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This paper is all about how to install solar power stations in space and collecting solar energy with an efficiency of 95% (as proven). And then by using either microwaves or LASER, sending that energy to the power grids on earth and converting it into electricity.

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  1. 1. SPACE BASED SOLAR POWER STATIONS AN ENERGY SOLUTION FOR TOMORROW Kaustubh S. Ilmulwar Computer Science & Engineering Shri Guru Gobind Singhji Institute of Engineering & Technology Nanded, India (M.S.) Abstract—the general principles and special components of space based solar power stations and various projects that are under development is discussed. The beamed system is defined as starting with a dc power source at the transmitting end, converting it to a microwave beam for transmission through space, and ending with the dc power output on the earth which is discussed. While electricity cannot power all our vehicles and homes today, current hybrids will soon evolve into plug-in hybrids which will, in part, use electrical energy from the solar- grids is also discussed. How SBSP deserves a place alongside ground-based solar collectors, nuclear power plants and wind turbines as potential solutions to energy dependence and global greenhouse gas induced warming is discussed. The efforts required for best transmission of power from SBSP’s to the Earth’s surface with minimal environmental impact is also discussed. I. INTRODUCTION The concept of space solar power (SPS) first emerged in the late 1960’s, invented by visionary Peter Glaser, a mechanical engineer, and then studied in some detail by the U.S. Department of Energy, and NASA in the mid-to-late 1970’s. However, at that time neither the technology nor the markets were ready for this transformational new energy option. Today, that has all changed. At an altitude of 22,240 miles above Earth, a great platform orbits, using vast, mirrored wings to collect a continuous torrent of sunlight always available in space. With few moving parts, the platform redirects and focuses this solar energy onto concentrating photovoltaic arrays- converting it into electrical power. In turn, the power is transmitted wirelessly- and with minimal losses- to highly- efficient receivers the size of airports on the ground. In an era when new energy options are urgently needed, SPS is an inexhaustible solution- and the technologies now exist to make it a reality. It is a seamless, endless transfer. The platform constantly gathers more than 5,000 megawatts (MW) of sunlight and delivers more than 2,000 megawatts (MW) of clean, near-zero carbon electrical power to customers as needed anywhere within an area the size of a continent. It can be routed directly into the electrical grid as base-load power – and divided across a half dozen or more receivers to meet local peak power needs. It can be used as well to power the annual production of hundreds of millions of gallons of carbon-neutral synthetic fuels. However, longer the path travelled, the more sunlight is absorbed or scattered by the air so that less of it reaches the surface. Altogether, these factors reduce the average energy produced by a conventional ground-based solar array by as much as a factor of 75 to 80 percent. And ground solar arrays may be subjected to hours, days or even weeks of cloud cover- periods when the array produces no energy at all. By comparison, the sun shines continuously in space. And in space, sunlight carries about 35 percent more energy than sunlight attenuated by the air before it reaches the Earth’s surface. No weather, no night time, no seasonal changes; space is an obvious place to collect energy for use on Earth. A SPS system consists of four subsystems: 1. Collection 2. Conversion 3. Transmission 4. Reception Current plans call for collection subsystems that are either photovoltaic or solar dynamic. Similarly, planned conversion, transmission, and reception subsystems for SPS tend to employ either microwaves or lasers. In both the cases the major trade- off to consider is power output vs. mass. Recent technology developments in SPS systems have focused on improving these subsystems by changing the method of incorporating microwaves and lasers into one unit, such as the combination of a transmitter and a solar cell. The advantages of integrated systems for space systems applications include: reductions in mass, size, and thermal losses; an increase in efficiency; and most importantly, a significant decrease in the size of solar power satellites. Also, each of the above subsystems is discussed in detail in the following sections: II. COLLECTION AND SOLAR TO ELECTRIC CONVERSION In this section technologies for power collection and power conversion have been reviewed. In particular photo-voltaic and solar dynamic systems have been examined in order to study the differences in design and how this affects the efficiency of the system, mass, cost and other factors which size the system. This has been divided into present technology and future technology so that trends can be identified and help us to
