LATENT HEAT STORAGE APPARATUS FOR COOLING

Abstract: In an apparatus intended to lower the temperature of fluid (1) like refrigerant which has received heat from a heating element, heat storage media having different phase change temperatures are used as a heat sink to utilize la­ tent heat associated with their phase change and contained in a plurality of cells (3a-e), the cells (3a-e) are disposed in such an order that the phase change temperature of heat storage media is gradually lowered in the flow direction of the fluid (i) to provide a certain temperature difference be­ tween the gradualy cooled fluid (I) and the media in the se­ rially disposed cells (3a-e), thereby achieving effective uti­ lization of latent heat stored in the medium and improving the fluid cooling rate or thermal response.

Description: TITLE OF THE INVENTION Latent Heat Storage Apparatus for Cooling BACKGROUND OF THE INVENTION This invention relates to a latent heat storage apparatus for cooling a fluid such as water and air, and more particularly, to a latent heat storage apparatus for storing heat in a heat storage medium in the form of a latent heat associated with its phase change. Among prior art heat storage installations for cooling a refrigerant circulating past a heating element in order to cool the heating element was known one using water as a low temperature source. Water has many advantages including economy and ease of handling, but has a low cooling capacity per unit volume because it absorbs heat in the form of sensible heat when used in the range of room temperature. A large volume of water is thus required to provide a desired cooling capacity so that the overall size of apparatus required becomes large enough to increase the cost of installation along with the cost required for installation space. In order to solve these problems, there was proposed cooling heat storage apparatus utilizing latent heat associated with phase change of a material including condensation and melting. When such apparatus of the latent heat type using a heat storage medium which undergoes phase changes are applied, it is preferred to divide the heat storage medium into sections of appropriate dimensions in order to provide ease of handling and transportation and allow for location in a limited space. When a refrigerant is circulated between a heating element and a cooling apparatus at a constant flow rate, the temperature of the refrigerant entering the cooling apparatus is always substantially constant provided that substantially a fixed amount of heat is supplied from the heating element. In order to carry out stable cooling of the heating element, the temperature of the refrigerant coming to the heating element, that is, the temperature of the refrigerant at the outlet of the cooling apparatus should be kept substantially constant. A cooling heat storage apparatus of segmented structure may be assembled by incorporating in such a circulation cooling system a plurality of cells each having latent heat storage medium charged therein. In cooling the refrigerant with this apparatus, the temperature of the refrigerant is gradually lowered from the inlet to the outlet side with the same temperature of phase change for all the heat storage media. Then those cells near the outlet experience a little temperature difference between the heat storage medium and the refrigerant and are thus poor in cooling efficiency per unit time, that is, thermal response. This is because quantity of heat transfer is in proportion to temperature difference, time, and heat transfer area. In a system having a plurality of cells of the same construction, there is the likelihood that the heat storage media of cells near the outlet has not undergone phase change even after the heat storage media of cells near the inlet has undergone phase change. Since phase change is completed in sequence from the heat storage medium in the cell nearest the inlet, the temperature of refrigerant at the outlet is progressively raised and the utilization of cooling heat stored as latent heat becomes inefficient. Such inconvenience may be overcome by changing the structure of cells in sequence of location to increase the heat receiving area of the downstream cells with the refrigerant to compensate for the lack of heat transfer quantity due to a reduced temperature difference, or by changing the amount of heat storage medium charged in cells in sequence of location. Such modifications need to prepare many different types of cells so that not only the apparatus becomes complicated with a reduced degree of freedom of design, but also the overall apparatus becomes large-sized and expensive. SUMMARY OF THE INVENTION A primary object of the present invention is to provide a novel and improved latent heat storage apparatus for cooling purposes including a plurality of cells having equalized thermal response and allowing the cooling heat stored to be effectively taken out, and thus of relatively small size. Another object of the present invention is to provide a latent heat storage apparatus for cooling purposes wherein a plurality of cells each containing a heat storage medium capable of storing heat in the form of latent heat associated with its phase change and adapted for heat exchange with a fluid to be cooled are located in such a sequence that the phase change temperature of the heat storage media is gradually lowered in the direction of flow of the fluid upon cooling. According to the present invention, there is provided a latent heat storage apparatus intended for cooling a fluid, comprising a cooling tank through which the fluid flows in a given direction. A series of cells are disposed in the tank. Each cell contains a heat storage medium for storing heat in the form of latent heat associated with its phase change and is capable of heat exchange with the fluid. The cells are disposed in such an order that the phase change temperature of the heat storage medium is lowered by degrees in the flow direction of the fluid. BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects, features and advantages of the present invention are partially obvious and partially become apparent from the following description taken in conjunction with the accompanying drawings, in which: FIG. 1A is a schematic plan view of a latent heat storage apparatus according to one embodiment of the present invention; FIG. 1B is a diagram illustrating temperature changes of fluid passing through a cooling tank of the apparatus of FIG. IA; FIG. 2 is an elevational cross-section of one exemplary cell; FIG. 3 is a diagram showing the freezing point of salt solution as a function of its concentration; and FIG. 4 is a diagram showing the melting point of nparaffins as a function of the number of carbon atoms. