Hydrogen in the atmosphere can exacerbate global warming by acting as an indirect greenhouse gas. But how much H2 currently leaks throughout the value chain — and would scaling production up to replace fossil fuels make this worse?

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A lack of real-world data means that estimates for total leakage — from production to end use — vary between 0.2% and 20%, according to a review of studies by researchers at US environmental advocacy group the Environmental Defense Fund published in peer-reviewed journal Frontiers in Energy Research.

Part of the problem is that hydrogen is an extremely small molecule that disperses quickly from the point of leakage.

Most commercially available sensors are built to spot high concentrations of H2 in the parts per million that pose an immediate safety risk, although the Environmental Defense Fund has worked with equipment manufacturer Aerodyne to develop a sensor that can detect hydrogen at concentrations as low as ten parts per billion.

The lack of monitoring equipment to date means that extremely small leaks that accumulate thoughout the value chain are never recorded, making it difficult to determine how much hydrogen escapes into the atmosphere based on empirical data.

And the estimates for hydrogen leakage vary wildly depending on method of production and how it is stored and transported.

For production, estimated emissions rates range between 0.5% and 1.0% for grey H2 made from unabated natural gas, 0.0%–1.5% for blue H2, and 0.03%–9.2% for green H2, according to the review.

Green hydrogen is expected to have a relatively high leakage rate “due to the higher emission rates associated with electrolysis compared to steam methane reforming and biomass gasification”, the paper notes.

However, blue H2 is expected to have far more of a climate impact due to high fugitive methane emissions from upstream gas production, previous studies have suggested.

The review notes that hydrogen leakage rates for commonly suggested end uses are estimated to be 0.01%–3.0% for power generation, 1.0%–2.3% for ships and ports, and 3.0% for aviation.

Meanwhile, emissions rates for storing and transporting hydrogen as a liquid vary wildly, with projected leakage during handling ranging from 2-20%.

Liquefaction is estimated to leak between 0.15-10%, while transporting liquid H2 via trucks could leak 2-13.2%. And leak estimates for refuelling stations storing and distributing liquid hydrogen range from 2-15%.

While storing, transporting and using gaseous H2 also varies, the upper estimates are much lower than for liquid.

The review tracks estimated leak rates of “0.02%–5.0% for transmission pipelines, 0.0003%–5.0% for distribution pipelines, 0.3%–2.3% for transportation on road tube trailers, 2.8%–6.5% for storage above ground, and 0.25%–3.0% for gas refueling stations”.

Liquefied hydrogen is expected to have higher emissions due to boil off. In order to stay liquid, the H2 has to be kept at minus 253°C (around 20°C above absolute zero), but hotspots within the tank can lead to the formation of vapour.

While planned venting is prohibited by the UN’s Agreement concerning the International Carriage of Dangerous Goods by Road (ADR), which regulates transport of industrial gases and other chemicals, a release of H2 is permitted as a safety precaution if pressure builds up due to the vapour expanding within the tank.

But while vented hydrogen can be recirculated, either into another tank to be taken back for liquefaction or as a system refrigerant, this hinges on whether the end-user is willing to pay for the additional cost of these processes and whether the H2 is needed at extremely high purity, which would prevent backflow.

Mainstream liquid hydrogen storage tanks, such as those used by Nasa, are said to lose 1-5% of their H2 every day.