Copyright 1995 by Ralph Nansen, reproduced with permission
Table of Contents
Chapter 13: Features of the Energy Beam
The miracle of the solar power satellite energy system is built around the concept of transmitting huge amounts of energy over thousands of miles without the use of wires. Wireless power transmission has been the dream of many people, but today technology is making it happen. The energy beam has no moving parts, it cannot be seen, it will pass effortlessly through the atmosphere and clouds, it will be very large to keep the energy density low, it will be safe, it will be environmentally clean, and it will be an efficient transmitter of energy.
As you saw on our tour of the satellite the transmitter is a flat phased-array antenna. The radio-frequency energy is distributed over the face of the antenna in slotted wave guides that allow the energy to be radiated into space in a tightly controlled beam. Even though it is mechanically pointed toward the earth, the final steering of the beam is accomplished electronically by controlling the phasing of the radio waves across the face of the antenna to guide it to the receiver on the earth. The actual controlling signals would come from the receiver site on the ground to keep the beam always pointing precisely at the receiving rectenna on the earth.
Safety of the Energy Beam Comes First
There are several important considerations to take into account when designing a wireless power transmission system for solar power satellites. The very first is safety. In order to ensure a safe environment on the earth the energy density is limited to a level that would be safe for all life forms. In the original studies an additional energy density limit was established to prevent excessive heating of the ionosphere. The maximum limit as initially established was 23 milliwatts per square centimeter, which is 230 watts per square meter. This level is one fourth of the energy in bright sunlight on the earth.
The next consideration is the selection of the best radio frequency to use. The most important factors are to select a frequency that will have minimum reaction with the atmosphere and one that will not interfere with wireless communication systems. Radio frequency bands are used for many purposes. They include broadcast radio, television, marine radios, commercial radios such as those used by taxicabs, police radios, military radios, radars, energy systems such as household microwave ovens, medical diathermy machines, industrial dryers, telephone systems such as communication satellites and cellular telephones, and amateur radio. Frequency allocation is made by the federal government in accordance with international agreements with other countries. Without control there would be chaos with users interfering with each other.
The frequency selected for the wireless energy transmitter is in the Industrial Scientific & Medical (ISM) band. The specific frequency selected was 2,450 megahertz, which is the same frequency that is used in microwave ovens and medical diathermy equipment. The atmosphere is nearly transparent at this frequency. An added advantage is the broad experience base developed through building and using huge quantities of radio frequency energy generators at this frequency. Another frequency that could be used is 5,800 megahertz, but it is subject to more atmospheric interference.
After selecting the frequency and the maximum energy density in the center of the beam as it reaches the earth, the engineers are then able to determine how big the transmitting antenna and the receiving rectenna must be. Radio-frequency beams follow fundamental laws of physics, with the key controlling factors being the distance the beam travels, its frequency, and the size of the transmitting antenna. These factors define the beam size and are independent of the how much energy is transmitted. The amount of energy transmitted then determines the magnitude of the energy density.
With geosynchronous orbit as the satellite location and 2,450 megahertz as the transmitter frequency, that leaves only one variable, the size of the transmitting antenna, to determine the size of the beam when it reaches the earth. If we were to reduce the size of the transmitting antenna, the beam would become larger on the earth and the energy density would be reduced. Conversely, in order to make it smaller and more concentrated on the earth the transmitter must be larger on the satellite.
After selecting the transmitter diameter, the antenna designer’s job is to tailor the profile of the energy density in the beam from the center to the outer edge. At the same time they would decrease the side-lobe energy. The side-lobes contain the energy that forms as concentric rings outside of the main beam. They are at a very low level, and the better the antenna design the less energy they contain.
The diameter of the satellite transmitter for a 5,000 megawatt output satellite was established at one kilometer (0.62 miles) in diameter in order to maintain a maximum of 230 watts per square meter of energy density at the center of the beam. This very low level was selected for all the studies to provide consistency of results. However, later tests by the government using the huge radio telescope at Arecibo in Puerto Rico demonstrated that excessive heating of the ionosphere would not occur, so that limit can be relaxed.
