Hartmut Michel won the Nobel Prize in Chemistry for his work on photosynthesis. So, it is fair to say that he knows a thing or two about energy transport and storage in plants. Today he is director of the Molecular Membrane Biology at the Max Planck Institute for Biophysics.
He recently penned an editorial in Angewandte Chemie International Edition in which he hammered the use of biofuels for alternative energy. Note that Angewandte Chemie International Edition has the world’s highest impact factor of all chemistry journals. His simple but pointed criticism condemns all varieties of biofuels and supports my previous posts on this subject.1, 2
The problem is the inherent inefficiency of photosynthesis. He points out…
“The photosynthetic pigments of plants can only absorb and use 47%(related to energy) of the light of the sun (“photosynthetic active radiation”). Green light, UV, and IR irradiation are not used…
Photosynthesis is most efficient at low light intensities. It is already saturated at 20% of full sunlight and 80% of the light is not used…In addition, high light intensities lead to photodamage of a central protein subunit of the photosynthetic apparatus…3.5 billion years of evolution have not been long enough to develop a mechanism for preventing the photodamage….
The dark reactions are limited by an insufficient discrimination between CO2 and O2 by the enzyme RuBisCO, which inserts CO2 into ribulose-1,5-bisphosphate. One third of the energy of the absorbed photons is believed to be required to remove the product of the O2 insertion…[and] photosynthesis depends on the availability of sufficient amounts of water, a condition that is not met during much of the day.”
Current biofuel technologies
Taken as a whole, conversion efficiency of sunlight to usable chemical energy in biofuels for commonly used technologies is extremely low.
“For German “biodiesel” which is based on rapeseed, it is less than 0.1%, for bioethanol less than 0.2%, and for biogas around 0.3%.”
But is it actually much worse than that when you consider
“… these values even do not take into account that more than 50% of the energy stored in the biofuel had to be invested in order to obtain the biomass (for producing fertilizers and pesticides, for ploughing the fields, for transport) and the chemical conversion into the respective biofuel.”
Michel confirms what I have pointed out before, biofuels of all stripes put a great burden on arable land. He says…
“Taken together, the production of biofuels constitutes an extremely inefficient land use. This statement is true also for the production of bioethanol from sugar cane in Brazil.”
“Second Generation” biofuels
Some people hold out hope for “second generation” biofuels where the whole plant is utilized. Michel explains that this is an illusion because the energy input for these types of processes in ever greater than for first generation processes. For example…
“in the production of biodiesel by the Fischer–Tropsch process, hydrogen has to be added because syngas obtained from biomass contains insufficient amounts of hydrogen.”
More distant possibilities
“Hydrogen production by photosystem II would reduce the number of photons required by more than 50%. However, this protein engineering task appears to be insurmountable at present.”
“Microalgae have been advertised as the ideal candidates for biofuel production. There are many unsupported claims about their efficiency, some even exceeding the theoretical limits of photosynthetic efficiency…the existence of photoinhibition and a poor RuBisCO will limit the advantages of microalgae together with the demands for growing and harvesting them.”
But biofuels will save us from CO2
“The production and use of biofuels therefore is not CO2-neutral. In particular, the energy input is very large for the production of bioethanol from wheat or maize, and some scientists doubt that there is a net gain of energy. Certainly the reduction of CO2 release is marginal.”
“Clearing rainforests in the tropics and converting them into oil palm plantations is highly dangerous because the underlying layers of peat are oxidized and much more CO2 is released by the oxidation of organic soil material than can be fixed by the oil palms…it would be even much better to reforest the land used to grow energy plants, because at a 1% photosynthetic efficiency, growing trees would fix around 2.7 kg of CO2 per square meter, whereas biofuels produced with a net efficiency of 0.1% would only replace fossil fuels which would release about 0.31 kg CO2 per m2 upon combustion!”
“Because of the low photosynthetic efficiency and the competition of energy plants with food plants for agricultural land, we should not grow plants for biofuel production. The growth of such energy plants will undoubtedly lead to an increase in food prices, which will predominantly hit poorer people.”
Read the entire editorial in context here (The Nonsense of Biofuels, Hartmut Michel, Angew. Chem. Int. Ed. 2012, 51, 2–4)
Here is a video of Michel making the same point.
His Noble Prize was for the determination of the three-dimensional structure of a photosynthetic reaction centre, as seen here