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Principles and operation of refrigeration and heat pump systems 41

to freeze, releasing the heat of solidification which raises the heat transfer fluid temper-
ature. The loop continues to run until about 90% of the water is frozen solid. During
periods of on-peak electricity pricing (typically daytime), the system operates in the

discharging mode (B). For the system shown, the fan coil unit needs 44 F (6.7 C)

cooling fluid to produce 55 F (12.8 C) air for the air conditioning system. The chiller

is driven to provide 52 F (11.1 C) fluid to the ice storage unit where it is cooled to

34 F (1.1 C). A bypass line sends some of the 52 F fluid to mix with the 34 F fluid,

thus making fluid at 44 F for the coil unit. If low-priced electricity happens to be
available during times of demand for cooling, the ice storage section may be
completely bypassed, thus saving ice for the high-priced hours.
1.9 Summary
This chapter opens with a snapshot of world energy usage with a special focus on
renewable energy sources. A brief summary is given of pertinent energy policies
around the world. Given the importance of achieving sustainable energy systems, pro-
cesses needed to create and assess energy efficient systems are described. Since one of
the main foci of this book is the use of heat pumps to enhance energy systems, the his-
tory of heat pumps and refrigeration is presented along with qualitative descriptions of
these machines. This is followed by quantitative analysis for ideal machines that set the
standard for real-world systems. Practical heat pump and refrigeration systems are
described in technical detail and analyzed using the principles of thermodynamics.
Both vapor-compression and absorption refrigeration systems are discussed. A heat
pump uses specialized working fluids to transfer heat between itself and its surround-
ings, and the selection of the working fluid depends on the constraints imposed by the
application and the environment. The working fluid selection process is described with
special emphasis on the environmental impacts and international protocols that place
limits of the use of certain refrigerants. The chapter concludes with a discussion of
modes of operation of heat pumps and cold storage systems.

Nomenclature

Cl clausius number
COP coefficient of performance
e specific exergy
E _ exergy rate
GWP global warming potential
h specific enthalpy
L characteristic length
_ m mass flow rate
P pressure
Q, Q _ heat, heat rate
s specific entropy