Broadly, there are two options for hydrogen supply - it can be delivered to the site or it can be generated on site:
- Delivered hydrogen - Hydrogen can be delivered in a liquefied form, by tanker truck, or as a compressed gas in cylinders.
- On-site generation - The two main options for generating hydrogen on site are reformation of natural gas (a process of splitting methane into molecules of hydrogen and carbon dioxide) and electrolysis of water (using electrical current to split water into hydrogen and oxygen).
If hydrogen is delivered as a liquid, a cryogenic storage vessel will be required on the site to maintain the temperature in the liquid range. Hydrogen will be decanted from a tanker into the storage vessel.
When hydrogen is delivered as a compressed gas, the storage vessel is usually dropped off at the site by a truck and then replaced when empty. For large volumes of hydrogen, a tube trailer will be delivered, which is a 40 foot trailer on which is mounted a bundle of pressurised tubes. For smaller volumes, the hydrogen can be delivered in cylinders, which can be bundled together into packs of varying numbers of cylinders, depending on the demand.
If hydrogen is generated on site, it will be transferred to a compressed storage vessel from the reformer or electrolyser via a compressor. Liquefaction of hydrogen is an energy-intensive process requiring large equipment and would only be considered at a location where very large quantities of hydrogen were being produced.
The refueller delivers hydrogen to the vehicle's tank in a controlled manner and to the correct pressure.
Hydrogen is dispensed to the vehicle through a flexible hose and nozzle connected to the vehicle's tank, in a similar fashion to refilling with petrol or diesel.
However, in addition to the dispenser, the refuelling system will comprise other components such as a compressor and, potentially, additional storage tanks.
For small-scale hydrogen demands, the dispenser and other components may be packaged into a single compact unit, sometimes referred to as a mobile refueller. These are units that can be rapidly deployed at a site, requiring only a connection to a suitable hydrogen source. In some cases, hydrogen storage can even be included in the unit to give a truly stand-alone and mobile refuelling solution for low hydrogen demands.
The hydrogen dispenser is similar in appearance to a typical petrol or diesel dispenser, with a nozzle at the end of a flexible hose that is connected to the vehicle's hydrogen tank.
The filling procedure involves transfer of gases at high pressure, which will be new for most drivers, who will require some training. In the past, filling facilities have required operators to wear protective clothing, but this is no longer necessary with newer designs.
Hazards (for further information refer to National Operational Guidance: Utilities and fuel and National Operational Guidance: Hazardous materials)
- Hydrogen can be a dangerous substance if mishandled or if an accidental release of hydrogen were to become ignited, although in this respect it is no different from all other fuels in common use
- A number of properties of hydrogen, which are distinct from those of hydrocarbon fuels, require special consideration:
- Mixtures of hydrogen in air are flammable over a wide range of compositions
- The energy required to ignite a hydrogen/air mixture can be very low
- Hydrogen burns with a flame that is invisible in daylight
- Hydrogen is a small molecule that can leak easily
- Hydrogen is often stored at high pressure, and if released and ignited burns with a rapidly moving or jet flame
References and further reading