Heat storage in eutectic alloys
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PHASEchange materials (PCMs) are being explored as heat storage media to reduce the costs of energy generation, conversion and distribution systems. At low temperatures, phase changes in Glauber's salt, Na2SO 4 9 10H20, and other salts have been studied for decades as alternatives to heat capacity storage in water or rocks. 1Rising energy costs and attempts to harvest solar radiation have intensified interest in high temperature storage. The suitability of PCM systems for this purpose has been recognized for years.2 However, systematic study of alloys instead of inorganic salts has been limited to very rough calculations on pure metals and a few binary alloys.2,3 This study explores the characteristics that make some alloys better than others. Experimental heat of transformation measurements on promising alloys of the relatively plentiful metals A1, Cu, Mg, Si and Zn provide values for the maximum possible heat storage densities in these alloys. The heat available from a phase transformation carried out reversibly at constant temperature and pressure is the enthalpy change, which is equal to the transformation temperature multiplied by the entropy change. A sample calculation applied to the fusion of an average close-packed metal illustrates the advantage of a PCM over heat capacity storage. The entropy of fusion of a normal fee or hcp metal is shown to be about 10joules per gram atom per kelvin in Fig. 1.4 The molar heat capacities are not so uniform, but average about 30joules per gram atom per kelvin. Fusion of a gram atom of such a metal at 800 K yields 8 k J, which could be stored in heat capacity only by a temperature rise of 270 K. In practice, the temperature rise is restricted by using much more storage material and a much larger container. In many heat storage systems, the cost of the containing structure and heat exchange surfaces is likely to exceed the cost of storage material, so heat storage density is very important. The advantages of even larger transformation enthalpies that might seem to accrue from the transformation of a C. ERNEST BIRCHENALL is Distinguished Professor of Metallurgy, Department of Chemical Engineering, University of Delaware, Newark, DE 19711. ALAN F. RIECHMAN is Materials Engineer, E. I9 duPont de Nemours and Co., Inc., Beaumont, TX 77704. Manuscript submitted July 16, 1979.
condensed phase to a gas are offset by the large system volumes that are required to handle the gases. Metals and alloys, because of their high thermal conductivities, offer an important advantage in that the ratio of heat exchanger area to storage volume can be much smaller, for a fixed cycling time, than it is for more poorly conducting materials.2 Any energy storage system having a large central unit or many smaller units must use cheap, plentiful materials. An analysiss of a number of elements indicated that nine elements, S, A1, Si, Zn, P, Na, Cu, Mg, Ca, should be given greatest consideration. Pb addition could also be considered in small amounts, while Sb, Cd, Sn were ruled out for large scale appli
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