Any way to use earth's magnetic field to "harvest" energy?

Question

Any way to use earth's magnetic field to "harvest" energy?

Answer 1

In principle it would be possible to use the dynamo effect and the earth's magnetic field to induce a usable electric current. However, the earth's magnetic field is not particularly intense and so utilising it to induce a current is not practical. Please allow me to expand upon this theme?

Wikipedia, comments, 'The strength of the field at the Earth's surface ranges from less than 30 microteslas (0.3 gauss) in an area including most of South America and South Africa to over 60 microteslas (0.6 gauss) around the magnetic poles in northern Canada and south of Australia, and in part of Siberia.

The field is similar to that of a bar magnet, but this similarity is superficial. The magnetic field of a bar magnet, or any other type of permanent magnet, is created by the coordinated spins of electrons and nuclei within the atoms. The Earth's core, however, is hotter than 1043 K, the Curie point temperature at which the orientations of spins within iron become randomized. Such randomization causes the substance to lose its magnetic field. Therefore the Earth's magnetic field is caused not by magnetized iron deposits, but mostly by electric currents in the liquid outer core.

Convection of molten iron, within the outer liquid core, along with a Coriolis effect caused by the overall planetary rotation that tends to organize these "electric currents" in rolls aligned along the north-south polar axis. When conducting fluid flows across an existing magnetic field, electric currents are induced, which in turn creates another magnetic field. When this magnetic field reinforces the original magnetic field, a dynamo is created which sustains itself. This is called the "Dynamo Theory" and it explains how the earth's magnetic field is sustained.

Another feature that distinguishes the Earth magnetically from a bar magnet is its magnetosphere. At large distances from the planet, this dominates the surface magnetic field. Electric currents induced in the ionosphere also generate magnetic fields. Such a field is always generated near where the atmosphere is closest to the Sun, causing daily alterations which can deflect surface magnetic fields by as much as one degree.'

Using the data, I will briefly explore an estimate of a simple dynamo's capability to generate an electromotive force (emf for short) from the strongest regions of the earth's magnetic field.

Thus the flux B of the earth’s magnetic field strength H close to one of the poles, is: -

B = μ0.H

or

B = 4π x 10^-7 x 60 x 10^-6

Hence: -

B = 7.536 x 10^-11 weber

Thus, a simple coil of N turns, area A, a rotational frequency f, and making an instantaneous angle of θ to the earth’s field lines - would induce an emf E in the wire, of: -

E = -2πfNABcosθ

Given that the flux of the earth’s magnetic field is at a peak value of B = 7.536 x 10^-11 weber, the induced emf would be a very small figure indeed for any practical form of dynamo generator. Furthermore, the earth’s field fluctuates continuously and slowly drifts towards magnetic reversals every 700, 000 years or so.

Thus, to summarise – tapping power from the earth’s magnetic field is just not practical from an engineering standpoint. Since, more energy would have to be put into driving the dynamo than would be tapped out (see above partial calculation).