Page 84 - Piston Engine-Based Power Plants

Piston Engine Cogeneration and Combined Cycles 75

cycle of the engine. Most reciprocating engines will show little fall in
overall efficiency when the electrical load on the engine falls from
100% to 50% but if it falls lower than 50% then efficiency will start to
fall more sharply. Engine and waste heat temperatures are likely to fall
too, and so a wide daily variation in load is likely to have an impact
on the effectiveness of the CHP system.

Since cooling systems are fitted to internal combustion engines
whether the waste heat is exploited or not, the use of these systems in
CHP applications offers a logical extension of their application.
Cogeneration systems based on small engines can provide power, space
heating and hot water for commercial offices while large engines can
produce power and low-grade process heat for small industrial opera-
tions. The economics of these systems can be quite favourable where
there is a use for the waste heat. As a consequence the cogeneration
market, particularly for small systems, is buoyant and is likely to
become more so if fuel costs continue to rise.

STIRLING ENGINE CHP SYSTEMS

From a CHP perspective, Stirling engines offer a different challenge to
the conventional reciprocating engine because these engines do not
normally have extensive cooling systems. However there is a flow of
waste heat from the engine and this can be captured and used. Most
commercial Stirling engines are relatively small, with typical sizes from
1 to 25 kW, although larger engines, up to 100 kW, are likely to
become available in the future. These small engines have a low power
to heat ratio so there is significant amount of heat available and this,
together with their size, makes them particularly suitable for domestic
or small commercial CHP systems. Residential CHP systems have
been built around the Stirling engine and this could provide an impor-
tant future market if technology costs can be reduced sufficiently.
Stirling engines are not very good at load following, so domestic sys-
tems are likely to be sized so that the engine can run continuously.
Depending upon design considerations, the engine might be small
(typically around 1 kW) and only supply a part of the domestic supply,
or it might be relatively large (3 5 KW) and supply most of the power
needed by the household, with surplus being exported to the grid.