Impact of suction and injection gas superheat degrees on the performance of a residential heat pump with vapor injection and variable speed scroll compressor

The first part of this paper focuses on the experimental results collected from a vapor injection and variable speed scroll compressor air-to-water residential heat pump. The unit is a 10 kW residential system working with R410a and capable of providing floor heating and domestic hot water. It is tested in a controlled environment in order to achieve a wide range of outdoor and indoor conditions. The impact on the system performance of the vapor superheat degrees at both injection and suction ports is discussed. It is shown that a better control of these variables could improve the system COP and heating capacity by respectively 10 and 15%. It is also shown that the control of the superheat degrees is a coupled problem and the use of standard gain-scheduled (Single Input Single Output) SISO proportional–integral–derivative (PID) controllers is not optimal.
The second part of this paper presents a multivariable decoupler-based proportional–integral (PI) controller using feedforward action in order to take into account the coupling between both superheat controls. System models are first presented, the first one is a MIMO (Multiple Input Multiple Output) first order transfer function based model in order to identify the decoupler and PI controller gains. The second one is a non-linear dynamic model of the injection line in order to first test the proposed controller in simulation. Finite-volume models are used for the heat exchangers and the split lines. A thermodynamic model of the vapor injection scroll compressor is developed using empirical correlations for the volumetric efficiency, isentropic efficiency and the ratio between the injection and suction mass flow rates. Three adapted multivariable controllers are tested in simulation and compared to a baseline PI controller. All proposed controllers show increased performances compared to the SISO PI controller.