A Tubular Integral Gas Exchange Photobioreactor for Biological Hydrogen Production

Abstract

Very low-cost closed photobioreactors are required for photobiological production of H₂ from water and sunlight using microalgae as catalysts. Assuming a 10% solar energy conversion to H₂ fuel, such photobioreactors should cost at most about $50/m² for single-stage processes, or somewhat over $100/m² for two-stage systems. The basic requirements for such photobioreactors are:

• • maximum light interception per unit area of reactor to minimize total costs;

• • large unit size, over 100 m², to minimize capital and operating costs;

• • low-cost, long-lasting, transparent materials to contain the culture and H₂;

• • low-cost assembly, site preparation, and erection of the photobioreactor system;

• • effective gas exchange to remove H₂ and O₂ or CO₂, as required;

• • effective temperature control for the local climate and specific organisms used;

• • minimum energy inputs for mixing, culture handling, and gas exchange;

• • lack of fouling and/or ability to clean the photobioreactor, as required; and

• • ease of operation, maintenance, and troubleshooting, for lowest possible costs.

Photobioreactors are of a few basic design concepts, such as tubular or flat plate with internal or external gas exchange, with myriad variations and elaborations described. However, no design has proved superior or commercially successful, even for high-value products. A tubular photobioreactor that can meet the above requirements has been demonstrated at the University of Florence. This near-horizontal tubular reactor (NHTR) has the following general attributes:

• • tubes about 4 cm (range 3–5 cm) in diameter, about 40 m long (20–60 m);

• • tubes placed on corrugated sheets to maintain straight lines and inclination;

• • inclination of about 10% slope (6–12% likely range), with reactors set on earth-works;

• • 40 (20–50) tubes connected by manifolds at the bottom and a degasser on top;

• • internal gas exchange using compressed gas, with gas flow also providing mixing; and

• • cooling with water sprays, with the cooling water collected and re-used.

In Florence, such NHTRs, ranging from 6–87 m, have been tested with Spirulina (Arthrospira platensis), with volumetric productivities of up to 1.5 g/L/day (for 4-cm diameter tubes). One important attribute of this reactor is little fouling, due to the scouring effect of the gas bubbles. Two photobioreactors, each with eight 20-m long tubes, were installed at the University of Hawaii in early 1997 under a collaborative project. A very preliminary cost analysis for a conceptual single-stage biophotolysis process is presented, based on many very favorable assumptions, such as 10% solar energy conversion efficiencies and a very low cost of capital. It suggests that such NHTRs might cost only $50/m², and projects H₂ costs of $15/GJ. This projection, however, does not consider many factors and a more detailed analysis is required.