 2450mAh cells from IKEA.

Pumped storage scales well though, as the stored volume scales squarely to the required container material (e.g. storing 25,000L will only take 10x the material while storing 100x the energy).

It can work, it's just a cost equation. You need a large amount of mass to get meaningful amounts of energy stored. So pumped hydro is nice because you are basically using natural reservoirs and some cheap pumps and pipes.

A water tower needs to be built first and only stores a limited amount of potential energy. The problem is not the efficiencies but the cost of building a big enough tower. A tower with the same capacity of a pumped hydro reservoir would be insanely expensive. Or alternatively building many thousands of smaller towers that add up to the same capacity would also not be cheap.

There are a few alternative gravity systems being tested out in the form of e.g. big cranes with weights that drive generators, mine shafts with similar systems, etc. Proving they work efficiently is not the issue. It's just scaling their deployment cost effectively.

Why would you need a water tower? If you're in flat geography at sea level, you could probably access tidal energy to drive generators. If you're flat and far from water, couldn't you dig a shallow hole and a deeper hole, and put the generator and pumps in between? I mean, even in Kansas it's not that hard to find an elevation difference of 50 feet.

It's just an example from the comment I replied to was using; I agree it's not a very feasible thing to do. But I used it to illustrate the difference in cost between that and pumped hydro using natural reservoirs. The point being here that you'd need a stupidly large amount of water towers to come close to the amount of energy you can store in a pumped hydro reservoir

Whether it's a water tower, a big crane, or whatever that holds the mass that you then use to drive a generator. You have to build it or use nature to have some natural height difference. Building infrastructure like that is expensive. So nature is preferable and way cheaper.

I come from a relatively flat part of the world; so that's not really an option there. Gravity based batteries are nice where they can be done cheaply. But otherwise probably not ideal.

> If you're in flat geography at sea level, you could probably access tidal energy to drive generators

There are projects testing this, but ocean water is highly corrosive and maintenance is a nightmare.

Digging 100m deep holes isn't cheap.

It is 100% possible.

The problem is that it's monumentally expensive. You will need to have a pretty big water tower to handle enough energy to run a microwave for a hour.

P.E. = mg(h)

So say we have 2,000 liters of water that is 50 meters high.

2000kg * 9.81m/s^2 * 50 * 80% efficiency = 784,800.0 joules.

Say you have a 1100 watt microwave. A watt is 1 joule per second. That gives you enough energy to run that microwave for about 11 minutes.

A better example would be existing municipal water towers. A 500,000 gallon water tower that is 129 feet tall costs about 2 million dollars. This is based on a public notice I found for a city in the USA.

That is 1892700 liters and 39 meters. I am guessing that the average height the water actually falls is 25 meters.

The average American household uses about 30Kwh per day. Which is 108,000,000 joules.

1892700 * 9.81 * 25 * .80 / 108,000,000 = 3.4

So 2 million dollars for 3.5 households-days worth of energy.

I don't know if having 1 days worth of energy is too much or too little per household.

And beyond the cost of the tower itself you need the generators and lines and holding pond for the water. Or underground tank or whatever you want to use.

It's very expensive.

However it is probably the cheapest option right now as far as current technology goes.

It would probably be cost effective for a off-grid shack in the woods, though.

> Or underground tank

Hmm that is an interesting idea, but it would be even more expensive.

If you take that water tower, place it on top of a 80 m hill, then dig down for say another 40 m for the second tank you now have a total of around 160 m of height difference. That would bring it up to 22 households-days or if my conversions are correct, 660kWh, which is not insignificant.

Then again if google is correct the price for an equivalent lithium bank would be only like 100k so it's not even close.

Big fan of gravity well storage, it seems simple and elegant. https://energyskeptic.com/2019/gravity-energy-storage/

The above link has some numbers (2019 had it bookmarked so maybe it has improved)

That website is full of deranged ramblings which are largely disconnected from reality. Case in point:

> MY NOTE: well whoop-dee-doo. China generates 16.2 trillion terawatt-hours (TWh) a day. That’s 64 billion times more than all of the open-cast mines can provide. Better start digging more holes!

16.2 trillion terawatt hours a day is ~4 million times the total global insolation or 35 billion times total global energy usage. This is more wrong than asserting that a single man on a bicycle generator could produce our entire primary energy worldwide.

Thank you for letting me know! Did a bit of checking and deleting this from my bookmarks!

