by G. Harry Stine
Copyright 1981 by G. Harry Stine
Chapters 1 and 2: Space Solar Power
It's the cornerstone of the universe, life, our individual existence, and our civilization.
We're being told by demagogues that we're running out of energy.
Nothing could be further from the truth.
An outstanding scientist and inventor, Dr. Henri M. Coanda, once told the author, "We are surrounded by energy; we must only learn how to use it."
We've done so in the past, and we'll continue to do so in the future, because the development of the human race from an animal-like existence to a condition where some of the members of the species are reasonably civilized is inexorably linked to the increasing use of energy.
According to anthropologist Carleton S. Coon:
This process perhaps began when early people learned how to use the energy of fire to heat their living spaces, cook their food, and fire-harden the points on their wooden hunting spears, and therefore operate more efficiently together in larger groups or "institutions" beginning with the tribal family. It has progressed to the point where today we have developed a wide variety of local, regional, national, and international institutions ranging from clubs to international associations. These have been made possible only because we've had the energy available for activities above and beyond the mundane task of surviving on a meal-to-meal and day-to-day basis.
Worldwide social organizations have recently evolved to serve a totally new function in the annals of history: to keep people from destroying the world using the massive amounts of energy available from nuclear fission and fusion.
Again, the operation and maintenance of these large, far-flung, sophisticated, and highly complex institutions require a large amount of energy. Being the newest, most fragile, and most energy-intensive human institutions, they are also likely to be the first to fail if we do indeed face an energy shortage.
Space power comes from space energy.
The major source of energy in the Solar System is the Sun.
The Sun probably formed long before the planets, planetary satellites, planetoids, and other celestial bodies in the Solar System. The forces of gravity and magneto-hydrodynamics caused the matter at the core of the proto-Sun to be squeezed hard enough to ignite the thermonuclear fires that are still burning and will continue to burn for millennia, radiating their energies outward into the Solar System and interstellar space as the nuclear particles of the solar wind and as electromagnetic radiation—radio waves, microwaves, infra-red, visible light, ultra-violet, X-rays, and gamma rays.
The manner in which the Sun and our Solar System were born is of little consequence for the purposes of this discussion. The fact that the Sun is there and providing energy right now is of prime consideration.
The Sun is the only operating nuclear fusion reactor known at the time of this writing.
The Sun is a star, and only an average, middle-aged star at that. Although we're learning more about it every day because of space science research, the following general data are of interest.
The Sun has a mean or average visual diameter of approximately 1,393,000 kilometers. It has a volume 1,300,000 times that of the Earth and a mass 330,000 times the Earth's. It contains most of the mass in the Solar System. In fact, an extraterrestrial astronomer looking at the Solar System from a distance of five light years would conclude that it consisted only of the Sun and the planet Jupiter, which in itself contains 72% of the planetary mass of the Solar System.
But the important thing about the Sun is the enormous amount of energy produced by the thermonuclear fusion that goes on within it. The Sun ' 'burns'' hydrogen into helium by nuclear fusion processes, consuming 657,000,000 tons of hydrogen every second. Its core temperatures run into the millions of degrees, but the temperature of the photosphere which we see is only about 6000° Celsius. Each square meter of its surface radiates energy at the rate of 80,000 horsepower continuously. At a distance of the orbit of the Earth, this energy amounts to 1.94 calories per square centimeter per minute. This quantity is known as the solar constant. It's not truly "constant" because modern astronomers have found small changes in the solar output from time to time for reasons as yet unknown. However, for all intents and purposes, we can use the number 1.94 cal/cm2/min as a "constant."
Geothermal energy—or more properly "planetothermal energy"—is the only energy in the solar system that doesn't originate in the Sun. Planetothermal energy comes from the gravitational squeezing of matter in planetary cores. It's the same sort of energy that lit the fires of the Sun eons ago. It didn't light the thermonuclear fires of Earth because Earth is too small to create enough squeezing to accomplish the thermonuclear light-off. However, the planet Jupiter is apparently right on the edge of being a proto-star, being just a little bit too small to generate enough thermonuclear energy by gravitational squeeze to become a star.
All the remaining energy in the solar system comes from or came at one time from the Sun.
