Supermarket refrigeration systems for demand response in smart grids

With an increasing share of intermittent renewable energy sources in the electrical grid, the need for adapting the demand to the available supply of electricity becomes increasingly important. Within this thesis, the demand response capacity by supermarket refrigeration systems are investigated and methods for enabling it is developed.Article 1 explores the temperature control system in depth, concluding that the majority (80.5 %) of return air temperature sensors in RDCs were located in an area where a thermal gradient interfere with the perceived temperature, i.e. the temperature readings falsely indicated a higher return air temperature than the actual mean temperature of the passing air. The issue is analysed in detail and mitigated through a strategic re-positioning of the affected temperature sensors.Article 2 presents a computationally efficient yet accurate dynamic hygro-thermal model of an RDC with the capability to include effects of door openings. Thus, the model contributes to enabling demand response by supermarkets as it could provide the forecasts of the necessary temperature constraints, limiting the duration for which the supermarket could attend to a demand response request.Article 3 presents a field study where wireless gyroscopes were attached to the RDC doors in an operational supermarket to record the speed, duration, angle and frequency that the doors are operated at. Novel insights in significant differences in behaviour between medium and low temperature RDCs could be concluded.Article 4 presents a method for the thermal characterisation of RDCs based on an adaption of the Co-Heating methodology. The method evaluates infiltration rates within the 10 % limit compared with the condensate collection method. In addition, data on thermal performance, such as the heat transfer coefficient for the envelope and its thermal inertia, can be measured in a systematic way. The thesis together with the four appended articles presents a suite for the evaluation of temperature development in refrigerated display cabinets in operational supermarkets, which represents the main constrain for the demand response capacity

Model predictive control for a thermostatic controlled system

Abstract — This paper proposes a model predictive control scheme to provide temperature set-points to thermostatic con-trolled cooling units in refrigeration systems. The control problem is formulated as a convex programming problem to minimize the overall operating cost of the system. The foodstuff temperatures are estimated by reduced order observers and evaporation temperature is regulated by an algorithmic suction pressure control scheme. The method is applied to a validated simulation benchmark. The results show that even with the thermostatic control valves, there exists significant potential to reduce the operating cost. I