Spatially explicit life cycle assessment of 5 sun-to-wheels pathways finds photovoltaic electricity and BEVs offer land-efficient and low-carbon transportation
4 January 2013
A new spatially-explicit life cycle assessment of five different “sun-to-wheels” conversion pathways—ethanol from corn or switchgrass for internal combustion vehicles (ICVs); electricity from corn or switchgrass for battery-electric vehicles (BEVs); and photovoltaic electricity for BEVs—found a strong case for PV BEVs.
According to the findings by the team from the University of California, Santa Barbara and the Norwegian University of Science and Technology, published in the ACS journal Environmental Science & Technology, even the most land-use efficient biomass-based pathway (i.e., switchgrass bioelectricity in US counties with hypothetical crop yields of more than 24 tonnes/ha) requires 29 times more land than the PV-based alternative in the same locations.
Furthermore, PV BEV systems also have the lowest life cycle GHG emissions throughout the US and the lowest fossil fuel inputs, except for locations with hypothetical switchgrass yields of 16 or more tonnes/ha. Including indirect land use effects further strengthens the case for PV BEVs, the researchers suggested.
Biofuels for ICVs and bioelectricity for BEVs use photosynthesis to convert solar radiation into transportation services, that is, they are sun-to-wheels transportation pathways. While photosynthesis has a theoretical maximum energy conversion efficiency of 33%, the overall conversion efficiency of sunlight into terrestrial biomass is typically below 1%, regardless of crop type and growing conditions. Therefore any biomass-based energy pathway is very land-use-intensive. As a result, biomass-based transportation pathways are increasingly seen as a threat to food supply and natural habitats.
A third type of sun-to-wheels pathway is the use of photovoltaics (PV) to convert sunlight directly into electricity for BEVs...Existing environmental assessments of biofuels and photovoltaic energy pathways use average biomass and PV yields, even though these yields vary widely between geographical locations. Spatially-explicit assessments are more informative, since pathway performance depends on location, and land use decisions are always local by nature. This article presents life cycle assessments of five different sun-to-wheels conversion pathways for every county in the contiguous U.S: Ethanol from corn or switchgrass for ICVs, bioelectricity from corn or switchgrass for BEVs, and PV electricity for BEVs using cadmium telluride (CdTe) solar cells. The assessments include the production and use of the transportation energy (the fuel cycle) and the life cycle of the vehicle.—Geyer et al.
The functional unit of the assessment was 100 km driven in a compact passenger vehicle during one year. The team calculated three environmental indicators for each county of the contiguous US:
Land area required for the corn and switchgrass fields or the PV installation—i.e., direct land use measured in m2/100 km driven.
Total global warming potential from the vehicle and fuel life cycles, measured in kg CO2 equiv/100 km driven.
Total fossil fuel consumption from the vehicle and fuel life cycles, measured in MJ of net calorific value (NCV) per 100 km driven.
The system boundary includes vehicle production, use, and end-of-life management, as well as fuel production and use. In the case of PV electricity, the fuel cycle consists of production, use, and end-of-life management of the PV system.
GHG and fossil fuel data for the production of corn and switchgrass and their conversion to ethanol are based on the EBAMM Model, which was combined with crop yield maps and updated with data from version 1.8c.0 of the GREET model and other recent literature. Among the assumptions were:
NCV of corn and switchgrass is 18 MJ per kg, and that 2.53 kg of corn and 2.62 kg of switchgrass are required to produce 1 L of ethanol with 21.2 MJ NCV.
Energy consumption and GHG emission values of the biorefineries include coproduction credits and in- and out-bound logistics. The crop-to-electricity conversion model assumes that half of the biomass is converted in biomass boilers and the other half is co-combusted with coal to generate electricity.
Inventory models for both product systems are based on Ecoinvent data and reports. A biomass-to- electricity conversion efficiency of 32% was used, and an electricity transmission and distribution efficiency of 92%.
The PV system life cycle is based on 2005 technology and production data.
Economic input−output life cycle assessment (EIOLCA) was used to derive energy and GHG values for the production of a compact ICV; data on 2005 Li-ion battery technology was added to model PHEVs of equivalent size. The resulting energy and GHG values are 102,000 MJ and 8,500 kg CO2eq per compact ICV, and 1700 MJ and 120 kg CO2 equiv/kWh of Li-ion battery.
A 150 km (93-mile) -range BEV model was derived by increasing the battery size in the PHEV model. This may overestimate GHG emissions and fossil fuel consumption of BEV production, the researchers noted, since they merely added the battery to an ICV and did not deduct the internal combustion engine or related components.
Together with the maximum range of 150 km and a maximum depth of discharge (DOD) of 0.8, the BEV energy demand translates into a required battery size of 33.75 kWh. The life cycle mileage of both vehicles is assumed to be 240,000 km (149,000 miles).
Among their findings were that relative to the gasoline baseline, the PV and switchgrass scenarios would also reduce associated GHG emissions and fossil fuel consumption from production and use of vehicles and fuels by 75−80% relative to gasoline ICVs. The PV-based pathway would reduce life cycle GHG emissions, including vehicle production, by almost 80%, from 1.92 to 0.41 billion tons of CO2 equiv.
Vehicles powered with switchgrass electricity or ethanol come second and third with 0.46 and 0.48 billion tons of CO2 equiv, yet these numbers do not include any GHG emissions from indirect land use change, the researchers noted.
The three sun-to-wheels pathways with the lowest fossil fuel requirements are switchgrass ethanol for ICVs; switchgrass electricity for BEVs; and PV electricity for BEVs; with 4.7, 5.4, and 5.2 trillion MJ.
For both BEV-based pathways, more than 85% of fossil fuels are consumed during vehicle production. Of all the 5 sun-to-wheels systems, corn ethanol for ICVs had by far the highest land requirements, GHG emissions, and fossil fuel requirements.
Assuming that the economics of PV and BEV technology will further improve and issues of material availability, and electricity transmission and storage can be resolved, PV offers land-efficient and low-carbon sun-to-wheels transportation. Unlike fuel crops, PV electricity does not have to compete with food production and biodiversity for fertile land and could potentially replace all gasoline used in US transportation.—Geyer et al.
Roland Geyer, David Stoms, and James Kallaos (2012) Spatially-Explicit Life Cycle Assessment of Sun-to-Wheels Transportation Pathways in the US. Environmental Science & Technology doi: 10.1021/es302959h
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