For Powering Cars, Solar-Electric Is ‘Orders Of Magnitude’ More Efficient Than Biofuels


Climate Progress recently reported on a study that found both economic and environmental benefits if homes in the northeastern United States upgraded older heating systems by moving from heating oil to switchgrass. However, one point to emphasize was the findings were specific to those circumstances — the region, the homes, and that particular use.

Switchgrass was not nearly as good an idea for electricity generation or transportation fuel. Further confirming the need for a diversity of renewable solutions to our energy needs, a recent study determined that electricity generated by solar beats out biofuels for powering cars under myriad scenarios.

The report, put together by a team from the University of California, Santa Barbara and the Norwegian University of Science and Technology, and published in Enviornmental Science and Technology, compared five different approaches to see what was the most efficient way to power a compact passenger vehicle for every 100 kilometers driven:

  1. Battery-electric vehicles (BEVs) run on electricity from solar power.
  2. Battery-electric vehicles run on electricity from switchgrass.
  3. Internal combustion vehicles (ICVs) run on switchgrass biofuel.
  4. Battery-electric vehicles run on electricity from corn.
  5. Internal combustion vehicles run on corn-based biofuel.

The analysis considered land-use, greenhouse gas emissions, fossil fuel use, and took into account the production and use life cycles of both the fuels themselves and the vehicles they power.

In terms of land-use, solar significantly out-performed all other options. It performed modestly better than switchgrass in terms of greenhouse gas emissions, and significantly better than corn-based biofuel. Solar was actually equal or slightly worse than switchgrass when it came to fossil fuel requirements over the totality of the life cycle, but it still out-performed corn-based internal combustion. (And, of course, gasoline.)

So all things considered, a pretty clear win for solar-powered electric battery vehicles:

A write up over at Green Car Congress has more details on the assumptions and variables in the study’s modeling.

“PV is orders of magnitude more efficient than biofuels pathways in terms of land use — 30, 50, even 200 times more efficient — depending on the specific crop and local conditions,” Roland Geyer, a UCSB Bren School of Environmental Science & Management Professor, told Science Daily. “You get the same amount of energy using much less land, and PV doesn’t require farm land.” The central bottleneck, as the report notes, is the low efficiency of photosynthesis:

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 percent, the overall conversion efficiency of sunlight into terrestrial biomass is typically below 1 percent, regardless of crop type and growing conditions.

“Today’s thin-film PV is at least 10-percent efficient at converting sunlight to electricity,” Geyer explained — hence solar’s superior performance. In fact, the WWF’s Solar PV Atlas found that as far as land-use goes, solar is so efficient that less than 1 percent of global land areas would be needed to supply all the world’s electricity needs in 2050.

Traditional corn-based biofuels are problematic on all sorts of levels: Carbon emissions from agricultural production over their full life cycle largely wipe out any carbon benefits at the point of actual vehicle use. They compete with human food supplies and food cropland, driving up global prices and contributing to global poverty and instability. And new cropland sequesters less carbon from the atmosphere than the grassland or forest it typically displaces.

Switchgrass and other cellulosic biofuels, while they avoid disrupting food supplies, are not immune to these other flaws either. On top of that, their commercial viability at any time in the near future is far from certain.

For the clean car fleet of the future, electrical and hybrid vehicles relying on a grid powered by solar — and presumably wind, hydroelectric, and such — still appears to be the way to go.

23 Responses to For Powering Cars, Solar-Electric Is ‘Orders Of Magnitude’ More Efficient Than Biofuels

  1. Mike Roddy says:

    Nice study, including the photo, thanks.

    Why in the world are we still burning biofuels in gas tanks? We’ve known about its problems for a while now, though this study clarifies things further.

  2. Anderlan says:

    Wonderful to see this fully fleshed out. Elon Musk always says “back of the envelope” but what I think he always means is, start by reckoning the envelope–the bounds, best and worst case–using the simplest undergrad* physics. Then you know what’s impossible. You do this for 2 solutions and you know if one is incredibly worse than the other. Biofuel is incredibly worse than solar.

