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:

Battery-electric vehicles (BEVs) run on electricity from solar power.Battery-electric vehicles run on electricity from switchgrass.Internal combustion vehicles (ICVs) run on switchgrass biofuel.Battery-electric vehicles run on electricity from corn.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.

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