The document discusses wireless power transmission (WPT), a technology that facilitates the efficient transfer of electrical power without wires, employing methods like microwaves, millimeter waves, or lasers. It highlights historical experiments and applications, particularly in remote areas and space solar power satellites, demonstrating the challenges, costs, and potential for WPT in providing power to hard-to-reach locations. Safety concerns regarding microwave exposure and competition with traditional power systems are also addressed, concluding that WPT remains viable under certain conditions despite higher costs.

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Briefing Paper
Peter Vaessen
KEMA
September 2009
Wireless Power Transmission

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1 Introduction
The definition of Wireless Power Transmission (WPT) is: efficient
transmission of electric power from one point to another trough vacuum or
an atmosphere without the use of wire or any other substance. This can
be used for applications where either an instantaneous amount or a
continuous delivery of energy is needed, but where conventional wires are
unaffordable, inconvenient, expensive, hazardous, unwanted or
impossible. The power can be transmitted using microwaves, millimeter
waves or lasers. WPT is a technology that can transport power to
locations, which are otherwise not possible or impractical to reach.
Maxwell's theory of electromagnetism, published in 1865 mentions
electromagnetic waves moving at the speed of light, and the conclusion
that light itself was just such a wave. In 1886 Hertz performed a
successful experiment with pulsed wireless energy transfer. He produced
an apparatus that produced and detected microwaves in the UHF region.
Also Tesla did experiments in the field of pulsed wireless energy transfer
in 1899. Tesla's Magnifying Transmitter, an early type of Tesla Coil that
measured 16 meters in diameter, could transmit tens of thousands of
watts without wires.
Tesla supposedly managed to light 200 lamps, without wires, from 40
kilometers away. No documentation from Tesla's own records has been
published validating that this actually happened. In 1897, he filed his first
patents dealing with Wardenclyffe tower. This aerial tower was ment to be
a pilot plant for his “World Wireless System” to broadcast energy around
the globe. [1] The core facility was never fully operational and was not
completed due to economic problems [4]

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Figure 1 tesla's colorado springs lab
The Raytheon Company did the first successful WPT experiment in 1963.
In this experiment energy was transmitted with a DC-to-DC efficiency of
13%. This company also demonstrated a microwave-powered helicopter
in 1964 [2]. The Jet propulsion lab of NASA carried out an experiment and
demonstrated the transfer of 30 kW over a distance of 1 mile in 1975.
They used an antenna array erected at the Goldstone facility. This test
demonstrated the possibilities of wireless power outside the laboratory.
Rockwell International and David Sarnoff Laboratory operated in 1991 a
microwave powered rover at 5.86 GHz. Three kilowatts of power was
transmitted and 500 watts was received [2].
This paper provides an overview of the technologies, possibilities and
uses of wireless power transmission. An overall view of the past present
and possible transmission systems is presented. In this paper also the
different systems; economical, ecological and social aspects are
discussed. The paper focuses on wireless power transmission systems
with microwaves in the power range of about 100 W to 100 kW. WPT
systems using optical technologies (laser) are not discussed.

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2 State of the art of WPT technology
In order to transport electricity is has to be transformed into a suitable
energy form. For wireless transmission, this has to be a form that can
travel trough air. Microwave frequencies hold this ability. The microwave
spectrum is defined as electromagnetic energy ranging from
approximately 1 GHz to 1000 GHz in frequency, but older usage includes
lower frequencies. Most common applications are within the 1 to 40 GHz
range.
A complete microwave transmission system consists of three essential
parts:
• Electrical power to microwave power conversion
• Absorption antenna that captures the waves
• (Re)conversion to electrical power
Figure 2 Microwave transmitter and rectenna

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The components include a microwave source, a transmitting antenna and
a receiving antenna. The microwave source consists of an electron tubes
or solid-state devices with electronics to control power output. The slotted
waveguide antenna, parabolic dish and microstrip patch are the most
popular types. Due to high efficiency (>95%) and high power handling
capacity, the slotted waveguide antenna seems to be the best option for
power transmission.
The combination of receiving and converting unit is called rectenna. The
rectenna is a rectifying antenna that is used to directly convert microwave
energy into DC electricity. It is an antenna includes a mesh of dipoles and
diodes for absorbing microwave energy from a transmitter and converting
it into electric power. Its elements are usually arranged in a multi element
phased array with a mesh pattern reflector element to make it directional
[6].
One of the disadvantages is that microwaves have long wavelengths that
exhibit a moderate amount of diffraction over long distances. The Rayleigh
criterion dictates that any beam will spread (microwave or laser), become
weaker, and diffuse over distance. The larger the transmitter antenna or
laser aperture, the tighter the beam and the less it will spread as a
function of distance (and vice versa). Therefore, the system requires large
transmitters and receivers. The used power density of the microwave
beam is normally in de order of 100 W/m2
. This is relative low compared to
the power density of solar radiation on earth (1000 W/m2
) and chosen this
way for safety reasons.

