Chapter hvac engineering Fundamentals: Part 1 Introduction

17.6
Coefficient of Performance
The
coefficient of performance
(COP) is defined as
useful heat moved or obtained
COP
⫽
energy required to drive process
For a refrigeration cycle, the useful heat is the refrigeration effect
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Engineering Fundamentals: Part 2
457
Q
evap
COP
⫽
cool
Q
in
In the classical sense, similar to the definition of efficiency, there is a
theoretical limit to the COP defined in terms of the temperature (low)
of the cold region
T
L
related to the temperature (high) of the warm
region
T
H
T
L
COP
⫽
(17.1)
max
T
⫺
T
H
L
For a building chiller evaporating at 40
⬚
F (500
⬚
R) and condensing at
100
⬚
F (560
⬚
R), the maximum COP would be
500
COP
⫽
⫽
8.3
(17.2)
max
560
⫺
500
The actual COP of these machines is on the order of 4 to 6, which
reveals the inefficiencies of the real world. Studying the defining equa-
tion reveals that it takes more energy to refrigerate at lower temper-
atures, and it takes more energy to drive a process with a higher
temperature differential between evaporator and condenser. We need
to minimize the differential (often called
thermal lift
) as much as pos-
sible for the purposes of energy conservation.
For the heat pump cycle, the energy input becomes a benefit.
Q
⫹
Q
evap
in
COP
⫽
⫽
COP
⫹
1
heat
cool
Q
in
In heat pump systems, the system designer should again try to work
with the smallest possible thermal lift to get maximum beneficial ef-
fect for least input.