Algal biomass. Photosynthetic algae have the potential to achieve very high biomass productivities, namely 93 gDW/m2/day (grams of dry weight per m2 per day). Indeed, photosynthesis requires 8 photons to fix a C atom from CO2, biomass is roughly 50% C, we assume a solar irradiance of 5.3 kWh/ m2/day as well as a high quantum yield of 0.83 seen in dark-adapted algal cells. PIARCS’s proposed photobioreactor design (proprietary) aims to reach 70% of this maximum theoretical productivity, namely 65 gDW/m2/day (or 96 tons/acre/year, which compares to 6 tons/acre/year for corn).
Algal biofuel. As has been known for decades, certain eukaryotic algal strains accumulate lipids (a biodiesel precursor) when subjected to limiting levels of nitrogen. Assuming that the chosen algal strain produces continuously biomass containing 20% recoverable lipids at a productivity of 60 gDW/m2/day (the lower biomass productivity would result from nitrogen limitation), the resulting biofuel production would be on the order of 5,400 gal biofuel/acre/yr.
Current shortcomings. Outdoor algal biomass productivities remain low, on the order of 5-18 gDW/m2/day (yearly average). Continuous lipid production in native algal strains (not engineered) has not been documented to date, as strategies rely on intermittent nitrogen exhaustion from the growth medium. This further impairs overall areal lipid productivity.
Important element. Efficient light utilization by algae (high quantum yield) is a necessary condition for high biomass yield, which drives high areal biomass productivities. Indeed, low quantum yields indicate that the absorbed incident photons are wasted as heat, and not processed for photosynthetic biomass production.
Problem addressed. Under increasing incident light, the photosynthesis quantum yield plummets. This phenomenon, which has been broadly documented in plants, is a major reason why the rate of photosynthesis plateaus in ‘PI curves’.
The solution. As early as 1953, numerous investigators have documented the ‘flashing-light effect’, which indicates that adequate mixing driving intermittent shading could alleviate this shortcoming. PIARCS’ mixing design aims to maintain dark-adapted quantum yields under high irradiance (outdoor levels).
Work in progress
Chemostat growth. Under high quantum yield and complete absorption of the incident light, algal growth models should become trivial. Namely, P = ΦDW · I0, where P is the areal productivity, ΦDW the constant autotrophic yield and I0 the known incident light per area. Using this simple growth model and corresponding mass balances, algae can be grown in chemostats with complete nutrients (nitrogen, phosphorus) utilization under high light.
Continuous lipid production. Lowering the nitrogen quotient should enable continuous lipid production under high light.
If successful, broad implementation of the technology and a cooperative mindset are both critical to collectively achieve a successful environmental transition. Currently, PIARCS is working on a low-cost licensing business model to help promote broad adoption by a variety of algal biomass producers.
PIARCS’ configuration would be implemented as a low-cost add-on to open algal pond, with a target licensing fee of 0.2% of the gross biomass sales for ponds over 50 acres. For example, a 100-acre farm could generate $1.4 million per year from algal biomass sold at $150/ton, with a yearly licensing fee of $1,400/year. A 10% penetration of the oil market would generate PIARCS $1.9bn yearly.
Economic resilience. Just to provide an order of magnitude, transforming about 2.9% of the non-arable continental US land into algae ponds would satisfy the country’s oil consumption, while supporting thriving rural communities.
Carbon-neutral biofuel. Curbing reliance on fossil fuels is a global priority, which could be achieved.
Carbon sequestration. As other companies have shown, algal biomass (50% carbon) can be stored.
Treatment of polluted (eutrophic) waters. Complete nutrient removal would lead to environmental restoration as an added benefit of large-scale algal growth.
Manageable evaporation. Assuming evaporative losses of 11 millimeters per day over 3% of the US (open ponds) would lead to the loss of 10 million gallons per day (MGD). The Los Angeles area alone dumps an average of 270 MGD of treated fresh wastewater into the ocean.
Holland AD, Dragavon JM and DC Sigee. Methods for Estimating Intrinsic Autotrophic Biomass Yield and Productivity in Algae: Emphasis on Experimental Methods for Strain Selection. Biotechnology Journal. 2011 6:572-583. https://doi.org/10.1002/biot.201000260
Holland AD and D Wheeler. Methods for Estimating Intrinsic Autotrophic Biomass Yield and Productivity in Algae: Modeling Spectrum and Mixing-Rate Dependence. Biotechnology Journal. 2011 6:584-599. https://doi.org/10.1002/biot.201000261
Holland AD., Dragavon JM. (2014) Algal Reactor Design Based on Comprehensive Modeling of Light and Mixing. In: Bajpai R., Prokop A., Zappi M. (eds) Algal Biorefineries. Springer, Dordrecht. https://doi.org/10.1007/978-94-007-7494-0_2