Photovoltaics beat biofuels at converting sun's energy to miles driven

Jan 17, 2013

In 2005, President George W. Bush and American corn farmers saw corn ethanol as a promising fossil fuel substitute that would reduce both American dependence on foreign oil and greenhouse gas emissions. Accordingly, the 2005 energy bill mandated that 4 billion gallons of renewable fuel be added to the gasoline supply in 2006. That rose to 4.7 billion gallons in 2007 and 7.5 billion in 2012.

Since then, life cycle assessments (LCAs) have shown that has modest if any effect on reducing CO2 emissions and may actually increase them, while posing a threat to and food supplies, as are turned to fuel and marginal lands are put under the plough to keep up with demand. In 2010, fuel ethanol consumed 40 percent of U.S. , and 2012 prices are at record highs. Since the U.S. also accounts for 40 percent of the world's corn, U.S. has affected around the planet.

As (EVs) increasingly enter the market and charging stations are built to serve them, EVs are competing with alternative-fuel vehicles. Using electricity generated by coal-fired plants to power the cars defeats the purpose to some extent, but what if the energy comes from the ultimate clean and – the sun itself? How would that compete with ethanol in terms of land use, life-cycle emissions, and even cost?

The question, says UCSB Bren School of Environmental Science & Management Professor and LCA expert Roland Geyer, is which makes more sense, growing fuel crops to supply alternative-fuel vehicles with ethanol and other biofuels or using photovoltaics (PV) to directly power battery electric vehicles (BEV)?

"The energy source for biofuels is the sun, through photosynthesis," he says. "The energy source for solar power is also the sun. Which is better?"

To find out, Geyer joined former BrenSchool researcher David Stoms and James Kallaos, of the Norwegian University of Science and Technology, to model the relative efficiencies of the technologies at converting a given amount of sunlight to miles driven.

The results, which appear in a paper titled "Spatially Explicit Life Cycle Assessment of Sun-to-Wheels Transportation Pathways in the U.S." and published in the Dec. 26 issue of the journal Environmental Science & Technology, showed photovoltaics (PV) to be much more efficient than biomass at turning sunlight into energy to fuel a car.

"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," says Geyer. "You get the same amount of energy using much less land, and PV doesn't require farm land."

The researchers examined three ways of using sunlight to power cars: a) the traditional method of converting corn or other plants to ethanol; b) converting energy crops into electricity for BEVs rather than producing ethanol; and C) using PVs to convert sunlight directly into electricity for BEVs.

Because land-use decisions are local, Geyer explains, he and his colleagues examined five prominent "sun-to-wheels" energy conversion pathways – ethanol from corn or switchgrass for internal combustion vehicles, electricity from corn or switchgrass for BEVs, and PV electricity for BEVs – for every county in the contiguous United States.

Focusing the LCA on three key impacts – direct land use, life cycle greenhouse gas (GHG) emissions, and fossil fuel requirements – the researchers identified PV electricity for battery electric vehicles as the superior sun-to-wheels conversion method.

"Even the most efficient biomass-based pathway…requires 29 times more land than the PV-based alternative in the same locations," the authors write. "PV BEV systems also have the lowest life-cycle GHG emissions throughout the U.S. and the lowest fossil fuel inputs, except in locations that have very high hypothetical switchgrass yields of 16 or more tons per hectare."

PV conversion also has lower GHG emissions throughout the than do cellulosic biofuels, even in the most optimistic scenario for the latter. "The bottleneck for biofuels is photosynthesis," Geyer says. "It's at best 1-percent efficient at converting sunlight to crop, while today's thin-film PV is at least 10-percent efficient at converting sunlight to electricity.

Finally, while cost was not a key component of the study, Geyer says, "The cost of solar power is dropping, and our quick calculations suggests that with the federal tax credit, electric vehicles are already competitive."

What does this mean for the future?

"What it says to me is that by continuing to throw money into biofuels, we're barking up the wrong tree," Geyer explains. "That's because of a fundamental constraint, which is the relative inefficiency of photosynthesis. And we can't say that right now, biofuels aren't so great but they'll be better in five years. That fundamental problem for biofuels will not go away, while solar EVs will just continue to get more efficient and cheaper. If they're already looking better than biofuels, in five years the gap will be even greater. A search for a silver bullet is under way through "synthetic photosynthesis," but using genetic engineering to improve the efficiency of photosynthesis is a pipe dream. If there is a silver bullet in energy, I think it's solar power."

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User comments : 17

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antialias_physorg
4 / 5 (3) Jan 17, 2013
showed photovoltaics (PV) to be much more efficient than biomass at turning sunlight into energy to fuel a car.
...
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

That's an astonishing result. Good job on this study.

Yes, it leaves out the fact that storage of biofuels is much easier than storage of electricity - so I'd say there is some use to such fuels (if only as a backup for generating electricity for the off days when solar/wind/hydro don't deliver). But the margin by which PV beats biofuels is pretty amazing.
kochevnik
3.7 / 5 (6) Jan 17, 2013
Ethanol doesn't even return the energy investment of fossil fuels used to grow it. Of course Monsanto wants to cover up that fact about their frankencorn
JRi
5 / 5 (3) Jan 17, 2013
Both need sunshine, but PV works without rain. Last summer showed that corn is susceptible to drought.
Lord_jag
3 / 5 (6) Jan 17, 2013
If PV can produce an order of magnitude (10 x) more energy, then storage isn't an issue. Dumping power into today's batteries is about 50% efficient.

So you throw away half of your production just for the privilege of storing it. I'm not sure why would would want to since the next day you get 10x the production again anyway.