    *(accreditted science and engineering undergrad, mind you)

  3. It seems fairly clear that the light-duty ground fleet should be largely directed to electricity, while biofuels are directed to areas where electrical alternatives are not currently feasible. Aviation stands out here, as do ships and heavy trucks, though hybridization can reduce fuel demands. Rail is a good candidate for complete electrification.

  4. Sasparilla says:

    Patrick I think you’re totally on target there – I wish we had those as guiding principles for future investments for the country we’d save ourselves so much money and time.

  5. Sasparilla says:

    It’s a very nice article detailing what alot of us guessed.

    The picture does bring to mind the question of what an array that size could actually charge in real time (each charger on a Volt charges as 3.3kWh I believe).

    One of the great things about solar is that the panel tech shows no signs of slowing down (the theoretical efficiency limit was breached in a laboratory according to a recent report) and while we may see a temporary firming of prices while the Chinese bubble works its way out of the system, long term the price is only downward.

  6. Mulga Mumblebrain says:

    You see, Mike, dumb, ignorant, vicious people, of whom there is a gargantuan excess, are very often stubborn. They are so because they are incapable of much thinking, have their opinions inserted into their ‘minds’ by relentless indoctrination, and resent those more intelligent and inquisitive than they. This latter type hold their opinions tentatively, and will change them if the evidence dictates that path. Changing your mind can be exhilarating or dismaying, but intellectual honesty requires it. The dullards hate it viscerally, their beliefs being more acts of faith than reason. They also hate being shown up by those they despise for ideological and psychological reasons. So they call their stubborn dullness ‘principle’ and make stupidity and ignorance into ‘virtues’.

  7. Ken says:

    PV and BEVs were clearly made for each other. The sooner we can get electric companies to use BEVs for energy storage and retrieval (i.e. load leveling) the better.

    But for at least the near future we need range extension. I wonder how a Stirling-cycle engine burning switchgrass pellets would work?

  8. Merrelyn Emery says:

    If we had an international agreement that all countries would go solar in 10 years with other renewables as a backup, if required, we could leave competition where it belongs, with the inventors and designers improving the technology, ME

  9. Dave Bradley says:

    Why is anyone even thinking that the electricity per unit area of land converted into trtansport energy a relevant thing in most of this country? Most of America is VERY sparsley populated. And most biofuel made is a by-product of PROTEIN production from plant life, where the nutritionally USELESS sugars and starches are converted into fuels.

    Besides, it is the COST of the energy used for transportation that is what is important for the transportation consumer. In cities, electric or plug-in hybrids make sense, assuming people can afford them (for a lot of non-rich people, PHEV costs more than a house…). In rural America, fuels rule, for cars, trucks and especially tractors/semis/big transport turcks. Trains are easy to electrify (again, assuming there is money), but since there is no money……

    If we have cars that get an average of 45 mpg for those that use liquid fuels (double the present pathetic US value), close to European/Japanese standards, we can cut fuel usage in half-down from 8.6 million barrels per day (mbd) to around 4.3 mbd of gasoline. Then if the miles traveled by liquid fuel powered cars is cut in half (ride sharing, mass transit, walking, biking, moving from sub-urbs to cities, PHEVs and EVs, or just not needing to go anywhere), then the required amounts of fuel for transport is down to about 2.1 mbd. And that is something that our country could easily grow, whether using crops where the sugars/starches are mostly extracted and used to make fuels or the conversion of cellulose into fuels.

    If you need 30 kw-hr per day for electricity for transport and get an average of 4 hrs of decent sun per day (10% average PV output), you’d need 7.5 kw of capacity, or around $35,000 worth of PVs, before subsidies. I guess there are plenty of people where that is chump change, but that is more than my house is worth, and if $35k worth of panels could be put on it, the house would still be worth $35,000. I’d much rather run a PHEV on a wind turbine off of the grid, where 15 c/kw-hr (mostly that high due to high grid “troll monopoly” costs) means that $4.50 worth of electricity can take care of avoiding a lot of fuel usage, and after buying a PHEV, (assuming that is possible), well, the PV electricity is just an exercise in absurdity. Besides, if the battery runs out (winter, stuck in traffic, have to go farther than normal), that’s where the EtOH kicks in…, as otherwise it’s a long walk….

  10. BBHY says:

    About 100,000 years ago prehistoric cavemen learned to make fire.

