REFRIGERANTS
Strictly speaking, the answer to the question is ‘they are already here’, in that alternate technologies are already used for specialist applications. Thermoelectric cooling (TE), is well-known for small drinks refrigerators and electronics cooling, where simplicity is an asset, and over-rides modest lift, low efficiency and high cost per kW.
A fundamental problem with TE cooling is the back conduction of heat opposing the refrigeration effect. To an extent this can be reduced by using TE materials containing heavy metallic elements such as bismuth and tellurium, which are toxic and have low abundances in the Earth’s crust, indeed far lower than fluorine. Magnetic (magneto-caloric or MC) refrigeration has received significant attention over the past 30 years. Achieving temperature lifts around ambient of even a few kelvin requires intense magnetic fields even when used with magnetic materials with highest MC effect.
However, these contain rare metals such as gadolinium and lanthanum, so are always likely to be very expensive. MC cooling works well in achieving temperatures within a few millikelvins of absolute zero. Absorption cooling has been successfully used for many years, both at small
domestic fridge and large industrial chiller scales. This commercial technology is based on refrigerant/absorbent pairs. Despite extensive research over many years, the long-established ammonia/water, water/LiBr and methanol/LiBr pairs dominate. Althou gh poor cycle efficiency and limited temperature lift are fundamental problems, where ‘free’, low grade ‘waste’ heat from an industrial process or thermal solar energy is available, then energy efficiency is not necessarily the major concern.
Even stretching and relaxing a rubber band and other elastic materials can generate cooling. A recent report describes a new take on this approach whereby metal wires are twisted and untwisted cooled a stream of water by 7.7°C and speculated that greater cooling might be achieved with an increase in coiling/uncoiling rate. This result is still far from the tens of kelvin lifts achieved routinely by VRC refrigeration. Thermoacoustic cooling using air or other non-condensable gases was glorified in the 1990s as a promising alternative to VRC to avoid HFCs, but efficiency is poor and power density low, a criticism also of related devices based on oscillating gas volumes on such as pulse and Gifford-Mahon tubes.
The latter are available commercially for cryogenic applications, but irrelevant to higher temperature cooling applications. It is clear that alternative cooling technologies exist, but VRC continues to be the dominant cooling/ heat pumping technology because low GWP refrigerants and progressive performance improvements have enabled it to stay ahead of its rivals.
Maybe exotic materials or physics are not the answer? Liquid air technology, espoused by Dearman for renewable energy storage, could be readily integrated into large commercial and industrial refrigeration.
For room air conditioning, M cycle, using water from renewable-energy driven desalination, might be practical in lower humidity, coastal climates. Because they are also storage media, liquid air and liquid water as refrigerants can usefully help buffer the variable output of renewables. Nor should we rule out the possibility of combining such technologies with conventional VRC to optimise overall system performance.
But if we ultimately make contact with aliens from a more advanced galactic civilisation, perhaps they will show us physics we are missing for a viable VRC alternative?
Natural refrigerants
for a cleaner tomorrow. Cool by nature.
BOC
BOC has been a leading provider of natural refrigerants for over 30 years. Whether your application is a ir conditioning, commercial refrigeration, process chilling or heat extraction, our range of high quality natur al
We can offer Technical assistan ce
Local stocking at over 350 retail outlets Delivery on our 300+ vehicles
Training on product handling, and safety awareness for facility staff
Natural refrigerants R717 (ammoni a) R744 (CO 2)
Full range of Hydrocarbon refrigerants (including ethylen e)
Visit
www.BOConline.co.uk/refrigerants, email us at
specialproducts@BOC.com or call us on 0800 02 0800 for more information. 27
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