Photobioreactor design and operation are discussed in terms of mixing, carbon utilization, and the accumulation of photosynthetically produced oxygen. The open raceway pond is the primary type of reactor considered; however small diameter (1-5 cm) horizontal glass tubular reactors are compared to ponds in several respects. These are representative of the diversity in photobioreactor design: low capital cost, open systems and high capital cost, closed systems. Two 100-m(2) raceways were operated to provide input data and to validate analytical results. With a planktonic Chlorella sp., no significant difference in productivity was noted between one pond mixed at 30 cm/s and another mixed from 1 to 30 cm/s. Thus, power consumption or CO(2) outgassing limits maximal mixing velocities. Mixing power inputs measured in 100-m(2) ponds agreed fairly well with those calculated by the use of Manning's equation. A typically configured tubular reactor flowing full (1 cm diameter, 30 cm/s) consumes 10 times as much energy as a typical pond (20 cm deep flowing at 20 cm/s). Tubular reactors that flow only partially full would be limited by large hydraulic head losses to very short sections (as little as 2 m length at 30 cm/s flow) or very low flow velocities. Open ponds have greater CO(2) storage capacity than tubular reactors because of their greater culture volume per square meter (100-300 L/m(2) vs. 8-40 L/m(2) for 1-5-cm tubes). However, after recarbonation, open ponds tend to desorb CO(2) to the atmosphere. Thus ponds must be operated at higher pH and lower alkalinity than would be possible with tubular reactors if cost of carbon is a constraint. The mass transfer coefficient, K(L), for CO(2) release through the surface of a 100-m(2) pond was determined to be 0.10 m/h. Oxygen buildup would be a serious problem with any enclosed reactor, especially small-diameter tubes. At maximal rates of photosynthesis, a 1-cm tubular reactor would accumulate 8-10 mg O(2)/L/min. This may result in concentrations of oxygen reaching 100 mg/L, even with very frequent gas exchange. In an open pond, dissolved oxygen rises much more slowly as a consequence of the much greater volume per unit surface area and the outgassing of oxygen to the atmosphere. The maximum concentration of dissolved oxygen is typically 25-40 mg/L. The major advantage of enclosed reactors lies in the potential for aseptic operation, a product value which justifies the expense. For most products of algal mass cultivation, open ponds are the only feasible photobioreactor design capable of meeting the economic and operating requirements of such systems, provided desirable species can be maintained.

No abstract

Harvested productivity of M. minutum MONOR2.

f o r each of the m a j o r subsystems: principally growth pond, harvesting and CO2 supply systems. supply to the ponds was carried out. Processing was only addressed cursorily. Operating costs (labor, electrical and nutrients) were also estimated. Four different cases of C 0 2 supply were cost estimated: recycle at high pH and no recycle at low pH. The cost of flue gas transport and transfer about equaled the projected costs of commercial CO , at CZ/pound. A cost routine was developed to convert t 2 e cost estimates to total product COS~SS. Capital and operating cost factors used for engineering, contractors fees, contingencies, insurance, maintenance, etc. were similar to those used on other DOE studies. Other factors used included land costs, labor and utility unit costs, overhead, and working capital. A return on investment (ROI, before ta#es) of 15% was used to calculate total product costs. capital charges and factors were 71% of total product costs. An engineering design and cost estimate was carried out A detailed analysis of COZ flue gas and pure C 0 2 , each with CAt that ROI, ii Total costs were estimated at about $125/barrel of o i l for the C recycle cases (pure C02 or flue gas); the higher costs of the nonrecycle case ($150/barrel) would likely be offset by the value of any byproducts. carried out to evaluate the effect on estimated costs by reduction of key cost elements. reduction of about 25%, on average, of capital and operating costs, and a similar reduction in the cost factors, resulted in an overall cost reduction to as low as $60/barrel of oil. This required the onsite use of coal for supplying (202. Higher productivities would lower costs further, but they cannot be forecast based on existing knowledge. Other options, such as methane or alcohol production or use of seawater as a water and carbon source have similar costs. In conclusion, even with optimistic projections,microalgae liquid-fuel costs are about twice those of current prices. biomass systems were explored. exist, principally wastewater treatment and specialty chemicals and feed production, the market potential of these byproducts is limited in terms of total energy production. e A sensitivity analysis was An optimistic, but still reasonable, c Alternative byproducts from microalgae Although a number of options 0 8 3 8 a Issues that must be addressed are the technical feasibility of the proposed fuel-production processes and the potential contributions to U.S. fuel supplies. The former requires research, ofabasic and applied nature, in the area of microal-gae physiology and population dynamics in open systems. involves an assessment of the key resourcescarbon overlap.

Several problems currently limit the relibility of algal cultivation systems for the production of aquaculture feeds. Many of these problems may be eventually solved through research in culture system design and operation. However, research into the isolation and development of new strains of microalgae may also directly lead to improved system performance.Through a collection and screening program aimed at isolating microalgae tolerant to high light,high salinity, and high temperatures, we have been able to isolate several new strains with enhanced production potential (30-35 g/m*/d). The overall range of environmental tolerance exhibited by these strains may result in enhanced culture stability, leading to the high production rates. The estimated costs of producing strains with these high production rates range from $1.18 to $1.71 per kg (unharvested), depending on the size of the production system. Total production costs for harvested algae (15% solids algal cell paste) range from $1.63 to $2.45 per kg. The major costs for producing harvested algae include the cost of CO,, as well as capital and operating costs for the harvesting systems. I Corresponding author.