Life Cycle Environmental Impact of Houston METRO System – Evaluation of Electric Alternatives

In the Greater Houston Area, mobile sources (on- and off-road vehicles) contribute the highest share of nitrogen oxide (NOx) emissions and second-highest share of volatile organic (VOC) emissions. The Houston METRO system is a key element in Houston's infrastructure that can be expanded to lower emissions of criteria air pollutants (CAPs) and greenhouse gases (GHGs) and improve regional air quality. Currently, there is no comparative study for relative emissions and environmental impacts between passenger automobiles and METRO routes in Houston. Our research addressed this critical gap and developed environmental life cycle assessment for conventional diesel buses, diesel hybrid buses, and alternative electric buses in Houston using the GREET model. The life cycle GHG emissions of electric buses are slightly lower than the other two types of buses. However, all the other major emissions such as CO, NOx, PM10, PM2.5, VOCs, SOx, N2O, CH4, black carbon and primary organic carbon associated with electric buses are higher than diesel buses, thus causing higher environmental cost of electric buses than diesel buses. The life cycle costs of buses are very sensitive to future diesel and electricity prices. The results from this project would serve as a guiding framework to evaluate the effects of the decision to expand the METRO system and estimate the contribution of the METRO system in realizing the Greater Houston Area's environmental objectives.在大休斯顿地区，移动源（道路及非道路车辆）是氮氧化物（NOx）排放的最大贡献源，同时也是挥发性有机物（VOC）排放的第二大贡献源。休斯顿大都会交通（METRO）系统是休斯顿基础设施的关键组成部分，对其进行扩容可有效降低常规空气污染物（CAPs）与温室气体（GHGs）的排放，改善区域空气质量。目前，针对休斯顿地区乘用汽车与METRO线路的相对排放及环境影响的对比研究仍属空白。本研究填补了这一关键缺口，采用GREET模型针对休斯顿地区的传统柴油巴士、柴油混合动力巴士及替代能源电动巴士开展了环境生命周期评估。研究结果表明，电动巴士的全生命周期温室气体排放略低于另外两类巴士。但电动巴士所产生的其余主要排放物，包括一氧化碳（CO）、氮氧化物（NOx）、PM10、PM2.5、挥发性有机物（VOC）、硫氧化物（SOx）、一氧化二氮（N2O）、甲烷（CH4）、黑碳及原生有机碳，均高于柴油巴士，因此电动巴士的环境成本高于柴油巴士。巴士的全生命周期成本对未来柴油与电力价格极为敏感。本项目的研究成果可作为指导框架，用于评估扩容METRO系统的决策效应，并量化METRO系统在实现大休斯顿地区环境目标过程中的贡献。