  2. 2. understand where technology will be in 10-15 years. Detailed designs of power collection and conversion systems are explained as follows: A. PHOTOVOLTAICS The most important element in photovoltaic systems is the solar cell. Nowadays, there are many kinds of solar cells, but crystal silicon (c-Si) is the most popular for space use. Gallium Arsenide cells (GaAs) were developed and have been qualified for space use in the middle of 1980’s. They have a higher efficiency than silicon solar cells. But the cost of GaAs is much higher than that of c-Si, so careful trade-offs between the two types of cell are needed in designing a solar array. Many kinds of solar cells are under development, for example amorphous silicon (a-Si), Indium phosphide (InP), copper indium diselenide (CuInSe2: CIS), cadmium Telluride (CdTe) etc. Moreover various research of multi-junction types have been performed in order to get more efficient solar cells, for example AlGaAs/Si, AlGaAs/CIS, GaInP/GaAs etc. Every solar cell can be characterized by its efficiency, cost, temperature characteristics, mechanical characteristics, degradation caused by radiation, etc. When designing a solar power satellite, we must investigate various parameters of solar cells based on the requirements of solar array wings such as power generation, operation temperature, mission life, space, spacecraft orbit, cost etc. The characteristics and the performances of the main types of solar cells and solar arrays are described first. Then the problems of large scale solar array construction and operation in space will be discussed. 1. C-Si & a-Si (SILICON) Almost all the solar cells used in space are silicon. This is the cheapest solar cell and they have been used for a long time in space. The research on single-crystal silicon is advanced in comparison with other types of cells and the actual efficiency is 24.2% (AM 1.5) which is very near the theoretical limit (27% - 30%). Recently polycrystalline silicon produced by cast method has also been developed. This type has high reliability and its electrical performance is stable for a long time but now the actual efficiency is about 17% (AM 1.5). For constructing many huge solar power satellites, silicon has a big advantage because there are large reserves of Si on both the Earth and the Moon. The general characteristics of c-Si are as follows:  Low cost  Flight proven  Lower specific power  Large degradation for strong radiation The typical performances of c-Si for space use mounted on polyimide film are as follows: (BOL: Beginning of Life EOL: End of Life) Cell Efficiency (@28 deg. C BOL) 14.5% Panel Efficiency (@28 deg. C BOL) 11.2% (@28 deg. C EOL) 6.5% Weight (@28 deg. C EOL) 119 W/kg Area (@28 deg. C EOL) 88 W/m sq. Cost $700/W Radiation degradation 0.86 From the point of low cost and light weight, a-Si mounted on thin film has high performance. The characteristics of a-Si are as follows:  Mass produceable  Lightweight and very compact stowed size  Better operation at high temperature  Very light absorbent  Excellent tolerance to radiation dose  Little degradation with temperature  Degradation of its efficiency by light Cell Efficiency (@28 deg. C BOL) 10.3% Panel Efficiency (@28 deg. C BOL) 8.3% (@28 deg. C EOL) 6.8% Weight (@28 deg. C EOL) 613 W/kg Area (@28 deg. C EOL) 92 W/m sq. Cost $150/W Radiation degradation 0.96 For the large scale SPS installations in Geosynchronous Earth Orbit (GEO), a-Si can reduce total mass and cost. Because radiation degradation of a-Si is lower than the c-Si it is highly efficient for large scale installations for SPS. 2. GaAs (Gallium Arsenide) The efficiency of individual cell has reached 25.7% (AMI 1.5) which is near the theoretical value of 27%. The efficiency of this cell is higher than that of Si, but the cost is much higher than that of Si. The typical performances of GaAs cells mounted on polyimide panel are as follows: Cell Efficiency (@28 deg. C BOL) 18.0% Panel Efficiency (@28 deg. C BOL) 13.4% (@28 deg. C EOL) 11.7% Weight (@28 deg. C EOL) 126 W/kg Area (@28 deg. C EOL) 158 W/m sq. Cost $1400/W Radiation degradation 0.92 3. InP (Indium Phosphide) This material has the advantage that its radiation degradation is very low. Also, the theoretical efficiencies are high, with laboratory cells demonstrating about 19% (AM0: Air Mass 0) efficiency at this time. Material costs are very high
  3. 3. and resources of Indium are rare at this time. The typical performances of InP cells mounted on a polyimide panel are as follows: Cell Efficiency (@28 deg. C BOL) 11.8% Panel Efficiency (@28 deg. C BOL) 8.9% (@28 deg. C BOL) 5.8% Weight 100 W/kg Area 79 W/sq. m Radiation degradation 0.94 4. CuInSe2 (Copper Indium Diselenide) This material can be a thin-film cell. The advantages of thin- film cells are: high radiation tolerance, high specific power, large area solar cells with integral series interconnections, flexible blankets, large body of array manufacturing experience and low cost. The disadvantages of thin-film cells are: lower efficiency, lack of spacecraft experience and no light weight substrates have been produced for space use. As a feature of CuInSe2, the band gap is 1.0 