Chapter 10 ̶ Electromagnetic
Energy Harvesting
10.1
Introduction
With increasing use of
electronic gadgets one needs portable and wireless power sources. One of the
noticeable properties of electronic gadgets and sensors is that, they need very
small amount of electric power for their operation. This small power can be accessed
through energy harvesting by such sources, which are easily available or
already well installed/established for other uses. In this context energy may
be harvested from the electromagnetic energy radiating from AC power lines. AC
power lines are available everywhere, on the street, outside and inside the
building.
10.2 Physics and
Mathematical model of Electromagnetic Energy
Harvesting
According to basic laws
of physics, i.e., Ampere’s law, the magnetic field generated from a group of
closely bundled wires is dependent on the net current flowing through them. Due
to presence of AC power lines magnetic field is present everywhere. Magnetic
field in homes varies from 0.01 to 10 Gauss near appliances and typically
exceeds 100 Gauss in industries with heavy electrical machinery [1]. Although
the live and neutral wires carrying current in opposite directions are usually
placed close together therefore the magnetic fields produced by them cancel each
other. Still there exists magnetic field due to separation distance between
live and neutral wires or imbalances in ground loop. The cancellation of
magnetic field is almost negligible if the separation between the wires is more
than a few inches. . A typical office space/industrial building has a dense
network of power line cabling, and some of those wires would carry currents in
the orders of 5-10 amperes. The ubiquity of power lines and the magnitude of
current running through them in any human occupied environment make energy
harvesting from the stray electromagnetic fields. This magnetic field can be
converted to electrical energy source using specific device.
The magnetic field at a point P at a
distance r from an infinitely long conductor carrying an alternating current
with peak amplitude of Io and frequency w is:
B =μI0sin(wt)/2pr............................................................(1)
where B is the magnetic flux density and
μ is the magnetic permeability given by μr ´
μ0 (μr is relative permeability). The magnitude of the
magnetic flux acting on a coil with N turns, cross sectional area A placed with
its plane perpendicular to the magnetic field is given by:
f = NBA
The induced voltage on the coil due to
the rate of change of the magnetic flux acting on it will be:
V =df/dt = NAμI0wcos(wt)/2pr.......................................(2)
The above equation shows that the net
voltage induced on the coil placed in a magnetic field increases proportionally
with frequency, number of turns, and area. It decreases proportionally with the
distance. It is interesting to note that the induced voltage can be increased
with a coil with high relative permeability.
Experimental
setup: Experimental setup consists of two parallel
conductors carrying the live and return current. An inductor placed in between
the two conductors will produce voltage. The voltage measured across the
inductors give estimation of the maximum power available from the
magnetic field. In a typical experiment following results have been obtained
[1]:
Separation between conductors-15 inches
Value of current flowing through
conductors-8.4 Amperes
Inductance value-15 Henry
Voltage across inductor=176 mV
10.3
Linear Generators
Linear generators are
the devices which convert mechanical energy into electrical energy
(alternators) and electrical energy into mechanical energy (motors). During
energy harvesting, often the source of energy produces its impact in the form
of mechanical energy which is converted into electrical energy.
10.4 Applications of
Electromagnetic Energy Harvesting
i. Wireless
sensor networks
ii. Portable
power source
iii. Power
source for electronic gadgets
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