Theoretical and experimental studies of impacts of heat shields on heat pipe evacuated tube solar collector

The heat pipe evacuated tube solar collectors, which is non-concentration, no-tracking and working at medium temperatures, suffer from low efficiency and thus rare application. The heat loss exerts serious negative influence on its performance at high operation temperatures. Thus, heat shields were introduced between the absorber plate and the glass tube to decrease the heat loss. Experimental and theoretical work were conducted to evaluate the thermal performance of the novel and traditional heat pipe evacuated tube solar collectors. According to the experimental results, the heat shields improved thermal efficiency at inlet water temperature of approximately 20 °C–150 °C, and the solar collector performed better at higher inlet water temperatures. The thermal efficiency of the novel one was enhanced by 11.8% at the inlet temperature and solar radiation of approximately 150 °C and 820 W/m2, respectively. Moreover, the novel one’s efficiency coefficients in the instantaneous efficiency curves, which can reflect the decrease rate of instantaneous thermal efficiency, resulted in decrease of 28.4% and 29.9%, respectively. Meanwhile, simulated results showed that, the thermal efficiency increments of novel collector over traditional one increased at weaker solar radiation and lower ambient temperature at each inlet water temperature.