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3 Applications
WTP for space solar
The largest application for microwave power transmission is space solar
power satellites (SPS). In this application, solar power is captured in
space and converted into electricity.
The electricity is converted into microwaves and transmitted to the earth.
The microwave power will be captured with antennas and converted into
electricity. NASA is still investigating the possibilities of SPS. One of the
problems is the high investment cost due to the space transport. The
current rates on the Space Shuttle run between $7,000 and $11,000 per
kilogram of transported material. Recently the idea of Space Solar Power
caught again the public attention e.g. by the Obama transition team [17]
and The Economist [18].
Figure 3 Space Solar Power satellite

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Power transfer, bridging applications
Using a powerful focused beam in the microwave or laser range long
distances can be covered. There are two methods of wireless power
transmission for bridging application. First is the direct method, from
transmitting array to rectenna. A line of sight is needed and is therefore
limited to short (< 40 km) distances. Above 40 kilometers, huge structures
are needed to compensate for the curvature of the earth. [16].
The second method is via a relay reflector between the transmitter and
rectenna. This reflector needs to be at an altitude that is visible for both
transmitter and rectenna. This method is not discussed further. Next three
bridging applications of WPT are discussed.
Alaska '21
WPT can be an option for power supply to rural areas. In 1993 was a
project presented about wireless power supply in Alaska. Because of
limited infrastructure, hundreds of small rural communities in Alaska must
provide their own electricity. These systems can be expensive not
standard or just not available. At the moment, the small communities
produce their own power with mainly diesel engines. These produce noise
and pollution. Also the needed fuel has to be transported over long
distances. All this results in an electricity price in excess of $40/kWh [14].
Cable connections trough water is no option because of ice. With the help
of WPT, the needed power production of the communities can be
combined. This can reduce noise, pollution and transportation of fuel.
WPT may be capable of transmitting electrical energy to Alaska’s remote
villages. To investigate these possibilities, a pilot project was conducted
named "Alaska'21”. The system used for the pilot project consisted of a
2.45 GHz phased array design. The distances that should be bridged are
between 1 and 15 miles. The status of the project is unknown

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Figure 4 Alaska’21
Grand Bassin - La Reunion
The Grand Bassin project [15] lead by the local university will supply
electricity to a remote isolated mountain village. The project supported by
CNES, the French space research centre, aims to beam electrical power
to the tourist village of Grand Bassin on the island of La Reunion. Grand
Bassin lies at the bottom of one of the deep canyons on the island. WPT
will be used to preserve the beautiful scenery of the valley. The plan was
to build a microwave link down the side of the canyon, operating at 2.45
GHz over a distance of 700 meters and delivering 10kW with an overall
efficiency of 57%. The system will work in combination with PV panels,
charging batteries of the system when the power is not being used
directly. A prototype was build and presented at the Wireless Power
Transmission conference in 2001, which was held on the island. The cost
of the project was estimated to be "1 million dollars for 10 kW” and the
project stopped.

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Figure 5 Grand Bassin – La Reunion
Hawaii demonstration
In May 2008, a long-range WPT demonstration was realized on one of the
islands of Hawaii. The demonstration was organized by Managed Energy
Technologies of the U.S. and involved the wireless transmission of energy
over a distance of 148 kilometers. Although the amount of power
transmitted, 20 watts, is barely enough to power a small compact
fluorescent light bulb, and most of it was lost in transmission, the system
was limited by the budget not the physics. If they had been able to afford
more solar panels, more phased array transmitters and a better receivers
(the one they had could only receive in the horizontal direction), they could
do much better - possibly up to 64% efficiency. [10] The costs of the
whole project where less then1 million dollar.
Figure 6 Hawaii demonstration

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4 Safety issues
Bio-effects
A general public perception that microwaves are harmful has been a
major obstacle for the acceptance of power transmission with microwaves.
A major concern is that the long-term exposure to low levels of
microwaves might be unsafe and even could cause cancer.
Since 1950, there have been thousands of papers published about
microwave bio-effects. The scientific research indicates that heating of
humans exposed to the radiation is the only known effect. There are also
many claims of low-level non-thermal effects, but most of these are
difficult to replicate or show unsatisfying uncertainties. Large robust
effects only occur well above exposure limits existing anywhere in the
world [5].
The corresponding exposure limits listed in IEEE standards at 2.45 or 5.8
GHz are 81.6 W/m2
and 100 W/m2
averaged over 6 minutes, and 16.3 or
38.7 W/m2
averaged over 30 minutes [11]. This low compared to average
solar radiation of 1000 W/m2
.
A clearly relevant bio-effect is the effect of microwave radiation on birds,
the so-called "fried bird effect". Research is done on such effect at 2.45
GHz. The outcome showed slight thermal effects that probably are
welcome in the winter and to be avoided in the summer [5]. Larger birds
tend to experience more heat stress then small birds [11].
The overall conclusion of bioeffects research is that microwave exposures
are generally harmless except for the case of penetrating exposure to
intense fields far above existing exposure limits [5]. Further discussions

