Performance optimization and analysis of solar combi-system with carbon dioxide heat pump

• We carry out a performance analysis on CO2 heat pump in solar combi-system.
• We carry out a multi-parameter optimization design for the solar combi-system.
• The average heating COP of HP was 2.38 for the entire heating season.
• The solar fraction of proposed heating system was 68.95%.
• The performance of the heating system was also discussed for net zero energy aim.

In this paper, a solar combi-system which consists of solar collectors and a carbon dioxide heat pump is proposed and investigated through simulation and optimization. Performance analysis and comparison are primarily conducted to show the feasibility and reasonability of using a CO2 heat pump as an auxiliary heater under local weather conditions. Then, a system model with a test building in TRNSYS is developed for performance optimization. The most influential variables are identified using influence and sensitivity analyzes of single parameters. Subsequently, a multi-parameter optimization using the high-weight parameters is carried out to obtain a final design result. The simulated results of the optimized case show that the average coefficient of performance of the CO2 heat pump is 2.38, and the solar fraction of the system is 69.0% for the entire heating season. The time when a comfortable temperature level can be achieved in the indoor environment accounts for 81.6% of the entire heating season. Furthermore, the performance characteristics of the proposed system are evaluated in terms of the thermal balance, fraction of the thermal energy saving, feasibility of net zero energy, economic factor, and CO2 emissions reduction.