Optimal Energy Management of Grid-Connected Solar-Heat Pump Hybrid Water Heating System

This paper presents an optimal energy control strategy for hybrid solar thermal/heat pump water heaters to address the issue of energy conservation commonly associated with conventional water heaters in residential applications. The pervasive use of fossil-fuel-based water heating technology incurs significant energy costs and contributes to the climate crisis. These have sparked a lot of interest in using renewable technology such as solar water heaters (SWHs). Amidst this, solar radiation intermittent and low efficiency due to PV cell temperature rise have been identified as a limitation to using SWHs. One potential solution to these issues is to explore hybrid technology, which would replace conventional heating technology with a more energy-efficient hybrid solar/heat pump water heater. The main objective of the controller is to reduce the electricity cost by optimizing the operation of the heat pump water heater. A medium-density family in the southwest of Nigeria is selected for the study. The mathematical model was formulated based on the energy balance equations of the sub-system components and then discretized using the Euler forward method. Furthermore, a cost optimization problem with continuous and binary control variables was developed, resulting in a mixed integer problem. This optimization problem was solved using the OPTI toolbox's mixed integer nonlinear programming (MINLP) optimization solver and the solving constraint integer programs (SCIP) algorithm. Simulations were run in MATLAB, and the results show that the optimal control (OC) has a potential energy savings of 61.14% and a cost savings of approximately 70.83% compared to the baseline model. Better utilization of the hybrid system with the OC strategy saves 45.4% more energy than conventional water heaters.