Browsing by Author "Sengupta, Shayak"
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Item Fuel cycle emissions and life cycle costs of alternative fuel vehicle policy options for the City of Houston municipal fleet(Elsevier, 2017) Sengupta, Shayak; Cohan, Daniel S.Municipal fleet vehicle purchase decisions provide a direct opportunity for cities to reduce emissions of greenhouse gases (GHG) and air pollutants. However, cities typically lack comprehensive data on total life cycle impacts of various conventional and alternative fueled vehicles (AFV) considered for fleet purchase. The City of Houston, Texas, has been a leader in incorporating hybrid electric (HEV), plug-in hybrid electric (PHEV), and battery electric (BEV) vehicles into its fleet, but has yet to adopt any natural gas-powered light-duty vehicles. The City is considering additional AFV purchases but lacks systematic analysis of emissions and costs. Using City of Houston data, we calculate total fuel cycle GHG and air pollutant emissions of additional conventional gasoline vehicles, HEVs, PHEVs, BEVs, and compressed natural gas (CNG) vehicles to the City's fleet. Analyses are conducted with the Greenhouse Gases, Regulated Emissions, and Energy use in Transportation (GREET) model. Levelized cost per kilometer is calculated for each vehicle option, incorporating initial purchase price minus residual value, plus fuel and maintenance costs. Results show that HEVs can achieve 36% lower GHG emissions with a levelized cost nearly equal to a conventional sedan. BEVs and PHEVs provide further emissions reductions, but at levelized costs 32% and 50% higher than HEVs, respectively. CNG sedans and trucks provide 11% emissions reductions, but at 25% and 63% higher levelized costs, respectively. While the results presented here are specific to conditions and vehicle options currently faced by one city, the methods deployed here are broadly applicable to informing fleet purchase decisions.Item Net greenhouse gas emissions savings from natural gas substitutions in vehicles, furnaces, and power plants(Inderscience Enterprises Ltd., 2016) Cohan, Daniel S.; Sengupta, ShayakWe compare the net greenhouse gas emissions impact of substituting natural gas for other fossil fuels for five purposes: light-duty vehicles, transit buses, residential heating, electricity generation, and export for electricity generation overseas. Emissions are evaluated on a fuel cycle basis, from production and transport of each fuel through end use combustion, based on recent conditions in the USA. To compare across sectors, the emissions difference between natural gas and its alternative is normalised by natural gas consumption to compute the net reduction in CO2e per MJ of natural gas used. Greatest emission reductions can be achieved by replacing existing coal-fired power plants (78gCO2-e/MJ natural gas) or fuel oil furnaces (66gCO2e/MJNG). Compressed natural gas in vehicles yields no significant reductions. Uncertainties arising from upstream emission rates for natural gas and the global warming potential of methane are quantified. The study demonstrates the critical role of deployment choice on the net climate impact of natural gas.