CPD Programme
> applications are electrically driven and based
Reverse cycle heat pump – heating cycle
on the “vapour compression cycle”. However,
4 way
heat driven systems are also available based on
reversing
the absorption cycle and use gas, or waste heat,
valve
or solar/geothermal heat.
Outdoor coil
The principle of the vapour compression
cycle is well documented but, for a reminder, Indoor coil
look back to the March CIBSE Journal and the
Heat in
cpd programme – “Refrigeration – inside the
box”. This referred to the cycle operating in the
Heat out
cooling mode – whereas, in heat pump mode,
the condenser heat exchanger is the useful
“heat output” component (the heat sink),
Heating
whilst the evaporator is the “heat collection” cycle
component (heat source). The majority of heat
expansion
Compressor
valve
pump applications in building services use a
reverse cycle system that is able to heat or cool
the space. Figures 1 and 2 show how this is
achieved with the introduction of a four-way
Check valve Check valve
reversing valve.
Referring back to the March issue, perhaps
High pressure gas Low pressure gas
the most significant section explains the
Figure 1
High pressure liquid Low pressure liquid
efficiency and performance characteristics
of the cycle. A heat pump coefficient of
performance (COP
h
) is: Reverse cycle heat pump – cooling cycle
4 way
Useful heating duty (kW)/power input to the
reversing
valve
compressor (kW)
Outdoor coil
Today, COP
h
average values between 3 and
Indoor coil
4 are achievable for air source heat pumps
over the heating season. This means that, for
Heat out
every 1kW of electricity consumed by the heat
pump unit, between 3 and 4kW of heating
Heat in
are produced. At this point the performance
characteristic of a vapour compression cycle
must be appreciated, whether operating
Heating Cooling
cycle cycle
in cooling or heating mode. Basically, the
expansion
Compressor
expansion
efficiency is a function of the pressure valve valve
difference over which the heat pump operates.
The greater the pressure difference, the lower
the efficiency, and the two pressures in the
Check valve Check valve
system are determined by the heat source
and heat sink temperatures – together with
High pressure gas Low pressure gas
the refrigerant fluid used. This means that
the lower the heat source temperature, say
Figure 2
High pressure liquid Low pressure liquid
ambient air or the ground, the lower is the
COP
h
and therefore the heat output is reduced. Heat pump types loop, containing an antifreeze solution, which
The higher the heat sink temperature, say for This is well documented now, but basically exchanges heat with the ground. Open systems
air or water heating, the lower the COP
h
. So there are a number of heat sources used by that take water directly from the ground can
to optimise maximum efficiency for any heat pumps and these, in simplest terms, can also be utilised – however, the requisite permits
heat pump application, the refrigerant heat be defined as air, ground and liquid. In the case from the Environment Agency are required to
exchangers must have high heat transfer of air, the source would normally be an ambient operate a system such as this. Heat pumps
rates and the condensing and evaporating temperature; however, more sophisticated that use liquid as a source will generally utilise
temperatures must be maintained at the systems may use recovered exhaust air to drive water. This may be in the form of closed loop
minimum possible temperature difference. the heat pump process where this is available. energy transfer systems or closed loops utilising
This is why high temperature heat sources, The ground as a heat source normally consists man made ponds, rivers or lakes with heat
like waste heat extract, and low temperature of coils of piping laid in a shallow trench or exchangers embedded in the source water.
heating systems, like underfloor heating, can deep boreholes with a U-tube inserted in each Other high-energy liquid sources that can be
give COP values greater than 5. bore. In each instance the piping forms a closed used, include slurries and effluent.
h
60 CIBSE Journal May 2009 www.cibsejournal.com
CIBSEmay09 pp59-62 cpd.indd 60 4/30/09 5:54:42 PM
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