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Micro CHP units are unable to provide
instantaneous domestic hot water without the addition of a high output
supplementary burner. This is required to meet the very high
output, typically between 20-30kWt to achieve a reasonable flow
rate. However, if a large amount of supplementary firing takes
place, no electricity is generated and the economic and environmental
benefits are reduced. Currently available products are therefore
designed to operate in conjunction with a conventional heating system
incorporating a hot water cylinder.
However, further enhancements in
performance can be achieved in conjunction with underfloor heating,
whilst even greater benefits can be achieved in conjunction with a
thermal store, which enables the user to run the engine to produce
electricity when heat is not required, and to obtain heat when there is
no demand for electricity. This de-coupling of heat and power
offers significant benefits in terms of matching power production either
to the electrical demand within the home, or to the grid as a whole by
matching generation to periods of high market price, which tends also to
coincide with high carbon generation from central plant.
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Underfloor heating has many benefits for home heating regardless of the
heat source. It provides a higher level of comfort for a given air temperature
(reducing heat loss and improving energy efficiency) and can make use of
lower flow temperatures. In the case of micro CHP this could be
utilised to improve the efficiency of the engine (Stirling engines are
more efficient if the return temperature is lower), although the engine
would still need a higher flow temperature for domestic hot water needs. Effectively, for pipes buried in the
concrete floor screed, the mass of the floor above the insulation
provides a significant thermal store and reduces cycling of the engine
leading to higher efficiency and longer service life. |
Although
a conventional hot water cylinder does provide a certain level of
thermal inertia in the system, a significant improvement can be achieved
by use of a dedicated thermal store. The Gledhill store comprises
a storage vessel together with pumps and controls. The bulk of the
water in the tank forms part of the primary circuit and how water is
produced by passing this water through a high output plate heat
exchanger, effectively the reverse of a standard
system. Field trials of this unit with the WhisperGen
micro CHP unit have clearly demonstrated the reduction of cycling and
longer periods of operation at full output.
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