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SOLAR POWER SATELLITE AKSHAY KUMAR BAJPAI
BBDNIIT
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ACKNOWLEDGEMENT
I express my sincere thanks to Mr. Arun Kumar Singh (Asst. professor of
E.C.E) for extending his valuable guidance, support for literature, critical
reviews and above all the moral support he had provided to me.
I am also indebted to all the teaching and non- teaching staff of the
department of Electronics & Communication Engineering for their cooperation
and suggestions, which is the spirit behind this report. Last but not the least, I
wish to express my sincere thanks to all my friends for their goodwill and
constructive ideas.
-AKSHAY KUMAR BAJPAI
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ABSTRACT
The new millennium has introduced increased pressure for finding new
renewable energy sources. The exponential increase in population has led to
the global crisis such as global warming, environmental pollution and change
and rapid decrease of fossil reservoirs. Also the demand of electric power
increases at a much higher pace than other energy demands as the world is
industrialized and computerized. Under these circumstances, research has
been carried out to look into the possibility of building a power station in space
to transmit electricity to Earth by way of radio waves-the Solar Power
Satellites. Solar Power Satellites(SPS) converts solar energy in to micro
waves and sends that microwaves in to a beam to a receiving antenna on the
Earth for conversion to ordinary electricity. SPS is a clean, large-scale, stable
electric power source. Solar Power Satellites is known by a variety of othernames such as Satellite Power System, Space Power Station, Space Power
System, Solar Power Station, Space Solar Power Station etc. One of the key
technologies needed to enable the future feasibility of SPS is that of
Microwave Wireless Power Transmission.WPT is based on the energy
transfer capacity of microwave beam i.e. energy can be transmitted by a well
focused microwave beam. Advances in Phased array antennas and rectennas
have provided the building blocks for a realizable WPT system.
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CONTENTS
INTRODUCTION
WHY SPS
SPS THE BACKGROUND
TRANSMISSION
CHALLENGES
SPS A GENERAL IDEA
WIRELESS POWER TRANSMISSION
MICROWAVE POWER TRANSMISSION IN SPS
RECENTLY DEVELOPED MPT SYSTEMS
CONSTRUCTION OF SPS FROM NON TERRESTRIAL
MATERIALS: FEASIBILITY AND ECONOMICS
MICROWAVES ENVIRONMENTAL ISSUES
ADVANTAGES AND DISADVANTAGES CONCLUSION
REFERENCES
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INTRODUCTION
The new millennium has introduced increased pressure for finding new
renewable energy sources. The exponential increase in population has led to
the global crisis such as global warming, environmental pollution and change
and rapid decrease of fossil reservoirs. Also the demand of electric power
increases at a much higher pace than other energy demands as the world is
industrialized and computerized. Under these circumstances, research hasbeen carried out to look into the possibility of building a power station in space
to transmit electricity to Earth by way of radio waves-the Solar Power
Satellites. Solar Power Satellites(SPS) converts solar energy in to micro
waves and sends that microwaves in to a beam to a receiving antenna on the
Earth for conversion to ordinary electricity.SPS is a clean, large scale, stable
electric power source.
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Solar Power Satellites is known by a variety of other names such as Satellite
Power System, Space Power Station, Space Power System, Solar Power
Station, Space Solar Power Station etc. One of the key technologies needed
to enable the future feasibility of SPS is that of Microwave Wireless Power
Transmission.WPT is based on the energy transfer capacity of microwave
beam i.e; energy can be transmitted by a well focused microwave beam.
Advances in Phased array antennas and rectennas have provided the
building blocks for a realizable WPT system.
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WHY SPS?
Increasing global energy demand is likely to continue for many decades.
Renewable energy is a compelling approach both philosophically and in
engineering terms. However, many renewable energy sources are limited in
their ability to affordably provide the base load power required for global
industrial development and prosperity, because of inherent land and water
requirements. The burning of fossil fuels resulted in an abrupt decrease in
their .it also led to the green house effect and many other environmental
problems. Nuclear power seems to be an answer for global warming, but
concerns about terrorist attacks on Earth bound nuclear power plants have
intensified environmentalist opposition to nuclear power. Moreover, switching
on to the natural fission reactor, the sun, yields energy with no waste
products. Earth based solar panels receives only a part of the solar energy. It
will be affected by the day & night effect and other factors such as clouds. So
it is desirable to place the solar panel in the space itself, where, the solar
energy is collected and converted in to electricity which is then converted to a
highly directed microwave beam for transmission. This microwave beam,
which can be directed to any desired location on Earth surface, can be
collected and then converted back to electricity. This concept is more
advantageous than conventional methods. Also the microwave energy,
chosen for transmission, can pass unimpeded through clouds and
precipitations.
