All of the many methods of catching sunlight suggested thus far have been found to be too complicated, too inefficient, or too expensive to be practical for industrial use. Most new-fledged solar energy hunters are smitten with the idea of concentrating sunlight with mirrors. A horsepower per square yard looks very attractive as an inflow of power. Let's put up a reflector 10 yards on a side and concentrate 100 horsepower to run a steam engine! But large mirrors are costly and fragile, must be kept clean and free of dust, and must be turned to hold the sun's image still as the earth rotates. A boiler with all the needed gadgets and a mirror large enough to produce only 2 horsepower on a sunny day costs about $1000.
In India, a government scientific agency has put on the market, at $14, a simple solar‑operated cookstove. This has a mirror about one yard square which concentrates energy on a pressure cooker. The cook need only move the mirror or the pot occasionally to keep the sun's image on it. Using this device may result in saving for fertilizer much of the cow dung now used for fuel, but if enough of this can be saved to enable eucalyptus trees to grow effectively, it may be found cheaper in the long run to let the trees store solar energy, and then burn their wood for cooking.
The trouble with large installations for solar power is that they cost too much to build and to keep up. The solar energy falling on a square mile in a day is worth $200,000 at present power rates. At 5 per cent efficiency of conversion this would give a daily income of $10,000. However, any apparatus yet suggested to capture and convert this much energy would cost more than $50 million a square mile, and the interest on this amount would be more than $10,000 a day. Keeping the glass of the mirrors used shiny and in repair would also be expensive.
There is some hope that better methods of conversion can be found, especially those like the thermopile and the photovoltaic cell, which convert solar radiation directly into electrical power. Greatly encouraging was the announcement in 1953 by scientists of the Bell Telephone Laboratories that a new cell made of thin strips of specially treated silicon gave about 50 watts per square yard when exposed to sunlight. This efficiency of 4 per cent has since been increased to 8 per cent. Though such solar batteries are not likely to be used as an industrial source of power until they can be made more cheaply, in this direction lies hope.
We may be on the verge of using solar energy far more widely for heating our homes and hot water for domestic use. Solar houses have been operated through the winter even in New England with only a 10 per cent addition of furnace heat. For such applications sunlight need not be concentrated, but can be allowed to fall directly on blackened metal absorbers on a house roof, tilted at the best angle to soak up heat during the winter months. These boxes must be well insulated, and covered with one or more layers of very clear glass to act as a heat trap, like a greenhouse. A non‑freezing water solution is circulated under the blackened surface of the collector, and carries the heat to an insulated storage bin and thence as needed to radiators. Such collectors now cost about $2 a square foot; if their cost could be cut in half, solar heating would become very attractive, and larger installations to generate power from solar energy might become feasible.
As much as 20 tons of water, gravel, or a solution of chemical salts is needed to store enough energy to heat a house for a single day, and at least 5 per cent of the useful space of the building must be devoted to insulated heat storage bins. But if at least a ten‑day supply of energy cannot be counted on, central heating must be provided for stand‑by use in cold and cloudy weather. This doubles the cost of a solar house‑healing plant. This extra expense may be justified if the standard heater is of the heat‑pump type, which can be used also for cooling in summer, or even if electric air‑conditioning and heating are used.
Nature's solution of the solar energy storage problem, photosynthesis in the cells of plants, though very inefficient is effective, for it stores energy in chemical form where it will stay indefinitely in the elastic insides of molecules. It has been suggested that furnaces could be run on algae or other simple plants grown for fuel. This would be a ridiculous waste of effort, involving building up at great expense very complicated molecules to do a job that much simpler molecules could do more effectively. Stoking stoves with starch or sugar is as silly as it sounds.
Energy from the sun, though it comes from the nuclei of atoms, by the time it reaches the earth has been made diffuse enough to be safely fed to the delicate complex molecules from which plants and later animals are formed. When nuclear energy is released on earth, it is concentrated and intense, and to gentle it we must control great quantities of dangerous rays. It is natural then for the scientist to plan to use nuclear energy in those cases where he must have high temperatures, pressures, and energy concentrations, and to use solar energy where gentle diffuse actions are required. For the ordinary purposes of industry, solar energy needs concentrating, while nuclear energy needs diffusing. At the moment, scientists are getting on faster with the latter, but both will have great importance in man's future.
