Lightcell: An engine that uses light to make electricity

I find the bandgap tuned cell interesting. It reminds me of a TPV https://www.nature.com/articles/s41586-022-04473-y which is tuned for infrared instead of yellow light.

less moving parts means it could work in contexts where moving parts demand lubrication, maintenance.

I felt it was a bit light on putting the system energy efficiency/losses up front. I am sure they're stated but it was hard to work out how it compared to normal PV efficiency, or steam turbine efficiency.

Heat exchangers are applicable to lots of things. I am skeptical that this is significant because almost any heat energy process does reclaim and preheat, and so the size of the thermal mass and efficiency here would be exceptionally well studied and if they have made improvements, they may be as, or more valuable as IPR overall. So while it looks amazing, unless they are spinning it out into wider industry it will be a small increment over things in deployment.

I read their statement of 40% efficiency would be compared to the currently available photovoltaics were generally 20% efficiency is normal.

Amazing idea. BTW, following Danielle on X, very insightful and bright minded person.

Bottom line: 40% efficiency, which is better than I expected but the competition is batteries at 80+% efficiency. It's a hard sell, especially as continual improvements in battery storage will continue to eat away at their niche.

5,000 W/kg sounds great on paper compared to 150 W/kg for batteries and is even in the same ballpark as gasoline at 12,000 W/kg, but I think that's just the figure for the fuel. I don't think it includes storage, the solar panels, the burner, etc... The cost is an open ended question as well. Maybe this will pan out for aircraft?

The energy densities listed are flagged as approximate, so grains of salt etc, but the numbers on the page aren't entirely consistent.

The stated energy density is "> 500 watthours/liter".

But higher on the page we see a relative-energy-density bar graph shows lightcell at 5x the energy density of lithium batteries, and (38/5 =) 7.6x less dense then petrol. This implies an energy density for lightcell of 1250 Wh/liter, as (according to Google) petrol clocks in just under 9500 Wh/liter, and (again according to Google) lithium batteries can reach 300 Wh/liter so let's call it 250 for the math to work out.

I'm curious which number is closer to truth: 500Wh/liter, or 1250? Is 1250 the theoretical max and 500 the current output in a test rig?

If that is 40% efficient as in 40% of the theoretical energy input comes out as electricity then it's quite incredible but I find that hard to believe. It would put it in the same range as diesel engines.

I believe the bar graph is showing relative energy densities of the raw energy sources so the 5x bar is just the energy density of hydrogen as H2. Your 1250 Wh/L number is right for compressed gaseous hydrogen so The 500Wh/L lines up with burning H2 at 40% efficiency. The "use fuel for extended duration" implies that they believe they can achieve a much higher Wh/L with other fuels.

The 40% figure is supposed to be "wire-to-wire", but they do list that as the "target efficiency" which suggests it may be somewhat aspirational. It presumably doesn't include the energy needed to extract and refine the oil into whatever kind of burnable fuel you are using, nor the energy necessary to extract and then blend in the sodium additive.

forbes to prison pipeline?

The better comparison is Fuel Cells and vehicle based electrical generators. So you could put this in a vehicle or remote location, run it off hydrogen or natural gas, and get better efficiency. Potentially this could be a much better option for longer term storage in remote areas as well, where excess solar/wind could be used to crack hydrogen which then gets stored and later burned in one of these instead of a much much larger battery installation.

I would think the energy density varies with that of the fuel they put in. They mention hydrogen, natural gas, gasoline, ammonia, butane, propane, alcohols, syngas…. That’s about anything that is or can easily be turned into a gas that burns.

also, “/liter”, for gases such as hydrogen, can be made larger by using higher pressures in your tank.

On the other hand, they also say “target efficiency: ≥ 40% wire to wire”, and 40% of 1250 is 500, so it may be that.

More likely than you think.

You still need to truck in the sodium additive even if you're cracking water on site to store the H2. Dunno if you need a couple of mg/kg or if it is like 5% of the fuel to make it burn at the right color.

The solar panel conversion of sunlight to usable energy to around 20%, with a theoretical max of 30%. So it's better than that.