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SPS THE BACKGROUND
The concept of a large SPS that would be placed in geostationary orbit was
invented by Peter Glaser in 1968 [1].The SPS concept was examined
extensively during the late 1970s by the U.S Department of Energy (DOE)
and the National Aeronautics and Space Administration (NASA). The DOE-
NASA put forward the SPS Reference System Concept in 1979 [2]. The
central feature of this concept was the creation of a large scale power
infrastructure in space, consisting of about 60 SPS, delivering a total of about
300GW.But, as a result of the huge price tag, lack of evolutionary concept
and the subsiding energy crisis in 1980-1981, all U.S SPS efforts were
terminated with a view to re-asses the concept after about ten years. During
this time international interest in SPS emerged which led to WPT experiments
in Japan.
RECENT NASA EFFORTS
Fresh look Study
During 1995-96, NASA conducted a re-examination of the
technologies, system concepts of SPS systems [2],[3].The principal objective
of this Fresh Look Study was to determine whether a SPS and associated
systems could be defined. The Fresh Look Study concluded that the
prospects for power from space were more technically viable than they had
been earlier.
SSP Concept Definition Study
During 1998, NASA conducted the SSP Concept Definition Study
which was a focused one year effort that tested the results of the previous
Fresh Look Study. A principal product of the efforts was the definition of a
family of strategic R&T road maps for the possible development of SSPtechnologies.
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SSP Exploratory and Research Technology Program
In 2000, NASA conducted the SERT Program which further defined
new system concepts. The SERT Program comprised of three
complementary elements:
System studies and analysis:
Analysis of SSP systems and architecture concepts to address
the economic viability as well as environmental issue assessments.
SSP Research and technology:
Focused on the exploratory research to identify system concepts
and establish technical viability.
SPS technology demonstration:
Initial small scale demonstration of key SSP concepts and / or
components using related system / technologies.
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TRANSMISSION
Solar power from the satellite is sent to Earth using a microwave
transmitter. This transmission is transmitted to the relevant position via an
antenna. The transmission is transmitted through space and atmosphere and
received on earth by an antenna called the rectenna. Recent developments
suggest using laser by using recently developed solid state lasers allow
efficient transfer of power. A range of 10% to 20% efficiency within a few
years can be attained, but further experimentation still required taking into
consideration the possible hazards that it could cause to the eyes. In
comparison to laser transmission microwave transmission is more developed,
has high efficiency up to 85%, beams is far below the lethal levels of
concentration even for a prolonged exposure.
The microwave transmission designed has the power level well
below the international safety standard (Frequency 2.45 GHz microwave
beam). The electric current generated from the photovoltaic cells is passed
through a magnetron which converts the electric current to electromagnetic
waves. This electromagnetic wave is passed through a waveguide whichshapes the characteristics of the electromagnetic wave.
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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 aflow 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.
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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 isnt uniform. It has a taper forum 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.
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The efficiency of energy transmission is expressed by the
functional 2. To increase the field distribution on radiating aperture is made
as a tapered distribution. High value of 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 . 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 . The meaning of these two parameters and is
discrepant because to increase 2 it is necessary to have the field falling
down to edges, but to increase it is necessary to have a uniform field.
To increase the effectiveness of WPT system it is necessary to
increase the product 2, though the requirements for each of both multipliers
are opposite. This product is named a generalize criterion! It is possible to find
the way out of this contradiction if the antenna is discontinuous (discrete) one.
Let us produce the field distribution in the radiating discrete antenna falling to
its edges not by means of creation of non-uniform distribution of the field butwith the help of irregular situation of identical sub apertures, each of them
having the uniform field distribution. It is supposed that the number of these
apertures is sufficiently high in order to admit the approximation of the integral
optimum monotonous Gauss distribution by means of step function. The
places of sub aperture disposition can be found by the differentiation of this
step function.
