Figure 22.8: A compressor-driven cooling circuit.
In the compressor (1), vapourised coolant is compressed to a higher pressure. On
release of its latent heat to the surroundings, it is condensed to its liquid state in the
condenser (2). Then the coolant passes a thermal expansion valve (3), such that its
pressure on the other side of this valve is so low that it can evaporate again. For the
evaporation, it must absorb latent heat from the cool reservoir A. Then, the coolant
containing the latent heat from A is compressed again. We see that this cooling circuit
utilizes the latent heat stored and released during phase changes.
Naturally, heat only flows from warm to cold reservoirs. To reverse the heat flow, as
happens in a heat engine, additional energy must be added to the system. This happens in
the compressor and traditionally is supplied as electric energy. Probably the simplest
option to realize solar cooling is to generate this electricity by a conventional PV system
or a solar thermal power system as discussed in Section 22.3. It is also possible to drive
the compressor with mechanic energy obtained directly from solar energy with a Rankine
cycle, which is discussed in Section 22.3.
The choice of coolant is based on the temperature regime in which the cooling system
will operate. However, its effect on the environment has to be taken into account. For
example, chlorofluorocarbons, which were widely used as coolants, were heavily
regulated in the 1980s because of their destructive effect on the ozone layer [186].
Another concept that can be driven directly by solar heat is that of an absorption
cooling machine. In contrast to compressor-driven cooling, no mechanical energy is
required for absorption cooling; the cooling process is directly driven by heat that can be
supplied by solar collectors. Instead of one coolant, here two substances are used: a
refrigerant and a solvent. Often, ammonia (NH 3 ) is used as refrigerant, while water is used
as solvent. Under atmospheric pressure, ammonia has a boiling point of -33 °C. Further, it
is very soluble in water, a property that is utilized in the cooling process.
In the compressor (1), vapourised coolant is compressed to a higher pressure. On
release of its latent heat to the surroundings, it is condensed to its liquid state in the
condenser (2). Then the coolant passes a thermal expansion valve (3), such that its
pressure on the other side of this valve is so low that it can evaporate again. For the
evaporation, it must absorb latent heat from the cool reservoir A. Then, the coolant
containing the latent heat from A is compressed again. We see that this cooling circuit
utilizes the latent heat stored and released during phase changes.
Naturally, heat only flows from warm to cold reservoirs. To reverse the heat flow, as
happens in a heat engine, additional energy must be added to the system. This happens in
the compressor and traditionally is supplied as electric energy. Probably the simplest
option to realize solar cooling is to generate this electricity by a conventional PV system
or a solar thermal power system as discussed in Section 22.3. It is also possible to drive
the compressor with mechanic energy obtained directly from solar energy with a Rankine
cycle, which is discussed in Section 22.3.
The choice of coolant is based on the temperature regime in which the cooling system
will operate. However, its effect on the environment has to be taken into account. For
example, chlorofluorocarbons, which were widely used as coolants, were heavily
regulated in the 1980s because of their destructive effect on the ozone layer [186].
Another concept that can be driven directly by solar heat is that of an absorption
cooling machine. In contrast to compressor-driven cooling, no mechanical energy is
required for absorption cooling; the cooling process is directly driven by heat that can be
supplied by solar collectors. Instead of one coolant, here two substances are used: a
refrigerant and a solvent. Often, ammonia (NH 3 ) is used as refrigerant, while water is used
as solvent. Under atmospheric pressure, ammonia has a boiling point of -33 °C. Further, it
is very soluble in water, a property that is utilized in the cooling process.