eV, which is on the low side of the AM0 efficiency maximum, but nearly ideal for the bottom cell of a multi-band gap cascade structure. An efficiency of 10.4% (AM0) has been achieved by Arco Solar (now Siemens solar). An efficiency of 12% has been predicted in the near term. The absorption constant of CuInSe2 is extremely high, allowing the possibility of cells as thin as one micron. Existing cells consist of a layer of the active CuInSe2, typically about 3 microns in thickness. So, the current specific power is relatively high, 7.0 kW/kg. 5. CdTe (Cadmium Telluride) CdTe is also being extensively studied for thin-film cells. The band gap of CdTe is 1.5 eV and is very well matched to the solar spectrum. The best CdTe cells to date have been manufactured by an atomic layer epitaxy process that uses a graded junction to enhance current collection. These cells have an AM0 efficiency of about 11.2%. A production run of 20kw of large area CdTe modules was done for SERI by Photon Energy Co., using electrode posited CdTe and a thinned CdS window. The best efficiencies for modules of this design are about 9.8%. b. THIN FILM CASCADES An important technology for the production of high- efficiency thin film arrays is the ability of thin films to be produced in multi-band gap “cascade” structures. In the cascade structure, short wavelength (high energy) photons are absorbed in a high band gap material on the top of the solar cell. The high band gap material is transparent to longer wavelength (low energy) photons, which pass through and are absorbed by a second layer consisting of a photovoltaic material with lower band gap. In a current-matched two- element cascade, the efficiency can be approximated, as equal to the top cell efficiency plus half the bottom cell efficiency. The best currently demonstrated thin-film cascade, reported by Siemens Solar, uses an amorphous silicon top cell on a CuInSe2 bottom cell. The achieved efficiency is 12.5% (AM0). c. SOLAR ARRAY PADDLE For the photovoltaic system, parameters of structures to support solar cell array are as important as for solar cells. Generally the structure is called solar array paddle or solar array wing (fig (a).) and classified by three types that are rigid paddle, semi-rigid paddle, semi-rigid paddle, and flexible paddle. The rigid paddle is the cheapest type and has the highest- reliability. Usually it consists of aluminum-honeycomb sandwiched by sheet of aluminum (Al) or Carbon Fiber Reinforced Plastic (CFRP). Lightweight Lattice Panel (LLP) which has lattice structure made by CFRP is the lightest paddle in the rigid paddle. When the power level is lower than about 5 kW, rigid paddle is lighter than the other types. The power to weight ratio of this type is about 30 W/kg in the case of 5kW class with Si cells. (Figure (b).) Fig (a). The semi-rigid paddle consists of CFRP lattice and CFRP sheet tensioned in the lattice. The weight is almost same as that of LLP and is employed by Japanese Earth Resources Satellite – 1(JERS-1), etc. (see figure (c).) The flexible paddle is suitable for large-power use in space. It is easy to fold in the fairing of the carrier. The weight of this paddle is lighter than the rigid paddle in the case of large power (more than 5 kW). This paddle consists of a thin blanket containing wire harness and usually is extended by a mast mechanism on orbit. When the power level is below 10 kW, the weight of the mechanism for holding and expansion is constant and is about 100 kg to 150 kg. Therefore, the higher is power, the bigger is the ratio of power to weight. There are two ways to expand this paddle. One is roll-type method and the other is fold-type method. The typical example of the roll-type method is the paddle of the Hubble Space Telescope which was launched in 1990. The power generation of this paddle is 4.8 kW (BOL), and the weight is 270 kg. The ratio of power to weight is 17.7 W/kg. The size of this paddle in expansion is 11.82 m x 2.83 m (without its mechanism), the ratio of power to area is 143 W/m2 . The typical example of the fold-type method is the paddle of International Space Station (ISS). 8cm x 8cm Si cells are mounted on this paddle and the generation
  4. 4. power is 27.8 kW. The ratio of power to weight is 43 W/kg (BOL) and ratio power to area is 95 W/m2 (BOL). (Figure (d).) Fig (b). Fig (c). Fig (d). d. PROBLEMS OF LARGE SCALE SOLAR ARRAY WING When the size of the solar array is large and its generation power is high, we are faced with a lot of problems with which we have not had experience until now. In this case, if we use current technology for the solar array paddle, we must use a high-voltage solar array to reduce transmission loss between solar cells and power transmission area in the spacecraft. It is ensured that the limit of operating voltage of solar array is determined by plasma induced discharge. The voltage threshold for breakdown is dependent on the plasma density and it may be about several hundred’s volts in Low Earth Orbit (LEO). The reason for this phenomenon is that the exposed interconnectors of high voltage biased solar cells make complicated electrical fields near the boundary between