太陽能空氣加熱器主要用於固體、農作物及海產之乾燥或室內溫度調節等等。研究太陽能空氣加熱器主要目的在提高其熱效率。目前已有許多不同型式之太陽能空氣加熱器被設計且提出；影響太陽能空氣加熱系統之集熱器效率的因素很多，其中不同吸收板型式與空氣加熱器通道之設計均能影響集熱器效率。 本文針對雙流型平板式、絲網式迴流型太陽能空氣加熱器，於一定的吸收板面積及相同操作條件下；在太陽能空氣加熱器系統，主要探討五個部份:一是迴流對總集熱器效率之影響，二是吸收板形式對效率值的影響，三是質量流率對效率值的影響，四是迴流型式對太陽能空氣加熱器的效率之影響，五是進口溫度對效率的影響。 研究結果顯示：太陽能空氣加熱器之總集熱器效率隨著迴流比及質量流率增加而增加。此外，在不同收集器中空氣加熱器效率提高百分率之高低依次為: 加絲網式，平板式。又在於不同質量流率下，質量流率越低則空氣加熱器效率提高百分率越高；而且四種迴流中，第三種迴流型式的經濟可行性是最好的。 An analytical study on the heat-transfer efficiency improvement of downward-type wire-mesh solar air heaters with external recycle has discussed. The purposes of this study were to enlarge the heat transfer area by attaching wire-mesh, to enhance the convective heat transfer coefficient by recycling operations, and to find out the best recycle types based on the heat-transfer efficiency. The thermal performance of recycling downward-type wire-mesh solar air heaters were also compared to that of the single-pass device and the flat-plate double-pass device with recycle. The mathematical formulation and analytical analysis to such a recycling wire-mesh double-pass solar air heater were developed theoretically. Two conflict effects are made by the recycle operations: the desirable effect of increasing the convective heat-transfer coefficient by increasing the air mass velocity in conduit, and the undesirable effect of decreasing the driving force (temperature difference) of heat transfer, due to remixing. The heat transfer efficiency can be improved while the desirable effect compensates the undesirable one. Conclusively, the recycling operations and the wire-mesh attached design can readily improve the heat-transfer performance of solar air heaters by comparing to the flat-plate double-pass and single-pass devices. An economic consideration in terms of the heat- transfer efficiency and power consumption increment for double-pass operation is also delineated.