Renewable Energy Installer December/January 2014 | Page 17

TECH
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Heat pumps and domestic
hot water
Heat pump trouble shooter Bob Long focuses on cost effective methods of
providing domestic hot water (DHW) from a heat pump
H
eat pump installations are steadily
on the rise I am pleased to say and,
provided our government sticks
to its promise of delivering the RHI in this
coming year, the continued rise in heat pump
installations will continue.
In previous issues of REI, I have
discussed a number of heat pump topics and,
hopefully, how to avoid some fundamental
problems. But DHW production is perhaps
the most difficult commodity to provide
economically from a heat pump.
Although there are a small number of
‘high temp’ heat pumps available in the
market, the most economical output is always
achieved at the lowest operating temperature.
Utilising low temperature water for
heating a property isn’t generally an issue,
as a suitable choice of emitter will ensure
adequate delivery of energy.
DHW is quite different, however,
requiring higher temperatures at the point
of use and even higher temperatures
when stored, to eliminate the possibility of
legionella, made safe through pasteurisation.
DHW supplying showers is generally
needed at a flow rate of somewhere between
10 and 15 litres a minute, although power-
showers are significantly more, and perhaps
should be considered ungreen due to their
high water and energy usage.
To create perspective with regard to
the quantity of energy consumed in the
production of DHW, a flow rate of only 10
litres/min will require an energy input of
around 25kW. This is equal to the energy
consumed by twenty five, single bar electric
radiators!
At 15 litres/min, the energy requirement
would be 37kW - assuming incoming water
at 10°C.
This level of energy exchange is easily
accommodated by a conventional type of
boiler, but not so easily with a small domestic
heat pump that is often dependant on a single
phase electrical supply.
To meet periodic on-demand needs,
DHW is heated by the heat pump over an
extended period of time and stored in a
stratification cylinder. This type of DHW
storage employs the principal of the warmest
water occupying the top section of the
cylinder, from which the DHW is drawn as
required. As hot water is drawn off the top of
the cylinder, cold water enters at the bottom,
heated by energy from the heat pump through
an internal heat exchanger.
In this type of system, it is usual for heat
pumps to operate at two different output
water temperatures, differentiating between
space heating and DHW, accomplished by a
motorised valve, directing the output from the
heat pump as required between the heating
system and the DHW cylinder.
The water stored in the DHW cylinder
can be maintained at a sufficiently high
temperature for normal usage but, periodically,
the contents of the DHW cylinder will require
pasteurisation - usually accomplished by an
electrical powered immersion heater capable
of raising the temperature of the whole
cylinder during the pasteurisation period.
The energy required to perform the
pasteurisation process is by comparison
expensive, and should be used sparingly.
With the advent of modulating-flame
boilers, DHW production in a heat pump
system can be made simpler and in some
instances more cost effective. By employing a
secondary energy source the required water
temperature can be produced on demand.
The incoming water to the secondary
boiler is of course economically pre-heated by
the heat pump reducing the amount of energy
required to reach the target temperature.
This method of DHW production
combines the economics of a heat pump
with the flexibility of a conventional boiler,
eliminating the necessity for pasteurisation, as
no DHW is actually stored.
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