May / June 2007
Global warming is now almost universally accepted as a serious problem caused by human activity – mainly burning fossil fuels – that demands strong remedial action as soon as possible. Past events, such as the temporary boycott by some of the major petroleum producers in the ’70s, showed that the US also has a national security problem related to both price and availability of one of our main energy sources. This note is intended as a contribution to the effort to devise a comprehensive solution to all aspects of the energy problem.
Many others have also recognized various aspects of the problem and the need for a rapid response. I have found that most workers in this field have not completely defined the problem, but nevertheless have some favorite solutions to be exclusively pursued.
When I began my engineering education long ago, I was lucky enough to have had the tutelage of experienced engineers, not scientists. They all said (preached, actually) that the indispensable first step in devising a solution in the real world was to define the problem.
What is the energy problem? It has several parts.
In the early ’70s, the temporary boycott of the world market by OPEC caused the price of petroleum to rise dramatically, as petroleum is the most common source of energy used in heat generation, production, commerce, transportation, and residential facilities. (1) The ability of major petroleum producers to withhold the supply reveals the importance of energy independence and price.
(2) More recently, global warming has become unmistakably important with widespread melting of ice, noticeable climate changes, and rising sea levels. This is now recognized by nearly everyone as caused by greenhouse gases, mainly carbon dioxide, produced by burning fossil fuels such as petroleum, coal, and natural gas. While nuclear power plants are being advocated by some, dealing with spent nuclear fuel is as problematic as greenhouse gases, and energy must be used to produce nuclear fuel. Note there is now a worldwide shortage of nuclear fuel.
Others are pushing ethanol, which is such a bad idea that it is hard to understand how its use has become as widespread as it has.
Ethanol’s production consumes nearly as much energy as it provides, and its use generates greenhouse gas. With only about 1% of gasoline now replaced by ethanol, some growers of corn have become rich, but many growers of domestic animals for food are in dire straits because of the unanticipated rise in the price of feed corn.
Solar power, wind power, hydroelectric power, nuclear power, hydrogen power, methane from buried organic material, and other renewable power sources are advocated by some, but so far, no solution has been proposed that would be both affordable and complete. The purpose of this paper is to propose such a complete solution, the development of which requires only resources that we already have in abundance.
Unless, by some miracle, we find a substitute for petroleum fuel that can be used with the same technology we use today, takes no energy to produce, has no noxious residue, and has no unexpected consequences (like raising the price of corn) its adoption will require rebuilding our entire energy infrastructure. This will be neither easy nor cheap, but if we hope to preserve the Earth for our descendents, we have no choice but to act now. This will involve diverting manpower and funds from current uses. If we examine how these resources are now being used, military applications will be found high on the list. Many of us believe that such diversions would make our world a better place in which to live. The decisions, of course, will be political, which is beyond the scope of this short paper.
Though expensive to build, the proposed system, which abandons fossil fuels, should be cheap to operate, as the fuel, which is sunlight, has no operating cost.
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All the energy the earth has stored and almost all of the energy it receives every day comes from the Sun. About 89,000 terawatts (1 TW = a million million (quadrillion) watts) falls on the Earth, while total usage (in 2004) was only 15 terawatts, of which 87% was provided by fossil fuels. Their use produces most of the global warming that has become so obvious. If we were to get most of our useable energy from the Sun, we would solve many of the most important problems, including the price and availability of petroleum as well as (3) the noxious by-products associated with using nuclear power and fossil fuels. (4) Relying on the Sun rather than petroleum would also permit us to be much less involved with events in the Middle East. (Anybody who does not realize how advantageous this would be is urged to read Seymour Hersh’s “Annals of National Security” in The New Yorker of 5 March 2007.)
Cleaning carbon dioxide (and other greenhouse gases such as water vapor) from the Earth’s current atmosphere is not one of my fields of expertise, but greatly reducing the rate at which we increase it is clearly a good idea. (Perhaps we shall discover that if we stop adding these gases to the atmosphere, the existing unwanted gases will slowly dissipate.) A way to do this is to move to an electrical economy, producing electricity from sunlight, and then replacing as much of other fuels as possible by electricity. There is cost associated with this, but mostly new technology is not required. The one field in which this is not yet completely possible is transportation, where better batteries (or their functional equivalent) are needed. Fortunately, we still have a lot of competence in developing new technology, in spite of losing a good part of our manufacturing skills. (A very promising battery project is underway at MIT.)
Solar power at present is faulted for being available only during clear days, for requiring expensive solar cells of limited efficiency and life, and for not having enough space for the receptors in crowded areas such as cities. This proposal concentrates on dealing with these issues.
When I was teaching in India in the ’60s, I learned that some irrigation pumps were solar-powered without using any electrical components. Small collectors concentrated sunlight sufficiently to produce steam of high enough temperature and pressure to operate water pumps. (The motivation was that pilferage of electrical components, even copper wire, was then a problem in the outlying areas where the apparatus was often located.) This idea is one of the elements in the proposal.
