Hydrogen tanks

Hydrogen is stored as compressed gas or in liquid, cryogenic form. This procedure makes it possible to increase energy density, but requires cooling to temperatures as low as -253 °C. Maintaining such conditions, in turn, requires large amounts of energy. Furthermore, storing hydrogen in liquid form requires the use of multi-layered tanks containing a vacuum layer and equipped with safety valves and appropriate thermal insulation. Hybrid methods are also in use, i.e. hydrogen in semi-liquid form (so-called slush) or cooled to -235°C and compressed to 30 MPa. Hydrogen can also be mixed with natural gas, as well as combined with organic carriers and ammonia.

Hydrogen penetrates most materials, so the interiors of tanks must be made to form an effective barrier. This provides them with the right structure to resist mechanical damage.

High-pressure tanks can withstand much higher pressure than the nominal value. For example, in the case of the Toyota Mirai passenger car, the tank can withstand as much as 225% of the nominal pressure. Such a tank is made of a layer of aluminium, a layer with helically twisted fibres and a layer with rim-shaped fibres. Composite tanks consist of a special anti-impact layer, an outer composite layer, an inner layer made of carbon fibre and a polymer layer. This makes the tanks extremely safe.

The pressure at which hydrogen is stored depends on the end use. Passenger cars usually use tanks with a pressure of 70 MPa, which corresponds to 700 bar. Buses and trains use 35 MPa, or 350 bar. If hydrogen is transported over longer distances, it can be stored in a tank capable of holding the gas in liquefied form.

The storage density of liquid hydrogen is approx. 71 kg per 1 m3, but this means additional costs in the form of 25-35 per cent of the hydrogen needed to cool it down. Therefore, this option is only viable for long-distance transport. A standard car-tanker combination can be refuelled with 300-500 kg of compressed hydrogen gas at a pressure of 200-250 bar. Modern tanks make it possible to load 900 kg of compressed hydrogen gas at a pressure of 500 bar or 3500 kg of liquid hydrogen at a time. Liquid hydrogen can also be transported in containerised tanks on ships or trains, or even in 'ordinary' lorries.

Supervision of hydrogen containers is exercised by the Transport Technical Supervision under the Act on Technical Supervision of 21 December 2000.Such tanks may only be used on the basis of an appropriate decision issued by the Director of the Transport Technical Supervision.

Requirements for testing and operation of specialised pressure equipment, including hydrogen tanks, are set out in the Regulation on Specialised Pressure Equipment, the so-called SUC Regulation. Under this name is the Regulation of the Minister of Transport of 20 October 2006 on the technical conditions for technical supervision in the design, manufacture, operation, repair and modernisation of specialised pressure equipment (Journal of Laws of 2014, item 1465).

Hydrogen tanks are subject to testing by the TDT. The SUC regulation clearly specifies that during the operation of this type of equipment, which is a source of power for engines in vehicles, periodic and ad hoc tests must be carried out. The latter are performed as: external and internal inspections, pressure and tightness tests. The deadline for the technical inspection of a hydrogen tank is set at 10 years with regard to the internal inspection and pressure test, i.e. similarly to the LPG tank popular in passenger cars. Once a year, they should undergo an external inspection and a leak test.

In the case of a serious collision or vehicle failure involving a hydrogen tank, they should be removed and submitted for emergency testing. In order for them to continue to operate, there must be no signs of deformation or other mechanical damage and a positive result of the test conducted by the OTD after such an event.