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# Solar/Geothermal Electric Stirling

Pitting the Sun against the Earth in a somewhat boring way.
 (+5, -1) [vote for, against]

To be succinct, a Sterling engine can convert a temperature differential into mechanical energy [see link]. This is usually a difference in air temperatures, but let us use water jacketed cylinders.

Now, pump water in a closed loop from a solar collector to the heating cylinder jacket of the engine. Meanwhile, another loop of water is pumped from the cooling cylinder jacket to a length of pipe that is buried in the earth a couple stories underground.

This should create a large and steady temperature differential for the engine because the ground stays relatively cool year round in most of the world, and the sun is hot. The Sterling engine will be used to generate electricity to run the two pumps and to produce surplus.

This will only work under daylight, and requires a kick-start in the morning… unless nighttime radiation toward outer space from the solar collector causes the whole thing to flip into reverse?

<Prior art note: In researching for this I did see an annotation on an idea suggesting a Sterling with direct contact to air and earth, but no discussion resulted.>

 — Laughs Last, Jul 14 2005

The Sterling Cycle http://science.hows...tirling-engine1.htm
from HowStuffWorks [Laughs Last, Jul 14 2005]

 Has anyone done any calculations on how the temperature underground will be affected? It seems to me that the underground pipe will soon be surrounded by warm rock. How well is heat conducted underground? How much volume of rock is going to be heated, and how will that heat spread?

The heat-pump houses in Missouri tend to use the ground for heating, then cooling, so it averages out over a year. If a Sterling engine did something like that, it might work--otherwise, you are going to loose your cool.
 — baconbrain, Jul 14 2005

I've seen information on units that use the ground under a house as a heat store for summer air conditioning and winter heating. Using it for a Stirling engine is new to me; I like it. [+]
 — david_scothern, Jul 14 2005

 good idea

anyone heard of 'heat wells' ? another strange way of cooling which is similiar but with no moving parts
 — supershnitzel, Jul 14 2005

Nice combo. I plan on using solar + geothermal heating/cooling in my next house. The caves around here are always 50F.
 — Shz, Jul 14 2005

//Has anyone done any calculations on how the temperature underground will be affected? //
I agree, I think you'd soon warm the surrounding rock, and coolth wouldn't permeate in fast enough. The fact that subsurface temperatures *are* uniform year-round suggests a very low thermal conductivity. It might be different if you had porous rock with moving groundwater.
 — Basepair, Jul 14 2005

Great idea. Not new.
 — Freefall, Jul 14 2005

 In watts-per-meter-kelvin, soil thermal conductivity ranges from 0.75 (low) to 1.5 (high) depending on soil type and wetness.

 Compare that to the following: Styrofoam 0.01 Air 0.026 Water 0.6 Quartz 8 Steel 50.2 Aluminum 205 Copper 385

 Underground temperature is usually equal to the annual average air temperature due to the combination of conduction and the lag-time of thermal mass. Earth is a bad insulator, but it certainly isn't a metal. Geothermal coupling systems are designed to be large, to better take advantage of the Earth’s bigness.

[Freefall], yes this is like posting an idea for a new way to turbo charge a car engine. It is some well trodden territory, but I think I’ve got a uniquely advantageous way of combining the same old widgets.
 — Laughs Last, Jul 15 2005

 It will work, but would probably only be cost effective in a remote place where power not available and where other forms of renewable energy (solar cells, windmills, etc.) were impractical for some reason.

 The low thermal differential and the need to use one or more heat exchangers to introduce heat into, and reject heat from the heat engine will make this system very large per unit of power delivered. It might take a long time before it produced an amount of energy equivalent to the energy necessary to build it.

I still give it a bun because fuel prices might one day rise to the point where this is cost effective in a few special cases.
 — RunVentura, Oct 04 2006

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