All-rounder in the start­ing blocks

On Earth, hydrogen is practically only found in chemically bound form, for example in water, methane or biomass. Before it can be used as an energy carrier, the hydrogen must first be released from these compounds. This is done using energy in the form of electricity or high-temperature heat. The so-called 'grey' hydrogen is mainly produced from natural gas and currently accounts for about 95 percent of global production. This produces considerable carbon dioxide emissions. With  'blue' hydrogen, these greenhouse gases are separated and stored – or hydrogen is produced by means of electrolysis and relies on electricity from nuclear power.

Only 'green' hydrogen is sustainable and climate-neutral. Its production uses water and energy from solar, wind, hydropower or biomass. Until now, its production in relevant quantities was considered too expensive. Karsten Lemmer, DLR Executive Board member responsible for Innovation, Transfer and Research Infrastructure, is confident that this will change in the future: "First of all, the expansion of renewable energies must move forward. In addition, large-scale electrolysis systems should be installed in Germany in the short term. This will take us the first steps on the path to making green hydrogen competitive."

Towards green hydrogen – electrolysis and solar processes

DLR is focusing on two methods for the production of hydrogen on an industrial scale – electrolysis and solar thermal processes.

Electrolysis is the most advanced form of this technology, and it is already commercially available. The principle, whereby water is split into hydrogen and oxygen molecules using electricity, has been known for over 200 years. At present, scientists are particularly interested in three technological implementations of electrolysis – alkaline, proton-exchange membrane, and high temperature electrolysis. DLR is involved in the development of all three.

Germany currently has a total electrolysis capacity of 30 megawatts in place, powered by electricity from renewable sources. This capacity would have to be massively expanded in order to make the transition to a hydrogen economy. A study by the German National Organisation for Hydrogen and Fuel Cell Technology (Nationale Organisation Wasserstoff- und Brennstoffzellentechnologie; NOW) envisages an increase in this capacity to 137–275 gigawatts by 2050. This will require both smaller, decentralised electrolysis systems – at filling stations, for example – and centralised, large-scale electrolysers with particularly high levels of efficiency.

Solar thermal processes for producing hydrogen promise higher efficiency, but they require lots of space. In this process, solar thermal power plants use solar energy to produce heat for thermochemical water splitting. DLR is continuing to develop components and systems that will allow these plants to be made as efficient, durable and suitable for industrial use as possible. The first pilot plants are already in operation, but it will take several more years before the solar hydrogen production processes are ready for the market.

Major hydrogen demand requires national production and imports

In order to meet the rapidly increasing demand for green hydrogen, it will be essential to significantly increase the power supply capacity from renewable energy sources. Germany's potential and available land area for this are somewhat limited. There are also issues with acceptance, such as those currently being experienced with wind power. "We will not manage to produce the amount of green hydrogen needed for the energy economy, industry and mobility in Germany. International solutions are needed. Large-scale hydrogen production should be established in sunny countries. Solar thermal processes have the highest potential to drastically reduce production costs. Global hydrogen logistics must then be devised for distribution," says Lemmer.

Within Europe, such hydrogen production is mainly suited for regions of Spain, Greece, and southern Italy. The production and export of hydrogen could be factored into a European 'green deal' and help to stimulate national economies following the Coronavirus pandemic. Countries in North Africa and the Middle East are also attracting interest from Germany and Europe for their potential in this area.

Transport, storage, and distribution – building and modifying infrastructure

In addition to production, cost-effective and reliable hydrogen transportation is essential for a future hydrogen economy. This involves both the transport routes from global production sites to nodes within the customer countries and local distribution to the end consumer. There are a number of possible approaches for this. The hydrogen could be transported in liquid form, or converted into ammonia, methane, or other liquid organic carriers. For now, it remains unclear which of these approaches will prove the most economically attractive. If hydrogen needs to be transported to an end consumer, it will probably be transported by lorry as a liquid or compressed gas.

Another means of transporting and distributing hydrogen is to gradually convert the existing natural gas network into a hydrogen network. The German gas network consists of a transport network that extends 40,000 kilometres, with a distribution network covering 470,000 kilometres. To a certain extent, it is already suitable for distributing hydrogen. However, the introduction of a larger proportion of hydrogen would require careful investigation and optimisation of the various materials, components, operating methods and user requirements.

