Comment by m463 | Hacker News Reader

Comment by m463 | Hacker News Reader

I get that sand is inexpensive and simple and I like the idea - especially since it scales to an entire town (!)

But I can't help but think of one "technology" that could make a scheme like this MUCH more effective.

a phase change.

for example, it takes 1 calorie for water to go from 30 to 31 degrees F, but but it take 80 calories to go from solid at 32 to liquid at 32 F.

A project I remember reading about that was really interesting was a house constructed with logs that had a resin that phase changed around room temperature. The idea was that the logs would be heated by sunlight, and the resin in the logs would absorb lots of energy without easily going above room temperature. Then at night, they would slowly solidify and give off heat at room temperature. This would be a cool way to stabilize house temperatures without needing equipment.

EDIT: it was called the enertia house

I'm uncertain of the history, it seems there are a lot of different "enertia house" search hits.

Here's one explaining the idea:

https://enertia.net/howitworks.html

Phase change heat storage is well established techology. It's in the product you buy when you want to have building scale latent heat storage. I've even seen a company trying their luck at the business model of selling heat in twenty foot equivalents, e.g. for building sites. So it's safe to assume that they knew about this option. But there's also the square-cube-law, at some point it will be cheaper to insulate a larger volume of the cheaper material than a smaller volume of a material that requires less volume per unit of energy at the heat range you aim at. It's well possible that this line is between building-scale and grid-scale storage.

There's a pretty cool coffee mug based off the idea too

https://joeveo.com/pages/the-temperfect-mug

Coffee Joulies work off the same concept

https://www.baristamagazine.com/coffee-joulies-engineered-so...

Dunno what 'wax like substance' they use, but if I were designing it I would use sodium acetate (vinegar+baking soda).

By adjusting the amount of water in the mixture, you can fine-tune the phase change temperature to whatever the customers like for their coffee.

It also boils above 100C so when the customer puts it in the microwave it won't burst unless they boil the cup dry.

And when the customer does break it, it's non toxic and food safe.

The Venera probes used this idea in the 1970s. [0]

> One new idea, for additional thermal protection, was the addition of phase-change material. Lithium nitrate trihydrate melts at 30° C, absorbing a large amount of heat, due to its high latent heat of fusion.

[0] http://mentallandscape.com/V_Lavochkin1.htm

> I get that sand is inexpensive

It depends where you are. Sand thieves are a thing in many places around the world.

Here's a story from 2015 about how residents in one area have lost 25 hectares of land to sand thieves.

https://tuoitrenews.vn/news/features/20150524/residents-losi...

Or a police captain whose legs were severed while trying to apprehend sand thieves.

https://www.vietnam.vn/en/dien-bien-moi-vu-dai-uy-cong-an-bi...

And here's a longer from article on the phenomenon:

https://www.mekongeye.com/2023/05/01/mekong-delta-sand-minin...

tl;dr sand is used in construction

It is still inexpensive, they just steal a lot

For construction you need a special kind of sand (still pretty common on average). For this battery you can use sand unusable for construction.

Sand used for construction should not have salt content.

For energy storage, any sand is sufficient.

The article points out that it doesn't actually use what people usually think of as sand:

"The sand itself will also be sustainably sourced – it’ll consist of crushed soapstone, which is a manufacturing byproduct of another local industry."

And this is key to the finances...

Waste material costs over €100/ton to dispose of across most of the EU. So therefore this material costs less than nothing!

That's why the project doesn't use water (which has a 4x higher heat capacity and can be pumped allowing the heat exchangers to be far smaller and eliminating the 'temperature decreases as it discharges' problem).

> The sand itself will also be sustainably sourced – it’ll consist of crushed soapstone, which is a manufacturing byproduct of another local industry.

The requirement is not really construction grade sand but any substance with a largish thermal mass. Sand/dirt/crushed rock, etc. whatever you have right on your doorstep. There have been plenty of prototypes using such various materials as thermal mass.

All you need is a big container, some insulation to keep the heat in and lots of mass in whatever form. And some pipes to get heat in and out via e.g. water. The bigger the container, the smaller its surface area is relative to its volume. So these things can be quite efficient. If you make them large enough, you can store enough energy in the summer to last for months during the winter.

Here's a story about a Dutch retiree who built a prototype using basalt as the thermal mass in his backyard: https://deepresource.wordpress.com/2020/03/08/cesar-seasonal...

  Claimed performance:

  Storage volume: 200-250 kWh/m3

  Storage cost: 2 cent/kWh

  Storage leak: 80% energy still present after 6 months

Awesome and very simple. There is a small community of eco housing that uses this system, in Boekel. They use solar panels in summer to heat the battery which lasts all winter.

On a diy scale, there is also this system for passive greenhouse called a climate battery which uses the same principle, storing heat underground and using it in winter. With almost zero energy we can use it to grow bananas in a greenhouse in the Netherlands.

Phase changes will probably work well for structures that would otherwise be used for insulation anyway - in the case of this particular idea though, I think heat is moved in and out using another medium, air in this case, so a phase change would complicate things greatly (hard to bubble air through lava). But yeah, something like a phase changing fluid (gas <-> liquid) will probably work well, as it does in air conditioners right now.

Now what would be suitable material with such phase change around 80 to 110 degree temperature? As that is what effective heating needs. Water with the safe change at 0, is clearly too low as then you could just run ground source or air source heatpumps...

> But I can't help but think of one "technology" that could make a scheme like this MUCH more effective.

I think this is one of those things that drive home the point that there are fundamental differences between physics and engineering.

The article states that the thermal silos are heated with excess energy from the power grid. This alone tells you right from the start that efficiency is not the primary requirement.

Sand is inert, doesn't decompose or degrade, is readily available, is easy to work with, and has no moving parts. You can make it work in a silo, or digging a well to fill it with sand. In fact, geothermal heat pumps are already used extensively in residential buildings to regulate temperature. You just have to drill a hole in the ground that's deep enough, run a water pipe through it to heat/cool the water, and run that water through your building to heat/cool the environment. The nifty trick of Polar Night Energy is that they introduce the extra step of actively heating the thermal source with cheap energy supplied by the electrical power grid.

This sort of argument is like complaining that a Formula 1 car is far more efficient than a Volkswagen Golf. Yes it is,but that's a mute point.

I can't endorse this perspective enough. The amount of energy storage we need is staggering and ever-growing. We've someone convinced ourselves that the 'baseline' is consuming with abandon millennia worth of stored energy and anything even slightly less responsive than that is too inconvenient. Given those parameters, we need any and all energy storage options and efficiency is not a priority. Tesla powerwalls were never going to power the world, but giant caverns full of sand might.

Excess energy from the power grid -- in times when renewables produce excess energy.

Otherwise, the heat to be stored comes as a byproduct of local industry. Something that would otherwise be vented out and not used.

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