Analysis and operation of Magneto Hydrodynamic Generators.
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Transcript of Analysis and operation of Magneto Hydrodynamic Generators.
Analysis and operation of
Magneto Hydro Dynamic (MHD) Generators
Graduate Project
December 12th, 2012
By Khalil Raza
Energy Conversion (ME 6530)
Course Instructor: Dr. James Menart
Table of Content
1. Introduction
2. Working Principle and Development of MHD Generators
2.1 Working Principle
2.2 Structure of MHD Generator
2.3 Development of MHD Generators
2.4 Basic Equations
2.5 Ionization of gas:
2.6 Ionization Method
3. Classification of MHD Generators
3.1 According to Working Fluid
3.1.1 Open Cycle Magnetohydrodynamic Generators (OCMHD)
3.1.2 Closed Cycle Magnetohydrodynamic Generator (CCMHD)
3.2 Classification according to Electric Field:
3.2.1 Conduction Type
3.2.2 Induction Type
3.3 Classification of MHD Generators according to their Design
3.3.1 Faraday MHD Generator
3.3.2 Hall MHD Generator
3.3.3 Disc Type MHD Generator
4. Efficiency of MHD Generator
5. Fuel of MHD Generators
6. Advantages/Disadvantages
7. Conclusion
8. References
1. Introduction
The word Magnetodydrodynamics (MHD) is a combination of three words Magneto – meaning Magnetic
Field and hydro mean Liquid and the dynamics which means the movement. The Magneto Hydro
Dynamic power generation technology (MHD) is the production of electrical power utilizing a
high temperature conducting ionized gas which is forced to pass through a high intensity
magnetic field. The primary working principle of MHD generators is similar to conventional electrical
generators. However, the MHD Generators uses Gaseous or fluid conductors instead of using solid
conductors. Therefore, MHD generators do not use mechanical parts unlike the conventional electrical
generators and that is why it is direct energy conversion process. The initial concept of MHD generator
was put forwarded by Michael Faraday in 1893. The first experiments on electrical energy
generation were performed by B. Karlovitz at the Westing-house laboratory, U.S. in 1938 using
an MHD generator working with nonequilibrium plasma [1]. MHD Generator uses thermal energy
and kinetic energy of fluids that are converted to electricity without going through any mechanical steps.
Figure 1: Schematic diagram of MHD Generators
2. Working Principle and Development of MHD Generators 2.1 Working Principle
MHD generator works on the principle described by the Faraday that when a conductor moves through a
magnetic field it creates an electrical field which is perpendicular to the magnetic field and the direction
of the movement of the conductor. Similarly, MHD generator uses an intense magnetic field that produces
Similarly, MHD generator uses an intense magnetic field that produces an electric field normal to stream
of current conducting medium or fluid which can be plasma or liquid metal. The magnetic field directs
the negatively charged electron and positive charged ions in direction of cathode and anode junctions.
Therefore, the positively charged particles electrons move towards cathode while negatively charged
electrons move towards the anode. As a result the Electromotive force (EMF) is induced in the channel
and electric current is generated and flows through conductors which are connect to the electrodes. The
velocity of the conducting fluid U is parallel to the channel and due to the presence of electric field the
electrons move towards the electrodes, which are installed opposite to the side walls of the chamber as
shown in figure 1. Electrons travel from one electrode to another and to the external load then back to the
conducting medium through an electrode thus, completing a circuit [2].
The MHD generator uses a high temperature conducting fluid, mostly high temperature combustion gases
generated by burning fossil fuel like coal in the combustion chamber of the generator.
Figure 2: Components of Open Cycle MHD Generator [3]
When the high pressurized conducting fluid passes through an expansion nozzle, the pressure of the fluid is reduced and this causes the increase in speed of the fluid that flows through the generator chamber thus maximizing the power output of the generator. However, because of the
reduction in the pressure the fluid temperature goes down which creates the resistance for the fluid. While the heat exhausted from the fluids drive the compressor that increases the rate of fuel combustion.
2.2 Structure of MHD Generator
A simple magnetohydrodynamic generator consists of a gas nozzle. The gas nozzle is a combustion
chamber that injects a pulse of gas into the channel/duct. The walls of the channel act as an electrode.
