A group of students from Washington State University (WSU) won the Hydrogen Education Foundation’s 2014 Hydrogen Student Design Contest with plans for a transportable hydrogen fueling station.
The hydrogen station is designed to support the early commercialization of fuel-cell electric vehicles (FCEVs). Introduction of these new light-duty vehicles began in earnest when the Hyundai Tucson Fuel Cell hit the California market in June, and will grow with Toyota, Honda, and Mercedes-Benz hydrogen FCEVs in the near future.
Hydrogen FCEVs have zero tailpipe emissions while bolstering many of the conveniences of conventional gasoline vehicles, such as 5-min fueling times and a driving range over 300 mi (500 km). Despite FCEVs’ many benefits, their widespread adoption has been hindered by a lack of hydrogen fueling infrastructure. There are only 12 hydrogen fueling stations open to the public in the U.S., mostly in southern California, compared to roughly 121,000 gasoline stations spread throughout the country. The WSU station design is intended for rapid, minimum-cost deployment.
The WSU team found an edge over the competing designs by employing liquid hydrogen (LH2) storage to maximize station capacity and reduce capital and operating costs. Delivering hydrogen in liquid form reduces the energy used for distribution, and 80-90% of small-merchant hydrogen is delivered via cryogenic liquid tanker truck.
“We asked ourselves, ‘What do we know better than anyone else in this competition?’ The answer was handling low-temperature hydrogen,” said Jake Fisher, a mechanical engineering graduate student whose research is focused on processing solid hydrogen.
The team designed a novel two step dispensing process that uses cryogenic compression to achieve the industry-standard FCEV tank pressure of 10,000 psi (900 bar). Cryo-compression, or “autogenous pressurization,” is the process of converting a closed volume of low-temperature liquid into high-pressure gas by adding heat while using minimal equipment and no moving parts.
The energy required to boil the liquid can be supplied by the surrounding environment, another benefit of cryo-compression. This compression method would eliminate the need for a 12,000-psi (800-bar) hydrogen compressor, which is typical of most hydrogen fuel stations. A 12,000 psi compressor adds considerable cost to the station and overheats the fuel, which requires active cooling, which in turn adds time to customer fill-ups.
“We then faced the dilemma of what to do with the leftover hydrogen after dispensing the cryo-compressed volume,” said Ian Richardson, the team leader and a material science and engineering graduate student at WSU. "We conceded to adding a relatively cheap 6000-psi (400-bar) compressor which made sense economically."
The compressor would store boil-off vapor from the liquid storage tank and the residual cryo-compressed gas in medium-pressure cylinders. This would make the station essentially 100% efficient at storing and dispensing hydrogen. The hydrogen from the medium pressure cylinders would be used in the first step of dispensing to bring the FCEV fuel tank to 75% of full charge before discharging the cryo-compressed gas from the high-pressure cylinder to complete the fill-up.
The team saw the two compression methods complementing each other in thermal management as well. The compressor-filled medium-pressure cylinders and the cryo-compressed high-pressure cylinders would all be held in a bath of heat-transfer fluid. This would create a thermal link between the cylinders so the heat added by the compressor could be absorbed by the cold liquid hydrogen charge, promoting cryo-compression. The bath would be maintained at -40°F (-40°C) by an industrial chiller. Dispensing gaseous hydrogen at -40°F keeps the FCEV tank from overheating, which allows for a quick refueling time of 2 to 3 min for a 5-kg (11-lb) tank at 10,000 psi. Competing designs and current stations store ambient temperature gaseous hydrogen at 3000 psi (200 bar), then compress and cool it on demand, requiring high-pressure hydrogen heat exchangers and adding several minutes to each fill-up.
The hydrogen fueling infrastructure has historically been technically infeasible and too expensive, but the WSU transportable station challenges that convention. The station was completely designed using actual vendor quotations totaling $423,000 of off-the-shelf components, 25-40% of the cost of permanent hydrogen stations, and could deliver hydrogen to vehicles at a price comparable to gasoline. An operational cost analysis shows that this design could fuel vehicles for $9.62/kg ($4.36/lb) of hydrogen, which is roughly $48 for a 5-kg fuel tank with a range of approximately 300 mi. This price of hydrogen is equivalent to paying $4 per gallon of gasoline for a car that gets 25 mpg.
The fueling station was designed to fit within a standard 40-foot ISO container to ease production and maximize mobility. It would be delivered via semi-truck, connected to 208-V electricity and local water, and ready for dispensing within 24 h. Tanker trucks would refill the hydrogen station’s bulk liquid storage tank once or twice a week depending on demand, just like existing gasoline stations.
Costumers would interact with the station via touch-screen tablet computer located next to each nozzle. The customer interface would walk the customer through the fueling process from payment to fill-up providing step-by-step instructions along the way. A “Helpline” button has been included in the design to allow customers to live-video-chat with a remote operator for additional assistance. Electronic payment options such as PayPal have also been included for customer convenience.
The fuel station was designed to operate autonomously so an attendant is not required. Offsite monitoring allows a remote operator to monitor and access the station to ensure the station is operating properly.
The hydrogen station was designed with top-of-the-line safety and fire-suppression systems to ensure public safety and meet codes and regulations.
A complete report of the design can be found at: http://hydrogencontest.org/pdf/2014/WSU_2014_HEF_CONTEST.pdf.
This article was written for SAE Magazines by Ian Richardson and Jake Fisher of Washington State University.Continue reading »