城市綠屋頂成本效益分析

Abstract

綠屋頂由於具有降溫節能、減碳、空污減量、截水等效益，已在不少國家推廣中，國內亦正起步中，唯相關研究甚少，且設置綠屋頂對於整個城巿的成本效益目前亦缺乏有系統的方法可估算之，本研究因而發展一套方法分析及估算國內城巿推廣綠屋頂的成本效益。 所發展的城巿綠屋頂成本效益評估方法，首先依屋頂可設置綠屋頂的最大面積及合理綠屋頂比率估算城巿可設置綠屋頂面積。效益部分主要評估降溫節能、減碳 、空污減量及截水等四項；降溫節能部分採用1D-HF法及eQUEST模式模擬分析增設綠屋頂所減少的建築耗能；減碳部分除了節能所造成的減碳效益以外，亦依植物減碳能力估算城市綠屋頂之減碳效益；空污減量部分則採用乾沈降模式推估綠屋頂空氣污染減量效益；截水部分採用綠屋頂截水效益經驗公式推估總截水量及依據綠屋的飽和截水深度推估尖峰逕流減少量。各效益的經濟效益則分別依據電價、碳稅、空污防治與損害成本及可能節省的雨水系統擴充成本估算之，且分別推估一般及綠屋頂成本，最後綜合評估城市綠屋頂之成本效益。 本研究以新竹巿進行案例研討，並依據10、15及20年三種不同的一般屋頂更新年限分析，結果顯示除了在一般屋頂年限為20年時綠屋頂的成本效益較差以外，其他均較佳，約在5百萬至6億6千5百萬元間，經濟效益主要以節能為最大宗約3億750萬元；而額外考量碳稅與雨水下水道使用費時可再增加約1億元的效益，碳稅約8千4百萬元，雖在一般屋頂更新年為20年時，綠屋頂成本效益仍較差，但主要是因本研究尚未把減緩熱島等其他效益納入，且國內屋頂的更新年限預期小於20年，每年節能量約1億4千9百萬度，減碳約11.1萬噸eCO2，加上可降低O3及其前驅物的形成等，故綠屋頂對新竹市而言有推廣之潛力，且未來碳稅之推行，預期有助於其推廣，本研究成果亦可供其他城市進行相關決策分析時之參考依據。

Green roof (GR) technology can reduce internal room temperature, save energy, decrease carbon emissions, remove air pollutants, and reduce runoff, and thus it is widely promoted in many countries. However, the method for evaluating the cost-benefit of GRs in a city is so far not available. This study was thus initiated to develop a benefit-cost analysis (BCA) method to evaluate major benefits of GRs for a domestic city. To implement the BCA method at the city scale, the rooftop area available for installing GRs is first estimated based on the maximal rooftop area and a reasonable GR ratio. Four major GR benefits are considered: energy saving, carbon emission reduction, air pollutant removal, and runoff detention. The one-dimensional heat flux (1D-HF) method and the eQUEST model are applied to compute the energy saving. The carbon emission reduction is determined based on the avoided emissions from energy saving and the amount of carbon sequestrated by GRs. The quantity of air pollutants removed by GRs is approximated by a dry deposition model. The overall rainfall detention capability of GRs is calculated by an experimental equation, and the peak runoff reduction is estimated by the potential saturated detention depth of GRs. And the cost information of GRs is obtained from a local survey. The economic values of all GR benefits are estimated and compared with the GR life cycle cost. A case study was implemented for Hsinchu City. The proposed BCA method was applied to estimate the potential benefits and cost for executing a city-wide GR program. The conventional roof is assumed to be replaced by three possible periods: 10, 15, and 20 years. The results show that the GR program worse than the conventional roof only with a replacement period of 20 years, for other two replacement periods the GR program can have economic benefit of about NT$5-666 million, of which the energy saving is about NT$300 million and is the largest portion among all benefits. If a carbon tax and a stormwater fee were applied, it would increase the GR economic benefit of about NT$100 million, of which the carbon tax is about NT$84 million. Although the GR program is worse than the conventional roof with the replacement period of 20 years, additional GR benefits such as mitigating head island are not considered and the domestic roof replacement frequency is usually shorter than 20 years. The energy saving is about 149 million KWH, and the carbon emission reduction is about 111 thousand ton eCO2. The GR program can also slightly reduce O3 and it precursors. Therefore, promoting a city-wide GR program in Hsinchu City is desirable. The incentive for implementing the program can be further increased if a carbon tax is applied. The proposed BCA method and the results are expected to facilitate related decision analyses for other cities.