Beamed Power Transmission From Solar Power Satelites

Introduction

The desire of the modern man for more and more amenities and sophistication led to the unscrupulous exploitation of natural treasure. Though nature has provided abundant source of resources. It is not unlimited. Hence the exhaustion of the natural resources is eminent. The only exception to this is sunlight.

Scientist who had understood the naked truth had thought of exploiting the solar energy and started experimenting in this direction even from 1970. But the progress was very slow. Much headway is yet to be made in this direction. However as the impotents source of non-conventional energy and due to the limited source of conventional energy emphasis has given for the better utilization of solar energy.

But the application of solar cell, photovoltaic cell etc, we are able to concert only a small percentage of solar energy into electrical energy. But by using beamed power transmission from solar power satellite we can envisage a higher percentage of conversion. By beamed power transmission, we can extend the present system of two dimensional transmission network to three dimensional, if does not have any environmental problem as well.

SOLAR POWER SATELLITE

The solar power satellite concept would place solar power plants in an above the earth where they could convert sunlight to electricity and beam the ground based receiving station. The satellite would be placed in sow called geostationary or earth synchronous orbit. 24 hour orbit that is thus synchronous with earth rotation. so the satellite placed their will stay stationary overhead from earths receiving antenna

The solar power satellite will consist of a large number of solar cells mounted on a frame of steel reinforced lunarcrete. the solar cell produces electricity from sun with no moving part. the only moving part of the satellite is the transmitter antenna, which slowly tracks the ground based rectenna while solar array keeps facing the sun. each transmitter antenna is connected to solar array by two sotary joints with slip rings.

Because solar panels generate low voltage dc, super conducting transmission line is used for transmission to microwave beamer instead of conventional copper conductors. operating super conducting transmission line at low voltage reduces the chance of voltage breakdown.

The vision is to generate a large Solar Power Satellite (SPS) system for space applications. In contrast to classical designs of solar power stations, the solar energy collected by the SPS system will not be beamed to the Earth's surface, but used to provide for the required electrical and/or propulsive power for vehicles in Earth orbit (LEO, MEO and GEO) and/or deep space vehicles. Transmission of the energy to these space vehicles is foreseen thought either beamed microwave or laser power at a power level that can substantially rise above todays power levels up to e.g. 20-50 kW per space vehicle (e.g. for a telecommunications satellite in GEO). This power is received by a relatively low mass microwave or laser collection and rectifying system on board of the satellite. 

The use of beamed energy from a solar power satellite offers space vehicles the advantage of omitting (part of) the heavy on-board power generation system and or maximizing propulsive performance (in terms of propellant consumption), thereby allowing for a reduced launch mass or an increased payload mass. It also allows us more flexibility in the power take-in thus providing us with a capability for e.g. short high power applications without the need of relying on costly batteries or to reduce power without the need to dissipate excess power. Other potential advantages for the receiving vehicle are

• Omitting the solar wings frees up two faces of the satellite for e.g. the installation of antenna's and/or other equipment; 
• Reduction of thermal problems; 
• Less complex structure. 

The physical separation of the power generation onto a separate platform allows for an independent optimization of the power generation, without the need to consider the spacecraft. For example, this could open up an opportunity to operate the power satellite at very low temperatures ensuring high solar cell efficiency or to use other (more efficient) ways of harnessing solar power, like dynamic heat engines (o.a. Stirling engines). 

• Market demands: Size, power, number of users (satellites), power need per satellite, etc.; 
• Economic viability/business plan: System life, life cycle, date in operation, laucher selection, development cost, etc.; 
• Efficient power conversion both at SPS and receiving satellite (e.g. dynamic or static conversion); 
• High power transmission by laser and/or microwave: Size/mass of receiver and/or transmitter, cooling requirements, etc.; 
• Light weight structures (flexible or rigid); 
• Pointing (accuracy & stability); 
• Orbit (LEO, GEO or other); 
• Impact onto space plasma environment; 
• Impact of space debris; 
• Safety (ISS, other spacecraft, launchers, etc). 

