INTRODUCTION TO HYDROGEN AND FUEL CELLS

Currently the annual world production of hydrogen is estimated to be around 500 billion Nm³ per annum which is used mostly as an industrial chemical, e.g. for ammonia production (for fertiliser manufacture), for desulphurisation and other processes in refineries, and methanol production.

Hydrogen can be produced from water by using a variety of energy sources, such as solar, nuclear and fossils, and it can be converted into useful energy forms efficiently and without detrimental environmental effects. The only by-product is water or water vapor (if air is used for flame combustion of hydrogen, small amounts of NOx are produced). When solar energy - in its direct and/or indirect forms - is used to produce hydrogen from water, both the primary and secondary forms of energy become renewable and environmentally compatible, resulting with an ideal, clean and permanent energy system - the Solar Hydrogen Energy System.

Main implication in the use of hydrogen for various energy applications is its safe storage. Tremendous amount of research and development global efforts are under way for safe hydrogen storage technologies suitable for various applications, e.g. Metal hydride technologies offer a variety of applications in refrigeration, air conditioning, hydrogen storage and purification.

Hydrogen Production

There are numerous ways of producing hydrogen from renewable energy sources. It can be produced from a variety of biomass feed-stocks, such as agricultural crops and wastes, sewage sludge or municipal solid waste, by thermo-chemical (pyrolysis or gasification) or biological processes that break down complex organic molecules into simpler molecules including hydrogen.

Hydrogen can also be produced from renewably generated electricity, via electrolysis, to split water into hydrogen and oxygen. Wind and solar resources are much larger than biomass resources and it would be possible to produce electrolytic hydrogen in most parts of the UK. This provides a way of storing renewably generated electricity on a much larger scale than is currently possible with existing battery technology. As some renewable sources are intermittent (for example, electricity is only generated when the wind is blowing at certain speeds or if the sun is shining), the electrical energy can be converted to chemical energy in the form of stored hydrogen for use when renewable sources are not available. However, the efficiency of the best large-scale electrolysers is only about 70 per cent, and the subsequent conversion of hydrogen into electricity may not exceed 50 per cent.

At present, the bulk of hydrogen (almost 50%) is produced most economically by steam methane reforming (SMR). Partial oxidation of hydrocarbon fuels can be competitive where a cheap source of oxygen is available. Both these processes result in the emission of carbon dioxide and need to capture and store carbon dioxide if a carbon emissions are to be controlled.

Hydrogen can be produced  by the following technologies:

• Steam methane reforming (SMR)
• Partial oxidation of hydrocarbons
• Clean coal gasification (with carbon capture and storage)
• Pyrolysis (decomposition of hydrogen in the absence of oxygen)
• Electrolysis of water (ideally using electricity generated from low carbon sources such as wind or nuclear)
• Photoelectrolysis (splitting of water with sunlight)
• Biological production of hydrogen (photosynthetic)
• Biological production of hydrogen (fermentation)
• Gasification of biomass
• Thermochemical cycles (powered by solar or nuclear heat)

Hydrogen Storage

Hydrogen needs to be stored on a wide range of scales to achieve a fully functioning hydrogen economy. Large, centralised storage would be required if hydrogen is produced in large plants for wider distribution; longer term or seasonal storage would be required in systems relying on large scale exploitation of renewable energy; comparatively small scale storage is required on board vehicles, for homes, and for portable devices.

A wide range of potential hydrogen storage technologies are under research and development. Particular interests is being shown in solid-state hydrogen storage as follows:

• Compressed gas  
• Liquefaction (LH₂)
• Reversible metal hydride
• Alkali metal hydrides
• Carbon nanotubes
• Methanol

Hydrogen use as energy carrier

Hydrogen can be used in two main ways:

• in a fuel cell, where it produces zero emissions at the point of use
• in normal combustion, where it produces lower emissions of pollutants than fossil fuels.

Fuel cells

A fuel cell is an energy conversion device that uses an electrochemical process to convert hydrogen into electricity without combustion. It produces electricity with a conversion efficiency of up to 50 per cent. In a combined heat and power (CHP) installation, an overall efficiency of up to 80 per cent may be possible by utilising the heat that is also produced as a by-product of this process.

Fuel cells principle was discovered over 160 years ago and until recently, their use was limited to the laboratory and in spacecraft applications such as the Gemini, Apollo, and space shuttles.

Potentially, fuel cells can be made in any size to power anything, from mobile phones to large power plants. However, at present the costs are between 10 and 100 times higher (depending on application) than existing polluting technologies. Possible applications include replacements for internal combustion engines for transport, powering portable devices, and electricity and heat for homes and buildings.

A fuel cell contains an anode and a cathode insulated by an electrolyte situated between them. Hydrogen is supplied to the anode while oxygen is supplied to the cathode. The two gases try to join, but because of the electrolyte, the hydrogen atom splits into a proton and an electron. The proton passes freely through the electrolyte. The electron takes a different route, creating an electric current before recombining with the hydrogen and oxygen, creating a molecule of water. This chemical process generates electrical and thermal energy but produces pure water as a by-product.

There are many different types of fuel-cell technology, with different characteristics such as power output and operating temperature, and each fuel-cell technology will only be suitable for certain types of application, for example large or small-scale stationary power generation, transport or portable battery replacement.

• Alkaline
• Proton exchange membrane (PEM)
• Direct methanol
• Phosphoric acid (operating temperature ~220^oC)
• Molten carbonate (operating temperature ~650^oC)
• Solid oxide (operating temperature ~500-1000^oC)