Abstract :
The vision of achieving
WPT on a global scale was proposed over 100 years ago when Nikola Tesla first
started experiments with WPT, culminating with the construction of a tower for
WPT on Long Island, New York, in the early 1900s. Tesla's objective was to
develop the technology for transmitting electricity to anywhere in the world
without wires.
A major
problem facing Planet Earth is provision of an adequate supply of clean energy.
It has been that we face “...three simultaneous challenges -- population
growth, resource consumption, and environmental degradation -- all converging
particularly in the matter of sustainable energy supply.” It is widely agreed
that our current energy practices will not provide for all the world's peoples
in an adequate way and still leave our Earth with a livable environment. Hence,
a major task for the new century will be to develop sustainable and
environmentally friendly sources of energy.
Two types of WPT:
Ground based power
transmission
Space based power
transmission
But
Space-based power transmission is preferred over Ground-based power transmission.
Ground is
(obviously) cheaper per noontime watt, but:
• Space
gets full power 24 hours a day
3X or
more Watt-hours per day per peak watt
No
storage required for nighttime power
Space
gets full power 7 days a week – no cloudy days
Space
gets full power 52 weeks a year
No long
winter nights, no storms, no cloudy seasons
• Space
delivers power where it’s needed
Best
ground solar sites (deserts) are rarely near users
Space
takes up less, well, space
Rectennas
are 1/3 to 1/10 the area of ground arrays
Rectennas can share land with farming or
other uses
Effectiveness
of Wireless Power Transmission (WPT) depends on many parameters. Only a part of
WPT system is discussed below, which includes radiating and receiving antennas
and the environment between them. The wave beam is expanded proportionately to
the propagation distance and a flow power density is increased inversely
proportional to the square of this distance. However the WPT has some
peculiarities, which will be mentioned here.
WPT systems require transmitting
almost whole power that is radiated by the transmitting side. So, the useful
result is the power quantity at the receiving antenna, but not the value of
field amplitude as it is usually required. Efficiency of WPT systems is the
ratio of energy flow, which is intercepted by receiving antenna to the whole
radiating energy.
Field
distribution on the receiving antenna usually is uniform because its size is
small comparatively to the width of the beam. For WPT systems this distribution
isn’t uniform. It has a taper form and it depends on the field distribution on
the transmitting antenna.
For
increasing of the energy concentration on the receiving antenna the phase
distribution on the radiating antenna has usually a spherical form with the
center in the point on crossing of the receiving plate and the radiating axis.
Radiating antenna of the WPT systems usually has a taper distribution of the
field. This distribution allows to increase the efficiency and to decrease the
field out of the receiving antenna.
The
efficiency of energy transmission is expressed by the functional Λ2. To
increase Λ the field distribution on radiating aperture is made as a tapered
distribution. High value of Λ is supposed to be in the majority of known
projects of the WPT systems.
However,
the effectiveness of the WPT system is defined not only by the value of Λ. It
is also determined by the rectangularity of the field distribution on the
radiating aperture, the rectangular distribution factor in the theory of
antennas is usually called the surface utilization factor χ. The meaning of
these two parameters Λ and χ is discrepant because to increase Λ2 it is
necessary to have the field falling down to edges, but to increase χ it is
necessary to have a uniform field.
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