Environmental Engineering Reference
In-Depth Information
turbine, the work
η g q f , where q f is the fuel heat added per unit mass of combustion
products. The amount of heat that can be utilized in the steam cycle is just q f w g =
w g is equal to
q f (
η g )
The steam cycle work output
w s is therefore
η s times this heat, or
η s q f (
η g )
. Thus we find the
combined cycle efficiency,
η cc w g + w s
q f
= η g + η s (
η g ) = η g + η s η g η s
The efficiency of the combined cycle is always less than the sum of the efficiencies of the component
cycles. Nevertheless, the combination is always more efficient than either of its components. For
example, if
In the combined cycle gas plus steam power plant, the thermal efficiency of the steam cycle
component is considerably lower than that for the most efficient steam-only power plant, because
the gas turbine exhaust gas is not as hot as the combustion gas in a normal boiler and because the
gas turbine requires much more excess air than does a steam boiler. Both restrictions limit the steam
cycle efficiency, but nevertheless the combined cycle plant provides an overall fuel efficiency that
is higher than that for any single cycle plant.
The combined cycle power plant burning natural gas or jet fuel is often the preferred choice
for new electric generating plants, rather than coal-fueled steam plants, for a variety of reasons
that overcome the fuel price differential in favor of coal. These reasons are mostly financial and
environmental, the latter including the reduced air pollutant emissions, especially carbon dioxide. 22
η g =
30% and
η s =
25%, then
η cc =
The use of mechanical power to move heat from a lower temperature source to a higher temperature
sink is the thermodynamic process that underlies the functioning of refrigerators, air conditioners,
and heat pumps. The process is the reverse of a heat engine in that power is absorbed, rather than
being produced, but it still observes the restrictions of the first and second laws of thermodynamics
that the heat and work quantities balance [equation (3.9)] and that the ratio of the heat quantities is
related to the temperatures of the heat source and sink.
The most common form of refrigeration system employs the vapor compression cycle. The
refrigeration equipment consists of an evaporator, a vapor compressor, a condenser, and a capillary
tube connected in series in a piping loop filled with the refrigerant fluid. The refrigerant is chosen
so as to undergo a change of phase between liquid and vapor at the temperatures and pressures
within the system. The fluid flows through these components in the order listed, being propelled
by the pump. The purpose of the capillary tube is to reduce the pressure of the refrigerant fluid
flowing from the condenser to the evaporator, resulting in a lowering of its temperature. The fluid
states in the ideal vapor compression cycle are illustrated in the temperature-entropy diagram on
the left of Figure 3.8. Vapor leaving the evaporator
the compressor to a higher pressure, where it enters the condenser at a temperature T 2 that is higher
than the environment to which heat will be transferred from the condenser. The condenser, a heat
is compressed isentropically
22 For further discussion, see Section 5.3.1.
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