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Discrete distribution of sub apertures presents non-equadistant antenna.array
consisting of the similar elements. Such optimization is optimal in
Chebyshevs sense since the maximum error tends to zero while the number
of sub apertures is tended to infinity. So the field in the place of observers
disposition would be similar to step and the monotonous signal source. The
falling to the edge field distribution is typical for the WPT problems. For the
discrete-step distributions that means the concentration of sub apertures in
the center and their gradual discharge on the edges. Thus all sub apertures
are similar and have the uniform distribution of the field with the equal
amplitude, which may reach the maximum admissible value.
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The dismemberment of continuous apertures and slight moving of them apart
in the space when all of apertures are equal and uniformly feed increases
their effectiveness (the generalized criterion is increased). The generalized
criterion determines the quality of the WPT Systems better than usual
criterion.
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The optimal distribution form may be reached for the large radiating
apertures where dismemberment at many parts is easily realized by
disposition of sub aperture clots in places, which correspond to high field
intensity (first of all it concerns the center of the radiator) and relieving sub
aperture density at edges of antenna. This construction allows to approach to
unite the value both of coefficients 2and . As a result the effectiveness of
the WPT system will be essentially increased.
For receiving these transmitted waves rectennas are set up at the
Earth.An antenna comprising a mesh of dipoles and diodes for absorbing
microwave energy from a transmitter and converting it into electric power.
Microwaves are received with about 85% efficiency and 95% of the beam will
fall on the rectenna but the rectenna is around 5km across (3.1 miles).
Currently there are two different design types being looked at- Wire mesh
reflector and Magic carpet. Wire mesh reflector type rectennas are built on a
rigid frame above the ground and are visually transparent so that it would not
interfere with plant life whereas in the magic carpet type material pegged tothe ground.
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CHALLENGES
The development and implementation of any new energy source present
major challenges. And it is acknowledged that bringing about the use of
Space Solar Power on the Earth may be particularly daunting because it is so
different. The major challenges are perceived to be:
The mismatch between the time horizon for the implementation of SSP
and that for the expansion of conventional energy resources.
The fact that space power is intrinsically global, requiring enterprise
models that give every player a suitable stake and adequate safeguards.
The potential for concerns over reliability, safety and environmental
implications.
The need to obtain publicly-allocated resources outside the normal
purview of the energy community
The prevailing mind set which tends to view the future energy
infrastructure as an extrapolation of the present one.
However great the challenges, it is important to enhance global energy
systems so they work for all the people of the Earth. It is asserted that a
prudent course would be to give serious attention to all plausible options and
prepare to implement several if needed.
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It is well understood that something as vast as the global energy system can
change only slowly. In fact, it takes from 50 to 75 years for one source to lose
dominance and be replaced by another. Even if it is recognized and agreed
that a shift to different sources is needed, penetration would be slow.
The time horizon for implementing Space Solar Power will be at least a couple
of decades. Current work being carried out in the US by the National
Aeronautics and Space Administration (NASA) and in Japan by the Ministry of
Economy, Trade and Industry (METI) indicate that demonstrations of space-
to-ground transmission of power could come in the current decade and initial
commercial power delivery in about 20 years. A significant contribution in
terms of global energy would clearly take substantially longer. The challenge
presented by this mismatch can be addressed in two ways:
1. First, governments will need to underwrite, to a major extent, the R&D
needed to bring the enabling technologies to maturity. Governments have
traditionally supported R&D efforts as a spur to new economic activity.
Examples can be found in the development of rail and air transport systems,
computers and, most recently, the internet.
2. Second, a near-term involvement by the users (the electric utilities and
their suppliers) should be promoted. It is very important for these prospective
users to keep abreast of progress as the technology matures.
The global scope of Space Solar Power will present another significant
challenge in terms of appropriate enterprise models that give every player a
suitable stake and adequate safeguards. International cooperation in the
energy area is commonplace and indeed the infrastructure for energy is highly
interdependent around the world. Energy acquisition, distribution, and
utilization tend to involve multiple countries and far-flung networks along
which various forms of energy flow.
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Similarly, international collaboration has been important in major space
ventures of which Space Solar Power would certainly be an example. Briefly,
there are several reasons for international collaboration. The most compelling
are:
The need for increased energy supplies is a global need
The impact on the environment of present energy practices is a matter of
worldwide concern
International coordination in energy provisioning is common today and the
interdependence will only grow in the future
The needed technology is widely distributed and no one country has all
the capability
The large scale of Space Solar Power will require international financing
International regulations control critical resources, specifically slots in
geosynchronous orbit and appropriate transmission frequencies
Recognition of Space Solar Power as a viable and safe approach to
energy will require an international consensus.
