Heat transfer enhancement in parabolic trough collectors: A comprehensive review

Improving the performance of solar collectors has been recently a subject of intense research because of its advantages such as a decrease in the size and cost of systems and an increase in the thermal performance. Among solar collectors, parabolic trough collectors (PTCs) are of great importance because of their applicability. PTCs are widely used in Concentrated Solar Power Plants (CSP), and Industrial Process Heat (IPH), which demand high and low-temperature heat, respectively. Thermal oils have been extensively utilized as the working fluids in solar thermal electric plants (STE) with PTCs. However, their operating temperature limitation and environmental pollution have caused employing alternative working fluids. The present study is conducted to evaluate the thermal efficiency enhancement methods in PTCs. The effect of parameters such as absorber tube coating and design parameters on the performance of PTCs is also discussed. Moreover, passive methods for heat transfer enhancement in PTCs are investigated and the effect of using nanofluids as a working fluid is reviewed. This review can open new horizons for further investigations in this field.