回填水型地埋熱交換器之性能模擬與分析

淺層溫能為一種綠色能源，利用地下淺層土壤終年恆溫的特性來進行熱交換，可有效減少溫室氣體的排放並達到節能之目的。在近年能源短缺及環境汙染日益嚴重的狀況下，淺層溫能技術逐漸受到重視。使用鑽井式地埋熱交換器(Borehole Heat Exchangers, BHEs)與地表土壤做熱交換的淺層溫能利用方式已經行之有年，目前大多數的系統都是採用U型管做為地埋熱交換器，以皂土或水泥等固態材料做為鑽井的回填材料，固態材料大多以熱傳導的方式與周圍土壤進行熱交換。而本研究將探討以液態水做為回填材料的狀況，期望透過強制對流或密度差所造成的自然對流現象提升熱交換的性能，並發展出另一種有別於傳統U型地埋熱交換器(U-shaped Borehole Heat Exchanger, UBHE)的大套筒型地埋熱交換器(Line Borehole Heat Exchanger, LBHE)。其中，大套筒型熱交換器兼具自然對流與強制對流的熱傳效益。本文主要以數值模擬的方式，透過計算流體力學套裝軟體Ansys Fluent分析回填水型的地埋熱交換器之熱傳性能，並輔以熱響應試驗(Thermal Response Test, TRT)的實驗數據驗證數值模型的準確性。模擬主要針對不同套筒材質、運行模式、操作流速和埋設深度等變化進行分析，並比較回填水型的熱交換器與傳統回填固態材料的熱交換器之性能差異。此外，亦針對新開發的LBHE進行參數分析，並從模擬結果歸納出提升LBHE性能的方案。研究結果顯示，套筒材質對UBHE和LBHE的熱傳效果有顯著的影響，使用鐵套筒的熱傳性能會高於使用PVC套筒，且在熱交換器進口流速越大的狀況下，套筒材質的影響越明顯。此外，模擬數據亦顯示間歇式運行模式能有效減緩土壤蓄熱所造成的熱傳衰退，尤其LBHE在使用間歇運行時能有較明顯的熱傳恢復效果。研究數據亦指出回填水型的熱交換器性能相較於回填土型的熱交換器有明顯的提升，尤其LBHE在不改變鑽井深度的前提下，可透過加長抽水管長度、抽水管外加絕緣層和縮短入水管長度等三種方式進行調整，使熱傳性能得到不同程度的增強。根據本研究模擬的案例可發現，使用LBHE在較短的鑽井深度下就可以達到與UBHE相同的熱傳效果，據估算在穩態時能節省12%的鑽井深度，能有效降低鑽井成本。

關鍵字

淺層溫能；地埋熱交換器；間歇運行；自然對流；強制對流

並列摘要

Shallow geothermal energy is a type of renewable energy that takes advantage of the stable temperature of the shallow ground to transfer heat. It could reduce the emissions of greenhouse gases and help reach the goal of saving energy. In recent years, energy shortage and environmental pollution have become worsen; therefore, the technique of shallow geothermal energy has increasingly attracted more attention. Using borehole heat exchangers (BHEs) to exchange energy with the ground has been the most common way to use shallow geothermal energy. The most typical type of BHEs is the U-shaped borehole heat exchanger (UBHE) backfilled with solid materials such as bentonite and cement. However, solid materials generally transport heat with the ground by conduction. This study focuses on the BHEs backfilled with liquid water, and it is expected that forced convection or natural convection driven by the density difference of the borehole water could lead to better heat transfer performance. This study also developed a newly innovative type of water-filled borehole heat exchanger called line borehole heat exchanger (LBHE) , which involves both natural and forced convection heat transfer mechanisms. In this study, the models of UBHE and LBHE were built and simulated by using the commercial computational fluid dynamics software Ansys Fluent, and the accuracy of the numerical models was validated by the thermal response test results. The effects of the borehole wall material, operation modes, inlet velocity and borehole depth were investigated, and then the performance of the water-filled BHEs was compared with the grout-filled UBHE. Finally, this research simulated various configurations of LBHE and summarized several methods of improving the heat transfer capacity of LBHE from the simulation results. The results show that the performance of the BHEs is significantly affected by the borehole wall material. The heat transfer rate for the use of iron as borehole material is larger than the use of PVC for both UBHE and LBHE, and it was observed that the impact of the borehole material is more significant with higher inlet velocity of the heat exchangers. It was also found that the intermittent operation mode can effectively alleviate the heat buildup in the ground, and the performance of BHEs is improved under the intermittent operation mode, especially for LBHE. The simulation results also indicate that the performance of the water-filled BHEs is better than the grout-filled UBHE. In particular, the efficiency of LBHE can be enhanced without increasing borehole depth by lengthening the outlet tube, adding thermal insulation on the outlet tube and shortening the inlet tube. According to the results of the simulation, it can reduce 12% of the required borehole depth that replacing UBHE with LBHE in the steady state condition, thus lowering the initial construction cost.

並列關鍵字

shallow geothermal energy ； borehole heat exchanger ； intermittent operation ； natural convection ； forced convection

參考文獻

[1]Energy Information Administration, Annual Energy Outlook 2021, U.S. Department of Energy, February 2021.

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