Joule Tries to Push the Photosynthetic Limit on Microbial Fuels

JouleThe graph above illustrates some of the limiting factors for production of biofuels and biomass using photosynthesis. The paper linked here provides more in-depth thinking on this topic. To achieve higher yields, it is necessary to feed much higher levels of CO2 to the bioreactors than is available in the atmosphere. Modifying the microbes to channel most of their effort toward biofuels production is required. And photon input must be maximised to provide high input energies.

Several emerging technologies are aiming to.....provide viable alternatives to fossil fuels. Direct conversion of solar energy into fungible liquid fuel is a particularly attractive option, though conversion of that energy on an industrial scale depends on the efficiency of its capture and conversion. Large-scale programs have been undertaken in the recent past that used solar energy to grow innately oil-producing algae for biomass processing to biodiesel fuel. These efforts were ultimately deemed to be uneconomical because the costs of culturing, harvesting, and processing of algal biomass were not balanced by the process efficiencies for solar photon capture and conversion. This analysis addresses solar capture and conversion efficiencies and introduces a unique systems approach, enabled by advances in strain engineering, photobioreactor design, and a process that contradicts prejudicial opinions about the viability of industrial photosynthesis. We calculate efficiencies for this direct, continuous solar process based on common boundary conditions, empirical measurements and validated assumptions wherein genetically engineered cyanobacteria convert industrially sourced, high-concentration CO2 into secreted, fungible hydrocarbon products in a continuous process. These innovations are projected to operate at areal productivities far exceeding those based on accumulation and refining of plant or algal biomass or on prior assumptions of photosynthetic productivity. This concept, currently enabled for production of ethanol and alkane diesel fuel molecules, and operating at pilot scale, establishes a new paradigm for high productivity manufacturing of nonfossil-derived fuels and chemicals. _Robertson, Jacobson et al

ImageSource SpringerlinkJoule claims the ability to produce up to 15,000 gallons of advanced biofuel per acre, using its modified cyanobacteria in "Helioculture" bioreactors. The research paper describing the researchers' thinking, published in Photosynthesis Research.
The approach described in the paper linked above and below allows for a more comprehensive look at the requirements for a high-yield system of production of biofuels using photosynthetic organisms.

Not all photons that enter a reactor are available for conversion. For instance, it may be too costly to maintain the reactor in a condition in which it can convert every photon, such as early in the morning and late in the day when solar radiation is very diffuse. Likewise, depending on how the reactor temperature is maintained, the organisms may not be at optimal production temperature early in the morning. In addition, at very high intensity levels, the organisms may not be able to convert all of the photons. Based on models that integrate solar and meteorological data with a thermal and production model, we estimate that about 15% of the incoming photons will not be available for conversion for the direct case. We assign a comparable loss to the algal open pond.
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The main fractional loss in photosynthetic conversion results from energy-driven metabolism. Because the photosynthetic process is ultimately exothermic, the available energy contained in the product formed by metabolism is a fraction of that contained in the incoming photons. The remaining energy is dissipated as heat into the culture. For the production of alkane, we calculated that ~12 photons are required to reduce each molecule of CO2. Assuming an average PAR photon energy of 226 kJ/mol and a heating value of 47.2 MJ/kg for alkane, the photosynthetic conversion efficiency is about 25% (equivalent to a loss of 74.8%). For the simpler triglyceride, we assume only eight photons are required to reduce each molecule of CO2, but that the product consists of half triglyceride (heating value ≈37 kJ/kg) and half simple biomass (heating value ≈15.6 kJ/kg), resulting in a photosynthetic conversion efficiency of about 29.8%. This value for algal open ponds is considered to be very conservative, with the actual value likely a few percent lower. Finally, for the theoretical maximum, we use the value computed in Zhu et al. (2008) for a maximum photosynthetic efficiency of 29.1% (obtained by combining the loss for photochemical inefficiency and carbohydrate synthesis). _Source

Brian Wang also looks at this paper.

There is no easy way to produce high yield advanced biofuels -- else it would have been accomplished decades ago. The density of sunlight and available CO2 -- as well as selfish microbes who only think about themselves -- present significant obstacles. And yet the prize for success is so great, that billions of US$ are being funneled into the attempt, and dozens of top notch research teams are devoting their time toward the goal.

Forget about peak oil. If you want to see peak energy for Earth, look at the sun and the planetary core, and at the nuclear energy of the atom. Better yet, expand your horizon to take in the enetire solar system. If you're smart, you are only at the beginning.