interconnectors and cover glasses, which are insulators. Therefore, this may not be serious in GEO. Another way to reduce the transmission loss is to employ an Alternating Current (AC) system. In this case, solar array must contain some inverters and this may reduce the power density of solar array. In GEO, there is a problem of charge-up to several kilovolts on the surface of the spacecraft and the electrical function will suffer from the discharge. In the case of the current satellites, it is well known that bonding is effective for protecting from charge-up. But it is no sure for large scale structure that there is no problem with bonding. In the design of a large scale solar array, we must pay attention to the direction of the current loop in the solar array. Because the high current loop induces strong magnetic field, we must design the direction of current loop not to influence attitude control of the spacecraft and not to produce Electric Magnetic Interference (EMI) problems. In LEO, high voltage solar array with Direct Current (DC) output collects plasma particles which produces leakage current, ion drag, plasma induced discharge, and contamination caused by sputtering of interconnectors material. These phenomena are dependent upon the array voltage and plasma density. e. SOLAR DYNAMIC SYSTEMS This section mainly presents an overview of solar dynamic technology as a method for providing power to an orbiting satellite. Solar Dynamic Systems (SDS) works by using a solar concentrator to collect solar radiation and focus it on a receiver to heat up the working fluid. A power conversion unit based usually on either the Brayton, Sterling or Rankine thermodynamic cycles convert the heat energy into electrical energy which then feeds it the satellite. First of all a short summary of the major elements of a solar dynamic system is provided which shows how the system operates. This includes a short description on the thermodynamic cycles. Also a review of the main elements of a solar dynamic system is given. A solar dynamic system (SDS) consists of the following elements: 1. A concentrator used to collect the solar radiations and focus it on a receiver. 2. A heat receiver used to convert the solar energy to heat energy and to store energy for eclipse. 3. A power conversion unit used to convert the heat energy to electrical energy 4. A heat rejection element used to reject any waste heat to deep space Other elements required include integration hardware to attach the solar dynamic system to the satellite, electrical control equipment, pointing equipment to accurately point the collector at the sun and a power distribution system.
  5. 5. 1. CONCENTRATORS Concentrators collect solar radiation using a large reflector system and focus it on the receiver. Different designs exist for collector assemblies which make use of standard reflector systems, e.g. Cassegrain or offset Parabolic mirrors with the design chosen depending on the overall system. The examples of four concentrator configurations: The first is a plane receiver with plane reflectors at the edges to reflect additional radiation onto the receiver. The concentration ratio is relatively low with maximum value less than 4. The second shows a parabolic reflector which could be a cylindrical surface or a surface of revolution which have much higher concentration ratios. The third shows a Fresnel reflector which uses a set of flat reflectors on a moving array. Alternatively, the facets of the reflector can also be individually mounted and adjusted in position. The concentration ratio, C, is defined as the ratio of the area of aperture Aa to the area of the receiver Ar. That is C = Aa / Ar This ratio has an upper limit that depends on whether the concentration is a three dimensional concentrator such as a paraboloid or a two dimensional concentrator such as a cylindrical parabolic concentrator. As the concentrator ratio increases, the receiver temperature increases and imposes an increasing requirement for precision in optical quality and positioning of the optical system. The concentrators correspond to collection efficiencies of 40- 60% and represent the usual range of operations. The maximum concentration ratio is limited by the optics and for circular concentrators is less than 104 Since the collectors focus light on the receiver, accurate pointing of the mirror with respect to the sun to the order of 0.1’ is required. This requires some kind of pointing mechanism. It is possible to use the satellites attitude control system to point the collectors but this can have a negative impact on other requirements of the satellite. 2. RECEIVERS OF SATELLITE The receiver is located near the focal point of the concentrator. The solar energy is used to heat up the working fluid which can be either by liquid or gaseous. Because the satellite will be in eclipse for at least part of its orbit (unless it is sun-synchronous orbit), energy needs to be stored to provide the required energy during the dark periods of its orbit. One method of doing this is to surround the receiver with canisters containing a eutectic salt mixture and use its heat of fusion as the energy storage method. Thus when an eclipse occurs the salt mixture changes state releasing heat and providing power. This greatly increases the mass and hence cost of the receiver. 