Hydrogen

Hydrogen is an ideal energy carrier and it can be used (in gas or liquid form) to store and transmit energy and can be used to generate electricity.

Electrolysis of water

Technology for the electrolysis of water already exists and is currently on the market. It is being developed for use with intermittent power sources such as wind and solar. When DC electricity is passed between 2 electrodes (anodes and cathode) immersed in water, hydrogen collects at the negatively charged cathode and oxygen collects at the positively charged anode. The electrolysers which are commercially available are based on alkaline water electrolysis. They have a bipolar or unipolar configuration and the main chemical reactions occurring at the two electrodes are:

• Anode : 2 H₂O ---> O₂ + 4 H⁺ + 4 e^-
• Cathode : 2 H⁺ + 2 e^- ---> H₂
• Global reaction : 2 H₂O ---> 2 H₂ + O₂

Example

5 MWh of electricity would produce 1000 m³ of H₂ and 500 m³ of O₂.
1000 m³ of H₂ will generate 1MWh of electricity and 1MWh of heat.

Sketch of the process

Electrolysis of seawater

The use of conventional electrodes, under normal conditions, for seawater electrolysis will result in the formation of chlorine in the form of sodium hypochlorite.

Chlorine is a green, highly reactive gas with a strong and irritating odour. Chlorine is used in water treatment and in the manufacture of PVC. It can be harmful and very dangerous for people and the environment and in this project we were interesting in processes which were environmentally benign.

It has been demonstrated that manganese dioxide coated electrodes are capable of producing oxygen with 99% efficiency therefore the direct electrolysis of seawater in this way to produce hydrogen and oxygen is advantageous.

Hydrogen is a near ideal energy transport medium. The transmission of hydrogen by pipeline is comparable to the present system of transmitting natural gas. Extensive testing indicates that natural gas pipe networks can carry a mixture of up to 20% hydrogen with no modification. The same diameter pipe can be used but at the expense of higher compressor energy (+50%). There are however problems with 'Hydrogen cracking' of the pipes, a technical problem which requires further analysis.

For long distance of transmission it was claimed that transmitting hydrogen in a pipe was cheaper than transmitting electricity in overhead cables; by around 4 times.

• Hydrogen as a compressed gas: use of a pressure vessel when small amounts are stored. This technology is available but becomes expensive for large amounts.
• Hydrogen absorbing materials (metal hydrides).
• Liquid storage: well know technology (NASA). However, it is costly and requires a lot of energy to be converted in liquid.
• Underground storage: depleted oil or gas reservoirs, aquifers, salt cavities.

Today, hydrogen is primarily used by industry in the manufacture of ammonia, oil refining (it is used in the process to convert crude oil into petrol) and in the synthesis of methanol. It is also used in power stations as a coolant in alternators, as well as having other uses in the chemical and pharmaceutical industry. Hydrogen is used in NASA's space program as a fuel for their space shuttles as well as in fuel cells that provide heat, electricity and drinking water for the astronauts.

Hydrogen produced by the electrolysis of water can be used either as an energy carrier (fuel or transmission) or as a storage medium. It is combusted and used as a fuel, in the similar way to petrol, in transportation: in public buses or in cars (in Germany for example). Alternatively, fuel cells could be used to power electric buses and cars (NECAR IV): a fuel cell uses reverse electrolysis where hydrogen and oxygen are used to produce electricity. It is a very efficient process (up to 70%) and the only by-products are water and heat and small amount of NOx.

Electrolysis is a well-established technology but for cost reasons is used mainly in small plant, however the hydrogen produced by electrolysis is attractive because it is very pure. The cost of hydrogen depends on the cost of the renewable energy source and the cost of transportation (in pipelines or ships). The hydrogen produced by electricity from renewable energy is very expensive at the moment. Hydrogen from electrolysis is ten times as expensive as natural gas and three times more expensive than gasoline.The most cost-effective way to produce hydrogen today is by using methane with a process called "steam reforming".

Smog, global warming, greenhouse gases, ozone depletion and acid rain are effects that have been attributed to an increase in carbon dioxide ad pollutants emission in the atmosphere essentially caused by the combustion of fossil fuels. Unlike fossil fuels, hydrogen when combusted with air does not produce pollutants or carbon dioxide. Although it is estimated that hydrogen is more expensive than fossil fuels it avoids the cost with environment and health problems associated with fossil fuels. Hydrogen made from renewable energy resources is a virtually inexhaustible, environmentally benign energy source that could meet most of our future energy needs. It's more versatile and has more uses than electricity.

• Safety: research and demonstrations have shown that under everyday conditions, hydrogen is safer than all of the other fuels that are available, liquid or gas, whether used in transportation or stationary uses.
• Hydrogen has a considerable potential to overcome the limitation of intermittent renewable energy sources (solar, wind) and can therefore benefit the development of them.
• At present the use of hydrogen as storage medium for solar power utilisation is at the research and development stage.
• Hydrogen is one of the most promising energy carriers for the future.
• Technological improvements in renewable electricity could make the production of hydrogen by electrolysis very attractive for the future.
• Hydrogen is a long term option: production, storage and distribution facilities must improve and be developed.
• Many countries such as USA, Canada, Germany, are investing money in the development of hydrogen.

Conclusion