Experimental studies of thermal performance of an evacuated tube heat pipe solar collector in Polish climatic conditions

This work presents the results of experimental studies on the energy performance of an evacuated solar collector, heat pipe type, consisting of 24 tubes, over the period of 2 months. The solar collector with a gross area of 3.9 m(2) is part the solar hot water test system located in Lublin (Poland). The effect of the weather conditions and operating parameters on the thermal and exergy efficiencies of the evacuated tube solar collector has been defined. The solar irradiation per month for July amounted to 80 kWh/m(2), and for August, it equalled 112.8 kWh/m(2). The average thermal gain was found to be in July 163 W/m(2) and in August 145 W/m(2), respectively. For the considered study period, the average value of energy yield in the solar collector was obtained at the level of 4.28 MJ/(m(2)·d). The average monthly energy efficiencies of the solar collector in July and August were 45.3% and 32.9%, respectively, while the average monthly exergy efficiencies reached 2.62% and 2.15%, respectively. Increasing the wind speed to 0.86 m/s decreases the thermal efficiency and the exergy efficiency by 67% and 41%, respectively.