I find it's a useful barometer of what crazy disinformation is going to be spread next because it has been SEO'd so well.

It also often covers interesting topics, and many of the stories are actually about something important or useful.

Additionally many of the things it 'debunks' are actually terrible ideas, and some of the arguments it uses are valid.

For gravity storage the costs and inefficiencies are a bit prohibitive for the use case of an unsubsidized high capacity (more than 1 day) option, and it's not really competitive with batteries for less (and is highly power limited).

Water is just too heavy. Pressurized air would be easier.

Joule for joule, pretty sure I'd rather be standing 10 feet from a failed water tower than a failed pressure vessel. 1/2 m*v^2. That squared gets you every time.

I think there's a start-up in Italy working on pressurized CO2 which apparently has some major benefits over using air.

Yes, higher liquifying point.

That's the problem with the US capitalist approach to renewable energy. "Can I downsize this to a scale that makes sense in our economic reality?" Dang, there dies another viable idea. Instead of changing the status quo we have some warm libraries and city halls.

If you then use the lithium ion to produce heat, things start looking less favorable for it.

Also, I think the “geothermal” part of this means your ‘battery’ gets partly charged for free.

This article is about storing energy generated using renewables in reservoirs; not about tapping into geothermal energy.

The issue with geothermal is not that the energy isn't there but that getting to it involves a lot of expensive infrastructure. It's cost effective in some parts of the world and not really cost effective (relative to cheaper solutions) in other parts of the world.

Using this instead would mean having to drill much less deep (so it's cheaper). You are sill throwing away more than half of the energy you put in though. Geothermal has similar inefficiencies except you get the energy that needs to put in for free. Not so with this solution: you have to generate the energy yourself.

> This article is about storing energy generated using renewables in reservoirs; not about tapping into geothermal energy.

I think it’s about both. They plan to create reservoirs that heat water _and_ use them to store warm water. Also for the latter the outside heat source will provide energy.

FTA: Enhanced geothermal systems (EGS) get around this geographical limitation by creating artificial reservoirs. Developers create fractures in hot, dry rock formations by drilling into or melting the rock, and then injecting water into the fissures. Production wells bring the heated water up for producing electricity. “For scales necessary to contribute to national or global electricity decarbonization, we need to be able to extract geothermal heat outside of conventional formations,” Ricks says.

[…]

Once these EGS systems are in place, they would be ideal for storing energy as well as producing electricity. Excess wind or solar energy could be used to inject water into the artificial reservoirs

Yeah that last line is key — they’re not creating heat with the excess energy, they’re pumping water. The article doesn’t have much specifics but the implication is they’re achieving much higher efficiency than if they were doing electricity → heat → electricity

It’s not clear from the article, but I think they’re aiming to do both: create new geothermal sources by drilling (running, I guess, some of the risks of fracking) _and_ storing excess energy as heat in those sources.

yeah they're creating geothermal reservoirs to pump with water to then extract steam. any excess energy generated from renewables gets used to pump additional water into the reservoir.

Counter point is if you have work (or high grade heat) you can use it to move heat rather than just thermalising it.

They claim 90% efficiency, so I doubt you can compare this to heating a container and them driving a stream engine afterwards.

I suspect they are here also utilizing geothermal heat and heatpumps which allows you to increase efficiency. If you heat water that is already quite hot, then you can get more energy back than you put in. Higher temperature differentials give better efficiency.

In reading, it's not clear which types of batteries this is referring - lithium like the Tesla power wall is definitely not the ideal chemistry to use full time -- but we have different electrolytic (flow) or catalyst (fuel cell) driven batteries that have similar advantages of the geothermal method without the drilling or maintenance of underground systems.

Flow batteries are minimally dangerous, have huge storage capacities, have a minimum maintenance per 1000 charge schedule, and while they are slow to "turn on" are faster than geothermal.

Disclaimer; I have a small startengine investment in a Vanadium Flow battery company

"Ricks and his colleagues’ simulations found that the systems could store energy with up to 90 percent efficiency over one cycle."

Heat, maybe, but electrical to heat and back to electrical, no way. You're limited by basic thermodynamics.

They have a web site.[1] Not that it has any useful information.

[1] https://www.fervoenergy.com/

Hot dry rock schemes have been around for a while (apparently dating back to a project in 1970 in Los Alamos NM), but for it to work it seems you need some specific rock types. For example, one project only recovered 3% of the injected water due to losses into fractures. Here's a historical overview:

https://www.sciencedirect.com/topics/engineering/hot-dry-roc...

