Playing it cool
13 May 2005
Andrew Gigiel of the Food Refrigeration and Process Engineering Research Centre has been looking at ways to make domestic refrigerators more energy-efficient.
About one billion refrigerators and freezers are in use worldwide. In an era of diminishing energy resources, finding ways to improve their efficiency is vitally important and domestic refrigeration manufacturers are under increasing pressure to improve the energy consumption of their refrigerators and freezers.
EU regulations requiring energy labels to be shown on all refrigerators and freezers have been in place for some time. However, recently the requirements have been significantly tightened and the sale of both refrigerators and freezers with poor energy consumption banned.
The design and development of a new refrigerator or freezer is initially driven by market requirements. These normally specify the internal storage capacity, features such as ice makers and drink dispensers, outside appearance and overall footprint. There is economic pressure to use existing carcass designs, manufacturing jigs and where possible existing evaporators. While there is some scope for developing new evaporators, the main freedom is in the choice of compressors and control circuits. Consequently, some white goods manufacturers are looking at more energy-efficient compressors.
The research aims to develop an understanding of the relationship between the factors that control the energy consumption of small refrigeration circuits
Currently the design and selection of components (primarily evaporator, compressor, thermostat and capillary tube) for the refrigeration system is empirical. Starting with an existing system, components are physically changed and the resulting effect on temperature control and energy usage experimentally determined. Logically, there must be a combination of existing components that will achieve the temperature requirements at the lowest energy consumption. However, the empirical process currently used to find the optimum is time-consuming, expensive and cannot be guaranteed to achieve its aim.
The objective of my research project was to develop an understanding of the relationship between the factors that control the energy consumption of small refrigeration circuits suitable for domestic refrigerators and freezers. The factors were principally the size and efficiency of the compressor, the temperature control system/cycle used and the evaporator size. This understanding would be expressed as a predictive model that would substantially simplify the optimisation process.
A mathematical model to investigate the factors was therefore developed and verified. It followed transients in the refrigeration cycle by modelling a series of steady states. The model was initially verified against experimental data obtained using a typical chest freezer. When the freezer was run continuously, the model predicted an internal air temperature of -33.0°C and an evaporator temperature of -35.6°C (measured results -30.1°C and -33.7°C respectively).
Use of the model has clearly shown that energy consumption decreases as the size of the evaporator increases
The energy consumption predicted by the model was in exact agreement ±0.0005 kWh over a 24h period of the experimental measurement.
Use of the model has clearly shown that energy consumption decreases as the size of the evaporator increases. Thus, for maximum efficiency and minimum energy consumption, the evaporator should be as large as physically and economically possible.
When the effect of compressor displacement was modelled on its own it showed that the larger the compressor, the greater the energy consumption. The optimum energy consumption was with the smallest compressor that would still maintain the required temperature. However, the efficiency of compressors is in practice a strong function of the size of the compressor with isentropic efficiency increasing from 0.6 to 0.8 as the compressor displacement increased from 3 to 12cc. When this function was included a larger but more efficient compressor produced the minimum energy solution.
The operation of the thermostat had only a small effect on the energy consumption. However, the thermostat did affect the number of starts that the compressor makes in a given period. The effect of average door openings on the amount of energy used was very small, only 1 - 2%. The effect of the amount of food in the freezer on the energy used was also shown by the model to be small. However, the effect on the number of starts of the compressor was very large.
It is believed that stop-start losses can be as large as 50% of the total energy consumption in poor designs
If we make the very optimistic assumption that all domestic refrigerators and freezers in the UK have been designed using the smallest possible efficient compressor then the model predicts that using the optimum (larger) compressor would produce a 7% (1225 GWh) annual saving. If we make a pessimistic assumption that all domestic refrigerators and freezers in the UK have been designed using the smallest possible efficient compressor then the optimum (larger) compressor would produce a 40% (6995 GWh) annual saving.
Currently the program does not model the redistribution of refrigerant around the refrigeration circuit during transits. In particular, it does not model the stopping and starting losses of the refrigeration circuit. It is believed that stop-start losses can be as large as 50% of the total energy consumption in poor designs, so their effect on the energy consumption could be substantial. Further studies could develop the model to cover these areas.