During the late 1970s a large number of environmental studies were conducted under the auspices of the Environmental Protection Agency to determine the effects of radio-frequency energy on life forms. After the termination of the satellite studies in 1980 many of these studies continued since there are so many uses of radio frequency energy. The US Air Force was particularly interested because of the great number of high-power radio and radar systems they operate. As a result they have been the chief sponsors of the recent studies. I had the opportunity to attend the First Annual Wireless Power Transmission Conference (WPT ‘93) in San Antonio, Texas, where an entire session was devoted to reports on the findings of these studies. They covered experiments at the biological cell level to determine if there were any carcinogenic effects that would cause mutation and cancer. They included investigation of higher life forms up to and including apes.
The findings of all the studies have been consistent. The only adverse effects found are due to heating when the energy level is high enough to overcome the ability of a cell or organism to reject the excess heat. This level is not reached until the energy density is in the neighborhood of 1,000 watts per square meter or higher. This far exceeds the maximum level of 230 watts per square meter established for all of the solar power satellite studies. The threshold is different for different life forms because of how they absorb the energy and how well they can reject the heat. Past experience with high power radars has shown the most sensitive parts of the human body are the eyes and liver. If we are exposed to high levels for extended periods of time, we could develop cataracts, or in extreme cases the liver could be damaged. There have been a few cases where people exposed to very high-power radar transmitters with energy densities much higher than the level in the energy beam have suffered such effects.
Experiments made at the cell level showed there was absolutely no evidence of carcinogenic effects at the energy levels considered for the wireless energy beam. These experiments were made by scientists who were very familiar with the known effects of ionizing radiation from x-rays and nuclear radiation and from chemical agent exposure. In addition, testing conducted on honey bees and birds have also shown no permanent effects after exposure to energy levels about four times greater than the 230 watts per square meter study limit and minimal effects during exposure to these levels.
The evidence is very good that the wireless energy beam will be totally safe. It is also likely that a level of 500 watts per square meter, about twice the study level, could be used safely. Increasing the allowable energy density to 500 watts per square meter would have definite economic and environmental benefits by reducing the size of the rectenna.
If someone by some chance found themselves on top of the antenna right in the middle of one of these beams, they would not suffer any damage. Bill Brown, the inventor of the concept, has been working with wireless power transmission research since he demonstrated his first working system in 1964. He is now retired from Raytheon but is continuing his work to further develop the system. He has a working demonstration model that he uses to show wireless power transmission in operation. His transmitter is about thirty inches square and has an energy density higher than our original study level of 230 watts per square meter. When he gives his demonstration he has a rectenna with lights that come on and change their brightness in response to the energy pattern they receive from the transmitter on the opposite side of the room. As he gives his talk he casually walks through the beam and the lights go out as his body absorbs the energy. He will stand there for a while as he talks and then as he moves out of the beam the lights come back on. It is a very graphic demonstration that he has done for many years, and he is an elderly gentleman. He has commented that he can just feel a slight warming effect.
So far we have examined the safety of the beam if something went wrong and people were exposed to the maximum level of radio frequency energy in the center of the beam. However, it is very unlikely that this could happen since the beam will always be pointing at the rectenna. The real question to address is the normal everyday environment near the rectenna site.
At the edge of the antenna the maximum energy level will be one watt per square meter or one tenth of the US standard of acceptable exposure to radio-frequency energy. To prevent casual exposure even at this low level, an exclusion fence will be placed outside of this area at the point where the energy level drops below one tenth of a watt per square meter or one hundredth of the US exposure standard. This would be the maximum level people would normally ever be exposed to. Even the side lobes of the beam, which form as concentric rings of ever-decreasing levels of energy around the beam, will not exceed this level.
The Rectenna — The Biggest Opportunity
The satellite (the power generating part of the system) is located in space, but the element that will be of greatest interest to us on the ground will be the receiving antenna. Since it is very large, we must ensure that it can be designed, built, and operated in a manner acceptable to all of us.
