利用淺層溫能之地道風節能系統數值模擬與性能研究

Abstract

地下淺層溫能空氣熱交換器(Earth air heat exchanger)是一種使用地表下的穩定溫度作為熱傳主體的低耗能外氣節能系統。其理論與實驗在全世界有許多的研究，但在台灣，其相關研究則相當稀少。本論文主要藉由數值模擬分析、探討，分別針對土壤式空氣熱交換器與水式空氣熱交換器進行研究。土壤式空氣熱交換器所進行的模擬主要設施包括了四支52.95 m長、直徑為0.15 m並埋於4.7 m深度的PVC空氣管道，規劃風量為3150 CMH，通風時間設定為9:00至21:00間啟動系統進行通風。水式空氣熱交換器則為三支75m長、直徑為0.15m並埋於9.5m深度的PVC空氣管道，以及規劃風量為990 CMH，通風時間為9:00至21:00間啟動系統通風。 本研究將使用ANSYS Fluent version 16.0進行模擬分析，得出系統隨時間之各物理狀態，包含進出口溫濕度變化，以計算出各地道風熱交換系統之節能效益。針對筏基水溫能系統與土壤溫能系統之各初始參數，設定為入風量、全年四季之引用外氣溫度、地道風系統隨外氣條件改變之入口相對濕度，經由計算結果得出全年各季引入外氣條件下，全年之地道風系統出口乾球溫度、系統出口相對濕度、熱傳導量、筏基水溫度分佈、風管周邊2 m的土壤溫度分佈。其中，可透過熱傳導量得知節能效率，例如本系統於夏季之冷卻效能與冬季之暖氣效果，以利往後相關系統於各場域之規劃，配合選定之場域氣候特徵，改變與調整相關系統之設置；亦可透過濕度之變化，得知潛熱熱傳對於本系統節能效率之佔比，以利往後相關系統之場域選定，以加強其節能效率。經由分析筏基水溫度分佈，可得出本地道風系統所影響之周圍筏基水庫，與系統最佳效率運轉週期，以維持周圍筏基水庫之較低水溫與較高之熱傳導率，保持本系統之節能效率。經由分析周遭土壤溫度分佈，可得出本地道風系統所影響之土壤範圍與其溫度影響幅度，規劃較合適之回填土種類，以維持周圍土壤之較低溫度、較高熱傳導率、以及較高自我溫度調節能力，保持本系統之最高節能效率。 透過數值模擬分析，對於管材鐵、銅、鋁、PVC進行探討，了解管材對於本系統之影響，以選定更為合適之管材，增加熱傳導效率與節能效率。亦可利用風管內引用外氣的溫度分佈，得出有效熱交換長度以利往後其他案例之應用；此外，雖然入風速增加可提升熱傳能力，但外氣與管材之熱交換時間也隨著減少，因此，透過數值模擬分析並得出最佳熱傳量之風速。本研究將對於風速2 m/s、5 m/s、9 m/s、12 m/s進行系統效能探討。最後，本研究探討筏基水溫21℃、23℃、25℃與其水量全滿、0.75滿、半滿，對於系統節能效率之影響，與系統停機時，水溫之回復能力。藉由以上設置，可透過模擬計算本系統之空氣溫濕度調節能力、本系統於各季之節能效率、系統最佳效率運轉週期、管材對於本系統之影響、有效熱交換長度、周圍土壤之影響範圍與較適合之回填土種類、最佳熱傳導量之風速、水溫與水量變化之影響。

The earth air heat exchanger (EAHE) is a low-energy ventilation technique using the stable temperature of earth. In the past decade, many experimental studies deal with the performance evaluation of EAHE system worldwide; however, in Taiwan these kinds of researches are still uncommon. This research focus on the performance of soil-based EAHE and water-based EAHE using numerical methods. Soil-based EAHE includes 4 PVC air pipes each with 52.95m in length, 0.15m in diameter and buried 4.7m beneath ground surface. Its arranged ventilation is 3150 CMH, working 9 a.m. to 9 p.m. Water-based EAHE consists of 3 PVC air pipes each with 75m in length, 0.15m in diameter and buried 9.5m underground. Its arranged ventilation is 990 CMH, working 9 a.m. to 9 p.m. This research uses ANSYS Fluent for simulating various system status over time. Initial conditions are ventilation, seasonal introduced air temperatures and seasonal introduced air humidity. After simulation, EAHE system’s seasonal outlet temperature, outlet humidity, heat transfer rates, temperature distributions in raft foundation water and soil of 2 meters around air pipes can be generated. Through heat transfer rates, seasonal energy saving efficiency can be recognized, such as cooling load in summer and heating load in winter, as well as the percentage of latent heat transfer rates through the examination of humidity difference. This analysis benefits future applications for selecting suitable areas to adjust and improve the energy saving efficiency of EAHE systems. By analyzing raft foundation water temperature distribution, the range of water affected by EAHE and its most efficient operating period can be recognized to maintain stable water temperature and its heat transfer rate for preserving EAHE’s energy saving efficiency. Through analyzing surrounding soil temperature distribution, the range and extent of soil affected by EAHE can be realized. Selecting suitable soil allows soil to maintain its stable temperature, heat transfer rates and its self-recovering ability to sustain EAHE’s energy saving efficiency. From simulation results, the influence of pipe materials, such as stainless steel, copper, aluminum and PVC on the performance of EAHE can be known, thus suitable pipe materials can be determined. From temperature distribution of introduced air within air pipes, effective length for regulating can be discerned for future applications. Besides, although increasing airflow velocities promotes heat transfer rates, heat exchange time between introduced air and air pipes also decreases. Therefore, through simulation, the optimal airflow velocities are discovered. Airflow velocities of 2m/s, 5 m/s, 9 m/s and 12 m/s are considered in this research. Finally, the influence of water temperature 21℃, 23℃, 25℃ and water storage of full, 0.75-full, half-full on the performance of EAHE and recovery during shut down period is discussed. By the simulation with mentioned settings, systems’ seasonal air regulation, energy saving efficiency, efficient operating period, influence of pipe materials, effective length, affected range of surrounding soil and water, suitable soil types, appropriate airflow velocities, the influence of water temperature and water storage can be acknowledged.