Renewable Energy Installer December/January 2016 | Page 14

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Waste not want not
Heat pump specialist, Bob Long, calls for thermal energy recovery from
wastewater to be included in the RHI
Just as I was coming to the end of writing
this month’s column, I learned of Amber
Rudd’s leaked letter indicating a 3.5 percent
shortfall in meeting 2020’s 15 percent
renewable energy target.
I couldn’t help but think of the potential
impact the domestic heat pump industry could
have had on these figures, and the opportunity
missed through poor handling of the RHI.
If financial support for heat pump systems
can be made more attractive, an increased
number of installations would make a major
contribution to reaching our target.
Until DECC understands the enormous
contribution that can be made through
heat pump technology, the subsidy will
probably not increase, and we must therefore
concentrate on applications that make
economic sense without subsidy.
Energy from wastewater could be a
viable commercial focus for heat pump
installers, and also an invitation for innovators
to develop new ideas to collect the energy
contained in the drain water from showers,
washing machines, bath tubs and wash
basins.
A kW of thermal energy in any form has
a monetary value, and this is true regardless
of source. Gas currently seems to be the
lowest cost fuel at around 5p/kWh, and
electricity, at around 12p/kWh, will probably
be the highest.
14 | www.renewableenergyinstaller.co.uk
With energy having such a high value,
it makes sense to minimise wastage, and the
energy contained in waste hot water usually
ends up down the drain.
Wastewater can contain significant
amounts of thermal energy, and can be
recovered by specially designed systems
using water to water heat pumps.
A heat pump, dedicated to the
production of domestic hot water (DHW), and
optimised to collect energy from wastewater
at temperatures of at least +15C and higher,
could quite easily produce a COP of over 5:1.
Obtaining DHW at below 2.4p/kWh is
an impressive figure, and should represent
an attractive investment, with big savings
in a variety of domestic and commercial
applications.
This valuable method of energy recovery
is not currently recognised by the RHI,
however.
Until this happens, the economic savings
must stand alone, and therefore need to be
appealing enough to attract the market.
In the field of domestic heating, the
RHI provides some financial assistance
to heat pump users but savings are often
compromised by low system efficiency.
The seasonal performance factor for any
heat pump eligible for RHI payments must
carry MCS certification and operate with an
annualised efficiency of 2.5:1 or higher.
If we look more carefully at heat pump
manufacturers’ literature, we can see that
higher efficiencies are achieved with lower
water temperatures.
Designs dependent on higher water
temperatures, such as retrofit installations
using existing radiators should be avoided
as they will never produce the best value
for money. Often requiring circulating
temperatures above 50C, the seasonal
performance factor is likely to plummet below
2:1.
In defence of the standard domestic heat
pump, it is primarily designed to produce
large amounts of cheap hot water, and is
optimised for the temperature range 35-40C.
A good quality heat pump should be
capable of producing COP figures of 4:1 and
above, at low water temperatures. However,
the same heat pump when in DHW mode will
have a much reduced efficiency, generally
dropping to very low and unattractive values.
Because of the low-efficiency-at-high-
temperature-scenario, it is not unusual to
have a separate heating arrangement for
DHW while using the main heat pump for
space heating.
Savings can be made by using different
resources for production of DHW, but none
are likely to be more efficient than converting
the energy from wastewater into DHW
through an energy recovery system.