Solar power collected in space and beamed to Earth could be an environmentally friendly solution to our planet's growing energy problems.

by Steve Price

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 US 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 Imagine providing the Earth or a moon base with harnessed solar power, or ravelling in space without returning to Earth for fuel. That's the idea behind space-based solar power generators such as this SunTower.

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 utilise well-known physical principles -- namely, the conversion of sunlight to electricity by means of photovoltaic cells. 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.

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.

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.

NASA 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.

"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 US 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.