ESO2 Optimization of Supermarket Refrigeration Systems: Mixed Integer MPC and System Performance

Supermarket refrigeration systems consists of a number of display cases, cooling cabinets and cold rooms connected to a central compressor pack. This configuration saves energy compared to placing a compressor at each cooling site. The classical control setup of a supermarket refrigeration system is highly distributed.

The cooling sites are equipped with an individual hysteresis controller that keeps the air temperature in the cooling site within a defined band by manipulating the opening degree of an inlet valve. The compressor bank maintains a desired suction pressure by adjusting the capacity to the given load from the cooling sites.

An illustration of the principle is shown in figure 1. This design is flexible and simple, but its major drawback is however that it introduces self-inflicted disturbances and the dynamic coupling of the display cases makes them synchronize. Synchronization results in low efficiency and high wear of the compressor, because the compressor has to work much harder for short periods of time.

The problem increases when the load is small compared to the available compressor capacity, which it is in the winter time, at night and when only a few cooling sites are present in the supermarket. The first approach to solve this problem is to design an overall control system which coordinates the compressor capacity and the current refrigeration load.

The drawback of this approach is the complexity of the single controller. The solution is investigated in the first part of the report. A second solution is investigated where only the compressor control is considered. This controller try to feed-forward the measured disturbances, i.e. opening and closing of the cooling site AKV’s.

Last a performance analysis of the refrigeration system is performed.