Page 81 - Piston Engine-Based Power Plants

72 Piston Engine-Based Power Plants

RECIPROCATING ENGINE COGENERATION SYSTEMS

When an internal combustion engine is used to generate electricity, a
large part of the energy supplied to the engine in the form of fuel
emerges as heat in the exhaust from the engine or is dumped to the
atmosphere by engine cooling systems. If this heat can be captured it
can be utilised for space heating or for heating water, potentially mak-
ing the energy usage much more efficient.
The efficiency of piston engine-based power generation varies from
25% for small engines to close to 50% for the very largest engines. This
means that between 50% and 75% of the fuel energy actually emerges as
waste heat. If this heat can be captured it can be utilised in a cogenera-
tion system. This type of system is often also called a combined heat
and power (CHP) system and it can raise the efficiency of fuel usage to
80% or more. The overall efficiency will depend on engine type and size.
Heat energy capture can be applied to all the main types of recipro-
cating engines including spark ignition engines and compression igni-
tion engines. The environmental demands in the 21st century mean
that in many cases the engines that are used will be gas engines using
spark ignition and natural gas as fuel but all these internal combustion
engines share similar components, whatever the fuel or ignition type,
and all can be adapted for cogeneration.

In the past reciprocating engine CHP systems have only been cost-
effective in installations where the electrical generating capacity of the
engine is 50 kW or above. Some smaller systems have been produced
but at lower electrical output the electrical efficiency falls and mainte-
nance costs rise. Small reciprocating engines are normally mass-
produced and they are not designed for continuous base-load opera-
tion. In the last 5 years there has been considerable development
directed at improving the efficiency and longevity of these smaller
engines and this has led to much smaller systems, in the range of
1 5 kW, being marketed for domestic and small commercial applica-
tions. This is a small but expanding market.

The larger reciprocating engine CHP systems will normally be used
to supply base-load electricity, often with the ability to sell surplus
power to a local grid. These engine systems are usually supplied as a
complete package. In the case of a gas engine, all that is required is to