3. POWER CONVERSION UNIT The power conversion unit converts thermal energy into mechanical energy which is then be converted into electrical energy using a generator. SDS based on heat engines use closed-cycle conversion systems of Rankine, Brayton and Stirling type. The energy is extracted from the working fluid by a mechanical device, usually either a turbine or piston. If we compare the merits of different dynamic power cycles, we come to know that the peak cycle temperature is increased the efficiency of the cycle also increases. However, as the temperature increases so does the use of non-conventional materials and hence the cost of the system. a. RANKINE CYCLE This thermodynamic cycle uses a two phase working fluid in a closed cycle and is shown in fig. The fluid phase is pumped to operating pressure, vaporized and expanded across a turbine. This turbine drives a generator to produce electric power. The discharge vapor from the turbine is condensed and the waste heat is rejected. If an organic fluid such of toluene is used as the working fluid, the discharge from the turbine is still superheated. A regenerator is incorporated in this cycle to remove the superheat and transfer the heat to the high-pressure liquid before entering the vaporizer. b. BRAYTON CYCLE The Brayton cycle, shown in figure, is a power producing thermodynamic cycle that functions by the mechanical compression of a gas, further heating of the gas at constant pressure, expansion of the gas to produce mechanical energy, and rejection of the waste heat at constant pressure. This operates with a gaseous single-phase working fluid but is similar in principle to the Rankine cycle. Experimental systems have used helium-xenon mixture to minimize compressor power consumption and enhance heat transfer.
  6. 6. c. STIRLING CYCLE This is a closed cycle reciprocating piston engine that uses helium or hydrogen gas as its working fluid and is shown in the figure. Heat energy is converted into electrical energy using a free-piston type engine integrated with a linear alternator. The regenerator is used as a heat exchanger to minimize waste heat and therefore increase thermal efficiency. TABLE: TECHNICAL DEVELOPMENT REQUIRED IN THERMODYNAMIC POWER CYCLES BRAYTON ORGANIC RANKINE FREE PISTON STIRLING High Temperature Level (Storage media and container materials) Stable working Fluid Dynamics (vibration, stability) Compressor efficiency Two Phase Flow Control Linear Alternator Recuperator Weight Turbine Efficiency Non-contact Stealing Gas bearings Fluid Bearings 4. HEAT REJECTION ASSEMBLY To reject any waste heat left over from the PCU, a radiator is required. The amount of energy radiated into space is proportional to the temperature to the fourth power, so the higher the temperature the more energy is radiated into space, i.e. the higher the Watts/m2 . However the efficiency of the SDS is dependent on the ratio of the turbine inlet temperature and the radiator outlet temperature, with the higher the ratio the more efficient the system will be. Thus although it is possible to reduce the radiator size by increasing the temperature within the radiator, it is at the expense of the thermodynamic efficiency. At 293 K the heat rejection capability of a radiator in orbit is -200 W/m2 to 250 W/m2 . The waste heat can be transferred from the PCU and other electrical equipment by a pumped liquid loop to the radiator which radiates the heat to space. The choice of method depends on thermal cycle and system trade-offs. F. COMPARISON OF PHOTOVOLTAICS WITH SOLAR DYNAMIC SYSTEMS FOR POWER COLLECTION This section provides an overview of the two main competing technologies for collecting solar radiation and converting it to electrical energy. Its purpose is to indicate the type of trade- offs that must be performed when designing a solar satellite power source. a. EFFICIENCY One of the main technical design drivers is the efficiency of the collection process since this will determine the overall mass of the system used, i.e. the higher the efficiency, the smaller the mass. Photovoltaic systems which are presently used for solar arrays on satellites, are typically 10-15% efficient using Silicon solar cells with GaAs cells being developed being 18-20% efficient. In the long term it is possible to increase the efficiencies up to 30% using tandem cell technology but this considerably increases the costs. Solar dynamic systems (SDS) tested on the ground have typical efficiencies of 25-30% depending on the cycle, used with up to 40-50% predicted in the longer term. Since SDS are a lot more efficient than photovoltaic cells then for generating the same power, the area required to collect the solar radiation can be considerably reduced. b. SPACE QUALIFICATION Photovoltaic cells based on silicon solar cells have a very good space heritage having been used for solar arrays for the last 20- 30 years. GaAs cells have been flown on satellites but mainly on an experimental basis to test the characteristics of these cells in a space environment. SDS’s have not been used in space although there is a ground database on the use of certain type of heat engines in particular the Brayton Heat Engine has been tested extensively by NASA. c. COSTING OF SYSTEM Photovoltaic systems are very expensive based on present manufacturing techniques. Using more efficient GaAs and tandem cell reduces the number of solar cells required to produce a given amount of power but increases the individual solar cell cost and so a trade-off must be performed as to the lowest. SDS are inherently less expensive since they use mirrors in the form of concentrators to collect the radiation. d. HEAT REJECTION
