Technology Heat pumps
Boiler unit
This Illustration
shows, in a simplified
format, how a heat
pump links to the rest
of the heating system
Outdoor unit
CO
2
Compressor
Exchanger
Evaporator
Water supply
> According to the Heat Pump Association (HPA),
the UK’s commercial heat pump market is valued Benefits of CO2
at around £300m, but the domestic sector has In addition to the benefits afforded by its thermodynamic
been much slower to adopt this technology. This is properties, CO2 is cheap, non-toxic, does not need to be
partly because heat pumps that are small enough reclaimed and is not covered by the F-Gas Regulations.
for domestic use have not generally produced Crucially, it has a global warming potential (GWP) of just
sufficiently hot water, with a limit of 50 degrees C. 1; many times lower than HFCs, as indicated in the table
But Terry Seward, secretary of the Heat Pump below, which also shows critical point temperatures.
Association, disagrees: “While that may be true of heat
pumps using R410A [refrigerant], those units using Refrigerant GWP Critical point (°C)
R134a can achieve a DHW [deliver hot water] temperature CO2 R744 1 31
at 60 degrees C, with a COP of 2 to 2.2.” Others argue R134a 1300 101
that such a temperature would require higher volumes of R410A 1900 72
refrigerant, resulting in larger units that would not be as R407C 1600 86
suitable for domestic applications. It has also been noted
that a temperature of 60 degrees C doesn’t give much
room for error when needing to achieve a sustained 58 central heating system with no need to change the
degrees C to eliminate legionella. radiators to low-temperature versions.
Certainly, the ability to achieve a water temperature “There are around 1.6m domestic boilers replaced
of 65 degrees C is important. This is not just because every year, so there is huge potential to replace these
of its potential to eliminate supplementary heating with heat pumps to reduce energy consumption and
for legionella ‘kill cycles’ but also because it enables CO2 emissions,” says Graham Wright of Sanyo.
the heat pumps to be retrofitted directly to an existing The key issue here, though, is getting the right
balance for each project. “CO2 is very good when you
need a high delivery temperature, but when lower
temperatures are acceptable it’s important to get the
HFC CO2
90
0
C
right balance between the significantly higher cost
Temp. of a CO2 heat pump, compared to HFC systems,
condensation
Ref. IN Temp. Ref. IN
of refrigerant 55
0
C
Water OUT and the performance of the system,” says Christian
Heerup of the Danish Technological Institute.
As heating loads fall in newer buildings, therefore,
Water OUT
Ref. OUT the heating capacity of the heat pump becomes less
Ref. OUT
critical. However, that still leaves millions of existing
Water IN Water IN
Inlet temp also
affects the system
systems that could benefit from a retrofitted, higher-
efficiency a lot.
temperature system; though it remains to be seen
how many homeowners will shell out the higher
costs for a CO2 heat pump, compared with installing
The respective benefits of using HFC and CO2-based heat pumps a new gas boiler. l
52 CIBSE Journal May 2009
www.cibsejournal.com
CIBSEmay09 pp48-50,52 pumps.indd 52 5/1/09 3:08:46 PM
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