It is hard to imagine a world without refrigeration. If all perishable food were to spoil naturally at warm temperatures it could not be stored long term or distributed long range: our diets would become more localised, less varied and much harder work. Our daily lives would be blighted by a proliferation of harmful bacteria. Research laboratories and medical progress would be hamstrung.
No wonder, then, that throughout history mankind has striven to find ways of lowering to safe and hygienic levels the temperature of containers used for storage and transport.
For as long as refrigeration has involved more than just the use of ice blocks, it has been mechanical. The idea of using a pump to create a vacuum – for the purpose of causing a chemical to boil and so draw heat from the surrounding air – was first demonstrated in the eighteenth century by the Edinburgh physician William Cullen.
Subsequent generations of scientists went on to develop the vapour compression refrigeration cycle, the first working prototype being built by the American Jacob Perkins in 1834. A repeated sequence of processes – the compression and discharge of a refrigerant vapour, the condensation of that vapour into liquid, and the cooling of the liquid ready for its evaporation (the phase of the cycle that absorbs heat from the region to be cooled) – demonstrated that a continuously refrigerating effect could be created by machine.
Ever since then commercial refrigerators and freezers, much larger and more hard-wearing than domestic ones and often built to order, have been indispensable to the functioning of the food and health care industries; they are to be found in all meat processing plants, supermarkets, restaurants, pharmaceutical factories, hospitals and in the road, rail and ocean-going carriers that move between them.
The work a refrigerator has to do in order to keep its contents chilled varies according to how fully loaded it is. And, although a system is designed to function at full capacity, over 90% of the time it operates below that level, creating significant potential for energy inefficiency. If the process cannot somehow be fine-tuned, it will either work unnecessarily hard or – by simply cycling on and off – only crudely reflect partial capacity as well as inflicting undue wear and tear on component parts.
One strategy for adapting system performance to partial loading is the use of two or more compressors in the refrigerator design. A twin-compressor rack is a simple way of enabling a unit to function as efficiently at half capacity as it does at full. Three compressors allow for three different capacity steps – or more if capacitors of different sizes are used in different combinations.
Variable Speed Drives (VSDs) – using only the energy required, smoothly accelerating up to full capacity from average conditions when necessary – have more than one role to play in commercial refrigeration systems: from compressor control to the operation of the fans in the condenser unit that cool the refrigerant vapour.
By supplying the operating system with a feedback pulse for each kWh, a variable speed AC drive on a compressor enables more precise monitoring and regulation of energy usage than would otherwise be possible, and, even as load levels fluctuate, maintenance of the desired temperature more or less to the degree.
When controlling the speed of condenser fans, VSDs continuously compare the condensing temperature with that of outside air and work only to meet the differential, so again optimising efficiency while conserving energy.
In larger systems a drive may control a single compressor or fan while running at base level; then start up more units, cascade-style, as energy consumption rises. (In such scenarios systems can be engineered to prevent uneven wear by rotating the choice of initial unit.)
One beneficial side-effect of the role of VSDs in supplying properly calibrated levels of power to refrigerating systems is the overall reduction in noise levels. With motor activity lower in general, and condenser fans in particular not now noisily accelerating up to full blast all the time, refrigerators require less sound-proofing (or architecturally remote positioning) than before.
The standout benefit of VSDs is, however, the marked reduction they bring to overall energy consumption. Thanks to the unique mathematical relationship between torque, speed and power, a reduction in fan speed brings about a greater than equal percentage reduction in energy used: 10% less speed, for example, brings with it an energy saving of 30%.
This equation has particular significance for the world of commercial refrigeration. A recent British study of electricity use in the commercial kitchen determined that refrigeration is the single most energy-hungry aspect of operations, accounting for 41% of electricity consumed. The financial implications of improving the efficiency of motor behaviour in this context are not trivial.
One UK food processing factory – to take an example from the Control Techniques case studies – found that by replacing its compressor’s soft starter with a variable speed drive, the energy saving of approximately 7,500 kWh represented about half the plant’s total annual power usage (or £23,400).
The market for commercial refrigeration equipment is predicted to grow vigorously over the next decade – by more than 5.1%, according to one report. The reasons go beyond simple population growth. Urbanisation continues to shape new food consumption patterns, especially in India and China. A wide variety of fresh, rather than preserved, foods is the preferred diet of more and more consumers. And as the globalisation of trade develops, food supply chains are becoming lengthier and more elaborate.
In these rapidly changing circumstances, all too often characterised by poorly coordinated systems and uneven technologies, the potential for waste – of food and of energy – remains high. Variable speed drives are one means by which, unit by unit, business by business, system-wide efficiency can be tightened from ground level up.