Determining Safety Parameters for Small Scale Passive Hydrogen Venting Schemes[Fuel Cell and Nuclear Enclosures]

Introduction: A hydrogen economy is proposed to mitigate the effects of climate change, using hydrogen fuel cells (housed in enclosures for protection) to generate heat and energy. There is the potential though for hydrogen leaks to develop in the enclosure. Nuclear waste stored in stacked boxes produces hydrogen as a by-product of water decomposition (through corrosion and radiolysis). Hydrogen is a buoyant gas with a flammable range of 4 to 75 % in air. In both cases, leaked hydrogen can build up in confined spaces, with fuel cells the protective enclosure and with nuclear waste boxes in the spaces between the stacked boxes. To prevent flammable mixtures forming, ventilation schemes are used to disperse the hydrogen. Fail-safe passive ventilation schemes are preferred to mechanical systems that are vulnerable to power outages.
Thesis: This thesis supports the development of small fuel cell systems by investigating buoyant gas removal from a small enclosure. The thesis hypothesis is that; ‘Safe, passive ventilation parameters can be determined that will manage hydrogen concentrations below the lower flammable limit, for hydrogen leak rates at or below 10 litres per minute in a passively ventilated 0.144m3 hydrogen fuel cell enclosure’. [A 0.144 m3 enclosure houses the BOC Ltd. Hymera small fuel cell environmental unit]. To obtain new data sets the thesis hypothesis, passive ventilation experimental tests using displacement or mixing regimes were undertaken. The tests studied the effects of plain, louvre, chimney and flue ventilation openings applied to the enclosure test rig.
Helium was used as a safe analogue for hydrogen in the tests.
Findings: The displacement passive ventilation regimes were more effective than the mixing regimes and managed helium concentrations at or below 4 %, but at a leak rate limit of 4 lpm. Louvre vents applied to the enclosure increased flow resistance compared to plain vents increasing enclosure concentrations. Tall chimneys enhanced flow through the enclosure, reducing concentrations compared to shorter chimneys. Horizontal flues can be useful for transporting gas away from the enclosure [through a building wall]. The experimental data was used to validate a series of SolidWorks Flow Simulation CFD models with a good correlation found between the experimental and CFD data sets, supporting its use in enclosure design. The
thesis response is that this thesis presents a range of new experimental datasets for passive ventilation in a 0.144 m3 fuel cell enclosure, setting out hydrogen LFL safety limits. In some
scenarios, the LFL is breached at a very low leak rate, demonstrating the importance of this safety investigation, as minor leaks can lead to devastating consequences.

A jointly funded research project supported by Sellafield Ltd. and London South Bank University and in collaboration with BOC Ltd.