This isn't the sole solution, at night you'll still need another power source, but free energy all day every day? I can learn to live with that.
antialias_physorg
3.7 / 5 (3) Jan 17, 2013
I'm not sure why would would want to since the next day you get 10x the production again anyway.

Because sometimes you need power NOW - not the next day.

(And dumping it in batteries is not 50% efficient but 80-90% efficint for Li-ion batteries)

If PV can produce an order of magnitude (10 x) more energy,

That's an idiotic demand, since today's PV cells produce between 20% (thin film) and 43% (monocrystalline multjunction). You can't get more than 100% - and the theoretical limit is somewhere in the 87% range.
RadiantThoughts
3 / 5 (2) Jan 17, 2013
Storage is key like antialias said. Simply the energy density of these fuels cannot be matched by todays battery technology. This is slowly getting better though and perhaps we'l see some solid magnesium core 18650 lion batteries with 25000mAh capacity.

Generation efficiency of PV also can be improved when they finally get around to making terahertz capable PN junctions from graphene tech allowing the infrared band to be rectified. Very exciting technology ahead, but as usual we need it now but may have to wait 30 years for it.
Steven_Anderson
1.5 / 5 (2) Jan 17, 2013
We have the technology to achieve CO2 free industry by utilizing Solar, Wind, and LFTR Reactors. The Solar for day, the morning and evenings for Wind, and LFTR reactors for the night. Let's do it now and start exporting our technology to the world from the USA.
Len44
5 / 5 (1) Jan 17, 2013
If PV can produce an order of magnitude (10 x) more energy, then storage isn't an issue. Dumping power into today's batteries is about 50% efficient.

So you throw away half of your production just for the privilege of storing it. I'm not sure why would would want to since the next day you get 10x the production again anyway.

This isn't the sole solution, at night you'll still need another power source, but free energy all day every day? I can learn to live with that.


How about using the extra stored in the day for the night.
Steven_Anderson
3 / 5 (2) Jan 17, 2013
Storage costs a lot with current technology. It also is an environmental hazard. Throwing it away for the near future reduces efficiency dramatically but the storage technology, from what I have seen, really isn't there...YET..
RealScience
5 / 5 (2) Jan 17, 2013
@AntiAlias LordJag meant "if PV can produce 10x more energy than biofuels", not if PV efficiency can be 10x higher than today.
PPihkala
5 / 5 (1) Jan 18, 2013
As they write, stop throwing money into corn ethanol. Better result comes when that money is used to build PV for energy creation. It does not matter where that energy is used, it will make CO2 emissions smaller, because it it will replace fossil fuels. Other benefits are smaller water and land use. Of course farmers might not be happy, but somebody will always lose when focus is shifted.
Steven_Anderson
1 / 5 (1) Jan 18, 2013
Besides the use of biomass production as a way of getting rid of waste products, the technology is useless, and even harmful.
Eikka
not rated yet Jan 20, 2013
Photovoltaics beat biofuels in miles driven in total, but biofuels beat photovoltaics in miles driven and work done by a single vehicle on a single fill-up.

So it's fundamentally a question of utility. The photovoltaics won't make trucks drive across the country, or ships to sail at seas, or tractors to plow fields and combine harvesters to harvest crops, but they can drive your granny to the mall and back.
RealScience
5 / 5 (1) Jan 20, 2013
@Eikka - photovoltaics beat current biofuels at gathering energy per acre by a huge margin. Given humanity's cleverness at transforming one form of energy into another, and how generally useful electricity is for anything that doesn't have to move, people will figure out how to use solar electricity to make liquid fuels.
Whether they will need to actually do so or not is another question, because there is also significant photochemical and solar-thermal-chemical research going on.
See http://www.hydrog...n.html#e
Also see Sandia's 5R process, and other ways to get the C from CO2 (I agree with you that hydrocarbons are a great way to store energy - nature itself uses fats/oils).
QuestionEverything___
not rated yet Jan 20, 2013
@RealScience, as @Eikka has pointed out, the study is correct about efficiency of converting sunlight, but that is only part of the utility equation of how useful a given delivery system is. All the efficiency in the world isn't going to overcome (a) the vast increase of energy density of gas (and biofuels) [46 Mj / kg] over batteries [1-2 Mj / kg] and more importantly, (b) how quickly you can transfer that energy.

Since gasoline contains about 34 MJ per liter, even if you assume an internal combustion engine vehicle has an abysmal 15% efficiency (fuel to wheels), its usable energy density is 5.1 MJ/l (19.3 MJ/gal). If you spend 3 minutes at the pump filling up 50 liters (13.2 gal), you're transferring energy at a rate of 1.42 MegaWatts. If you then assume the electric vehicle is 100% efficient (socket to wheels), to reach 1.42 MW with the 220 V circuit found in most homes, you'd need 6440 Amps. More than 40x the amperage which feeds into the typical home.
RealScience
5 / 5 (1) Jan 20, 2013
@QuestionEverything: Exactly the point of my post:
people will figure out how to use solar electricity to make liquid fuels. ... there is also significant photochemical and solar-thermal-chemical research going on.
...
(I agree with you that hydrocarbons are a great way to store energy - nature itself uses fats/oils).
Telekinetic
1 / 5 (1) Jan 21, 2013
Photovoltaics beat biofuels in miles driven in total, but biofuels beat photovoltaics in miles driven and work done by a single vehicle on a single fill-up.

So it's fundamentally a question of utility. The photovoltaics won't make trucks drive across the country, or ships to sail at seas, or tractors to plow fields and combine harvesters to harvest crops, but they can drive your granny to the mall and back.

But ships at sea don't run on biofuels- they run on diesel. One super-sized container ship chugs out in a year as much pollution as 50 million cars. Now apologize to my granny.