Performance of the unglazed solar air collector with air curtain jets

This study aims to address the issue of reduced thermal efficiency of traditional Unglazed Solar Air Collectors（USAC）under crosswind conditions. The low specific heat capacity and high flow velocity of air in flat-plate solar air collectors with covers often lead to insufficient heat transfer. A novel USAC with an air-curtain jet is proposed to enhance the collector's ability to resist crosswind interference and improve its thermal performance. A combination of numerical simulation and experimental methods was adopted. Initially, SolidWorks and FLUENT were used to conduct numerical simulations to determine the reasonable range of slot widths for the air-curtain jet structure. Subsequently, an experimental platform was constructed, consisting of a porous aluminum plate, slotted rectangular air ducts, insulation boards, and so on . The experimental setup was placed in a controlled laboratory environment at Yanshan University. A TRM-PD1 solar simulator provided a stable radiation intensity of 700 W/m². A hot-wire anemometer was used to monitor the outlet flow rate, maintaining it at 0.018 kg/s. An Agilent DAQ970 recorded temperature data at 20 measurement points on the collector plate surface at 10-second intervals. To further reduce the influence of the environmental temperature, the experiments were conducted from 5:00 p.m. to 10:00 p.m. every day, and the experimental period lasted for one month. The results show that the air-curtain jet improves the collector's thermal performance significantly. When the air-curtain jet is applied, the pre-heated air absorbed by the small holes on the collector plate increases the initial temperature of the inlet air, enhancing the heat utilization efficiency of the collector plate. The double-sided jet demonstrates the best performance, achieving the highest outlet temperature (33.12℃) at the lowest jet velocity of 1.2 m/s. The optimal structural are determined as a slot width of 10 mm and a double-sided jet mode. Under a crosswind speed of 0.6 m/s, the air-curtain jet increases the collector efficiency by 14.57%. However, as the crosswind speed increases to 1.2 m/s and 1.8 m/s, the efficiency improvement decreases to 10.64% and 7.82%, respectively. Additionally, the study reveals that both extremely narrow and wide slot widths are unfavorable for heat collection. Narrow slots result in insufficient heat absorption due to thin jet layers, while wide slots cause heat loss as the upper part of the thick jet layer diffuses into the atmosphere. Under crosswind conditions, the double-sided jet with a slot width of 10 mm exhibits the best performance in enhancing the thermal efficiency of the collector. Moreover, as the crosswind speed increases, the effect of the jet on improving the thermal efficiency gradually decreases, and the jet velocity required to make the outlet temperature reach the optimal value increases accordingly. The significance of this study lies in the fact that the proposed air-curtain jet provides a new approach for USAC to resist environmental interference.