Efficiency enhancement of a solar trough collector by combining solar and hot mirrors

Parabolic trough collector is an up-and-coming technology for urban energy supply. To further enhance the optical-thermal efficiency of the collector, a modified receiver enhanced by solar and hot mirrors was proposed. In this receiver, two symmetrical solar mirrors were designed to collect more solar irradiance, and a transparent hot-mirror shield was employed to lessen the thermal loss. After the design, the collector performance was studied by developing an optical-thermal coupled model. Based on the model, firstly, the proposed receiver was compared with the conventional receiver and two existing modified receivers. Results indicate the proposed receiver can achieve an optical efficiency of 78.02%, which is higher than those of the existing modified receivers and is just 0.22% lower than that of the conventional one. Moreover, compared with the conventional receiver, the two existing modified receivers cannot improve the optical-thermal efficiency at sufficiently high direct normal irradiance or relatively low fluid temperature, which can be overcome by the proposed receiver. Furthermore, compared with the conventional receiver, the proposed receiver can lessen the thermal loss by 11.2–89.6 W·m−1, thus improving the receiver efficiency and the optical-thermal efficiency by 0.03–5.26% and 0.03–4.70%, respectively, under the typical ranges of the inlet temperature (573–823 K) and the direct normal irradiance (0.3–1 kW·m−2). Then, parameter analysis indicates the hot mirror with low emissivity is necessary for efficiently improving the optical-thermal efficiency. Finally, the proposed receiver can improve the optical-thermal efficiency more effectively at higher fluid temperature or larger coating emissivity. The above results signify that the proposed receiver can be helpful for enhancing the efficiency of the trough collector.