Page 113 - Advanced thermodynamics for engineers

CHAPTER
5
RATIONAL EFFICIENCY

OF POWER PLANT

5.1 THE INFLUENCE OF FUEL PROPERTIES ON THERMAL EFFICIENCY
The thermal efficiency of a cycle has been defined previously, in terms of specific quantities, as
w net
h ¼ (5.1)
th
q in
where
w net ¼ net work output from the cycle per unit mass of fluid
and

q in ¼ energy addition to the cycle per unit mass of fluid.
In this case q in is the energy transfer to the working fluid, and does not take into account any losses
(irreversibilities) in the boiler or heat transfer device. Equation (5.1) can be rewritten for the whole
power plant, including the boiler or heat transfer mechanism, as
u net
h ¼ h h ¼ (5.2)
B th
0
Dh 0
where
h 0 ¼ overall efficiency of power plant,
h B ¼ efficiency of boiler,
h th ¼ thermal efficiency of cycle,
u net ¼ net work output from the cycle per unit mass of fuel and
Dh 0 ¼ specific enthalpy of reaction of fuel.
This might be considered to be an unfair, and possibly misleading, method of defining the effi-
ciency because the energy addition cannot all be turned into work, as was shown when considering
exergy and availability (Chapter 4). Another definition of efficiency can be derived based on the
Second Law, and this relates the work output from the cycle to the maximum work output obtainable.
The efficiency of the power plant has been related, in Eqn (5.2), to the amount of energy that has
been added to the cycle by the combustion of the fuel. In the past this has been based on the enthalpy of
reaction of the fuel, or usually its calorific value, Q . It was shown previously (Chapter 4) that this is
0
p
not the energy available for the production of work, and that the maximum available work that can be
Advanced Thermodynamics for Engineers. http://dx.doi.org/10.1016/B978-0-444-63373-6.00005-8 99
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