Space Solar Power is perceived as very different from all other power sources
because of its wireless delivery. A significant challenge will be to allay
concerns about the safety of this transmission mechanism.
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A substantial body of theoretical and experimental work exists and this
work indicates that, for the power density levels being considered for
importation of power from space, there are no troublesome effects to life
forms. Since radio frequency power is non-ionizing, the only likely effects are
thermal and these should be modest in view of the fact that the intensity of
the transmitted beam Space Solar Power is perceived as very different from
all other power sources because of its wireless delivery.
A significant challenge will be to allay concerns about the safety of this
transmission mechanism. A substantial body of theoretical and experimental
work exists and this work indicates that, for the power density levels being
considered for importation of power from space, there are no troublesome
effects to life forms. Since radio frequency power is non-ionizing, the only
likely effects are thermal and these should be modest in view of the fact that
the intensity of the transmitted beam.
Developing any substantial source of energy requires the dedication ofsignificant amounts of capital, land, technical skills, etc. The exploitation of
Space Solar Power will require all of these plus some that are unique. As
noted before, SSP systems will likely operate in geosynchronous orbit. This
orbit is at an altitude such that the platform appears to be stationary over a
specific point on the surface of the Earth. As a result, this particular orbit is
highly desirable for Earth-oriented activities, for example communications,
hence international control is exercised over the assignment of positions or
"slots" in this orbit.
The changes in dominant source over time were noted in an earlier
figure and we see a continuing change. Considering the relative role of the
various sources over just the last century, we have seen the prominence of
wood vanish and that of coal diminish greatly. At the same time, the
contributions of oil and gas rose from virtually nothing to dominance, and
nuclear became a significant contributor in a matter of only 25 years.
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Considering the changes washing over our world in almost all areas of
life and the economy, can we expect anything less dramatic in the energy
arena over the 21st century?
Today we the opportunity and the challenge to create a future that is
energy-rich and sustainable, but we must be open to a departure from past
and present practices and expect that the energy situation in 2100 will be very
different from that of today. The prudent response is a pro-active assessment
of all reasonable options and pursuit of those that appear most viable,
however futuristic they may seem at present.
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SPS A GENERAL IDEA
Solar Power Satellites would be located in the geosynchronous
orbit. The difference between existing satellites and SPS is that an SPS
would generate more power-much more power than it requires for its own
operation.
The solar energy collected by an SPS would be converted into
electricity, then into microwaves. The microwaves would be beamed to the
Earths surface, where they would be received and converted back into
electricity by a large array of devices known as rectifying antenna or
rectenna.(Rectification is the process by which alternating electrical current
,such as that induced by a microwave beam , is converted to direct current).
This direct current can then be converted to 50 or 60 Hz alternating current .
Each SPS would have been massive; measuring 10.5 km long and
5.3 km wide or with an average area of 56 sq.km.The surface of each
satellite would have been covered with 400 million solar cells. The
transmitting antenna on the satellite would have been about 1 km in
diameter and the receiving antenna on the Earths surface would have been
about 10 km in diameter .The SPS would weigh more than 50,000 tons.
The reason that the SPS must be so large has to do with the
physics of power beaming. The smaller the transmitter array, the larger the
angle of divergence of the transmitted beam. A highly divergent beam will
spread out over a large area, and may be too weak to activate the rectenna.In
order to obtain a sufficiently concentrated beam; a great deal of power must
be collected and fed into a large transmitter array.
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The day-night cycle ,cloud coverage , atmospheric attenuation etc.reduces
the amount of solar energy received on Earths surface.SPS being placed in
the space overcomes this. Another important feature of the SPS is its
continuous operation i.e,24 hours a day,365 days a year basis. Only form a
total of 22 in a year would the SPS would be eclipsed for a period of time to a
maximum of 72 min.If the SPS and the ground antenna are located at the
same longitude, the eclipse period will centre around midnight.
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The power would be beamed to the Earth in the form of microwaves
at a frequency of 2.45 GHz. Microwaves can pass unimpeded through clouds
and rain. Microwaves have other features such as larger band width , smaller
antenna size, sharp radiated beams and they propagate along straight lines.
