Improved superheat control of variable-speed vapor compression systems in provision of fast load balancing services

Utilization of renewable energy resources, such as solar and wind, has seen rapid growth on the power grid worldwide. To mitigate the adverse impact of these volatile generation on the reliability of the power grid, fast balancing services are increasingly procured by system operators from various resources, such as flexible airconditioning and space heating loads. Recent work of the authors has demonstrated technical feasibility and various benefits of using variable-speed vapor compression equipment for two fast load balancing services, namely frequency regulation and solar photovoltaic power smoothing. A major control issue was also identified in prior experimental tests: the drastic changes of the compressor speed for fast load balancing have caused severe superheat regulation issues including oscillations and even wet compression. To this end, a gain-scheduled feedforward and proportional integral (PI) controller is proposed and presented in this paper, which uses the real-time compressor speed in determining compensating control actuation of the expansion valve to mitigate the disturbance effect. Both simulation and experimental validations of the proposed control strategy were carried out with a 3-ton variable-speed heat pump. Test results have shown that the proposed controller is effective in improving the superheat regulation performance for three load balancing scenarios, i.e., compressor speed step changes, frequency regulation and photovoltaic smoothing. The demonstrated performance gains include fast post-disturbance recovery of the superheat with the settling time shortened by 63% to 83%, complete avoidance of wet compression and enhanced transient energy efficiency.