This is also true of the planet Earth. Coal represents solar energy converted to organic matter by plants millions of years ago and stored up within the Earth. The same can be said for petroleum. Tidal forces are caused by the combined gravitational fields of the Moon and the Sun. Wind forces are caused by unequal heating of the Earth and its atmosphere by the radiation from the Sun. Wood and other organic fuels are of solar origin; the trees that produced the wood could not have grown without the process of photosynthesis within them producing cellulose and other carbohydrates with solar radiation as the energy to drive this complex system.
For the past several millennia, the human race has been using that vast quantity of converted solar energy stored within plants from season to season and stored for eons underground in the form of coal and petroleum. The human race has also tapped wind power and tidal power, but most of our energy has come from the "fossil fuels": coal, petroleum, and natural gas.
In accordance with Coon's energy thesis, we've been converting more and more of this natural energy of the Earth into social structure at an ever-increasing rate. Depending upon which futurist you talk to and the particular computer program he's developed to provide a mathematical model of the world, you'll be told that we'll use up all the fossil fuel reserves in anywhere from twenty years to five hundred years.
According to the best current data I can get my hands on, we have enough fossil fuel to last well into the twenty-first century.
The amount of coal in the United States alone would last this country more than 2,000,000 years at our current rate of consumption. The entire known world reserve of coal amounts to 560,000,000,000,000 tons, enough to last the whole world for at least 250 years at current consumption rates.
In the case of petroleum, there have been twenty-five serious forecasts of world petroleum reserves made since 1942. By 1980, six of these estimates had already been exceeded by known, proven, and producing petroleum fields discovered since the forecasts were made. It's currently estimated that the world has 4,000,000,000,000 barrels of petroleum, a ten-fold increase in forecast reserves over the last thirty-five years. At the current rate of consumption, this currently estimated reserve will last until 2150 A.D.
The United States alone has 11,900,000,000,000 cubic feet of natural gas in reserve, enough to last at current consumption rates for another 196 years.
However, the current rate of fossil fuel consumption is increasing in perfect agreement with Coon's energy thesis. Even though this increase in consumption rate means a quicker end to the Earth's fossil fuel reserves, we can forget about running out of fossil fuels because over the next fifty to seventy-five years the human race will not be using coal, petroleum, or natural gas for fuel. People will be conserving and recycling these precious non-renewable resources for chemical feedstocks that can be used over and over again.
Where will we get the energy we need if not from these fossil fuels?
From where that fossil energy came from in the first place.
We will get the energy from the Sun.
And we will get that solar energy in space.
We will have to get it in space because of the manifold problems of getting enough of it here on the surface of the Earth.
With a solar constant of 1.94 calories per square centimeter per minute, solar energy is diffuse. It takes a large solar collector to bring together enough energy to be of either domestic or industrial use. An increasing number of buildings in the United States are being heated or cooled by solar energy, and this requires that they have a very large area of solar collectors. These large collectors are expensive because they require structural bracing and support against Earth's gravity. In addition, because human beings have voluntarily crowded themselves together in densely-populated centers called ' 'cities,'' there isn't very much surface area available for these large collectors near the place where the solar energy is to be used.
One other major problem faces earthbound solar energy technology: the sun doesn't shine all the time on a given location in the densely-populated temperate zones. Therefore, some manner of energy storage must be used to make the solar energy collected during daylight hours available when it's needed at night. This in turn means larger solar collectors because the collectors must bring together 100% more energy beyond immediate demand during the day so that this excess can be stored for nighttime use. Probably the best widely-available heat storage material is water, which creates another problem because sunlight is most abundantly available in locations where water usually isn't: the deserts of the world.
These many problems of harnessing solar energy on the Earth's surface are engineering problems, and they will be solved. They require the development of known technology, not the invention of new technology. However, because of the Earth's gravity which places limitations on very large structures and because of the Earth's rotation which creates the day-night cycle, solar energy usage on Earth in the foreseeable future is likely to be confined to small, decentralized, local applications. In this regard, terrestrial solar power offers some promise in the area of decentralization, which is certainly an admirable goal. It is particularly useful in the Third World nations whose energy needs may be too small to justify the large decentralized energy collection, generation, and distribution systems. Decentralized terrestrial solar energy technology will be useful in the isolated regions of the most highly developed countries, too.
