31 January 2008

Why Not Cellulosic Electricity? Bio-EtOH vs. EVs

The ongoing debate between promoters of cellulosic ethanol (or butanol) and electric vehicles (EVs) is heating up. Can you get more energy from biomass by converting to ethanol (or butanol) and burning it in flex-fuel vehicles, or by burning the biomass with (or in place of) coal in power generators--and run the vehicle (EVs) on the generated electricity?
According to the NREL report referenced above, converting biomass into cellulosic ethanol can be done at about a 45% efficiency (i.e. 45% of the energy of the biomass makes it into the fuel.) In contrast, biomass can be converted at 33-37% efficiency [pdf] when cofired. Combining this with the 5x improvement of drivetrain efficiency that comes with electric propulsion, and the same amount of biomass converted to what I'll call "cellulosic electricity" will take a vehicle 3.8x as far as it would in the form of cellulosic ethanol. In a more recent article on Biomass, Vinod Khosla states "we consider [Energy Return on Investment] a less important variable than carbon emissions per mile driven." If carbon emissions per mile driven are the most important variable, a 3.8x increase in miles driven on the same energy source will lead to a less than 27% of the carbon emissions per mile driven.

While cellulosic electricity is still not sufficient to displace all of our current petroleum use, it comes much closer than cellulosic ethanol. Biomass cofiring with coal also tends to reduce SOx and NOx emissions.

Biomass is a distributed resource, seldom available in large quantities in any one place. This will be a problem for the cellulosic ethanol and cellulosic electricity industries. Only a fraction of the available biomass will be close enough to existing coal plants that it will be practical to transport for cofiring. Cellulosic visionaries see a system of distributed ethanol plants, yet that still leaves the problem of getting the fuel to market, since the current pipeline system for petroleum products has difficulty accommodating ethanol.

On the other hand, while distributed direct- fired biomass generation of electricity is probably twice as expensive as cofiring with coal, distributed generation leads to opportunities for Combined Heat and Power (CHP), or cogeneration. CHP can displace heating fuels such as natural gas, propane, or electricity, and often have combined efficiency from 50% to 80%. In addition to the potential of displacing additional fossil heating fuel, cellulosic electricity is identical to the fossil fuel derived kind. Therefore, unlike cellulosic ethanol, cellulosic electricity is completely compatible with the existing electric grid, leading to far fewer difficulties in transport.

The calculations are still in flux. But it is important to include "cellulosic electricity" in the total planning, when looking at biomass power. More here and here

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