The induced electric current is fed to the load by an external circuit that supplies the generated
electricity to the desired destination. The MHD generators can be constructed in various designs like
the Faraday generator, Hall generator and disc generator. Faraday generator was the first designed
MHD generator. It was made by Michael Faraday in 1831. The Faraday generator used copper disks
and a horse-shoe magnet to generate electricity [4].
2.3 Development of MHD Generators
Michael Faraday demonstrated motional electromagnetic induction in a conductor moving through
Earth’s geomagnetic field. He set up in January 1832 a rudimentary open-circuit MHD generator, or flow
meter, on the Waterloo Bridge across the River Thames in London. His experiment was unsuccessful
owing to the electrodes being electrochemically polarized, an effect not understood at that time.
Faraday soon turned his attention to other aspects of electromagnetic induction, and MHD power
generation received little attention until the 1920s and ’30s, when Bela Karlovitz, a Hungarian-born
engineer, first proposed a gaseous MHD system. In 1938 he and Hungarian engineer D. Halász set up
an experimental MHD facility at the Westinghouse Electric Corporation research laboratories and by
1946 had shown that, through seeding the working gas, small amounts of electric power could be
extracted. The project was abandoned, however, largely because of a lack of understanding of the
conditions required to make the working gas an effective conductor.
Interest in magnetohydrodynamics grew rapidly during the late 1950s as a result of extensive studies
of ionized gases for a number of applications. In 1959 the American engineer Richard J. Rosa
operated the first truly successful MHD generator, producing about 10 kilowatts of electric power.
By 1963 the Avco Research Laboratory, under the direction of the American physicist Arthur R.
Kantrowitz, had constructed and operated a 33-megawatt MHD generator, and for many years this
remained a record power output. The assumption in the late 1960s that nuclear power would
dominate commercial power generation, and the failure to find applications for space missions, led to
a sharp curtailment of MHD research. The energy crisis of the 1970s, however, brought about a
revival of interest, with the focus centred on coal-fueled systems. By the late 1980s, development had
reached the point where construction of a complete demonstration system was feasible. However, the
performance and economic risks have deterred electric power utilities from making deep investments
in such systems. This situation may change if energy prices or environmental considerations shift
significantly [5].
2.4 Basic Equations
MHD working principle is basically governed by Faraday’s law and Lorentz describing the effects of a
charged particle moving in a constant magnetic field can be stated as
F =qE+qv x B (2.1)
Where, F is the net force acting on the charged particle.
q is charge of particle
v is velocity of particle
B is magnetic field
Normally in gaseous state the free electron flow in circular motion but in MHD systems, these electrons
collide with ions and neutral particles due to presence of magnetic forces. This collision changes the
behavior of motion of electrons into a certain sideways direction which is the primary reason for electrical
current generation. This collision effect between the electrons and free particles ions and neutrals is called
the Hall Effect or Hall Parameter.
Mathematically, Hall Parameter can be stated as !!!
the force on the electrons to generate current is 𝐹𝐵=𝑞𝑢𝐵 (2.2)
We can look at 𝑢𝐵 as an equivalent electric field 𝐸𝑥,𝑛=𝑢𝐵 (2.3)
The net current density can be determined from 𝐽𝑥=𝜎𝐸𝑥,=𝜎(𝑢𝐵−𝐸𝑥) (2.4)
The channel load factor, K
𝐸𝑥=𝐾𝐸𝑥,𝑛=𝐾𝑢𝐵 (2.5)
𝐾𝑥=𝐸𝑥/𝐸𝑥,𝑜𝑝𝑒𝑛
The current density is given by: 𝐽𝑥=(1−𝐾) (2.6)
Power density delivered to load:
𝑃𝑙𝑜𝑎𝑑/∀ = 𝜎𝑢2𝐵2(1−𝐾) (2.7)
2.5 Ionization of gas:
Ionization is an endothermic whereby one or more electrons are removed from an atom. The most
convenient source of a high velocity, ionized gas stream is the exhaust stream from a high intensity
combustion system feeding into convergent divergent nozzle. This has been done by adding
substances with low ionization potentials, alkali metals usually; cesium is the best material with an
ionization potential of 3.89 eV. However, caesium is too expensive to use in an open cycle system, so
potassium is used in open cycle system this has an ionization potential of 4.33 eV [6]
There are primarily three types of Ionizations:
