A novel evacuated receiver improved by a spectral-selective glass cover and rabbit-ear mirrors for parabolic trough collector

Highlights: • A trough receiver with a spectral-selective cover & rabbit-ear mirrors is proposed. • Receiver efficiency can be improved at any solar irradiance or temperature. • Receiver optical loss can be reduced by 13 W per meter length receiver. • Absorber thermal loss can be reduced by 3.3–41.6 W per meter length receiver. • Increase in the receiver efficiency of 0.30–2.72% is gained at typical conditions. Improving the efficiency of the evacuated receiver, which is a vital element of the parabolic trough solar plant, can increase the plant performance significantly. To improve the receiver efficiency, four novel modified receivers were designed. In each modified receiver, a spectral-selective glass cover partially coated by a heat-reflecting film on its top half was designed to decrease the thermal loss from the absorber. Meanwhile, two rabbit-ear mirrors were employed to concentrate some solar rays lost by the traditional receiver. After the design, the optical and thermal characteristics of the modified receivers were analyzed by using ray tracing method and computational fluid dynamics. Results indicate that the mirrors are effective for improving the optical efficiency. Furthermore, the heat-reflecting film on the cover inner wall was found to be effective to lessen the thermal loss, but the film on the cover outer wall not only cannot lessen the thermal loss but also causes additional optical loss. Moreover, it was found the modified receiver that only employs the inner film and the mirrors can obtain higher optical and thermal efficiencies than other modified receivers and the traditional receiver. Therefore, it was suggested as the optimal design. Then, simulation indicates the optimal design can enhance the receiver efficiency under any fluid temperature or solar irradiance compared with the traditional receiver. When the inlet fluid temperature is within 573–823 K, and the direct normal irradiance is within 300–1000 W·m⁻², the optimal modified receiver could increase the receiver optical efficiency by 0.23% and reduce the thermal loss by 3.3–41.6 W·m⁻¹(3.1–6.1%). As a result, it can improve the receiver efficiency and the collector efficiency by 0.30–2.72% and 0.28–2.56%, respectively. In addition, parameter study indicates the emittance of the heat-reflecting film has significant effects on the relative reduction in the thermal loss. Therefore, it is expected to be as low as possible for improving the efficiency to the best extent possible. The proposed receiver and results from this study should be useful for promoting the performance of the trough plant.