Magnetohydrodynamic (MHD) Power Generation
Magnetohydrodynamic power generation provides a way of generating electricity directly from a fast moving stream of ionised gases without the need for any moving mechanical parts - no turbines and no rotary generators. Several MHD projects were initiated in the 1960s but overcoming the technical challenges of making a practical system proved very expensive. Interest consequently waned in favour of nuclear power which since that time has seemed a more attractive option.
MHD power generation has also been studied as a method for extracting electrical power from nuclear reactors and also from more conventional fuel combustion systems
The MHD generator can be considered to be a fluid dynamo. This is similar to a mechanical dynamo in which the motion of a metal conductor through a magnetic field creates a current in the conductor except that in the MHD generator the metal conductor is replaced by a conducting gas plasma.
When a conductor moves through a magnetic field it creates an electrical field perpendicular to the magnetic field and the direction of movement of the conductor. This is the principle, discovered by Michael Faraday, behind the conventional rotary electricity generator. Dutch physicist Antoon Lorentz provided the mathematical theory to quantify its effects.
The flow (motion) of the conducting plasma through a magnetic field causes a voltage to be generated (and an associated current to flow) across the plasma , perpendicular to both the plasma flow and the magnetic field according to Fleming's Right Hand Rule
Lorentz Law describing the effects of a charged particle moving in a constant magnetic field can be stated as
F = QvB
F is the force acting on the charged particle
Q is charge of particle
v is velocity of particle
B is magnetic field
The MHD System
The MHD generator needs a high temperature gas source, which could be the coolant from a nuclear reactor or more likely high temperature combustion gases generated by burning fossil fuels, including coal, in a combustion chamber. The diagram below shows possible system components.
The expansion nozzle reduces the gas pressure and consequently increases the plasma speed (Bernoulli's Law) through the generator duct to increase the power output (See Power below). Unfortunately, at the same time, the pressure drop causes the plasma temperature to fall (Gay-Lussac's Law) which also increases the plasma resistance, so a compromise between Bernoulli and Gay-Lussac must be found.
The exhaust heat from the working fluid is used to drive a compressor to increase the fuel combustion rate but much of the heat will be wasted unless it can be used in another process.
- The Plasma
The prime MHD system requirement is creating and managing the conducting gas plasma since the system depends on the plasma having a high electrical conductivity. The plasma can be likened to the fourth state of matter after the solid, liquid and gaseous states, in which the atoms or molecules are stripped of their electrons leaving positively charged ions. Suitable working fluids are gases derived from combustion, noble gases, and alkali metal vapours.
The Gas Plasma
To achieve high conductivity, the gas must be ionised by detaching the electrons from the atoms or molecules leaving the positively charged plasma. The plasma flows through the magnetic field at high speed, in some designs, more than the speed of sound, the flow of the positively charged particles providing the moving electrical conductor necessary for inducing a current in the external electrical circuit..
Methods of Ionising the Gas
Various methods for ionising the gas are available, all of which depend on imparting sufficient energy to the gas. It may be accomplished by heating or irradiating the gas with X rays or Gamma rays. It has also been proposed to use the coolant gases such as helium and carbon dioxide employed in some nuclear reactors as the plasma fuel for direct MHD electricity generation rather than extracting the heat energy of the gas through heat exchangers to raise steam to drive turbine generators. Seed materials such as Potassium carbonate or Cesium are often added in small amounts, typically about 1% of the total mass flow to increase the ionisation and improve the conductivity, particularly of combustion gas plasmas.
Since the plasma temperature is typically over 1000 °C, the duct containing the plasma must be constructed from non-conducting materials capable of withstanding these high temperatures. The electrodes must of course be conducting as well as heat resistant .
Note: The plasmas used in nuclear fusion reactors have a different function and run at much higher temperatures.
Note that 90% conductivity can be achieved with a fairly low degree of ionisation of only about 1%. (Note also logarithmic scale)
- The Faraday Current
A powerful electromagnet provides the magnetic field through which the plasma flows, and perpendicular to this field are installed the two electrodes on opposite sides of the plasma across which the electrical output voltage is generated. The current flowing across the plasma between these electrodes is called the Faraday current. This provides the main electrical output of the MHD generator.
- The Hall Effect Current
The very high Faraday output current which flows across the plasma duct into the load itself reacts with the applied magnetic field creating a Hall Effect current perpendicular to the Faraday current, in other words, a current along the axis of the plasma, resulting in lost energy. The total current generated will be the vector sum of the transverse (Faraday) and axial (Hall effect) current components. Unless it can be captured in some way, the Hall effect current will constitute an energy loss .
Various configurations of electrodes have been devised to capture both the Faraday and Hall effect components of the current in order to improve the overall MHD conversion efficiency.
One such method is to split the electrode pair into a series of segments physically side by side (parallel) but insulated from eachother, with the segmented electrode pairs connected in series to achieve a higher voltage but with a lower current. Instead of the electrodes being directly opposite eachother, perpendicular to the plasma stream, they are skewed at a slight angle from perpendicular to be in line with the vector sum of the Faraday and Hall effect currents, as shown in the diagram below, thus allowing the maximum energy to be extracted from the plasma.
- Power Output
The output power is proportional to the cross sectional area and the flow rate of the ionised plasma. The conductive substance is also cooled and slowed in this process. MHD generators typically reduce the temperature of the conductive substance from plasma temperatures to just over 1000 °C.
An MHD generator produces a direct current output which needs an expensive high power inverter to convert the output into alternating current for connection to the grid.
Typical efficiencies of MHD generators are around 10 to 20 percent mainly due to the heat lost through the high temperature exhaust.
This limits the MHD's potential applications as a stand alone device but they were originally designed to be used in combination with other energy converters in hybrid applications where the output gases (flames) are used as the energy source to raise steam in a steam turbine plant. Total plant efficiencies of 65% could be possible in such arrangements.
Demonstration plants with capacities of 50 MW or more have been built in several countries but MHD generators are expensive. Typical use could be in peak shaving applications but they are less efficient than combined-cycle gas turbines which means there are very few installations and MHD is currently not considered for mainstream commercial power generation.
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