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Carbon Cutting Refrigeration - Blue sky, bright future?

Judith Evans concludes her guide to the Refrigeration Road Map with a glimpse into the future of fridge technology

The researchers at London South Bank University have investigated ‘blue sky’ technologies that may be relevant to UK supermarkets at some point in the future.

When we examined these blue sky technologies we considered it almost impossible to gauge the carbon savings that they could achieve. Many of the technologies were currently only being developed or were in the prototype stage and so the savings were not fully quantified. Therefore we could not place them on the ‘bubble maps’ that we have produced for the existing technologies, which showed levels of carbons emissions savings in visual form.

Overall we examined 17 different new or emerging technologies that we felt had potential to reduce carbon emissions in supermarkets. These could be broadly divided into: refrigeration system technologies, refrigeration system add-ons, food based technologies and some general alternative retailing concepts.

Refrigeration System:

Acoustic refrigeration

Air cycle

Electro caloric

Hydraulic refrigeration

Magnetic

Optical cooling

Peltier

Stirling cycle variations

Thermionic refrigeration

Vortex tube cooling

Water

Refrigeration add-on:

Ammonia (sealed hermetics)

Pulsed electrical thermal de-icers

Food technology:

Eutectic packaging

New food

Alternative retailing concepts:

Automation

Leasing concepts

The majority of the technologies considered were new forms of refrigeration system. Most of these have the advantage that they use either an environmentally friendly refrigerant such as air or water or have some alternative method of producing cooling (that does not use a substance with Global Warming Potential). They are all characterised by the fact that they each have challenges that must be overcome before they can be fully commercialised.

The refrigeration systems that have undergone most development are acoustic, air cycle, magnetic, electrocaloric, Peltier, Stirling and water. Of these, the systems that have most applicability to retail are probably the first four.

Acoustic refrigeration uses sound waves to produce cooling. The technology is currently less efficient than conventional vapour compression technologies but potential improvements are possible. There are a number of prototype systems reported which are undergoing further development with the view to future commercialisation.

Magnetic cooling has potentially high efficiencies - above those of vapour compression technologies. Magnetic refrigeration takes advantage of the magnetocaloric effect: the ability of some metals to heat up when they are magnetised then to cool when demagnetised. The majority of prototypes developed have been based on the use of the rare metal gadolinium that are quite expensive. More recent work has looked for new materials that are cheap, have suitable transition temperatures and exhibit a large magnetocaloric effect. There are already interesting developments in the domestic arena understood to be at early commercial field trial stage.

Electrocaloric cooling uses the ability of a material to change temperature by applying an electric field. Potentially such a device can be thermodynamically very efficient and could outperform classic direct expansion refrigeration systems. The electrocaloric effect has been known for many years; however, until recently only small temperature differences have been possible. In addition the thin films used are expensive and this is an area where research is investigating the potential for new materials.

Systems such as air cycle have already been developed in the laboratory for retail applications. The use of air as a refrigerant has been demonstrated in chiller and freezer cabinets using components from the aircraft industry. There were a number of advantages above those of the use of an environmentally benign refrigerant, including low operating pressures (3-4 bar absolute) and the ability to use heat from the compressor for space or water heating. Currently equipment availability and costs limit the use of the technology.

Another more ‘extreme’ cooling concept is optical cooling which has recently been trialled by scientists at the Los Alamos National Laboratory in the US. Here lasers are aimed at certain materials to produce a phenomenon called ‘anti-Stokes fluorescence’ where more light energy is emitted than the energy absorbed. The radiating of this energy cools the material. The upside is that Los Alamos researchers produced a cooling effect of an impressive 47 deg C, but the downside is that the success depended on the use of the rare earth element Ytterbium in the fluorescent glass compound.

Although most of the novel systems revolve around a new means to produce cooling, we also examined some concepts that could change the ‘business as usual’ scenario for retail refrigeration, by changing the way the retailer operates. These include the development of alternative foods and greater automation in supermarkets.

It is possible that the foods themselves could be engineered to have longer shelf life and therefore may need less refrigeration. In addition some conventional products do not need refrigeration if they are cured or dried. Certain processes such as irradiation or high pressure processing have been put forward as a way to increase the shelf life of meat with minimal refrigeration. There is therefore a possibility that some, if not all, foods may require less or even no refrigeration in the future.

Among what is termed as ‘refrigeration add-ons’ is the current hot topic of ammonia as a cooling agent. An environmentally benign and efficient refrigerant, the use of ammonia is common in large industrial vapour compression systems, where the risk to workers can be minimised via confinement to the plant room and several levels of failsafes. However, the Roadmap researchers believe its use in retail applications is likely to be limited due to its potential risk to the public through leakage or component failure, and equally importantly the public perception of that risk. However, ways to use the gas in small  hermetic or semi-hermetic systems to reduce the system charge and therefore the risk of leakage are being developed. The Roadmap researchers believe that the application of ammonia to leak-tight integral cabinets could present efficiency potential for retailers.

At the same time, the increasing willingness of retailers to consider ‘safer’ secondary distribution systems to pump round the store(see Retail Question Time report in this issue), could open up the possibility of ammonia as a primary refrigerant in the future.

Further information on each of the technologies evaluated is provided in the road map document which can be downloaded from the Carbon Trust web site at: http://www.carbontrust.co.uk/Publications/pages/publicationdetail.aspx?id=CTG021.

Judith Evans was lead researcher for the LSBU, which was commissioned by the Carbon Trust, the Institute of Refrigeration and the British Refrigeration Association to develop a technical road map to support retailers in their efforts to reduce the carbon footprint of their refrigeration systems.

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