    As the needs of our planet's ever-increasing population grow to unprecedented highs, the search for a new and more efficient way of powering our industries, businesses, and homes is becoming a very pressing priority. The techniques we use today to generate power are simply detrimental in the long run; burning fossil fuels or splitting atoms generate a lot of power, but also damage the planet with pollution, and alternatives like wind and hydro power can be limited both geographically and seasonally.  As replacement technologies are pondered, one stands out. Instead of manipulating existing elements of Earth, this inventive proposal plans to collect solar energy from space and transmit it back to the surface using solar power satellites (SPS). Directly harnessing the energy of the sun allows mankind to preserve the well-being and resources of Earth while producing enough energy to satisfy the needs of the growing human race hundreds of times over.   First proposed in 1968 by Dr. Peter Glaser of NASA, these satellites would use large solar panels in space to collect the sun's light energy. Once collected, the energy would go through two conversion phases. First, it would be sent through onboard photovoltaic cells to be converted into electrical energy. Afterwards, the electrical energy would be channeled into large microwave generators where, using the principles behind the wireless power transmission of energy (WPT), it would be converted into controllable microwaves and beamed down to earth. On the surface, large antennas would pick up the beam and reconvert the microwaves into electrical power, which could then be plugged into the local power grid to use.   

At first, solar power satellites were nothing more than hopeful dreams of scientists, but recent advances have propelled them into the reaches of reality. Satellite technology has developed to the point where telecommunication companies use satellites for everything from cell phones to television transmission. The WPT system has also gone through comparable progress, especially since the development of adequate antenna technology. Though energy conversion efficiencies from electrical energy to microwaves and then back to electricity are currently around 54% , WPT transmissions to a helicopter, a small aircraft, and a satellite from a launched rocket have all been successful, demonstrating that WPT can indeed be used to power or receive power from flying bodies. Computing technology is also advanced enough to control satellites and allow them to change position without interrupting a running microwave beam. 

Solar power satellites are attractive ventures for many reasons. Foremost, by using the sun, humans could acquire all the necessary energy without causing pollution, threatening species, or generally damaging the Earth. Second, the sun's power is limitless and perpetual.   By carefully placing the satellites on certain points over the equator, they would be exposed to sunlight 24 hours a day, ensuring a constant reception and flow of energy. The power output would then only depend on how many satellites were in space; a whole network could potentially generate exponential amounts of usable energy. A third reason solar power is attractive is because the constant energy flow and bulky, expensive storage facilities would no longer be needed, minimizing costs and increasing availability. Fourth, since there are no weather or atmospheric disturbances in space, the solar panels would be exposed to more sunlight and have a greater efficiency than panels placed anywhere on the Earth's surface . 

Unlike x-rays or ultra-violet radiation, microwaves are non ionizing and are one million times too weak to cause harm (SUNSAT Energy).   The only perceivable effect is heating, but since the power density of the beam near the receivers on Earth is about 20 milliwatts per square centimeter, one-fourth of natural sunlight, the heat generated is so slight that a person walking through would feel nothing. Manufacturing costs are not a problem either, since materials would be negligible Photovoltaic cells, satellite technology and microwave beams have all been explored and researched sufficiently to operate solar power satellites. 

Solar Power in Space

Although solar energy is abundant in the inner solar system, collecting enough of it to provide electricity for a large population of humans is a non-trivial matter.  The most comprehensive studies of large-scale solar power generation in space were conducted over two decades ago. Legend has it that the concept of Solar Power Satellites was first envisioned by Dr. Peter Glaser as he sat in an early-1970's gas line. At the time there was an Arab oil 

embargo, an energy crisis, and global concern about increasing use and decreasing availability of energy. People sometimes waited hours in lines at the few stations that had not run out of gas, sitting in cars that got 15 miles per gallon and had a range around 200 miles. There was plenty of time to think. 

The idea of using solar cells to generate electricity in space was nothing new. Communications satellites had been doing that for years. Indeed, the most distinguishing characteristics of most Earth-orbiting satellites, even today, are their arrangements of solar cells. A common configuration is a cylindrical shape with the entire exterior covered in purplish-blue solar cells. Non-cylindrical satellites have large "wings" covered with solar panels. The crewed laboratories Skylab, Mir, and International Space Station all had or have large solar cell arrays that generate power for the satellites' use. 

