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576 CHAPTER 39 1959 Guide greater or lesser degree. In the case of a building, the struc tural materials are almost always in the process either of absorbing heat from or delivering heat to the interior space. This effect is more pronounced in cooling operation where greater air temperature variation is tolerated. Storage tends to reduce the rate of temperature change and helps in some measure to reduce the peak equipment requirements. In this sense every heating and cooling system, can be said to in volve heat storage in some degree. Many attempts have been made, particularly in recent years, to increase the heat-storage effect by using special heat-storage materials as part of the heating or cooling sys- Table 5 .... Properries of Various Specific Heat Type Storage Materials Material Specific Heat Btu per (lb) (cu ft) (F deg) Density Ib/cu ft Heat Capacity Btu par (cu ft) (F deg) Water Iron Concrete Brick Gravel 1.00 0.11 0.27 0.20 0.20 62.4 490.0 140.0 120.0 100.0 62.4 53.9 37.8 24.0 20.0 CONTROL SYMBOLS -------- HEATING SUPPLY -------- HEATING RCTURN -- C-- COOLING SUPPLY --C-- COOLING ACTUM -W-- WELL SUPPLY --W-- WELL RETURN >va AM) V MOOULATE AT COMMAIA OF T2 THROUGH WtU. WATER EXCHANGER AS REQUIRED BY tOAO. -T-J-- CHECK VALVE THERMOSTAT AUTOMATIC WLVC Refrigemnf circuit not shown Fig. 9___ Row Diagram of Central-Type Heat-Pump System heating; or on the low side as an intermittent heat source at temperatures lower than the heated space. Sensibfe Heat Storage Water, with the highest sensible heat storage capacity on a weight baas, is an obvious first choice and has the ad vantage of low cost, easy storage, and' good heat exchange properties. The danger of freezing can be reduced by various types of antifreeze solutions. In some cases, however, freez ing may be desirable. The maximum temperature is limited by the cost of maintaining water under pressures when over 212 F, but this temperature is high enough for most heat ing systems. Rock and gravel beds have also been employed, and have a heat content per unit volume approximately 20 percent of the value for water. The earth has been utilized for beat storage, both for heating and cooling in a number of instances. A problem with most solid media such as these is one of heat transfer into and out of the medium. Table 5 lists the thermal properties of some of the com mon materials used, or considered, for sensible-heat storage. Latent-Heat Storage Systems Because of the relatively large volumes required for all ' sensible-heat storage systems, many attempts have been made to utilize the latent beat of fusion of certain materials. Most materials have some capacity to absorb heat when changing from a solid to a liquid, and some materials have a large capacity to do so. Ordinary ice changing to water is a prime example of this kind and the possibility exists of utilizing it as low-ride storage. For most heating applications. tem. The result is to reduce the size of the heating or cooling equipment necessary to take care of peak demands. In the case of the heat pump, a provision for beat storage can serve not only to reduce the size of the heat pump neces sary for a given load, but also to provide a more desirable electrical load by shifting part of the load to the time of day when the cost of power is least. The off-peak electric hot water heater is a common example of such a heat-storage application. In general, there are two types of heat-storage systems that have been employed: (1) sensible heat-storage systems and (2) latent heat-storage systems. Usually the latter is actually a combination of the two, making use of some sensi ble beat storage in addition to the latent effect. Heat storage in a heat-pump system may be utilized on the high side, when heat is available at a temperature suitable-for direct Fig. 10 .... Row Diagram of Air-to-Air Heat Pump for Simultaneous Hearing and Cooling The Heat Pump 577 Table 6 .... Materials for neat Storage listed According to latent Heat per Unit Volume* of All Materials Having Melting Points between 72 F and 176 F, listed in Chemical Handbooks Material Forawfa MefJing Point F / Heat Content G-caf per g Specific Gravity latent Heat of Fusion per Uait Volume (Tbeoreticof) Thousand* of G-cal per cc Btu/ai ft Nitrogen pentoxide Gallium nhosnhate. dodecahvdrate Sodium sulfate, decohydrate Water Nickel nitrate Calcium chloride Osmium tetroxide Zinc nitrate Calcium nitrate N<0 Ga NatHPO12HjO N&jSOriOHjO H,0 Ni(NOi)*6HjO CaCljOHiO OsO, Zn(NO)*6H0 Ca(NOt)*4HiO 85 76.7 1.63 125.0 14.0- 86 19.0 5.88 113.8 12.7- 94 66.8 1.52 101.5 11.4 90 57.1 1.46 83.4 9.4 