About
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 other names such as Satellite
Power System, Space Power Station, Space Power System, Solar Power Station,
Space Solar Power Station etc.
Klystron
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.
SPS,
as mentioned before is massive and because of their size they should have been
constructed in space. 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.One often discussed road to lunar resource utilization is to start
with mining and refining of lunar oxygen, the most abundant element in the
Moon’s crust, for use as a component of rocket fuel to support lunar base as
well as exploration mission. The aluminum and silicon can be refined to produce
solar arrays.
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. 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. 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.
Rectenna
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 . 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.
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.
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