Extremum Seeking Control of Hybrid Ground Source Heat Pump System

The Hybrid Ground Source Heat Pump (GSHP) system features the combination of ground heat exchanger (GHE) and cooling tower for rejecting the cooling load, which enables the use of the renewable resource of geothermal energy while reducing the cost of installation for applications of high cooling demand. However, model based control for efficient operation of the GSHP system can be intricate due to variations in ambient conditions and equipment characteristics, as well as the cost, accuracy and reliability of sensors required. In this study, a nearly model-free self-optimizing control strategy is proposed for the efficient operation for GSHP based on the Extremum Seeking Control (ESC) scheme. The proposed ESC scheme is based on the feedback of the total power consumption of heat pump compressor, cooling tower fan, and water pump of ground loop and the control inputs are cooling tower relative flow rate. The heat pump is controlled with an inner-loop proportional-integral (PI) controller to regulate the evaporator0 leaving water at 7C. The proposed control strategy is simulated on a Modelica based dynamic simulation model. The vertical GHE model is adopted from the Buildings Library developed by Lawrence Berkeley National Laboratories (LBNL), for which the transient heat transfer process is implemented with the finite volume method inside and outside the borehole. Simulation was conducted for a small office building in Dallas area under two scenarios: one is a change in evaporator inlet water temperature (i.e. load change), and the other is a change in ambient air condition. The simulation results demonstrated the effectiveness of the proposed ESC strategy, and the potential for energy saving is also evaluated.