Solar Hydrogen

The Innovation Pool project "Solar hydrogen: high purity and compressed" aims to improve both the scientific knowledge and the technological maturity of various viable technologies for the conversion of solar energy into hydrogen (H2). At the halfway point of the project, a review paper has been published in the journal "Angewandte Chemie International Edition" that focuses on the technological methods for producing compressed and high-purity hydrogen from solar energy. The aim of this extensive work, which involved several Helmholtz centers, was to provide a clear statement on the status quo of technologically achievable H2 purity and pressure levels. In this way, future development opportunities can be identified and potentially pursued.

The energy transition is one of the most important future projects of our time, in which the generation and use of renewable and sustainable energy is an important driver for a decarbonized economy. In this context, hydrogen - and in particular so-called "green" hydrogen from renewable energies - plays a crucial role as a "game changer" in the entire energy system and as an important factor in the societal transformation. However, the share of green hydrogen is still too low to reach the net-zero target, while the demand for high-quality hydrogen continues to grow. These factors reinforce the need for economically viable hydrogen production technologies.

Schematic representation of photoelectrochemical water splitting.

The sun, as one of the most powerful natural energy sources, provides free and abundant energy that can be captured and converted into hydrogen (also called solar hydrogen) through various technological routes. To achieve its highest possible value as a fuel for power generation and the mobility sector, and as a raw material with broad industrial applications, hydrogen must meet strict criteria in terms of its purity and pressure. The technologies pursued in the Innovation Pool project, which are the subject of the recently published review article, enable the production of solar hydrogen with the highest purity and pressures that in some cases far exceed atmospheric pressure.

Five major technological pathways for green hydrogen production are considered in the review: Low- and high-temperature water electrolysis, photoelectrochemical and thermochemical water splitting, and hydrocarbon (e.g., biomethane) conversion in liquid metal and plasma reactors. They are all powered by solar energy in the form of photons, electricity, or heat, and are surveyed in terms of their current stage of development, technical limitations, and future potential in terms of the quality of hydrogen produced. These technologies differ in their maturity level (Technology Readiness Level, TRL from 2 to 9) and can deliver hydrogen with a purity of ≥99.97 and a pressure of >500 bar. Moreover, coupling with electrochemical compressors may be a necessary downstream element to further increase the hydrogen output pressure, currently to about 1300 bar. In addition to the technological perspective, numerical simulations on different scales are also considered as an overarching approach and provide insights for the development and optimization of materials, components, reactors and systems.

Purity and pressure ranges achieved with various solar powered technologies. Marked with (*): Purity after cleaning.

Although few of the solar-powered technology pathways have reached sufficient maturity to date, and some of them still have a wide playing field for innovation and basic research, they all show significant potential for achieving hydrogen quality goals. To achieve the goal of low cost price for hydrogen production, the associated capital and operating costs of deployed technological solutions must be reduced through increased scalability, efficiency, and longevity of equipment and facilities. Ultimately, renewably powered technologies for higher quality hydrogen production are already a compelling alternative to existing solutions. Reducing system complexity and increasing installed capacities will contribute in a special way to effective climate protection, energy supply security and economic efficiency.

In summary, solar energy, in conjunction with various technologies being pursued within the innovation pool project "Solar Hydrogen: Highly Pure and Compressed", forms the core of a technological platform for the production of high-quality "ready-to-use" hydrogen. In this context, the review paper clearly addresses the technical challenges, but also highlights the importance of accelerating development to advance such promising technological pathways beyond the current state of the art to achieve the highest possible value for a sustainable energy system in Germany and worldwide.

Ivanova, M., Peters, R., Müller, M., Haas, S., Seidler, M. F., Mutschke, G., Eckert, K., Röse, P., Calnan, S., Bagacki, R., Schlatmann, R., Grosselindemann, C., Schäfer, L.-A., Menzler, N. H., Weber, A., van de Krol, R., Liang, F., Abdi, F. F., Brendelberger, S., Neumann, N., Grobbel, J., Roeb, M., Sattler, C., Duran, I., Dietrich, B., Hofberger, C., Stoppel, L., Uhlenbruck, N., Wetzel, T., Rauner, D., Hecimovic, A., Fantz, U., Kulyk, N., Harting, J., Guillon, O., (2023): Technological Pathways to Produce Compressed and Highly Pure Hydrogen from Solar Power, Angewandte Chemie International Edition published by Wiley-VCH GmbH. https://doi.org/10.1002/anie.202218850