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Commercially available refrigerators are being furnished with additional storage capacity for the investigations in the form of latent heat storage units on the back of the devices.
© Bayerisches Zentrum für angewandte Energieforschung e.V. - ZAE Bayern
Latent heat storage systems
17.10.2017

The modified refrigerators are being tested in the laboratory with different latent heat storage materials
© Bayerisches Zentrum für angewandte Energieforschung e.V. - ZAE Bayern

Refrigerators stabilise the electricity grid

Refrigerators that consume electricity precisely when the wind and the sun provide excess electrical energy, and which during slack periods maintain their cooling temperature without additional energy input, would greatly relieve the grid. Scientists from ZAE Bayern are investigating the costs and benefits of such a load shift together with industrial partners. For this purpose they are increasing the thermal inertia of an efficient standard refrigerator by installing latent heat storage units. The test device can thereby bridge longer standstill times.

Nearly every household these days has at least one refrigerator – overall there are about 40 million in Germany. If only a fraction of these were used to shift loads in the power grid then a large number of devices would be available as indirect power storage unit. However, the cooling storage capacity of conventional refrigerators is limited. Many foods do not tolerate temperatures that are either too high or too low. In addition, refrigerators operate less efficiently at low storage temperatures. This is where latent heat storage systems come into play. These store large amounts of energy without significant temperature changes. This makes them ideal storage systems for refrigerators.

To ensure that new refrigerators with high storage capacities are accepted by customers they must not be limited in terms of their function and storage capacity. Rather an incentive needs to be created to ensure that people nevertheless buy the devices despite their increased cost due to the additional integrated technology. In the DiTES4Grid project, scientists at ZAE Bayern are exploring the technical and economic possibilities together with the project partners Bayernwerk AG and BSH Hausgeräte GmbH.

For their experiments the scientists are modifying a conventional no-frost refrigerator with the currently highest energy efficiency class (A+++). In particular, they have integrated a receptacle for the storage material in which they are testing different eutectic salt solutions. The charging capacity and therefore the shiftable load amounts to 120 W. The tests show that the refrigerator can span a period of seven to nine hours without being in cooling mode. During this time only a small amount of power is required for the fan, the discharging device and the device controller.

The costs for the additional integrated technology could be below that for battery storage systems, but the operation still needs to be optimised. “Domestic refrigerators provide a promising option for shifting loads in the power grid. The theoretical potential for Germany amounts to 2 GW if every second refrigerator is integrated into the demand side management,” says Andreas Krönauer, who heads the research project. „With a flexible, market-oriented electricity price for end customers, such a refrigerator could also be interesting for private customers.” However, further research is still needed. For instance, work still needs to be done on the stability of the eutectic salt solutions and the charging operation needs to be made more efficient.

The project is being funded by the German Federal Ministry of Economic Affairs and Energy as part of the Energy Storage research initiative. The research work and findings are continuously updated on the research initiative’s website.

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Addresses

Project management
ZAE Bayern - Garching

Project partner
BSH Hausgeräte GmbH

Project partner
Bayernwerk AG

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