The other [idea] is to collect the sunlight on large steerable, focusable mirrors in geostationary orbit that would direct the reflected light onto much smaller receptors on the ground.
(The orbits would be inclined so that the mirrors would never be in the shadow of the earth.) Initially, the receptors would be located near existing hydroelectric plants, where solar-powered pumps would be used to move water up into the lake(s) behind the dam(s) for energy storage. At NASA, we have the skills to develop such devices as the mirrors and perhaps even have the money if we give up such projects as the space station, which produce no noticeable benefits for mankind. Should the initial installations prove workable, new plants could be built in more remote locations.
Solar power like the kind I saw in India is still used to some extent in the U.S. Heating of swimming pools seems to be the largest application. Some is used for domestic hot water and some for space heating. Numerous small companies are in the business of making and selling the collectors and the receptors for the various applications. The same is true today in India.
The orbiting mirrors would be, perhaps, a mile in diameter. They would be constructed as transparent inflatable thin balloons, one of the inside surfaces of which would be aluminized to provide the reflecting surface for the required concave mirror. The mirrors would be lifted into orbit while folded, the inflated shape being determined by the thickness of the plastic or other material and by the pressure. It is likely that spherical reflectors would be adequate, and the focal length could be adjusted by the pressure, thus avoiding high precision in their manufacture. Communication satellites already use slanted orbits and incorporate sufficiently accurate steering mechanisms.
Note that since the Sun apparently moves through the sky while the mirror apparently remains fixed to viewers on the Earth, the angle of incidence of the sunlight on the mirror changes. Thus the mirror must be constantly redirected. This is preferably done by using feedback from small sensors located around the edge of the mirror to the steering mechanism of the satellite carrying the mirror. These same sensors can also be used to adjust the focal length of the concave reflector by adjusting the air pressure inside the plastic balloon so that the incident beam just fills the receptor surface.
At the surface of the Earth, incoming solar radiation in clear weather averages something over 300 watts/sq. meter, but it is much higher and nearly constant above the atmosphere. Measurements show the “solar constant” to be about 1366 watts/sq. meter.
A reflector about 5000 feet in diameter thus collects about 3000 megawatts, which is comparable to the capacity of a typical terrestrial electric power plant.
I am guessing that collectors might be 500 feet in diameter, but this must be verified. The fraction of the collected power that would be received by the collectors depends on the weather, and the fraction of that which becomes useful heat to make steam and drive pumps remains to be seen.
Close to populated areas, it may be necessary to stop the transmission at night. For these reasons, storage of the collected energy is essential, which makes the use of dams holding pumped water a vital part of these systems. The ability to defocus the mirrors is also important.
One of the reasons for using the solar energy directly to produce steam and drive pumps is that solar electric cells, besides being expensive, are not very efficient in converting light into electricity, and need replacement from time to time. At best, the efficiency is about 20%, the rest of the light energy appearing as heat, which limits the intensity of light that can be handled. There is no such limitation when converting the incoming power into steam, but there probably are some limitations from safety considerations. However the efficiency is surely higher than that of solar cells.
It has been known for some time that thousands of pieces of debris, some very large but most very small, abandoned from previous launches, are in orbit around the Earth. Some objects that have been returned, such as shuttle vehicles, have been found to have suffered minor damage from impact with small pieces. This raises concern for us, since the mirrors we propose to place in orbit are actually quite fragile. Fortunately, almost all space junk is in much lower orbit, where it will eventually burn up as it enters the Earth’s atmosphere.
There are two possible approaches to deal with this problem. One is to make the mirrors less fragile by abandoning the balloon approach and providing a structure to support a single-surface properly shaped mirror. The other is to provide redundancy by placing two or more mirrors in orbit for each receiving location on the ground. The balloon approach is very attractive because it enables focus to be controlled by pressure, rather than making and then placing in orbit a very precise mirror.
Although the redundancy approach seems better to me, my inclination is to leave the final decisions to the engineers who will do the actual design, hopefully from NASA.
This proposal need not be the only scheme used. Higher efficiency in systems that do burn carbon-containing fuels would lessen, but not eliminate contamination of the atmosphere. Conservation, wind power, tidal power, and any other schemes that do not burn fossil or carbon-containing fuels may also be used. I have no special knowledge about hydrogen fuel cells, except to note that water vapor is also a greenhouse gas. Carbon sequestration seems to involve significant new technology and does not free us from the grip of OPEC.
Many of the numbers used here are from Wikipedia, “World energy resources and consumption.” en.wikipedia.org/wiki/Energy:_world_resources_and_consumption
This piece also has a very good list of additional references. It is well written and apparently accurate. However it uses the words “energy” and “power” as synonyms in many instances, much to the discomfort of technically trained persons, such as myself. In this paper, I have used these two terms only in their technical sense. Power (typical unit is watt) is the rate of providing energy (typical units are BTU – British thermal units – or joules).
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