Cost-Benefit Analysis of Green Roofs in a City
|關鍵字:||綠屋頂;降溫;節能;減碳;空污減量;截水;成本效益分析;建築耗能模式;乾沈降模式;永續環境系統分析;green roof;thermal reduction;energy saving;carbon emission reduction;air pollutant removal;runoff detention;cost-benefit analysis;building energy consumption simulation model;dry deposition model;sustainable environmental systems analysis|
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.
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