    Now it’s the 21st century. It is finally time to move on from burning stuff for our energy needs.

  11. Joe Romm says:

    You have the wrong price for PV.
    Also arable land is going to be as precious as water in the coming decades.

  12. Sasparilla says:

    We’ll need better battery technology for load leveling though – the current tech has issues with capacity loss as the batteries are cycled and people won’t stand for cycling their batts at this point.

    I’m sure that battery tech will come (beginning next decade I’d guess), but its not here yet – for now people want their batteries cycled less so their cars don’t loose as much range over time.

  13. Sasparilla says:

    Wonderful way to look at it BBHY…I love that perspective.

  14. Pat Hansen says:

    Everyone misses the point that cars are 95% inefficient and no matter how you power them the energy is wasted and the whole system is unsustainable. Slow congested and environmentally unacceptable. The system will have to be superseded sooner rather than later.Start now is the message.

  15. Joan Savage says:

    Arable land in the US is currently around 174 million hectares (1.74 x 10^8 hectares). The units in the first section of the republished graph are also in 10^8 hectares. Had the graph had a bar for US total arable land we would have gotten the point right away.

    The title for the graph in the linked article says,
    “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.”

  16. polymerase says:

    According to the bar chart, the GHG emissions of switchgrass EtOH looks to be about only 10-15% more than that of CdTe PV (note to editor: if you’re going to reprint a figure, please include the caption!). What would the GHG emissions be with Corn Stover EtOH, which uses ag residue instead dedicated, cultivated land for an energy crop? Did the authors favor one “straw man” over another (pun intended) to prove their thesis?

    “Switchgrass and other cellulosic biofuels, while they avoid disrupting food supplies, are not immune to these other flaws either. On top of that, their commercial viability at any time in the near future is far from certain.” Can you please elaborate? I’m a big fan of EVs, but is their commercial viability certain? I do agree with Patrick above that EVs for light duty and cellulosic biofuels for unelectrifiable heavy duty (ships and planes and bulldozers) are perhaps our immediate best bet.

  17. Mike Roddy says:

    I agree, Pat, but our politicians are bought. Stimulus money in Los Angeles was used to expand freeways and build overpasses. Full light rail is a dream, and it’s worse in Oklahoma City and Fresno.

  18. Joan Savage says:

    How much could that graph be nicked back in all categories of fuel just by efficiencies in transportation?

    I’d like a version of Patrick Mazza’s vision of the light duty ground fleet, as we might further economize on number of vehicles, and thus trim back on the blue-black part of the graph, vehicle life cycles.

  19. Mulga Mumblebrain says:

    Public transport and leg power ne’er failed me yet. Well, public transport in car-bedeviled cities is often less than it ought to be, particularly when it has been ‘privatised’.

  20. JPWhite says:

    Internal Combustion cars maybe be very inefficient, but EV’s are very efficient in comparison. That’s why this study came to the conclusions it did.

    It matters less where the fuel comes from when very little is wasted.

  21. Uncle B says:

    White Zombie Dragster proves the torque is available, Google nano carbon super capacitors for the Power Densities approaching, even exceeding that of gasoline to prove that 1000 km trips are reasonable, and if still not convinced of the “Electric Takeover”, Google Israel’s electric 1000 km car, and the Chreos. All carbon sequestering hemp bodies next, from University of Alberta’s hemp molding technologies – and Thorium heat and electric power will yield Aluminium undercarriage parts for as cheap as U.S. steel. 21st Century technologies will not be refused. Global Village moving ahead very fast. Three moving part electric power trains, far lower failure rates, much longer lasting. Huge electric efficiency increases for electric sources through the “ballast” effect or storage effect of more efficient electric storage.

  22. euroflycars says:

    “Start now…” ought to be only the beginning of the message, which should continue reading: “…setting up mass-production of Personal Ultra-Light Electric Tilt-Rotor Aircraft with a detachable cockpit serving as a mini EV to drive downtown after landing the VERTOL airframe anywhere at the urban periphery.”

  23. euroflycars says:

    200 million years ago, reptiles crawling on all fours got airborne — we’re still crawling on our four wheels…

    It’s time for us all to get individually airborne on a regular basis!