Technology Heat pumps
Captivating
carbon
The more environmentally-friendly carbon dioxide-based
heat pump has been slow to take off in the domestic market
in Britain. Paul Haddlesey looks at the technology and recent
developments that could help to buck the trend
>
T
he benefits of carbon dioxide as a refrigerant Another characteristic of CO2 refrigerant is the
(known as R744) have been recognised for higher pressure that is generated: up to 100 bar,
many years, and there are already thousands compared with 25 to 35 bar for HFC heat pumps.
of CO2-based cooling systems throughout “CO2 systems are very different to the more familiar
Europe. The key difference between a CO2-based HFC systems. I feel there is more risk of an accident
heat pump and a hydrofluorocarbon (HFC)-based with CO2 systems, so training is going to be a major
one is that the refrigeration cycle with HFCs is issue,” Gartshore warns.
‘subcritical’. This means that heat rejection takes Many of the measures that are being taken to
place below the critical point, so that condensation reduce dependence on fossil fuels are encouraging
occurs. the increased use of electricity for heating and hot
CO2 heat pumps operate with a ‘transcritical’ cycle, water. “If we are really serious about achieving
where heat rejection occurs above the critical point. a low-carbon economy we have got to do it with
Consequently there is no condensation, the CO2 electricity through a low-carbon grid,” says Ant
remains in a gaseous state, and higher temperatures Wilson of consulting engineers AECOM, formerly
can be achieved. Faber Maunsell | AECOM. “Therefore, any electric >
As well as generating higher temperatures, CO2
has other properties that improve heat transfer. “With
HFC heat pumps the temperature remains constant,
so heat transfer reduces with time,” notes Jane
With CO2, the temperature is
Gartshore, president of the Institute of Refrigeration
always falling, so heat transfer is
and a strong advocate of CO2 refrigerants. “With
maintained better than with HFC
CO2, the temperature is always falling, so heat
transfer is maintained better,” she adds.
– Jane Gartshore
www.cibsejournal.com May 2009 CIBSE Journal 49
CIBSEmay09 pp48-50,52 pumps.indd 49 5/1/09 10:14:15 AM
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