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Defrosting in a flash

Frigesco’s flash defrost technology promises to dramatically reduce running costs for retail refrigeration equipment. David Walter explains how

Eskimos are wise about snow and frost. Not for nothing do they make their homes out of compacted snow. While it may be -25 deg C outside, temperatures inside an igloo can be in the balmy 10 to 20 deg C range.

An igloo makes a good shelter because of the excellent insulating properties of snow. Whereas solid ice makes a very poor insulator, compacted snow contains a matrix of small air pockets that work very effectively, along similar lines to modern polymer-based, closed-cell insulation. Being highly reflective also helps keep out radiated thermal energy.

While the insulating properties of snow are a boon for eskimos, they present a serious problem for the refrigeration industry and retailers. The rapid build-up of frost that occurs on the cold surface of evaporators significantly reduces the efficiency of refrigeration systems. The cold, bare metal of an evaporator is good at transmitting its “coolth” to the surrounding air (or, indeed, absorbing thermal energy from it).

However, it is not long before water vapour from the air passing over the cold surface is deposited, as air reaches dew point, resulting in a dusting of frost over the metal surface. Unless active steps are taken, frost build-up will continue to accumulate to many inches in thickness, rendering the refrigeration system ever more inefficient, and in the worst cases putting food safety at risk.

To overcome this, and restore the refrigeration plant to proper functioning, frost build-up must be removed through the application of energy. This process, better known of course as defrosting, has to be carried out on each system a number of times each day – depending on humidity levels in ambient air and the level of activity in the cabinet or local cold store environment.

Energy for defrosting is traditionally obtained from so-called ‘hot gas’, via the refrigeration system itself, or directly from the mains in the form of electric defrost. The latter uses small heaters within the cabinet, near the evaporator, to warm the local environment and melt frost build-up.

But there are a number of downsides to these conventional approaches.

Hot gas defrosting carries a significant energy and performance penalty, as it requires the refrigeration system to operate outside its optimum envelope for significant amounts of time in order to generate the additional heat for defrosting. This reduces the efficiency of the refrigeration system and increases running costs for end-users.

Electric defrost incurs a direct energy penalty, through the consumption of electricity to melt frost using dedicated heaters. The use of electric fans in defrost systems adds further to energy consumption.

In both cases, the addition of significant quantities of indiscriminate heat energy to refrigerated spaces must be offset by increased refrigeration input, again leading to higher electricity consumption. On the legal and safety front, there is growing concern that repeated heating of food in refrigerated spaces may lead to hygiene risks.

A touch of frost

Due to its high specific heat capacity and good insulation properties, accumulated frost requires a considerable amount of energy to melt it. Defrosting a refrigerated display case typically accounts for around 30 per cent of total energy use. When multiplied across a supermarket’s estate, or indeed the combined refrigeration systems in the food supply chain across a whole country, this amounts to an immense drain on energy supplies and a huge additional cost for end-users.

Despite this, over the past five decades there have been few significant developments in the field of defrosting technology. During this time, the cost of energy has continued to rise, grid supplies have become “maxed out” in some cities, and concern has grown about the risks of exposing refrigerated food to repeated surges of heat over long periods.

Frigesco has for some time been working on a new approach to defrosting refrigeration systems that overcomes the downsides of conventional approaches. Following an intensive period of development and testing, our new low-energy flash defrost system was recently granted full patent protection. We are now finalising trials in working UK supermarkets prior to a planned global roll-out of the system.

Energy benefits

Our flash defrost system dramatically reduces the amount of energy required to maintain optimum frost-free conditions in refrigeration systems. Lab trials have shown that it can cut the overall energy consumption of refrigeration systems by up to 25 per cent. It uses a specialist food-safe phase-change material to store heat energy generated by the refrigeration system in the course of normal operation. It then uses the latent heat energy stored in the phase-change material to defrost evaporators, as and when required.

Efficiency is improved, as it does not require electrically driven components to deliver heat energy to the frosted evaporators, although small pumps can be used to improve the speed of the process if required. In addition, as it sequesters heat produced by the refrigeration system during normal operation, it adds a sub-cooling effect, improving performance and yielding further energy savings. It can be used on both integral and remote refrigeration systems, with equal success.

Traditional defrosting methods are very energy-inefficient, with as little as 20 per cent of the energy being used in the defrosting process. In addition, it adds a significant unnecessary heat gain to the refrigerated space. The new system uses much lower temperatures, better targeted to effect a complete defrost of the evaporator. This means that the temperature of stored product is maintained within much narrower limits, rather than fluctuating wildly as in the case of a traditional defrost.

The flash defrost system has been developed by a team headed by Professor Tom Davies in the UK. It uses a heat store containing a waxy phase-change material, calibrated to change phase at a specific temperature. This is placed in the refrigeration circuit after the condenser, so that heat from the warm liquid leaving the condenser is collected and stored for use during a defrost.

This has two benefits. First, heat that would otherwise be wasted is harvested and put to later use for defrosting. And second, the resulting sub-cooling of the liquid arriving at the expansion device has a beneficial effect on the overall efficiency of the system. In thermodynamic terms, the sub-cooling gain effectively pays for the post-defrost re-chilling required.

Therefore, with no additional energy required for defrost and little extra energy needed for re-chilling, the defrost process is virtually energy-free. It has been calculated that if supermarkets adopted the system worldwide, it could save around £1.6bn in energy costs. This reflects adoption on display cases only. If flash defrost was also applied to store freezer rooms, the saving could potentially be doubled.

With energy prices increasingly sharply, pay-back times are getting shorter and the scale of potential energy savings increasing with each passing day.

Safety and stability

Although the main benefit is a reduction in energy use and lower running costs, the Frigesco system delivers other attractive benefits. These include:

n Improved food hygiene and safety due to more effective defrosts, and more stable temperatures in the refrigerated space;

n Increased working life and fewer breakdowns for refrigeration plant, due to more complete defrosts and improved equipment operation;

n Potential reductions in equipment capital costs due to design optimisation possible with the use of flash defrost;

n Reduction in a retail store’s overall maximum electrical draw, giving “head room” in locations where power supplies are near the limit. This may be vital in extreme weather conditions, as experienced in hot summer months;

n The more efficient defrost process stops the build-up of frost on floors and walls in the immediate environment of refrigeration plant, eliminating the so-called “Santa’s grotto effect”, and reducing slip hazards for store staff.

Frigesco was awarded a Department of Energy and Climate Change (DECC) grant to develop and commercialise the technology. Earlier this year in Clean Equity Monaco 2014, an international showcase of the most promising sustainable and energy-saving technologies from around the world, Frigesco’s flash defrost system won an award for Excellence in the field of Environmental Technology Development.

Having proven the technology, Frigesco is working with a number of major UK food retailers on in-store trials. Once proven in cold rooms, the next stage will be to apply the system to refrigerated display cases.

Bob Arthur, former president of the British Refrigeration Association, and a respected international authority on retail refrigeration, has joined the Frigesco team to help develop and commercialise the technology.

“The Frigesco flash defrost system is a genuine innovation, and the savings are substantial and proven,”says Mr Arthur. “Retailers are understandably keen to carry out trials, to prove the system for themselves. Given the potential savings and relatively low capital cost, I believe Frigesco flash defrost technology will be widely adopted by food retailers and supermarkets in the near future. It deserves to become a mainstream technology across the industry.” 

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