US startup begins producing 40%-efficient thermophotovoltaic cells • Hacker News Board

Re: US startup begins producing 40%-efficient thermophotovoltaic cells

#171

Post by goodpoint » Fri, Oct 06, 2023, 10:09 PM UTC

bluGill wrote: post #133

ChrisRR wrote: post #77

Earlier quoted context omitted.

I don't know why people always comment this on posts about renewables as if it's some kind of gotcha. When there's no sun, then other countries probably have sun. When there's no sun, there's hydro. When there's no hydro there's wind. When there's no wind, there's tidal. When there's no tidal there's geothermal. And when there's none of those, there's stored energy in batteries. And if for some reason all of those co…

While other countries have sun when it is night here, realistically we cannot build the needed wires to get that energy here. We need 20,000km of wire to get power around the earth at the equator. Wind suffers the same problem as the sun in that sometimes there is no wind anyplace close, and it is even less predictable. While tide power is predictable and consistent, I don't live anywhere close to the ocean so we sti…

> realistically we cannot build the needed wires

On the contrary, Europe, Africa, Asia, the Americas are all wide enough that they can do plenty of solar trading during the daytime. Of course that does not provide energy in the night and that's OK.

Re: US startup begins producing 40%-efficient thermophotovoltaic cells

#172

Post by lambdatronics » Sat, Oct 07, 2023, 6:25 PM UTC

pbmonster wrote: post #45

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…

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...

Re: US startup begins producing 40%-efficient thermophotovoltaic cells

#173

Post by lambdatronics » Sat, Oct 07, 2023, 7:26 PM UTC

svantana wrote: post #16

Could this be used to create a nuclear power generator without moving parts? Some radioactive material in the center, some coating to absorb the radiation, and a shell of these cells to generate electricity.

No-moving-parts is probably not realistic - the core would be much hotter than the radiating surface due to the thermal resistance of the shielding. More likely, you would have a working fluid to transfer the heat. Most nuclear reactors operate at much lower temperatures where TPV wouldn't be efficient or cost-effective. It's certainly possible to go much higher. Nuclear-thermal rocket propulsion tests ran with exhaust temperatures up to ~2200 C [0]. Whatever fluid is used, you would need to avoid radioactivity in it, b/c that would probably degrade the TPV. Also you would probably want to avoid having heat exchangers because each one incurs a temperature drop. So helium would fit that bill. That's what the "high temperature gas reactors" use [1]. IDK if helium-compatible plumbing/pumping could be made to work at >2000 C though.

Edit: BTW, there are radioisotope thermoelectric generators used for space applications primarily, but they are not true nuclear reactors - they produce short-range radiation that doesn't require much shielding. Nuclear reactors produce neutrons and gammas that require thick shielding.

[0] https://en.wikipedia.org/wiki/Project_Rover#Kiwi_A

[1] https://en.wikipedia.org/wiki/High-temperature_gas_reactor

Re: US startup begins producing 40%-efficient thermophotovoltaic cells

#174

Post by vlovich123 » Sat, Oct 07, 2023, 9:19 PM UTC

nine_k wrote: post #131

vlovich123 wrote: post #121

Earlier quoted context omitted.

A sphere rotating in a vacuum should still experience black body radiation and gravitational drag. Damn kids and their pesky attempts to violate the 2nd law.

Cooling down would not slow down the sphere, if the radiation is uniform. Gravitational drag, maybe, but only if there is something to drag nearby, and we postulate a vacuum. A sphere is symmetric and thus does not emit gravitational waves. I wonder if an effect akin to black hole evaporation could play a role: if one of the two virtual particles gets accelerated towards the sphere, it may bring some momentum to it.…

A vacuum doesn’t exist in reality. In addition to virtual particles, gravity travels forever afaik. Which means your rotating sphere will interact with matter no matter how far away it is. Unless you’re hypothesizing a universe that doesn’t exist. In which case, we can also hypothesize one where the 2nd law isn’t an issue.

Also, I’m not so sure that black body radiation wouldn’t have an impact on rotation. At some point, the object will have radiated all its heat and have no energy left. If the particles have no energy, how is the sphere rotating which implies kinetic energy? That implies that even when radiating uniformly, black body radiation must take away angular velocity, no?