  7. 7. In the process of generating heat energy, a SDS produces waste heat which must be rejected using some kind of radiating surface. For large systems, a heat transport system using pumped-liquid or two-phase fluid loop is necessary to transport heat to the large radiator that is required. For a photovoltaic system no waste heat is produced and therefore no radiator is required making it a simpler system. III. TRANSMISSION AND RECEPTION OF ELECTRICAL ENERGY Wireless power transmission can be presently performed through two possible technologies, microwave and LASER. They are presented in this section discussing the generation of the transmission beam and beam propagation. An overview on the microwave sources technology is discussed. A. MICROWAVE TRANSMISSION In this section the general principles of microwave power beaming and reception are discussed. Different types of antennas suitable for microwave power transmission are examined. Special attention is paid to phased array antenna technology. The atmospheric effects on the transmitted beam are addressed. a. PHASED ARRAY ANTENNA TECHNOLOGY The basic radiation characteristics of any antenna are determined by its dimensions and the transmitting wavelength. If the transmitter is emitting an equal amplitude and phase wave front, the far-field radiation intensity will be simple diffraction pattern. Approximately 90% of the transmitted energy will be within the main lobe of the antenna, as shown on fig. while the rest is spread out into the grating side lobes. The angle defined by the main lobe is roughly ƛ/Dt (radians), where Dt is the diameter or width of the antenna. In the case of a perfectly circular aperture the beam width is given by 2.44 ƛ/Dt. For an antenna located at a distance H, the transmitting antenna will produce a footprint with a diameter Dr . Dr = 2.44H ƛ/Dt The above equation illustrates the fundamental relationship between transmitter antenna and rectenna sizes. For a given wavelength and transmission distance, the product Dt Dr is constant. In order to collect all the radiated power within the main lobe of the antenna, the rectenna diameter must be equal or larger than the footprint diameter. If the receiver antenna is smaller than the footprint the efficiency of the link decreases. B. ATMOSPHERIC EFFECTS Power transmission by means of microwave technology is presently used for many ground applications and it is considered for space power beaming. The choice of the frequency depends on the beam interaction with the atmosphere, the size and the mass of the instruments, the environmental and safety issue, the efficiency achievable and the technology availability and reliability. The frequency employed since the early utilization of microwave transmission is 2.45 GHz. The reason was mainly the poor influence of the earth’s atmosphere on the processes of electromagnetic propagation in S-band. The ionosphere of the earth is usually opaque to frequencies below 10 MHz and has strong effect on signals with frequencies up to 1 GHz. At 2.45 GHz the diameter of an orbiting transmitting antenna is 1 km and one of the ground receiving antenna is 10 km. Using higher frequencies the antenna and rectenna can be considerably reduced (the antenna size is 10 times smaller at 35 GHz) and therefore also the cost decreases. The interaction of microwave radiation with the earth’s atmosphere is a very complex problem to be analysed. When a microwave beam propagates through the earth’s ionosphere many phenomena occurs. The most important are summarized in the following: a) The electromagnetic radiation is influenced by absorption of the oxygen molecules and water vapour. At higher frequencies, molecular absorption in the earth’s atmosphere produces remarkable losses as the waves propagate through the atmosphere. b) Rain, mist and cloudiness produce an attenuation of the radiation. c) The presence of the earth’s magnetic field leads to Faraday rotation of the electric vector affecting any polarization measurement. d) The ionosphere’s in-homogeneities and turbulence has as a consequence changes in the amplitude and phase of the propagating wave. At 2.5-3 GHz the loss of energy due to the precipitation are estimated around 2-6%. At 35-38 GHz those losses are 8-11%. In this range a very strong loss (up to 12dB) occurs of dense cumulus-rain clouds and heavy rain. The total atmospheric attenuation of a 35 GHz beam in traversing vertically through the atmosphere in clear weather is 