Page 73 - Piston Engine-Based Power Plants

Stirling Engines and Free Piston Engines 65

Another issue with the Stirling engine is that it will not start imme-
diately but requires a time to warm up. While this applies to many
engines it tends to be more of a problem with external combustion
engines. Steam engines, which are also external combustion engines,
require extended start-up periods too.

ENERGY SOURCES

One of the attractions of the Stirling engine is that the heat energy is
applied externally. Thus the energy can, in theory, be derived from any
heat source. Unlike an internal combustion engine, heat is supplied
continuously to the heat absorber of the engine and this makes the
engine design simpler than that of a conventional internal combustion
engine which requires the repetitive release of energy.

The heat energy can be supplied from a combustion fuel. This can
be one of the many fossil fuels available; coal, oil and gas can all be
used, although these are unlikely to be the main energy source for a
Stirling engine power system. However other combustion fuels includ-
ing biogas and some types of biomass can be used.
A Stirling engine can also operate on waste heat. This could be
exhaust heat from a power plant or it could be heat released during an
industrial process. The engines are more efficient, the higher the tem-
perature of the heat supply, so a high-quality heat source will normally
be needed to make the engine economical.

Another potential source of heat is nuclear power. While a Stirling
engine is unlikely to be used in a terrestrial nuclear power plant today,
they could potentially offer a means of generating power from a
nuclear source in space. For the future there is some interest in Stirling
engines in terrestrial nuclear power plants because they would elimi-
nate the need for a heat transfer system and could greatly simplify the
design. This would be particularly true of advanced nuclear power
plants that use exotic heat transfer fluids such as molten sodium.
Geothermal power is also a potential heat source. However this tends
to be a low-temperature source which can limit efficiency.

Another important heat source is solar energy. This can be focussed
using optical systems to provide a very high-temperature energy source
and this has been successfully used to drive Stirling engines.