It's laughable this was published citing: "the storage capacity effectively comes free of charge" when the whole mess we're in is the tragedy of the commons.

FWIW, the link to the Google article is much more interesting: https://cloud.google.com/blog/products/infrastructure/google...

Every time I think of geothermal, I can't help but wonder what the at-scale externalities are. Sure, if a few people do it in their backyard, who cares...but hundreds of millions of people pumping heat into the ground to store for later? What terrible ecological outcomes will this have?

I guess we'll see.

It is not clear to me how they use the solar and wind power.

Are they heating the steam from a geothermal source, higher than the source's temperature, and sending it back down the geothermal well when power is cheap? And then later run the steam through a turbine when power is expensive?

Or are they using cheap power to pump water into the well at high pressure, to force it it deeper into the fractured rock? And then later use the extra steam to produce power?

Or something else?

It's because of design efficiency. The most distilled and finest design utilizes each part for many things. Like a unibody car, or a fuel tank in the wing in an aircraft.

In this case the material doing the thermal storage is also part of the building. It's not like there is a building and a separate 100 tons of thermal storage stuff, just make the house and foundation out of that 100 tons of stuff and then you only have to buy 100 tons of stuff, not 200. It's the same economy of scale with the labor and digging. You can dig 1 hole and put all this geothermal stuff in it along with the foundation, or you can dig 2 holes and have twice the everything. If you just combine all the items into one big effort, look for design efficiencies where you can combine parts and systems, and these systems/buildings can basically run themselves with just what is provided by the environment. They don't change the environment inside the building, they just regulate it between the 2 extremes, taking the edge off the high and lows.

More the competition the better for the world.

I still feel a well designed Lithium Iron Phosphate or Sodium based battery with > 5000 cycles can prove very cost effective if used properly. The high no of cycles reduces the levelized cost of storage significantly. Difficulty is scaling this to beyond 1-2 days at most.

The interesting thing is when they've been around for 30 years (providing they don't decay too much with time).

A sodium battery with 100 useful cycles and 10 years left in it is great for cycling once a month.

But even without that, if we see costs of $50/kWh then you only need to cycle 300 times to make it more worthwhile than some generation technologies..

Don’t some battery chemistries have much higher self-discharge rates as they get older? If so, it might be uneconomic/wasteful to try to use old batteries.

That's one consideration (and if storage gets as cheap as variable generation it will be the most important one). With the advent of cheap solar the value of moving the energy is a fair bit higher than its cost. So the economics become mostly a matter of cost of storage vs. duration.

If you can move energy from a time where it's the least valuable generation of a system that costs $15/MWh net (or even less in future systems) to a time when the cheapest scalable generation is $50/MWh then you've got a lot of room for losing some.

There's incentive for the solar system to overbuild 2x or 3x just to sell that 3pm-sunset energy so from their point of view they can sell the sunrise-3pm energy at any price over $0 and be profitable.

A hypothetical bad second hand battery might be the most valuable (and thus highest bidding) use, but it will have to compete with other opportunistic loads.

For places with high car ownership, you can use electric cars as the battery storage. Just need lithium iron phosphate batteries to be standard in EVs. Telsa is using them in some of their models in China.

Competition is just a euphemism for fighting. You're saying that more fighting is better.

Isn't that crazy? You'd think that fighting would be wasteful and upsetting. That peace would be better.

It's a crazy world.

I'm not convinced competition is a euphemism for fighting. Competition can be friendly.

You got any examples of "friendly, nonfighting type competition" in the business world?

https://www.cnet.com/tech/tech-industry/microsoft-to-invest-...

All of the oligopolies? Do you really believe that, e.g, tech companies are "fighting" each other? What about Big Pharma? The Food industry?

In any case, you are arguing semantics. Competition can be friendly.

The tech, food and pharma industries fight each other tooth and nail.

I don't think this "friendly competition" happens in the business world. It's just an ordinary fight.

I guess we have different ideas for "fighting tooth and nail".

- Steve Jobs and Eric Schmidt would regularly discuss matters together.

- They have pretty solid "do not poach" agreements.

