Maintaining hydrogen gas purity for hydrogen refuelling
The quality of hydrogen fuel, used to power hydrogen cars, trucks, buses and other fuel cell vehicles, is critical for proper operations of the fuel cells. Contaminants originating in the hydrogen supply can damage fuel cells, therefore it is critical that purity is effectively managed during the hydrogen refuelling processes to reduce the risk.
Stewart Anderson, Engineering Manager at Haskel, explains how Haskels’ H-Drive hydraulic compressor, the technology at the core of Haskel’s large fleet Geno refuelling stations, has been designed to maintain hydrogen gas purity and stop contamination.
Minimising contamination
Haskel understands the importance of maintaining gas cleanliness during compression activities, we know fuel cells are sensitive to any type of gas contamination which is why it is imperative that the gas purity is maintained within the levels outlined in ISO 14687 and SAE J2719 standards.
We recognize that gas contamination is a particular concern within the industry, especially with hydraulically driven technology. Haskel’s full range of hydrogen compressors are designed specifically to prevent any cross contamination of the gas to ensure gas purity is maintained throughout the refuelling process.
Designing out risks during hydrogen compression
During the refuelling process hydrogen is compressed, transported and stored until point of use. Compression is achieved by moving hydrogen through a compressor that decreases the gas’ volume between the inlet and discharge, sometimes in several stages. Decreasing volume increases the gas’ static pressure and makes it possible to transport Hydrogen economically so that it can be stored in tanks, while compression also boosts the gas’ pressure up to levels that are required to fuel a vehicle. Gas purity must be maintained throughout the compression process.
In order to achieve this, our H-Drive and all Haskel hydraulically driven boosters have two distinct sections, the hydraulic cylinder assembly and the gas section.
The hydraulic cylinder assembly gives the drive to the piston via high pressure hydraulic oil which pushes the pistons, this in turn compresses the gas in the gas section increasing the pressure. Between the gas piston and the hydraulic cylinder assembly is a designated separation or distance piece (section 1), which is vented maintaining atmospheric pressure to isolates the gas section from the hydraulic drive, and prevents any cross contamination.
The hydraulic cylinder assembly in the middle of the booster includes three types of seals on either side of the piston rod to keep the oil inside the cylinder assembly. The primary seal, buffer seal and a wiper seal prevent any oil escaping that could lead to contamination.
The gas sections of the booster are at either end and inside the gas cylinder is a reciprocating piston with a gas seal. The gas seals are designed to keep the process gas in front of the piston.
The seals on the gas piston are consumable items and over time these seals will start to wear and gas will leak past the front of the piston to the rear. Any gas that leaks around the gas seal will vent out through the seal bypass line (item A) in the distance piece. Given the pressure difference between the positive gas inlet and the atmospheric seal bypass line it is impossible for the process gas to mix and cross contaminate.
The distance piece also includes a number of hydraulic seals, including a rod seal which is the final barrier ensuring that any hydraulic oil is kept away from the process gas. Like the gas seals, these are consumable items and will wear over time, but if serviced correctly will never fail.
In the very unlikely event of a failure or that some oil does start to pass, the system has been designed so that any oil that leaks out of the hydraulic drive will drain out the oil bypass port in the bottom of the distance piece (item B), again the positive pressure from the gas side will prevent any oil carryover entering the gas section.
Testing and quality assurance
Through extensive hydrogen testing and operation, Haskel has found that our sealing technology is effective in keeping the process gas from being contaminated. Gas purity is maintained within the levels outlined in ISO 14687 and SAE J2719 standards.
Of course, as a consumable item, seals will degrade over time and usage profile, however with regular maintenance intervals efficiency can be maintained for the life of the system.
As a leading manufacturer of highly engineered fluid and gas handling equipment designed to generate, store and control high-pressure gases and liquids for a wide range of mission critical applications for over 75 years, customers can be confident that Haskel’s hydraulically driven boosters are fit for hydrogen refuelling applications.
If you have any further questions or would like to discuss this matter further – contact us.