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folder 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. 

Although 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 different 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 efficient than VRC and to avoid the perceived environmental problems of fluorinated 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 define 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 field, 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 benefits 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 fluids. 

Each component (evaporator, compressor, condenser and expander), under steady state conditions, runs at approximately constant temperature. Only the temperature of the refrigerant changes significantly 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 offer 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 fluorinated 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 fluorinated and natural refrigerants are manufactured from abundant, readily available elements. The possible exception is the iodine in CF3I should this fluid become a mainstream refrigerant. 

The energy efficiency 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 efficiencies for their systems compared to VRC, what really matters is the practical efficiency of an actual unit. 

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. Although 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?

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