- None of them actively blocks their products from running on each other platforms

- None of them launch products with the sole intent of destroying each others cash cow. E.g, Google could release flagship products at budget prices forever, just to take from Apple's market share. They certainly wouldn't lose money over it. Why don't they do it?

Competition? Ok, but it never comes to the point where they actually want to inflict damage on one another. This is as friendly as it gets.

Pepsi not releasing coke's formula despite knowing it

Bill Gates bailing out apple

Forget the word competition. Just think of it as smart people having an incentive to produce something with beneficial effects or externalities for once.

Even low grade geothermal is pretty awesome. Example: "Nebraska retiree uses earths's heat to grow oranges in snow" - https://youtu.be/ZD_3_gsgsnk

Arlanda airport outside of Stockholm uses a similar system: https://www.swedavia.com/about-swedavia/the-aquifer/ But in this case, the aquifer is naturally occurring and not man-made. Constructing underground aquifers requires a lot of energy so I wonder how many charge-and-drain cycles are needed before more energy is saved than spent.

It's interesting to wonder how profoundly different the world would be if everyone had access to geothermal energy at the scale of somewhere like Iceland.

I know they heat their homes with the geothermally warmed water, but their electricity rates don't seem to be significantly different than most places.

"Iceland, March 2022: The price of electricity is 0.139 U.S. Dollar per kWh for households."

Iceland has the cheapest electricity in the world. They run heat under their roads so they don't need to be plowed. Aluminum is shipped to Iceland to be smelted just because the energy is so much cheaper.

Iceland's electricity usage per capita is 4x that of the United States, and more than 2x the next highest country (Norway). They're the largest producer per capita as well. The population is not large, mind you, but it's noteworthy.

https://en.wikipedia.org/wiki/List_of_countries_by_electrici...

The rate for businesses is 0.066per kWh vs 0.128 in the US.

All of which is obfuscated by taxes and subsidies of course. https://www.globalpetrolprices.com/Iceland/electricity_price...

Technically it may be cheaper in some small petrol states but that feels misleading.

Mind you I'm just regurgitating a quick google's worth + tourism fun facts.

> Iceland has the cheapest electricity in the world.

Come again? If the parent post about about $0.139/kWh for households is correct, I can assure you there are many places in the US cheaper than that.

If you're asking for some contextual explanation why that $0.139 is not a sufficient figure to explain the issue, consider reading the rest of my post past the first sentence.

There were only 2 markets nationwide with retail rates lower than this [1] in Aug 2022.

[1] https://www.bls.gov/regions/midwest/data/AverageEnergyPrices...

The best part about geothermal is it's one of the cheapest ways to make electricity, about as much as burning coal, with none of the pollution. Using it for power storage is just the bonus.

https://en.wikipedia.org/wiki/Cost_of_electricity_by_source

An excellent discussion on the matter on the excellent Volts podcast: https://www.volts.wtf/p/the-extraordinary-potential-value#de...

One wonders what the limits of "Thermal Heat" are from small "wells"(???) up to the heat content of the Earth's ore (Heat relative to sea level)

How much heat is generated by Earth's gravity onto itself? How "renewable" is Geothermal energy, truly?

From the Wikipedia article on "Geothermal energy":

"Geothermal power is considered to be renewable because any projected heat extraction is small compared to the Earth's heat content. The Earth has an internal heat content of 10^31 joules (3·10^15 TWh), approximately 100 billion times the 2010 worldwide annual energy consumption.[...] About 20% of this is residual heat from planetary accretion; the remainder is attributed to past and current radioactive decay of naturally occurring isotopes." -- https://en.wikipedia.org/wiki/Geothermal_energy#Renewability...

The total budget for any given power source is surprisingly close to 100TW.

Geothermal, you'd cool the crust and have to drill far deeper than is possible to exceed 50TW for long.

Wind, if you exceed somewhere in the 1-10W/m^2 you slow down the wind and create turbulence.

Solar, if you use more than ~1% of the land you're radically altering the albedo and habitat (so maybe up to 200TW).

Fossil fuels create GHG and run out quickly.

Nuclear creates enough waste heat in the 100-200TW range to cause as much radiative forcing as GHG does now.

We each get our 10-100kW and that's about it for living on a planet that isn't one giant factory/heatsink.

