20 Principles of Energy Conversion Using Magnetic Fields

Mr. D. Leo

1. Introduction

Magnetism is a property of material and magnetic field produced by a magnet or a current carrying conductor is the space where the magnetic lines of force is experienced. The magnetic field at any given point is specified by both a direction and a magnitude; as such it is a vector field. The term is used in two distinct but closely related fields denoted by the symbols B and H, where H is measured in units of amperes per meter (A/m) and the unit of the B is Tesla (T). The relationship between magnetic field generated B to the magnetization force H and the induced electric current I at the conductor was defined by the Biot-Savart law.

Where the integral sums over the conductor length where dℓ is the line vector element, μ0, μr are the magnetic constants, r is the distance between the location of dℓ and the location where the magnetic field is calculated, and r̂is a unit vector in the direction of r. The Ampere’s law relates the electric current to the magnetic field accounting time varying electric fields.

Where the line integral is over any arbitrary loop and Ienc is the current enclosed by that loop. The relation between the induced voltage in a closed circuit and the rate of change of the magnetic flux it encloses was defined by the Faraday’s law of electromagnetic induction.

Where E is the induced Voltage and the negative sign is due to the direction of the induced Electromotive force (emf). Generating electric power in a wire loop can be achieved by varying magnetic field B, varying the number of conductor loops and by varying the intersecting area of the conductor loop.

2. Electric generators

Electric generators are rotating electric machines and converting mechanical energy to electricity using the laws of electromagnetism and every electric generator must possess the following parts.

1. Stationary member called the stator.
2. Rotating member called the rotor.
3. Shaft and bearing.
4. Slip ring/ Commutator /Brush assembly.
5. Armature and field Windings.

An arrangement of the winding which is used as a primary source of flux when current is passed through it. This is called field winding or exciting coil. In some applications, permanent magnets are used to produce main flux. An arrangement of conductors to form a winding in which emf is induced is called an armature winding or armature coil. The current flowing through the field winding, used to produce main flux is called magnetizing current, exciting current or field current. The armature current varies as the load on the machine varies. So it is called load current or armature current. In case of synchronous machines used in power plants, the current in the exciting winding is always DC.

There is an air gap between stator and rotor of the generator as shown in fig.1. In rotating field-synchronous machines, the machine armature windings are placed on the stator and field windings placed on the rotor. To provide a low reluctance path to the magnetic field produced at stator and rotor, stator and rotor are made up of high grade magnetic materials such as silicon steel. In practice, a brush assembly or slip rings are required to take out the generated power from the rotor which is the rotating member of the turbo alternator. In case of DC generators, a set of switches mounted on armature called commutators was used to convert the generated AC power to DC power. The ventilating ducts are also provided with the machines for cooling purposes. For large turbo alternators instead of air, hydrogen is used as a coolant. A closed circuit ventilating system is preferred for large capacity machines. The hydrogen is circulated over the machine parts and is cooled with the help of water cooled heat exchangers.

2.1. Classifications of Electric Generators

Electric generators are classified based on the output current into two major types, namely Alternating Current (AC) generators or alternators and Direct Current (DC) generators as shown in fig-2. Alternators are used in power stations and all the grid connected electric generators are alternators.

2.1.1 Classification of AC generators

AC generators are otherwise known as alternators and these generators are widely used in power stations. Based on the operating principles alternators are classified as Synchronous and asynchronous generators.

2.1.1.1 Synchronous Generators

A synchronous machine consists of a stationary armature winding (stator) with conductors connected in series or parallel to obtain the desired terminal voltage.

As the magnetic flux developed by the rotating DC pole crosses the air gap of the stator windings as shown in fig.3, a sinusoidal voltage is developed at the generator output terminals. This process is called electromagnetic induction. The magnitude of the AC voltage generated is controlled by the amount of DC excitation current supplied to the field. The frequency (F) of the voltage developed by the generator depends on the synchronous speed (N) of the rotor and the number of field poles (P).

Synchronous generators are used in power stations and reactive power compensation at grid stations. Permanent Magnet Synchronous Generators (PMSG)s have permanent magnetic poles. So there is no external field excitation source(DC) needed. PMSGs are widely used in renewable energy applications such as wind turbine systems and can be used for ocean   etc.