The difference between existing satellites and Solar Power Satellites (SPS) is that an SPS would generate more power--much more power--than it requires for its own operations. Studies in the 1970's by Glaser, NASA, and major corporations produced a myriad of design concepts. Their single most distinguishing characteristic is that they were huge-with up to 60 square miles of surfaces covered with solar cells.A common goal of designers was to put enough solar cells on a structure in space to generate 10 gigawatts, approximately equal to the output of ten nuclear power plants. The idea was not entirely far-fetched; advantages over Earth-based solar power facilities were that the GEO locations typically proposed for SPS were almost always in sunlight and only rarely eclipsed, and the amount of energy available to a unit area of solar panels is seven to ten times greater than for the same area of solar panels on Earth, because sunlight in space is not filtered by atmosphere. 

Once having generated electricity in space, however, it is necessary to get the power to where it is needed on Earth's surface. The solution selected in the 1970's, and still valid today, was to convert power into microwave energy that could be beamed to Earth's surface. Microwaves pass through atmosphere, clouds, and precipitation with no loss of energy. Experiments on Earth with transmission and reception of energy converted to microwaves proved the concept. The antennas designed to transmit the huge amounts of SPS power were, however, huge (although dwarfed by the sizes of the solar panel arrays). Typical designs were a half mile or kilometer across; examples can be seen near the ends of the design shown in the figure. 

Antenna sizes were probably dictated not so much by constraints of materials or technology as by concern for safety. A highly concentrated power beam would be a tough sell for people concerned about airplanes being zapped out of the sky or entire migratory flocks of birds being cooked en route. Large antennas in geosynchronous orbit, combined with the physics of an expanding microwave beam, resulted in receiving antenna (rectenna) designs six to eight miles across (10 to 13 kilometers), with maximum intensity at the center of the microwave beam less than five times greater than standards for kitchen emissions from a microwave oven. These facilities would convert the microwaves back to energy, and contribute their power to the energy grid in the same manner as a hydroelectric dam, coal-fired plant, nuclear reactor, ground-based solar facility, geothermal plant, or field of wind generators.

The benign radiation environments under these widely-dispersed beams would enable air traffic, radio, TV, and birds to continue their normal activities with no impediments. Even so, safety concerns (and the importance of not wasting power by beaming it away from the rectenna) dictated that the microwave beam was kept centered on the target rectenna by a "guide beam" reflected back to the SPS. Because the rectenna structures would be ten to twelve feet off the ground and designed to capture all of the microwave 

energy in the beam, the land under the rectenna would be available for agriculture. It was speculated that birds would avoid the rectennas during Summer months and congregate in them during Winter months, because they would experience a slight warming sensation. Dr. Peter Glaser himself offered a standing bet that he would provide fine wine and salad to the person who would eat the first fowl to venture into the microwave beam; his point was that the bird would be very much alive and unwilling to be eaten until recently. 

Occasional threats to energy supplies, projections that coal and oil reserves will eventually be depleted, and concerns that burning hydrocarbons contributes to environmental damage are providing inspiration for new interest in Solar Power Satellites. New technologies have, however, changed some of the parameters involved in constructing a viable SPS. Solar cells can now convert sunlight to power much more efficiently than when the first designs were envisioned, resulting in new designs about half the size of the originals for the same amount of power generation. Even so, any viable SPS of the future will still be huge. 

The implication for a space settlement is that the need for power--assuming it is provided from a solar source--will be a significant factor in the settlement's configuration and a major feature of its design. The need to orient solar panels toward the sun or a rectenna toward its power source will determine how the entire settlement is positioned in space. The Space Settlement Design Competition organizers anticipate that future non-industrial human communities will require about 10 megawatts per 5000 people. Industrial communities will require more. The physics of microwave transmission have not changed. Whether a space settlement is designed with its own solar panels or with a rectenna for receiving power generated elsewhere in space, the equipment for providing power will be a major part of what is seen when the settlement is viewed by approaching spacecraft. 

PRINCIPLES OF MICROWAVE POWER TRANSMISSION SYSTEM

BEAMED MICROWAVE POWER TRANSMISSION SYSTEM

BEAM FORMING ANTENNA

FREE SPACE TRANSMISSION

DC TO MICROWAVE CONVERSION

RECEPTION CONVERSION TO DC

 70 – 90 % 70 – 97 % 5 – 95 % 85 – 92 %

MAXIMUM POSSIBLE DC TO DC EFFICIENCY ---- 76%

EXPERIMENTAL DC TO DC EFFICIENCY ----- 54%

The basic parts of a micro wave power transmission system: 1) DC to microwave conversion (2) a beam forming antenna (3) free space transmission and (4) reception and reconvertion to DC.

