The Power and Transportation Future - Hydrogen and Fuel Cells: Advances in Transportation and Power - page 274

Space Station (ISS). They are 73 meters long and 11 meters wide. These
panels make the ISS one of the brightest objects in the night sky.
A proof of concept satellite could use the station's solar panels. These
solar panels are 14% efficient, but advances with solar cells and solar con-
centrators allow panels that are up to 50% efficient.
A solar power satellite in orbit uses microwave power transmission
to beam solar power to a very large antenna on Earth. The system consists
of three parts: a solar collector, made up of solar cells and a microwave an-
tenna on the satellite, aimed at the Earth antenna to collect the power. The
Earth-based receiver antenna called a rectenna would consist of a series of
short dipole antennas.
Microwaves broadcast from the SPS would be received with about
85% efficiency. A conventional microwave antenna is even better, but the
cost and complexity are much greater. Rectennas would be several kilo-
meters across and crops and farm animals could be raised under the rec-
tenna, since the thin wires used only slightly reduce sunlight. For best
efficiency the satellite antenna would be between 1 and 1.5 kilometers in
diameter and the ground rectenna around 14 kilometers by 10 kilometers.
This would allow the transfer of 5 to 10 gigawatts of power.
The satellite would need 50 to 100 square kilometers of collector area
using 14% efficient monocrystalline silicon solar cells. More expensive tri-
ple junction gallium arsenide solar cells with an efficiency of 28% would
reduce the collector area by half. In both cases the solar station's structure
would be several kilometers wide, making it much larger than most man-
made structures on Earth. Building structures of this size in orbit has nev-
er been attempted before.
Although the advantage of placing the solar collectors in space is an
unobstructed view of the Sun, the costs of construction are very high, and
SPS may not be able to compete with conventional sources unless lower
launch costs can be achieved, or unless a space-based manufacturing in-
dustry develops and they can be built in orbit from off-Earth materials.
A major problem for the SPS is the current cost of space launches.
Current rates on the Space Shuttle are $3,000 to $5,000 per pound ($6,600/
kg and $11,000/kg). Launch costs of less than $400-500/kg are thought to
be necessary for SPS. Economies of scale on expendable vehicles could pro-
vide some large reductions in launch costs. Thousands of rocket launches
could reduce the costs by ten to twenty times based on experience with
similar technical achievements. This places the costs into the range where
this system could be conceivably attempted. Large reusable vehicles could