The German industrial giant Siemens is investigating the use of ammonia as a way to store and transport hydrogen in energy systems with high penetration of renewables.
The company this month opened a £1.5 million ($2 million) proof-of-concept plant in Harwell, Oxfordshire, U.K. to test the efficiency of converting electricity to hydrogen, and then to ammonia, and then back.
The plant, funded one-third by Siemens and two-thirds by government agency Innovate U.K., is thought to be the first of its kind in the world.
The U.K. Science and Technology Facilities Council, University of Oxford and Cardiff University are also attached to the project, which includes a wind turbine, a nitrogen generator, a water electrolysis system, a Haber-Bosch reactor and a 30-kilowatt electric genset.
Ian Wilkinson, program manager for the project within Siemens, told GTM that the research into ammonia was complementary to Siemens’ work on other energy storage technologies, such as batteries.
But batteries are primarily useful for electricity, which in the U.K. only accounts for around a quarter of all energy use, he said. “Chemical fuels have a [use case], including energy storage of electricity but also beyond it,” he said.
“It’s pretty apparent that we will need a range of energy storage solutions to decarbonize our electricity generation," Wilkinson added. "I think a lot of different storage technologies will be required.”
For short-term, low-capacity applications, it is likely that batteries would be the dominant storage technology, he said.
But where longer-duration, large-scale storage is needed, ammonia could play a role, particularly if the energy has to be transported from one place to another or stored in a location devoid of hills for pumped hydro or caves for compressed air.
“For big-capacity, long-duration storage, chemical fuels are hard to beat,” Wilkinson said. “Of course, we use chemical fuels a lot today, and they are ubiquitous for a reason. It’s just that all of our fuels right now are fossil-based.”
Ammonia has similar storage and transportation characteristics to fossil fuels but without the potential to release carbon into the atmosphere, he noted. Hydrogen, which is the prime focus of current non-carbon chemical fuel efforts, is not so easy to store or move around.
Another point in ammonia’s favor is that the gas is already manufactured, stored and transported at industrial scale, so it is a familiar and low-cost compound to handle.
The boiling point of the gas is -33 degrees Celsius, so although it needs to be kept cold when in a liquid state, the level of refrigeration necessary is not excessive.
The ammonia industry produced around 140 million metric tons of the compound worldwide in 2016, according to the United States Geological Survey.
Most of this came from fossil fuels, with the hydrogen being split out by steam reforming and then combined with nitrogen via the Haber-Bosch chemical process.
In the future, though, it is envisaged that large amounts of hydrogen might be produced directly from renewable electricity, using water hydrolysis.
Where possible, Wilkinson said, this hydrogen should be used directly so as to minimize the energy losses inherent in chemical transformation.
For bulk storage or transport, though, “the energy penalty of going to ammonia is pretty small,” he said, at around 10 percent of the total cost of an electricity-to-hydrogen-to-ammonia process.
The cost of ammonia production would largely depend on the cost of the electricity used to power the process, although Wilkinson said some of the more recent ultra-low bids in the renewable energy sector could help the pathway pencil out financially.
“You don’t need to store it too long or transport it too far for the cost of going to ammonia to be significantly less than the cost of storing or transporting hydrogen,” he said. “It’s a piece in the decarbonization puzzle. It’s not a silver bullet, but it’s got a lot of potential.”
Whether or not ammonia production would be worthwhile would be dependent on specific applications, Wilkinson said. For use in fuel cells, for example to power vehicles, ammonia would have to be converted back to hydrogen, with a further loss in efficiency.
However, there is also increasing research into burning ammonia directly in gas turbines. At present, “it looks quite feasible,” said Wilkinson.
