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I believe the heat will be used to drive a Stirling engine. Most concentrated solar power rigs use steam engines of some kind. https://en.wikipedia.org/wiki/Co

by dougk16 10y ago

I believe the heat will be used to drive a Stirling engine. Most concentrated solar power rigs use steam engines of some kind. https://en.wikipedia.org/wiki/Concentrated_solar_power https://en.wikipedia.org/wiki/Concentrated_solar_power

pstuart 10y ago
But are usable Stirling engines available? Dean Kamen's been promising a refined version for over a decade and has yet to release anything to production.
- dougk16 10y ago
  No idea about Stirling engines but you could always use other kinds of steam engines or turbines, albeit with lower theoretical efficiency. There's a very niche hobby for this kind of stuff, see https://www.youtube.com/watch?v=uJGpbvvJA2I https://www.youtube.com/watch?v=uJGpbvvJA2I for example. The one thing that is problematic overall probably is the noise that any steam-based solution will make.
- semi-extrinsic 10y ago
  Stirling engines are used in some niche applications. In particular, several Swedish and Japanese submarines are powered by Stirling engines running on diesel and liquid oxygen. The ability to run on a separate oxygen supply from breathing air is apparently a main advantage. NASA and DOE did some tests with Stirling powered cars in the 70's and 80's. They got up to 50 MPG. In the end, retooling, cheaper fuel and different UX when driving was the negatives that prevented mainstream adoption.
splawn 10y ago
In that case.... there goes their moonlight claim. https://what-if.xkcd.com/145/ https://what-if.xkcd.com/145/
- teraflop 10y ago
  You could definitely use moonlight to heat up an engine, if you have a big enough concentrator. You just can't reach a temperature higher than the surface of the moon itself. Stirling engines are capable of operating with relatively small temperature differences.
  - LeifCarrotson 10y ago
    > You just can't reach a temperature higher than the surface of the moon itself. False. You can't reach a temperature hotter than the black body which emitted the light. The moon is a reflector which is as cold as -153 C, not a black body emitter. Try reflecting sunlight with a large mirror cooled by air to 23 C through a magnifying glass onto an object in the shade. You can still light things on fire.
    - caspianm 10y ago
      A better analogy is a piece of paper. Like a mirror, it reflects rather than thermally emits the light, but unlike a mirror, it scatters the reflected light, limiting how much it can be focused.
- FiatLuxDave 10y ago
  I'm a huge Randall Munroe fan, and so I was hugely disappointed when he posted that "What if", because it is 100% wrong. Moonlight is pretty much partially reflected sunlight. The moon is acting more like a dirty mirror than like a blackbody emitter. This is easily apparent through two simple facts: 1) If the moon was emitting light due to its temperature rather than reflecting light from a higher temperature object, we couldn't see it all. Objects at 120 C don't really emit light in the visible range. 2) The color temperature of moonlight is around ~4000K, about 1000 K lower than sunlight. It's not like we haven't measured this stuff. So, yes, you could start a fire with moonlight. I leave this as an exercise for the Mythbusters-inspired reader. [1] https://en.wikipedia.org/wiki/Color_temperature https://en.wikipedia.org/wiki/Color_temperature [2] http://physics.stackexchange.com/questions/244922/why-does-moonlight-have-a-lower-color-temperature http://physics.stackexchange.com/questions/244922/why-does-m...

by dougk16 10y ago

pstuart 10y ago

But are usable Stirling engines available? Dean Kamen's been promising a refined version for over a decade and has yet to release anything to production.

dougk16 10y ago
No idea about Stirling engines but you could always use other kinds of steam engines or turbines, albeit with lower theoretical efficiency. There's a very niche hobby for this kind of stuff, see https://www.youtube.com/watch?v=uJGpbvvJA2I https://www.youtube.com/watch?v=uJGpbvvJA2I for example. The one thing that is problematic overall probably is the noise that any steam-based solution will make.
semi-extrinsic 10y ago
Stirling engines are used in some niche applications. In particular, several Swedish and Japanese submarines are powered by Stirling engines running on diesel and liquid oxygen. The ability to run on a separate oxygen supply from breathing air is apparently a main advantage. NASA and DOE did some tests with Stirling powered cars in the 70's and 80's. They got up to 50 MPG. In the end, retooling, cheaper fuel and different UX when driving was the negatives that prevented mainstream adoption.

splawn 10y ago

In that case.... there goes their moonlight claim. https://what-if.xkcd.com/145/ https://what-if.xkcd.com/145/

teraflop 10y ago
You could definitely use moonlight to heat up an engine, if you have a big enough concentrator. You just can't reach a temperature higher than the surface of the moon itself. Stirling engines are capable of operating with relatively small temperature differences.
- LeifCarrotson 10y ago
  > You just can't reach a temperature higher than the surface of the moon itself. False. You can't reach a temperature hotter than the black body which emitted the light. The moon is a reflector which is as cold as -153 C, not a black body emitter. Try reflecting sunlight with a large mirror cooled by air to 23 C through a magnifying glass onto an object in the shade. You can still light things on fire.
  - caspianm 10y ago
    A better analogy is a piece of paper. Like a mirror, it reflects rather than thermally emits the light, but unlike a mirror, it scatters the reflected light, limiting how much it can be focused.
FiatLuxDave 10y ago
I'm a huge Randall Munroe fan, and so I was hugely disappointed when he posted that "What if", because it is 100% wrong. Moonlight is pretty much partially reflected sunlight. The moon is acting more like a dirty mirror than like a blackbody emitter. This is easily apparent through two simple facts: 1) If the moon was emitting light due to its temperature rather than reflecting light from a higher temperature object, we couldn't see it all. Objects at 120 C don't really emit light in the visible range. 2) The color temperature of moonlight is around ~4000K, about 1000 K lower than sunlight. It's not like we haven't measured this stuff. So, yes, you could start a fire with moonlight. I leave this as an exercise for the Mythbusters-inspired reader. [1] https://en.wikipedia.org/wiki/Color_temperature https://en.wikipedia.org/wiki/Color_temperature [2] http://physics.stackexchange.com/questions/244922/why-does-moonlight-have-a-lower-color-temperature http://physics.stackexchange.com/questions/244922/why-does-m...