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The most efficient way of moving heat from one place to another, short of sunlight, is the Heat Pipe.
Basically at the heat source, water in an evacuated tube, is evaporated, taking it’s latent heat of evaporation with it. Then it condenses at the colder “heat sink” end giving up that latent heat. Different fluids can be used for different temperature ranges, but water is one of the best. Since external power isn’t needed, the Alaska Pipe Line has heat vertical finned heat pipes all along its way, to keep the permafrost frozen.
The sealed pipe is first filled about 1/3 with water, heated to boiling, so the vapor will blow the air out of the “evacuation tube.” While the pipe is still hot, the evacuation tube is sealed shut so the air cannot reenter.
With only water and water vapor in the sealed pipe, the fluids are nearly always at their vapor pressure, and will evaporate and condense with the slightest bit of heat, regardless of temperature. The limits are freezing, and the explosion temperature of the pipe itself. After it cools, place your hand on the previous hot place, and it will feel cold, because even the heat of your hand will evaporate some of the water inside, spread it through the whole pipe, where it condenses, giving up your heat.
If a toarch is put on the “Heat Input” end, the whole pipe, up to the point where all condensation takes place, will be really hot. See Fig. 1. Insulation is placed on the outside of the pipe to reduce heat flow where you do not want it to go.
In Fig. 2, the wick will pull the needed condensate water to the heat-supplying end of the pipe. That is, as far as that wick substance is able to pull it.
In Fig. 3, heat flow is Up Hill. However the water condensate is trapped at the cooler end, and only allowed to return down to the heat source when the thermostatically controlled electric valve is open. This effectivly shuts off the Heat Pipe, except when needed.
If the temperatures happen to switch while the valve is closed, heat will flow back up hill until it runs out of water.
Fig. 4 is useful then the heat source and sink are too far apart for a wick to pull up the water, and a pump must be used. Here the electric pump can be thermostatically controlled, or just on a switch.
The reason electric valves and pumps are used is to allow operation without rotating mechanical items that may break the seal of the pipe.
One application of heat pipe technology is in storing heat in the earth. Here heat is taken underground in the daytime and returned at night.
The plan contains a high effieicncy electric generator that operates at night, from solar heat saved during the day, and durring the day while storing solar heat.
Thanks for your interest.