Generating Electricity By Earth Magnetic Field

Submitted By Group-5

Nahian, Ahnaf Tahmid 12-20178-1Ananta, Mahmud Hossain 12-20415-1Mozumder, Turza Dhiman 12-21132-1

Course Instructor- RETHWAN FAIZ

Magnetic BasicsMagnets always have two poles: a north pole and a south pole.North and South are always attracted.Two similar poles will repel one another.

Earth as a Magnet

• The Earth is like a giant bar magnet—it has a north magnetic pole and a south magnetic pole.

• These are different than the geographic north and south poles.

• The magnetic poles move a little (10 km) each year.

In 1600, William Gilbert published De MagnEarth Magnetic field is also known as Geomagnetic ete, which showed that the Earth itself is like a giant magnet, rather than the magnetism arising from an extraterrestrial source as supposed by others since 1939, the geomagnetic field has been believed to originate by convective motions in the Earth’s fluid, electrically-conducting core. The idea is that the convective fluid interacts with the Coriolis forces produced by planetary rotation and acts like a dynamo which is a magnetic amplifier []

Earth Magnetism

Why is Earth Magnetic?

• Earth’s core is iron, a magnetic material• Earth’s outer core is liquid iron. • When this liquid iron circulates around the core, a magnetic field is developed.• This is known as the Dynamo Theory

What Benefit is a Magnetic Field?• The Earth’s magnetic field protects us from the sun’s charged particles.• The magnetic field acts like a force field—without it, our atmosphere

would be ripped off.• And we can produce electricity by magnetic field

Nasa

Nasa can take electricity from Earth’s magnetic field. The technology is called electrodynamic tethers(EDT). This energy is electricity created by a long conductor moving in orbit through the planets geomagnetic field. It is a proven fact that free electricity can be made from this technology.

Electrodynamic tether

Electrodynamic tethers (EDTs) are long conducting wires, such as one deployed from a tether satellite, which can operate on electromagnetic principles as generators, by converting their kinetic energy to electrical energy, or as motors, converting electrical energy to kinetic energy. Electric potential is generated across a conductive tether by its motion through a planet's magnetic field.

Fig: Medium close-up view, captured with a 70mm camera, shows tethered satellite system deployment

Electrodynamic tether fundamentals

An electromotive force (EMF) is generated across a tether element as it moves relative to a magnetic field. The force is given by Faraday's Law of Induction:

Without loss of generality, it is assumed the tether system is in Earth orbit and it moves relative to Earth's magnetic field. Similarly, if current flows in the tether element, a force can be generated in accordance with the Lorentz force equation:

In self-powered mode (deorbit mode), this EMF can be used by the tether system to drive the current through the tether and other electrical loads (e.g. resistors, batteries), emit electrons at the emitting end, or collect electrons at the opposite.

In boost mode, on-board power supplies must overcome this motional EMF to drive current in the opposite direction, thus creating a force in the opposite direction, as seen in below figure, and boosting the system.

Fig:Illustration of the EDT concept

EDT concept

the NASA Propulsive Small Expendable Deployer System (ProSEDS) mission as seen in above figure. At 300 km altitude, the Earth's magnetic field, in the north-south direction

Here, the ProSEDS de-boost tether system is configured to enable electron collection to the positively biased higher altitude section of the bare tether, and returned to the ionosphere at the lower altitude end. This flow of electrons through the length of the tether in the presence of the Earth's magnetic field creates a force that produces a drag thrust that helps de-orbit the system.

EDT Concept

Fig:Illustration of the EDT concept

The top of the diagram, point A, represents the electron collection end. Point C, is the electron emission end. Similarly, and represent the potential difference from their respective tether ends to the plasma. Finally, point B is the point at which the potential of the tether is equal to the plasma. The location of point B will vary depending on the equilibrium state of the tether, which is determined by the solution of Kirchhoff's current law (KVL)

and Kirchhoff's voltage law (KCL)

Tethers as generatorsA space object, i.e. a satellite in Earth orbit, or any other space object either natural or man made, is physically connected to the tether system. The tether system comprises a deployer from which a conductive tether having a bare segment extends upward from space object. The positively biased anode end of tether collects electrons from the ionosphere as space object moves in direction across the Earth's magnetic field. These electrons flow through the conductive structure of the tether to the power system interface, where it supplies power to an associated load, not shown. The electrons then flow to the negatively biased cathode where electrons are ejected into the space plasma, thus completing the electric circuit.

NASA has conducted several experiments with Plasma Motor Generator (PMG) tethers in space. An early experiment used a 500-meter conducting tether. In 1996, NASA conducted an experiment with a 20,000-meter conducting tether. When the tether was fully deployed during this test, the orbiting tether generated a potential of 3,500 volts.

This conducting single-line tether was severed after five hours of deployment. It is believed that the failure was caused by an electric arc generated by the conductive tether's movement through the Earth's magnetic field.

When a tether is moved at a velocity (v) at right angles to the Earth's magnetic field (B), an electric field is observed in the tether's frame of reference. This can be stated as:E = v * B = vBThe direction of the electric field (E) is at right angles to both the tether's velocity (v) and magnetic field (B). If the tether is a conductor, then the electric field leads to the displacement of charges along the tether. Note that the velocity used in this equation is the orbital velocity of the tether. The rate of rotation of the Earth, or of its core, is not relevant.

Voltage and current

Voltage across conductor

With a long conducting wire of length L, an electric field E is generated in the wire. It produces a voltage V between the opposite ends of the wire. This can be expressed as:

where the angle τ is between the length vector (L) of the tether and the electric field vector (E), assumed to be in the vertical direction at right angles to the velocity vector (v) in plane and the magnetic field vector (B) is out of the plane.

Tether current

The amount of current (I) flowing through a tether depends on various factors. One of these is the circuit's total resistance (R). The circuit's resistance consist of three components:

1. the effective resistance of the plasma,2. the resistance of the tether, and3. A control variable resistor.

Advantages

1.The Operational advantages of electrodynamic tethers of moderate length are becoming evident from studies of collision avoidance

2.High efficiency and good adaptability to varying plasma conditions .

3.Substantially reduce the weight of the spacecraft.

4. A cost effective method of reboosting spacecraft, such as the International Space Station(ISS)

References

1. NASA, Tethers In Space Handbook, edited by M.L. Cosmo and E.C. Lorenzini, Third Edition December 1997

2. Johnson, L., Estes, R.D., Lorenzini, E.C., "Propulsive Small Expendable Deployer System Experiment," Journal of Spacecraft and Rockets, Vol. 37, No. 2, 2000, pp. 173–176

3. Tether power generator for earth orbiting satellites. Thomas G. Roberts et al.

4. Lieberman, M.A., and Lichtenberg, A.J., "Principles of Plasma Discharges and Materials Processing," Wiley-Interscience, Hoboken, NJ, 2005, pp. 757.

5. Fuhrhop, K.R.P., “ Theory and Experimental Evaluation of Electrodynamic Tether Systems and Related Technologies,”University of Michigan PhD Dissertation, 2007, pp. 1-307.

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