Thermo-hydraulic performance improvement inside parabolic trough receiver tube using passive techniques: A review

Parabolic trough collector (PTC) converts the incident solar radiation into useful energy used in many applications. Researchers in the past two decades focused on improving thermo-hydraulic performance inside a parabolic trough receiver (PTR) using various passive methods. These methods do not use an external power source to enhance the heat transfer. Twisted tapes, screws, rings, rods, and fins are passive turbulators inserted into the receiver tube to enhance thermo-hydraulic performance of PTR. On the basis of a study of published research, the present review evaluates, in depth, these passive methods used in enhancing the performance of a PTR and the consequent overall thermal performance of PTCs. Among the most remarkable findings, the ratios of the Nusselt numbers ($∆$Nu) under different parameters varied between 1.1 and 7.42 with the values of the performance evaluation criteria (PEC) falling between 0.44 and 1.73 in various turbulator set-ups. As an example, a 2.5-fold enhancements in 5Nu are attained with twisted tapes with optimum twist ratios (23) and full porous discs that have up to 60 % enhancements in PEC. The heat transfer is also enhanced further by longitudinal fins (10–15 mm length, 2–4 mm thickness) with approximate values of 10Nu of 3.2. These results show the possibility of verification of a passive method to as well enhance the PTC efficiency largely. The review can be helpful to the researchers trying to maximize the utilization of solar energy in PTC systems.