企業綠屋頂節能與環境效益評估

Abstract

由於綠屋頂具有隔熱降溫、減緩暴雨逕流等功能，而企業在展現社會責任、節能減碳與環境美化上，建構綠屋頂是適切且容易著力的方向，因而國外企業已積極推廣綠屋頂(Cooperate Green Roof , CGR)。然而針對CGR的效益，目前國內並無適當的方法可用以評估CGR效益，本研究因而以CGR較顯著之節能及環境效益為主軸，建立一套方法評估CGR在隔熱降溫上對節能的助益，以及環境上對降低温室氣體排放及截水之效益。 節能評估主要採用eQUEST 軟體模擬以及熱傳導係數法估算CGR建置前後的能源消耗差異，亦考量不同CGR型式，包括介質厚度、植栽特性等環境狀況及熱阻等對CGR隔熱降溫節能效益的影響。環境效益則主要包括温室氣體減量及截水，温室氣體減量主要包括二部分，一部分是由於節能所造成，另一部分是由於植栽吸收及固定二氧化碳的能力，前者依所減少電力的排放係數來推估，後者則以可能種植的植物吸收及固定二氧化碳的係數來推估。而降低降雨逕流的截水效益則以其他研究所建立的截水公式推估。且將分別參考碳稅及國外都巿逕流收費費率，探討這些CGR效益可展現的成本效益。 本研究針對企業辦公大樓就薄層型及密集型CGR進行研究，依所建立方法估算之單位成本分別為212及301 NT$/m2-yr，而10、15及20年三種不同更新年限之傳統屋頂的單位成本分別為241、182、134 NT$/m2-yr，密集型CGR因初設成本較高，年均成本亦較高，薄層型CGR雖然較15及20 年更新年限之傳統型屋頂高，主要是因國內電價偏低及未考量綠屋頂其他效益，但已比10年更新年限低。本研究除了探討及示範所建議方法的可行性與實用性，亦期所得的結果可作為企業進行建置CGR相關決策分析時的重要考參考依據。

Green roof technology has the functions of roof insulation, room temperature decreasing, and rainwater runoff detention, corporate green roof (CGR) establishment is thus widely promoted in many countries and is regarded as an appropriate approach for demonstrating the corporate social responsibility in energy conservation, carbon-emission reduction and landscape aesthetics. However, the method for evaluating the cost-benefit of domestic CGRs is so far not available. This study was thus initiated to develop a cost-benefit analysis method to evaluate major benefits of CGRs including the heat reduction for energy saving, carbon-emission reduction and rainwater runoff detention for a local corporate building. The eQUEST simulation program and thermal conductivity coefficient (U-value) method are applied to estimate the energy savings of CGRs, with consideration of different CGR type and thickness of soil medium. The environmental benefits of CGRs are mainly carbon-emission reduction and rainwater runoff detention. The carbon emission reduction is approximated based on the avoided emission from energy saving and the amount of carbon sequestrated by CGRs. The rainwater detention of CGRs is calculated by an experimental equation. The benefits are estimated based on the local electricity fee, a suggested carbon tax, and a stormwater charge. A case study for a corporate office building was implemented for both extensive and intensive CGRs. The proposed benefit-cost analysis method was applied to estimate the potential benefits and costs of CGRs, and compared with the conventional roof with a life span of 10, 15, or 20 years. The annual costs for extensive and intensive CGRs are 212 and 301 NT$/m2-yr, while the costs for the conventional roof for different life spans are 241, 182, and 134 NT$/m2-yr, respectively. The results show that the intensive CGR is not cost effective due to its high initial installation cost. Although the extensive CGR is more expensive than the conventional roof with a life span of 15 or 20 years, it is superior to the latter with a life span of 10 years. The CGR will be more cost effective if the electricity rate increases and other benefits such as heat island mitigation are also considered. This study has demonstrated the applicability of the proposed CGR cost-benefit analysis method, and the results are expected to facilitate related decision-making analyses for a company to install CGRs.