Page 85 - Piston Engine-Based Power Plants

76 Piston Engine-Based Power Plants

COMBINED CYCLE

The waste heat from the exhaust of an internal combustion engine is
generally hot enough to generate medium-pressure steam at up to

400 C and 15 30 bar (more often at the low end of this range). In the
case of small engine installations, steam production is not normally an
economical option unless there is a local use for low-quality steam. In
the case of a very large diesel installation, however, the engine exhaust
can be used to generate steam in a boiler, steam which can drive a
steam turbine to produce additional energy. This forms the core of a
diesel engine-based combined cycle plant.
Diesel engine CHP systems are rare because they are generally
only economical on very large engines. Typical of this sort of applica-
tion is a generating plant which was installed in Macau in 1987. This
plant was equipped with a slow-speed diesel engine with a capacity of
24.4 MW. The engine exhaust was fittedwithawasteheatboilerand
steam turbine which could generate an additional 1.34 MW when the
engine was operating at full power, thus contributing around 5% of
the plant output. As a result of this and other measures a fuel-to-
electricity conversion efficiency of close to 50% was achieved. More
modern installations aim to exceed 50% electrical efficiency.
Large engines of this type are frequently derived from marine engines
and the original engines upon which they are based are not normally
optimised for combined cycle operation. In particular, the cooling sys-
tem is designed to keep the engine as cool as possible. For best com-
bined cycle performance, however, it is preferable to run the engine as
hot as possible because the higher the exhaust gas temperature, the
more efficient the steam turbine cycle. High-temperature operation can
also improve engine efficiency because the potential thermodynamic
efficiency will increase with operating temperature.
Combined cycle performance of a large diesel engine can therefore be
improved by modifying engine components such that they can operate
continuously at a higher temperature. Such modifications may require
more expensive materials capable of withstanding the more extreme con-
ditions. For example, the top of the piston may be made from an alloy
that allows it to remain uncooled while exhaust valves are treated with
advanced coatings able to resist the high exhaust gas temperature.
These modifications allow a higher temperature exhaust which can
be used to generate higher quality steam to drive a steam turbine.