Performance analysis and optimization of a solar-air source heat pump heating system in Tibet, China

Owing to its great potential for solar energy and uniquely low relative humidity, Tibet is an ideal region for the application of solar-air source heat pump (SASHP) heating systems. This study aimed to evaluate the performance and optimize the key design parameters of a SASHP in Tibetan climatic conditions. A SASHP parallel system was designed for providing space heating in office buildings, and was modeled and simulated in transient system simulation (TRNSYS) software. The simulation results showed that for a reference SASHP system, the solar collection efficiency, with a daily average value of 0.27 and a maximum value of 0.44, is low during the entire heating season. The solar collectors could afford a high percentage of heating (even 100%) at every initial heating period. However, the overall solar fraction for heating was only 42.79% for the entire heating season. The SASHP operated with a high efficiency for most of the heating period, with a maximum coefficient of performance (COP) of 2.68 and an average COP of 1.95. Furthermore, the hot water temperature of the water tank generally varied near 45 °C, indicating that the water temperature satisfied most of the heating requirements with regard to the heating time. In the performance optimization, the optimal values for the area of the solar collector, angle of the solar collector, and volume of the water tank were found to be 225 m2, 45°, and 30 m3, respectively. The average solar fraction increased to 0.793, an improvement of 85.3% over the reference system. Furthermore, orthogonal experiment results indicated that the solar collector area and water tank volume were the priority factors for the design of a SASHP system in Tibet.