Taking the Long View
Hydrogen produced using algae, water and sunlight could be a long-term energy source.
While some ORNL researchers strive to develop a cost-effective means of converting green plants to ethanol and other liquid transportation fuels, their colleagues are exploring another potential plant-based source of renewable fuel for the longer-term goal of hydrogen-powered vehicles.
The development of hydrogen as a major energy carrier envisioned for the latter half of this century includes the use of green plants to produce hydrogen to power fuel cells in electric cars, replacing internal combustion engines and dramatically slowing the buildup of atmospheric carbon dioxide. The Department of Energy views the use of hydrogen as one way to address the agency's energy security and environmental missions.
"Green" hydrogen sources
After America's first major energy crisis in 1973, some researchers at the national laboratories began exploring photosynthesis to extract hydrogen from water in the search for energy alternatives. Three decades later, two common methods for producing hydrogen use fossil fuels and release carbon dioxide, a greenhouse gas. In one method, hydrogen is produced by reacting natural gas with steam at high temperature. Splitting water molecules using electrolysis—which often uses electricity from power plants that burn coal, natural gas or oil—also produces hydrogen.
ORNL was the first research institution to demonstrate the sustained simultaneous production of hydrogen and oxygen by illumination of green algae, a biological version of electrolysis, for hydrogen production. However, the efficiencies of the process must be improved before the research will have practical application.
The research was pioneered by Eli Greenbaum, an ORNL corporate fellow who heads the group in the Chemical Sciences Division that demonstrated a way to split water into hydrogen and oxygen using algae in the presence of light. The team is currently using genetic engineering to make a mutant form of algae with the goal of boosting photobiological hydrogen production 10 times using light with a tenfold increase in intensity.
In 1999, 60 years after a scientist observed that pond scum algae alternated between hydrogen production and normal photosynthesis in different atmospheres, researchers at the University of California at Berkeley and DOE's National Renewable Energy Laboratory in Colorado determined that depriving algae of sulfur and oxygen enables the production of hydrogen for an extended period. Plants require sulfur to survive because they make proteins from the element. Eventually, researchers were able to switch hydrogen production on and off repeatedly by changing the algae's chemical environment. With support through Robert Hawsey, manager of ORNL's Energy Efficiency and Renewable Energy program, and Tim Armstrong, manager of the Hydrogen, Fuel Cells and Infrastructure program, James W. Lee, a senior ORNL scientist, leads the ORNL photobiological hydrogen research effort using a new approach: proton-channel designer algae for enhanced hydrogen production from water. Lee developed this new approach using molecular genetics based on the fundamental understanding of biological hydrogen systems that has accumulated in the field, including Greenbaum's pioneering contributions.
In Oak Ridge, Greenbaum's group discovered that Chlamydomonas reinhardtii can produce sustained simultaneous production of hydrogen and oxygen from water using light for extended periods of time. These algae normally grow new cells by photosynthesis, using carbon dioxide from the air in the presence of sunlight. But after placing the aquatic organisms in a large flask of water illuminated by lamps, the ORNL researchers "tricked" the algae by depriving them of carbon dioxide and oxygen. As a result, a normally dormant gene becomes activated, leading to the synthesis of the enzyme hydrogenase. The algae use this enzyme to produce both hydrogen and oxygen from water.
"Our group performed some of the original basic research studies on the kinetic rates and mechanism of photosynthetic hydrogen production in which green algae split water into molecular hydrogen and oxygen," Greenbaum says. "We were the first to demonstrate that illuminated green algae can be used to sustain the simultaneous production of hydrogen and oxygen. We performed the first measurements of the photosynthetic unit size and turnover times of hydrogen and oxygen production." Greenbaum made another important discovery. His research also demonstrated that the presence of carbon dioxide inhibited photosynthetic hydrogen production.
With support through Robert Hawsey, manager of ORNL's Energy Efficiency and Renewable Energy program, and Tim Armstrong, manager of the Hydrogen, Fuel Cells and Infrastructure program, James W. Lee, a senior ORNL scientist, leads the ORNL photobiological hydrogen research effort using a new approach: proton-channel designer algae for enhanced hydrogen production from water. Lee developed this new approach using molecular genetics based on the fundamental understanding of biological hydrogen systems that has accumulated in the field, including Greenbaum's pioneering contributions.
The proton-channel designer alga approach could solve the following four problems that currently challenge researchers and investors in the field: (1) restriction of photosynthetic electron transport needed for hydrogen production by the increased proton concentration gradient, (2) competitive inhibition of photosynthetic hydrogen production by carbon dioxide, (3) requirement of bicarbonate binding at photosystem II (PSII)—a protein in the algal chloroplasts which absorbs light energy—for efficient photosynthetic activity, and (4) competitive drainage of electrons by oxygen in algal hydrogen production. Lee says that, unless these four physiological barriers are overcome, impressive amounts of hydrogen will not be produced from green algae.
"For example, the biomolecular machinery needed for hydrogen production severely stalls when the algal thylakoid membrane is quickly jammed with too many protons, inhibiting photosynthetic electron transport," Lee explains. "In addition, natural algae do not produce hydrogen efficiently because of the competitive inhibitions by carbon dioxide and oxygen. Furthermore, because the binding of bicarbonate at PSII is required for efficient photosynthetic activity, the inhibition of hydrogen production by the presence of carbon dioxide cannot be solved by removal of this source of carbon."
However, according to Lee, these four problems could be simultaneously solved by the proton-channel designer alga approach. The reason: all four hurdles are associated with the proton concentration gradient.
In supporting Lee's project of genetically engineering wild algae to leap over all four hurdles, Dong Xu used the award-winning PROSPECT computer software tool he helped develop at ORNL to analyze the structure of mellitin, or bee venom, which conducts protons. With this model, Lee and his collaborators designed a proton-channel gene and coupled it with another DNA segment that serves as a promoter, turning on only when oxygen is absent. Thus, the proton-channel gene is expressed only when not exposed to oxygen. During those times, protons pass through the channel the gene creates, preventing proton accumulation and simultaneously eliminating the other three proton-associated problems as well.
"We have made our first set of proton-channel genes and introduced them into algae cells successfully," Lee says. "We could improve renewable production of hydrogen from water by a factor of more than 10 if the designer alga gene works as theory indicates. When we remove oxygen, the designer gene should switch on, causing the mutant algae to produce hydrogen in large amounts."
Lee, who received the FuelCellSouth 2006 Crystal Flame Innovation Award for this research, is realistic about the challenges that remain: "In addition to creating the designer algae, we will also have to solve engineering problems, such as separating hydrogen from oxygen and suppressing the explosive potential of hydrogen in the presence of oxygen," Lee says. "Our ‘switchable PSII designer alga' concept, which has recently been explored through our ORNL seed money project, could provide an alternative solution to the problems posed by the presence of oxygen, including the need to separate oxygen from hydrogen immediately after both gases have been produced from biological splitting of water molecules. Calculations indicate that the designer algae hydrogen production technology, when fully developed, could annually support 140 fuel-cell cars per acre of enclosed algal ponds. This estimate represents a theoretical upper limit, and much more work is needed to achieve such a challenging goal."—Carolyn Krause
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