Frozen food Retail Display Cabinets, RDCs, in UK supermarkets account for up to 30% of total UK refrigeration power consumption and defrosting these RDCs accounts for up to 25% of that power. Cold rooms and other freezer applications also account for material energy usage. Supermarkets aim to maintain the frozen food in an RDC at below -18°C. Air is circulated around the cabinet using a fan that blows air through a cold heat exchanger (evaporator) where refrigerant boils at below -32°C. Moisture in the air freezes on the evaporator surfaces leading to loss of heat transfer efficiency and increase in product temperature. This ice has to be removed on a regular basis and in the UK this is done using electrical resistance heaters. Only 20% of the heat supplied by the electrical defrost system is used to melt ice, the rest is transferred to cabinet components, the air in the cabinet and the food. This wasted heat then has to be removed after each defrost to bring the cabinet and its contents back below -18C.
The Frigesco™ system uses a small thermal store to store waste heat that is used to defrost an evaporator using a thermo siphon. In addition the thermal store sub cools the liquid entering the cabinet and so has added benefits during the period when the cabinet is refrigerating. The Frigesco™ defrost system is virtually energy-free and rapid with much less unwanted heat entering the cabinet. Tests have shown that reduction in total refrigeration power consumption of up to 25% is possible when the Frigesco™ system is deployed.
The potential for the Frigesco™ system to make a major contribution to the reduction in worldwide CO² emissions is significant.
In the UK alone annual savings would be 230,000 tonnes of CO² if all the freezer cabinets running in 2010 were converted to the Frigesco™ defrost system.
The corresponding worldwide effect of modifying all 4.8 million existing supermarket frozen food cabinets would lead to an 18 million tonne per annum reduction in CO² emissions.
The Frigesco™ System is the first major change of defrost technology in 50 years and is a truly disruptive and innovative technology. The only other commercially available defrost technology which has been introduced in the last 50 years is a control system known as “defrost on demand” which attempts to minimise the number of daily defrosts by monitoring ice build up. This has not been widely adopted and data suggests that the take-up has been less than 5%.
The alternative system of using hot gas from the compressor exhaust or warm gas from the receiver is no longer used in the UK for remotely connected RDCs because of the difficulties associated with extra pipework, large pressure swings (a source of refrigerant leaks), operational complexities and energy penalties. Hot gas defrost essentially uses the compressor as a source of heat, which is far from thermodynamically desirable (a very inefficient way of using grid power to melt ice).
The advantages of the Frigesco™ innovation are clear and indisputable – it will save up to 25% of the energy currently used by a typical RDC in a UK supermarket, is simple and cheap to install and is the only available engineering solution to the problem of defrost energy consumption.
It is protected by a UK patent Pt.No. GB 2 495 672. “Flash defrost system” and national phase application processes in all the major market areas have been initiated. There is no other competitive defrost technology and the Refrigeration Road Map (Carbon Trust, 2010) which provides an exhaustive list of all possible energy saving technologies in this sector clearly shows that defrost energy consumption is the largest and most intractable of the energy consuming processes.
At a time when supermarkets and other sectors of the cold chain are striving to reduce energy consumption and their carbon footprint, and given that a very large amount of grid power is used by the cold chain in the UK (3% of UK output) the introduction of a simple change to the refrigeration system which can produce a large reduction in energy use has enormous potential.