The gasoline vs H2 ballpark is a little wider because storage is not trivial for H2 -- you need to carry around a cryogenic and/or high pressure vessel instead of a plastic box -- which will detract from your p/w ratio. It also wants to leak out, so H2 is maybe better for fleet vehicle applications where they can refill daily. Granted, anything is better than burning more hydrocarbons!

I don't think they are claiming an efficiency breakthrough on their heat exchanger, just that they've made a competitive heat exchanger that also blocks light very effectively.

I've periodically seen lightcell and danielle fong in various news / reddit /forums over the last few years and it always seems to be steeped in controversy.

I know next to nothing about the field / tech, but a portion of folks seem to be like "incredible visionary etc. etc." and the another portion like "fringe science / complete bullshit / this is as realistic as cold fusion" kind of thing.

Very interested to hear from folks more in the know of like, high level long term viability / what the implications are etc.

My initial thought about this was that it's using fuel to make electricity, right? Rather than using sunlight/hydro/etc -- kinda like a generator, but without the mechanical aspect?

And at the bottom they seem to indicate they are still in the "proving feasibility" stage.

I read this all as: "this is a POC we have, and if we can get it to 40% efficiency than it might make sense (otherwise who cares, just use a conventional generator)"

That can't be true. The current record for non-concentrating cells is 39.5% efficiency using triple junction cells [1]

Concentrating cells are at 47.6% [2]

[1] https://www.cell.com/joule/fulltext/S2542-4351(22)00191-X

[2] https://publica-rest.fraunhofer.de/server/api/core/bitstream...

the 40% efficiency is a claim about how much energy contained in the fuel can be converted into electricity*. It would make the most sense to compare this against either combustion engines or hydrogen fuel cells. Compared to those 40% is not breaking any records but could be extremely useful given the size, flexibility, weight, power output, etc.

Basically big if true, but this thing's 40% and photovoltaics' 20% aren't comparable efficiency numbers.

* They say wire to wire, IDK exactly what that means, but if it includes the losses from green hydrogen production then it seems like pretty wild efficiency. This doesn't line up with the numbers though, as H2 with 1250Wh/L * 0.4 = 500 Wh/L claimed density.

Often I imagine storing light as fuel. Compared to hydrogen, it doesn't weigh much at all, and you can fit a lot in the same space.

(Yes, I know where the halfbakery is.)

To my limited understanding yes, that's what they claim.

Basically burning fuel (any fuel, really) with added sodium to create very bright monochromatic light that can then be converted into electricity using very high efficiency solar cells.

But sunlight is wide spectrum, and a lot of the reasons why the efficiency of regular solar panels is low, is that they don’t absorb all of the spectrum equally well. That’s why there’s all this talk of tandem solar cells with perovskites these days. The two solar cells can be tuned to extract energy from different wavelengths of light.

Since the light they’re making is nearly monochromatic, it’s a lot easier to get higher efficiency. That’s kind of the whole point of the invention.

Do you mean watts or watt-hours?

this was done by a company in Alberta,late 90's early 2000's, except burning diesel, same idea of tuned photovoltaics outside a quarts cylinder,where a flame was buring @ one specific coulor temperature, they were marketing an initial model for sailboats, and had working devices in service. published efficiencies wrre also 40%+ lost track of them and could not find again this effort uses excited sodium,though there will be a number of other possibilities

That's not really relevant. They have a light source that runs on a fuel and are putting multiple PV cells around it. The efficiency they care about is the fuel in to electricity out. If you can put more cells around the light, the system efficiency goes up.

It's a very good idea that is worth pursuing, they are pursuing it. There are many many many problems that need solving between here and "this is a better way to make energy from heat at scale than turning water into steam and spinning a turbine". The science is fundamentally sound but we're nowhere near economic viability.

Isn’t that for sunlight though? I imagine if you have a source that only radiates a single wavelength, you could make a collector for that specific wavelength that’s more efficient than some general case one. Could be wrong though.

My understanding of fuel cells is they are rather sensitive to the purity of the fuel and oxygen. I wonder if this system is less sensitive such that, say, piped hydrogen can be used.

