Biologically-generated Hydrogen as Fuel
by Amanda Marshall
"What happens when we run out?" For decades, engineers, politicians and frustrated gasoline customers have grappled with this question regarding fossil fuels. Coal and oil, today's main energy sources, are being consumed much faster than available supplies can sustain. From California's energy crisis in the year 2000 to skyrocketing prices at the gas pump to fervid political debates about the global petroleum trade, it is no secret that homes, automobiles and industry cannot continue to be powered by fossil fuels indefinitely. Dr. James Swartz's Chemical Engineering research group at Stanford is working on a novel solution to this problem by engineering bacteria to produce large quantities of hydrogen for fuel.
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| Energy from sunlight is harnessed for a hydrogenase enzyme to produce H2 (the target product) and O2 (a byproduct) from H20. |
Swartz aims to use bioengineered organisms to 'farm' molecular hydrogen, which could then be used to generate electricity in a fuel cell. Certain types of bacteria contain hydrogenases, enzymes that generate hydrogen from electron-transport chains used for metabolic processes. Like photosynthesis in plants, these electron-transferring processes in bacteria are driven by sunlight energy.
The most convenient electron source for large-scale energy production is water. Swartz explains that only three main components are involved in the pathway of H2 production from water, allowing for "the shortest possible path that biology gives us between absorption of sunlight and production of molecular hydrogen." First, a photosystem - a molecular system that promotes electrons to higher energy levels - captures light energy to splits water, releasing oxygen, protons and energized electrons. The energized electrons are transferred to ferredoxin, an iron-containing protein that serves as an electron carrier. H2 is produced when ferrodoxin transfers these electrons to the hydrogenase. When used for fuel, the combustion of H2 uses O2 and gives off heat and water, which can then enter the cycle again to produce H2. Thus, water-splitting offers a direct, efficient path between sunlight absorption and H2 production.
Engineering Bacteria
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| Chemical Engineering Professor James Swartz (right) and graduate student researcher Marcus Boyer. |
One hurdle to this method is that the O2 produced through water-splitting inactivates hydrogenase. Therefore, one of the foremost challenges Swartz faced as he began his research was to find a way to make hydrogenase resistant to O2 inactivation.
To create an O2-resistant hydrogenase, Swartz's group is using a novel approach. He uses a cell-free production process to rapidly and efficiently evaluate the biochemical properties of hydrogenase. Without the need for living organisms, Swartz's group can quickly design, isolate and examine tens of thousands of mutant enzymes in order to identify those that are O2-tolerant. In a cell-free environment , hydrogenases require 'helper' proteins to become biologically active. The class of helper proteins currently used was identified in 2004.
According to Swartz, "We can make high level s of active hydrogenase with [these helper proteins], but we still are not quite sure how they work." Consequently, a significant portion of the Swartz group's research involves not only characterizing the hydrogenases, but also understanding the chemical mechanisms and biological functions of the specific types of helper proteins that are working to assemble and activate the hydrogenases.
Once oxygen-tolerant hydrogenase enzymes are discovered, they can be reintroduced into the organisms from which they came, or transferred into new organisms. Particularly, Swartz wants to put genetically engineered hydrogenase into photosynthetic organisms in order to maximize the capture of solar energy, and also to add helper proteins to these organisms. As a result, the potential fuel-generating bacteria will be hybrids composed of elements from at least three different organisms.
From Research to the Real World
Swartz estimates that H2 will be introduced as a fuel in as little as five to ten years. Ideally, it will eventually be used to power automobiles, but the most immediate applications will likely be 'point sources' such as electrical power plants, from which large quantities of power can be distributed. For H2 to be produced in mass quantities as fuel, bioreactors need to be designed to house the bacteria. They will need to efficiently collect sunlight, harvest H2, and remove O2 waste.
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| Hydrogenases for producing H2 are obtained from E.coli such as those pictured above. |
Swartz envisions a three-layered bioreactor. In this design, temperature would be controlled by circulating fluid in the lower layer, captured sunlight and bacteria would produce H2 in the middle layer, and gaseous waste would be pulled into a vacuum chamber in the upper layer. Verifying a bioreactor's effectiveness would entail measuring rates of H2 production and light absorption efficiency, as well as determining its ability to control such parameters as temperature and pH. Swartz characterizes bioreactor development as an "iterative process, by which we identify what [needs] to be modified to make it into a better overall system."
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| Diagram of a proposed system for hydrogen production. |
Wind Is Another Clean Source of Hydrogen Production?
Wind is another promising means of generating hydrogen. Associate Professor Mark Z. Jacobson of the Civil and Environmental Engineering Department, along with former postdoctoral fellow Cristina Archer, published a study in 2005 that mapped global winds and showed the world, especially the United States, has more than enough wind to meet all its energy needs. Wind farms, linked in a network of electricity-generating wind turbines, would ensure energy production even when parts of the network have days without wind. The electricity would travel through transmission lines to filling stations where it would enter an electrolyzer, which applies current to water, splitting it into oxygen and hydrogen. The hydrogen would then be compressed and stored.