Page 69 - Analysis and Design of Energy Geostructures

Energy geostructures 39

Figure 2.10 Schematic of a heat pump.

2.4.3 The heat pump or reversed heat pump
Heat pumps are thermal devices that convert mechanical work into heat. The thermo-
dynamic principle behind heat pumps is that fluids become warmer when they are
compressed into a smaller volume. The opposite is true for reversed heat pumps,
that is fluids become cooler when they are expanded. This fact indicates that heat
pumps and reversed heat pumps are characterised by the same operating principle
(Yunus and Michael, 2006).
Simple heat pumps (e.g. compression heat pumps) comprise four main devices: the
evaporator, the compressor, the condenser and the expansion valve (cf. Fig. 2.10).
Heat pumps work with a refrigerant, which is a special fluid that (1) circulates in a
closed circuit in the heat pump, (2) undergoes phase transitions from a liquid to a gas
and back again and (3) evaporates at low temperatures.
In the evaporator the refrigerant is put in its liquid form in contact with the heat
carrier fluid circulating in the pipes of the energy geostructures in the primary circuit
and is evaporated to a gas, with its temperature being lower than that of the heat car-
rier fluid and its boiling point (at relatively low pressure) below the entering heat car-
rier fluid temperature. The phase change from liquid to gas of the refrigerant fluid
decreases the temperature of the heat carrier fluid, which is then reinjected into the