Practical maximum yield by site, with different cell oil content. Weyer et al. (2009) Click to enlarge.

The calculated theoretical limit is lower than the 53,000 gal·ac^-1·yr^-1 figure presented by Solix’s Dr. Kristina Weyer, co-author of the paper, at the 2008 Algae Biomass Summit in Seattle, Washington. (Earlier post.)That figure, however, assumed 70% oil content in the algae, among other factors. The practical range remained the same.

The equations, calculations, and discussions in this paper have shown that, because physical laws dictate the theoretical maximum, it represents a true upper limit to production that cannot be attained regardless of new technology advances. However, if algal biofuel production systems approach even a fraction of the calculated theoretical maximum, they will be extremely productive compared to current production capability of agriculture-based biofuels.

—Weyer et al. (2009)

A number of studies have assessed the maximum theoretical efficiency of photosynthesis, but they have not specifically examined algal biofuel production or calculated maximum instantaneous efficiency and maximum annual production yield, the authors note.

The limits calculated in the paper apply to any large-scale algal production system that relies only on solar energy input to drive growth and oil production; the authors did not consider systems that use artificial lighting or other additional energy inputs, such as sugars for heterotrophic growth (e.g., earlier post.)

The calculation for theoretical maximum yield is based on physical laws; an established value for quantum yield; solar irradiance assuming perfectly clear weather and atmospheric conditions; and assumes 100% for unknown efficiencies.

Thus, the theoretical maximum yield is a true upper limit: a value that cannot be surpassed without breaking fundamental physical laws. Due to the numerous assumptions of perfect efficiency employed in the theoretical calculation, it is an unattainable goal. A practical case is also calculated, in order to provide designers with a realistic goal, which employs solar irradiance data for several sites and reasonable but conservatively high values for some efficiencies that were assumed to be 100% in the theoretical case. The practical case therefore represents what may be possible with system optimization.

—Weyer et al. (2008)

The primary physical law that limits the production capabilities of algae is the first law of thermodynamics, which states conservation of energy for any system. For a system of photosynthesizing algae, it is the rate of incident solar irradiance on the production area and the rate of chemical energy storage by the algae as oil and other biomass. The amount of stored chemical energy is directly limited by the amount of solar irradiance available.

For the theoretical case, total solar irradiance was calculated assuming year-round clear skies and minimal atmospheric absorption. For the practical case, total solar irradiance was calculated using weather data for six global climates, because the actual amount of irradiance is greatly reduced from the theoretical by clouds and other absorptive atmospheric conditions.

Only a portion of the solar spectrum is utilizable for photosynthesis; PAR (photosynthetically active radiation) is commonly defined as 400-700 nm.

Cell oil content is the portion of the cell that can be refined into a usable biofuel. A theoretical maximum value is not yet known, and oil content is highly specific to species and growth conditions. Studies have cited algal lipid contents ranging from 15 to 85% (dry cell weight), although the highest values can correspond with reduced biomass productivity.

The authors selected 50% oil content was chosen for both the theoretical and practical maximum cases, “though it is acknowledged this may be an overestimate of what will be achievable for production systems.”

While the practical case includes the estimates for efficiencies that may be improved with optimization of the growth system and chosen algal strain, the theoretical case includes no estimates and thus continues to represent an unattainable limit despite system optimization and even genetic improvements to algal strains. Any possible genetic improvements would be aimed at improvements in the efficiencies included in the practical case. These might include decreasing photoreceptor antennae to reduce photoinhibitive effects, increasing temperature tolerance, or improving resistance to predatory species. These effects are already assumed to be nonexistent in the theoretical case.

Despite any discrepancies among approaches, all estimates affirm the productive potential of algae as a biofuel feedstock. The lowest projection in this paper, is 4,900 gal·ac^-1·yr^-1, for Kuala Lumpur, is drastically higher than reported yields for corn (18 gal·ac^-1·yr^-1), canola (127 gal·ac^-1·yr^-1) or even oil palm (637 gal·ac^-1·yr^-1). Thus, the bounds on algal production presented in this paper should not be viewed as unpleasant news about physical realities, but as a realistic check that confirms its potential and will serve the industry in its pursuit of maximum algal biofuel production.

—Weyer et al. (2009)

Resources

• Kristina M. Weyer, Daniel R. Bush, Al Darzins and Bryan D. Willson (2009) Theoretical Maximum Algal Oil Production