A spectral self-regulating parabolic trough solar receiver integrated with vanadium dioxide-based thermochromic coating

A significant decrease in the solar-thermal conversion efficiency of parabolic trough collectors occurs at high operating temperatures, mainly due to the massive radiant heat loss from the parabolic trough solar receiver incurred in this case. In this paper, a novel parabolic trough solar receiver integrated with vanadium dioxide-based thermochromic coating is proposed to effectively reduce the radiant heat loss and improve the overall performance of solar receivers. The thermochromic layer, deposited on the glass envelope's inner surface in the negative thermal-flux region, has a reversible transition from a monoclinic (M) phase to a rutile (R) phase at the critical temperature of 68 °C. The occurrence of the phase transition in the thermochromic coating induces beneficial self-regulation of the spectrum-selectivity characteristics of the glass envelope, i.e., transmittance, reflectance, and absorptance (emittance) of glass envelope to solar and infrared radiation, for enhancing the thermal performance of solar receivers. Comprehensive heat transfer models based on the finite volume and spectral radiant heat transfer methods are established in this study. It is validated the models can predict the thermal performance of the solar receivers with high accuracy. In this framework, the total heat loss, thermal efficiency, and exergy efficiency of parabolic trough collector systems integrated with the proposed novel solar receivers are investigated and analyzed. Besides, the effects of transmittance of thermochromic coating (M phase) and emittance of thermochromic coating (R phase), two key parameters, on the overall performance of proposed solar receivers are also studied. The results show that no matter under the M phase or R phase, the thermochromic coating can play unique advantages to improve the thermal performance of the proposed solar receivers. The heat loss and thermal efficiency of the proposed solar receivers are effectively reduced and enhanced by 18.2% and 4.8% at the absorber temperature of 600 °C and inlet fluid temperature of 580 °C, respectively.