Hydrogen economy and storage

This hydrogen economy promotes hydrogen as a fuel for transportation, for the energy needs of buildings and industry; and for portable electronic devices. The current interest is understandable since the waste product from hydrogen combustion is simply water. However free hydrogen does not occur naturally in the atmosphere in any quantity and must be produced from other energy sources. Hydrogen is, therefore, an energy carrier like electricity, and not a primary energy source, such as petroleum. The benefits of a hydrogen economy, therefore, depends on issues of energy sourcing, including fossil fuel use, climate change, and renewable energy generation.

Hydrogen energy applications can span a wide power range: from small scale use in fuel cells which could power your mobile phone; to hydrogen-powered vehicles; and to large scale applications in electricity generation and storage.

The power plants of hydrogen vehicles convert the chemical energy of hydrogen to mechanical energy either by burning hydrogen in an internal combustion engine or by reacting hydrogen with oxygen in a fuel cell to produce electricity to run electric motors. However, there are much higher purity requirements (99.99% or better) for fuel cell applications. Widespread use of high purity hydrogen for fuelling transportation is a key element of a proposed hydrogen economy.

With these mobile applications, fuel storage becomes one of the major challenges. The current technology includes the storage of liquefied or pressurised gas. Liquefaction requires large amounts of energy and also maintaining cryogenic temperatures using thermal insulation. Pressurised hydrogen gas has good energy density by weight, but poor energy density by volume versus hydrocarbons, hence it requires a large tank size. A large hydrogen tank will be heavier than the small hydrocarbon tank used to store the same amount of energy. Also, tanks of hydrogen, especially in vehicle applications have inherent safety issues.

Hydrogen can also be stored as a chemical hydride phase where the hydrogen reacts to form the hydride and then decomposed when required, to provide the gas phase. Another well-researched approach uses porous metal alloys as a solid-state storage method working as a sort of “hydrogen sponge”. This solid-state storage may potentially use carbon nanotubes as storage media. The widespread use of hydrogen in vehicles will, therefore, need a widespread distribution network including hydrogen filling stations.

Remember that hydrogen is just the energy carrier and that a primary source of energy is needed. This can be from a renewable source such as biomass gasification or can be from fossil fuel sources (natural gas or coal) using the chemical reactions of steam reforming and water gas shift. Other methods can use electricity for water electrolysis. This electricity can again be from a (non-renewable) power station source or from a renewable photovoltaic solar cell system.

Also, high-temperature processes are possible (electrolysis and catalysis) using sources of high-temperature heat. These can include nuclear fission reactors; concentrating solar mirror collectors; and geothermal sources.  There are also biotechnology approaches to generating hydrogen such as fermentation processes using bacteria, microbes and enzymes which can work on waste materials.

Hydrogen therefore only becomes a renewable low carbon fuel if it is generated from a renewable primary energy source. However, it is still possible to produce a truly clean source of hydrogen from fossil fuels if some form of carbon capture or carbon sequestration is performed during the hydrogen production process.

Major relevant Inspec thesaurus terms:

• alkaline fuel cells
• bioenergy conversion
• biofuel
• biotechnology
• catalysis
• combustion
• direct alcohol fuel cells
• direct carbon fuel cells
• direct ethanol fuel cells
• direct methanol fuel cells
• electrolysis
• fermentation
• fuel cell power plants
• fuel cell vehicles
• fuel cells
• fuel gasification
• fuel processing
• fuel processing industries
• hydrogen
• hydrogenation
• hydrogen economy
• hydrogen production
• hydrogen storage
• microbial fuel cells
• molten carbonate fuel cells
• phosphoric acid fuel cells
• proton exchange membrane fuel cells
• solid oxide fuel cells
• steam reforming
• syngas
• water gas shift

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