IEC
An emerging water-splitting technology called solar thermochemical hydrogen (STCH) promises a more energy-efficient and carbon zero method for producing H2 as a green fuel.
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Hydrogen (H2) is considered one of the most promising sources of clean energy to tackle climate change but producing it economically and carbon free from end to end is a huge challenge. “We’re thinking of hydrogen as the fuel of the future and there’s a need to generate it cheaply and at scale,” said Ahmed Ghoniem, a Professor of Mechanical Engineering at MIT.
Hydrogen energy sources are being promoted as part of a mixed economy of green power options by governments. In 2020, the European Commission published A hydrogen strategy for a climate-neutral Europe, aiming to accelerate widespread H2 use and achieve a carbon-neutral European Union.
In its Combined Heat and Power Act, the German government requires new gas power plants to be H2-ready. States in Australia are investing in hydrogen refuelling stations along the country’s busiest freight highway between Sydney and Melbourne in a push to see more zero-emissions technology used in the heavy-vehicle industry. In the US, the Department of Energy’s Hydrogen Energy Earthshot set a target to cut the cost of clean hydrogen by 80% to USD 1/kg in a decade (and to USD 2/kg by 2025 as an interim step).
As it stands though, more than 90% of the world’s H2 is produced from fossil fuels through processes such as steam methane reforming, methane partial oxidation and coal gasification. This alone generates emissions of around 830 million tons of CO2 per year, which accounts for over 2% of global annual CO2 emissions.
That’s hardly a sustainable way forward. An alternative is to substitute methane and coal with a carbon-free source such as water (H2O). Using a process called electrolysis, electricity is used to split water into hydrogen and oxygen and in theory produce H2 with zero greenhouse gas emissions. The catch is that this depends on the electricity source also being carbon free and, in many regions of the world, the infrastructure is simply not yet suitable or economically viable for this pathway. In addition, different types of electrolysers required for the process make use of metals such as nickel and platinum group metals (PGMs) that are associated with high cost, environmental impact and supply chain concerns.
Solar thermochemical hydrogen
Attention has turned to an emerging technology which offers a completely emissions-free solution. This is solar thermochemical hydrogen (STCH), which relies on heat, rather than water, generated from renewable solar energy to drive H2 production.
In this method, the power to drive STCH hydrogen production comes from concentrating solar power (CSP). These are typically arrays of hundreds of mirrors that gather and reflect sunlight to a central receiving point. The heat from the receiver is then absorbed by a STCH system, which directs it to split water and generate hydrogen. Temperatures greater than 1 400 °C can be used to boil water for steam to run a turbine, which in turn can generate electricity.
But there’s another catch. To date, STCH designs have had limited efficiency: Only about 7% of incoming sunlight is used to make hydrogen, rendering such systems low-yield and high-cost.
In October, a team at MIT claimed a breakthrough. Their concept for a system of reactors could harness up to 40% of the sun’s heat. According to MIT researchers, this increase in efficiency could drive down the system’s overall cost, making STCH a potentially scalable and affordable option to help decarbonize industries like transportation.
“We have to think of every bit of energy in the system, and how to use it, to minimize the cost,” Ghoniem said. “And with this design, we found that everything can be powered by heat coming from the sun. It is able to use 40% of the sun’s heat to produce hydrogen.”
“It could drastically change our energy future – namely enabling hydrogen production 24/7,” said Christopher Muhich, Assistant Professor of Chemical Engineering at Arizona State University. “The ability to make hydrogen is the linchpin to producing liquid fuels from sunlight.” The next stage is to build a prototype which will be tested in concentrated solar power facilities.
A number of IEC Technical Committees prepare international standards for solar systems and installations. IEC TC 117 works on international standards for systems of Solar Thermal Electric (STE) plants for the conversion of solar thermal energy into electrical energy. One of its standards, published in 2022, IEC 62862-3-1 specifies the requirements for the design of parabolic-trough solar thermal power plants. Future standards would also address issues of connectivity and interoperability with the power grid related to connections, bi-directional communicates and centralized control (Smart Grid) and environmental aspects.
IEC TC 82 prepares standards for solar PV energy systems, and TC 105 for fuel cell technologies.
Other approaches also being developed
Another approach to improving thermochemical technology comes from a team of engineers at ETH Zurich, funded by the Swiss Federal Office of Energy. They tackled the challenge of maximising the transfer of heat from a CSP system to the interior of the reactor.
At the heart of its production process is a solar reactor that is exposed to concentrated sunlight delivered by a CSP array and reaches temperatures of up to 1 500 °C. Inside this reactor, a thermochemical cycle takes place for splitting water and CO2 captured previously from the air. The product is synthesis gas or syngas: a mixture of hydrogen and carbon monoxide, which can be further processed into liquid hydrocarbon fuels such as kerosene (jet fuel) for powering aviation.
Two ETH spin-off companies (Climeworks and Synhelion), are further developing and commercialising the technologies. “This technology has the potential to boost the solar reactor’s energy efficiency and thus to significantly improve the economic viability of sustainable aviation fuels,” said Aldo Steinfeld, ETH Professor of Renewable Energy Carriers.
Hydrogen-producing solar panels
Researchers at KU Leuven in Belgium have developed rooftop panels that capture both solar power and water from the air. Hydrogen panels are like conventional PV modules, but instead of an electric cable, they are connected via gas tubes. The researchers claim one panel produces 250 litres of H2 per day, at an efficiency of 15% and are now preparing to bring the tech to the mass market via a spinoff company.
Project researcher Jan Rongé explained, “The hydrogen panels themselves do not store hydrogen and work at very low pressure. This has several safety and cost benefits. The hydrogen is collected centrally from the hydrogen panel plant, and then compressed if needed.” It is expected that the product will be commercially available from 2026 and that prices will drop in line with that of PV modules today.
Piece of a bigger puzzle
While hydrogen produced by solar shows promise, development remains at its early stages, and it should not be considered a silver bullet green energy solution. Dr Kim Beasy of Australia’s Swinburne University Hydrogen Hub said, “We’re coming to understand that hydrogen is going to be one piece of the puzzle. What we really need is more government support and subsidies in bringing down the cost of getting this technology on the ground.”
It is a view echoed by the International Energy Agency. In its Global Hydrogen Review 2023 it concludes that “low-emission hydrogen production can grow massively by 2030 but cost challenges are hampering deployment”. It also stresses that “governments need to urgently implement these programmes and make funding available to enable a scale-up compatible with their decarbonisation ambitions”.
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