> Solar, if you use more than ~1% of the land you're radically altering the albedo and habitat

Both of these may actually turn out to be beneficial. Putting up solar panels above agricultural land ("Agri-PV") is not just avoiding clobbering up arable land with PV panels, but also has benefits for the plants [1]: they need less water because less sun directly hits the plants and heats up the soil which means both more resilience in drought periods and more capacity to regenerate groundwater planes, and the soil erosion from wind is reduced as well.

I wonder if putting up panels in the desert could help out de-desertification efforts as well for the same reasons.

As for albedo - given that we're already having issues with global warming, it is not that far-fetched to say that solar radiation being reflected off to space is a good thing overall.

[1] https://www.ise.fraunhofer.de/content/dam/ise/de/documents/i...

Agri-PV is one way of using the thermal budget (and a great one from a habitat-harm perspective as it causes a net reduction in human-occupied land).

The albedo problem is the opposite of what your are thinking as it is another source of radiative forcing from putting a mostly black thing in the sun.

A Shockley-Queisser limited solar panel mostly only reflects IR. So where your plants/dirt might have an albedo of 0.3, the solar panel absorbs 80-95% of the light (albedo 0.05 to 0.2). 20-30% becomes electricity (and later thermalises when used).

This gives you a net forcing on the order of 100-250W or so _somewhere_ on earth for every m^2 of pv. If you cover too much land the radiative forcing is on the same order as GHG, hence the ~1% limit. 200-300W of work for 100-250W of new heat is a pretty fantastic deal compared to other options though.

Putting it above existing asphalt or a similar surface is 'free' because albedo is already 0.1 there. Similarly wind is free from a radiative forcing perspective.

Putting the panel in existing (light coloured) desert is much worse because deserts have an albedo around 0.4-0.6 so you are making 400-550W of new heat for your 200-300W of work.

This also leads to the interesting thought of placing bifacial tracking modules sparsely on low albedo dead surfaces and painting the surface white for a net reduction in thermal forcing (is there a 1000km patch of volcanic rock somewhere? Gobi desert?)

In the same framework, every joule of nuclear energy is a new watt of heating, and if it is from a steam engine, it's more like 3W (or even 5W once you include post-reactor heat as well as xW of heat for every 1W of pre-reactor work inputs of mining, milling, and enriching low quality ore).

Of course these all only become relevant if we slash carbon intensity to under 2% of what it is today and continue trying to grow our energy usage exponentially.

> Putting the panel in existing (light coloured) desert is much worse because deserts have an albedo around 0.4-0.6 so you are making 400-550W of new heat for your 200-300W of work.

Yeah, but the heat doesn't penetrate to the ground any more, which should help with water retention?

Local effects vs. global effects.

You might make your desert nice, but you're still doing a climate change if you try to install more than a few hundred TW of solar panels.

Ideally you make the desert nice and then take a chill pill before covering all of the deserts and settle for somewhere in the range of 2-100kW per human as a steady state economy powered by a mix of wind, solar, geothermal, and maybe some nuclear.

Doesn’t the nuclear heating happen regardless of us harnessing the energy or not?

Unenriched uranium will fission eventually, but you're talking trillions of years. In a fission reactor, the U238 atoms are placed close enough to a minority of U235 atoms and to each othekr that neutrons from one fission will trigger the next at a rate just short of running away exponentially.

There are some ore bodies that are concentrated enough to fission faster, but they are rare (and not the uranium that is typically mined for fuel).

The uranium undergoes alpha decay first. Alpha decay is not fission.

Yes, this is correct. Thanks.

Majority of geothermal enery comes from the decay of radioactive isotopes. It's not renewable, but there will be enough heat for millions of years.

Surprisingly it's about 50/50 radioactive decay and heat left over from the forming of the solar system: https://en.m.wikipedia.org/wiki/Earth%27s_energy_budget#Eart...

It's not renewable, but exploiting it doesn't reduce the overall output. It's "not depletable", you could say.

At the macro scale, correct. Humans might cool the outer crust somewhat in localised regions. Odds we'll be able to significantly change overall core and mantle thermodynamics are exceedingly slight.

At the micro scale, that is, for an individual geothermal well or source, not so much. Single wells or geothermal fields may be depleted or degraded.

Because heat conductivity of rock is very limited, extraction of that heat by some mechanism will eventually cool that rock below viable levels for power generation. For enhanced intensive geothermal energy --- drilling holes in rock to depths of multiple kilometers and circulating a working fluid (typically water) through the substrate --- that is thought to be on the order of 1--3 decades. After which the borehole is no longer viable and must be left to recover for some period of time, perhaps centuries.

