Heat Pumps - low grade heat in soil, lake, air boosted at high efficiency to higher temperature.
Heat pumps could become the preferred method of house heating in the future. A liquid (e.g. freon) passes through underground pipes outside the house where the liquid gains heat by evaporation to become gas; it is then pressurized by the pump, raising temperature and pressure. The gas condenses- to become liquid again - and in the process heat is released in radiators in the house. After passing through a reducing valve, where pressure and temperature drop to below the ground temperature, the liquid returns to the underground pipes to complete the cycle.
Efficiencies of up to 500% can be achieved. The source of low grade heat could be water in a river/lake or pipes laid underground or it could be air sourced.
A heat pump is used in refrigerators where the cooling is inside the refrigerator and the heat released through pipes outside the refrigerator. In practice, evaporation and condensation may all take place inside the heat pump unit which includes heat exchangers connected to the pipes outside and radiators/underfloor pipes inside the building respectively.
Underground pipes for larger or commercial buildings can be located vertically, typically 50 to 100 feet deep.
Air source heat pumps give a reasonable efficiency in the UK where the temperature doesn't often drop below 0 deg C. At lower temperatures the efficiency would fall (see example in RH column) and auxiliary electrical heating arranged to come in.
Where space is limited, to prevent the ground steadily cooling as heat is taken out, heat could be pumped back in summer from solar heating and /or air conditioning, or air heat pumps used.
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