Adoption of simplified parabolic trough collectors for domestic hot water applications: numerical and experimental investigation

Compared to evacuated systems, stationary and non-evacuated parabolic trough collectors (PTCs) are less expensive and require less maintenance, making them a potential option for home hot water applications. But without vacuum insulation and sun tracking, thermal heat losses increase and efficiency declines. This study proposes tube geometry modification for a low-cost, small-scale, non-tracking, and non-evacuated PTC to overcome these limitations. A simple (R1), a helical tube (R2), and sinusoidal tubes oriented perpendicular (R3) and parallel (R4) are examined to adopt this technology for hot water production systems. Using a combination of numerical and experimental methods, 3D Computational Fluid Dynamics (CFD) simulations are coupled with Monte Carlo Ray Tracing (MCRT) optical analysis. After the validation of numerical models against experimental data, CFD results are used to compare the performance of various geometries. Results revealed the distinct role of receiver geometry in driving air movements between the tube and the glass cover. Moreover, all the modified receivers reflect an increased thermal performance compared to the conventional tube design, while the pressure drop penalty also increases. The helical receiver displays the strongest internal fluid mixing, achieving the maximum water outlet temperature rise. Moreover, the R2 tube outperformed other designs, achieving the maximum thermal efficiency of 40% and exergy efficiency of 2%. In addition, the helical receiver improves the average N u by 360% and the overall efficiency by 38.6% compared to the simple design. As a result, the helical receiver was selected as the optimum design for boosting the efficiency of the stationary non-evacuated PTC system.