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Spatially explicit life cycle assessment of 5 sun-to-wheels pathways finds photovoltaic electricity and BEVs offer land-efficient and low-carbon transportation

Direct land use, life cycle GHG emissions (excluding indirect land use change), and life cycle fossil fuel requirements to generate the transportation services provided by 17.8 × 1012 MJ NCV of gasoline, the amount used in transportation in the US in 2009. Credit: ACS, Geyer et al. Click to enlarge.

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:

  1. Land area required for the corn and switchgrass fields or the PV installation—i.e., direct land use measured in m2/100 km driven.

  2. Total global warming potential from the vehicle and fuel life cycles, measured in kg CO2 equiv/100 km driven.

  3. 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



Harvey, storage needs for solar power can be quite low depending on the end use of the electricity. For example, a solar-powered server farm will have more stringent electrical power back-up requirements than solar-powered air conditioner because the consequences of the loss of power is greater for servers than air conditioners. If your air conditioner loses power due to clouds, no sweat – at least for a couple of hours, anyway. Furthermore, air conditioning can also be provided by thermal storage of excess solar or off-peak electricity in the form of phase change materials – ice cubes et al. Similarly, daytime charging of BEVs with solar power will require an alternate charging source, which is not necessary for PHEVs.

Commercial and residential air conditioning represents about 13% of the electricity used annually in the US. EV charging demand and PV generated electricity will not come near this level or at least a decade. However, any peak grid capacity freed up by solar-powered AC should be available for daytime charging of EVs.

The heat pumps with a SEER in the 26 range would represent a significant threat to natural gas in one of its main markets - heating (space and hot water) and a potential 10% increase in electricity demand with moderate electrical storage requirements.

Kit P

" forgot"

No, I did not forget Harvey's fantasy. Calculations are easy, making power 24/7 is a bit more of a challenge.

Thomas Lankester

"No, I did not forget...making power 24/7 is a bit more of a challenge. "
I think you forgot that the object of the exercise is to deliver power 24/7 which allows quite a bit of flexibility compared to a dogmatically making it 24/7.


FYI, cooling is a major energy consumers in a server farm. Every Watt consumed by CPUs and disk arrays needs to be removed using air conditioning. If you know a way around this, please tell Google/Amazon/Apple/Facebook, etc. You will be a billionaire in no time.



The Gemasolar plant in Spain cost $260 million US for a plant with a rated output of only 20 MW ($13,000/kW) and a capacity factor of only 73%.  It makes nuclear power look positively cheap, even from EPRs.


Let's not forget that Nuclear power cost is going up fast and that Solar power cost is going down even faster. The two will cross soon and Solar power cost has a good chance of being cheaper by 2030 or even before.

Meanwhile, Wind power is getting more and more attention. Our $175B public pension fund has recently invested $5.5+B in USA/Canada 1500 mega-watt Wind power installations. The same fund will invest into another 1500+ Mega-watt capacity being built locally where the potential high quality Wind capacity is about 95,000 mega-watt.


NorthernP....yes, Solar power storage required may vary depending on how much of the total power demand can be supplied during sunshine hours etc.

Of course, SEER 26+ Heat Pumps and LEED type homes, LED ligths, improved appliances, TV, PC, tablets etc can reduce energy required for residence by 75+%. The saved energy would be more than enough for 2+ EVs per family.

Eventually, charging many million EVs during sunshine hours could further favor Sun Power installations.


If China can build AP-1000's at an overnight cost of $2300/kW, there is no problem with escalating nuclear costs:  there is a problem with escalating political interference.

Charging millions of EVs during sunshine hours is great, until you get a week of cloudy days.  This happens very often where I am.

Kit P

“The Gemasolar plant in Spain cost $260 million US for a plant with a rated output of only 20 MW ($13,000/kW) and a capacity factor of only 73%. ”

Is that actual performance or projected? Last I check they found a red rubber stamp 'EPIC FAILURE'
across a picture of this experiment.

“Nuclear power cost is going up fast ”

Actually it is coming down. First the cost of construction is coming down because of standard designs. Modern construction also includes modular designs where whole sections are built in a factor and set in place with a massive crane

Second the initial cost is amortized over a longer period. Many nuke plants were designed for 40 years and will run for 60 or 80 years. New reactors are being designed for 60 years so they may have a higher cost but not over the life of the power plant.

Of course the same principles apply to solar thermal like the Gemasolar plant. If they can make the solar plant work they can build many more at lower costs.

“problem with escalating political interference. ”

Again E-P is wrong. There is a surprising lack of opposition to new nuke plants where they are being built in the US. Both the democrats and republicans leadership supports nuclear power. Locally where the plants are being built, new plants has strong support.

“compared to a dogmatically making it 24/7 ”

That is correct. The engineering truth is that we produce power on demand. While we are always experimenting with better ways of doing things.

Based on my understanding of the practical application of the second law of thermodynamics, the engineering is not going to change. However, it is free country. Thomas can buy a bunch of solar panels and batteries and disconnect from the grid.

There is a surprising lack of opposition to new nuke plants where they are being built in the US.
Yet there are continuing efforts to shut down perfectly good plants such as Vermont Yankee, and the war of attrition has succeeded in getting a utility to declare one will be closed in Wisconsin.
Kit P

One can only wonder part of the word 'new' E-P does not understand. Furthermore E-P was ranting about politics at the NRC but the NRC has yet to not extend the operating license including the one E-P brings up. It is rather surprising that more of the smaller old nuke plant have not closed.

Score card: Local political opposition has failed to close VY. Second there is not political opposition in Wisconsin but the plant might be mothballed for a few yeas.


Are there new plants under construction in Wisconsin and Vermont?  I must not have gotten the memo.  Sure, if you restrict your claims to places where there's enough support to START construction, you won't have any opposition to old ones... but where in carbon-conscious California are they planning to build more carbon-free power?

Removing perfectly-functioning nuclear plants from the grid is a huge waste of capital as well as fossil fuel.  It's a scandal that this is happening anywhere (Wisconsin and Spain, to name two).

Kit P

E-P likes to put his horse in front of his cart because he has misconceptions about how things work.

We build new power plants because we project we will need the energy at some point in the future. Then we, the power industry that is, decide what the good choices are. Next a public debate is held since making power is a public service.

The Southeast is projecting increased demand and new nuke plants are being built. As I said earlier, very little political position.

“but where in carbon-conscious California are they planning to build more carbon-free power? ”

Fresno for one. Of course it is not a power reactor but a reactor to desalinate water and the huge amount of electricity produced will just be a byproduct. Nuclear friendly Idaho is a being considered by two different groups.

“Wisconsin and Spain, to name two ”

There is no indication Wisconsin is anymore than the economics of keeping a smaller plant running.

Hey dude, you know, dude, Spain is like another country dude. The US NRC does not regulate in Spain. Spain does have anti-nuke politics and has huge economic problems. A tax was imposed on the nuke plant equal to the value of the power produced.

The evidence is clear. We are building new nukes in the US to meet future needs. The usual suspects are against it. However, usual suspects are against everything and fight everything in court.

When it comes to making power, you have to show that it is safe and does not have significant environmental impact. If you can not handle a few over priced lawyers, you should not be in the business.


Anyone interested in trying the PV/EV marriage out for themselves is encouraged to enter the Real Goods Solar Smart Electric Sweepstakes. The winner will be given a 3-year smart electric drive lease and a 1.5 kW home solar system to charge it!

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