• The reverse reaction must release significant quantities of energy—at least 100 calories per gram if the energy is released as heat. • Ideally, the energy should be released as electrons, rather than heat, to avoid conversion losses.
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RESEARCH
Solar Energy Problems Chemical methods are a promising ovenue of research for storing solar energy for nonsunlight hours A
MAJOR ROADBLOCK o n
the
way
to
large-scale utilization of solar energy is what to use as an energy source in nonsunlight hours. A number of mechanical methods have been suggested. Elevated tanks or reservoirs could be filled with water during the daylight hours by a solar pump and the water released at night, producing hydroelectric power. A variation of ihe same idea: produce power by dropping water into abandoned mine shafts, emptying them hy day with a solar pump. Enormous quantities of water must be stored to give a significant number of kilowatt hours. Still another variant i s pumping air into diving bells during t h e light hours, using the compressed air to turn motors during the dark. But these methods are expensive, and there is another w a y of storing energy—by chemical means, electrochemical or photochemical. I n his Ira Remsen Lecture in Baltimore, University of Wisconsin's Farrington Daniels pointed out some promising avenues of research. To store solar energy electrochemically, the sun's heat would b e used to run a steam turbine to produce electricity. This "solar" electricity could be stored in these ways: • Electrolyze water t o hydrogen and oxygen and utilize the energy of the stored gases in two ways—burned in a heat engine at 2 5 to 30 9fc efficiency, or recombined in a fuel cell in which hydrogen is bubbled at one electrode, oxygen at the other, t o produce electrical energy at an efficiency of 70%. Fuel cells are being studied in the U. S. and Europe but not yet on a large scale. •Produce sodium b y electrolysis. Dump the metal into water. Use the hydrogen released a s an energy source. This might be practical in small units. • Produce aluminum by electrolysis. Use it as one of the electrodes in a battery—a sort of Leclanche cell using aluminum instead of zinc—and recover the energy as electricity and recover the aluminum in the next cycle. The first step involves high temperatures 3250
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and to be economical would have to be done on a large scale. • Make an improved storage battery —cheaper and longer lived. Electrical energy can be stored and released very efficiently in electrochemical storage batteries of small size. The present types meet the demand for heavy drains. They involve a change of phase and in time deteriorate. Electrodes involving oxidation and reduction of polyvalent ions—which do' not involve a change of phase—should be studied. T o be practical, these new batteries would have to be 1/10 the price of present storage batteries, but they would not have to be portable. Any of these methods of storing solar energy may prove effective, but they suffer from this disadvantage: They involve conversion of solar heat first to steam then to electricity, and such conversions are limited by the efficiency of the Carnot cycle: Thermal efficiency =
T —T 2 —l
T 2 is the temperature of the source and T \ the temperature of the receiver. Small engines in use today convert only about 10% or less of heat to useful work, although 30% conversions can be obtained. Photochemical storage, which is not limited by Carnot's efficiency, therefore, holds interesting possibilities, says Daniels, even though a maximum of only 50% of the sun's energy can theoretically be stored in this way (50% of the sun's rays—those in the infrared —do not have enough energy to activate chemical reactions). What is needed is a chemical to store the energy. Daniels outlines these requirements for "Compound X:" • It must be cheap. • It must be colored, so that it absorbs most of the visible sunlight. • This light absorption must produce a heat-absorbing chemical reaction. • The dark, or reverse, exothermic reaction must not occur spontaneously but must give up its energy when desired—by heating, for example.
Difficult as it sounds, finding such a compound is not impossible, Daniels says. It must not absorb too much energy, for then sunlight will not make the reaction go. It must not absorb too little, for then the storage of energy will be insufficient. Chelates, dyes, and fancy inorganics are possibilities. Daniels points out that both hydrogen and oxygen have been obtained by photolysis with ultraviolet light of eerie and cerous perchlorate in water. Research might turn u p other solutes which would decompose water with light of all wave lengths. And, Daniels observes, nature has shown it can be done in chlorophyll, a compound that meets many of the requirements for Compound X. • Encouraging Research. The interest developing in the potentialities of photochemical storage is witnessed by the National Research Council's action in forming a Committee on Photochemical Storage of Energy. Daniels is chairman of the new committee. Other members are Robert Livingston of University of Minnesota, Lawrence Heidt of MIT, and Eugene Rabinowitch of University of Illinois. Other members may be named to the committee. Meanwhile, Daniels, as chairman of the committee, welcomes suggestions for new lines of research to be encouraged.
Henry C. Freimuth ( l e f t ) , chairman of the Maryland Section, presents the Ira Remsen Memorial Lecture Award to Farrington Daniels (right) of University of Wisconsin. * Daniels was introduced by Paul H. Emmett (center), of Johns Hopkins University
