建築是全球溫室氣體排放的三大來源之一，建築日常使用時為維持建築內空間舒適，在運行過程中消耗大量的化石燃料，儘管台灣冬季暖氣需求不高，但夏季空調需求仍是造成建築二氧化碳排放量居高不下的主因。為降低日常使用的能源消耗，減少所排放的二氧化碳量，具有調控室內溫度功能的新型建築材料，利用熱能儲存(Thermal energy storage)技術降低建築耗能與排碳量。相變材料的建築外殼藉由材料吸收或放出的熱量，維持室內溫度恆定，減少室內受到外界溫度影響。
本研究挑選適合台灣夏季氣溫的正十八烷作為相變材料，並突破一般實驗侷限於小尺度的限制，使用流體力學計算軟體(Computational fluid dynamic, CFD) ANSYS Inc. Fluent準確模擬正十八烷相變情形，第一部分建構全尺度虛擬立體3D模型，液化比例顯示，正十八烷層隨時間增加逐漸融化，表示成功模擬相變材料利用潛熱儲存熱量的過程；接著探討建築外殼材料對於整棟建築室內溫度影響，包括對照組傳統水泥牆以及具有隔熱效果的保麗龍進行對比，平均溫度差為對照組0.25 K、隔熱組1.26 K、相變組2.29 K，相變組表現維持室內舒適度表現最佳。第二部份利用簡化平面2D模型進行正十八烷的熱傳導係數分析並探討中空外牆效果，低、高熱傳組降溫效果幾乎相同，但相變化比例分別為31.3 %和29.0 %；台灣氣候下，低傳熱能力的相變材料作為建築外殼具有較好的利用率。中空外牆可降溫2.43 K，但室內空氣些許上升，對於調控室內溫度仍效果不佳。
本研究利用流體力學計算，可以更準確模擬相變材料儲存熱量的過程，對於相變材料功能性改良能夠提供正確的方向。

Housing is one of the three major sources responsible for the global greenhouse gas emissions, mainly due to the consumption of large amounts of fossil fuels during daily use. Although central-heating system is not used in Taiwan, summer air-conditioning demand is still the main reason of high carbon dioxide emissions. In order to reduce daily carbon dioxide emissions, phase change materials used in the buildings envelope can regulate indoor temperature for human comfort.
In this study, we use computational fluid dynamic software, ANSYS Inc. Fluent, to simulate the building performance and construct a real size 3D model to emulate the phase change process and temperature regulation.
In the 3D building envelope model, three different kinds of materials are incorporated, they are concrete, expanded polystyrene and n-octadecane respectively. Concrete is the traditional wall material and it reduces 0.25 K from outside temperature. Expanded polystyrene is heat insulator, and it reduces 1.26 K. We chose n-octadecane as the phase change material and it reduces 2.29 K. Results show that phase change material can stabilize indoor temperature and its thermal performance is the best. In the second part, we used simplified 2D model to study the relationship between thermal conductivity of n-octadecane and its thermal performance. We concluded that n-octadecane with higher thermal conductivity is more efficient to use. Finally, hollow wall can reduce 2.43 K but there will cause a rise in indoor air temperature.
As a result, n-octadecane is an effective energy saving material used in a building envelope in subtropical climate.

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