Green hydrogen and unicorns
- Feb 27, 2021 2:02 am GMT
Question: what do green hydrogen and unicorns have in common?
Answer: they are both mythical.
Well, OK. Not entirely. Green hydrogen, as defined by proponents, does have at least the theoretical potential to be a real thing. Unicorns (as in magical creatures, not high-valued private startups), not so much.
The thing is, nearly all the hydrogen that’s currently labelled as “green”, isn’t. It’s a dark olive drab, shading toward black. AKA dirty.
Don’t get me wrong, I’m not dissing the concept of green hydrogen. Electrolytic hydrogen produced using electricity from resources with zero atmospheric carbon emissions would be great! Stored fuel to replace natural gas for generating power on demand; chemical reducing agent to replace coked coal in steel production; chemical feedstock to produce clean ammonia for fertilizer and long term energy storage, … the list goes on.
Unfortunately, the conditions required to produce truly green hydrogen are currently rare. It wouldn’t be far wrong to say nonexistent. Nor is that situation likely to change anytime soon.
To be truly green, it’s not sufficient for electrolytic hydrogen to be produced using electricity from zero-emissions energy resources. It must be electricity that would otherwise be wasted. If there is any competing use for the electricity consumed by electrolytic hydrogen production, then the hydrogen produced takes on the footprint of the added generation needed to satisfy the competing use.
That may sound contrived, but it’s not. It’s basic accounting. Say a given amount of clean energy is available; it can be used to produce some amount of hydrogen, or it can be used to satisfy a competing demand. Option A: use it for the competing demand instead of producing hydrogen. Option B: use it to produce hydrogen, leaving the competing demand to find its supply elsewhere. Choosing option B requires a marginal increase in generation somewhere on the grid. Marginal increases in generation almost always translate to increased output from fossil-fueled power plants.
Unless they’re in curtailment, wind and solar resources have no ability to increase output on demand. But if they’re in curtailment, there’s presumably no competing use for the electricity that electrolysis would be using. The conditions for truly green hydrogen production would be satisfied.
More often, renewables will already be delivering everything their nameplate capacity and current weather conditions allow. Any marginal increase in supply to the grid will have to come from elsewhere. “Elsewhere” is at present almost always a fossil-fueled power plant. Hence, option B incurs the carbon footprint of fossil-fueled generation in an amount matching the electricity consumed for hydrogen production.
In the best case, the power plant would be a natural gas combined cycle plant with a net thermal efficiency of 60%. It would then take 5.4 kg of natural gas to produce the 50 kWh of electricity for one kg of electrolytic hydrogen. The carbon footprint would be 14.8 kg CO2. In the worst case, the plant would be coal-fired, net thermal efficiency 40%. Then it would take ~13.7 kg of coal for the electricity to make one kg of hydrogen. The footprint would be 50 kg of CO2 per kg H2.
By comparison, it only takes ~2.86 kg of natural gas to produce one kg of H2 by conventional steam methane reforming (no CCS). The carbon footprint would be 7.86 kg CO2. Clearly, there’s a stiff penalty for violating the “no competing use” condition for green hydrogen production.
Why did I say earlier that the situation for production of green hydrogen won’t be changing any time soon? It’s because the cost for grid-scale battery storage is dropping. Installed capacity will be increasing rapidly, and charging all that capacity will be a higher-value competing use for otherwise excess wind and solar production. That situation will hold for at least the next decade.
I conclude that if you’re expecting to see (truly) green hydrogen taking off in a big way anytime soon, you might think about taking up unicorn spotting. I think you’ll have a better chance of success.
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