What guidance is available for using an autonomous solar hydrogen electrolyzer to produce and vent small amounts of hydrogen to the atmosphere, with a goal to eventually demonstrate compressing the hydrogen into a storage tank? Because solar is intermittent, there needs to be an operating scheme that keeps the unit running when power is low. The electrolyzer runs 24 hours per day, albeit with relatively low output.
The vent system for the excess hydrogen should be vented in accordance with NFPA 2 and CGA G-5.4 and G-5.5. The oxygen also must be vented safely and should be in accordance with NFPA and CGA G-4.4.
Please be extremely cautious with compressing hydrogen. NEC/NFPA 70 and its Articles 500/505 address electrical equipment in flammable atmospheres. Please also consider the information in NFPA 2, Hydrogen Technologies Code. NFPA 2 has many prescriptive requirements for use, generation, and compression of hydrogen with properly rated equipment and containers. There have been many accidents where equipment and systems not rated for hydrogen have:
- leaked near ignition sources such as static electricity and standard electrical equipment,
- used vessels not intended for hydrogen,
- used vessels improperly purged of air or other reactive gases, and d) located equipment in areas with inadequate air ventilation that allows hydrogen to accumulate.
Valuable insight comes from an incident that occurred in May 2019, where an outdoor hydrogen tank exploded at a research and development venture complex during test of a water electrolyzer coupled to a renewable energy system. The estimated 100 lb. TNT equivalent blast killed two and injured six. The cause of the tank explosion was reported to be auto-ignition of a hydrogen-oxygen gas mixture within the storage tank. The gas source, a 9 bar H2 and O2 rated pressurized alkaline water electrolyzer, was under test to evaluate intermittent renewable energy duty cycle performance.
Technical reports state that potential electrolyzer cell membrane degradation permitted excessive oxygen gas crossover rate through the electrolyzer cell membranes into the product hydrogen gas. This condition may have been made worse by an extended period of low electrolysis gas generation rate prior to the incident, resulting in low hydrogen flow rates insufficient to dilute diffused oxygen below O2 in H2 flammability/explosibility limits. Corrective actions reported include use of catalytic gas purifiers to remove O2 from the product H2 and waste O2 lines.
Lessons learned:
- Understand interrelation of electrolyzer membrane gas permeability, membrane degradation, and dynamic operating range when establishing process safety controls;
- Consider automatic gas storage isolation and stopping gas generation when safety limits are exceeded, such as flammable gas mixture, excessive cross-cell differential pressure, low dynamic range, and other applicable process safety limits.