1. Thermal Ionization
2. Ionization by Irradiation
3. Cumulative ionization, also known as ionization by stages.
2.6 Ionization Method:
When an electron collides with an atom of gas, the ionization energy is release off from another
electron from that atom. Here, the most important factor is cross-section which is the basis of
ionization. The ionization cross-section determines the chances that a certain electron with some
energy will ionize with a collision with an atom. Cross-section is a direct function of energy of an
electron that is being collided with an atom and energy state of that atom. This is the minimum
energy required for ionization of an atom. In order to ionize an atom, the incident electron must have
equal energy to that of ionization energy of an atom. If the incident electron has an excess energy
than required, then that energy could be hold by the electron itself or shift that energy to an electron
released in ionization process or may be used to further excite the atom [7].
3. Classification of MHD Generators
MHD Generators can be classified according to their working fluid or their design or construction.
3.1 According to Working Fluid:
Following are the two types of the MHD Generators classified according to their working fluid
3.1.1 Open Cycle Magnetohydrodynamic Generators (OCMHD):
In open cycle system the working fluid after generating electrical energy is discharged to the atmosphere
through a stack. In open cycle the working fluid is air. An open cycle MHD generator looks like a rocket
engine surrounded by a magnet. The coal is burnt to produce the hot has. Which, is further seeded with an
ionized alkali metal either cesium or potassium to enhance the electrical conductance of gas. This ionized
gas is called as Plasma or sometimes combined with the liquid metal [8]. The hot gases are expanded
Figure 3: Schematic of Open Cycle MHD Generator [9]
through generator bounded by a magnet. While the hot gas is passed through the generator duct the
positive and negative ions travel to the electrodes and produce an electric current. The seeding material is
recovered through a recovery mechanism for consecutive use. During the process, there is production of
nitrogen and sulfur which are captured before exhausting the gas to the atmosphere. The exhaust gases are
still at high temperature therefore, it can be used for additional power generation through a steam
turbine. This increases the efficiency and this kind of cycle is referred as Hybrid MHD-Steam
Plant Cycle. The open cycle MHD generators are not viable for commercial usage.
There is a lower limit of working temperature that has to be maintained in order to continue the
electrical conductance. Below that lower limit temperature the electrical conductance becomes
zero. In this case of Open Cycle MHD generator the lower limit is 2300 K [10]. However, there is
not specific upper limit for working temperature and normally temperature range from 3000-3500K
performs the operation at sustainable level
Figure 4: Taxonomy of MHD Systems [11]
3.1.2 Closed Cycle Magnetohydrodynamic Generator (CCMHD):
Closed Cycle MHD use a plasma consisting of seeded noble gas or a Liquid metal in
combination with a gas. Thermodynamically, plasma generators follow a Brayton Cycle; liquid-
metal generators, either a Brayton or Rankine cycle depending on the choice of working fluid.
The flow through the generator can be either single or two phases.
In any MHD Cycle, the working fluid in the generator must be a satisfactory electrical conductor. The
open cycle MHD system work under high temperature and chemical active materials which, is considered
as a major drawback of open cycle MHD generator. However, with close cycle MHD generator this
limitation can be resolved. For closed cycle MHD channel, the ionization instability is considered to be
major factor, while for some experimental channel the electrode potential drop and the shortage of a Hall
field mask, more or less, effects of the ionization instability. From this point of view, it is important to
reduce effects of ionization instability in a full scale closed cycle MHD generator which must be operated
with a high adiabatic efficiency [12]. While there have been experiments carried out where the
Helium and xenon mixture were used as coolant for their excellent heat transfer when flowing,
and performs well as thermal insulator at stagnant state. Therefore, the in CCMHD generators
mixed inert gas of helium and xenon can be used as a working medium without any use of alkali-
metal. This combination can increase the plant efficiency up to 60 % when used with single
cycle CCMHD [13].
Figure 5: Schematic of Nuclear Brayton CCMHD Space Power System [14]
3.2 Classification according to Electric Field:
MHD Generators according to electric field are classified into:
3.2.1 Conduction Type:
In this type of MHD generator the electrical current is produced in the working fluid medium due to
presence of electrical field potential and the current flows through a load [15]. Conduction MHD
generator can generate direct or alternating current. It has been observed that conduction type MHD
generators are more feasible technically and sophisticatedly much advanced regarding research and
development.
