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finnh | on: Lightcell: An engine that uses light to make electricity
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?

enragedcacti | parent | next
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.
Someone | parent
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.