MadSci Network: Physics

Re: What are some waves that can be used to create electricity?

Date: Tue Dec 8 17:28:04 1998
Posted By: Adrian Popa, Directors Office, Hughes Research Laboratories
Area of science: Physics
ID: 912811110.Ph


The proposed system that you ask about is called Solar Power Satellites 
(SPS) and you will find many references to SPS on the Web. Systems of this 
sort have been proposed ever since satellites were first orbited in the 
late 1950s. After 40 years scientists, engineers and politicians are still 
arguing about the concept. The issues argued about include: How efficient 
will it be? How much will it cost? How safe will it be? 

A recent article entitled "Beam It Down: How the New Satellites Can Power 
the World" (
hoffert.html)  by Martin I. Hoffert and Seth D. Potter in the Massachusetts 
Institute of Technology’s Technical Review Web magazine brings the reader 
up to date on SPS and also presents a picture of a new concept that they 

To explain the SPS concept I’ll quote a part of the Hoffer & Potter 

“The skies will soon fill with low-orbiting satellites
providing communications links to every point on earth. 
We should press these fleets into double duty as solar energy
collectors that would relay uninterrupted beams of nonpolluting electrical 
power to earth.

In outer space,the sun always shines brightly. clouds never block the solar 
rays, and there is no night time. Solar collectors mounted on an orbiting 
satellite would thus generate power 24 hours per day, 365 days per year. If 
this power could be relayed to earth, then the world's energy problems 
might be solved forever. 

Solar power satellites (SPS) were originally proposed as a
solution to the oil crises of the 1970s by Czech-American
engineer Peter Glaser, then at Arthur D. Little. Glaser
imagined 50-square-kilometer arrays of solar cells deployed
on satellites orbiting 36,000 kilometers above fixed points
along the equator. A satellite at that "geosynchronous"
altitude takes 24 hours to orbit the earth and thus remains
fixed over the same point on earth all the time. 

The idea was elegant. Photovoltaic cells on a satellite would
convert sunlight into electrical current, which would, in
turn, power an onboard microwave generator. The
microwave beam would travel through space and the
atmosphere. On the ground, an array of rectifying antennas,
or "rectennas," would collect these microwaves and extract
electrical power, either for local use or for distribution
through conventional utility grids. 

The technology, as originally envisioned, posed daunting
technical hurdles. Transferring electrical power efficiently
from a satellite in geosynchronous orbit would require a
transmitting antenna on board the satellite about one
kilometer in diameter and a receiving antenna on the ground
about 10 kilometers in diameter. A project of this scale
boggles the mind; government funding agencies shied away
from investing immense sums in a project whose viability
was so unclear. NASA and the Department of Energy, which
had sponsored preliminary design studies, lost interest in the late 1970s.”

The article then proceeds to discuss a new SPS concept. For cost and safety 
reasons it would be best to place the large 10 km (6 mile) diameter 
antennas in remote areas such as the deserts. This would provide low cost 
land and keep the super powerful microwave beam from injuring people and 

Other scientists argue that if you place the antennas in the desert why not 
place the solar cells in the desert and convert the sunlight to electricity 
on the ground and not use costly satellites and harmful, less efficient 
microwave beams. However; the power generating solar cells would be useless 
at night and on cloudy days but you could store excess energy generated 
during the day in batteries or fly wheels to provide power during darkness.

Next February cosmonauts in the MIR Space Station are expected to deploy a 
Russian space mirror. The 25 meter (82 ft) diameter mirror will reflect 
sunlight upon a 2.4 km (1.5 mile wide swath on Earth as it circles in low 
Earth orbit.The goal is to light up northern regions during the long dark 
winters. Perhaps mirrors of this type could illuminate solarcells at night 
for generating power.

Others have proposed building huge solar energy collector arrays on the 
moon using lunar materials and then beam the power to earth with very 
narrow beams from ultrahigh power lasers. 

As you can see the problem is complex and the cost of the system will play 
a major part in any decisions made. While oil is as cheap as it is today it 
will be difficult to justify the cost of an SPS system; however, when the 
oil reserve begins to end and nuclear generated power becomes a necessity, 
perhaps the SPS concept will be more cost effective and better for earth’s 
environment. Read the article and some of the links that are referenced and 
develop your opinion about Solar Power Satellites!

Best regards, Your Mad Scientist
Adrian Popa 

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