Figure 6: Conduction Type MHD Generator [16]
3.2.2 Induction Type:
An MHD induction generator consists of either a flat rectangular channel or an annular channel
surrounded by a stator which produces a magnetic field that travels in the same direction as the fluid.
Current is induced in the fluid as in the rotor windings of a conventional induction machine. If the
velocity of the fluid stream exceeds the synchronous velocity of the traveling magnetic field, some of the
mechanical energy of the working fluid is converted into electrical power. The MHD induction generator
has several advantages over DC MHD generators in that there is no need for D.C. –A.C inverters, the
stator windings serves as a step up transformer to increase the terminal voltage and reduce the armature
current, and there is no need for electrodes and connections for heavy direct current [17].
Figure 7: MHD generator channel with traveling magnetic field created by external conductors [18].
3.3 Classification of MHD Generators according to their Design:
3.3.1 Faraday MHD Generator
The Faraday generator is named after Michael Faraday who was the pioneer of the idea of using
this type of generator. This generator is composed of a insulating pipe or tube. When the working
fluid flows through that pipe or tube, a charge is produced in the field due to the presence of a
magnetic field. This electrical charge or the current is then passed to the electrodes and utilized
for external load.
Figure 8: Schematic of Segmented Faraday Generator [19]
The power is proportional to the cross sectional area of the tube and the speed of the conductive
flow. The basic flaw in this design of MHD generator is the voltage difference and shortage of
the current via electrodes. There currents also short and is wasted from the Hall Effect current.
That is why the Faraday duct is considered inefficient. Therefore, this kind of generator has been
refined by using the saddle shaped duct. Moreover, attempts have been made to incorporate the
superconducting magnets, as in order to generate the field, a massive powerful magnet is needed
for a large sized generator.
3.3.2 Hall MHD Generator
Since, there is wastage of charges due to the presence of Hall Effect; therefore, Hall Effect is
used to create a current which has same direction of motion as the fluid. Normally, an array of
short and vertical electrodes is used on the sides of the channel. The first and last electrodes of
the channels are connected to the load. These shorts of faraday current induce a powerful
magnetic field within the fluid, but in a chord of a circle at right angles to the faraday current.
The main advantage of Hall Effect generator is that the losses are minimized and induced
currents do not shorts off and thus the voltages are higher. However, there is on disadvantage of
this type of generator that when load fluctuated, fluid velocity changes then there are the
misalignment in the faraday currents which makes the efficiency of this generator sensitive
respective to its load.
Figure 9: Schematic of Hall Generator (Encyclopedia Britannica)
3.3.3 Disc Type MHD Generator
Disk type MHD generator is considered to be the most efficient generator among the other types.
This generator has good efficiency as well as better energy density for MHD generation. This
generator uses the center of a disc and fluid flow between the center of a disc and its channel.
The Helmholtz coils are used above and below the disc to create the magnetic field. The faraday
currents flow around the edges of the disk. The electrodes are connected at the center of the disc
and one of the electrodes is connected at the edges of the disc near the periphery. The hall effect
currents flow between those electrodes one at the center and another at the periphery.
This type of generator has several advantages. Firstly, the field lines are parallel. Secondly, the
fluid the fluid flow in the disk therefore, the distance between the fluid and the magnetic field is
less thus the magnetic flied is strengthened 7 times the power of distance. Because of its compact
design the size of the magnet is smaller. And, overall the Disc type MHD generator becomes
more efficient.
F
Figure 10: Disc Type MHD Generator [20]
4. Efficiency of MHD Generator
MHD generators are less popular as compared with other technologies using fossil fuels, such as
combined cycles in which the exhaust heat of the gas turbine is used for steam generation. The conversion
efficiency of a MHD system is about 50 – 60 % as compared to less than 40 percent for the most efficient
steam plants [21][22].
