Page 80 - Piston Engine-Based Power Plants

CHAPTER 7 7

Piston Engine Cogeneration and
Combined Cycles

Reciprocating engines can be relatively efficient at least compared
to other types of heat engine at converting fuel into electricity. The
best diesel engines can achieve 50% efficiency. Similar sized gas tur-
bines are several percentage points less efficient though the largest can
match this. However even if an engine is 50% efficient it means that
roughly 50% or more of the energy in the fuel is not converted into
electrical power and instead emerges as heat.
If there is a use for it, this heat can be captured either in the form of
hot water or as steam. This can raise the overall efficiency of energy
usage to 80% or more. There are two broad ways in which the waste
energy can be exploited. The first is to capture and use the heat directly.
This will often provide space heating or hot water but the heat from
some parts of the engine can be of high enough quality to generate low-
or medium-pressure steam which can be used in some industrial and
commercial processes. Engines are necessarily equipped with cooling
systems of various types to ensure engine components do not overheat
and these can be exploited easily to provide a heat output.
The second approach is to use the waste heat as an additional source
1
of electric power. This requires a second (bottoming ) turbine generator
to be added, one that can use the waste heat to drive its cycle. Some
large engines are equipped with steam generators and small steam tur-
bines to create diesel engine combined cycle plants. This is normally
only economical for the largest of installations. For smaller engines it
might be cost-effective to use an organic Rankine cycle (ORC) turbine
that can exploit lower grade heat economically. The use of ORC bot-
toming cycles is not widespread but interest is growing.

1 A bottoming cycle is so called because it takes rejected energy after it has emerged from the ‘top’
cycle which in this case is a diesel engine.
Piston Engine-Based Power Plants. DOI: https://doi.org/10.1016/B978-0-12-812904-3.00007-0
© 2018 Elsevier Ltd. All rights reserved.