2.1.1.2. Asynchronous (Induction) Generators

Induction generators operate at the super-synchronous speed by mechanically turning their rotor faster than the synchronous speed, giving negative slip. A regular AC asynchronous motor usually can be used as a generator, without any internal modifications. Induction generators are useful in applications such as mini hydro power plants, wind turbines, or in reducing high-pressure gas streams to lower pressure, because they can recover energy with relatively simple controls. To operate, an induction generator must be excited with a leading voltage; this is usually done by connection to an electrical grid, or sometimes they are self-excited by using phase correcting capacitors. The induction generator requires one additional item before it can produce power – it requires a source of leading VAR’s for excitation. The reactive power may be supplied by the capacitors or from the utility grid. Induction generators are inexpensive and simple machines, however, they offer little control over their output. The induction generator requires no separate DC excitation like synchronous generators, regulator controls, frequency control or governor.

2.1.2   Classification of DC Generators

DC generators are usually classified on the basis of their field excitation method into three broad categories (i) Separately excited and (ii) Permanent magnet DC generator (iii) Self-excited

2.1.2.1 Separately excited DC generator

In separately excited dc generators, the field winding is electrically separated from the armature circuit. Due to its high cost from the additional power supply source, separately excited DC generators are not commonly used. They are used in laboratories for research work, for accurate speed control of DC motors with Ward-Leonard system and few other applications where self-excited DC generators are unsatisfactory.

2.1.2.2 Permanent Magnet DC (PMDC) Generator

Basic configuration of a permanent magnet DC generator is very similar to that of a normal DC generator. The working principle is that a force is produced when a current carrying conductor is placed in a magnetic field.

2.1.2.3 Self excited DC generator

In this type, the field coils are excited from the current produced by the armature coil. Initial emf generation is due to the residual magnetism in field poles. The generated emf causes a part of current to flow in the field coils, thus strengthening the field flux and thereby increasing emf generation. Self excited DC generators can further be divided into three types.

(a) Series wound – field winding in series with the armature winding so that field winding carries load current.

(b) Shunt wound – Field winding is connected in parallel with the armature winding and full voltage is applied across the field winding.

(c) Compound wound – In this type, one set of field winding is connected in series and the other is connected in parallel with the armature winding. Compound wound machines are further divided as

Short shunt – Field winding is connected in parallel with only the armature winding

Long shunt – Field winding is connected in parallel with the combination of series Field winding and armature winding.

3. Magneto hydrodynamic power generation

The word magneto hydrodynamics (MHD) is derived from magneto- meaning magnetic field, hydro-meaning water, and -dynamics meaning movement.

A magnetohydrodynamic generator (MHD generator) is a magnetohydrodynamic device that transforms thermal energy and kinetic energy into electricity. It differs from electric generators from its operation at high temperatures without moving parts. This is capable of heating boilers of steam power plant. It also improves its overall efficiency. This is generally applied along with electric generation from coal or natural gas.

A magneto hydrodynamic (MHD) generator directly extracts electric power from moving hot gases through a magnetic field is based upon Faraday’s law of electromagnetic induction, which states that energy is generated due to the movement of an electric conductor inside a magnetic field. The MHD generator does not have any rotating coils are magnets unlike conventional rotating electric machine.

MHD generators were originally developed because the output of a plasma MHD generator is a flame, well able to heat the boilers of a steam power plant. Significant progress has been made in the development of all critical components and sub component technologies. Coal burning MHD combined steam power plants premises significant economic and environmental advantages compared to other coal burning power generation technologies. It will not be long before the technological problem of MHD systems will be overcome and the MHD system would transform itself from non-conventional to conventional energy sources.

The conversion process in MHD was initially described by Michael Faraday in 1893. The first known attempt to develop an MHD generator was made at Westinghouse research laboratory (USA) around 1938. Dr. Hannes Alfven, a Swedish scientist received Nobel Prize in physics in 1970 for his work on MHD.

The first MHD-steam power plant U-25 was put into operation was of 75MW unit in USSR of which 25MW is generated by MHD means in early 1970’s & this work has been progressing fruitfully. A 500MWeThe solid fueled MHD power system (Sakhalin) was constructed by Russia. The first pilot plant was set up by BARC-India in Tiruchirapalli, Tamilnadu. A five year plan was signed in February 1975 which included 22 spheres of applied science and technology connected with the MHD energy generation.