The major factor that makes MHD generator more efficient is recycling the energy from the hot
conducting fluid to the steam turbines. When the hot conducting fluid flows through the MHD generator,
it does not lose its thermal energy and still at very high temperature to convert water into steam to
generate additional power through a steam turbine. Therefore, the power generation from MHD
generators can be segmented into three different stages for enhancing its efficiency up to 60-7523. First of
all, power generated the thermal energy of the gas itself. Then, passing the hot gas through generator’s
chamber by any method including Faraday, Hall or Disc. At the last stage since the conducting fluid has
attained high temperature so it can be utilized through gas turbine to produce maximum power.
5. Fuel of MHD Generators:
The MHD generators primary operation is to generate power by passing hot conducting gas
through its chamber. The hot gas or its ionization can be processed with several kinds of fuel or
source of heat can be obtained from different fuels including coal, gas, nuclear or solar etc.
However, the choice of selecting a particular source of heat from a fuel depends on the
application for which the MHD generator is to be used. The plenty of coal reserves in the world
has made a significant progress in developing coal-fired MHD generators for electrical
generation. As mentioned earlier, when the gas expands in channel its temperature goes down
resulting the drop in electrical conductivity. Whereas, to continue the electrical conductance in
the MHD generator there is lower limit of temperature which has to be maintained in order to get
the power generation. Therefore, the coal fired MHD generators are combined with the
conventional steam power plants. The hot gas is passed through the MHD generator, this stage is
called as topping and then in second stage, it is passed through conventional steam plant which is
called the bottoming phase [24] and such an arrangement is called the open cycle or once-
through system.
Figure 11: Comparison of Heat Sources and Energy Conversion Technologies [25].
Moreover, the coal fired MHD systems has many advantages. Such as, the coal when burnt produces slag
which flows through MHD system, is in molten state that acts as a film surface and protects the insulator
and electrode walls [26]. This layer or film surface over the wall also helps in reduction of thermal losses
resulting in higher efficiency of the system.
In addition to coal, MHD power generators have used with conventional nuclear reactor that uses argon or
helium, as the conducting fluid are used.
For space application Laser can also be used to drive the MHG systems to generate electrical power. This
system has the best power to weight ratio because of lesser use of components. The laser driven MHD
converter has high conversion efficiency, high power density and closed cycle operation [27].
Solar concentrator can also be used to provide thermal energy at high temperature for thermal ionization.
Therefore, Solar powered MHD system is viable option to generate the electrical power generation if they
operate considerably well for extensive time at high temperatures.
6. Advantages of MHG Generators:
• Conversion efficiency of about 50-60%.
• Direct Energy Conversion Process
• The MHD process requires less cooling water requirement hence, industrially attractive.
• Less fuel consumption.
• MHG generation is flexible in differed modes such as peak load, semi peak load or base
load.
• It can us all kinds of fuels such as coal, nuclear, gas, solar and nuclear energy.
• Pollution free power generation.
• Ability to reach full power level as soon as started.
• Plant size is considerably smaller than conventional fossil fuel plants .
• Less overall generation cost.
• No moving parts, so more reliable.
Disadvantages of MHD Generators:
• Suffers from reverse flow (short circuits) of electrons through the conducting fluids
around the ends of the magnetic field.
• Needs very large magnets and this is a major expense.
• High operating temperature.
• The use of the superconductive coils to generate the external magnetic field.
7. Conclusion:
MHD generators being more efficient in terms of converting the thermal energy directly to
electrical energy, the losses of energy from thermal to mechanical and then to electrical are
reduced drastically. Thus, it has a significant importance and potential to employ these kinds of
devices to generate the electrical energy to get more power with greater efficiency.
However, due to technical complexities of materials that could sustain the high temperature and
the methods of ionization have kept this generator out of commercial use.
The MHD generators can possibly be used by three different arrangements: (a) Open Cycle
System, (b) Closed cycle system with liquid metal working fluids and (c) closed cycle system
with gaseous fluids. Whereas the choice of fuel and generator type such as Faraday, Hall or Dick
can be used according to the application of the system being used for.
Moreover, nuclear reactor are considered as a suitable source of heat for combined closed-cycle
MHD generator capable of generating thousands of megawatts of electricity. Another major
practically viable prospect is to use the closed cycle systems using molten liquids for large scale
production. The ionization techniques and seeding methods are being improved with the
experience.
With ongoing research on MHD generators while keeping the complexities in mind, new
efficient methods are coming in that will make this device feasible to use on commercial levels.
