Uses
Fuel cells
The devices that use hydrogen to directly produce electric energy are called “fuels cells”. Hydrogen fuel cells are electrochemical generators where electric energy is produced from threaction between a fuel (hydrogen) and a gaseous oxidizing compound (oxygen and air). Together with electricity, heat and water are produced.
A fuel cell is made up of two electrodes of porous material, namely the cathode (negative pole) and anode (positive pole). Electrodes act as catalytic sites for the cell reactions that mainly consume hydrogen and oxygen, with water being produced and electric power running in the external circuit. Between the two poles there is an electrolyte, that has the task to drive the ions produced by a reaction (the one occurring in the anode) and consumed by the other reaction (the one occurring in the cathode), closing the electric circuit inside the cell (see image).
The electrochemical transformation is accompanied by the production of heat, which has to be extracted in order to keep the cell functioning temperature constant. This structure is completely similar to the structure of ordinary electric batteries but, differently from them, hydrogen fuel cells consume substances that come from outside and therefore can work without interruptions as long as they are supplied with fuel and oxidant.
The cell has a flat three-layer structure: the central layer, between the cathode and the anode, is made up of or contains the electrolyte. Individual cells overlap and are connected in such a way to obtain the desired voltage. A pile of cells is called stack.
Usually a fuel cell plant includes, as well as the electro-chemical part, a power converter and a transformer that convert continuous power generated by the stack into alternated power.
Fuel cells are different according to the chemical nature of the electrolyte and their functioning temperature. Those cells that release temperatures between 60 and 200°C are called low-medium temperature cells, while high temperature cells release a temperature up to 1000 °C. The latter are often used for applications that require both electricity and heat.
Low-medium temperature cells cause fewer technological problems than high temperature ones, but have a worse performance.
The technology that exploits hydrogen as an energy source is now rapidly developing both for stationary applications (which do not move, like industries, households) and moving systems (transport).
Fuel cells are extremely interesting as far as electric energy production is concerned, as they have advantageous energy and environmental characteristics:
• high electricity performance, with values between 40-48% (referring to the minimum calorific value of the fuel) for low-temperature cell plants, up to 60% for high temperature cell plants
• extremely low environmental impact, both with regard to gaseous emissions and acoustic emissions. Therefore plants can be located in residential areas, making the system particularly suitable for the production of the electric energy to be distributed
• cogeneration possibility (associated production of electric energy and vapour): co-generated heat can be available at different temperatures, as vapour or hot water, and used for sanitary aims, air conditioning, etc.
One of the biggest “fuel cell” electric power plants is located in Bicocca, Milan (1.3 MW power).
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