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Comment by pbmonster | Hacker News Reader

I don't get why they go for storage with this. Storing a block of carbon or tungsten at 2000°C for hours or days does not sound like something that will ever be economical. A battery leaking energy this quickly (and it will leak₎ will need to be incredibly cheap to ever make sense.

I wonder if you could use this with parabolic mirrors, though. Build a large mirror array, focus sunlight onto a big carbon sphere (maybe coat it with one of those new materials that are transparent for visible light but pretty reflective for IR), cover the top of the sphere in those new panels. They are more efficient than practically all solar cells and get much more power out the same area than solar cells. This should beat a photovoltaic parabolic mirror setup, right?

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You won't get the claimed efficiency that way: it only applies to an enclosed heat source. It's critical that they're reflecting light outside of the PV bandgap back into the thermal mass, where it's re-absorbed and re-emitted again.

If you try that with solar radiation, you'll lose most of it back into the sky.

https://www.nature.com/articles/s41586-022-04473-y.pdf

(Equation 1 and surrounding discussion, and the energy-flow diagrams in Figure 1).

If you shine solar radiation on a nice black carbon sphere, practically all of those photons are absorbed, right? The sphere gets hotter, and starts emitting IR photons.

My idea was to now enclose this sphere in TPV modules - except in places where the parabolic mirror puts sunlight on it Let's say that's half of the sphere (but you could cut this number down if you chose a parabolic mirror with a long focal point).

Now you lose IR photons in across the part of the surface that you didn't cover in TPV modules. But you could but a IR mirror there, that is transparent for most of sunlight.

Wouldn't this process also contribute to climate change by trapping energy on Earth that would have been reflected back into space?

Edit: Apparently, yes, current PV solar may also do this:

> We found temperatures over a PV plant were regularly 3–4 °C warmer than wildlands at night

https://www.nature.com/articles/srep35070

But I suspect/conjecture that turning sunlight directly into heat, then turning some of that heat into electricity, would capture even more heat from sunlight.

Because some of the heat in the sphere will re-radiate, but much of it will inevitably be lost to convection and conduction.

There is an "albedo effect", yes, but it's not cumulative while CO2 in the atmosphere is.

Arguably there should be a "white roof" campaign for urban areas, it would be a cheap way of reducing the urban heat island effect.

Anecdotally, my apartment building's roof was painted silver-white during the latest renovation of it. I live on the top floor, and I did notice the change: it's less hot in the summer.

Most desert buildings already do white roofs for this reason (new ones anyway), same with commercial buildings. It usually pays for itself in short order. Most available roof coatings now are also white + extra reflective additives.

[https://www.homedepot.com/p/Henry-887-Tropi-Cool-100-Silicon...] [https://maps.app.goo.gl/TwrV57Jt2fGo2obw9?g_st=ic]

Many major cities already have such regulations even outside desert areas.

There are also a lot of HOAs that require black roofs. Something else to write your representative about.

Isn’t that what all solar energy devices do?

Making the earth’s surface darker contributes to climate change, yes (that’s why sea ice loss is particularly bad). When you use the sunlight to make electricity usually the net effect is still positive.

That area over PV plants is tiny relative to the size of the world though even with enough of them to power world electricity usage.

Carbon or tungsten I don't know, but sand as a thermal energy storage medium can be quite economical. Energy loss scales with the surface area (^2), energy stored with volume (^3).

With grid scale, above certain dimensions, you can store energy for months while maintaining economic viability.

Even for single days or weeks it makes sense. You need hot water in your home 24/7, but sun doesn't shine every day in most regions.

Molten salt heat storage [1] is used relatively widely, with the advantage that the salt is liquid and can directly work as a heat transport fluid (usually to heat water or gas that actually turns an engine).

The same geometry that allows heat to be effectively trapped also prevents its fast extraction as radiation. A fluid that can relatively easily change its volume to surface ratio could have an advantage here, too.

[1]: https://en.wikipedia.org/wiki/Thermal_energy_storage#Molten_...

Is the sand flowing or stationary?

Flowing sand has issues with blockages and erosion.

Stationary sand has pretty low conductivity, so getting all the energy out of your 200 yard cube of hot sand might be a challenge.

Stationary, this has already been done. It does require a heat exchanger aswell rather than getting a direct current out.

Just have rods similar to nuclear (well the opposite)

You can progressively sink them in as the sand cools/ you want more energy out.

You don't really need to get the energy out fast. You typically have 8-12 hours every day where you want energy out, and similar amounts where energy goes in. Just put some pipes through the sand and you can get plenty of transfer.

Possibly this is targeting trying to make solar thermal storage work again; focus sunlight onto an object and then extract the energy later in the evening as it cools. There are a few of these plants around but they're getting beaten economically by good old PV+batteries.

Yeah, tungsten is far too expensive. Sounds like more realistically you would have molten silicon in graphite plumbing [0]. This article claims the self-discharge rate could be made 1%/day.

There's been talk of doing solar concentration -> hot object -> thermophotovoltaic converter. IIRC concentrated solar already exceeds 40% efficiency so it doesn't make sense to add the extra step, unless you are using the solar concentration to "recharge" a heat storage system.

I buy the argument that thermophotovoltaics can become cheaper on a $/kW basis than comparably-efficient fluid/mechanical heat engines. The power per unit area is intrinsically orders of magnitude higher than for direct solar, so even if these cells are pricier than regular solar cells, they have a fighting chance. Also, both the power density and efficiency increase with temperature, and in principle the operating temperature can be higher than that of a turbine (since the materials don't have to simultaneously withstand crazy mechanical stresses and reactive chemical environment).

[0] https://pubs.rsc.org/en/content/articlelanding/2019/EE/C8EE0...

Maybe my physics knowledge is off here. Why would storing a block of material at a constant high temperature be infeasible? Can't we surround the heated material with a forced vacuum to nullify any heat loss, or am I underestimating something like the rate of diffusion through a pinprick in such material?

The fact we don't seem to be able to do this yet suggests I'm missing something, probably several things.

Sure, you can built a thermos (vacuum around the hot storage volume, then highly reflective walls to put the radiating heat back into the storage volume).

The problem is that a tungsten tank filled with molten salt is heavy. If you want to store a couple of MW/h, we're quickly talking about several thousand tons. You have to suspend it somehow inside that thermos. With materials, that don't conduct heat well, but still have some strength at 2000C.

Conversion to/from heat and electricity is usually quite lossy. There are numerous approaches to attempt to do this effectively, but often things like pumped storage is cheaper per watt hour and similarly (or more) efficient.

Batteries are faster to charge/discharge and more efficient per wh.

Both usually have less loss over time - it’s very difficult/expensive to avoid significant heat loss over time due to radiative heat loss, if nothing else.

Most batteries or pumped storage have much lower losses over time.

All bodies at temperatures higher than 0K radiate heat, the vacuum around them doesn't stop that from happening.

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

Couldn't it be just a shell of carbon or tungsten, but the bulk of material inside be molten sand, or something else "dirt cheap"?

Like this? https://polarnightenergy.fi/sand-battery#:~:text=It%20is%20a....

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