Members of the two teams, at DLR’s Institute of Future Fuels, and at the industry partner Grillo that will trial the electrolysis step
A sulphur-based method for making green hydrogen using a solar hybrid technique that combines concentrated solar heat and electrically driven electrolysis has proven to be more efficient than today’s state-of-the-art electrolysis-only method.
“Our studies have shown that you can reach more than 20% conversion with the Hybrid Sulphur cycle,” explained Dennis Thomey, who heads the department developing the HyS cycle technology at Germany’s DLR Institute of Future Fuels.
“PV plus conventional water electrolysis is the state of the art for making renewable hydrogen. And if you look at the annual efficiency of water electrolysis, you can convert about 11% of your solar input into hydrogen, which is good. But with our HyS cycle, we have the potential to reach more than 20% conversion – an increase of more than 50% compared to state-of-the-art electrolysis.”
How this hybrid electrolysis method of making green hydrogen works
The chemistry of how the Hybrid Sulphur cycle (HyS cycle) works is superficially similar to today’s green hydrogen electrolysis when you look at the entire cycle:
– Water goes in
– Hydrogen and oxygen come out
– Sulphuric acid acts as a recyclable intermediate, continuously being broken down to SO2 and reformed back to H2SO4 for reuse
There are two main steps: A thermal step and an electrochemical step. The first step is driven by heat from a concentrated solar thermal reactor (How solar fuels work) the second step by electricity.
In the thermal step:
The solar reactor supplies heat at over 800°C, which causes the sulphuric acid (H2SO4) to break down into its three components:
– Oxygen gas
– Water vapor
– Sulphur dioxide (SO2) gas
In the electrochemical step:
The SO2 gas is combined with water, and electricity is applied to drive a chemical reaction, producing two main outputs
– Hydrogen gas (the desired product)
– Sulphuric acid (recycled back to the thermal step)
From lab bench to industry trial
The HyS Cycle team has successfully operated a lab bench prototype in the DLR lab at Cologne over the last few months. Now, an industrial partner, GRILLO-Werke AG (Grillo), is interested in collaborating on a 50 kW demo project to make green hydrogen using the HyS sulphur cycle, first described here by Christian Sattler who heads the German Aerospace Center’s Institute of Future Fuels.
“We are lucky. We have the industry already involved in our projects,” said Thomey.
“It would be smart for them to use the SO2 they produce on their site to make hydrogen as an additional product of their sulphuric acid recycling plant. Then, they could use this hydrogen to fuel their sulphuric acid splitter onsite.”
DLR’s particle receiver to provide the solar heat
The pilot will be carried out in two German locations. The solar-driven thermal step will be performed at the DLR on-sun solar tower test site at Jülich, while the second electricity-driven step will be performed at the GRILLO-Werke AG (Grillo) facility in Duisburg.
“The idea is here to use our centrifugal particle receiver technology, which can provide heat at up to 900 Celsius,” explained Thomey.
“And this temperature fits very well to the sulphur acid splitting to SO2. Then, this SO2 will be transported to the industrial site of our industrial partner, Grillo, to be used in their electrolyzer to make hydrogen. It requires only a seventh of the electricity needed for water electrolysis. Our SDE electrolyzer is very similar to a PEM electrolyzer. It needs several modifications because, as you can imagine, sulphuric acid is more corrosive, so you need some coatings.”
The researchers do not have to design new solutions to deal with corrosion. Such coatings are readily available because the sulphuric acid industry is extensive and well-developed, and has long ago solved corrosion problems, so there are many off-the-shelf products and materials available.
In commercial operation, both steps would be performed together on one site. A solar tower (with a solar field of heliostats) would generate the heat to run the first step, and a trivial amount of grid electricity would be used to run the second electrolysis step.
“You feed in high temperature, heat, coming, for example, from CSP at around 1000°C, and electricity, but much less than in conventional water electrolysis,” noted Thomey.
“We replace the electrical energy with thermal energy, which is more efficient thermodynamically; that is how we come to these higher efficiencies.”
Direct solar heat is more efficient
The annual solar to hydrogen efficiency is 22% compared to 11% for state-of-the-art electrolysis. The DLR-developed green hydrogen technology needs only 14% of the electricity demand of conventional water electrolysis.
HyS cycle researchers continue to refine the technology further. For example, one disadvantage of today’s standard PEM electrolysis is that it requires very pure water.
“So something we are looking at starting next year is what happens to our SO2 electrolyzer if you have some impurities in your feed,” he said.
“This is quite important because there are many processes like copper mining where you cannot avoid having these impurities in your feed. Therefore, having an electrolyzer that can operate successfully over long durations while accepting some impurities would be good because this SO2 electrolysis could then be integrated into mining processes.”
New Sulfur-Based Solar Reactor Makes Cheap Green Hydrogen for Copper Mines
A solar sulphur cycle to make unlimited thermal energy storage