Because of competing factors such as increasing atmospheric attenuation but
reducing size for the transmitting antenna and the other components at higher
frequency, microwave frequency in the range of 2-3 GHz are considered
optimal for the transmission of power from SPS to the ground rectenna site. A
microwave frequency of 2.45 GHz is considered particularly desirable
because of its present uses for ISM band and consequently probable lack of
interference with current radar and communication systems.
The amount of power available to the consumers from one SPS is 5
GW.the peak intensity of microwave beam would be 23 mW/cm.So far, no
non thermal health effects of low level microwave exposure have been
proved, although the issue remains controversial. SPS has all the advantage
of ground solar, plus an additional advantage; it generates power duringcloudy weather and at night. In other words SPS receiver operates just like a
solar array. Like a solar array, it receives power from space and converts it
into electricity. If the satellite position is selected such that the Earth and the
Sun are in the same location in the sky, when viewed from the satellite, same
dish could be used both as solar power collector and the microwave antenna.
This reduces the size and complexity of satellite.
However, the main barrier to the development of SPS is social, not
technological. The initial development cost for SPS is enormous and the
construction time required is very long. Possible risks for such a large project
are very large, pay-off is uncertain. Lower cost technology may be developed
during the time required to construct the system. So such a large program
requires a step by step path with immediate pay-off at each step and the
experience gained at each step refine and improve the risk in evolutionary
steps.
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WIRELESS POWER TRANSMISSION
Transmission or distribution of 50 or 60 Hz electrical energy from
the generation point to the consumer end without any physical wire has yet to
mature as a familiar and viable technology. However, the reported works on
terrestrial WPT have not revealed the design method and technical
information and also have not addressed the full-scale potential of WPT as
compared with the alternatives, such as a physical power distribution line.
However the main thrust of WPT has been on the concept of space-to-ground
(extraterrestrial) transmission of energy using microwave beam.
Fig.3 shows the block diagram of a conceptual WPT system
annexed to a grid.
Figure : conceptual model for a WPT system annexed to a grid.
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The 50 Hz ac power tapped from the grid lines is stepped down
to a suitable voltage level for rectification into dc. This is supplied to an
oscillator fed magnetron. Inside the magnetron electrons are emitted from a
central terminal called cathode. A positively charged anode surrounding the
cathode attracts the electrons. Instead of travelling in a straight line, the
electrons are forced to take a circular path by a high power permanent
magnet. As they pass by the resonating cavities of the magnetron, a
continuous pulsating magnetic field i.e., electromagnetic radiation in
microwave frequency range is generated. After the first round of cavity-to-
cavity trip by the electrons is completed the next one starts, and this process
continues as long as the magnetron remains energized. Fig.4 shows the
formation of a re-entrant electron beam in a typical six cavity magnetron. The
output of the rectifier decides the magnetron anode dc voltage. This in turn
controls the radiation power output. The frequency of the radiation is adjusted
by varying the inductance or capacitance of the resonating cavities.
Figure : Re-entrant electron beam in a six-cavity magnetron
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The microwave power output of the magnetron is channeled into an array of
parabolic reflector antennas for transmission to the receiving end antennas.
To compensate for the large loss in free space propagation and boost at the
receiving end the signal strength as well as the conversion efficiency, the
antennas are connected in arrays. Moreover, arrayed installation of antennas
will necessitate a compact size.
A series parallel assembly of schottky diodes, having a low
standing power rating but good RF characteristics is used at the receiving end
to rectify the received microwave power back into dc. Inverter is used to invert
the dc power into ac.
A simple radio control feedback system operating in FM band
provides an appropriate control signal to the magnetron for adjusting its
output level with fluctuation in the consumers demand at the receiving side.
The feedback system would switch of the supply to the oscillator and
magnetron at the sending end if there is a total loss of load.
The overall efficiency of the WPT system can be improved by :
Increasing directivity of the antenna array
Using dc to ac inverters with higher conversion efficiency
Using schottky diode with higher ratings.
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MICROWAVE POWER TRANSMISSION IN SPS
The microwave transmission system as envisioned by NASA and
DOE would have had three aspects :
1. The conversion of direct power from the photovoltaic cells, to microwave
power on the satellites on geosynchronous orbit above the Earth.
2. The formation and control of microwave beam aimed precisely at fixed
locations on the Earths surface.