Just be careful or you might make a Kugelblitz

Two questions I have:

1. How much of the fuel's energy is released as heat? They have a heat recapture device, but that's only used to preheat air/fuel, and not used to generate electricity. Is the energy in the heat simply discarded?

2. Can this be made to work without the process of burning? i.e. can it function purely from heat? If it can, it might be able to replace steam turbines in, for example, nuclear plants or CSP plants. That could be hugely beneficial.

The innovation here is you have a system that emits monochromatic light, and you have solar cells tuned specifically for that bandgap, plus the system is also "naturally" concentrating because the light output is incredibly bright. 3000 suns vs 500-1000 suns in typical CPV, plus they also do waste heat recycling. End-to-end efficiency of 40% is definitely feasible as advertised.

This seems like a hydrogen fuel cell with extra steps.

1. The countercurrent heat exchanger achieves exactly that: exhaust gases are cooled while the inflowing fuel mixture is heated up.

2. Thermophotovoltaics in general can operate with any heat source, though this device is clearly optimized for combustion. However, the efficiency is far too low to compete in the large-scale power generation segment. This is almost certainly aimed at light aviation, heavy drones, military applications, etc., where there are not a lot of alternatives that combine small size, high power density and good efficiency.

It's not like cold fusion, the lightcell is based on well-understood physics. The author may be too optimistic with efficiency claim, but those are relatively easy to verify independently.

It's only true for a single junction. https://en.wikipedia.org/wiki/Shockley%E2%80%93Queisser_limi...

Multi-junction cells beat that limit, but they're still horribly expensive to manufacture which confines them to niche uses like spacecraft.

It's a different way to capture energy stored in fuel.

Normally to produce electricity from fuel you would spin a turbine, either with a mechanical engine or using vapour. But here the energy is captured through a photo cell, and the author claims that mixing sodium into certain fuels leads to a very significant part of fuel energy going into light at specific wavelength.

It probably doesn't help that the website looks like an American Science & Surplus catalog

I suppose for aviation at least this is no less efficient than a gas turbine or a piston, and it's certainly a good deal quieter, has fewer moving parts, and requires less precision engineering than a jet engine. This feels tailor-made for attritable low->medium performance aviation, aka loitering munitions and drones. Strip away the "green" talk, and you're left with something that can burn just about anything (including hydrocarbons like avgas) without the complexity of a turbine.

This seems possibly not crazy. If you can have one of these powered by natural gas and scale it to 20 kW then you have a nice home generator that is "whisper quiet" according to TFA and also: simple, easy to maintain, with few moving parts, perhaps even durable. The hydrogen aspect of this is not as interesting as the other fuels, though it'd be nice to know the efficiency numbers for different fuel types. That said, having to supply sodium might be a problem.

1. It's hard to capture all the waste heat. If you could run this indoors (but vent outdoors if the fuel is anything other than H2, naturally) then you could use some of the waste heat to heat a building.

2. There are thermovoltaic generators, but they're limited by the need to cool one side of the material. These are typically used in deep space probes that use Pu 240 to power them. To my knowledge thermovoltaic generation is not scalable or practical on Earth at this time.

It might not be a hard sell compared to home generators. Forget hydrogen. Think natgas.

Rechargeables/battery packs have inefficiencies due to the grid and/or solar cells though, in terms of where to measure inefficiency?

I suspect this needs some moving parts to function - without a turbine's suction, you need some sort of a fan to pump air into the thing, and also a fuel pump. Most things with internal combustion require some kind of active cooling as well.

From the "wire-to-wire" mention it seems that they're proposing using electric power to generate and store hydrogen till it's needed, then burn it to get electric power back.

But they say other fuels work, in which case it wouldn't be "wire-to-wire", and then it'd be more appropriate to compare this to a power generator fueled by natural gas or gasoline. A generator with no pistons or turbines, just a fuel pump, sounds fantastic, if they can make it work. But you'd have to supply sodium.

What does "wire to wire" even mean? The input isn't a wire! (Do they mean they think they can synthesize fuel and burn it at 40% overall efficiency? If so, that's pretty good.)

If using compressed natural gas you might not need a fuel pump at all.

