地埋熱交換器回填材料性能研究與分析

空調系統可透過淺層溫能作為冷源或熱源，提供室內冷卻或加熱負荷之需求，其中地埋熱交換器為空調系統與土壤熱交換之關鍵元件，是影響系統性能之主要因素。地埋熱交換器裝置常使用回填材料回填於套筒中，用於固定套筒內之U型管或螺旋管，而回填材料的性質對於地埋熱交換器系統有明顯影響。本研究使用目前常用的回填材料進行測試，建立一實驗室尺度的測試平台，用以模擬現地埋熱交換器系統，並以土壤熱響應方式(Thermal Response Test, TRT)進行實驗分析，探討不同回填材料對於系統之影響。此外，本研究亦使用相變材料(Phase Change Material, PCM)作為回填材料，因相變材料含有潛熱變化，可提高地埋熱交換器之儲存熱量，減少地埋熱交換器土壤影響範圍。最後，實驗測試結果可透過Kelvin線熱源理論推論出現地埋熱交換器系統之性能表現。本研究使用之回填材料為碎石、矽砂、皂土、皂土加石墨四種材料，相變材料則是石蠟RT27，其由35%月桂酸及65%癸酸組成。本研究比較PVC與鐵兩種套筒材質對於性能之影響，實驗結果顯示，當現地埋熱交換器之加熱量為60W/m時，PVC與鐵兩種材料的套筒壁溫度上升量分別為7.8°C及8.0°C，可得知套筒材質對於系統熱傳效益影響不顯著，故選擇耐用及抗腐蝕之材質即可。當採用傳統常見的回填材料進行測試時，在固定相同加熱量下，碎石、矽砂、皂土及皂土加石墨排至周圍土壤的熱量分別為396.4 kJ、393.2 kJ、414.3 kJ及431.4kJ，推論至現地埋熱交換器的套筒壁升溫分別為10.5°C、10.8°C、11.4°C及11.2°C，此說明以皂土為基底的回填材料有較佳之散熱性能，且皂土加入石墨後可少許提升熱交換器之散熱量。當使用相變材料作為回填材料時，因相變材料之潛熱特性，可使現地埋熱交換器的套筒壁升溫量減少，僅上升9.1°C，且可有效減少系統之土壤影響熱半徑(thermal radius)。因此，相較於傳統回填材料，使用相變材料可使土壤長時間維持低溫，使地埋熱交換器的熱傳能力不會因土壤溫度升高而下降，藉此拉長地埋熱交換器運作時間，減少土地使用面積。

關鍵字

淺層溫能；地埋管熱交換器；回填材料；相變材料；熱響應

並列摘要

The air-conditioning system can use shallow geothermal energy as a cold or heat source to provide indoor cooling or heating loads. Among them, the borehole heat exchanger is the key element of the air-conditioning system and the soil heat exchange, and it is the main factor that affects the performance of the system. Borehole heat exchanger devices often use backfill materials to backfill the borehole to fix the U-shaped tubes or spiral tubes in the borehole. The nature of the backfilling materials has a significant impact on the buried heat exchanger system. In this study, the commonly used backfill materials were used for testing, and a laboratory-scale test platform was established to simulate the heat exchanger system, and the soil thermal response test (TRT) was used for experimental analysis to analyze the influence of different materials on the system. In addition, this research also uses phase change material (PCM) as the backfill material. Because the phase change material contains latent heat, it can increase the heat storage of the borehole heat exchanger and reduce the soil influence range of the heat exchanger. Finally, the experimental test results can be used to infer the performance of the actual large-scale borehole heat exchanger system through the Kelvin’s line source theory. The backfill materials used in this study are crushed stone, silica sand, bentonite, and bentonite plus graphite. The phase change material is paraffin RT27, which is composed of 35% lauric acid and 65% capric acid. This study compares the effects of PVC and iron as borehole material. The experimental results show that when the heating capacity of the borehole heat exchanger is 60W/m, the temperature rise of the borehole wall of PVC and iron are respectively 7.8°C and 8.0°C, it can be known that the borehole material has no significant effect on the heat transfer efficiency of the system, so it should be chosen durable and corrosion-resistant materials. When using traditional and common backfill materials for testing, under the same heating amount, the heat discharged to the surrounding soil by crushed stone, silica sand, bentonite and bentonite plus graphite is 396.4 kJ, 393.2 kJ, 414.3 kJ and 431.4kJ, respectively. The temperature rise of the borehole wall of the actual borehole heat exchanger is deduced to be 10.5°C, 10.8°C, 11.4°C and 11.2°C, respectively. This result shows that the bentonite-based backfill material has better heat dissipation performance. After graphite is added, the heat dissipation capacity of the heat exchanger can be slightly increased. When phase change materials are used as backfill materials, due to the latent heat characteristics of the phase change materials, the temperature rise of the borehole wall of the borehole heat exchanger can be reduced by only 9.1°C, and the soil-influenced thermal radius of the system can be effectively reduced. Therefore, compared with traditional backfill materials, the use of phase change materials can keep the soil in low temperature for a long time, so that the heat transfer rate of the borehole heat exchanger will not decrease due to the increase of soil temperature, thereby lengthening the buried heat exchanger operation time, and decrease the use of land area.