3. The collection of the microwave energy and its conversion into electrical
energy at the earths surface.
The ability to accomplish the task of efficiently delivering electrical
power wirelessly is dependent upon the component efficiencies used in
transmitting and receiving apertures and the ability to focus the
electromagnetic beam onto the receiving rectenna.
Microwave WPT is achieved by an unmodulated, continuous wave
signal with a band width of 1Hz. Frequency of choice for microwave WPT hasbeen 2.45GHz due to factors such as low cost power components, location in
the ISM band, extremely low attenuation through the atmosphere [2]. The
next suggested band centered at 5.8GHz system reduces the transmitting
and receiving apertures. But this is not preferred due to increased attenuation
on higher frequency.
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The key microwave components in a WPT system are the transmitter, beam
control and the receiving antenna called rectenna .At the transmitting
antenna, microwave power tubes such as magnetrons and klystrons are used
as RF power sources. However, at frequencies below 10 GHz, high power
solid state devices can also be used. For beam safety and control retro
directive arrays are used. Rectenna is a component unique to WPT systems.
The following section describes each of these components in detail.
TRANSMITTER
The key requirement of a transmitter is its ability to convert dc
power to RF power efficiently and radiate the power to a controlled manner
with low loss. The transmitters efficiency drives the end-to-end efficiency as
well as thermal management system i.e., any heat generated from
inefficiencies in the dc-RF conversion, should be removed from the
transmitter as it reduces the life time of RF devices and control electronics
[2]. Passive inter modulation is another field which requires critical attention.
Filtering of noise and suppression of harmonics will be required to meet the
regulatory requirement.
The main components of a transmitter include dc-to-RF converter
and transmitting antenna. . The complexity of the transmitter depends on
the WPT application. For the large scale WPT application such as SPS,
phased array antennas are required to distribute the RF power sourcesacross the aperture and electronically control the power beam. Power
distribution at the transmitting antenna= (1-r), where r is the radius of
antenna [7].
There are mainly three dc-to-RF power converters: magnetrons, klystrons
and solid state amplifiers.
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Klystron
Fig.5 shows the schematic diagram of a klystron amplifier [15].
Figure 5 Klystron amplifier schematic diagrams.
Here a high velocity electron beam is formed, focused and send down a
glass tube to a collector electrode which is at high positive potential with
respect to the cathode. As the electron beam having constant velocity
approaches gap A, they are velocity modulated by the RF voltage existing
across this gap. Thus as the beam progress further down the drift tube,
bunching of electrons takes place. Eventually the current pass the catcher
gap in quite pronounce bunches and therefore varies cyclically with time.
This variation in current enables the klystron to have significant gain. Thus
the catcher cavity is excited into oscillations at its resonant frequency and a
large output is obtained.
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Fig.6 shows a klystron transmitter [2]. The tube body and solenoid operate at
300C and the collector operates at 500C. The overall efficiency is 83%. The
microwave power density at the transmitting array will be 1 kW/m for a typical
1 GW SPS with a transmitting antenna aperture of 1 km diameter. If we use
2.45 GHz for MPT, the number of antenna elements per square meter is on
the order of 100. Therefore the power allotted to the individual antenna
element is of the order of 10 W/element. So we must distribute the high power
to individual antenna through a power divider [1].
Figure 6 Klystron transmitter
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BEAM CONTROL
A key system and safety aspect of WPT in its ability to control the power
beam. Retro directive beam control systems have been the preferred method
of achieving accurate beam pointing.
As shown in fig.7 a coded pilot signal is emitted from the rectenna
towards the SPS transmitter to provide a phase reference for forming and
pointing the power beams [2]. To form the power beam and point it back
forwards the rectenna, the phase of the pilot signal is captured by the
receiver located at each sub array is compared to an onboard reference
frequency distributed equally throughout the array. If a phase difference
exists between the two signals, the received signal is phase conjugated and
fed back to earth dc-RF converted. In the absence of the pilot signal, the
transmitter will automatically dephase its power beam, and the peak power
density decreases by the ratio of the number of transmitter elements.
Figure 7 Retro directive beam control concept with an SPS.
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RECTENNA
Brown was the pioneer in developing the first 2.45GHz rectenna [2].
