Two-step high-temperature water splitting

Hydrosol
Hydrosol, © DLR

Hydrogen certainly has the potential to become a mainstay across all kinds of energy markets. It can be produced entirely from renewable energy, requiring not even a tiny amount of carbon-based fuel. It is currently being brought to market to power cars, buses, trucks and trains through fuel cells driving their engines. It is likewise used in ground and material handling equipment for logistics and airport operations, in stationary units, such as CHP installations, and in uninterruptible power supply systems.

The gas also continues to be in high demand as an intermediate product in industrial settings. Around the world, several hundred billion cubic meters of it are consumed each year in the plastics, petrochemical, glass and metal fabrication, and fertilizer industries, as well as in agriculture and food processing. Additionally, the plans to inject hydrogen into natural gas pipelines are becoming more concrete. The aim is to replace part of the fossil fuel stored and transferred throughout the network with a non-carbon alternative produced from sustainable and renewable resources.

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Japan, the prime mover

Despite Japan’s very ambitious strategy, the country has not wavered in its years-long commitment to make hydrogen the country’s main energy source. In April 2014, Japan’s cabinet approved a roadmap, revised in March 2016, to sketch out the path it intends to take to become a prime example of a hydrogen society. The roadmap outlines not only technical aspects of manufacturing, transport and storage but also societal factors, such as market adoption and public acceptance. It is not surprising then that it has been and is being developed in close collaboration between research organizations, manufacturers and suppliers, the government and the public.

Japan’s way to a hydrogen-based economy is divided into three stages. The first has already been set in motion and is planned to last until the end of this decade. The aim of this stage is to expand the range of fuel cell applications to realize energy savings and secure a position on the global market. The 2020s, on the other hand, are to see the buildup of a supply infrastructure based on imported hydrogen, mainly from renewable production. A prominent example is hydrogen generated by solar installations in Australia and elsewhere. The principal objective is supply security.

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How a solar power tower works

Solar alternatives to water electrolysis are closely associated with the use of concentrating technologies, especially power towers. The solar thermochemical system, or its later industrial version, will include an array of mirrors focusing the sun’s rays onto what is called a receiver at the top of the tower. The high temperatures reached at the focal point will then be used to convert energy into heat. Solar power towers have already been available on the market and are in operation in many sun-rich countries, where they generate heat and, subsequently, electricity.

These towers also make it possible to meet the heat requirements of energy-intensive chemical processes, for example, when splitting water into oxygen and hydrogen. During the first step in the thermochemical cycle, the sun heats a metal oxide in a reactor on the tower to around 1,400 °C. Consequently, oxygen is removed from the substance, a chemical process known as reduction.

Written by Dr. Martin Roeb, Deutsches Zentrum für Luft- und Raumfahrt, Cologne, Germany

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