Energy in supermarkets - An overview on the energy flows and refrigeration controls

Licentiate thesis, 2016

The electrical energy used by supermarkets represents approximately 3% of Sweden's annual energy use. Of this, the refrigeration system accounts for 50%. To reduce greenhouse gases introduced by energy generation, a larger share of renewable energy sources must be implemented. Energy sources such as solar and wind are predictable but not controllable. Hence, if these sources are implemented, there is a need to make the energy users more flexible e.g. adapt the demand to the production instead of vice versa, as is the case today. Here, supermarkets have great potential to utilise their refrigeration system as an electrical energy sink by transforming electricity to cooling when there is a surplus of energy in the grid. In this thesis, the energy flows within the supermarket were analysed from a conceptual systems' perspective to find potential leverage points for increasing the power flexibility and energy efficiency. From the systems, it was confirmed that the refrigeration system holds great potential for increasing the power flexibility. The indoor air conditions \it(temperature and humidity)\rm are connected in multiple ways to the supermarkets' energy demand and should, therefore, be included as an important parameter in any energy efficiency measure. From a numerical CFD model developed in this thesis, the temperature field within a refrigerated display cabinet was visualised and analysed. It was found that the current area for temperature sensor placement in the air return canal might be exposed to warmer air streams, heated by the ambient air in contact with the glass door of the display cabinet. The findings were validated and confirmed in a laboratory. The findings initiated a field study to map and correct the position of the return air temperature sensor in 235 positions. From the measurements of the electrical energy use and temperature in the affected refrigerated display cabinets, a decrease in energy use and temperature variation could be concluded. The thesis concludes a demand of having a systematic and holistic approach when implementing measures for increasing the energy efficiency and power flexibility of supermarkets. The thesis also concludes a need for further research on the energy flows within the supermarket to enable the full potential of energy efficiency and power flexibility measures.