U型地埋管熱交換器在外部強制對流下之性能增益研究

Abstract

隨再生能源的興起，淺層溫能之應用被視為有效的節能方法。淺層地表之溫度相較於地表附近環境溫度，溫度變化較小，其熱容量相當可觀。本研究於地表鑽孔，至地表下深度25m，並埋置U型之PVC管做為熱交換器。管內以水做為工作流體，可將VRF、熱泵…等之冷或熱帶走，與地下水隔PVC管做熱交換。 本研究著重於當對U型管外抽水，井內強制對流對系統性能的助益。以殼管式熱交換器模型之雷諾數Re為基礎，分析其與U型管外部熱對流係數h_o之關係。同時，由於有上述所言對井內抽水之設計，為避免抽水流率過大，造成井內水面不斷下降，或使得熱交換器與地下水接觸的面積減少過多，分析井內地下水因抽水造成之遞降曲線有其必要性。 最後，在本研究的實驗之中，抽出水跟補注水的溫差可達0.7℃，向井外抽水在流量20LPM時帶走了熱源輸入C.V.中熱能達38%。此外，與未使用管外抽水系統做比較，系統因管外抽水系統減省了21%的鑽井深度。而Re與h_o之關係趨勢線採乘冪形式，為h_o=114.81〖Re〗^0.1876。

Shallow geothermal Energy has been considered an effective way to save energy as renewable energy becomes popular. The thermal capacity of shallow earth is considerable. Its change of temperature is small relative to the surrounding temperature above the ground. A borehole is drilled in the experiment of this study in a depth of 25 meters, and U-type PVC tubes are placed as heat exchanger. Water as the work fluid can exchange heat with groundwater and facilities like heat pumps or VRFs. This study focuses on the efficiency improvement because of some forced convection in the well. Definition of Reynolds number in Kern Method is adopted to show the relation between the forced convection and the convection coefficient outside the U tube. Meanwhile, to avoid excessive pumping, analyzing the dropping curve because of pumping is necessary. Last, in the experiment of this study, the temperature difference of extracted water and compensating water reached 0.7℃, while 38% of injected energy is carried away because of the pumping system. Besides, compared with the system with no pumping, this system reduced the required borehole length to21%, and the relation between Re and h_o is h_o=114.81〖Re〗^0.1876。