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Alternative Refrigerants: Microporous solids-the refrigerant of the future?

Following a three-year investigation into alternatives, Thermal Energy Systems under John Poole has opted for the development of the compressor-driven adsorption/desorption of CO2 on microporous solids.
Alternative Refrigerants: Microporous solids-the refrigerant of the future?
INTRODUCTION of the F-Gas Regulation and a directive on mobile air conditioning in Europe has caused uncertainty concerning the long-term future of hydrofluorocarbons (HFCs) owing to their high direct global warming potential (in comparison to carbon dioxide).

Substantial taxes in Scandinavia have been imposed on HFCs, and Denmark and Austria have imposed more rigorous controls. HFCs are included in the Kyoto Protocol and the British government has stated publicly that HFCs are not sustainable technology in the long term.

There is an air of déjà vu about all of this for the acr industry which, having switched away from CFCs to HCFCs, is now in the process of completing the change to HFCs.

Understandably, there is considerable concern arising from the mixed messages about the viability of HFCs as replacements for ozone depleting substances. This has been accelerated the search for alternatives which have similar properties but without the high GWP of HFCs.

Natural refrigerants

Some well known major companies, eg Coca-Cola, Uniliver, McDonalds, Marks and Spencer, Tesco and others, have announced that they are looking towards the use of 'natural' refrigerants as the long term solution to this problem.

Unfortunately, the known alternatives to HFCs which have zero or close to zero GWP=1) are either flammable (hydrocarbons), toxic (ammonia, sulphur dioxide) or both (ammonia) or operate at high pressures (CO2). More sophisticated fluorinated molecules are theoretically possible but they will be high cost and, being fluorinated products, they will have a GWP albeit considerably lower than the current range of HFCs.

If fluorine is discounted as the chemical building block to replacing HFCs due to its inherent property of greenhouse warming, we are left with fluids which are flammable, toxic or high pressure.

Other possibilities

Various non-Rankine technologies have been investigated. For many years the thermally powered absorption cycle, based on ammonia/water or water/lithium bromide, has found limited applications where its specific advantages outweigh its poor energy efficiency. Stirling Cycle, thermoacoustic, magnetic refrigeration and others have not been commercialised for a variety of reasons amongst which are cost and energy efficiency.

Under the technical guidance of technical director Dr Dick Powell, Thermal Energy Systems Ltd (TESL) has completed a three-year r&d programme to identify and develop technologies to meet what the acr industry would expect in a viable HFC replacement technology.

In addition to combining hazard and good energy efficiency at acceptable cost, future cooling technologies need be sustainable in the long term to avoid yet another round of technology substitution.

Furthermore, a new technology should as far as possible employ existing components to facilitate the change over by making best use of existing expertise to minimise the cost.

From the range of innovative options generated by TESL the company has selected the compressor-driven adsorption/desorption of CO2 on micro-porous solids to develop first. Gas adsorption and desorption is analogous in many respects to condensation and evaporation respectively.

In the initial TESL system, based on a commercial porous solid, the enthalpy of adsorption is comparable with the condensation latent heat of HFC refrigerants.

Most importantly this means that the operating pressures are comparable with the HFCs avoiding the problem of very high pressure transcritical CO2 systems and enabling existing components from the acr industry to be used.

All of us who work in the acr industry view refrigerants as liquifiable fluids containing simple molecules whose relevant properties are provided by their thermodynamic tables.

Chemical expertise

In TESL we now consider the combination of CO2 with the microporous solid to be the refrigerant, not the CO2 alone. Obviously the physical properties of CO2 are fixed - it is the specific properties of the selected adsorbent that enables TESL to tailor the thermodynamics of the combination to achieve the pressures required. By choosing different adsorbents TESL can vary the intrinsic properties of its 'refrigerants'.

By using chemical expertise both in house and in academia, TESL is already looking at options to synthesise microporous solids with superior properties.
Could this be the Holy Grail? Time will tell.

Thermal Energy Systems
0161 926 9876

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