MICROWAVE POWER GENERATION

The DC power must be converted to microwave power at the transmitting end of the system by using microwave oven magnetion. The heat of microwave oven is the high voltage system. The nucleus of high voltage system is the magnetron tube. The magnetron is diode type electron tube, which uses the interaction of magnetic and electric field in the complex cavity to produce oscillation of very high peak power. It employs radial electric field, axial magnetic field, anode structure and a cylindrical cathode.

The cylindrical cathode is surrounded by an anode with cavities and thus a radial electric field will exist. The magnetic field due to two permanent magnets which are added above end below the tube structure is axial. The upper magnet is North Pole and lower magnet is South Pole. The electron moving through the space tends to build up a magnetic field around itself. The magnetic field on right side is weakened because the self-induced magnetic field has the effect of subtracting from the permanent magnetic field. So the electron trajectory bends in that direction resulting in a circular motion of travel to anode. This process begins with a low voltage being applied to the cathode, which causes it to heat up. The temperature rise causes the emission of more electrons. This cloud of electrons would be repelled away from the negatively charged cathode. The distance and velocity of their travel would increase with the intensity of applied voltage. Momentum is provided by negative 4000 V DC. This is produced by means of voltage doubler circuit. The electrons blast off from cathode like tiny rocket.

As the electrons move towards their objective, they encounter the powerful magnetic. The effect of permanent magnet tends to deflect the electrons away from the anode. Due to the combined affect of electric and magnetic field on the electron trajectory they evive to a path at almost right angle to their previous direction resulting in an expanding circular orbit around the cathode, which eventually reaches the anode. The whirling cloud of electrons forms a rotating pattern. Due to the interaction of this rotating space chare wheel with the configuration of the surface of anode, an alternating current of very high frequency is produced in the resonant cavities of the anode. The output is taken from one of these cavities through waveguide. The low cost and readily available magnetron is used in ground.

The same principle would be used but a special magnetron would be developed for space use. Because of the pulsed operation of these magnetrons they generate much spurious noise. A solar power satellite operating with 10 GW of radiated power would radiate a total power of one microwatt in a 400 Hz channel width.

TRANSMITTING ANTENNA

The transmitting antennas are large active electronically steerable phased array. These arrays are composed of radiation module that consists of a high gain phased locked magnetron and directional amplifier that supplies microwave power to slotted waveguide array.

The antenna must have the ability to match the transmission line (source impedance) and load (atmosphere 377W). If impedance match is correct, the energy being transferred will be radiated into the atmosphere.

An antenna is used to convert high frequency current into electromagnetic waves. It must have the ability to transfer energy alternatively from electrostatic to electromagnetic.

A co axial cable is used to connect the microwave source to a waveguide adaptor. The adaptor is connected to a ferrite circulator, which protects the microwave source from reflected power. 

A phase shifter is used to produce a difference in shift between the radiation modules. Even though total difference in shift between radiation modules may be great, only a low power level phase shifter of 360o is needed in each module. Phase reference at each module is adjusted to some integral multiple of 360o relative to the source of reference.

The slotted waveguide antenna consists of 8 waveguide section with 8 slots on each section. These 64 slots radiate power uniformly through space to antenna in ground.

FREE SPACE TRANSMISSION

There is no economic burden for the transmission through space. The transmitting and receiving apertures are needed for transmission. The size and expense of this aperture has a direct relationship with the wave length that is being used, the distance over which energy is being sent and the desired efficiency of transmission. The parameter T in defined as

At is the transmitter aperture area.

Ar is the receiver aperture area.

D is the separator distance between two apertures.

  is the wavelength that is being used. 

We assume that transmitter and receiver areas are equal. Under these conditions

At = Ar = T D  

Since aperture area varies with wavelength the advantages of going to higher frequency are diminished if the aperture areas are approximately equal as they to be for total overall economy.

When the radiation area may be limited and a particular intensity of the incident microwave illumination is desired; we use the expression

Pa = A Pt /  

Pa  is the power density at the center of receiving location.

