Page 20 - Low Temperature Energy Systems with Applications of Renewable Energy

Principles and operation of refrigeration and heat pump systems 9

had a rapid development only in the 1920s and 1930s when the first heat pump unit
was set up in England. In 1930 Holden described the testing of a domestic heat
pump designed for heating and hot water supply, while using the heat of ambient
air. After that, work started in the United States and resulted in demonstration plants,
but relatively few projects were brought to this stage, since all of them had only private
funding. The first large heat pump plant in Europe was put into operation in Zurich in
1938e9. It used the thermal energy of river water, a rotary compressor, and a refrig-

erant. It provided heating for the city hall with water at a temperature of 60 C and at a
thermal power of 175 kWt. There was a system of heat accumulation with an electric
heater to cover the peak load. In summer months, the installation was operated for
cooling [14].

1.5 Efficiency of heat pump and refrigeration systems

This section will develop equations for assessing the performance of heat pumps (HPs)
and refrigeration machines (RMs). It is important to first examine ideal thermodynamic
systems so that we may compare real systems to these to find where improvements
may be made in any particular design [15,16].

1.5.1 Ideal measures of performance

Heat pumps are energy-consuming devices used to create heating or cooling effects.
This is the opposite of power plants that use temperature differences to drive cycles
that produce power output at the expense of high-temperature energy. Carnot intro-
duced a cycle with the highest thermal efficiency possible given a fixed set of heat res-
ervoirs e the Carnot cycle. That efficiency formula is well-known to all who
understand thermodynamics, namely,

T L
h ¼ 1 (1.1)
C
T H
where T L is the absolute temperature of the heat sink and T H is the absolute temperature
of the heat source. Figure 1.5 shows the Carnot cycle in a temperature-entropy (T-s)
diagram. All processes are reversible in this ideal cycle. Knowing that the heat transfer
during a reversible process is equal to the area under the process in a T-s diagram, it is
easy to derive Eq. (1.1) from its basic definition as the ratio of the net work to the heat
input.
Next we will find an ideal measure of performance for a heat pump. Given that a
heat pump is built to deliver heat at the expense of work input, the coefficient of per-
formance for a heat pump, COP HP ,isdefined as the ratio of the heat delivered to the
net work input. Figure 1.6 shows the ideal heat pump cycle in a T-s diagram, where the
cycle is traversed in a counter-clockwise direction, opposite to a power cycle. By anal-
ogy with the Carnot efficiency, it is easy to determine that the ideal COP HP is given by