Environmental Engineering Reference
In-Depth Information
For a solar power satellite in 666 km orbit, if we accept the 7.5 km needed
rectenna size from36 000 km, one can estimate the required rectenna size by linear
scaling. Using the same 500m diameter phased array antenna, described for
the power satellite of Figure 5.7, one would need a (666 km/36 000 km)
ΒΌ
139m diameter dedicated ground rectenna. This area is equivalent to 1.89
football
fields or 3.75 acres, a reasonable size for urban satellite power system,
small enough to be cordoned off to allow a higher microwave power density. A
rectenna is an array of dipole antennas with diodes mounted to produce direct
current. The spacing of themany elementswould be on the order of thewavelength,
5.2 cm. This array could be elevated above ground so that the ground beneathwould
receive sunlight (possibly could have solar cells) with near-zero microwave power
density at ground level. Admissible legal occupational 5.8GHz power density is
50W/m
2
, while the power density to deliver 1GWto the proposed area is 66 kW/m
2
.
As we know, full sunlight is about 1 kW/m
2
and full power inside a typical
microwave oven is 10 kW/m
2
. The envisioned beam would be deadly, 6.6 times
more intense than a microwave oven.
On this basis, it seems unlikely that such a satellite power system could be
implemented near New York, although the receiver area would be more dangerous
but cover less area than tolerated hazards of miles subway tracks, heliports, airport
runways, and freeways. A reasonable microwave power density, equivalent to full
sunlight, 1 kW/m
2
, would deliver only 15.2MW, which seems not enough to be
useful. (The Indian Point Nuclear Reactor 40 miles north of New York has three
reactors totaling 1.9GW.) Fromthe solar power satellite, oneGWcould be delivered at
the 1 kW/m
2
intensity level if the beamwere split equally onto 66 areas of the 3.75 acre
size. A phased array antenna on the solar power satellite could, in principle, address
66 separate rectennas in rapid sequence. This might offer also a solution to the
problemof buildingmore power lines, as part of the distribution can be done directly
from the satellite.
We can make a more detailed model for a power satellite constellation in sun-
synchronous orbit, extrapolating from the Radarsat-1 and -2 satellites developed and
launched by the Canadian Space Agency, with 100.7min orbital period, total cost
$1.145 billion. Radarsat 1 (mass 2750 kg) was launched in 1995 and has completed
15 years of data collection in a sun-synchronous orbit near 807 km altitude. As
suggested in Figure 5.8 (Radarsat-2, 2200 kg, launched in 2007), these satellites
access a 1000 kmwidth of land along their path with a synthetic aperture radar (SAR),
which uses a phased array of transmitting elements. The high-resolution mode
addresses at minimum a swath 45 km in width, larger than the 7.5 km rectenna
envisioned for the geosynchronous earth orbit (GEO) satellite of Figure 5.7. The
SAR antenna elements are stationary and the directionality is accomplished by
timing or phasing of the signals from those elements. The satellite always faces the
sun at approximately the same angle, and the advantage, if this were turned into a
solar power satellite, is that tracking of the sun to optimize power would be facilitated.
The satellite does 14 7/24 orbits in its
7.5 km
fixed plane as the earth rotates once
underneath it, in 24 h. A small effect associated with the particular angle 98.594
tilts the orbital plane a tiny amount each day so that during the course of the year the
