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 (with suitable condensation and evaporation points) passes through a heat exchanger which is also connected to underground pipes outside the house. The liquid gains heat by evaporation to become gas; it is then pressurized by the pump (driven by and using electrical power), raising temperature and pressure. The gas condenses- to become liquid again - and in the process heat is released via further heat exchangers 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 complete the cycle.
Efficiencies from 200% up to 700% may be achieved.
A heat pump is used in refrigerators where the cooling takes place inside the refrigerator and 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.
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.
Types of heat pump and their efficiency.
Ground source. Described above. Closed loop of probably 600 foot pipe laid under the ground (lawn)surface. At 6 foot below, ground temperature is well above freezing (in UK) probably 10 deg C. A domestic system could cost £9000 plus depending on earth works. Underground pipes for larger or commercial buildings can be located vertically, typically 50 to 100 feet deep. If used with underfloor heating generating say 40degC the efficency could in practice be 600%. Theoretically it could be (273 + 40) / (40-10) = 10 times or 1000 %.
With ground source and using radiators at 60 deg C the efficiency would be approximately 330 %.
Air source. Draws heat from air outside. Lower cost than ground source. The efficiency could be between 350 c% and 200%. It is at the lower end at lower outdoor temperatures. For the higher efficiencies the gap between top (condensation) and low (evaporation) temperatures must be low. Below 0 deg C auxiliary electrical heating may be arranged to come in.
Water source. Applicable where a heat vcan be taken from a lake or other large water source.
Exhaust air source. Also called a Mechanical ventilation and heat recovery (MVHR) system where heat is extracted from stale air inside the building before venting to outside.
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