Rectenna is the microwave to dc converting device and is mainly composed
of a receiving antenna and a rectifying circuit. Fig .8 shows the schematic of
rectenna circuit [2]. It consists of a receiving antenna, an input low pass filter,
a rectifying circuit and an output smoothing filter. The input filter is needed to
suppress re radiation of high harmonics that are generated by the non linear
characteristics of rectifying circuit. Because it is a highly non linear circuit,
harmonic power levels must be suppressed. One method of suppressing
harmonics is by placing a frequency selective surface in front of the rectenna
circuit that passes the operating frequency and attenuates the harmonics.
Figure 8 Schematic of rectenna circuit.
For rectifying Schottky barrier diodes utilizing silicon and gallium arsenide are
employed. In rectenna arrays, the diode is the most critical component to
achieve higher efficiencies because it is the main source of loss. Diode
selection is dependent on the input power levels. The breakdown voltagelimits the power handling capacity and is directly related to series resistance
and junction capacitance through the intrinsic properties of diode junction and
material .For efficient rectification the diode cut off frequency should be
approximately ten times the operating frequency.
Diode cut off frequency is given by =1/ [2RsCj], where is the cut off
frequency, Rs is the diode series resistance, Cj is the zero-bias junction
capacitance.
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RECENTLY DEVELOPED MPT SYSTEM
The Kyoto University developed a system called Space Power
Radio Transmission System (SPORTS) [1]. The SPORTS is composed of
solar panels, a microwave transmitter subsystem, a near field scanner, a
microwave receiver. The solar panels provide 8.4 kW dc power to the
microwave transmitter subsystem composed of an active phased array. It is
developed to simulate the whole power conversion process for the SPS,
including solar cells, transmitting antennas and rectenna system.
Another MPT system recently developed by a team of Kyoto
University ,NASDA and industrial companies of Japan , is an integrated unit
called the Solar Power Radio Integrated Transmitter (SPRITZ),developed in
2000 [1]. This unit is composed of a solar cell panel, microwave generators,
transmitting array antennas and a receiving array in one package. This
integrated unit as shown in fig.9 could be a prototype of a large scale
experimental module in the orbit
Figure 9 SPRITZ (Solar Power Radio Integrated Transmitter 2000)
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CONSTRUCTION OF SPS FROM NON TERRESTRIAL
MATERIALS: FEASIBILITY AND ECONOMICS
SPS, as mentioned before is massive and because of their size they should
have been constructed in space [5]. Recent work also indicate that this
unconventional but scientifically well based approach should permit the
production of power satellite without the need for any rocket vehicle more
advanced than the existing ones. The plan envisioned sending small
segments of the satellites into space using the space shuttle. The projected
cost of a SPS could be considerably reduced if extraterrestrial resources are
employed in the construction [9].One often discussed road to lunar resource
utilization is to start with mining and refining of lunar oxygen, the most
abundant element in the Moons crust, for use as a component of rocket fuel
to support lunar base as well as exploration mission. The aluminium andsilicon can be refined to produce solar arrays [12].
A number of factors combine to make the concept of using non
conventional materials appear to be feasible. Among them are the shallow
gravity wells of the Moon and asteroids; the presence of an abundance of
glass, metals and oxygen in the Apollo lunar samples; the low cost transport
of those materials to a higher earth orbit by means of a solar-powered
electric motor; the availability of continuous solar energy for transport,
processing and living. Transportation requirement for SPS will be much more
needed for known for known commercial applications. One major new
development for transportation is required: the mass driver [12].The mass
driver is a long and narrow machine which converts electrical energy into
kinetic energy by accelerating 0.001 to 10 kg slugs to higher velocities.
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Each payload-carrying bucket contains superconducting coils and is
supported without physical contact by means of dynamic magnetic levitation.
As in the case of a linear synchronous motor-generator, buckets are
accelerated by a magnetic field, release their payload, decelerate with return
energy and pick up another pay load for acceleration. The power source can
be either solar or nuclear. The mass driver conversion efficiency from
electrical to kinetic energy is close to 100 percent.
The mass driver can be used as a launcher of lunar material into
free space or as a reaction engine in space, where payloads are transferred
from orbit to orbit in a spiral trajectory. The performance of the mass driver
could match that of the space shuttle main engines. But the mass driver has
the advantage that any material can be used as fuel and continuous solar
power in space is the common power source.An alternative to the use of lunar
resources for space manufacturing is the use of earth-approaching a steroidal
materials.