Decentralized terrestrial solar energy technology can't provide us with enough energy to satisfy the demands of our growing planetary culture and its social institutions, however. In order to meet the 1978 demand for heat energy obtained from coal and petroleum alone, one would need to cover over a thousand square miles of the Earth's surface with solar collectors at various points in order to cover the contingencies of bad weather plus the day-night cycle of solar energy. We need that land for other purposes: people, animals, plants, and water storage.
Although terrestrial-based solar energy collection can't do the job all by itself, there's a possibility that it can be augmented or assisted by the use of devices located in space around the Earth.
The simplest apparent approach to this is the use of mirrors.
For many years, one of the pioneer space thinkers and planners, Dr. Krafft A. Ehricke, has been considering (among other things) a system called "Soletta" or "little sun." A soletta unit is a collection of large mirrors built in orbit and reflecting the raw sunlight of space to the surface of the Earth below. A soletta unit would be large. In order to reflect to Earth 50% of the daytime solar intensity, it would be approximately 650 meters in diameter. A cluster of three soletta units would be required in order to reflect 50% daytime illumination onto an area of approximately 2700 square kilometers (1047 square miles) on the Earth.
The soletta illumination could be used for photosynthesis enhancement—providing an environment of constant sunlight in which plants would grow, thus theoretically increasing crop yields. Soletta illumination would also provide a means for operating solar collectors on an around-the-clock schedule to provide a constant source of heat or electrical power derived from photovoltaic cells (solar cells). The problem is the cost of doing this—approximately 80 billion 1980 dollars per soletta system. This cost is high when compared to other space power systems.
Another alternate space power system that has been considered is the location of fast breeder nuclear reactors in orbit to eliminate any possible safety hazard of operating them on Earth.
But by far the most promising space power system under consideration in 1981 is the Solar Power Satellite (SPS) invented and patented in 1968 by Dr. Peter E. Glaser of Arthur D. Little, Inc., Cambridge, Massachusetts.
The SPS offers an elegant solution to the problem of getting diffuse solar energy from space to the Earth's surface with an energy density high enough to be economical.
As we've seen, solar energy is diffuse—1.94 calories per square centimeter at the Earth's orbit. Furthermore, on the Earth's surface the sun doesn't shine constantly. The SPS takes advantage of the fact that the sun shines continuously in Earth orbit—except for brief occasions when an SPS would be momentarily in the Earth's shadow. And the SPS concept offers at least two "buckets" for getting the solar energy down to Earth in a concentration that's economical.
The orbital segment of an SPS system is the powersat itself. It would be located in "geosynchronous orbit" (GSO or GEO), an orbit 22,400 miles above the Earth's equator where a satellite goes around the Earth once every 24 hours, thus appearing to stand still in the sky when seen from the Earth below. Many communications and weather satellites are currently positioned in "geosynch" orbit.
The advantages of locating a powersat in geosynch orbit should be obvious: since the powersat appears to stand still in the sky, a ground antenna working with the satellite can be positioned and locked into alignment without having all the technical problems involved with tracking the satellite across the sky.
The type of powersat that appears to be most economical and most technically feasible at this time would be made up mostly of huge arrays of photovoltaic cells or ' 'solar batteries" that are very similar to the type you can buy for experimental purposes in most radio/electronic stores.
The photovoltaic powersat converts sunlight directly to electricity by the use of solar cells made either from silicon or from gallium arsenide—two possibilities for use of different solar cells types exist with this design.
Another type of powersat that has been studied would concentrate solar energy by means of huge mirror arrays to boil a working fluid. The thermal cycle powersat—so-named because it uses a thermodynamic cycle in the making of electricity from heat—would require the use of turboelectric generators to convert heat energy into electric energy in a manner similar to the way it's done at most coal-fired or oil-fired electric plants on Earth. However, the thermal cycle is a closed system that recycles the working fluid rather than exhausting it into the environment as is done on Earth.
