Today, most hydrogen is produced from fossil fuels via thermochemical processes. Although currently cost-effective, the production of hydrogen from fossil fuels results in the release of large quantities of carbon dioxide and will, in the future, be constrained by the availability of feedstock. The inherent environmental advantages of hydrogen may not be sufficient for its acceptance as an energy carrier in the world economy. Successful introduction of hydrogen into the national energy sector is dependent on the continued development of sustainable production and storage technologies as well as the development of essential infrastructure and end-use applications. Sustainable production technologies utilizing water or photosynthetically produced biomass as the feedstock have been identified, but technological developments and cost reductions are necessary for these processes to become viable on large scales.
In 1986, under contract to the National Renewable Energy Laboratory (then the Solar Energy Research Institute), HNEI together with the Florida Solar Energy Center conducted an assessment of hydrogen production technologies and the economic feasibility of the production and use of hydrogen from renewable resources. Today, the Institute has a nationally recognized hydrogen research program focusing on the production of hydrogen from renewable resources and the development of improved high-density hydrogen storage technologies. Under this DOE program, significant advancements have been made in the direct production of hydrogen using photoelectrochemical techniques, by the gasification of biomass using supercritical water, and by biological techniques.
Photoelectrochemical Hydrogen Production
Development of high-efficiency photoelectrochemical systems to produce hydrogen directly from water, using only sunlight as the energy source, is a major goal of the DOE Hydrogen Program. Since 1995, a number of thin-film photoelectrode configurations have been explored in HNEI's Thin Films Laboratory. In 1997, a small scale reactor based on monolithically-stacked triple junction amorphous silicon/germanium alloy (a-Si:Ge) thin film solar cells was used to demonstrate solar-to-hydrogen efficiencies up to 7.8%. These solar cells were modified with cobalt-molybdenum and iron-doped nickel oxide thin film catalyst coatings developed at HNEI. In separate tests, these thin-film low-cost catalysts were operated in KOH electrolyte for over 5,000 hours with no evidence of significant degradation.
Current efforts are focusing on a unique 'hybrid' photoelectrode structure developed at HNEI which combines a tandem a-Si:Ge solar cell monolithically series connected to a thick, photoactive over-coating of nano-structured metal oxide. In early testing, these devices have shown great promise for the development of long-life, high-efficiency hydrogen production systems. Working with key industry and academic partners, HNEI expects to successfully demonstrate solar-to-hydrogen efficiencies in excess of 10% and exceptional stability in a working version of the hybrid photoelectrode by 2015. For more information, click here.
Biological Hydrogen Production
HNEI has carried out R&D on biological hydrogen production since the early 1990's. Initially, this project investigated the genetics of cyanobacterial (blue green algae) hydrogenases. A new R&D phase was initiated in 1996 to develop a microalgal indirect biophotolysis process, in which water is converted in separate stages into O2 and H2. The organism chosen for initial work on this project was a strain of Spirulina (Arthrospira platensis) already being commercially grown in Hawaii and used in the prior biohydrogen research at HNEI. Laboratory work confirmed that Spirulina produces H2 by dark fermentations, but not in the light.
The major part of the research carried out under this project from 1996 to 2000 was the operation and engineering studies of the photobioreactors. While this initial work demonstrated the ability to produce Spirulina in the reactors and significant advances were made, an indirect biophotolysis process using cyanobacteria in the photobioreactors was not demonstrated. Proposals for future biohydrogen research at HNEI aim to maximize the yield of H2 from endogenous substrates by dark fermentations in microalgae and, in particular, by bacteria using exogenous waste substrates. Such processes could produce H2 fuel in small-scale amounts at acceptable costs in the near-term, and larger quantities in the long-term.
Gasification of Biomass in Supercritical Water
Wet wastes (sewage sludge and paper sludge) are available in large quantities at negative cost around the world. Likewise, many aquatic biomass species grow rapidly and are particularly attractive because their cultivation does not compete with land-based agricultural activities centered on food production. In spite of these many attractive features, wet wastes and biomass have not been regarded as promising feedstocks for conventional thermochemical conversion processes because of the high cost associated with drying the material prior to entering the reactor. Under this project, HNEI has developed a process for hydrogen production by the catalytic gasification of biomass in supercritical water (water at high temperature and pressure). This "steam reforming" process produces a gas at high pressure (>22 MPa) that is unusually rich in hydrogen. The results of this work, conducted in the Renewable Resources Research Laboratory between 1990 and 2001, are summarized in a series of peer-reviewed publications.
While much of the early research on high-density storage systems was conducted under the auspices of HNEI, in 2000 this component was placed under the direction of the University of Hawaii's Department of Chemistry. The latest information on this area can be found at http://www.chem.hawaii.edu/UH_Chem/faculty/jensen.html.