Large storage facilities will be an essential part of the overall hydrogen infrastructure. They will be necessary to reliably cover seasonal peaks in demand, such as during colder and darker months. In Germany, underground salt caverns are considered to be particularly suitable for this purpose. DLR is investigating the safety, hydrogen quality and durability of the materials used in such storage facilities. It is also conducting research into possible business models for production and storage, and analysing the potential of different locations, particularly in northern Germany. For geological reasons, these areas are particularly suitable for the type of infrastructure required.

Sustainable hydrogen mobility for roads, rail, air and sea

Green hydrogen represents a sustainable alternative for many applications powered today by petrol, diesel, kerosene or heavy oil. At the same time, it preserves the conveniences to which we have become accustomed, allowing for long range travel and fast refuelling. Hydrogen fuel cells are characterised by high levels of efficiency and, unlike the direct combustion of hydrogen in engines and turbines, produce only water vapour as emissions.

DLR is developing special fuel cells and new types of hydrogen tanks for mobile use and integrating them into the propulsion systems of cars, buses, lorries, trains, aircraft and ships. Hydrogen-based propulsion solutions have significant advantages over battery concepts when it comes to transporting heavy loads over long distances.

Private transport vehicles powered by hydrogen fuel cells are already available on the market. DLR experts are analysing their market and application potential. The DLR Safe Light Regional Vehicle (SLRV), a concept vehicle, will have a highly efficient hydrogen drive and is scheduled to make its first trips in autumn 2020. Trains powered by fuel cells provide an emissions-free alternative to diesel locomotives or multiple unit trains on stretches of track without overhead lines. DLR conducted a study in which it examined the market for trains with hybrid drive concepts and, together with the rail vehicle manufacturer Alstom, developed and tested the world's first fuel-cell-powered multiple unit train. Additional trains and test regions are set to follow. The first buses with fuel cells are already on the streets as part of pilot projects, while several manufacturers are developing lorries with this type of drive.

One focus of the new DLR Institute of Maritime Energy Systems is the use of hydrogen to power ships. The scientists are conducting research into aspects such as service life, suitability for everyday use, and the efficient integration of such systems. One example of such integration could be the simultaneous use of electricity produced using hydrogen to drive the ship’s propulsion and refrigerate its cargo. DLR is also working with companies and research institutions to launch the world’s first sea-going ferry to be powered with hydrogen fuel cells.

In aviation, hydrogen can be used as a fuel in modified gas turbines. This is of particular interest for large classes of aircraft, but requires the development of hydrogen storage systems that are compatible with aircraft and new combustion chambers. Powering flight with hydrogen fuel cells and electric propulsion systems has so far posed a highly complex technical challenge, but promises to be remarkably quiet, efficient, and emissions-free if successful.

The use of liquid synthetic fuels based on hydrogen could also improve the sustainability of flight. In future, they could be deployed not only in aviation, but also anywhere that conventional drive systems cannot easily be replaced with climate-friendly alternatives such as batteries or hydrogen fuel cells. These fuels would require only minor adjustments to drivetrain components and infrastructure. In the DLR cross-sectoral project 'Future Fuels', 11 institutes are investigating the chemical and physical properties of such climate-neutral fuels and their performance, composition and cost-effective production methods.

Green hydrogen for power and heat

The energy sector is also set to benefit from this versatile source of energy. Hydrogen fuel cells and gas turbines can be used to generate a controllable supply of power and heat. Both are key requirements in tomorrow's energy system, which is based on fluctuating renewable sources. In this way, consumption peaks can be balanced out. The aim is to achieve the highest possible levels of efficiency.

Only minor adjustments are required to convert current efficient gas-fuelled power plants to hydrogen usage. DLR is currently working with turbine and power plant manufacturers to investigate fuel cell versatility and devise concepts for making the combustion of natural gas and hydrogen mixtures as stable and low in emissions as possible.

Sector coupling – networking as a key to success

Coupling of the mobility, energy, and industrial sectors will play a key role in this process. The more technologies and applications that are integrated into the system, the more flexible and stable it will become as a whole. Green hydrogen is crucial for sustainability here.