The widely accepted view that electrolysers need to operate as close to 24 hours a day seven days a week as possible to produce green hydrogen at the lowest possible cost is false, according to a new scientific study.
A team led by Stanford University professor Mark Z Jacobson found that the lowest-cost renewable H2 in 2035, assuming a 100% renewables electricity grid, will be produced with an optimum electrolyser use factor — ie, the amount of energy consumed annually compared to the theoretical maximum — of between 20% and 65%, depending on location and use cases.
“From a business point of view, rectifiers [to convert AC to DC electricity], electrolysers, and compressors should be operated full time (with a use factor of unity [ie, 100%]) to minimise cost,” Jacobson and colleagues write in the Smart Energy journal.
“However, we hypothesise that, at high WWS [wind-water-solar] penetrations and hydrogen penetrations, running this equipment intermittently (with sub-unity use factors) reduces overall system cost.”
The study goes on to explain that “the nameplate capacities of rectifiers, electrolysers, and compressors must be increased to ensure the same annual quantity of hydrogen is produced as with a unity use factor” and that “higher nameplate capacities mean higher capital costs”.
But it adds: “Fortunately, the impact of higher capital costs on overall hydrogen cost is largely offset by the longer lifetimes of rectifiers, electrolyzers, and compressors with lower use factors.”
In other words, the cost of buying the equipment can be amortised over a longer period of time, thus reducing the levelised cost of green hydrogen (ie, the per-kilogram cost of H2 over a project's lifetime, including all upfront and operating expenses).
The paper adds that reducing the amount of time H2 equipment is in operation also means that fewer solar panels and/or wind turbines would be needed, as well as smaller volumes of hydrogen storage — and no battery storage to keep equipment operating when the wind isn't blowing and the sun isn't shining.
“Additional cost is offset because, either a lower nameplate capacity of wind and solar or less hydrogen storage is needed with a lower use factor,” it explains.
“In sum, the lowest cost of hydrogen production integrated with 100% WWS occurs at a hydrogen-equipment use factor below unity, between 0.2 and 0.65 [ie, 20-65%] in the test cases provided.
“As such, it is more cost-effective from an overall system perspective to produce hydrogen intermittently rather than continuously. The results here are subject to model uncertainties, but [the] conclusion appears robust.”
The study — entitled Impacts of green hydrogen for steel, ammonia, and long-distance transport on the cost of meeting electricity, heat, cold, and hydrogen demand in 145 countries running on 100% wind-water-solar — concludes that using dedicated renewables projects solely for hydrogen production means that wind, water or solar power generators “cannot be called upon to provide grid electricity, heat, or cold in a time of high electricity, heat, or cold demand or low overall WWS supply, increasing the need for electricity and heat storage”.
“Economies of scale suggests that interconnecting all WWS generators and adding loads (such as green hydrogen production) that can absorb excess electricity, results in the most cost-effective system.
“Thus, grid operators should incorporate intermittent green hydrogen production in grid planning.”
As the extremely long title of the paper implies, the study was based around the amount of renewable electricity and electrolysis that would be required to meet the global demand in 2050 for green hydrogen from three sectors — steel, ammonia and long-distance transport (including, controversially, all aviation and shipping) — which it put at 78.7 million tonnes, 31.7 million tonnes and 91.2 million tonnes a year, respectively, requiring a peak renewable power demand of 7,057GW (including its use to compress H2) and 6,212GW of electrolysers.
The 145 countries are responsible for more than 99.7% of global fossil-fuel emissions.
Jacobson et al argue in the paper that biofuel and synthetic aviation fuel are not valid decarbonisation methods for ships or airplanes because they still produce gases and particles that are bad for human health and/or the planet.
“Despite the challenges of implementing green hydrogen for long-distance aircraft and ships, it is important to evaluate this solution because no other clean, renewable energy solution is on the horizon,” they write.
Jacobson is no stranger to controversy, having previously co-authored a study claiming that blue hydrogen was worse for the planet than natural gas, which was based on upstream US methane emissions, prompting companies in Europe, where such emissions are far lower, to claim it was misleading. He also attempted to sue the publisher and authors of a 2017 paper that criticised a previous study he had written, causing outrage among academics. Jacobson eventually dropped his lawsuit but was faced with a six-figure bill for legal costs.
This article was published first by Hydrogen Insight