3.1. Working principle of the MHD system

The working principle of MHD system is based on Lorentz force law. It states that movement of electric conductor cuts the magnetic induction lines and the charged particles in the conductor produce a force in a direction mutually perpendicular to the applied B field and to the velocity of the conductor.

The vector F is perpendicular to both v and B according to the right hand rule. As the negative charges tend to move in one direction, and the positive charges in the opposite direction due to the induced electric field or motional emf and this phenomenon provide the basis for converting mechanical energy into electrical energy as shown in fig.4.

The complete set of magneto hydrodynamic equations for a Newtonian, constant property fluid flow includes the Navier-Stokes equations of motion (i.e., momentum equation), the equation of mass continuity, Maxwell’s equations, and Ohm’s Law. In differential form they constitute the following system of equations.

The complete set of magneto hydrodynamic equations for a Newtonian, constant property fluid flow includes,The Navier Stokes equations of motion (i.e., momentum equation), The equation of mass continuity,Maxwell’s equations Ohm’s Law In differential form they constitute the following system of equations:

Where,

 = Fluid density

u = Fluid velocity

μm = Magnetic permeability

μf = Fluid dynamic viscosity

B = Magnetic field intensity

E= Electric field

σ = Electrical conductivity

J = MHD body force

3.2. Types of MHD systems

It is the generation of electric power utilizing the high temperature conducting plasma (stream of high temp working fluid) moving through an intense magnetic field. It converts the heat energy of fuel (thermal energy) directly into electrical energy. The fuel burns in the presence of compressed air in combustion chamber. During combustion seeding materials are added to increase the ionization & this ionized gas (plasma) is made to expand through a nozzle into the generator. Magnetic field, a current is generated & it can be extracted by placing electrodes in a suitable stream. The ionization can be produced by thermal or nuclear means. Materials such as Potassium carbonate or Cesium are often added in small amounts, typically about 1% of the total mass flow to increase the ionization and improve the conductivity, particularly combustion of gas plasma. 90% conductivity can be achieved with a fairly low degree of ionization of only about 1%.

3.2.1. Open Cycle

The working fluid used in open cycle for generating a high temperature and after generating electrical energy is discharged.

To achieve an adequate level of ionization at higher temperatures, potassium seeded combustion product is used as a working fluid and the working fluid is heated up to 2400-2600 0C to form the gas plasma is about 25000C. This gas plasma of high mass flow rate (ton/s) was sent to the MHD generator where DC Superconducting magnets are used to ionize the plasma entered. This induces the Direct Current on the electrodes

3.2.2. Closed Cycle

Temperature of closed cycle MHD plants is very less compared to open cycle MHD plants. It’s about 1400 0C. DC Superconducting magnets of four to six Tesla are used. Here exhaust gases are again recycled & the capacities of these plants are more than 200MW.

3.2.3. Comparison of MHD cycles

4. Advantages of MHD

• No moving parts-less maintenance-more reliable.
• Conversion efficiency is about 50%.
• Pollution free power generation.
• Ability to reach full power level as soon as started.
• Less overall generation cost.

Applications of MHD

• Power generation possibilities in space crafts.
• Hypersonic wind tunnel experiments.
• Defense applications.
• The plant is small compared to the fossil fuel plants.

5. Disadvantages

• MHD without superconducting magnets is less efficient.
• Difficulties may arise from the exposure of a metal surface to the intense heat of the generator and form the corrosion of metals and electrodes.
• Construction of generator is uneconomical due to its high cost.
• Construction of Heat resistant and non conducting ducts of generator & large superconducting magnets is difficult.
• Since the power produced is DC,Inverter (DC to AC) is required for grid interface.

4. Summary

Energy conversion using magnetic fields is a convenient and efficient method to generate electricity. The electric generator woks based on the electromagnetic principles and the alternators are used globally to cater the grid demands over decades. In other hand, the MHD based electricity generator can be an attractive non-conventional solution and the overall energy conversion efficiency of the MHD generator is better than the coal based power plants. The closed cycle requires a lesser fluid temperature compared to the open cycle and there is a second stage power generation using waste heat from the MHD generator. For the long time operation of the MHD generator the walls are required to be designed for extreme conditions. The super magnets and electrodes need to withstand the electrochemical stress and corrosion.