Hydrogen looks like one of the best ways to store and transport renewable energy – but as a flammable, leaky gas, there are still big questions around how hydrogen itself should be made, stored and transported.
Pure hydrogen, H2, can be transported as a compressed gas, or super-cooled until it’s a liquid. It can also be reacted with other chemicals to become more stable. Two of the most promising ways to do this are as ammonia (NH3) or methanol (CH3OH).
Which of these will work best?
“I don’t really see one specific chemical to be the only thing. The future will be a diversified market,” says Dr Kaveh Khalilpour, an associate professor at University of Technology Sydney’s Faculty of Engineering and IT.
But the way you count hydrogen does make a big difference.
Khalilpour is co-author on a recent paper in Energy Conversion and Management that uses a new model, developed by the researchers, to predict which technologies will work best for renewable energy supply chains.
“What we tried, in this project, was to develop an agnostic tool, which in a very unbiased way, takes the numbers and analyses, from the given options, which can be the best technology.”
Khalilpour and colleagues took this tool to study how Australia, with its abundant sun and wind, might export green hydrogen to three countries that have net energy deficits and are interested in our hydrogen: Singapore, Japan and Germany.
“In this work, we started looking at compression of hydrogen, liquefaction of hydrogen, and two chemistries of hydrogen: one conversion of hydrogen to ammonia, and the other conversion of hydrogen to methanol,” says Khalilpour.
The researchers found that the method that worked best varied, depending on whether importers wanted hydrogen by the kilogram, or energy by the joule.
“It makes a totally different story if the objective of export is hydrogen, or the renewable energy that we want to export elsewhere,” says Khalilpour.
“In one scenario, they need the hydrogen and they don’t need other commodities. Even if we send methanol, they still need to make a reverse reaction to make hydrogen out of it.”
In this scenario, ammonia became the cheapest way to transport hydrogen to all three destinations. Importers would then turn that ammonia back into hydrogen, before using it in their energy systems.
“But in the other scenario, they need energy in diverse ways. You send them methanol, you send them ammonia, they have a demand for that. So they don’t precisely need the hydrogen – they need energy in any form,” says Khalilpour.
And in this scenario, methanol, with its high energy density, became the cheapest export for all three destinations. Liquefied hydrogen was the second-best option in both scenarios.
This raises another question: methanol has carbon in it, and produces CO2 when it’s being combusted for fuel.
It can also be made with CO2 taken from the atmosphere via carbon capture – and in fact, other researchers have suggested that methanol might become a very powerful carbon-negative commodity.
But even if methanol is made with carbon captured directly from the atmosphere, which the researchers assumed it would be, this can cause complexities in the supply chain.
“In the exporting country, we are taking CO2 from nature and reacting with hydrogen to generate methanol, but in the receiving country, they will burn methanol and they will emit CO2,” says Khalilpour.
“You are not adding CO2, you are just using CO2 in one part of the world and you are emitting it in another part of the world. But from an economic [standpoint], it is incentives in one country and it is liability in another country.”
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Where does this leave Australia? Khalilpour says that there are “multiple dimensions” including market conditions and policies that will inform which technologies take off best.
“But based on my understanding, given that we are a little bit distant from the rest of the world, then the more high-density renewable chemicals we develop, the better position we will have in the global renewable chemical supply chain.”
He’s hoping other researchers will make use of their model as well.
“A framework like this can be useful for others as well to come and test their own chemical to see how they innovate that compared to the existing options,” he says.
“The future of energy storage will be diversified versions of storage technologies, including hydrogen, including batteries, and everything.
“And regarding the future of the renewable energy supply chain, I don’t believe that one specific chemistry [will] fit all purposes.”