Hot Where it Matters, Cool When its Needed
Colin Maidment at London South Bank University is leading a research effort to investigate more environmentally friendly air-conditioning and refrigeration systems. One option, ironically, is to use carbon dioxide to replace the synthetic HFC refrigerants used in such systems and using the heat generated by CO2-based air-conditioning systems and fridges in supermarkets, for example, to provide hot water for nearby homes.
By Helen Knight in New Scientist (27 July 2010):
Green machine is our weekly column on the latest advances in environmental technologies
Will people’s environmental intentions wither in a heatwave? With much of the US, eastern Europe and Asia sweltering, it may be tempting to crank up the air conditioning to make homes and offices more tolerable.
That would be bad news for the planet, with aircon playing a significant role in the greenhouse-gas emissions produced by residential and commercial property – roughly 10 per cent of the world’s total emissions, says Graeme Maidment at London South Bank University.
“With global warming that will tend to increase even further, because people will use more air conditioning, while existing systems will have to work harder,” he says.
Rebirth of the cool
Maidment is leading a research effort, funded by the UK’s Engineering and Physical Sciences Research Council, to investigate more environmentally friendly air-conditioning and refrigeration systems. One option, ironically, is to use carbon dioxide to replace the synthetic HFC refrigerants used in such systems, he says: such gases can have around 4000 times the global warming potential of CO2. Around 2 per cent of the UK’s greenhouse gas emissions are attributed to them, says Maidment.
In conventional vapour-compression systems used by aircon units, the refrigerant is compressed and condensed and then expanded and evaporated to cool the room. The heat generated in the compression phase is normally radiated into the air outside the building.
Compressing CO2 generates much higher temperatures than HFCs – around 150 °C compared with around 60 °C. So Maidment and colleagues are investigating the idea of using the heat generated by CO2-based air-conditioning systems and fridges in supermarkets, for example, to provide hot water for nearby homes.
Cool sounds
Meanwhile, Matt Poese and colleagues at Pennsylvania State University in University Park are developing an HFC-free air conditioning system based on sound.
The team uses a linear motor to move a loudspeaker-type plate back and forwards, generating a sound wave in helium gas – which doesn’t add to the greenhouse effect. The sound waves create areas of compression and expansion within the gas, causing it to alternately heat up and cool down, says Poese. “It’s like a sponge – the gas sucks up heat in one location and then gets transported by the sound wave to another location where the heat gets squeezed out of the gas,” he says.
The team have previously built a fridge for ice-cream maker Ben & Jerry’s based on this so-called thermoacoustic technology. They say they can scale it up for use in air conditioning, and aim to build 3.5-kilowatt devices – equivalent to a basic home model – which could be dotted around buildings. Having several units could reduce the energy used by air-conditioning systems by only cooling those rooms currently in use, says Poese.
Water work
Others are also looking at refrigerants that don’t warm the atmosphere. One of these is a mixture of ammonia and water. Srinivas Garimella at the Georgia Institute of Technology in Atlanta is developing a system in which external heat is used to boil off ammonia vapour from a solution. The ammonia circulates through a condenser and then an evaporator, where it provides the cooling, before being reabsorbed by the water.
Known as absorption refrigeration, the technology was invented before today’s vapour compression systems, but it is bulky, requiring multiple heat exchangers, and so has mostly been used to cool only large buildings such as hospitals and universities.
Garimella has got around this problem by developing micro-scale heat exchangers, in which the solution flows through channels only 0.5 millimetres in diameter, taking advantage of the high heat-transfer efficiencies at this scale. His team have so far developed a book-sized prototype that generates the equivalent cooling to a 300 watts system, and plan to increase the cooling power of the device further with funding from the US Advanced Research Projects Agency – Energy.
Since the process is driven by heat rather than electricity, it could reduce greenhouse-gas emissions even further by using energy-efficient sources of heat. Air conditioning could be combined with solar power – which has one obvious advantage, says Maidment. “When the sun shines, you need air conditioning.”
Source: www.newscientist.com