NASA 
Above:
Imagine providing the Earth or a moon base with harnessed solar power,
or traveling in space without returning to Earth for fuel. That's the idea behind
space-based solar power generators such as this SunTower. Credit: NASA
March
23, 2001 -- It's December 2000 and the governor of California flips a switch illuminating
the state Christmas tree on the capital lawn. Twenty minutes later, he orders
aides to pull the plug. Why? Statewide power shortages.
The United States
energy secretary ordered a dozen out-of-state power companies to sell electricity
to California to avert blackouts. But it's not just California.
In metropolitan
areas across the country, residents are being asked to limit power consumption
during peak periods of the day. Last November, in the midst of the closest presidential
election in history, Tom Brokaw referred to the electricity shortages as "The
Real Power Struggle."
So what's going on here?
"The United States
consumption of energy is almost flat," says Dr. Neville Marzwell, technical manager
of the Advanced Concepts & Technology Innovations program at NASA's Jet Propulsion
Laboratory. "But, we are decommissioning nuclear plants across the country and
they are not being replaced." Twenty-three states have joined California in deregulation
of the power industry, a step which is forcing companies to take a longer look
at investing billions in construction of new power plants.
With the world's
population projected to skyrocket to 10 billion people by the year 2050, supplying
cheap, environmentally friendly electricity to meet basic needs will be a daunting
challenge.
"We need new sources of electrical power," said John Mankins,
Manager of Advanced Concepts Studies at NASA Headquarters Office of Space Flight,
"and we have been studying a variety of space solar power concepts. Tremendous
advances have been made in many relevant technologies in the last fifteen years."
NASA's
involvement in space solar power, or SSP for short, began after the oil embargo
of the mid-1970's when the space agency (working under the leadership of the US
Department of Energy) began to study alternative energy sources that might result
in less dependence on foreign oil.
Proposed space solar power systems utilize
well-known physical principles -- namely, the conversion of sunlight to electricity
by means of photovoltaic cells. (You can see such cells on many neighborhood rooftops
and on small sidewalk lighting fixtures.) Giant structures consisting of row after
row of photovoltaic (PV) arrays could be placed either in a geostationary Earth
orbit or on the Moon. A complete system would collect solar energy in space, convert
it to microwaves, and transmit the microwave radiation to Earth where it would
be captured by a ground antenna and transformed to usable electricity.
According
to an April 2000 article in the Electric Power Research Institute (EPRI) Journal,
photovoltaic arrays in a geostationary Earth orbit (at an altitude of 22,300 miles)
would receive, on average, eight times as much sunlight as they would on Earth's
surface. Such arrays would be unaffected by cloud cover, atmospheric dust or by
the Earth's day-night cycle.
 Right:
Space solar arrays would enjoy more exposure to the Sun than similar
arrays do on our cloud-covered planet. Credit: National Renewable Energy Laboratory.
When
the idea was first proposed more than 30 years ago, PV technology was still in
its infancy. The conversion efficiency rate -- the fraction of the sun's incident
energy converted into electricity -- was only 7 to 9 percent.
"We now
have the technology to convert the sun's energy at the rate of 42 to 56 percent,"
said Marzwell. "We have made tremendous progress."
Even so, launching thousands
of tons of solar arrays into space will be expensive. But there may be a way to
reduce the needed area of the arrays -- by concentrating sunlight.
"If
you can concentrate the sun's rays through the use of large mirrors or lenses
you get more for your money because most of the cost is in the PV arrays," said
Marzwell.
A drawback to concentrated sunlight is that it is hot. Focused
radiation that's not converted to electricity turns into heat -- enough to damage
the arrays if there's too much excess warmth. Marzwell and his colleagues at JPL
are studying ways to capture waste heat and convert it to electricity by means
of thermal voltaic processes. Special coatings on the mirrors and lenses can also
reject portions of the sun's spectrum that PV arrays don't use, further reducing
excess heat.
 Left:
Who will assemble and maintain an orbiting solar array? Possibly robots
like these, under development at NASA. "We don't need humans for assembly anymore,
except to supervise," said Marzwell.
Once
the Sun's energy is captured in space, what do you do with it?
One possibility
is to convert stored solar energy to microwave radiation and beam it down to a
combination rectifier-antenna, called a rectenna, located in an isolated area.
The rectenna would convert the microwave energy back to DC (direct current) power.
According to Marzwell, the dangers of being close to the microwave beam would
be similar to the dangers of cell phone transmissions, microwave ovens or high-power
electrical transmission lines.
"There is a risk element but you can reduce
it," said Marzwell. "You can put these small receivers in the desert or in the
mountains away from populated areas."
Lasers are also under consideration
for beaming the energy from space. Using lasers would eliminate most of the problems
associated with microwave but under a current treaty with Russia, the U.S. is
prohibited from beaming high-power lasers from outer space.
All in all,
the positive aspects of such a system appear to outweigh the negative ones. Space-based
solar power offers energy from an unending source with no emissions and very little
environmental impact.
According to Marzwell, using today's technology
a space solar power system could generate energy at a cost of 60 to 80 cents per
kilowatt-hour. This estimate includes construction costs for the first system.
"We believe that in 15 to 25 years we can lower that cost to 7 to 10 cents
per kilowatt hour," said Marzwell. The market price today is around 5 to 6 cents
per kilowatt-hour.
"With funding and support, we can continue developing
this technology," said Marzwell. "We offer an advantage. You don't need cables,
pipes, gas or copper wires. We can send it to you like a cell phone call -- where
you want it and when you want it, in real time."
Mankins agreed. With a
dedicated effort and resources, he says, space solar power --just a dream today--
could become a reality in the decades ahead. |
