|Adrian Narvaez of Hawaii Hydrogen Carriers (HHC) observes a metal hydride storage tank, part of a project led by Sandia National Laboratories. (Photo by Dino Vournas) Click to enlarge.|
Sandia National Laboratories and Hawaii Hydrogen Carriers (HHC) are partnering to design a solid-state metal-hydride hydrogen storage system for forklifts; the storage system can refuel at low pressure four to five times faster than it takes to charge a battery-powered forklift. The tank will be combined with a fuel cell system to create a fuel cell power pack.
Current hydrogen storage units require high pressure (5,000 pounds per square inch, or psi) to achieve a short refueling time, and high pressure refueling requires an on-site compression system. A low-pressure on-board storage system could reduce fuel system cost and expand the market to facilities that can’t accommodate conventional high-pressure fueling systems.
That can be a big expense, especially for a small company. If we can provide a storage system that meets the target refueling time at, say, 500 psi, companies can get a break in the up-front costs. Plus, they no longer have to purchase battery rechargers or dedicate space for recharging. Instead, companies can simply purchase and store hydrogen tanks as needed.—Adrian Narvaez, Hawaii Hydrogen Carriers (HHC)
HHC, founded in 2003 by Dr. Craig Jensen, is a spin-off from the research effort on hydrogen storage materials that has been on-going since 1987 in his laboratories in the Department of Chemistry at the University of Hawaii at Manoa.
HHC, which is developing technologies for the fuel cell forklift market, obtained a grant from the Energy Department’s Office of Energy Efficiency and Renewable Energy and asked Sandia to help improve the design of such a hydrogen storage system for fuel cells.
Designing a storage system that meets HHC’s specifications and can be integrated into a fuel cell power pack required overcoming some key challenges. Among these are identifying optimal metal hydride materials, determining an optimal shape and size for the storage tank and ensuring thermal management to achieve and maintain the temperatures required for fast refueling and supply of the hydrogen.
Work to identify the right metal hydride for the system focused on Hy-Stor 208, a misch metal-nickel-aluminum alloy that meets targets for hydrogen storage capacity, density and thermal conductivity. The material also provides sufficient hydrogen pressure for refueling at an operating temperature of 60 degrees Celsius.
While this type of metal hydride is heavy, the weight acts as needed ballast and thus is a benefit in forklifts. To increase thermal conductivity, the team also explored adding to the metal hydride two forms of expanded natural graphite, flakes and “worms” (called this because of their tubular shape).
Sandia’s project manager Joe Pratt and Narvaez drew on modeling and simulation results from an earlier project led by Sandia engineer Terry Johnson to identify a small-diameter tube as the best design for storing the metal hydride. They then varied several tube characteristics, such as the hydrogen distribution channel and the amount and type of thermal enhancement material used. Next, they conducted experiments to evaluate the effects of these variations on a range of performance parameters, including hydrogen storage capability, refill time, durability, discharge ability and residual capacity at a minimum discharge point.
As the models predicted, we saw only minor differences in performance when we varied the graphite types. Likewise, the presence or absence of the hydrogen distribution channel had little effect on performance. These findings show that this application is not aggressively pushing the performance of the metal hydride storage to the point where these variations would make a difference. In fact, this is good, because it means we can use the lowest-cost solution and still expect good performance.—Adrian Narvaez
Using findings from their experiments, Pratt and Narvaez developed an optimized storage-system design.
During this time, the team also began to conceive of a tube array that would allow efficient thermal management (via water flows around the tubes).
With Sandia’s and HHC’s design complete, project activity will transfer to Hawaii, where HHC will produce the first prototype metal hydride storage system. HHC will work with Hydrogenics, which will integrate the new storage system into its proton exchange membrane (PEM) fuel cell power pack, designed to fit into a forklift.
DOE catalyzed the market for fuel cell forklifts, using industry cost-sharing to deploy more than 500 units through the American Reinvestment and Recovery Act. The private sector recognized the advantages of fuel cell forklifts and deployed more than 5,000 additional units since then without government funding. If successful, the HHC project will lead to lower cost, improved-performance fuel cell forklift systems that will lead to even greater market growth.—Joe Pratt
Sandia has worked with the fuel cell forklift industry for several years to help get clean, efficient and cost effective fuel cell systems to market faster. Standards developed by Sandia soon will be published so industry can develop new, high-performing hydrogen fuel systems for industrial trucks.
Pratt has spearheaded other Sandia efforts to introduce hydrogen systems into the marketplace. He served as technical lead, for example, for studies on the use of fuel cells to power construction equipment, personal electronic devices, auxiliary equipment and portable generators. Most recently, he led a study and subsequent demonstration project on commercial use of hydrogen fuel cells to provide power at ports.