Efficacy of natural circulation loop based parabolic trough collector- An analytical approach

Abstract

ABSTRACT Realizing the importance of the natural circulation loop (NCL), which is a passive heat transport device, an analytical study is undertaken to demonstrate the functioning of an NCL-based parabolic trough collector (PTC). Multiple validated analytical models were incorporated to evaluate the system performance. The parametric study involved variations in loop height, loop length, and solar radiation. The loop flow rate enhances by increasing its height or the receiver length. At maximum radiation, 1.3 kg/s flow rate was observed for loop geometry of 5 m height and 7.8 m length against 0.79 kg/s for 1 m height. The flow rate predicted for a 10 m length and 1 m height loop was 0.87 kg/s. Since instability rises with height and heat loss and receiver temperature increase with receiver length, a trade-off between the two is necessary to function NCL-based PTC properly. Otherside, though the higher heat output with an increase in the receiver length is evident, a noticeable drop in thermal efficiency (≈7%) during the peak hours of solar radiation is a matter of concern. Further, the outlet temperature was not affected by the loop height for the defined flow condition of the process fluid. Like in conventional PTC systems, the process fluid’s exit temperature or flow rate can also be maintained constant during a day operation by varying the flow condition. The proposed model can deliver ≈ 1500 liters of hot water in six hours with an average 68% thermal efficiency without external power, in contrast to the necessity of notable pumping power and associated overhead charges in conventional PTC. Further, the system’s stability was predicted using linear stability analysis, and stability maps were generated for different loop heights and found that as the loop height decreases, the unstable zone diminishes. Though the steady-state data points are in the unstable flow region, stable operation is expected by incorporating pertinent instability restraining technique, that is, loop tilt.