32 79.1 1.00 80.0 9.0 132 36.4 2.05 74.7 8.4 86 40.7 1.68 68.4 7.7- 104 13.5 4.91 66.3 7.5 ' 98 31.1 2.06 64.3 7.2 108 33.9 1.82 61.7 6.9 Nise --hvin* meltioi poiota between 7J and 176 F listed eooordinf to latent heat of fusion. Selected es higbeet on a list of 100 organic and inorganio ma terials listed in handbooks. however, the temperature of this change is too low to be useful. Prolonged searches have been made for materials which pass through a change of state as reliably and regularly as ice and water, but at temperatures high enough to be used for direct application to space heating. In order to be much superior to water, the latent heat capacity of such materials must be high. Most organic materials are eliminated from consideration on this basis alone. Furthermore, the absorp tion and release of heat must occur in predictable fashion. Many materials which have been tried exhibit the defect of subcooling, delayed change of state, or decomposition through iimft and repeated cyclee. A study of all materials, both inor ganic and organic, listed in chemical handbooks resulted in a list of over 100 materials having a fusion temperature be tween 72 F and 176 F. Most of these materials are extremely unpromising as heat storage media because of cost, corro siveness, instability, or unavailability. For many of them, data on their heat of fusion are unavailable. All of these materials, where such data are available, have been listed in order of heat capacity per unit volume. Table 6 gives the nine materials which were highest on this list, and it is in teresting to note that there are few common or low cost materials among this group. Others, notably the first two, are academic curiosities. The material disodium phosphate is high on the list and has been used in a number of experimental installations. Re sults for the most part indicated a considerable unreliability at least in the particular form of the material which was used. Such factors as the rate of heat addition or release, size and shape of container, presence of nucleating media, and temperature gradients proved important in determining the percentage of heat storage actually available compared with the theoretical. In several small house heating installa tions employing heat pumps with condenser-side storage, the amount of disodium phosphate actually used was 3300 lb with theoretical latent heat storage capacity of 400,000 Btu. Indications were that the material was only partially ef fective in actual heat-storage performance. A method which makes use of a mixture of this material suspended in oil has been successfully tested in the laboratory, and through use of forced circulation of the oil, has exhibited high rates of heat transfer. Low-Side Storage With a heat-pump system the possibility also exists of storing heat at relatively low temperature on the low side (heat source side) of the system. The freezing of water- has already been mentioned as one possibility. Heat pumps with solar energy as the heat source have been studied with.lowside storage.** A limitation present with this type of storage system as compared to high-side storage is that the peak heating ca pacity of the system is limited to the heat-pump capacity when operating with the low-temperature storage reservoir as the heat source. In many structures, particularly in colder /Hmates where the benefits of heat storage are potentially greatest, this is less than the peak heating load. REFERENCES 1 William Thomson: On the economy of heating and cooling of buildings by means of currents of air (Glasgow Phil. Soc. Proc^ Vo!. 3, December 1852). *T. G. N. Haldane: The heat pump--an economical method of producing low-grade heat from electricity. (Journal of In stitution of Electrical Engineers, London, June 1930). A. R. Stevenson, Jr.: Refrigeration (Journal Franklin In stitute, Vol. 208, August 1929). `Philip Sporo and D. W. McLenegan: An all electric heating cooling and air conditioning system (ASHVE Journal Section, Heating, Piping and Air Conditioning, August 1935). `Experience with a reversed-cycle heating system (Power, August 1941). * Philip Spom, E. R. Ambrose, and Theodore Bannister: Heat Pumps (John Wiley & Sons. New York, 1947). * E. N. Kemler and Sabert Oglesby, Jr.: Heat Pump Appli cations (McGraw-Hill Book Co., New York, 1950).. * Bibliography of the Heat Pump through J951 (Edison Elec tric Institute Publication No. 53-4). *Heal Pump Bibliography (compiled by Southern Research Institute for Southeastern Electric Exchange). "Philip Sporo: Electric Heating and Year-Round Air Con ditioning and the Heat Pump (Edison Electric Institute Bulle tin, May 1955). n