For conventional (geyser / steam vent) geothermal, the limiting factor tends to be groundwater. The instance I'm most familiar with is The Geysers powerplant in northern California, which saw a roughly 40% reduction in capacity over several decades as the groundwater feeding the geyser system was depleted. That would have to be restored by some means.

Note too that there may be contamination issues in repeatedly cycling deep-layer water to the surface, particularly of heavy metals or radioactive isotopes. These include "sulfur, vanadium, silica compounds, chlorides, arsenic, mercury, nickel, and other heavy metals".

<https://www.ucsusa.org/resources/environmental-impacts-geoth...>

At some point you're increasing the conductivity of the crust and mantle.

You have a bunch of bigger problems that come to roost much sooner than this if you're using that much geothermal though,

How is that different from renewables?

It's not. Renewable on energy discussions basically means "not depletable".

About 47 terawatts

https://en.wikipedia.org/wiki/Earth%27s_energy_budget#Earth'...

That seems terribly low, but apparently it's correct (humans need right now about 18TW). This means a lot of Earth's heat is actually coming from the sun itself.

One small nuance: the OP asked about the heat generated by Earth's mass. The 47TW is split between the original heat from the formation of the Earth and the nuclear decay of radioactive elements.

So can we combine this with the subway tunnels that are getting hotter and hotter each year?

The the ratio of area A to volume V for a sphere of radius R is 3/R. The larger the sphere, the less surface area to volume. A corollary of this is that a sufficiently large thermal tank has little in the way of heat loss to its surroundings in relation to its heat capacity.

At large enough scale, it's probably the most efficient way to store energy for general heating purposes. Simple water has significant thermal capacity. 1 tonne (roughly 1 kilolitre, 1000 litres) of water at 80 °C holds ~50 kWh of useful heat relative to normal indoor temperatures. Enough to heat a Canadian home overnight on a bad winter day and then some. 1 tonne of water isn't really that big; a typical car's interior volume would hold several tonnes of water. Home hot water storage, for heating over several days or a week, is quite viable with a large basement tank, and commercially available. Used with intermittent heat sources, and sometimes to save money when heat costs more at different times, like with electric time of use.

A standard Olympic swimming pool holds several thousand tonnes, the largest indoor swimming pools tens of thousands of tonnes. 20,000 tonnes of water at 80 °C = about 1 gigawatt-hour of usable energy storage. Assuming no losses, That could heat many dozens of typical Canadian suburban homes all winter. What about bigger? Some lakes are so large their thermal capacity causes their temperature to significantly lag behind the seasonal changes, moderating the local climate. The largest fully artificial reservoirs are measured in cubic kilometres. That's billions of tonnes. 1 cubic kilometre of hot water would have 30 terawatt-hours or so. That's in the range of the yearly energy consumption of small cities. And at that scale, even a lightly insulated tank would lose an insignificant fraction of its heat, even over a timescale of months.

Getting into less conventional territory, with much higher temperatures (perhaps a molten metal instead of water) and some real big tankers, it might be feasible to literally ship heat. This rarely made sense in the past except in some edge cases like using industrial waste heat. Fossil fuels were almost always the source of the heat to begin with. Just ship the fossil fuels. Far more energy dense and no conversion losses. But the economics are changing.

(As an aside, the effect of the ratio of area to volume is just not intuitive to me, for some reason. But it's a very powerful scaling force in nature. It is why a bucket of water takes days to evaporate while tiny droplets of mist take seconds. Area for evaporation relative to volume. It even limits the maximum size of a terrestrial animal. Elephants push that limit. About a kilowatt just for base metabolism. In a thickly-walled airtight tank. A really big tank. They'd cook from the inside out without active cooling in a matter of hours. Their hearts are heat pumps and their ears are radiators.)

> Assuming no losses, That could heat many dozens of typical Canadian suburban homes all winter.

This honestly seem incredibly optimistic to me. Even a high-end thermos will lose its heat in less than a day. Faster in a cold environment. And afaik, temperature loss is proportional to temperature difference, so the hotter the liquid the faster it cools.

While literally zero loss is a violation of thermodynamics, with a really large tank and good insulation, it gets to self-discharge rates lower than most battery chemistries. A few percent per month. There's a small district heating system in Alberta driven with solar thermal power. It uses a large insulated soil heatsink. Most of the energy input is in the summer and it took four years to fully charge up to operating temperature. It holds enough heat to provide all required heating through the winter for ~50 large houses.

