NASA is taking a fresh look at the possibility of generating power in space
and beaming it down to Earth, despite deciding more than 20 years ago that the
idea was not feasible.
Collecting solar energy in space is attractive because sunlight there can be
eight times stronger than on Earth, and there鈥檚 no shortage of real estate in
which to place solar panels. The problem is getting the power station up and the
energy down.
With an estimated price tag of around $250 billion, NASA鈥檚 first
designs for orbiting power stations in the late 1970s were hopelessly expensive.
鈥淎ll serious work on space solar power in the US stopped around 1980,鈥� says John
Mankins at NASA鈥檚 Advanced Projects Office in Washington DC.
Advertisement
These early designs envisaged vast metal structures of over 50 square
kilometres. Building them would have required factories in space and teams of
astronauts, not to mention dedicated 鈥渉eavy lift鈥� launch vehicles weighing
around 11,000 tonnes to get them into orbit.
Now NASA has come up with an alternative that is just one-thirtieth the price
of the previous design. Dubbed the SunTower, it is radically different from
other designs, including a recently proposed Japanese project consisting of
solar panels covering 3 square kilometres
(快猫短视频, 10 February, p 6).
In place of vast rigid panels, the SunTower uses pairs of solar cells less
than 10 metres across. These would be strung off a central cable up to 15
kilometres long hanging 鈥渧ertically鈥�, with one end pointing towards Earth.
The flexible backbone will be kept extended by the slight gravity gradient
between the top and the bottom. Electricity produced by each of the solar cells
will be fed down the backbone to a grid of transmitters that will beam the
energy back to Earth as 5.8-gigahertz microwaves, which pass easily through the
Earth鈥檚 atmosphere.
The transmitters will be arranged in a 鈥減hased array鈥�, which allows the beam
to be steered by adjusting the phase of the radio waves from each transmitter.
Antennas based at storage facilities on Earth could then collect the radio waves
and convert them into electrical power.
Mankins calculates that each SunTower could transmit up to 300 megawatts of
power to Earth. But safety and cost are still a problem. Any creatures caught in
the powerful beams would be fried. And at today鈥檚 prices the power would cost 80
cents per kilowatt-hour once construction and launch costs are included,
compared with around 5 cents for more traditional sources.
The towers would be sent into orbit in modules weighing around 10 tonnes. To
make the project viable, launch costs would have to come down to $400 per
kilogram, says Mankins, compared with at least $10,000 per kilogram
today.