Seminar by Ak111 - Copy

7/28/2019 Seminar by Ak111 - Copy

1/42


2/42

SOLAR POWER SATELLITE AKSHAY KUMAR BAJPAI

BBDNIIT

2

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


3/42


BBDNIIT

3

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.


4/42


BBDNIIT

4

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


5/42


BBDNIIT

5

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.


6/42


BBDNIIT

6

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.


7/42


BBDNIIT

7

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.


8/42


BBDNIIT

8

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.


9/42


BBDNIIT

9

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.


10/42


BBDNIIT

10

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.


11/42


BBDNIIT

11

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.


12/42


BBDNIIT

12

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.


13/42


BBDNIIT

13

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.


14/42


BBDNIIT

14

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.


15/42


BBDNIIT

15

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.


16/42


BBDNIIT

16

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.


17/42


BBDNIIT

17

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.


18/42


BBDNIIT

18

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.


19/42


BBDNIIT

19

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.


20/42


BBDNIIT

20

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.


21/42


BBDNIIT

21

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.


22/42


BBDNIIT

22

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.


23/42


BBDNIIT

23

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.


24/42


BBDNIIT

24

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.


25/42


BBDNIIT

25


26/42


BBDNIIT

26

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.


27/42


BBDNIIT

27

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


28/42


BBDNIIT

28

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.


29/42


BBDNIIT

29

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.


30/42


BBDNIIT

30

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.


31/42


BBDNIIT

31

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.


32/42


BBDNIIT

32

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


33/42


BBDNIIT

33

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.


34/42


BBDNIIT

34

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.


35/42


BBDNIIT

35

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)


36/42


BBDNIIT

36

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.


37/42


BBDNIIT

37

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.


38/42


BBDNIIT

38

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.


39/42


BBDNIIT

39

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.


40/42


BBDNIIT

40

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.


41/42


BBDNIIT

41

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.


42/42


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/

Documents

Seminar by Ak111 - Copy