4.1 The influence of fuel properties on thermal efficiency
The thermal efficiency of a cycle has been defined previously, in terms of specific
net work output from the cycle per unit mass of fluid,
energy addition to the cycle per unit mass of fluid.
In this case
is the energy transfer to the working fluid, and does not take into account
any losses in the boiler or heat transfer device. Equation (4.1) can be rewritten for the
whole powerplant, including the boiler or heat transfer mechanism, as
overall efficiency of powerplant,
efficiency of boiler,
thermal efficiency of cycle,
net work output from the cycle
specific enthalpy of reaction of fuel.
This might be considered to be an unfair, and possibly misleading, method of defining
the efficiency because the energy addition cannot all be turned into work, as was shown
when considering exergy and availability. 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 powerplant has been related, in eqn (4.2), 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,
shown previously (Chapter 2) that this is not the energy available for the production of
work, and that the maximum available work that can be obtained from the fuel is based on