Simulation Study of Solar Flat Plate Collector Absorber Plate over the Primary Risers with Different Heights of External Radial Rectangular Fins

Abstract

Solar water heaters are an established renewable-heat technology, although traditional flat-plate collectors suffer moderate thermal efficacies and heat losses, which may additionally be enhanced by operation and therefore encourage absorber-plate heat-transfer enhancement through geometric alterations, including fins. The aim of the numerical study was to isolate and quantify the effect of transverse fin height over the primary risers on the thermohydraulic performance to identify a suitable design that serves to optimize the useful heat gain without compromising on the acceptable hydraulic behaviour. A transient, pressure-based, finite-volume CFD model with conjugate heat transfer was simulated with automated, adaptive mesh refinement cut-cell meshing and grid-independence benchmarking on 3D absorber configurations (0, 5, 10, 15, and 20 mm fin height). A time-dependent temperature boundary of the top surface was considered to represent heating by the sun, and the remaining surfaces of the plates were maintained at 300 K; the simulations were done between 0-21,600 s (10:00-16:00). The increase in height of the fin increased normalized surface area in a monotonic way (40% at 0 mm and 100% at 20 mm; geometric enhancement of approximately 150%), whereas thermal performance was non-monotonic because of the fin-efficiency and conduction-resistance. The highest normalised efficiency is obtained on 15 mm fins (5% above the baseline), and maintaining Thermal efficiency About 38% During quasi study Time alongside (~13.1 kJ/kg of maximum specific exergy. 20 mm Fins minimized heat losses the most and have better temperature-field uniformity, but with diminishing returns and lower efficiency than 15 mm, in line with the decreasing fin effectiveness and increasing external losses. On the whole, the optimization in the absorber is dictated by thermogeometric considerations instead of surface-area considerations and 15 mm was found as the best fin height in the explored range to obtain maximum energy and exergy efficiency when operating under transient conditions.