Applicability of Heat Mirrors in Reducing Thermal Losses in Concentrating Solar Collectors

Abstract

Flux distribution around the parabolic trough receiver being typically non-uniform, only a certain portion of the receiver circumference receives the concentrated solar irradiance. However, radiative and convective losses occur across the entire receiver circumference. The present work attempts to introduce the idea employing transparent heat mirror to effectively reduce the heat loss area and thus improve the thermal efficiency of the solar collector. Transparent heat mirror essentially has high transmissivity in the solar irradiance wavelength band and high reflectivity in the mid-infrared region thus it allows the solar irradiance to pass through but reflects the infrared radiation back to the solar selective metal tube. Practically, this could be realized if certain portion of the conventional low iron glass envelope is coated with Sn-In2O3 so that its acts as a heat mirror. The spatial flux distribution around the receiver dictates the region of the glass envelope which needs to be coated with Sn-In2O3 so that thermal losses could be significantly reduced without hampering the optical efficiency of the collector. In the present study, a parabolic receiver design employing the aforesaid concept has been proposed. Detailed heat transfer model has been formulated. The theoretical modelling results have been compared to the results pertinent to a corresponding conventional concentrating parabolic solar collectors in the literature. It was observed that under similar operating conditions the heat mirror-based parabolic trough concentrating solar collector has about 3-12% higher thermal efficiency as relative to the conventional parabolic trough design. Furthermore, steady state heat transfer analysis reveals that depending on the solar flux distribution there is an optimum circumferential angle ( ), where θ is the heat mirror circumferential angle) up to which the glass envelope should be coated with Sn-In2O3. For angles higher than the optimum angle, the collector efficiency tends to decrease owing to increase in optical losses. Finally, the analysis revealed that the magnitude of efficiency enhancement is pronounced at high receiver temperatures. Thus as a whole, the proposed receiver design promises higher thermal efficiencies and lower thermal losses as compared to the conventional parabolic trough receiver designs particularly at high receiver temperatures.