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REFRIGERANTS


Where are they?


Professor Dick Powell notes that according to Edward Teller, this question was famously posed over lunch by fellow physicist Enrico Fermi in 1950 when colleagues argued that our galaxy, containing billions of stars, must be teeming with space-faring civilisations far in advance of ours, yet we have not been contacted by them.


A


lthough the ‘Fermi paradox’ has been discussed extensively over the past 70 years, the discovery over the past two decades that many stars have planets, some potentially conducive to life, has made it even more


pertinent. Unfortunately, short of making contact with galactic aliens, perhaps Fermi can never be answered. I would like to apply the title question to a very diff erent matter: replacements for reverse Rankine Cycle based, vapour recompression (VRC) cooling and heat pumping, the technology that has dominated our industry since its beginning in the mid-19th Century. Various technologies have been reported over the past three decades, promising to be more energy effi cient than VRC and to avoid the perceived environmental problems of fl uorinated refrigerants. This standard mantra, often used to justify research in this area, undoubtedly sounds good when applying for public funding from governments who, quite understandably, wish to show they are contributing to the reduction of global warming. So, where are these amazing new technologies? Before speculating on the reasons, I need to defi ne the term ‘refrigerant’. For the purposes of this piece I have assumed any material is a refrigerant if it undergoes a temperature change when work is done on it, for example by changing pressure, mechanical stress, magnetic or electric fi eld, electric potential etc.


This article does not allow detailed descriptions of the various alternative


cooling methods, but I believe that some general reasons can be adduced why progress has been less rapid than proponents predicted. Perhaps the most important is the inherent robustness and potential for


continued development of VRC. It has the following benefi ts that need to be at least matched, if not surpassed, by a successful rival:  The temperature change (‘lift’) induced by swinging the refrigerant pressure is more than adequate for many applications, because the heats of condensation and evaporation are large compared to the heat capacities of refrigerant fl uids.


 Each component (evaporator, compressor, condenser and expander), under 26 December 2019


steady state conditions, runs at approximately constant temperature. Only the temperature of the refrigerant changes signifi cantly in passing from one component to the next.


 When the lift is greater than can readily be achieved by a single stage, double staging can easily be applied.


 VCR is applicable over a very wide range of scales from small domestic refrigerators to large commercial building air conditioning and industrial plant refrigeration.


 Commercially and technically, VRC has been evolved over around 150 years, so is a long way down its learning curve, optimising cost and performance. To be overcome this accumulated experience an alternative technology must off er an overwhelming advantage.


 VCR has responded to environmental concerns by adapting to refrigerants with lower impact, like HFOs and CO2


, which combine very low GWPs


with low hazards. Avoiding the environmental impact of the fl uorinated refrigerants is no longer a convincing argument for the development of a new cooling technology. Some alternative refrigerant technologies rely upon exotic, expensive low abundance, toxic metals that potentially have their own environmental and hazard problems.


 Both fl uorinated and natural refrigerants are manufactured from abundant, readily available elements. The possible exception is the iodine in CF3I should this fl uid become a mainstream refrigerant.


 The energy effi ciency of VRC continues to improve, driven by regulations to minimise the indirect global warming from CO2


emissions by fossil


fuel power stations and scope exists to improve further. In the longer term, as the electrical power supply is decarbonised by the increasing use of renewables, the energy of cooling systems becomes less important environmentally, because their indirect contribution to global warming decreases.


 Although proponents of some alternatives cite higher theoretical cycle energy effi ciencies for their systems compared to VRC, what really matters is the practical effi ciency of an actual unit.


www.acr-news.com


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