This burns fuel at very high temperature, and I wonder how they plan to deal with NOx production. They could attempt to burn the fuel in pure-ish oxygen (with an oxygen concentrator?), but that would increase the complexity of the design and compromise the "quiet" part.

I agree that 40% "wire-to-wire" seems wild. But if it was 40% nat gas to wire that'd be quite nice considering how simple such a generator would be, and it might be more efficient (perhaps significantly more) than traditional internal combustion generators. I.e., if you ignore the green aspects of this it sounds quite nice. Though you have to supply sodium. Hmmm.

A couple video interviews with the inventor:

https://www.youtube.com/watch?v=aMQYAqIxK1s

https://www.youtube.com/watch?v=1U_KbgF-sAc

In context, an important innovation. Perhaps this technology can retrofit into hot gas heat exchangers like in steel works, but they use the thermal energy directly so it may be robbing Peter to pay Paul.

Thanks for a clarification which makes sense.

She seems like someone with an eye for a clever solution to an existing problem, an eye for funding (her compressed air "LightSail" thing raised over $70 million), and maybe a somewhat shaky relationship with practicality.

Reminds me of the TimeCube page…

hey! this is the inventor, danielle fong.

thanks to curl-up who posted this, whoever you are.

since it came up, "wire-to-wire" efficiency is what I intended to coin a synonym for electrical to electrical efficiency, with hydrogen storage. for example, an 80% electrical to hydrogen efficiency, and a 50% hydrogen to electrical efficiency, would yield a 40% wire to wire (electrical to electrical) efficiency. of course, people are working on 95% electric to hydrogen efficiency, and 50% fuel to electrical efficiency is a target.

here's an illustrative energy flow diagram for us trying to hit 60% -- even more aggressive. https://x.com/DanielleFong/status/1775595848887677138

we'll have to fire the web dev (me)

I wonder if they recover the sodium and run it back through the process.

For that matter, could you maybe put sodium in a sealed container and then heat the whole container? Like a sodium vapor lamp but causing it to glow by throwing heat at it instead of passing electricity through it.

it can work purely from heat, however our process requires high temperature heat for power density.

maybe so. i don't know about attritable for the first applications though. may long range or duration oversight. a large % of the cost is these specialty cells which have not been scaled up to mass production. in the denominator is the intensity of light we can produce, which is based on how high a temperature we can drive, there's a very nonlinear brightness vs temperature. but at 100 suns or so we can get near to $1/W on the cells at startup scale

we think it will be, it's a good bet

correct

indeed yes; the sodium is added as sodium chloride. in molten form, it wicks along sapphire and alumina surfaces, similar to a candle. it reforms into sodium chloride as the temperature drops below its boiling point -- 1400 C.

we're exploring fully sealed experiments, but, you have to get the heat into the sealed cell somehow.

https://patents.google.com/patent/US12136898B2/en?oq=US12136...

oh god

come on guys

correct

we're bootstrapping off the multijunction production while using just a single junction that matches the sodium D light well

thanks!

let me know if you can remember the name or a reference, thanks!

fuel cells have trouble being cheap, lightweight, high efficiency, and long lasting, all at the same time. I think this could have better scaling on all those dimensions, plus could use natural gas or propane or other fuels for when you don't have hydrogen

Does the tube become less transparent because of contaminants? Over what time scale? Is this issue exacerbated before the system is operating at full temperature (e.g., coking)? Is the sodium vapor kept in the closed cavity or is it a consumable? If a consumable how much is needed? How is it stored and dispensed?

you need at least valves/regulators, but for self pressurized fuels like propane, butane, or even natural gas (CNG or LNG) you can probably get away with only that, and fans for air intake and cell cooling.

we're not aiming to break records with the absolute heat exchanger efficiency, which can get into the high 90s (%) if you're willing to devote a lot of space and mass, but we are innovating in the heat exchanger area. to capture more of the waste heat up to a higher temperature, and preheat the incoming air and possible fuel to a higher temperature, we have to exceed 1000 C and want to drive towards the 1600-1800C maximum working temperature of the high alumina 3d printed material we're using. Thankfully Formlabs has already done some of the preliminary development on the material, but it's bleeding edge both as a material and in use in heat exchangers.

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Why would I use this instead of a fuel cell?