In our paper published today in Environmental Science and Technology, Kyle Meisterling and I analyze and discuss the life cycle greenhouse gas emissions from plug-in hybrids (PHEVs) and the policy issues concerning transforming the vehicle fleet and energy system. We use three energy scenarios- baseline, carbon-intensive, and low-carbon to let policymakers and utilities think about what our electricity system should look like if we want major GHG reductions from PHEVs.
The paper examines a conventional sedan (CV) a traditional hybrid (HEV) and PHEVs with 30, 60, and 90 km of electric range. We begin by estimating the life cycle energy use and GHGs from vehicle production, storage battery production, combustion and production of liquid fuel, and combustion and production of electricity to power the vehicle. Vehicle production (~8.5 t CO2-eq) includes making the vehicle, as well as the supply chain and materials impacts and was estimated with EIO-LCA. For batteries, we looked at lithium-ion (Li-ion) and nickel-metal hydride (NiMH) batteries and used values from the existing peer-reviewed literature to estimate the energy required for materials mining and production, transportation, battery production, and recycling. We then applied emissions factors to arrive at GHGs per battery; Li-ion (0.8-2.4 t CO2-eq) and NiMH (1.6-4.6 t CO2-eq) for the different PHEV batteries. Li-ion GHGs are about 2-5% of life cycle GHGs from PHEVs, if the battery lasts the life of the vehicle, so they don't appear to be a major contributor. Different factors can influence GHGs from batteries (e.g. where they're made, evolving battery chemistries and performance characteristics, etc.) so we should continue to think about battery impacts (and recycling issues) as systems progress.
We estimated the amount of driving that would occur using gasoline vs. the amount using electricity using a CDF constructed from the data from the NHTS. Electricity would power 47%, 68%, and 76% of travel for a PHEV30, 60, and 90 km electric range, respectively. Values from the literature using a "well-to-wheels" approach was used to estimate GHG impacts from gasoline, corn ethanol, cellulosic ethanol, and different types of electricity production, which we input into our model.
Current annual U.S. passenger vehicle gasoline demand is about 17 Exajoules (EJs) and the U.S. imports about 27 EJs of crude oil and petroleum products (some gasoline). Using the assumptions in the analysis, hypothetically (for illustration only) replacing the current U.S. passenger fleet with PHEVs would reduce light-duty vehicle gasoline demand from about 17 EJs to 4-9 EJs. This scenario would drastically reduce oil imports, and also might provide a method to leverage limited biofuel resources. The U.S. currently has about 180 million hectares (ha) planted with crops. 10 million hectares could provide about 1 EJ of cellulosic ethanol, using a higher assumption of 90 GJ of ethanol per ha. So there might be a possibility of using a up to a few EJs of cellulosic ethanol with PHEVs, reducing gasoline demand further. This becomes even more interesting if LIHD plays a role in biofuel production, because then we don't necessarily need prime farmland. Of course, there are environmental and economic impacts and tradeoffs in biofuel use, which we'll save for another day. But back to PHEVs-
Not surprisingly, the carbon intensity of the electricity used to power the PHEVs greatly affects the life cycle GHGs (duh!). The 2004 CO2 intensity of U.S. electric power (combustion only) was ~ 615 g/kWh. When upstream impacts from fuel extraction, production, processing, and transportation are included, total GHGs per kWh get closer to ~670. Using power that looks like this still affords PHEVs a 32% reduction in life cycle GHGs compared to a regular sedan, and a 5% reduction compared to efficient gasoline-electric hybrids. In areas where coal is or could be the dominant fuel for charging (~950 g/kWh), PHEVs would still edge out sedans on GHGs but they would have 9-18% higher GHGs than hybrids. On the other hand, with a low-carbon portfolio of ~ 200 g/kWh, PHEVs would have large GHG reductions compared to sedans and hybrids (51-63% and 31 to 47%, respectively).
So why does all this matter? The electricity sector is going to get cleaner in the next few decades, not dirtier, right? We all hope so, but the magnitude of this shift depends on policy. If PHEVs are going to be predominately charged overnight, we need to think about how renewable portfolio standards could be updated to help reduce the carbon intensity of cheap off-peak power, which in many regions is coal. We also should think about how smart chargers could not only reduce grid impacts of PHEV charging (as is being done in ongoing work in various places), but potentially the CO2 impacts as well, either by prices or credits for the user.
Power plants that get constructed stick around for a long time (30-40 years+). Vehicles have a much shorter time horizon (~10-15 years). So if you want to buy a PHEV two or three vehicles down the line, the types of power plants we build in the next few years will be part of the mix that charges those future vehicles. While there are many plans to replace retiring coal plants with new coal plants without carbon capture, the carbon and environmental risks of CCS-less coal are beginning to concern various state governments. If we instead replace retiring plants with more natural gas, we might seriously increase reliance on imported LNG, which has higher life cycle GHGs than domestic natural gas. If our electricity system in 2030 (and 2050) looks similar to today's system from a carbon perspective, not only will we obviously have lost the opportunity of achieving large GHG reductions from the electricity sector, but potentially from the transportation sector as well if we're going to use plug-in hybrids. Efficiency will help reduce demand and a probable modest implicit carbon price via a cap and trade policy will increase the likelihood of some low-carbon generation investment. But a low-carbon portfolio we describe above and in the paper to charge PHEVs will only be achieved by a serious commitment to transforming the electricity system.
Update: See a post about this paper on Green Car Congress.
Source: Samaras, C., Meisterling K., 2008. Life cycle assessment of greenhouse gas emissions from plug-in hybrid vehicles: Implications for policy. Environ. Sci. Technol. 42 (9) 3170-3176. DOI: 10.1021/es702178s
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25 page Supporting Information (no subscription necessary)
A note about funding: This research was funded by the NSF through the Carnegie Mellon Climate Decision Making Center with additional support from the Alfred P. Sloan foundation and EPRI through the Carnegie Mellon Electricity Industry Center, and the Teresa Heinz Scholars for Environmental Research program. Any opinions, findings, and conclusions expressed do not necessarily represent the views of these organizations.
