Technological differentiation as SENER’s strategic advantage - June 2014 - Number 47 /Up-to-Date

Production of hydrogen from water using renewable energy

Decomposition of water by photoelectrolysis.

Decomposition of water by photoelectrolysis.

Water is one of the world’s most widely-available natural resources. The decomposition of water produces hydrogen: a clean fuel that does not generate CO2 as it burns. Furthermore, it is a practically inexhaustible source of renewable non-fossil fuel that can be transported using gas pipelines and stored in certain conditions.

Yet water molecules are highly stable, which means that over 2000 oC are necessary to break them down, requiring an enormous amount of energy. Therefore, scientists and technicians have been working for years on how to overcome the main obstacles for producing hydrogen from water, trying to overcome its resistance to decomposition and to obtain the primary energy, required for the decomposition of water, from an abundant, renewable, non-fossil fuel source.

Part of this effort has been the use of electrolysis: a method that breaks down water molecules into hydrogen and oxygen by passing an electrical current through the water, which contains substances (electrolytes) that increase its conductive nature. Electrolysis is already being applied in prototype installations that use excess electric energy in wind or photovoltaic power plants, especially in periods of low consumption. This is the case of the emblematic installation of Berlin Airport, which uses excess electric energy from a wind power unit.

Photoelectrochemical decomposition of water, in which at least one of the electrodes (in this case acting as a semi-conductor) is illuminated so that the photons provide the energy needed to split the molecule is still in the scientific and technological development stage. Along these same lines, new materials (such as graphene doped with semiconductor materials or nanometric metals that also act as semiconductors) are being researched as catalysts for the photonic splitting of water molecules. Similarly, processes are being researched that are based on the photosynthesis performed by plants, which use sunlight, or on genetically-modified micro-algae or catalytic cycles of organometallic molecules.

Thermosolar energy is also being studied as a power source for decomposition of water using chemical cycles. For example, in this line of research, the reaction of water with zinc or metallic ferrites is also being investigated. These reactions are conducted in central tower thermosolar power systems with volumetric receptors at temperatures between 800 oC and 2000 oC. For the time being, the complexity of working with volumetric receptors at such high temperatures is making it difficult to develop these technologies.

It is not easy to predict when the obstacles for using renewable energies to decompose water will be overcome by technological developments, making the hydrogen obtained from this process cost-competitive with fossil fuels. Yet that mere possibility is pushing the transportation, the energy production and consumption sectors to prepare themselves for the future. Thus hydrogen transportation is already being practiced either by direct injection of hydrogen in natural gas pipe lines or, indirectly, obtaining methane by the already industrially proven reaction of H2 with CO2, obtained from fossil fuel combustion gases. Similarly, electricity production from hydrogen (instead of natural gas) in large combined cycle plants has been proven at the Fusina power plant in Venice; as well as distributed electrical power, generated from hydrogen in fuel cells, is a commercial reality.

Gallery

Electro-catalytic cycle for decomposition of water.  Thermal cycle experimentation with ferrites in the PSA solar power plant Almería (Spain).

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