Magnetohydrodynamic generator

An MHD generator is a generator for the direct transformation of thermal energy into electricity. It consists of a long pipe of non conductive material, by which one lets hot gas, liquid metal or an other electrically conductive substance flow through. At the wall of this pipe electrical contacts are appropriate in one level. Perpendicularly to these electrical contacts the arrangement is flowed through by a magnetic field. If an electrically leading substance flows by this arrangement, then an electrical tension develops at the electrodes. This is proportional the volume throughput, why this application is used frequently as volumetric flow meter. It has to arrive in relation to other procedures the advantage without mobile parts. Besides also an application is for energy production on a large scale, both in connection with conventional and with atomic power plants, conceivable. For an application in a conventional power station those must approx. by the pipe. 3000 degrees are called incineration gases flow through. A so high temperature is necessary, in order to make the gas sufficiently electrically conductive. Nevertheless is still another additive of easily ionizable substances, as salts of alkali metals necessary, in order to increase the electrical conductivity. Because of the high gas temperature the walls of the channel from very heatproof materials must be manufactured. As material for this yttriumoxid or zirconium dioxide would be applicable for example. Also the electrodes must be manufactured from very heatproof material such as tungsten. Behind the channel a device is to be planned, behind which the alkali salts must be separated from the exhaust gas. The efficiency of the magnetohydrodynamischen generator amounts to only 10 to 20 per cent. Since however the exhaust gases of the magnetohydrodynamischen generator have still another temperature of over 1000 degrees Celsius, one can use them still for a normal steam power plant with an efficiency of up to 50 per cent. With such an arrangement one can convert thus fossil fuels with an efficiency of up to 65 per cent into electricity. Up to now this arrangement was not used because of high wear yet in practice. Also with nuclear power stations with high coolant temperature the employment of the magnetohydrodynamischen generator is conceivable. Because of the necessary electrical conductivity of the cooling agent the reactor with liquid metal would have to be cooled and be operated with high Brennstabtenperaturen (approx. 2000 degrees Celsius). The hot liquid metal flows through the magnetohydrodynamischen generator directly behind the reactor before it the heat exchanger reached, at which it delivers its waste heat at a cooling circuit, in which a normal steam power plant is. In this way the efficiency of the nuclear generation of current up to 60% can be increased. Because of the enormous difficulties the enterprise with reactors with liquid-metal cooling (for liquid-metal cooling because of the necessary energy expenditure for the circulation of the liquid metal only the alkali metals in question, violently reacting with water, give it high safety problems come there) such arrangements were not carried out yet. Possibly they are once used with fusion reactors.

Usage as engine

The MHD generator can be operated also as engine, in which one lets a river flow by the electrodes. An application for this is the medical technology. It can be used also for the increase of the output speed of exhaust gases by rocket engines, in order to make thus rocket engines more efficient, however this is not practicable because of the high weight of magnets. A further application of the Magnetohydrodynami generator than engine lies in the drive of ships. Since for this the water must have as good electrical a conductivity as possible, this form of the drive for ships, which drive in waters with fresh water, is unsuitable. In Japan relevant studies were already accomplished. Whether appropriate prototypes were built, is not well-known.

Technical problems

The employment of the Magnetohydrodynami generator for the energy conversion failed so far because of the following problems:

• Production of the necessary high magnetic fields (because of the necessary fields with field strengths of if possible over 1 T only superconductors are economical)
• of the life span of materials for the channel and the electrodes

Model test

One leads the exhaust gases of a model rocket propellant charge, which one locks in a suitable arrangement by the pole pieces of a Magnetens. In the right angle to these two electrodes between those rule the produced tension to measure itself can. The produced tension is read off from a measuring instrument set up at safe distance. With this attempt is to be paid attention a stable structure and to the necessary safety margins!