Magnetohydrodynamics: The Opportunity For High-Efficiency Power Generation

 

The field of magnetohydrodynamics has proven to be a very interesting one. It involves the study of electrically charged fluids and their interaction with magnetic energy. In this model, the motion, flow and dynamics of charged fluids such as liquid metals, electrolytes, and plasma are studied in their interactions with magnetic fields. Today we can admit that magnetohydrodynamic phenomena are very important on Earth and elsewhere in the universe. You might have read or heard that there exists a magnetic field in the earth’s core that shields and protects us from the hazardous solar wind. This magnetic field is due to magnetohydrodynamics. The iron in the earth’s core generates this field creating a magnetic dipole and thereby deflecting the solar radiation. This leads to a distortion of the field lines as shown in the picture above. This is the basis of the protective effect of the magnetic field. Also the complex system which exists in the sun is a very interesting case of a MHD system. Space as we know it is filled with electrically charged particles many of which emanate from the sun as solar winds. The sun is composed of light atomic gases (primarily hydrogen and helium) in their ionized forms. The fusion and nucleosynthetic reactions produce highly energetic particles that pervade the solar system as magnetic solar winds. 

A central figure in this field of magnetohydrodynamics is the Nobel prize-winning physicist Hannes Alfven who is considered a pioneer in the field. He discovered many theories of plasma, describing their dynamics in the upper atmosphere, and the Milky Way galaxy. He also studied the interactions of extraterrestrial solar storms and the Earth’s magnetic field. Today, we can appreciate that his contributions have been widely applied in astrophysics, geophysics, atmospheric science, and electrical engineering. 

charged particles create electric fields around them. This is similar to magnetic fields.

The principles governing MHD are Faraday's electromagnetic laws, Ohm law and other important laws of electricity. These laws are used in the studies of space plasma, earth’s magnetosphere, and in power generation.

An exciting application of MHD is its use in the production of electricity. Hot plasma is used in special generators in the conversion of heat and kinetic energy into electrical energy. How is this possible? Do you know that this is a highly efficient way of generating electricity? This is the main focus of this article as I wish to explain how MHD can be an option to yield high energy output; but of course with some technical challenges.

The conventional solid metallic conductors generate electricity. When current flows through a metal rod or electrical wire, in the presence of a magnetic field, a force is generated perpendicular to this plane as demonstrated by Flemings's right-hand rule and described mathematically by the Lorentz force law. Likewise, an electrical field or current can be generated by a moving conductor in a magnetic field (called a dynamo). For traditional generators, conductors are used but in MHD generators, plasma is used as the moving conductor (recall that plasma is composed of ionized gases which are excellent electricity conductors and alkali metal vapors, inert gases or combustion gases of fossil fuels can be used for this purpose). We can consider it to be a fluid dynamo. MHD generators produce direct current (DC) and require powerful inverters to convert to an alternating current(AC).

MHD setup involves magnets placed on the tails of hot plasma (about 1500°C) emanating from jet engines. This can be obtained from the coolant gases used in nuclear reactors or gases generated in combustion chambers and ionized using high heat or ionizing radiation. This hot plasma ionizes gases along the path generating currents. This current is collected by charged plates positioned 90°. The resulting ions are deflected by the strong magnetic field according to their charges, opposite charges going in opposite directions (on the same perpendicular axis). The plasma passes through the magnets at high speeds. This process can be very efficient because of the high temperature of operation. As there is no turbine or moving parts, there is no loss due to friction, vibration, or noise. The efficiency can be as high as 65% compared to the approximately 40% obtained in ordinary generators.

The current flows through the plasma is the desired electrical output and is collected by conductors. This is called Faraday's current. 

Plasma, the fourth state of matter is composed of positively charged gas or vapors and electrons.

Hall Current

There is a mechanism that can lead to energy loss in the system and this is the hall current. It occurs when the Faraday current interacts with the magnetic field. The hall current is perpendicular to the Faraday current and cannot be collected by the conductors leading to energy loss if not collected. The method practically employed to tap the Hall current is to position the electrodes along the vector addition of the Faraday and Hall current. This is the method to obtain maximum energy output from a MHD generator.

What Is The Potential Of MHD In Power Generation?

MHD generators today, are combined with combined with modern electricity generators to improve efficiency. The hot exhaust gases can be used to turn steam turbines increasing the energy yield and efficiency of the process.

Where a stable supply of electricity is needed, an MHD generator is a good choice. They can be used to power industrial machinery and other big devices and machines for example air-crafts and submarines. While being environmentally friendly, they offer greater energy outputs with minimal losses and can enable an energy economy.

Despite the technical drawbacks such as high operating temperatures and high costs, many engineers still believe in the potential of MHD generators. Perhaps in the future, with further developments, it could become a mainstream source of sustainable electricity.