Let’s take a look at what the rectenna (receiving antenna) would look like for a satellite like the one we toured. The receiver will convert the radio-frequency beam from the satellite into 5,000 megawatts of electricity. If we want to keep the energy density low and still receive that much power, the beam has to be spread out over a large area. The actual size was determined when we established the maximum energy density at 230 watts per square meter and applied all of the natural laws that apply to radio-wave beams. Using this criteria we ended up with a beam 5.9 miles in diameter when it reaches the earth. This is a variable that can change as the system evolves in the final design phase—in fact, we are currently thinking of reducing the output to 1,000 megawatts, which reduces the beam diameter to two miles.
The profile of the energy in the beam when it reaches the earth would be shaped like a bell. In the middle of the beam the energy is at its maximum. As we move from the center the density falls off rapidly, and by the time we reach the perimeter the density is down to one milliwatt per square centimeter. This is the minimum density that would be economical to recover. For a 5,000-megawatt satellite, the exclusion fence would be another 0.8 miles from the edge of the rectenna; for a 1,000-megawatt satellite, the exclusion fence would be one-third of a mile from the edge of the rectenna.
Because the beam comes from geosynchronous orbit around the equator, it intercepts the earth at an angle in the United States. The angle of the beam would be the same as the degrees of latitude of the receiving site. Because of the angle the rectenna must be oval in shape with the long dimensions running north and south.
In its simplest form, the rectenna would be made up of a wire-mesh back-screen, mounted perpendicular to the beam with antenna elements mounted at regular intervals in front. There are many ways that these elements could be designed, but typical would be small half-wave dipoles mounted a few inches apart with a rectifying diode for each element. The dipoles receive the radio-frequency energy, and the rectifying diodes convert the radio energy directly to electricity.
All of this would be supported on columns and beams above the ground. Because of the angle of the energy beam to the earth, the rectenna can be built in rows with each row of antenna elements inclined towards the satellite, leaving space between each row. It will intercept more than 99% of the microwave energy and convert it directly to electricity by use of diodes, while allowing most of the sunlight to pass through. As a result, it would be possible to use the land underneath the antenna for agriculture.
An ideal location for a large rectenna would be in the desert near Las Vegas, where we find only a few lonely roads and some coyotes, very few people, and nearly level land of very little use today. About the only objections we might find would be from the military for cluttering up their lonely test areas, or from a few old prospectors still looking for the end of the rainbow.
Another site might be further north in a cold, dry area in the west, possibly near Cheyenne, Wyoming. At this site there are rolling hills and grazing land for cattle; hay land along river beds, a few ranches, some back-country roads, and a trail or two. Even though there may not be many inhabitants, they are undoubtedly fiercely independent and will need some powerful persuasion to be willing to leave their land.
These remote sites will disturb few people, but it is going to be difficult to find areas large enough for the receiver sites close to population centers where there is the largest demand for power.
If we are going to be part of the problem-solving team that is developing the system, let’s see how we might approach this obstacle. First let us consider the problems of finding a site that covers about 30 square miles (for 5,000 megawatts) or 6 square miles (for 1,000 megawatts) near a big city where we would like to locate a receiving antenna. Population densities make this a problem. More than likely, when we found a potential area in the northeast it would be covered with farms, a stream or two, some scrub land, several patches of trees, and a few roads. If this is the case, the people who live there will be pretty upset if we suggest moving them from the farms where they grew up and spent their lives just so that New York City can have bright lights. But what if we could offer them a great opportunity for the future and possibly a new place to live and work without having to move?
Even though the amount of land required for the receiving antennas is large, it only takes one fourth the land required by coal strip-mining for an equivalent amount of power. Site selection would be a compromise between the advantages of a remote location and the distance required for the transmission lines to bring electricity to the populated centers. Further development of superconductors would make it possible to place all the rectennas in remote areas since the length of the transmission lines would no longer be a problem. However, with a little change in the design of the receiver, we could have an exciting new concept that would build for the future in more ways than just providing energy.
John J. Olson, an industrial engineer, artist, and designer, let his imagination look at the problem in a very creative way. His resulting idea is fascinating.
What would happen if there were a series of greenhouses with glass roofs made up of wire screen and antenna elements imbedded in heavy glass, permanently tilted toward a satellite in geosynchronous orbit? With a simple modification to the design, the rectenna could become rows of greenhouses covering the entire site of the rectenna. Since the energy in the radio-frequency beam would be converted to electricity with only the sunlight passing through the antenna, we could safely use the area under the glass roof for agriculture. We could install tracks to handle automated farm machinery and build in the capability to collect rain water. Tempered glass would protect the antenna from damage from snow, hail, and the occasional bird that might think a half-wave dipole antenna element was a fat, juicy worm.
