Published on Apr 02, 2024
A major problem facing Planet Earth is provision of an adequate supply of clean energy. It has been that we face "...three simultaneous challenges -- population growth, resource consumption, and environmental degradation -- all converging particularly in the matter of sustainable energy supply." It is widely agreed that our current energy practices will not provide for all the world's peoples in an adequate way and still leave our Earth with a livable environment. Hence, a major task for the new century will be to develop sustainable and environmentally friendly sources of energy.
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
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 r ecently 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 which shapes the characteristics of the electromagnetic wave.
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 a flow 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.
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 isn't uniform. It has a taper form 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.
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.
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 Earth’s 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 Earth’s 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.
Figure 1 Configuration of SPS is space.
The day-night cycle ,cloud coverage , atmospheric attenuation etc.reduces the amount of solar energy received on Earth’s 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 for ma 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 center around midnight.
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 rectenna arrays would be designed to let light through, so that crops or even solar panels could be placed underneath it. Here microwaves are practically nil.
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 earth’s 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 has been 2.45GHz due to factors such as low cost power components, location in the ISM band, extremely low attenuation through the atmosphere. 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.
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.
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