Document nmvkyQQmpV1JaG8pNDXKmzL0m

794 CHAPTER 36 1951 Guide Refrigeration 795 consequently, at the present time there are several hundred residential in stallations and probably a greater number of commercial systems. How ever much research is needed before the residential heat pump installation can successfully emerge to compete economically and with equal reliability with the more common forms of heating and fuels. From an analysis of the equation for the coefficient of performance, it is evident that the economical adaption of the heat pump as a practical means of heating, requires that the temperature of the source from which the heat is extracted be as high as possible, and that the temperature of the sink to which the heat is rejected for heating purposes, be as low as possible. Thus, with a small temperature spread between the evaporator and the con denser, six or more times as much heat may be obtained theoretically (and three to five times practically) as the heat equivalent of the work necessary to operate the system. There are a number of limitations, however, the most serious of which is the lack of ready availability of a practical source of heat. One of the major problems in the development of the heat pump involves research on, and the compilation of reliable design data for, the various heat sources and sinks available. The four principal potential sources of heat are air, water, earth, and solar energy. Of these, the first three are primary sources of heat which may be used alone. The fourth, solar energy, while of tremendous potentiality, will probably be developed in most localities as auxiliary to the other three. In addition, there are other minor sources such as process waste heat, sewage, etc., which may be used under special circumstance. There are also a number of industrial applications of heat pumps, for purposes other than space heating, which are practical largely through economic considerations of the particular process involved. Table 4 pre sents a summary of the advantages and disadvantages of each of these major heat sources. By reference to Table 4, it will be seen that, to date, the most satisfactory heat sources are air, water, and earth, and that air and water are the most satisfactory heat sinks. There are, therefore, six possible combinations of source and sink in application: air to air, air to water, water to air, water to water, earth to air, and earth to water. In addition, it should be recog nized that heat, storage devices may be used with any of these systems involving either a single or a dual heat source. One promising possibility involves the utilization of a storage pit or cistern operating upon a heat of' fusion cycle and supplied by supplementary heat from air or solar sources. Heat of fusion may, for example, also be utilized with city water and sewage disposal to increase the practicability of these sources. When heat is to be obtained from a ground coil it should be emphasized that, in general, the heat extracted must be replaced by heat from the sun, received by radiation to the ground or by heat carried into the ground by rain. Combinations of heat sources, such as air and water, air and ground, air and solar energy, or ground and solar energy, may also be used and will frequently improve the coefficient of performance over an entire heating season; but they will probably result in considerably greater initial cost. A typical arrangement of a heat pump system with air as the heat source is shown in Fig. 7. Both water and air are practical media to which the condenser heat may be rejected; but the generation of steam requires too high a condenser