What is this thing called energy, which man can use for his destruction or to set himself ever freer from control by his environment? We recognize it in many forms, as light or heat or sound or electric power, but all have in common the capacity for doing work. They are merely different external manifestations of three basic forms that exist in the realm of protons, neutrons, and electrons: energy resulting from electrical, magnetic, and gravitational forces.
The deeper we dig into the structure of matter, the greater the amounts of energy we find. The events of the world we contact with our senses are only leftovers, which result from residual forces not balanced out on the more fundamental levels of matter. We have seen that protons combine with neutrons in the nucleus with forces measured in millions of volts. Forces corresponding to a few thousands of volts remain unbalanced, and with these the nucleus collects a family of electrons to form an atom. With still smaller residual forces, from a dozen volts down, these atoms cluster into molecules and crystals. It is the rest of this remnant of a remainder, measured, in thousandths of a volt for each material, that determines whether these molecules shall form "shoes or ships or sealing wax"; and "cabbages and kings" are formed by still more subtle residues.
The flowing of a waterfall, the heating of a house, and the hitting of an enemy with a club utilize only minor routine residues of energy. With the coming of the Industrial Age men learned how to bring the energy of the molecular world, through chemical reactions, directly to bear on mechanical problems, at the same time learning better how to harness the older residual forms of energy. With gunpowder, and later gasoline, they were able to release energy in much more concentrated form and in greater quantity. In the early years of this century scientists sensed the even more concentrated and vaster supply of energy in the nucleus of the atom, and in 1942 they learned how to release this energy directly.
Mastery of the nucleus will mark a much bigger step in man's ability to control his environment than any he has taken before, for it gives him the opportunity to be at home on more fundamental levels of the material world, instead of merely working in the outskirts. Calling the release of nuclear energy a new Promethean fire" is more than just a metaphor or an analogy. The new fire of atoms presents to man an opportunity similar to that he received when he first drew back a burned finger from the fire of molecules, now raised in intensity to a hidden but transcendent power.
When nuclear fission occurs, only about one thousandth of the energy content of an atom is released, and scientists know how to set this energy free from only a few of the heavier kinds of natural atoms. But with the processes of nuclear fusion, up to ten times as much of an atom's energy can be released. In one of its simplest forms, fusion involves the combination of two protons with two neutrons to form the nucleus of a helium atom. The sun gets most of its energy in this way, and most matter in the universe is probably thus formed as a first step. Though the sun is an excellent hydrogen fusion reactor, it is early yet to decide whether we can ever have on earth a small one so well controlled ‑‑ one that will operate at a strong simmer instead of exploding in a millionth of a second, or that, need not he kept a thousand miles away to be of any use, If the speed, instead of merely the extent, of a thermonuclear reaction can ever be controlled, water may well furnish a superfuel better than any of those of which men have dreamed.
Inventors who have tried to develop pills that would make water burn were on the wrong track, for water is already ashes of hydrogen. When hydrogen is burned with oxygen their molecules combine to produce water, and release much chemical energy in the flame. The water remains after the energy is dissipated, and new energy must be used to "unburn" the water as plants do in photosynthesis, or as electric power can be made to do in a hydrogen generator. But if protons are collected from the hydrogen in water and then assembled with neutrons to produce helium nuclei, a vastly greater release of energy results ‑‑ energy from nearer the base of the universal supply. At present no one appears to know how to control this reaction beyond setting it off with an atomic bomb as a primer, thus raising nuclear temperatures to those needed to start a fusion reaction. If this process could be slowed down and controlled, the oceans would provide an inexhaustible reservoir of energy, and man would never need to worry about power again. But he has much to learn before this can come about.
Of equal importance to man's rapidly improving ability to control energy of the most intense forms, like that of the multibillion‑volt cosmic rays, is his increasing mastery of the most subtle forces, those that hold atoms together in complex living molecules. When these forces can be better controlled the physical hungers of all humanity may well be filled by further gentling of the energies of the atom bomb into those of food and warmth. Then will it become apparent that energy from the atom, far from being the evil creation of a few clever but dangerous men, is a beneficent force of nature that has been lying in wait since the beginning of time, until man could awaken to awareness of its availability and learn properly to use and control it.