Thermo-economic Performance Comparison of Sand-based Absorber Collector and Conventional Flat Plate Collector

Flat plate solar collectors are widely used for low-temperature heating, but heat transfer from the absorber plate to the riser tubes still limits their performance. In this study, the stagnant air under the absorber of a conventional collector was replaced with low-cost, locally available dried sand to improve this heat transfer. Two collectors of identical dimensions, one conventional and one sand-based, were built and tested side by side under real outdoor conditions at mass flow rates of 0.3, 0.4 and 0.5 kg/min over nine test days. Thermal performance was assessed in terms of outlet temperature, useful heat gain and instantaneous efficiency. An economic analysis was also carried out using the levelized cost of energy, return on investment and payback period. The sand-based collector gave an average outlet temperature 0.96–1.97 °C higher than the conventional collector, depending on flow rate, and about 20–35% more useful heat gain. Its efficiency was up to 11.42 percentage points higher, and the overall average efficiency improved by about 30%. The levelized cost of energy fell from 6.39 to 5.05 BDT/kWh, the return on investment rose from 14.32% to 18.90%, and the payback period was reduced from 7.0 to 5.3 years. The improvement is attributed to better conduction and thermal contact between the absorber plate and the riser tubes through the sand, together with the added thermal mass of the sand layer. The results suggest that the sand-based collector is a practical, low-cost improvement for solar water heating.