Hydrogen can be delivered in one of two ways for on-site storage and dispensing. Gaseous hydrogen deliveries are suitable for a very small fleet (1 to 3 vehicles) or for backup to an on-site hydrogen generation system. A standard tube trailer at 2,400psig with 60,000 cubic feet of capacity contains hydrogen with about the same energy content as 140 gallons of gasoline. The hydrogen is compressed on-site in order to be dispensed into a typical vehicle tank that stores hydrogen at 5,000psi. A system such as this would be simple and uses existing, proven technology, however it yields hydrogen at a relatively high per unit cost.
Another way to deliver hydrogen is in liquid form. Some hydrogen fueling stations are using delivered liquid hydrogen. The California Fuel Cell Partnership station in Sacramento, and Shell's first hydrogen station in Washington, D.C. use liquid hydrogen provided by industrial gas companies. It is an existing, proven technology. However, up to 40 percent of the energy content of hydrogen is required to convert it from a gas to a liquid. Also, the transportation and storage of liquid hydrogen adds considerable cost and there is a continuous boil-off of liquid hydrogen, which must be vented into the air.
On-site Generation of Hydrogen: Electrolysis
An on-site electrolyzer can generate relatively small quantities of hydrogen required to fuel two or three cars. Theoretically, the electricity for these stations could be provided by renewable solar energy or wind power, but they are typically operated from electricity purchased from the utility power grid. While the electrolysis process is very clean, the original source of the electricity is almost certainly at least partially derived from coal or other hydrocarbon fuels. The current generation of electrolyzers require between 55--70kWh of electricity to produce one kilogram of hydrogen (approximately the same energy content as a gallon of gasoline). A PEM fuel cell will then convert the hydrogen back to electricity at about 60 percent efficiency. This is a very expensive way to generate hydrogen and, while possibly appropriate for small fuel cell demonstration projects, it has significant cost barriers.
On-site Generation of Hydrogen: Natural Gas Reforming
A promising alternative for generating hydrogen for airport fleet applications is to convert natural gas to hydrogen on-site. Most of the hydrogen currently produced in the world today is generated using a process called steam methane reforming. A steam methane reformer (SMR) can be scaled (most generate hydrogen in quantities up to several tons per day) down to a rate of 50--150 kilograms per day that is adequate for a small to medium sized fleet of hydrogen-fueled vehicles. On-site SMR's provide hydrogen that is lower in cost than either delivered hydrogen or hydrogen from electrolysis. Excess hydrogen generated by an on-site SMR can be used to generate electricity via a fuel cell for power generation. The biggest downside for this alternative is that SMR's are not widely available in this size range and are only just beginning to be deployed for fueling station applications.
A Hydrogen Fleet and Fueling Configuration for Airports
Airports are important early adopters for hydrogen technologies. In addition to environmental reasons such as emissions reduction for an airport ground support fleet, a hydrogen energy system, as depicted in Figure 7 can provide backup electricity for critical electronic systems in case of grid power failure. The system shown includes a basic hydrogen system which would provide "backup hydrogen" along with hydrogen generated from natural gas and dispensed to both on-road and off-road vehicles that are found at airports.
Long identified with emerging technologies, from the latest aircraft systems, security systems and even WIFI communications, airports are a good choice to deploy emerging technologies. Unique in their combination of quasi-government status, controlled access, high profile, multiple applications and availability of trained technicians, airports will be instrumental in proving the reliability and safety of hydrogen systems.
Clearly, embracing hydrogen has become a societal issue in which government has made the decision that economic, environmental and national security issues dictate the formation of a robust hydrogen industry. Airports and the GSE industry appear destined to play an important role in this process.