Page 82 - Piston Engine-Based Power Plants

Piston Engine Cogeneration and Combined Cycles 73

connect hot water and a gas supply and the system is ready to go.
However large systems, particularly where steam is being generated for
process use, may be customised to each installation.

The electrical generation efficiency of a reciprocating engine usually
increases with engine size. Typically, an engine with 100 kW electrical
output will be expected to have an electrical generating efficiency of
around 27% while for a 1 MW system efficiency rises to 37% and at
2
10 MW efficiency approaches 42%. The smaller engines also typically
have a higher exhaust gas temperature than the larger engines. The
lower electrical efficiency is not necessarily a handicap in a CHP sys-
tem. It simply means that there is relatively more heat energy available
to capture and reuse: For a small engine the heat may account for
over 70% of the total energy input while for a larger engine it will be
less than 60%. So, for a 100 kW engine, the heat recovery might be
close to 200 kW a heat to electricity ratio of 2:1 while for a
10 MW engine the total heat recovery may only be 8 MW a heat to
electricity ratio of 0.8:1.
While these figures cover off-the-shelf systems, it is possible to
adapt engine design to meet different thermal and electrical require-
ments. So, for example, a larger engine might be modified so that it
was less efficient, electrically, but produced more heat that could be
captured and used in some industrial or commercial process.

ENGINE HEAT SOURCES

There are four primary sources of waste heat in an internal combustion
engine: the engine exhaust, the engine case water cooling system, the
lubrication (lube) oil water cooling system and, where one is fitted, the
3
turbocharger cooling system. Each of these can be used as a source of
heat in a reciprocating engine cogeneration system. Fig. 7.1 shows the
first three of these, schematically, for a natural gas engine cogeneration
system.

2
Figures are taken from the Energy Solutions Center website: http://understandingchp.com/
chp-applications-guide/4-chp-technologies/.
3 A turbocharger is sometimes used to compress air before it is admitted into the cylinder of an
internal combustion engine. This can lead to improved performance by generating greater power
from the engine.