0.2dB. Therefore, the range 35-38 GHz can be used but only if the receiving ground antenna is placed in the earth’s regions with small quantity of rain and cloudiness. But the factor that hampers the practical use of the millimetric waves is in the limited availability of powerful and efficient microwave sources in this range. This was the problem of concern before introduction of Gyrotrons. C. RECTENNA The term “rectenna” is most used to indicate the whole receiving part of a transmission system. From this point of view, the rectenna refers to the structure that intercepts the main lobe of the transmitting beam, in order to collect the major part of its power. This structure is an array of elements, where each of those is converting the locally incident RF power. The term “rectenna” originally referred to each of these elements. It stands for “rectifying antenna” as the RF to DC conversion is conducted by rectifying the wave.
  8. 8. The way the rectenna converts the RF signal to DC power is shown in figure. The RF energy is collected by an antenna, rectified by a diode, and low-pass filtered in order to extract the DC component. A microwave low-pass filter can be found between the antenna and the diode, to attenuate the harmonic radiation, and to store energy for the rectification process. B. LASER TECHNOLOGY Laser technology for power beaming is a good possibility. Benefits of laser beaming compared to microwave beaming are smaller receiving site, radiation outside the beam is negligible and side lobes don’t interfere with other electromagnetic radiation. On the other hand, lasers have quite low system efficiency and higher losses on atmosphere. Higher power density on the beam can reduce the atmospheric attenuation by boring a hole through light clouds. a. CONVERSION A laser consists of a laser tube containing the laser medium, or lasant, a system for pumping the lasant with energy and a system for removing waste heat. As a laser emits monochromatic, coherent light, a laser beam propagates for great distances with very little beam spreading. The state of art in laser technology has concentrated on infrared lasers in the band of 2-11 micro-metres. The state of art technology in photovoltaic cells requires much shorter wavelengths for power generation. Main problems in scaling are gas purification and cooling. Gas purification is required to maintain the efficiency of the laser. Some laser technologies are described below: 1. SOLAR PUMPED LASERS Solar energy pumped laser is a good choice for space laser power system. Due to direct use of solar energy, no conversion from solar to electric energy is needed for beamed power. The system itself requires some electricity for controlling purposes. An active cooling system needs much energy that can be either electric or mechanical. In both cases some secondary power converter is needed. Direct solar pumped lasers can use only few narrow bands of total solar energy. To avoid excess heating of laser system, unused bands of solar energy have to be filtered or reflected out to reduce the heating on the lasant. The amount of useful solar energy in the total spectrum is at most 0.5%. This means that pumping energy has to be concentrated over a large area. Using solar energy value of 1.3 kW/m2 the minimum required collection area Ar for output power Po is the following: Ar = Po/6.5 m2 For a 100 kW laser beam this means a mirror at least 140 m in diameter is required. An indirect solar pumped laser is one possible way to avoid the inefficient use of solar energy. Solar energy is stored to a black body and the laser is pumped by blackbody radiation. Walls of the black body emit radiation in spectral distribution depending on the temperature of the black body. Temperature range required for wavelengths used for lasant pumping is 2000-3000 K. The lasant absorbs much of pumping wavelength from radiation passed through lasant. Rest of the radiation spectra is then absorbed by other wall of the black body. The efficiency of an indirect solar pumped laser can rise very close to the theoretical value of the lasant. The factors to decrease the efficiency are the energy required for cooling and the leak emission of the black body. The solar pumped laser is a good alternative because it don’t need any solar energy converter. However it requires quite large reflectors. 2. CHEMICAL LASER In this laser type the energy of molecules is produced by chemical reaction. An example is Hydrogen fluoride (HF) that has a reaction as shown in the following table. Another possible chemical lasant is Deuterium fluoride (DF). Efficiency for an HF laser is 2.5-3.0% and for DF laser is 3.5-4.0%. H + F2  HF* + F F + H2  HF* + H HF* + hv  HF + 2hv The third reaction shows that a photon stimulates the HF* (energized HF molecule) to emit a second photon of the same frequency. A chemical laser consumes fuel and oxidizer and produces reaction product. The laser is operable only until fuel and oxidizer is consumed. A space chemical laser has to be closed so that fuel and oxidizer can be reproduced from the reaction product. This reproducing can accomplished by electrolysis or by direct sunlight.
