Experimental Investigations on Photovoltaic-Thermal Arrays Designed for the Use as Heat Pump Source

摘要

Uncovered, liquid-based photovoltaic-thermal (PV-T) collectors can be combined with ground-coupled heat pump systems for different purposes: to increase the evaporator temperature, to regenerate geothermal resources or for pre-heating domestic hot water. This paper experimentally investigates the behavior of a novel PV-T panel with enhanced ambient heat transfer properties and primarily conceived for the operation as the sole heat source for brine-to-water compression heat pumps. After presenting the results of the thermal efficiency measurement of the panel, which attest its unusual parameters (c_1= 23 W/m~2K, c_3 = 7.6 J/m~3K, c_6 = 0.1 s/m), we report on the long-term performance tests carried out on large roof-mounted PV-T fields at two different locations in Germany. The investigation focuses on the impact of real environmental and installation conditions on the performance of the field as ambient heat exchanger, operating at fluid temperatures below ambient air temperature. The results confirm the high overall heat transfer coefficient U of the field, up to 40 W/m~2K, which is reduced by about 20% in comparison to the U-value determined for the single panel, however. The reduction can be attributed to a large extent to the high sensitivity of the collector design to convective heat transfer mechanisms and to the different wind exposure during the tests. For the same reasons we report a 30% decrease in the U-value compared to the basic configuration by using joint plates to cover the gaps of the field with the aim of improving the reliability and aesthetics of the installation.