Optimizing discharge pressure control in carbon dioxide heat pumps using particle swarm optimization

The provision of hot water constitutes a substantial fraction in the composition of household energy consumption. The deployment of carbon dioxide (CO₂) heat pump technology for water heating shows notable benefits, which are critical for enhancing the energy performance of buildings. An effective control strategy for CO₂ heat pump systems is indispensable for stable operation, system safety, user comfort, and optimal energy conservation. However, the majority of existing control strategies primarily investigate system performance under steady-state conditions, thus limiting their practical applicability. Consequently, this study conducts a dynamic experiment of a circulating heating heat pump water heater system. The findings indicate that maintaining a discharge pressure of 8.5 MPa during the heating stage can sustain the system’s coefficient of performance (COP) close to 3. Conversely, increasing the discharge pressure beyond 9.0 MPa during the frequency reduction stage mitigates the COP’s decline. During the start-up stage, the compressor speed is increased from 50 to 60 rps to expedite the start-up process in the shortest possible time (120 s), while ensuring system safety. Based on the experimental results, a proportional–integral–derivative (PID) control strategy is introduced, integrating both electronic expansion valve and compressor regulation. This strategy enhances total heating capacity by 20 % and the overall COP by 12 %. Subsequently, the PID parameters are optimized using the Particle Swarm Optimization (PSO) algorithm to achieve precise control and minimize overshoot of target parameters. Experiments evidence the superiority of the optimized PSO-PID control strategy, with a stable stage deviation of less than 0.1 MPa.