  9. 9. b. TRANSMISSION Electromagnetic radiation loses energy in plasma through linear (ohmic) and nonlinear (anomalous) absorption. In altitudes of 120 km to 340 km the absorption by plasma is 0.17 nW/m3 at maximum using a 100 MW, 5 micro-m laser beam. As plasma has maximum density in altitudes of 200-500 km the absorption due to plasma in space is negligible. The total efficiency of power transmission from space to earth is highly affected by the availability of the beam and attenuation in atmosphere. To avoid wasting of available power there has to be many power receiving sites in view of the power satellite. If possible these receiving sites should have different kinds of weather probabilities. When the received power in receiving site reduces below some threshold level the power has to be refocused to another receiving site. The selection of the threshold level effects greatly to the total effectiveness. The threshold level is not only depending on the transmission effectiveness exceeds some useful value. c. LASER RECEIVER Laser energy receiver and converter may be either energy to electricity converter as photovoltaic cell or it may transfer the laser energy to heat. This section first discusses the conversion to heat and on the second sub-section conversion to electricity or mechanical energy. 1. RECEPTORS Heat exchanging devices need very highly concentrated laser radiation. Concentrating optics is not desirable since the environmental effect will decrease the reflectivity and large high power density reflectors are very expensive. To avoid optical concentrator, the laser spot on receiver site should be reduced as small as it is possible considering the effect of diffraction, turbulence, thermal blooming, pointing accuracy and jitter. However reduced spot requires very dense power beam that in turn may cause other problems. One method to maximize the absorption of incoming radiation is an open sphere with a cone heading to the entrance opening of the sphere as shown in figure. The meaning of the cone is to minimize the escape of re-radiated energy. Convective losses due to air heating are minimized by purging dry air. 2. CONVERTERS In a paper written by Boeing Aerospace it was mentioned that an optical rectenna is entirely analogous to the microwave rectenna. It consists of micro miniature 10 micro-m wavelength dipole antenna and rectifier elements. It was estimated by Boeing Aerospace that efficient operation requires intensities of almost gigawatts/m2 to overcome forward voltage drop in the rectifying diode. This amount of intensity can be easily reached with pulsar laser beam and using efficient optics to amplify the laser beam. The thermoelectric laser energy converter (TELEC) generates electricity by collecting electrons diffused out of the plasma. Electrons are energized by absorbing laser energy through inverse bremsstrahlung. Electrons are collected using different area and temperature for anode and cathode. Theoretical efficiency using 10.6 micro-m laser beam is above 42% but experimental values are much lower. REFERENCES [1] International Space University Gregg Maryniak Space Studies Institute USA and Masamichi Shigehara Kyushu Institute of Technology Japan “(1992-spacesolarpowerprogram part-I)” pp. 147–232, April 1992. [2] International Space University Eric Dahlstrom Lockheed Corp. USA Barbara Mckissock NASA Lewis Research Center, USA “(1992-spacesolarprogram part-IIA)” January, 1992, pp.56-89. [3] Ad Astra Magazine by National Space Society published in 2008. [4] Sky’s no limit- Space Based Solar Power a new step in renewable energy Peter A. Garretson, IDSA, USA. [5] Beam It Down- An article published in National Geographic Magazine in 2011. [6] SPS-ALPHA: The First Practical Solar Power Satellite via Arbitrarily Large Phased Array (A 2011-2012 NASA Phase 1 Project) by Mr. John C. Mankins, Principal Investigator at Artemis Innovation Management Solutions, Santa Maria, California, USA.