8. References
[1] MHD Generator – History – Wikipedia <> http://en.wikipedia.org/wiki/MHD_generator [2] K. C. Weston, (1992), “Energy Conversion,” Energy Systems Alternatives, (1992), PP 477. [3] Magnetohydrodynamic (MHD) Power Generation, <>http://www.mpoweruk.com/mhd_generator.htm [4] How do MHD Generators Work <> http://www.buzzle.com/articles/how-do-mhd-generators-work.html [5] magnetohydrodynamic power generator <>http://www.britannica.com/EBchecked/topic/357424/magnetohydrodynamic-power-generator/258876/Other-MHD-systems [6] I. Fells, University of Sheffield, U.K. “Ionization processes in Gases and their application to Energy Conversion Systems” - International Union of Pure and Applied Chemistry PP 514. <> http://pac.iupac.org/publications/pac/pdf/1962/pdf/0503x0513.pdf [7] D.S Chauhan, S.K. Srivastava, “Non Conventional Energy Resources” PP-134 [8] R. Y. PEI, R.W. HESS (1978)“ Liquid Metal Closed Cycle System of Magnetohydrodynamic Genertor”, US DOE, December, PP-4 [9] G.D Rai, Inamdar, C.L Wadawa, “Open Cycle MHD System” Bhabha Atomic Research Center <> http://ffden-2.phys.uaf.edu/645spring2010_web.dir/srikamal/mhd_openfig.JPG [10] University of Alaska-Fairbanks, “Basic features of open-cycle generator” <> http://ffden-2.phys.uaf.edu/645spring2010_web.dir/srikamal/page6.htm [11] R. Y. PEI, R.W. HESS (1978) “Liquid Metal Closed Cycle System of Magnetohydrodynamic Genertor”, US DOE, December, PP-5 [12] H. Yamasaki, S. Shioda (1997), “Power Density and Electrical Efficiency in Closed Cycle MHD Generator with Fully Ionized Seed” Journal of Nuclear Sciences and Technology, PP-313 [13]. N. Harada, L.C. Kien, and M. Hishikawa, (2004) “Basic Studies on Closed Cycle MHD Power Generation System for Space Application”, 35th AIAA Plasma dynamics and Lasers Conference [14] “High Energy Density Nuclear Power for Space”, (2011), <>http://1.bp.blogspot.com/LTWkDyWfnbM/Th3bzbIxqOI/AAAAAAAAMB0/o_9iBgFGu_k/s1600/nuclearspacemhd.png
[15] Medin, S.A. Thermopedia – A-Z Guide to Thermodynamics, Heat – Mass transfer and Fluid engineering <>http://www.thermopedia.com/content/934/ [16] Thermopedia <>http://www.thermopedia.com/content/934/ [17] S.J. Dudzinsky, T.C. Wang, (1968), “MHD Induction Generator” Rand Corporations <> http://www.rand.org/pubs/papers/2008/P3837.pdf [18] Thermopedia <>http://www.thermopedia.com/content/934/ [19] Encyclopedia Britannica, Inc 1994 <> http://www.britannica.com/EBchecked/media/92959/MHD-generator-configurations-Segmented-Faraday-generator 20 Disc MHD Generator – Wikipedia <> http://upload.wikimedia.org/wikipedia/commons/1/14/Disk_MHD_generator.png [21] MHD Generators <> http://concentratingsolarpowerutility.com/ [22] MHD Generators – Images Scientific Instruments <> http://www.imagesco.com/articles/mhd/index.html [23] R. Pintus “Development Of An Inductive Magnetohydrodynamic Generator” University of Cagliari, PhD thesis, PP-16 [24] Coal-fired MHD systems, Encyclopedia Britannica Inc. <> http://www.britannica.com/EBchecked/topic/357424/magnetohydrodynamic-power-generator [25] R. Y Pei, R.W. HESS (1978),“Liquid-Metal Closed Cycle System of magnetohydrodynamic Power Generation” Department of Energy, PP- 17 [26] P. K. Nag, (2008), Power Plant Engineering, Third Edition “Non-Conventional Power Generation: Direct Energy Conversion” PP-860 [27] N. W. Jalufka, (1986), “Laser-Powered MHD Generators for Space Application” NASA Technical Papers, Longley Research Center Hampton, Virginia.