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about the maximum microwave power density are necessary. A range of
environmental issues and safety-related factors should continue to receive
consideration because of public concern about radio wave and micro
wave exposure.
Compatibility with other radio services and applications
It is assumed that WPT systems working with microwaves use frequency
bands around 2.45 GHz or 5.8 GHz. These bands are already allocated in
the ITU-R radio regulation to a number of radio services. They are also
designated for industrial science and medical (ISM applications). The ISM
band is, as presently defined, for local use only. The 2.45 GHz is further
more used for radio LAN and microwave ovens. The 5.8 GHz is also used
heavily for various applications like Radiolocation service and DSRC
(Dedicated Short-Range Communications). More investigation is needed
to get an image of the possible influencing between the systems [11].
Costs
For the very short range (1-10 meters), preliminary demonstrations of
WTP at low power levels (less then 1 kW) were in general quite costly,
however, the cost estimates are coming down for larger power levels and
longer ranges. The slightly over 1million dollar Goldstone test in 1975
delivered 34 kW at 1.6 km. The costs of the test in Hawaii in 2008, where
just under 1 million dollar and delivered 20 Watts over 148 kilometers.
Tens of km WPT systems are in the range of several $1,000,000/ MW-km,
whereas similar range wired systems are of order $10,000/MW-km, at
least two orders of magnitude less.
Involved organizations
NASA and the National Space Development Agency of Japan are still

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working on Space Power Satellites. France is at the forefront of European
interest in wireless power transmission. University La Reunion and CNES
(French space research centre) worked on the demo project in the village
Grand Bassin. The Raytheon company (military and Technology
Company) is also working on the development and application of WPT
systems.
5 Conclusions
It is clear that WPT systems in the range of 100 W to 100 kW to cannot
compete with traditional systems just looking at the costs. At places where
economic competition is not the prime consideration, it can be an option.
Microwave wireless power transmission can supply power to places that
are difficult to reach. Especially small communities in rural areas could be
supplied with power using WPT.
More successful demonstration projects can help the further development
and utilization of this technology. Further investigation concerning
compatibility and safety is needed to clarify these issues.
Space Solar Power has gained public interest during the last year
because of global warming and the energy independency goals of US and
EU.
References
[1] Colorado springs notes
[2] Status of international experimentation in wireless power
transmission, Gregg E. Maryniak Sunset energy counsel, Solar
energy Vol. 56 1996
[3] International cooperation for the acquisition of space based energy,
R. Bryan ERB, Solar Energy Vol. 56 1996

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[4] Cheney, Margaret (1999), Tesla Master of Lightning
[5] Health and safety issues for microwave power transmission, John M.
Osepchuk, Solar energy Vol. 56, 1996
[6] Research Activities and future trends of microwave wireless power
transmission, Djuradj Bubimir and Aleksandar Marincic, International
symposium Nikola Tesla, 2006
[7] The results of NASA "Fresh look" at the feasibility of Space Solar
Power, John C. Mankins, 1997
[8] Solar power? Ok but why capture it in space? R. Bryan Erb, Fourth
international symposium on space means for power utilities, 1997
[9] Comparative analysis of wireless systems as alternative to high
voltage power lines for global terrestrial power transmission, Andrey
P. Smakhtin, Valentin V. Rybakov, IEEE, 1996
[10] http://www.nss.org/settlement/ssp/index.htm
[11] White paper on Solar Power Satellite Systems, URSI, September
2006
[12] Advanced receiver/converter experiments for laser wireless power
transmission, Joe T. Howell, 5th
Wireless power transmission
conference , pp 1-8, Granada Spain 2004
[13] Laser Power Beaming Market Fact Sheet, Tom Nugent,
Lasermotive, july 2008
[14] Alaska '21: A terrestrial point-to-point wireless power transport
system
[15] Journal de l'Ile de La Réunion - Article of 09 February 2002
[16] Comparison of intercontinental wireless and wired power
transmission, Richard M. Dickinson, Jet Propulsion Laboratory, John
C Mankins, NASA, 1999
[17] http://otrans.3cdn.net/38b615154ce6479749_p9m6bn37b.pdf
[18] Space Solar Power - Let the sun shine in, The Economist print
edition, December 4th
2008