Modern Engineering Thermodynamics

14.18 Second Law Analysis of Refrigeration Cycles 579
WHAT IS THE SUPER EFFICIENT REFRIGERATOR PROGRAM?
By the 1990s, refrigeration consumed over 20% of all U.S. household electricity. The Super Efficient Refrigerator Program
was a contest sponsored by a consortium of 24 electrical utilities, which offered a prize of $30 million for the manufacture
and marketing of a new domestic refrigerator that was at least 50% more efficient than current models and used a chlorine-free
refrigerant. Whirlpool Corporation in Evansville, Indiana, easily won the award with a refrigerator that was 80%
more efficient than current models, using R-134a as the refrigerant. Its design modifications included
■
■
■
■
■
■
Foam insulation throughout the cabinet.
Fuzzy logic controlled defrost cycle, which defrosts only when needed.
Freezer vacuum insulation panels three times as effective as foam.
Thicker door with foam insulation.
Redesigned efficient fan motors.
Redesigned efficient compressor and drive motor.
Initially, improvement in system efficiency can be had by simply applying what we already know in the way of
better thermal insulation, more efficient compressors and pumps, more effective use of proportional control systems,
and so forth. The Super Efficient Refrigeration Program (SERP), developed in the 1990s, is a step in this
direction (see the box). This program and others like it may produce the energy-efficient products that will be
needed in the future.
14.18 SECOND LAW ANALYSIS OF REFRIGERATION CYCLES
A logical method for maximizing (or optimizing) system performance during the design stage of a new product
can be based on minimizing the losses (irreversibility rate) within a system. In Chapter 10, we define the irreversibility
rate of a process to be the product of the local environmental temperature T 0 and the entropy production
rate inside the system _S P as
Irreversibility rate = _I = T 0
_S P ≥ 0 (10.15)
This method involves computing the irreversibility rate of each of the components within a system over a range
of component parameters (size, efficiency, materials, and so forth) and system operating conditions. Then, you
specify the component parameters in the design that minimize the irreversibility and entropy production rates
within the system. Equations for the direct calculation of the entropy production rate for various component
processes are given in Chapter 7. Some examples follow:
Irreversibility rate due to heat transfer
Z
_q
= T 0 ð _S P Þ heat
= − T
dT
0
T 2 dV
dx
transfer
_I heat
transfer
Irreversibility rate due to fluid viscosity
Z
μ
_I fluid
= T 0 ð _S P Þ fluid
= T
dV 2dV
0
T dx
viscosity
viscosity
Irreversibility rate due electrical resistance
Z
Je _I electrical
= T 0 ð _S 2 P Þ electrical
= T ρ e
0
T dV
resistance
resistance
and so forth. Use of these equations requires a detailed understanding of the operation of the system.
However, if the system contains simply connected flow loops, it is much easier to determine the entropy
production and irreversibility rates using a simple entropy rate balance equation (recall that this is called the
indirect method for determining entropy production). Entropy production rate equations using the indirect
method are developed in Chapter 9 for heat exchangers (i.e., boilers, evaporators, and condensers), fluid mixing,
and transient operations.
V
V
V