Performance investigation of direct-expansion solar mediumand high-temperature heat pump systems with vacuum tubes
With the continuous growth of global energy demand and increasing emphasis on environmental protection, solar energy has garnered significant attention due to its clean and renewable characteristics. Solar thermal utilization technology has consequently emerged as a key research focus in sustainable energy conversion. The direct-expansion solar-assisted medium- and high-temperature heat pump (DX-SAMHTHP) system integrates the advantages of solar thermal utilization and medium-/high-temperature heat pump technology. It efficiently upgrades low-grade solar thermal energy into high-grade heat energy, suitable for industrial heating, domestic hot water, and other fields. This paper presents the design and construction of an experimental bench for a vacuum tube-based DX-SAMHTHP system in a laboratory setting. This experiment adopted a configuration with two vacuum tube collector-evaporators connected in series, using R1233zd(e) as the refrigerant. The thermal output temperature was set at 70-85 ?, and the system was experimentally investigated under multiple operating conditions to analyze its specific effects on energy performance. The effects of solar irradiance (564.9-1123.6 W·m-2), heat output temperature (70-85 ?), ambient temperature (28.31-32.15 ?), and collector area (1.5 m2, 3.0 m2) on the system were investigated using the coefficient of performance (COP) and collector efficiency as performance indicators. The results demonstrate that the proposed system can stably supply heat at 70–85 °C with a COP of up to 6.11, validating its technical feasibility. A key finding is the inherent trade-off between system COP and collector efficiency when increasing the collector area or solar irradiance. This work provides a foundational experimental study for the development of efficient solarassisted medium-to-high temperature heat pumps.