Pt is the radiated power from transmitting antenna

Power density distributions across the transmitting and receiving antenna aperture for various values of T are shown as:

R is the radius of transmitting or collecting antenna.

r is the radial distance from the center.

To achieve a desired valve of at the receiver site, while constrained by a transmitted power level, the transmitting aperture area varies as the square of the wavelength of radiation. When area available for transmitting is limited the short wavelength are attractive.

When microwaves are used to transmit power in the vacuum of space there is no resistive loss, no limitation to power handling probabilities. The receiving and transmitting aperture areas sediate any waste heat resulting from their inefficiencies directly to space. The radiation of waste heat in space is directly proportional to the radiating area and the fourth power of temperature at which heat is radiated. In case of the transmitter aperture in space the relation between the microwave power per unit area to the generator efficiency and radiating temperature is

Pr = (n/(n-1) (5.67 K T4 X 10-8)

Pr = radiated microwave power density 

T = temperature in degree Kelvin.

K = emissivity of radiating surface.

n = transmission efficiency

For the selection of best frequency for power transmission, the items that would have to be considered are 

• The size of aperture.
• The depending of overall system efficiency upon frequency.
• The heat radiation problem in space.
• Whether the transmission is all in space or in path through earth atmosphere. 
• Existing state of the art of available components.
• The impact of the use of the selected frequency upon other users of electromagnetic spectrum.

Transmission efficiency through the atmosphere as related to frequency and condition of the atmosphere are shown as.

 RECTENNA

The rectenna is a unique device that was conceived and d eveloped for beamed microwave power transmission. The functions of rectennal are power collecting harmonic filtering and rectification into DC power. Rectenna rectifies received microwaves into DC current. It spread out over the receiving aperture area and combines the function of an antenna and a rectifier. In it’s simplest from rectenna consist of a collection of rectenna elements, each with a half wave dipole that feeds a low pass filter circuit terminated in a rectifying diode. The output of the diode in the local region feeds into a common DC bus.

The efficiency h can be expressed as product of three partial efficiencies: the efficiency of the microwave beam energy intercepted by the rectenna hi, the efficiency of rectenna rectification hi, and the DC power collection circuit efficiency hc.

    h =  hi hr hc

PHF = HF energy extracted by rectenna from the beam.

PE = is the power reedited by transmitting antenna multiplied by efficiency of transmission line.

PRM = Sum of output DC power obtained from all RRE under the condition of each RRE working into the matched load.

PR =DC power in the rectenna load.

The incident power flux density P  over the rectenna with a circular aperture obeys the Gauss law

 pr = p max  

Where

r is the variable radius.

p max is the energy flux density in the center of the rectenna

PROPERTIES OF MICROWAVE POWER TRANSMISSION SYSTEM.

As a mean of transferring energy from one point to another. Beamed microwave power transmission has these features.

• No mass either in the form of wire or ferrying vehicle.
• Energy can be transferred at velocity of light.
• The direction of energy transfer can be rapidly changed.
• No energy is lost in its transfer through vacuum of space and little is lost in the earth atmosphere at the longer microwave length.
• The mass of power converters at the system terminal can be low because of operation at microwave frequency.
• Every transfer between points is independent of difference in gravitational potential between these points.

APPLICATIONS

Several applications of beamed power are

• High Altitude Long Endurance (hale) aircraft.
• Platforms for cellular voice and data services.
• Electric powered inter orbital vehicles.
• Satellite station keeping and maneuvering.
• Industries in orbital and on the moon.

The demand for wireless power is likely to unfold according to the following 

• Ground to special satellite.
• Ground to inter orbital vehicle.
• Ground to HALE platform.
• Power utility satellite to inter orbital vehicle.
• Power utility satellite to industry in space.
• Power utility satellite to earth based consumers.

It is hoped that this technology, which at present has an efficiency of 56%, will emerge as an effective substitute for the existing technology in the near future.

CONCLUSION

The Beamed Power Transmission will surely shower the mankind with an inexhaustible energy source and at the same time will lead to development of in-space industries. As per the proposed design SPS and Rectenna array are quiet efficient and would posses no threat to ecological imbalance. There is no significant advance in this technology till now in spite of the major research works. Once the technology is developed Power Satellites will become the premier energy source for Earth. The sooner, the better.