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| Schematic comparison between direct photosynthetic (top) and algal biomass (bottom) processes. Source: Robertson et al. Click to enlarge. |
Joule Unlimited’s direct, single-step, continuous process for the production of solar hydrocarbon fuels (earlier post) can produce the areal equivalent of up to 15,000 gallons of diesel per acre annually, according to a new open access paper by a Joule team published in the journal Photosynthesis Research. Harvard Medical School Professor of Genetics George Church, Joule co-founder and chairman of its technical advisory board, is a co-author.
The paper, which examines Joule’s advances in solar capture and conversion, direct product synthesis and continuous product secretion, finds that the solar-to-product conversion efficiency of the direct, continuous process for producing diesel, ethanol and chemicals is between 5 and 50X greater than any biomass-dependent process, and gains additional efficiencies by avoiding downstream refining. In addition, the analysis counters prior assumptions about the viability of industrial photosynthesis, addressing the barriers overcome by Joule to achieve unprecedented photosynthetic efficiency.
Joule was formed not to improve upon existing biofuel processes, but to create a new and transformational process altogether. We have channeled photosynthesis, the most productive energy-capture process on earth, at efficiencies previously thought unattainable. At the same time we’ve eliminated dependence on biomass, the Achilles heel of biofuel production, and the prohibitive costs, processing and logistics it entails. The result is a new paradigm for renewable fuel production with unrivalled productivity targets that are fully supported by actual, measurable gains we’ve achieved at every layer – from photon capture through product synthesis, secretion, separation and collection.
—Bill Sims, President and CEO of Joule
Joule’s process, called Helioculture, combines an engineered cyanobacterial organism
supplemented with a product pathway and secretion system to
produce and secrete a fungible alkane diesel product continuously in a
SolarConverter designed to efficiently and economically collect
and convert photonic energy. The process is closed and uses industrial waste CO2 at concentrations 50–100 times higher than atmospheric. The organism is further engineered to provide a switchable control between carbon partitioning for biomass or product.
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| Sum of individual contributions and accumulated photon losses for two fuel processes and a theoretical maximum for energy conversion. Log scale. Source: Robertson et al. Click to enlarge. |
The engineered microorganisms thus function as biocatalysts that use only sunlight, waste CO2 and non-fresh water to directly and continuously produce diesel-range hydrocarbons, which are compatible with existing infrastructure. The paper says that Joule’s combined advances in genome engineering, solar capture and bioprocessing result in photosynthetic conversion efficiency of more than 7% relative to available yearly solar energy striking the ground, many times greater than prior industry assumptions.
The paper compares the efficiencies for an algal pond biomass-to-biodiesel and a cyanobacterial direct-to-fungible diesel
process, presenting a theoretical maximum and computed practical maximum efficiencies. Both processes convert solar energy into fuel, but unlike the direct process, the indirect production of fuel from algal biomass requires the costly culturing, harvesting and processing of algal biomass. Moreover, Joule’s process directly yields hydrocarbons that are fungible with existing diesel infrastructure.
By using the photosynthetic efficiency calculated above, the extrapolated metric of barrel energy equivalents (bble) is equal to 6.1 x 109 joule) and any product density expressed in kg/m3 and
energy content, e.g., heating value in MJ/kg, the output of
this analysis can be converted to areal productivity for any
molecule produced from either an endogenous or an
engineered pathway.
For example, the direct process, operating at the calculated 7.2% efficiency would yield
350 bble/acre/year. This equates to 15,000 gal alkane/acre/year where a C17 alkane has a heating value of 47.2 MJ/kg
and density of 777 kg/m3. Given the flexibility of genome engineering to construct production organisms that make
and secrete various fuel products, a similar calculation can be applied for any product synthesized via a recombinant
enzymatic pathway and a productivity value extrapolated.
By comparison on an energy basis, the practical efficiency
of the algal biomass process would equal about 3,500 gal/
acre/year of the target triglyceride (71 bble; heating value
41 MJ/kg; density 890 kg/m3). Note that 1 gal/acre/year is
equivalent to 9.4 l/hectare/year.
The areal productivity estimate for the direct process
surpasses the best estimates for fuel productivity potential
by any biomass-derived fuel process, e.g., for grain or
cellulosic ethanol, for algal or vegetable oils for biodiesel,
or biomass gasification and Fischer–Tropsch reforming for
hydrocarbons. The photon energy densities and process
productivities, plus the advantage of no arable land or
freshwater displacement, create a scenario in which a
minimal dedication of marginal land can serve to meet US
renewable fuel standards.—Robertson et al.
Resources
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Dan E. Robertson, Stuart A. Jacobson, Frederick Morgan, David Berry, George M. Church and Noubar B. Afeyan (2011) A new dawn for industrial photosynthesis. Photosynthesis Research doi: 10.1007/s11120-011-9631-7