Aggregation and Control of Supermarket Refrigeration Systems in a Smart Grid

In this work, control strategies for aggregation of a portfolio of supermarkets towards the electricity balancing market, is investigated. The supermarkets are able to shift the power consumption in time by pre-cooling the contained foodstuff. It is shown how the flexibility of an individual supermarket can be modeled and how this model can be used by an aggregator to manage the portfolio to deliver upward and downward regulation. Two control strategies for managing the portfolio to follow a power reference are presented and compared. The first strategy is a non-convex predictive control strategy while the second strategy consists of a PI controller and a dispatch algorithm. The predictive controller has a high performance but is computationally heavy. In contrast the PI/dispatch strategy has lower performance, but requires little computational effort and scales well with the number of supermarkets. Two simulations are conducted based on high-fidelity supermarket models: a small-scale simulation with 20 supermarkets where the performance of the two strategies are compared and a large-scale simulation with 400 supermarkets which only the PI/dispatch controller is able to handle. The large-scale simulation shows how a portfolio of 400 supermarkets successfully can be used for upward regulation of 900 kW for a two hour period.