10.4: Other Ways of Harnessing Plant Energy- “Bioelectricity

In the opinion of some scientists converting corn to ethanol in order to use it as a fuel for automobiles is not a good idea. In an article published in 2009 in the SCIENCE magazine¹ the authors present the following arguments, backed by graphic illustrations. Because of the tough copyright rules imposed by the journal, we cannot show the graphic part here. Let’s consider the corm plant. The entire sunlight energy recieved by the plant is used by it for building its complete body: the stalk, the leaves, the husk, the cob, the grains and the roots). There is something called the harvest index HI, defined as the ratio of the corn grains mass to the total above-the-ground dry biomass:

\[ HI = \dfrac{\text{dry grain weight}}{\text{dry grain weight + dry weight residue}}

The average \(HI \) value of maize in the US is close to 0.5. So, when the grains is separated from the rest of the plant’s body, about 50% of the biomass and its energy is lost. Then, the starch from the grains has to go through the enzymatic conversion to glucose and then to be converted to ethanol by yeast fermentation – at each step some energy is lost. And finally, the alcohol fuel powers the internal combustion (IC) engine in an automobile – the thermal efficiency of an IC using gasoline or ethanol fuel is about 20%, so at the last step, propelling the vehicle, 80% of the energy is lost. It is difficult to tell exactly how much of the original energy contained by the “input biomass” is lost. According to the estimates of Ohlrogge et al. only about 10% of the initial energy stored by the plant through the photosynthesis process is used for propelling the vehicle (according to the “gut feelings” of the author of this text, it’s probably a pretty optimistic estimate).

As Ohlrogge et al. point out, there is an alternative worth considering. Arguably, the IC engine will not dominate the passenger car sector forever. There is a growing number of zero- and low-emission vehicles such as electric car and plug-in hybrids. At some foreseeable future they may even start outnumbering vehicles powered exclusively by IC engines. So, it makes sense to switch the attention from biofuels to “bioelectricity”.

Bioelectricity ? Is it possible to generate electric power from plant ma- terial? Yes, absolutely! And much of the needed infrastructure allready exists. Instead of growing plants that can provide feedstock for making corn bioethanol, one should cultivate plants with a high biomass yield (such as Napier Grass, or Switchgrass) – and then simply use the biomass as a fuel in a thermal power plant. Either as the principal fuel – as, for instance, in this 32 MW biopower plant fueled by Napier Grass, or by co-firing biomass with coal. The efficiency of modern thermal power plants can exceed 30%. Some power will be lost in transmission, and some in the process of charging car batteries. But the efficiency of motors in electric cars is very high, 90% or even more – so, after taking into consideration all possible losses, we finf that 20-25% of the original biofuel energy will be available for propelling the car.

The paper of J. Ohlrogge et al. is not new – it has been published in 2009. But the conclusions are certainly no less relevant today. Actually, taking into account that some biofuel technologies that seemed to have a “bright future” in 2009, but have badly disappointed the planners over the next decade – the conclusions of J. Ohlrogge et al. may be even more relevant today than they were in 2009.

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1. John Ohlrogge, Doug Allen,1 Bill Berguson, Dean DellaPenna, Yair Shachar-Hill, and Sten Stymne, Driving on Biomass, SCIENCE magazine, vol.324,pp. 1019-21, May 22, 2009.