A photovoltaic or thermal cycle powersat will be big. The Baseline Reference System used by the U.S. Department of Energy in its 1978-1980 SPS design study would produce 5,000,000,000 watts of electricity—that's five billion watts or, in technical shorthand, "five gigawatts." For comparison, Hoover Dam on the Colorado River can produce a maximum of 1.835 gigawatts (Gw) of electricity.
A five gigawatt SPS powersat would have a solar collection array 10.5 kilometers (6.5 miles) long and 5.25 kilometers (3.42 miles) wide having an area of 55.125 square kilometers (22.23 square miles). The thickness of this structure would be only about 500 meters (310 feet), making its thickness to area ratio thinner than the sheet of paper of this page. It can be constructed and it can maintain its structural integrity because it will be in the weightless condition of orbit.
The structure would be fabricated of graphite-reinforced composite plastic beams and girders formed in space by a "beam builder" device using raw materials brought up from Earth by Earth-to-orbit cargo rockets.
On one end of the array—in the middle of some designs other than the DOE baseline design—is the power transmitter. Two methods are under consideration for the transmission of electric power from the powersat array to the ground. One method would convert the array-generated electricity of the powersat into a high-energy laser beam. The other method which is considered as part of the DOE baseline system would convert the electricity into a radio beam. The radio frequency selected by DOE (but not yet approved by international treaties which allocate the electromagnetic frequency spectrum) is 2.45 gigaHertz up in the electromagnetic frequencies utilized by radars.
Either the laser beam or the radio beam would be aimed toward a ground-based receiving unit. In the case of the radio beam power transmission method, this is a rectifying antenna called a "rectenna."
If the rectenna were located at the Earth's equator directly under the powersat whose power beam it was receiving, a rectenna would be approximately 10 kilometers (6.2 miles) in diameter.
The size of the ground rectenna is based upon (a) the acceptable level of radio energy required to insure the safety and health of living organisms on the ground around and under the rectenna, and (b) an acceptable level of energy density in the power beam that will not cause beam/atmosphere interactions or radio-frequency interference. The bigger the rectenna, the more diffuse the radio frequency (r-f) energy in the power beam. The design guidelines for the rectenna were set by DOE at 23 milliwatts per square centimeter at the center of the rectenna and one milliwatt per square centimeter at the edge.
The energy in the middle of the radio power beam at the surface of the Earth is orders of magnitude below anything that will cause harm to living beings or the environment. This statement is based upon proven fact.
The accepted United States standard for r-f energy safety for human beings is 10 milliwatts per square centimeter, a figure that was arrived at with great care as a result of data accumulated for nearly half a century. However, some recent experimentation indicates that the beam energy density at the center of the rectenna could be safely increased to as much as 40 milliwatts per square centimeter without incurring either safety hazards or ecological damage.
The "spill-over" radio energy from the edge of the rectenna will be concentrated in "side lobes," but the energy density in the first and most powerful side lobe located approximately 8 kilometers (5 miles) from the center of the rectenna has an energy density of less than 0.8 milliwatts per square centimeter—more than a hundred times less than the U.S. safety standard.
A small prototype of the rectenna was tested at the Jet Propulsion Laboratory (JPL) of NASA in 1975. Tests conducted with the Arricebo radio telescope in Puerto Rico have shown that there would be no significant affects on the Earth's atmosphere—non-linear self-focusing instabilities and thermal runaway conditions—caused by the passage of a radio power beam of the size and energy level planned for a powersat.
The energy density in the power beam for rectennas not located on the Earth's equator is going to be less because in the temperate zones at, say, 35-degrees North latitude, the rectenna is tipped with respect to the beam and therefore occupies a larger area because the circular power beam has an elliptical footprint on the Earth's surface at these higher latitudes. Thus, a rectenna built in the southwestern United States would have an elliptical ground dimension of, say, 10 kilometers (6.2 miles) wide east-to-west by 17.4 kilometers (10.8 miles) long north-to-south.
The reason why radio power beams are being considered over laser power beams is the greater efficiency of the radio power beam transmission concept. Laser power transmission suffers from losses due to distances and to interferences with the Earth's atmosphere because the laser frequency will, of necessity, have to be in the infra-red portion of the spectrum rather than in the visible light portion, the power beam must be able to get through to the rectenna even if there's a cloud cover over the rectenna, and the power beam must not interact with this atmospheric moisture.
