A heat pump is an electrical device that transfers natural energy from a relatively low temperature reservoir to one at a higher temperature.
Natural energy sources are renewable, but their low temperature energy cannot be directly used in heat and cooling installations. Heat pumps transform the energy without both changing the quality of the source and polluting the environment.
Heat pumps use a small amount of external power to accomplish the work of raising the energy level from a lower (energy of a heat source) to a higher temperature level (energy of a heat sink). By using the energy of the environment, heat pumps generate three to five times higher amount of energy compared to the used amount of external power. The energy of the environment represents renewable natural heat source. Heat sources available in the nature are water, ground and air.
Water has a high specific heat index (Cp = 4.186 kJ / kgK) and represents a stable source of energy throughout the year. Those characteristics make water the most suitable fluid for heat pump applications. Temperature of natural water resources changes slightly during the year; therefore, it can be considered constant. The natural water resources are:
12 - 30 meters-deep wells with the average temperature of the water of 12°C to 18°C
30 -100 meters-deep wells with the average temperature of the water of 15°C to 22°C
The thermal springs and spas with the average temperature of the water of 20°C to 40°C
Rivers and lakes with the average temperature of water of 4°C to 12°C at a depth of a few meters
Heat pumps transform the energy without both changing the quality of the source and polluting the environment. Heat pumps reduce (the heating mode) or increase (the cooling mode) the temperature energy of the water without polluting it and affecting its quality. After the temperature exchange process the water is returned to the same depth from which it was taken preserving the over-all ecological balance.
Moreover, water form a variety of installations, swimming pools, and sewage represents a high-quality energy-source which can be used very effectively in heat pump installations (co-efficient of performance (COP) of 4 to 6).
Ground is an unlimited renewable energy source; the temperature of the ground, and consequently its usability for heat pump installations, increase with increased depth. Energy of the ground can be extracted using heat exchangers laid:
Horizontally at a minimal depth of 1.8 meters
Vertically at a depth of 80m to 120m
Heat pumps installations which use the ground as energy source have long lifespan and ensure a high return of investment. The disadvantage of ground source heat pumps arises if the installations is not well designed, i.e. for its proper installation a larger ground surface is needed. The correct design and initial installation of a ground source system increase investment costs. However, regardless of the higher investment costs, ground source heat pumps are very economical and have COP of 3.5 to 4.5.
Air as a source of energy is universally; however, the problem of using air as energy source is reflected in the large temperature difference during the year: from -18°C to +40°C.
Waste air form ventilation system has a constant temperature of 20°C to 30°C; hence, its use in air based heating pumps systems reduces installation costs.
Similarly, in the objects that require dehumidification, moist air represents a high-quality energy source for air based heating pumps systems a high COP of 4 to more than 6.
An example of the heat pumps application is ventilation installations of indoor swimming pools where return on investment can be expected within a few years.
Industrial waste air also represents a great potential for heat pump application.