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MICROWAVES ENVIRONMENTAL ISSUES
The price of implementing a SPS includes the acceptance of
microwave beams as the link of that energy between space and earth.
Because of their large size, SPS would appear as a very bright star in the
relatively dark night sky. SPS in GEO would show more light than Venus at its
brightest. Thus, the SPS would be quite visible and might be objectionable.
SPS posses many environmental questions such as microwave
exposure, optical pollution that could hinder astronomers , the health and
safety of space workers in a heavy-radiation (ionizing) environment , the
potential disturbance of the ionosphere etc.The atmospheric studies indicate
that these problems are not significant , at least for the chosen microwave
frequency [13]. On the earth, each rectenna for a full-power SPS would be
about 10 km in diameter. This significant area possesses classical
environmental issues. These could be overcome by sitting rectenna in
environmentally insensitive locations, such as in the desert, over water etc.
The classic rectenna design would be transparent in sunlight,
permitting growth and maintenance of vegetation under the rectenna.
However, the issues related to microwaves continue to be the most pressing
environmental issues. On comparing with the use of radar, microwave ovens ,
police radars, cellular phones and wireless base stations, laser pointers etc.
public exposures from SPS would be similar or even less.
Based on well developed antenna theory, the environmental levels of
microwave power beam drop down to 0.1W/cm [12]. Even though human
exposures to the 25 mW/cmwill, in general, be avoided, studies shows that
people can tolerate such exposures for a period of at least 45 min.
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So concern about human exposure can be dismissed forthrightly
[4]. Specific research over the years has been directed towards effects on
birds, in particular. Modern reviews of this research show that only some birds
may experience some thermal stress at high ambient temperatures. Of
course, at low ambient temperatures the warming might be welcomed by birds
and may present a nuisance attraction [13].
Serious discussions and education are required before most of
mankind accepts this technology with global dimensions. Microwaves,
however is not a pollutant but, more aptly, a man made extension of the
naturally generated electromagnetic spectrum that provides heat and light for
our sentence.
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ADVANTAGES AND DISADVANTAGES
The idea collecting solar energy in space and returning it to earth
using microwave beam has many attractions.
1. The full solar irradiation would be available at all times expect when
the sun is eclipsed by the earth. Thus about five times energy could
be collected, compared with the best terrestrial sites.
2. The power could be directed to any point on the earths surface.
3. The zero gravity and high vacuum condition in space would allow
much lighter, low maintenance structures and collectors.
4. The power density would be uninterrupted by darkness, clouds, or
precipitation, which are the problems encountered with earth based
solar arrays.
5. The realization of the SPS concept holds great promises for solving
energy crisis
6. No moving parts.
7. No fuel required.
8. No waste product.
The concept of generating electricity from solar energy in the
space itself has its inherent disadvantages also. Some of the major
disadvantages are:
1. The main drawback of solar energy transfer from orbit is the storage of
electricity during off peak demand hours.
2. The frequency of beamed radiation is planned to be at 2.45 GHz and this
frequency is used by communication satellites also.
3. The entire structure is massive.
4. High cost and require much time for construction.
5. Radiation hazards associated with the system.
6. Risks involved with malfunction.
7. High power microwave source and high gain antenna can be used to
deliver an intense burst of energy to a target and thus used as a weapon.
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CONCLUSION
The SPS will be a central attraction of space and energy
technology in coming decades. However, large scale retro directive power
transmission has not yet been proven and needs further development.
Another important area of technological development will be the reduction of
the size and weight of individual elements in the space section of SPS. Large-scale transportation and robotics for the construction of large-scale structures
in space include the other major fields of technologies requiring further
developments.
Technical hurdles will be removed in the coming one or two
decades. Finally, we look forward to universal acceptance of the premise the
electromagnetic energy is a tool to improve the quality of life for mankind. It is
not a pollutant but more aptly, a man made extension of the naturally
generated electromagnetic spectrum that provides heat and light for our
sustenance. From this view point, the SPS is merely a down frequency
converter from the visible spectrum to microwaves.
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REFERENCES
1. www.WiKipedia.com
2. Kennedy Electronics Communication Systems, Tata McGraw
Hill.
3. International Encyclopaedia of Energy, Vol.4, pp.771.
4. David M. Pozar, Microwave Engineering, Wiley
http://www.wikipedia.com/http://www.wikipedia.com/
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