Process analysis and economics of biophotolysis of water. IEA technical report from the IEA Agreement on the Production and Utilization of Hydrogen [report]

J.R. Benemann
1998 unpublished
PREFACE This report is a preliminary cost analysis of the biophotolysis of water based on the assumptions that such biophotolysis systems can be developed using cyanobacteria or green algae and can work at an overall 10% sunlight conversion efficiency. Subtask B and some 'uncommitted' members of Annex 10 have discussed this report. Some controversies arose due to differences in strategies or approaches to developing photobiological hydrogen production systems. These points are described in the
more » ... e described in the Appendix, along with responses by the author. We believe that this report is a useful contribution to the challenging R&D field of practical photobiological hydrogen production. Yasuo ASADA Leader of Subgroup B Annex 10 The views presented in this report do not necessarily represent the views of the International Energy Agency, nor the governments represented therein. i SUMMARY Biophotolysis is the conversion of water and solar energy to hydrogen and oxygen using microalgae. Biophotolysis has been a subject of applied R&D for a quarter of century without, however, achieving system scale-up beyond a few square meters or sunlight conversion efficiencies of even one percent. In laboratory experiments at low light intensities, algal photosynthesis and some biophotolysis reactions exhibit highlight conversion efficiencies that could be extrapolated to about 10% solar efficiencies if photosynthesis were to saturate at full sunlight intensities. The most promising approach to achieving the critical goal of high conversion efficiencies at full sunlight intensities, one that appears within the capabilities of modern biotechnology, is to genetically control the pigment content of algal cells such that the photosynthetic apparatus does not capture more photons than it can utilize. Assuming the achievement of high solar conversion efficiencies through such genetic technologies, it is plausible to extrapolate practical processes of biophotolysis. Biophotolysis processes can be classified based on the type of hydrogen producing enzymes used, nitrogenases or reversible (bidirectional) hydrogenases, and the electron transport pathways that reduce these enzymes. However, nitrogenase requires a large amount of metabolic energy, which would reduce the potentially achievable photosynthetic efficiencies by about half. This makes these enzymes unattractive in the development of practical biophotolysis processes. The reversible hydrogenases could be reduced either directly by the photosynthetic apparatus or indirectly through an intermediate CO, fixation step. The direct process results in simultaneous 0, production which requires a currently unavailable 0, resistant hydrogenase reaction, as well as separation of the H, from 0,. Indirect biophotolysis processes can be extrapolated based on currently known biochemistry, although an integrated or sustained process remains to be demonstrated even at a laboratory scale. Indirect biophotolysis would involve cyclic processes in which microalgae grown in a first step on CO, produce biomass high in carbohydrates. These would be metabolized to H, by first keeping the algal culture in the dark under anaerobic conditions, and by then exposing the cells to the light to complete the process of anaerobic hydrogen production. Only one photon per hydrogen evolved is postulated to be required in the light-driven reaction. An overall efficient hydrogen production process will require highly metabolically engineered microalgal strains that exhibit a host of desirable properties, in addition to high solar energy conversion efficiencies and high rates of hydrogen production in the dark and light. These would include the ability to recycle the cultures repeatedly, regulation of photosynthetic 0, production, and utilization of excreted fermentation products during light-driven H, evolution. A two stage indirect biophotolysis system was conceptualized and general design parameters extrapolated. The process comprises open ponds for the CO, fixation stage, an algal concentration step, a dark adaptation and fermentation stage, and a closed tubular photobioreactor in which hydrogen production would take place. A preliminary cost analysis for a 200 hectare (ha) system, including 140 ha of open algal ponds and 14 ha of photobioreactors was carried out. The cost analysis was based on prior studies for algal mass cultures for fuels production and a conceptual analysis of a hypothetical photochemical processes, as well as the assumption that the photobioreactors would cost about $100/m2. Assuming a very favorable location, with 21 megajoules (MJ)/m2 total insolation, and a solar conversion efficiency of 10% based on CO, fixation in the large algal ponds, iii an overall cost of $lO/gigajoule (GJ) H, is projected. Of this almost half is due to the photobioreactors, one fourth to the open pond system, and the remainder to the H, handling and general support systems. It must be cautioned that these are highly preliminary, incomplete and optimistic estimates. Biophotolysis processes, indirect or direct, clearly require considerable basic and applied R&D before a more detailed evaluation of their potential and plausible economics can be carried out. For example, it is not yet clear which type of algae, green algae or cyanobacteria, would be preferred in biophotolysis. If lower-cost photobioreactors can be developed, then small-scale ( 4 ha) single-stage biophotolysis processes may become economically feasible. A major basic and applied R&D effort will be required to develop such biophotolysis processes. iv V vi
doi:10.2172/776255 fatcat:wrvlyl3pwney3h7e7x4gvbygi4