Page 18 - Low Temperature Energy Systems with Applications of Renewable Energy

Principles and operation of refrigeration and heat pump systems 7

1.4.2 Qualitative description of vapor-compression heat pump
operation
Most people have a heat pump of some sort in their house, even without knowing it. A
refrigerator or freezer operates as a heat pump, only with a different focus. The refrig-
erator uses the “cold” side and the heat pump mainly uses the “hot” side. The heat
pump takes thermal energy or heat from the environment, i.e., soil, water or air,
“pumps” the working fluid (cooling agent or refrigerant) to a higher temperature,
which then delivers heat to the heating system. All this happens in a closed circuit
in which the cooling agent circulates; an evaporator, compressor, condenser and
expansion valve are the significant components of this circuit (see Fig. 1.3).
The components perform the following tasks:

Evaporator: An evaporator is a heat exchanger in which thermal energy is absorbed by the
working fluid, thereby cooling its surroundings. Thus there is a transfer of heat from the sur-
roundings to the working fluid inside the evaporator. A cooling agent flows through the evap-
orator with low pressure and at near-ambient temperature. As it receives heat from the
ambient air, which must be at a higher temperature than the cooling agent, it evaporates.
The temperature of the air must be higher than cooling agent otherwise there will be no
heat transfer from the air to the agent by the Second Law of thermodynamics.
Compressor: A compressor receives the cooling agent in the vapor state from the evaporator
and increases its pressure and, consequently its temperature. For this purpose, the compressor
requires power input, usually supplied by a motor driven by electricity from the power supply
network. Upon compression, the now hot refrigerant vapor goes from the compressor into the
condenser.
Condenser: A condenser is a heat exchanger, like an evaporator, but performing the opposite
process. Inside the condenser, the superheated cooling agent releases heat energy to a colder
heat carrier, for instance, water in a heating coil. Under the action of temperature difference,
heat from the hot refrigerant goes to the heat carrier. As a result, the refrigerant condenses and
a heat carrier is heated, while the pressure of the refrigerant remains high.

Fig. 1.3 Simplified schematic of heat pump/refrigeration system. P, pressure; EV, evaporator;
CN, condenser; CP, compressor; TV, throttle valve.