Environmental Engineering Reference
In-Depth Information
becomes larger, and becomes zero at a sufficiently large value of the temperature difference,
depending upon the characteristics of the refrigerant fluid.
A heat pump is a refrigerator operating in reverse; that is, it delivers heat to an enclosed space by
transferring it from an environment having a lower temperature. Heat pumps are commonly used to
provide wintertime space heating in climates where there is need for summertime air conditioning.
The same refrigeration unit is used for both purposes by redirecting the flow of air (or other heat
transfer fluid) between the condenser and evaporator.
For a heat pump, the coefficient of performance
)
hp
is defined as the ratio of the heat
q
h
transferred to the higher-temperature sink, divided by the compressor work
(
COP
w
,
q
h
w
q
c
+
w
w
q
c
w
h
2
−
h
4
h
2
−
h
4
(
COP
)
hp
≡
=
=
1
+
=
h
1
=
(3.46)
h
2
−
(
h
2
−
h
4
)
−
(
h
1
−
h
4
)
where we have used the first law relation that
q
h
=
q
c
+
w
and equation (3.45) to simplify the
right-hand side of (3.46).
The heat pump's coefficient of performance is always greater than unity, for the heat output
q
h
always includes the energy equivalent of the pump work
w
. But in very cold winter climates,
(
)
hp
may not be very much greater than unity and the heat delivered would not be much greater
than that from dissipating the compressor's electrical power in electrical resistance heating of the
space, a much less capital intensive system. It is the year-round use of refrigeration equipment for
summertime air conditioning and wintertime space heating that justifies the use of this expensive
system.
COP
3.12
FUEL CELLS
In Section 3.10 we considered several different systems for converting the energy of fuel to me-
chanical energy by utilizing direct combustion of the fuel with air, each based upon an equivalent
thermodynamic cycle. In these systems, a steady flow of fuel and air is supplied to the “heat engine,”
within which the fuel is burned, giving rise to a stream of combustion products that are vented to
the atmosphere. The thermal efficiency of these cycles, which is the ratio of the mechanical work
produced to the heating value of the fuel, is usually in the range of 25% to 50%. This efficiency is
limited by the combustion properties of the fuel and mechanical limitations of the various engines.
Thermodynamically speaking, the combustion process itself is an irreversible one, and it accounts
for a large part of the failure to convert more of the fuel energy to work.
Is there a more efficient way to convert fuel energy to work? The second law of thermodynamics
places an upper limit on the amount of work that can be generated in an exothermic chemical
reaction, such as that involved in oxidizing a fuel in air. In a chemical change that proceeds at a
fixed temperature and pressure,
the maximum work that can be extracted is equal to the decrease in
Gibbs free energy of the reactants as they form products in the reaction. For most fuels the change
of Gibbs free energy
f
, defined in equation (3.16), is only slightly different from the fuel heating
value (see Table 3.1), but this limit is certainly much greater than the work produced by practical
heat or combustion engines. But thermodynamics alone does not tell us how it might be possible
to capture this much greater amount of available energy in fuels.
An electrochemical cell is a device that can convert chemical to electrical energy. It con-
sists of an electrolyte filling the space between two electrodes. In a
battery,
the electrodes are
