Water and Greenhouse Gas Tradeoffs Associated With a Transition to a Low Carbon Transportation System

Abstract

Transportation fuels are heavily dominated by the use of petroleum, but concerns over oil depletion (e.g., peak oil), energy security, and greenhouse gas emissions from petroleum combustion are driving the search for alternatives. As we look to shift away from petroleum-based transportation fuels, most options consume and withdraw more water during their life cycle. Thus, shifting to alternative fuel and energy supplies for transportation will likely increase water use for the transportation sector. Previous work suggests that water consumption for transportation could reach 10% of total U.S. water consumption when meeting the Federal Renewable Fuels Standard mandate at modest irrigation levels for feedstock crops (corn, cellulosic grasses) in combination with other alternative fuels and vehicle technologies (electric vehicles, natural gas vehicles, etc.), but more refined analysis is needed. It is important to understand when and where these new water demands for transportation are anticipated to occur. This paper presents results from simulations of the U.S. 9-region (EPAUS9r) MARKAL (MARKet ALlocation) integrated energy systems model for mapping the changes in water withdrawal and consumption during a transition to a low carbon-emitting U.S. transportation fleet. The advantage of using a bottom-up, multi-sector model like MARKAL is the ability to look at consistent scenarios for the full energy system, and endogenously capture interactions between different sectors (e.g. electric power production, biorefineries, and the LDV fleet). MARKAL can simulate a baseline scenario driven by assumptions for biomass feedstock and fossil resource costs and availability, as well as the costs of converting those resources to liquid fuels and electricity. We investigate alternative scenarios both with and without carbon constraints, while varying the pace of vehicle electrification. We compare these scenarios to assess regional differences in water needs as well as aggregate water demand for transportation energy, and how those trade off against greenhouse gas emissions reductions. Our results indicate that the regional water demands and interregional transfers of embodied water could be significant as the light-duty vehicle fleet moves away from petroleum-based fuels, with exports of embodied water on the order of hundreds of billion gallons of water per year for ethanol coming from the Midwest. Interregional transfers of water embodied in electricity may also reach tens of billion gallons of water per year. However, these water requirements will vary substantially based on the light-duty vehicle mix, carbon policy, electric power generation mix, biofuel production levels, and feedstock characteristics.