Hacker News Reader

Comment by zaarn | Hacker News Reader

It reads a bit overhyped, considering they claim to power a house with a generator the size of an envelope; that's a hard no since there is a limit on how much energy hits every square meter and it's not THAT much.

View on HN

That is talking about heat and power cogeneration, not solar power if that's what you're thinking. The paper they site [0] says the density can 300-400kW/m^2 and so at 300kW, 50% efficiency and a small C5 envelope you're looking at over 5kW.

[0] https://scholar.google.com/scholar_lookup?author=M.+Bianchi&...

Solid-state cogen that is simple to include in heating furnaces would be very interesting for old stock high density residential in cold regions. Mire modern heating options than gas furnaces (various variations of heat pumps, usually coupled with solar) don't scale down to individual flats and anything more central (ideally district heating) is effectively impossible to retrofit in a free market economy. But the very high efficiency electricity conversion component of cogen is just too good to skip, so a smallest-scale cogen option would be a welcome addition to the energy mix.

The device is mentioned in this talk (https://youtu.be/lDxJsa8miNQ?t=3105) and provides 1kW of electricity and about 1kW worth of "waste" heat used for hot water. The key idea is that the amount of light would be the equivalent of 280 suns so you don't need a large panel.

To elaborate, solar energy hitting the Earth’s surface is about a kilowatt/m^2 if conditions are perfect. If you were able to convert solar power at the maximum theoretical PV efficiency (the Shockley-Queisser limit, 33.7%), you’d need about 3 square meters to just to make toast.

For those confused by that oddly low theoretical limit: it applies to single junction cells. Panels that achieve higher efficiencies do it by stacking multiple cells.

Your broad point is right, but the SQ limit only applies to conventional PV cells, it's not a theoretical limit on PV efficiency and in fact has been exceeded by other cell architectures.

Is this supposed to sound bad or good? Well, 3m² to make toast intuitively sounds bad I guess. Well, consider this: It's enough to run 2 small office PCs with all peripherals. Considering that it sounds much more positive. Really just anything producing heat would sound bad because of power dissipation. A toaster is probably one of the most extreme ... (Imagine putting 1kW into a slice of bread without power dissipation!)

Someone else wrote the comment I meant to, essentially doing the math to show that one would need an awkwardly large solar panel to power something like a quadrotor drone.

The idea behind individual solar energy is not consuming it real time but associating it with a storage solution to allow bigger throughput. You're not going to make toasts all day

>You're not going to make toasts all day

I bet you're doing something throughout the day. Maybe not making toast, but cooking, ironing, heating, cooling, watching TV etc. I'm not sure you can rely on any excess energy being available for storage if you're not connected to the grid.

>The idea behind individual solar energy is not consuming it real time but associating it with a storage solution to allow bigger throughput.

If that's the idea then you are exposing a major issue with solar. Namely: in order to rely solely on solar (+battery), you need a solar deployment that covers your energy needs now + energy to charge battery for when the sun isn't shining (taking into account the associated loss of storing and retrieving power from battery). This means that you need to oversubscribe/over capitalize solar to charge up the battery (which then lowers your total efficiency) in order to bridge daily and seasonal variability in solar output. This means that you need generation capacity that probably exceeds the surface area available to what a typical house or apartment can provide and explodes your costs.

This has major implications for large scale solar+battery deployments because at that scale, there is no grid to fall back on (which is why solar ALWAYS needs reliable backup generation - which is typically gas or biofuels). This is why solar+battery is never going to be cost-effective because you're always going to be forced to over-build infrastructure that will sit idly doing nothing most of the time.

This doesn't even touch on the fact that there is no actual battery technology that keep enough load to power a modern city overnight, much less to bridge seasonal variability at that scale (where the battery would be expected to keep enough energy for weeks at a time).

“Forcing over building of” solar is still cheaper than building a new coal or gas plant in some areas. I am positive there are plenty of people who live off grid and do not always need gas or biofuel backup.

>there are plenty of people who live off grid and do not always need gas or biofuel backup.

I'm sure there are plenty of such people, but those people don't really factor when you take into account the society as a whole. The vast majority of people and businesses cannot live off grid.

Absolutely agree and vast majority are not economically justified to live off grid in North America. Parent comment used all caps so just pointing out it is not absolute.

Yes, the max. energy from sunlight on earth hitting an envelope should be around 20-30 Watts (at 100% efficiency).

Also, the reported 29.1% efficiency have been reached at a temperature of 1,207 °C.

This isn't about sunlight, but about putting this with a furnace or something else generating heat.

Yes, the original article is about thermophotovoltaics.

Regarding the article posted here (and since a drone with a furnace is not feasible) I went with solar.

The corresponding author to be fair did say unmanned aerial vehicles.

True, though it is combined (in the article) with a maximum power consumption: "those that require as little as 100 watts, [such as] a lightweight unmanned aerial vehicle"

The original paper says "unmanned vehicles" (no aerial) and has a reference [0] to "unmanned undersea vehicles".

[0] https://www.pnas.org/content/early/2019/07/15/1903001116#ref...

The article says "a wide range of applications, from those that require as little as 100 watts, [such as] a lightweight unmanned aerial vehicle, to 100 megawatts"

I read the 100 watts as referring to lightweight unmanned aerial vehicle.

100 megawatts is followed by "[providing] electricity for 36,000 homes."

The original paper does not mention lightweight aerial vehicles as an application, but a drone is mentioned in the title (and shown in the top image) of the article.

Applications mentioned in the paper are: hybrid cars, unmanned vehicles, deep-space probes, energy storage, enabling efficient cogeneration systems for heat and electricity.

(A less-misleading title might even be: "New thermophotovoltaic engine for deep-space probes")

How about one of the authors talking about a credit-card sized device supplying 100 watts to quad-coptors using liquid fuel and staying airborn for 16+ hours. (https://youtu.be/lDxJsa8miNQ?t=3202)

Is this something you could use a laser for too? To bring heat to the flying drone? Or would that be super inefficient?

Beamed energy always has attenuation problems that make it inefficient.

Also, the reported 29.1% efficiency have been reached at a temperature of 1,207 °C.

Another 12% and they can compete with a standard steam turbogenerator.

Wouldn't you slap this on top of a steam boiler to get the best of both worlds, though?

Steam turbines have row after row of blades, each successive blade wheel larger and lower pressure than the last. Exit temperatures from modern turbogenerators are around 120° C, where the steam is about to turn into water, and pressure is usually below atmospheric. Almost all the energy which thermodynamics permits to be extracted has been extracted.

I probably should have phrased it better - I was picturing [Furnace] | [Thermophotovoltaics] | [Boiler], so the PV part gets first crack at the light/IR from the furnace and then the boiler cools the PV gear as it boils the water. This is also how it's suggested to be used in a paper linked elsewhere in the discussion above: https://www.sciencedirect.com/science/article/pii/S030626191...

If you make it harder to cool the turbine by putting those neat the radiator, efficiency will go down; if you make it harder to keep the heater hot, by putting those near the fire, efficiency will go down.

As animats already said, turbine efficiency on power plants is very near the theoretical optimum. The only way left to improve it is by increasing the temperature.

The furnace burns at over 900C but the superheated steam (which is your ‘hot’ temperature for your heat engine) is only at ~400C, you might lose some power output but you won’t be losing efficiency.

Perhaps for small scale applications this could be cheaper and more reliable? (if they can also get to 50% and if the materials used are cheap enough)

Reply on news.ycombinator.com