The feature of the r-f power beam that permits it to be closely controlled, directed, and concentrated is the ability to "phase-lock" the beam. This is a technical term meaning that all the transmission elements radiate r-f in a precise split-second sequence from the powersat. The phase-locked power beam can thus be directed to the rectenna 22,400 miles or more away with the greatest precision. It assures not only the maximum efficiency and least degree of loss within the power beam, but also permits the direction of the beam to be controlled.
The power beam would be further controlled by a ' 'pilot beam" transmitted up from the rectenna. This pilot beam is a coded radio signal to the powersat telling the control circuits in the powersat beam transmitter where to direct the beam and how to control the phase-locking. If the power beam wanders slightly away from the rectenna, radio frequency sensors would detect this, tell the pilot beam control circuits, and thus cause the power beam to be directed back into confluence with the rectenna. All of this happens within microseconds.
If the pilot beam fails, circuitry in the powersat "defocuses" the power beam or turns it off, thus rendering it harmless by spreading its energy out over a very large area with a power beam density many orders of magnitude below the safety limit.
The rectenna itself would be a collection of structures that would look like roofed sheds with an open steel mesh mounted horizontally on steel framing structures supported by steel columns on concrete footings. Solid-state diode rectifier elements would be mounted directly on the rectenna elements, and DC electricity collected from the rectenna diodes would be carried to a central point where the electrical energy could be converted from DC to whatever the local electrical power grid standard happens to be—in the United States, 22,000 to 330,000 volts at 60 Hertz.
Each powersat in orbit would have its dedicated rectenna which would in turn have a coded pilot beam that would be recognized only by the proper powersat.
The overall efficiency of the SPS system is about 7%, and this is almost totally determined by the efficiency of the solar cells used in the powersat. The radio power beam transmission system has an efficiency of 63% from the point where the electricity is taken off the bus bars of the powersat solar array to the point where the electricity is delivered to the local electric power grid at the rectenna output.
It's a very good bet that more highly efficient solar cells will be developed in the future, thereby permitting a significant increase in the efficiency of the overall SPS system.
Even at an overall system efficiency of 7%, the SPS system doesn't fare badly in comparison with other electrical energy generation systems currently used on Earth.
The DOE Baseline SPS System was predicated on 1990 technology, which means that there are still a few technical items that haven't been tested yet. But there is no deep secret of nature that needs to be uncovered before an SPS system can work. We need only to test and develop portions of the system that are new to determine the best way to make them or operate them most efficiently.
The rectenna system element based on Earth is simple. Most of it can be prefabricated. It can be assembled, erected, and put on-line with relatively unskilled labor.
The powersat itself requires high technology, new elements, and the construction and operation of a large space transportation system in order to lift the materials from Earth to orbit, fabricate them in orbit, and assemble the powersat.
The first pilot powersat will be the biggest engineering job attempted in space thus far and the construction of the SPS system will rank with the greatest engineering tasks of all time.
The first SPS unit will be the most difficult to build and the most expensive, because that's where all the mistakes will be made. Once the first one is on line, it will be possible with the space transportation system in place and the facilities available to construct two 10-gigawatt powersats per year in Earth orbit.
Big numbers impress only small people, and engineers deal with big numbers in their everyday work. Therefore, it will come as a surprise to most non-technical people to leam that a single 10-gigawatt powersat will weigh more than 38,000,000 kilograms (83,800,000 pounds or about 42 thousand tons). This isn't even as heavy as one of today's oil tankers and less than a Great Lakes ore carrier ship. It will come as a surprise to non-technical readers who are still amazed and astounded at the engineers' ability to put a one-ton satellite in orbit. Many readers may remember the dark days of 1958 when the launching of the 38-pound Explorer-I was hailed with delight as the first U.S. earth satellite. Well, things have come a little ways since that day . . .
To lift all of this mass into geosynch orbit will require a very large space transportation system, and that's one of the great spinoff benefits of building an SPS system. We'll get more than an answer to our energy needs; we'll also open the door to the solar system in the process.