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The first true micro CHP systems were based on external combustion
technology as their characteristics are best suited to this stationary,
constant running application. External
combustion engines separate the combustion process (which is the energy
input to the engine) from the working gas, which undergoes pressure
fluctuations and hence does useful work. As the combustion process is used to
provide a continuous heat input to the working gas, it is inherently more
controllable and, in theory at least, more
efficient, cleaner and quieter than internal combustion engines.
External combustion engines have the potential for long life and
service intervals similar to the annual maintenance of a gas boiler,
typically around 3,000 operational hours. However, the inherent
benefits of this technology have yet to be fully realised; early
products have suffered from reliability
problems, and have failed to deliver the expected
efficiencies. The two most common external
combustion micro CHP technologies are the Stirling engine, in which a working gas is
alternately heated and cooled to create pressure changes which in turn
drive a power piston, and the Rankine engine in which a fluid is heated
to evaporate and expand against a piston or turbine, and then cooled and
condensed prior to the next heating and cooling cycle.
For further discussion of
the relative merits of engine types see section on
PAPERS; greater detail and
description of operating principles can be found in the book,
micro CHP For information on products
under development, click on the respective
RANKINE
and
STIRLING links below.
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