太陽能集熱器主要可用於社區及家庭用水之加熱、室溫調節、蒸餾系統及乾燥等用途；其優點是結構簡單、維護容易且較少故障。對太陽能集熱器效率的影響因素甚多，尤以集熱器之流體流動組數、流體總流量及迴流比值所造成的影響最顯著。 本文針對迴流型式的矩形通道之平板型太陽能集熱器來討論：(一)迴流效應、(二)集熱器之流體流動組數、(三)翅的數量、(四)流體通道的形狀、(五)矩形通道之縱橫比、(六)流體總質量流率、(七)進口溫度、(八)太陽光入射強度等參數對集熱器的影響，並將結果與傳統單行程之太陽能集熱器相比，且進一步討論其集熱效率提升率與水力消耗增加量，而得到經濟考量下之最佳設計與操作變因。研究結果顯示，矩形的流體通道會比傳統圓管之管板式太陽能集熱器有較佳的集熱效率，且加翅型及具迴流型比無翅無迴流型之太陽能集熱器有較高之集熱效率，最後由實驗結果來驗證數學摸型的正確性。 The study presents theoretical and experimental results of the device performance on flat-plate solar water heaters with rectangular flow ducts under recycle operation. Comparisons of the collector efficiency were made between the present device and the conventional type collector. The theoretical and experimental results show that the collector efficiencies can be effectively enhanced by using recycle operation. The theoretical and experimental results of flat-plate solar water heaters with rectangular flow ducts have been carried out with as the recycle ratio, number of pair ducts, number of fins attached, aspect ratio of flow ducts, mass flow rate, inlet fluid temperature and incident solar radiation as the operating and design parameters. The collector efficiencies for flat-plate solar water heaters with external recycle increase with increasing recycle ratio, mass flow rate and incident solar, but decrease with increasing inlet water temperature and number of pair ducts. The effects of the recycle ratio and number of fins attached on the collector efficiency enhancement are represented graphically, where the power consumption increment under such recycle operations has also delineated.