Scheduled feedforward control of superheat through hardware-in-the-loop load emulation

Variable speed refrigeration systems have the potential for improved energy savings compared to conventional on-off systems. A single-input-single-output (SISO) control structure does not adequately regulate control parameters due to strong coupling inherent to the vapor compression cycle (VCC). With the use of an electronic expansion valve (EEV) a feedforward control configuration may be implemented to remove the effects of compressor speed changes on evaporator superheat, thus allowing better regulation during transients. Due to the nonlinearity of the VCC a scheduled feedforward compensator is proposed to effectively reject a compressor disturbance over the wide range of operating conditions typically experienced by variable speed systems. The proposed control structure allows superheat regulation at a low level without risking compressor damage, therefore helping maximize system efficiency. This paper presents experimental results obtained through load emulation. This is a novel approach which performs a task similar to the automotive dynamometer, where various size loads and environmental conditions may be placed on a VCC through an on-line experimental simulation.