Page 74 - Piston Engine-Based Power Plants

66 Piston Engine-Based Power Plants

THE APPLICATION OF STIRLING ENGINES FOR POWER
GENERATION

Stirling engines have been used to exploit solar energy and for biomass
applications. However their use is not widespread. Typical engines
sizes under development and in use range from 1 to 150 kW. In solar
thermal applications a Stirling engine could theoretically achieve close
to 40% energy conversion efficiency. The best so far recorded is 32%
which is still high for solar conversion.

The most common solar application is in a solar dish power system.
This type of power plant has a large reflecting dish, up to 25 m in
diameter which focusses the sun’s energy onto an absorber placed at
the focal point. The heat energy collected at this point is transferred
to the hot side of a Stirling engine to provide the thermal input to drive
the engine. The cold side is provided by air at ambient temperature.
Solar systems based on dishes are relatively small with generating
capacities of 25 50 kW. A 10 m dish can provide energy for a 25 kW
Stirling engine.

Another area for which Stirling engines have recently been devel-
oped is small combined heat and power. These systems are usually
aimed at domestic electricity and heat supply with the heat energy pro-
vided from natural gas, although other sources are possible. In these
systems the Stirling engine provides electricity while the heat that is
not used by the engine is used to heat hot water for domestic hot water
and space heating systems.

FREE PISTON ENGINES

Free piston engines are so called because the pistons in the engines
move linearly inside their cylinders without any mechanical constraint
such as a crankshaft. This simplifies engine design and the engines are
potentially more efficient because they do not have frictional and other
losses associated with more normal mechanical linkage designed to
convert the reciprocating motion into rotary motion. However it is
more difficult to extract energy from a free piston engine.

The first modern free piston engine was designed by the Argentine
engineer Raúl Pateras Pescara. The first design was for a compressor
developed and marketed by Pescara Auto-compressor Company which