Efficient refrigerators: heat storage devices help to save electricity

Although the electricity consumption of refrigerating appliances seems small, they present considerable potential for energy saving on account of their constant connection and widespread use. In Germany, domestic refrigerators account for around 4 per cent of total electricity consumption. Scientists at the Kompetenzzentrum für nachhaltige Energietechnik (KET) (Competence Centre for Sustainable Energy Technology) at Paderborn University are therefore researching the use of integrated heat storage devices to increase energy efficiency.

In recent years, the electricity consumption of domestic refrigerating appliances has been considerably reduced – mainly through improved insulation. However the KET has now developed special heat storage devices based on phase change materials (PCM), which are particularly efficient and environmentally friendly. The basic material is paraffin, which liquefies when heated, storing the heat in the process. Critical for its efficient operation is the influence of the PCM on the heat transfer in the refrigeration unit. It can reduce the liquefaction temperature and also raise the evaporation temperature. As the difference between these temperatures determines the energy requirement of the cooling process, the energy requirement of these new appliances can be significantly reduced.

Engine for progress:

  • Development of a phase change material as heat storage
  • Improving refrigerator efficiency by up to 17%
  • Use of renewable raw materials and avoidance of refined paraffins
  • Enabling load management in the smart grid

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Photo: G. Sonnenrein

Temperature distribution of the condenser; top: before the start of the compressor run time,  middle: at the end of the compressor run time, bottom: 15 minutes after the end of compressor run time; Column 1: without heat storage, Column 2: with sensible heat storage (water), Column 3: with PCM in PE-HD foil, Column 4: with PCM in aluminium film, Column 5: with copolymer compound.

Newly developed heat storage devices with improved properties

Heat storage technologies used up to now often have disadvantages, such as low thermal conductivity, risk of leakage or toxicity. These disadvantages prevented their use in practice. The KET heat storage device offers an innovative solution, being based on a special blend of materials – paraffins, polymers and graphites. This has enabled thermal conductivity to be raised by a factor of 20. Additionally, the dimensional stability of the materials developed prevents leakage, which in the case of liquid storage materials used up to now had to be prevented by expensive encapsulation of the material.

Up to 17 per cent less energy consumption

A classic combined fridge-freezer, depending on efficiency and size, consumes around 150 kilowatt-hours of electricity per year. By integrating the heat accumulators developed here, the energy requirement of domestic refrigeration appliances can be reduced by up to 17 per cent. Improving the energy efficiency of all domestic refrigeration appliances in Germany (around 50 million) by only 10 per cent would lead to an annual energy saving of more than two terawatt-hours of electricity – enough to drive round the earth in a Tesla about 415,886 times – and lead to a reduction of over a million tonnes of CO2-equivalent. The technology could also be extended to other uses such as cold stores or other refrigeration equipment, providing for further energy saving.

The environment also benefits from this new technology. The production of the materials entirely dispenses with partly and fully refined paraffins, which are very energy-intensive to produce. In addition, the polymers used in the process are based entirely on renewable raw materials.

Enabling load management

As well as enabling energy savings, the large number of refrigeration appliances on the market makes it possible, with the help of integrated heat accumulators, to generate a significant adjustable load for a future smart grid, without any reduction in convenience for the consumer. In this way, the load can be switched to times of high electricity generation by renewable energy sources. With all refrigeration appliances in German households, a possible energy saving potential of around 500 megawatts would result.

Photo: private

“Phase change materials enable refrigerators to use less electricity and store energy.”

Professor Dr.-Ing. Jadran Vrabec, Paderborn University

Partners and sponsors

  • Universität Paderborn - Kompetenzzentrum für nachhaltige Energietechnik
  • Miele & Cie. KG
  • Andere Förderung