With the rectenna modified to become a vast series of greenhouses we will be able to grow ten times as much food as is possible in open farming, using only one tenth as much water—enough food to feed more than a million people. Some crops, such as wheat, do not grow much better in a controlled environment, but most do much better. For example, some crops like cucumbers are phenomenal; they produce about fifty times more in a controlled environment than they do in the open. We could flood the world with pickles! Even “crops” that are not presently practical can be grown in a controlled environment. Did you know that shrimp can be raised that way? Not in the earth, of course, but in controlled environment tanks where their growth rate is spectacular. It would even be possible to grow plants to produce liquid fuel with biomass conversion.
With the rectenna built as greenhouses, instead of taking valuable land out of production, we will be multiplying its production potential. The possibilities are endless. Think of the opportunity to satisfy many of the needs of a populated area with one facility. Providing not only electrical energy, it could also supply most of the food and part of the fuel for a city of several million people. The long-range implications for the world are staggering. It could someday be possible to feed starving people at a level that would mean freedom through abundance. And if, in desert areas adjacent to an ocean, you add a desalinization plant that uses the abundant electricity to produce large quantities of potable water, great fields of greenhouses could turn an arid area such as the Baja Peninsula into a garden spot.
You might ask, “If this is so good, why don’t we build huge greenhouses now?” We do in a few places, but the capital cost of a greenhouse is very high. By the time a farmer borrows the money, builds the greenhouse, buys his equipment, and then pays the mortgage, he just about breaks even. But if we need to build the structure anyway for a receiving antenna, the additional costs of a greenhouse become quite reasonable.
Even the Displaced Will Prosper
Let’s go back and consider the problem of procuring the site near a big eastern city. With the greenhouse option, we can now offer the displaced farmers a chance to keep farming their land, but with greatly enhanced potential. With plentiful low-cost energy and reasonably priced food, the benefits could far outweigh the disadvantages even to those directly affected.
How about those sites out west? That cold weather around Cheyenne could be tamed when we combine the attributes of solar electricity generated in space with direct solar heating on the ground in our greenhouses. The cattle might become very lazy basking in the warmth of their glass-roofed homes eating to their heart’s content while the snow lies three feet deep outside. Even the most independent-minded of ranchers could appreciate the advantages of raising cattle in that environment. Some of the finest tasting beef in the world feeds on the warm luxuriant grass of coconut palm plantations in the tropics of Vanuatu, in the South Pacific.
Meanwhile, down near Las Vegas, they would no longer be gambling with nature when green things start growing out there on the desert in glass houses. Since the amount of water necessary to grow plants in a greenhouse is reduced to 10% of what it takes in the open, even the meager amount available in the desert could be enough to grow crops in places never before possible.
Another exciting possibility would be to grow plants for biomass conversion to liquid or gaseous fuels. Since plants use carbon dioxide to grow, would it be possible that they could use it at the same rate they would later be releasing it in the burning process? Sounds a little bit like perpetual motion. It might even be possible to raise fast-growing plants to provide raw material for the manufacture of paper products and reduce our dependence on rapidly disappearing forests.
Another option for minimizing the impact of land use for the rectenna sites is to enlarge the satellite transmitter and allow the maximum energy density to be increased to 500 watts per square meter. If this were done, the area of the rectenna could be reduced to about 15 square miles for 5,000 megawatts and 3 square miles for 1,000 megawatts. Then we need to challenge the antenna designers to tailor the antenna characteristic to make the profile of the energy density more like a barrel rather than a bell, which would make the energy density more uniform over the area of the rectenna without increasing the maximum level. The result would be another decrease in area required, maybe as small as 10 to 12 square miles for 5,000 megawatts or 2 to 2.5 square miles for 1,000 megawatts. At these sizes the number of available sites would increase dramatically, so that rectennas could be located nearly anywhere.
Sun Power: Chapter 14 Table of Contents