Micro CHP units are unable to provide
instantaneous domestic hot water as is common in homes equipped with a
combi-boiler, 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 only the Remeha Evita product is
configured in this fashion, resulting in a significantly
reduced economic benefit.
Currently available products, although usually
incorporating some supplementary boiler capacity to meet peak demands, are therefore
designed to operate in conjunction with a conventional heating system
incorporating a hot water cylinder. Indeed, all low carbon heating
systems including solar thermal, heat pumps, biomass boilers and so on,
all require some degree of thermal storage to operate effectively.
However, further enhancements in performance can be
achieved in conjunction with underfloor heating,
whilst even greater benefits can be achieved in conjunction with a
primary thermal store (where the stored volume of water is part of the
primary heating circuit), 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.
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, enhancing the performance of the heat source.
In the case of micro CHP the high thermal inertia combined with low flow
temperatures can 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.
However, although it is possible to retrofit
floor heating providing the householder is prepared to tolerate the
disruption and cost, it is much better suited to new-build homes where
it can be incorporated optimally during construction.
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 primary thermal store.
The bulk of the
water in the tank forms part of the primary circuit and domestic hot water is
produced by passing this water through a high output plate heat
exchanger, effectively the reverse of a standard
system. Alternative configurations pass the cold water
through a high efficiency coil running the entire height of the cylinder
or in some cases comprise a "tank-in-tank" (as illustrated
below). In order to minimise
the bulk of such systems, the inclusion of a quantity of phase change
material utilising the high latent heat capacity of, for example,
paraffin wax can result in a compact but high capacity unit.
Field trials of this
concept with the WhisperGen
micro CHP unit have clearly demonstrated the reduction of cycling and
longer periods of operation at full output, so that the same level of
comfort and higher annual electricity production can be achieved for a
given heat load than is possible with higher nominal efficiency micro
CHP units without such storage.