By Daan ten Have, Commercial Manager – Energy, Kiwa
The low energy density and molecular structure of hydrogen require it to be compressed to a significant pressure, which is one of the key challenges in designing a hydrogen cylinder. Meeting these difficult requirements with a product that has a limited number of components may not be easy, but it is crucial. So, what exactly are these requirements?
First of all, strength. The aim is to create a cylinder that is strong enough to withstand enormous pressure. Focussing on type IV cylinders, the laminate and its winding pattern rest as a sort of shell around the gas-tight inner part of the cylinder. This laminate is a combination of resin and fibre materials, and after processing, it should be able to withstand the forces and dynamics that a cylinder experiences throughout its lifespan.
To ensure the safe containment of a gaseous energy carrier, a cylinder must be completely leak tight. However, due to the specific characteristics of hydrogen, as discussed in an article on permeation featured in the February issue of the Hydrogen Tech World magazine, achieving complete leak tightness can prove to be a challenge, too.
It may appear implausible, but there is always some extent of leakage present due to the permeation of the liner material. Typically, the liner is made of a blow-moulded material that is integrated into the cylinder’s design, providing the necessary gas-tight properties of the product.
The principle of elongation refers to the ‘flex’ of a cylinder, or the increase in internal volume when the cylinder is pressurised from atmospheric pressure to, for example, 700 bar. The most cost-efficient solution is a cylinder that is able to maintain strength and gas tightness with a minimal amount of fibre material. Of the few components that form a cylinder, the fibres are the most valuable part.
During the lifetime of a cylinder, it will be pressurised numerous times, causing stress on the material in a mechanical way and also being influenced by the dynamics of temperature increase and decrease. Fast filling, which is essential in vehicle applications such as cars, trucks, and buses, involves filling the cylinder with high flow and pressure while pre-cooling the gas to -40°C to prevent temperature increase inside the cylinder and ensure safe filling.
While the above summary touches on some of the key parameters and characteristics, there are of course many more details to elaborate on. But what happens when a cylinder manufacturer aims to introduce its product to the market? How can they ensure that the market and its users can trust in a safe and well-engineered design? The answer is testing.
Standards and homologation
Several standards are applicable throughout the market, each accepted or harmonized by a structure of testing, certification, and product control in various ways.
The applicable standard and test program depend on the geographical area where the cylinder will be marketed.
National standards are required for cylinders in emerging markets like China and India, and existing standards are mainly adopted as they have been in place for quite some time. The automotive market for hydrogen cylinders is by far the most established in terms of standardisation and adoption.
The North American market has its own method of certification and does not necessarily involve an independent party for certification and product control. However, the standard used (HGV 2) is well formed and extensive.
In the European market, the UN ECE R 134 is obligatory for vehicle-level certification, and a national authority is involved in certification and production control (conformity of production).
The structure for certification of cylinders in the transportation sector, with hydrogen as cargo, is organised differently. A notified body, accredited and qualified, is involved in the technical assessment of the design and production control.
In any case, extensive testing is required.
Returning to the requirements for a cylinder to be considered fit for use, let us have a brief look at the test methods used. Hydraulic testing is used to determine the strength of the cylinder. One of the first parameters required is the burst pressure of a cylinder to align the theoretical burst pressure (by means of software modelling) with the practical burst pressure.
The cylinder testing process is relatively simple yet critical for manufacturers to obtain initial data. The dynamic hydraulic load cycle test, which involves the ingress of a liquid into the cylinder, is essential and required by many standards. The test starts at a predetermined low pressure and gradually increases to an upper limit, which depends on the maximum allowable working pressure (MAWP), the standard used, and manufacturer requirements.
The number of load cycles can vary, but 30,000 cycles are common. To simulate the cylinder’s lifetime, several external influences are added. Chemical exposure (automotive fluids), dropping from a height, impact tests, and even creating a flaw in the outer materials of the product are just some examples of what a cylinder endures during the test phase of certification. Additionally, low- and high-temperature testing is performed at -40°C and +85°C during the load cycle test.
Some standards require a hydrogen gas leak rate test to be conducted at working pressure. This test is static, meaning that the pressure remains constant and does not create any stresses on the material. The permeation is measured over a longer period of time, which can take up to 3–4 weeks.
During the simulation of the fuelling of a high-pressure hydrogen system the cylinder is pressurised with pre-cooled hydrogen for 500 pressure cycles at different temperatures, and intermediate permeation is measured. However, very few test laboratories can comply with these demanding parameters, especially as cylinders are increasing in size (>400 liters/700 WP).
When testing the cylinder for external influences, such as fire and high velocity impact, it is pushed to its limits.
For instance, when simulating a fire, the cylinder and its safety components must withstand a temperature of at least 800°C for 12 minutes without rupturing. The safety components must also ensure a controlled blowout.
In the high-velocity impact test (or shooting test), the cylinder is filled with gas and shot at to simulate the impact of a firearm. The cylinder must maintain its form after being hit, and any gas must blow out through the hole(s) caused by the penetration of the bullet.
Quality control by a third party
Certification or type approval can only be issued when the cylinder complies with the tests described in the appropriate standard and the quality of the product can be assessed and controlled.
A third party is involved in independently checking and controlling the production of the cylinders and quality management. The manufacturer is audited at pre-determined intervals to verify the quality management system. Additionally, the quality of the product is assured through batch testing of the produced cylinders using load cycle tests and burst testing.
New mobility and storage solutions
After considering all of the above, we can conclude that the hydrogen storage market is well defined in terms of safety, quality, production, and quality assurance. However, there is a constant learning curve throughout the market as new technologies, improvements in materials, and new applications emerge.
These new developments are continuously monitored and implemented in updated standards formed by all parts of the market, including manufacturers, legislators, users, and test facilities. All these stakeholders are involved in implementing these impulses in standardization.