The thermal time constant for a sphere scales as radius squared (one factor of r due to volume/surface area, and one factor due to the temperature gradient scaling as 1/r). Your thermos is quite small.

Geothermal smartphones when?

That puts real meaning into the term "landline"

This is an example of "how can we keep X relevant in a world where it is not?" Geothermal is falling away because maintaining a steam turbine is expensive.

There are plenty of efficient storage options that don't depend on a steam turbine. Those are favored except where the storage medium is also a transportable, valuable product, as for ammonia. (Ammonia is burned in combined-cycle turbines already built to burn NG.)

It depends what you want to use the geothermal heat for. In Norway, in the winter, a lot of the energy use is going to be heating. Geothermal heating systems are quite common in northern and don’t require lossy conversion

And pure thermal uses are not good storage for high-grade energy forms, e.g. electric, because of conversion losses. In Norway, pumped hydro is a good place to park high-grade energy.

That said, for rarely drawn-upon storage, conversion efficiency doesn't matter very much. It is why back when hydrogen electrolysis still lost 40% off the top, it was considered fine for long-term storage. Now that it has exceeded 90%, it is useful in more places.

EV batteries will store energy.

As long as EV scale faster than solar/wind we will have little need for storage batteries.

EV are moving in the direction of bidirectional charging and will thus become generator in peak demand period, the grid will become smarter and will offer differential price in function of offer/demand

Energy storage is the monorail of energy technology, expensive, unnecessary and conventional technologies like nuclear, hydro and geothermal is better in every regard.

Nuclear has enormous political hurdles that translate into extreme project costs. Hydro and Geothermal are both geographically limited. Hydro especially is effectively tapped out at this point. You are chasing progressively less suitable locations for each install. Most of the best remaining places are far away from civilization and would require hundreds of kilometers of high tension wire to get to the nearest grid connection.

Meanwhile battery prices are dropping year over year and installed capacity is still skyrocketing. Eventually the curve will level off, but we are not to the point of diminishing returns yet. Solar and Wind + Battery is shaping up to be the dominant power source of the future. It's all down to economics. Solar cells and wind turbines are not only getting cheap to buy, but once installed they require little maintenance and no ongoing fuel costs. Batteries are similarly low maintenance and the purchase price continues to drop as economies of scale and advances in technology make them cheaper and better.

Batteries is not an alternative to firm energy, you still need firm energy to balance the grid wether it's from nuclear/hydro/geo or fossil.

That's why all decarbonized grids have hydro/nuclear as their main source of energy.

Nuclear has screwed up its costs even with political support.

The politics argument is really "people won't let us get away with papering over horrific design and construction snafus."

Batteries are not cheaper than nuclear.

Batteries are getting cheaper quite rapidly, so don't get stuck in the dead past there.

Battery costs are literally rising

Short term rise due to commodity prices, but shifts to LFP and other cheaper approaches will allow the dramatic long term price trend to continue.

https://about.bnef.com/blog/battery-pack-prices-fall-to-an-a...

If you look at PV there were similar "ah ha, costs have stopped falling!" moments in the past (mid 2000s, for example), but they didn't last long and were followed by even more dramatic cost declines. It's important not to be selective here, like climate change deniers are looking at global temperature data.

Well it doesn't really change the fact that batteries: 1. Doesn't scale to grid level 2. Isn't interchangeable with firm energy sources

Why don't batteries scale to grid level? I don't see how this assertion could be justified. Do you mean a specific KIND of battery?

Renewables + storage certainly could provide 100% of the needs for the grid, in a properly designed system. At this point it's just a matter of quibbling over cost. Since the world could be spending upwards of a quadrillion current dollars on energy in the next century there's a hell of a lot of financial headroom to scale things up.

So far there is no proven technology that scales to grid level? We are closer to achieving fusion than large scale energy storage.

There's no proven nuclear technology that scales to power the world either. Do you accept that as an argument that nuclear should be disregarded?

It's entirely reasonable to assume that some storage technologies will survive being rolled out to the scale needed to allow renewables to reach 100% penetration. After all, there are thousands of different battery chemistries, and many other non-battery storage technologies, including ones using 19th century levels of science (pumped thermal storage, for example). Your negativity requires that all fail. I think you doth whine too much.

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