The first true micro CHP systems were based on external combustion
engines as the characteristics of this technology are well suited to this stationary,
constant running application.
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 more
controllable and generally more
efficient, cleaner and quieter than internal combustion engines and can
be relatively easily adapted to make use of a wide range of
External combustion engines have the potential for long life and
service intervals similar to the annual maintenance of a gas boiler.
combustion engines inject fuel and air into the cylinders where
The resulting temperature and pressure changes
of the fuel/air mixture (which is also the working gas) act on the
piston to produce useful work.
As the combustion process is cyclical,
rather than continuous, it is more difficult to ensure complete combustion of the
fuel, and noise and pollutant emissions tend to be higher than for
external combustion engines.
However, the inherent technical
challenges of ICE technology are being gradually overcome and current
engines outperform external combustion engines significantly in
efficiency terms, although with significant cost and size penalties..
a fuel cell, the chemical energy within the fuel is converted directly
into electricity (with by-products of heat and water) without any
mechanical drive or generator.
This can result in high electrical conversion efficiencies and
low emissions. However, numerous additional components are
required to condition the fuel and to convert the DC electrical output
into AC suitable for domestic installations, adding to the cost and
complexity of fuel cell micro CHP.
Commercially available domestic products,
based on PEM technology typically achieve electrical efficiencies of
around 35% whilst SOFC based products have demonstrated up to 60%.
Total efficiencies tend to be somewhat lower
than for engine based technologies.
are numerous experimental technologies which may at some future date
result in useable products.
These include thermo-electric
technologies which utilise temperature difference acting on metals or
produce electricity and thermo-photovoltaic units which convert the
radiant energy emitted by the burner to produce electricity from
infra-red sensitive PV cells.
There are also novel engine designs
such as the MTT pico-turbine illustrated above.
Although many of these concepts are relatively inefficient and
produce little power, there may be applications, for example, in "self-powered
boilers" for which such concepts are of value.
Micro CHP has implications not only for the immediate
installation, but also for the electricity system to which it is
Gas fired micro CHP technologies may play
a key role in supporting the widespread introduction of electrically
driven heat pumps and of supporting intermittent renewable generation.
This section covers technologies, products and concepts which are either essential to
enable the operation of micro CHP within the energy system, or
which enhance the performance or value of micro CHP products themselves.
These include thermal and electrical
energy storage systems and "smart" load management concepts,
intended to optimise self-consumption of generated electricity.