Author Topic: Power generation using a Sterling Engine  (Read 2897 times)

Offline G-pit

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Power generation using a Sterling Engine
« on: 09/04/2007 03:30 am »
This idea popped into my head recently, and I wanted to post about it because I believe members on this forum can tell me why it won't work, or why it isn't being used.

Sterling engines generate energy as long as there is a delta-T between two surfaces, and the larger the differnce in temperature, the more energy they produce. In space, there is a massive difference in temperature between objects in the sunlight, and objects in the shade. I recently saw a video clip about the extreme temperatures that EVA suits needed to withstand. The video showed a tool being handed between two astronauts on an EVA, one was in the shade and one was in the direct sunlight. The tool went from +several hundred degrees (Over 275° F) to -several hundred degrees (under -300° F).

Now, with this *massive* delta-T between objects in the direct sunlight, and objects in the shade, wouldn't sterling engines be a pretty good way to generate power? If it was necessary to further increase the delta-T, it may be possible to use a parabolic mirror to focus the sunlight on a panel.

I have a few nice model sterling engines at home that can spin a disk off of the heat from my hand. If you light a candle underneath them, they *really* can get going. with a candle underneath and an icecube on top, they spin rediculously fast  :bleh: I could only buy a sterlingengine with standard air inside, but I filled the cavity on a couple of them with helium gas to increase the efficiency, and it suprised me how much mechanical power was created. I'll post a picture of the sterling engine design I was working with at the bottom of this post.

I'm not advocating using this in the design of a spacecraft, or in comparing sterling engines to solar panels -- so please don't get sidetracked with that  ;)

I'm just wondering, will this work and will it work well?
"Find a job you love, and you'll never work a day in your life" - Confucius

Offline Lee Jay

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Re: Power generation using a Sterling Engine
« Reply #1 on: 09/04/2007 03:59 am »
- All thermodynamic engines move heat from hot to cold.  After a while, your cold in-the-shade reservoir will get warm.  In space, it can only dissipate energy via radiation.  If you want to keep it cold, you're going to need a HUGE radiator since the dissipation rate is very low at cold temperatures.

- The delta-T isn't very much different on the ground.  You can concentrate the sun and generate over 5000K if you want to.  Whether the cold reservoir is at 50K or 300K doesn't matter much if your hot reservoir is at 5000K.  It's much easier to cool things here on Earth than in space (convection, conduction, evaporation...).

RTGs effectively work this way, though the hot reservoir is a nice chunk of decaying radioactive material.  They work for a long time, but  they're relatively inefficient and low in power (watts to hundreds of watts - no kilowatts to gigawatts).

Offline kkattula

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Re: Power generation using a Sterling Engine
« Reply #2 on: 09/04/2007 04:24 am »
Good on the moon or an asteroid though, where you have a big 'cold sink'.

Offline G-pit

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Re: Power generation using a Sterling Engine
« Reply #3 on: 09/04/2007 05:24 am »
Quote
Lee Jay - 3/9/2007  11:59 PM

- All thermodynamic engines move heat from hot to cold.  After a while, your cold in-the-shade reservoir will get warm.  In space, it can only dissipate energy via radiation.  If you want to keep it cold, you're going to need a HUGE radiator since the dissipation rate is very low at cold temperatures.

Ah, Thanks for the clear (and quick) answer!
There really isn't a great way to release all of that heat during a space mission. Theoretically, if the 'cold in-the-shade' resevoir is continually radiating any heat away, and the 'hot-in-the-sun' side is continually absorbing more jewls of heat there will always be a delta-T between the two sides. Let me rephrase the garble: while the hot side gets hotter -- the cold side gets hotter, but never as hot as the hot side. You may be able to keep a substantial delta-T for months or even years if there is good insulation between the two sides, and the engine operates efficiently.
"Find a job you love, and you'll never work a day in your life" - Confucius

Offline yinzer

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Re: Power generation using a Sterling Engine
« Reply #4 on: 09/04/2007 07:32 am »
People have actually done a lot of research into using Stirling engines in space.  Usually using a radioactive heat source - this is frequently known as "DIPS" for Dynamic Isotope Power Supply.  The Stirling engine is more efficient than the thermocouple used in an RTG, so the power supply can be much smaller.  The problem is that it has moving parts, which are never popular in space missions, as they can break.  Sometimes they are considered using the sun as a heat source, but then in addition to moving parts there is usually a mirror that has to track the sun pretty accurately, especially compared to a solar panel.  This is a bit of a pain.

As the power usage increases, thermoelectric power sources like the Stirling engine become more attractive, but so far there haven't been any spacecraft in the several-hundred-kilowatt range.
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Offline yinzer

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Re: Power generation using a Sterling Engine
« Reply #5 on: 09/04/2007 07:34 am »
Also, the moon is a horrible cold sink.  The lunar rock is a very, very good insulator, so if you bury pipes in it and run hot fluid through them, the bit of rock right next to the pipe will melt before the rock a meter away warms up in the slightest.  Once you get below the immediate surface, it's also packed very tightly, and is very difficult to dig through.
California 2008 - taking rights from people and giving rights to chickens.

Offline tnphysics

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Re: Power generation using a Sterling Engine
« Reply #6 on: 09/30/2007 04:06 am »
I didn't realize that.

It would be important in designing any underground structures on the Moon.

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