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Air source heat pumps can deliver significantly higher COPs when
combined with heat recovery from other processes. *Chris Ree of Hoval
suggests some useful heat sources to exploit.
Despite the sustained pressure to minimise energy wastage, there are still
many buildings that waste heat from a range of processes – including space
heating, process heating and ventilation. Yet many of these offer an ideal
opportunity for recovery and re-use of this waste heat for space heating, feed
water pre-heating and domestic hot water (DHW) processes.
Mechanical ventilation systems are an obvious example, with the potential for
the exhaust heat to atmosphere to be recovered through, for instance an airto-
water heat pump, transferring the recovered heat to the hot water circuit for
space heating or DHW supply.
Just to put some realistic figures to this, many applications have ventilation
requirements of 3-15 air changes per hour, with 60-70% heat recovery
through an air-to-air heat exchanger. Thus, the exhaust to atmosphere is
often around 7șC – considerably above ambient for much of the heating
season.
This air can be passed through the heat pump heat recovery coil to provide a
substantial increase in COP. For instance, an air-to-water heat pump will have
a COP of approximately 3.2 at -4șC, but this will increase to 4.0 to 4.3 at 7șC
and 4.5 to 5.0 at 20șC - based on a 35șC water flow rate for the heating
system.
The heat pump output will effectively increase by over 30% with 7șC flow
(compared to –4șC) onto the heat recovery coil. The typical ventilation flow
rate through the heat pump coil closely matches the typical ventilation
requirements of many buildings These include schools, colleges, offices,
supermarkets, department stores, restaurants, sports facilities.
Beyond HVAC
If we cast our minds further afield there are many other instances where heat
is being wasted from non-HVAC systems and there is no reason why this heat
source cannot be harnessed for space heating and DHW in the same building
or complex. Indeed, there is a vast range of commercial and industrial
process applications that exhaust heat to atmosphere through air or water
outlets.
One common example is refrigerated food storage, where recovered heat
from the condenser heat discharge is used for building services heating. This
technique is already in use in many supermarkets and can also be applied to
other process areas, such as milk cooling. Similarly, plastic injection moulding
machines require cooling circuits and the heat can be recovered for space
heating.
Other industries where heat can be recovered from liquid effluents include the
food processing industry with the production of soft drinks, packaging, cooking
processes and sterilisation; the textile industry with washing and dye
processes; the paper production industry; paint processes and plating. All of
these areas have the potential to contribute heat to other systems and are
worthy of exploration by any building services engineer working in these
sectors.
Just as there is huge variation in the range of applications, there is potentially
a vast range of temperatures to deal with. The temperature of the exhaust
fluid to atmosphere may range from 0°C to 200°C. It is possible to recover a
high proportion of this heat through fluid to water heat exchangers, air to air
heat exchangers, air to water and water to water heat exchangers such as the
heat pump, thermal heat and moisture transfer through the use of thermal
wheels and exhaust flue gas to water heat recovery.
Thermal wheels provide a simple cost effective solution for heat recovery with
efficiencies in excess of 80% and the ability to recover moisture from the
exhaust air and transfer this moisture to the supply air.
These recovery systems can be combined to optimise on the total heat
recovery process with facilities for buffer storage, the use of compatible
heating, ventilation and heat recovery units for space heating and calorifier
storage for DHW and heat recovery / transfer exchangers for process pre
heating.
It is clear that if we are to achieve our objectives of minimising use of fossil
fuels and associated carbon emissions, we need to explore every opportunity
to reduce such wastage. With the supply of all these heat recovery products
and the controls interface it is possible to provide a cost effective solution for
clients in these wide ranging industries with each application being assessed
for its fuel cost saving and payback potential.
*Chris Ree is Divisional Manager Indoor Climate Systems at Hoval Ltd
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