Synchronization and desynchronizing control schemes for supermarket refrigeration systems Lars F. S. Larsen and Claus Thybo, Rafael Wisniewski & Roozbeh Izadi-Zamanabadi Abstract— A supermarket refrigeration system is a hybrid system with switched nonlinear dynamics and discrete-valued input variables such as opening/closing of valves and start/stop of compressors. Practical and simulation studies have shown that the use of distributed hysteresis controllers to operate the valves leads to synchronization, meaning that the opening and closing actions of the valves coincide. Consequently, the compressor periodically has to work hard resulting in low efficiency, inferior control performance and a high wear on the compressor. In this paper we propose two control schemes of low complexity for desynchronizing the valve operations while improving performance. Simulation results indicate the potential increase in efficiency and reduction in wear comparing with traditional control schemes. I. I NTRODUCTION A supermarket refrigeration system consists of a cen- tral compressor bank that maintains the required flow of refrigerant to the refrigerated display cases located in the supermarket sales area. Each display case has an inlet valve for refrigerant that needs to be opened and closed such that the air temperature in the display case is kept within tight bounds to ensure a high quality of the goods. For years, the control of supermarket refrigeration systems has been based on distributed control systems, which are flexible and simple. In particular, each display case used to be equipped with an independent hysteresis controller that regulates the air temperature in the display case by manipulating the inlet valve. The major drawback, however, is that the control loops are vulnerable to self-inflicted disturbances caused by the inter- action between the distributed control loops. In particular, practice and simulations show that the distributed hystere- sis controllers have the tendency to synchronize [4], [5], meaning that the opening and closing actions of the valves coincide. Consequently, the compressor periodically has to work hard to keep up the required flow of refrigerant, which results in low efficiency, inferior control performance and a high wear on the compressor. The control problem is significantly complicated by the fact that many of the control inputs are restricted to discrete values, such as the opening/closing of the inlet valves and the stepwise control of the compressors. Furthermore, the system features switched dynamics turning the supermarket refrigeration system into a hybrid system. L. Larsen, C. Thybo, R. Wisniewski are with Advanced Engineering - Refrigeration and Air Conditioning, Danfoss A/S, DK-6430 Nordborg, Denmark, lars.larsen@danfoss.com R. Izadi-Zamanabadi is with the Department of Electronic Systems, Aalborg University DK-9220 Aaborg, Denmark Previous work [5] has proven that using a hybrid Model Predictive Control (MPC) approach leads to a desyncronizing control scheme that reduces pressure variations and the number of compressor switches significantly. However, this approach leads to highly complex solutions and a resulting long computational time making the implementation of the solution somewhat impractical on a typical supermarket system. Motivated by these difficulties, we present in this paper two low complexity solutions for desynchronizing the operation of the display cases in a the supermarket refriger- ation system and hence improving the system performance. II. SUPERMARKET REFRIGERATION SYSTEM In a supermarket many of the goods need to be refrigerated to ensure preservation for consumption. These goods are normally placed in open refrigerated display cases that are located in the supermarkets sales area for self service. A simplified supermarket refrigeration circuit is shown in Figure 1. The heart of the system are the compressors. In most supermarkets, the compressors are configured as compressor racks, which consist of a number of compressors connected in parallel. The compressors supply the flow of refrigerant in the system by compressing the low pressure refrigerant from the suction manifold, which is returning from the display cases. The compressors keep a certain constant pressure in the suction manifold, thus ensuring the desired evaporation temperature. From the compressors, the refrigerant flows to the condenser and further on to the liquid manifold. The evaporators inside the display cases are fed in parallel from the liquid manifold through an inlet valve. The outlets of the evaporators lead to the suction manifold and back to the compressors thus closing the circuit. In Figure 2, a cross section of an open display case is depicted. The refrigerant is led into the evaporator located at the bottom of the display case, where the refrigerant evaporates while absorbing heat from the surrounding air Fig. 1. Basic layout of a supermarket refrigeration system with 2 display cases. 16th IEEE International Conference on Control Applications Part of IEEE Multi-conference on Systems and Control Singapore, 1-3 October 2007 WeC02.3 1-4244-0443-6/07/$20.00 ©2007 IEEE. 1414