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Comment by DontGiveTwoFlux | Hacker News Reader

Not mentioned in the article is power density. How quickly can the energy be released? Consider solar panels, you need a table sized cell to get 100W. That can make for a big battery to get grid scale power output if these cells are only as power dense as solar panels. The energy density of a heat based solution can be very high- metals can get very hot and they are dense enough to store a lot of energy. But if you can’t get the energy out of the battery fast enough that limits the applications. By comparison lithium ion batteries can dump power out extremely quickly, which is what makes them great for cars. Hydro is even better.

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The article in Nature quotes an energy density of 2.38 w/cm^2. Which means a Gw battery would require 10e5 m^2 of absorber surface, exposed directly and at close range to the radiation from molten metal (which is the heat transfer fluid they propose). It has to be direct, and at close range, because the efficiency they quote relies on the absorber reflecting non-absorbed photons directly back into the emitter, where they are re-absorbed as heat and potentially re-emitted.

That's about 25 acres of absorber, and an implied 25 acre surface area of the liquid metal emitter pool.

There is a basic challenge here to the design - the energy storage density for the thermal battery they envision scales as the cube of the characteristic dimension of the plant, but the power density that can be delivered scales only as the square of dimension. Not saying that can't be dealt with in engineering, but it ain't going to make this easier or cheaper.

Surface area is relevant for solar because the sun is so far away. A local heat source allows you to surround it with 3D shapes not just a flat plain.

As to temperature this thing is for very high temperatures: can generate electricity from a heat source of between 1,900 to 2,400 degrees Celsius. At 40% efficient you need a wide temperature difference which would suggest a high energy density.

For long term energy banking and if we can get them working, flow batteries seem vastly superior to all alternatives, by scaling storage with regards to tank volume. Instead of some difficult-to-manufacture structure.

This design is photovoltaics, just like solar, but optimized for infrared photons. There is no avoiding the reality that energy storage density will scale as the cube of the facility size, but power density only as the square. And at 2.38 w/cm^2, the scale coefficient is not all that great.

Picture a stack of flat panels with each layer consisting of: (Cold)(Panel)(Hot)(Panel)(Cold) held vertically. So: (Cold)(Panel)(Hot)(Panel)(Cold)(Panel)(Hot)(Panel)(Cold)(Panel)(Hot)

Now you add hot gas at the bottom and have say 4 layers per m. So a 3mx3mx3m cube would be 4 layers * 2 panels per layer * 3m * 3m * 3m = 216m2 of panels taking up a 3mx3m section of floor. At 2.38w/cm2 * 10000cm2/m2 * 216m2 = 5.14 MW of power.

I think their application is grid scale and you can scale across hundreds of batteries to provide the throughput you need. I don't know how I feel about having a small molten ball of metal inside the hood of my car. Turns my car into the most dangerous gusher in the case of an accident (for those who aren't familiar, gushers are a gummy like candy with juice inside).

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