He starts by looking at the energy efficiency of the process. It's not always appreciated that for all its rightly deserved glory, photosynthesis is not as efficient as we think, which would indeed be the case for something that's been tuned by evolutionary fits and starts and historical contingency. For one thing, UV, IR and green light cannot be utilized by plants so that leaves out a pretty high-energy part of the spectrum. Then there's all the wonderful machinery of electron transfer and light-harvesting proteins involved in the dark and light processes. The light step essentially captures photon energy and generates NADPH and ATP, and the dark step uses this energy source and reducing potential to synthesize carbohydrates from CO2. Considering the inefficiencies inherent in using the energy of massless, transient photons, only about 12% of energy from sunlight is stored as NADPH.
Then there's the question of light intensity which seems to invoke a classic catch-22 situation. At low intensities where the process is most efficient you don't have a lot of photons by definition. But try to improve the efficiency by bumping up the intensity and you get photodamage which, in Michel's words, 3.5 billion years of evolution hasn't been able to circumvent. To avoid this photodamage, plants have to recycle one of the key proteins in photosystem II about thrice every hour which inherently limits the efficiency. Finally, the key protein in the second step, RuBisCO, has a hard time distinguishing between CO2 and O2. A significant amount of energy has to be spent in getting rid of the product formed from O2 insertion.
All these hurdles lead to a rather drastic lowering of photosynthetic efficiency which gets watered down to a rather measly (but still staggeringly efficient by human standards) 4% or so.
It's pretty clear from this description that any kind of efforts to get better efficiency from biofuels will have to overcome enormous protein engineering hurdles. This does not bode well for current studies aimed at such goals. While these are very exciting from an academic standpoint, they will have to lead to a very drastic retooling of the basic photosynthetic apparatus, involving re-engineering numerous genetic pathways and their products, to be of large-scale commercial value. It's all too easy to underestimate the sheer amount of energy that we want to generate from these technologies. I feel the same about the synthetic biology efforts that seek to produce all kinds of valuable industrial chemicals and drugs from engineering bacteria. These efforts are undoubtedly promising, but getting bacteria to do something which they have not evolved to and that too on a scale rivaling the fossil fuel industry is a very long shot indeed. Michel doesn't even seem optimistic about the recent excitement regarding biofuel production from red algae, and reading his prognosis one wonders how much collaborations such as the one between Exxon and Craig Venter are actually going to yield. And finally, there's all the alternatives which that land for biofuels and the biofuel feedstock itself can be put to, which is a question still being pondered.
Michel is much more optimistic about photovoltaics which already promise energy conversion efficiencies of more than 15%. When the end product is used in a car, photovoltaic batteries also funnel about 80% of their energy into propelling the vehicle. In addition, Michel sees promise in recent advances in battery technology leading to much higher energy density.
Personally I am a proponent of context-specific energy use. I think that considering the vast variation of resource distribution, geography, energy requirements, paying capacity and economics, it doesn't make much sense to search for any one-size-fits-all solution. But that seems to usually be the case every time someone touts a single, seemingly miraculous solution as being universally applicable around the world. I have similar thoughts about solar energy. The solutions currently available don't seem to solve the problem of transmission and availability in regions where the sun doesn't shine that much. Part of Michel's "vision" is the widespread deployment of superconducting cables which even now (more than 25 years after the discovery of "high-temperature" superconductors) seems like a minor fantasy. Notwithstanding these issues though, solar power certainly seems to have a much larger role to play in our economy than it currently does, especially in regions which get plenty of sunlight.
But biofuels? The problems there seem much more grounded in fundamental biological constraints rather than technological ones. And it's hard to overturn 3.5 billion years of evolution so I am not sure I should hold my breath. Time will tell.