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Heat-pumped storage of thermal energy

Cool something at night, heat during day.
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This is an idea I was reminded of by Archimerged's two ideas:

Store coldness at night (when it's more easily available), and heat during the day, using a heat pump and heat engine, so the warm thermal mass (maybe 25 degrees C) and cold night air (maybe 0) power a heat engine, which powers a heat pump (refrigerator) to get another thermal mass colder than the night, (maybe -20). Then in the 30 degree heat of the next day, power the heat engine between the cold mass and the warm air, and use that to power a heat pump to heat the other thermal mass to high temperature (70 or 80 degrees), and so on.

After the temperature difference gets as big as you want (there will be some limit anyway), stop putting all the heat engine output into the heat pump, use some as electricity instead.

caspian, Apr 21 2006


       In order to pump heat, you must be able to reject some heat to the outside, so that the thermodynamic ball rolls downhill, so to speak. When a system gets its temperature differential from the ambient temperature around it, this is not possible.   

       A basic system would need to be well-insulated from ambient at the correct times in order to prevent it from losing heat gained from the environment, but at other times be able to gain heat from that same environment. You need effectively a one-way thermal blanket, which is a difficult undertaking.   

       I'm afraid you're going to have to flesh the thermodynamics out a bit more here, I'm quite suspicious that a stepped system wouldn't work at all. At the end of the day, you can't invent energy from nothing.
RayfordSteele, Apr 22 2006

       That is kind of baked; google for "solar ice maker". The ice makers work like absorber refrigerators that run on a gas flame. The solar version use the sun instead of a flame.
kbecker, Apr 22 2006

       //At the end of the day, you can't invent energy from nothing.// This machine does not do that. It takes energy from ambient which came from the sun, and returns it to the ambient which then goes out into space away from the sun.   

       Heat pipes, as is well known, do provide one-way thermal conductivity. Especially if they avoid using copper for the full length. If the metal doesn't conduct much heat, and the liquid refrigerant has no way to go upward except by evaporating, it is hard to see how heat is going to move downward through such a pipe.   

       The trans-Alaska oil pipeline uses such heat pipes to keep the permafrost from melting under its foundations and breaking the pipeline.   

       I misread heat pipe for heat pump. But you could use just heat pipes initially to create a large hot and cold reservoir.   

       The idea does work (ignoring mechanical efficiency problems). This machine transfers heat from a hot reservoir to a cold reservoir, as any good heat engine does. When cooling below ambient, it is accepting heat from the thing it is cooling, and rejecting heat to ambient, and using another heat engine to provide work to compress the refrigerant. When heating above ambient, it is accepting heat from ambient and rejecting it into its internal cold reservoir, again using a second heat engine to provide work to compress the refrigerant.   

       So it's not surprising to hear it has already been done.
Archimerged, Apr 24 2006

       For heat transfer I was thinking of fluid, maybe water, flowing through a radiator, and ambient temperature blowing through that either as natural wind or with a fan.   

       For heat storage I was thinking of large volumes of water in insulated containers, from a cubic metre on up.   

       The water could be pumped (with controls that stop it when the end we want to heat is hotter) or some parts could use the thermosiphon effect (have the end of the hose or pipe we want to heat higher up).   

       A heat pipe could work too.   

       A simpler idea I had was simply to extract work from the difference between a resovoir and the atmosphere using heat engine, and no heat pumps. It is basicly a heat engine between night air and day air, so I think that should obviously obey the laws of thermodynamics. There are two main limits to this: transfering a lot of atmospheric heat to and from the device, and having the capacity to store a lot of heat (or cold).   

       If storing a lot of heat is the bottleneck, you can either get a larger or better thermal mass, or you can store more heat by increasing the temperature you store it at, which is what I did for this idea.   

       I'll definitely admit my idea has a lot of opportunities for inefficiency, but at the ideal Carnot efficiency it increases the amount of energy you can extract using a given sized thermal mass.
caspian, May 04 2006

       This is the general concept upon which a Geo Thermal installation works, A heat pump moves daytime heat down into the soil and heats it up then at night the system is reversed and the heat is drown out of the ground and released back into the building. This is also used on a smaller scale for industrial Ice makers. heat is dumped into a large mass of water that then slowly radiates the heat out into the environment(which it could be redirected if desired)
jhomrighaus, May 04 2006


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