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1A, there is illustrated a latent heat storage apparatus according to one preferred embodiment of the present invention The apparatus includes an elongated cooling tank 2 through which a fluid 1 to be cooled is passed in a given direction, that is, from its inlet 2a to its outlet 2b as shown by arrows. The fluid may be a refrigerant which has received heat from any heating element. A series of (five in the illustrated embodiment) cooling cells 3a, 3b, 3c, 3d, and 3e are disposed in the tank in the flow direction of the fluid such that heat transfer may take place between the fluid 1 and the cells 3a-3e. All the cells 3a-3e are of the same structure and dimensions. In one preferred embodiment as shown in FIG. 2, each cell includes a container 4 filled with a latent heat storage medium 5 such as polyethylene glycol (PEG). A plurality of heat pipes 6 extend into the container 4 in contact with the heat storage medium 5. In the illustrated embodiment, four heat pipes 6 are horizontally (as viewed in FIG. 2) extended throughout the container 4 with their opposite end portions protruded out of the container. The heat pipe end portions are thus exposed to the fluid 1 in the tank 2 so that heat exchange may take place between the fluid 1 and the heat storage medium 5 through the heat pipes 6. All the heat storage media 5 in the cells 3a-3e have different phase change temperatures, for example, different melting points. The cells are disposed in such an order that the melting point Ma of heat storage medium in the most upstream cell in the fluid flow direction, that is, cell 3a disposed near the inlet 2a of the tank 2 is the highest and the melting point of heat storage medium is then lowered by degrees toward the cell disposed near the tank outlet 2b. The heat storage media in cells 3a, 3b, 3c, 3d, and 3e have melting points Ma, Mb, Mc, Md, and Me, respectively, with Ma > Mb > Mc > Md > Me. This stepwise setting of melting point may be achieved by a variety of methods, for example, when the heat storage medium is polyethylene glycol (PEG), by varying the average molecular weight of PEG. When the heat storage medium is an aqueous solution of a salt such as potassium chloride (KCl) and sodium chloride (NaCl), the concentration of solution may be varied. When the heat storage medium is n-paraffin generally represented by CnH2n+2, the number n of carbon atoms may be varied, or the proportion of mixed nparaffins may be varied. More particularly, for polyethylene glycols (PEG), their melting point (mp) is correlated to their average molecular weight (MW) as shown in Table 1. A PEG having a proper average molecular weight may be chosen for a particular cell depending on a melting point difference M to be set between the cells 3a-3e. Table 1 MW 480 470 460 450 440 430 420 410 400 390 mp, C 15 14 13 12 11 10 9 8 7 6 The freezing point or melting point of water is lowered with the concentration of a salt dissolved therein as plotted for both potassium chloride and sodium chloride in FIG 3. As for n-paraffins, their melting point is increased with the increasing number of carbon atoms as plotted in FIG. 4. Depending on a melting point difference AM to be set between the cells, an aqueous salt solution of a suitable concentration or an n-paraffin having a suitable number of carbon atoms may be chosen. The heat pipes 6 are designed such that when the temperature of the heat storage medium 5 is at its melting point, heat of the fluid 1 is transferred to the medium 5 to lower the temperature of the fluid. The operation of the above-arranged apparatus will be described. The diagram of FIG. 1B shows how the temperature of the fluid 7 passing through the tank 2 changes when the fluid 1 is cooled with the latent heat stored in the heat storage medium 5. The fluid 1 is passed through the tank 2 at a temperature TO and a flow rate both predetermined from a design choice, with the heat storage media 5 in the cells 3a3e being at the temperatures corresponding to their melting points Ma-Me, respectively. Since the temperature of heat storage medium 5 in the first cell 3a is at its melting point Ma, the fluid 1 is cooled on the basis of the temperature difference AT1 (= TO - Ma) between the fluid and the medium. The temperature of fluid is lowered to T1 in the region of the tank 2 where the first cell 3a is located. The fluid 1 having its temperature decreased to T1 is further deprived of heat by means of the second cell 3b. Since the melting point Mb of heat storage medium in the second cell 3b is set lower than the melting point Ma of heat storage medium in the first cell 3a, irrespective of the somewhat decreased temperature of fluid 1, the temperature difference T2 between the temperature of the fluid 1 and the melting point Mb of heat storage medium in the second cell 3b is not so small. Then the fluid 1 is cooled by means of the second cell 3b to approximately the same extent as done by means of the first cell 3a. The temperature of fluid is thus lowered to T2 in the region of the tank 2 where the second cell 3b is located. This means that the thermal response of the second cell 3b is maintained substantially equal to that of the first cell 3a. The fluid 1 is further cooled during passage of the third and succeeding cells 3c-3e. Since the melting points Mc-Me of heat storage media in the third and succeeding cells 3c-3e are set to become lower by degrees in the order of cell disposition, irrespective of progressive cooling of the fluid, the temperature differences AT3-AT5 between the temperature of the fluid 1 and the melting points Mc-Me of heat storage media in the cells 3c-3e are not so small. The fluid 1 is thus cooled to a similar extent across the successive cells. The temperature of the fluid 1 is eventually lowered to T5. In the apparatus of the invention, the temperatures of heat storage media in the cells 3a-3e are maintained at their melting points Ma-Me, respectively until the heat storage media have completely melted. The temperature differences between the respective cells 3a-3e and the corresponding fluid are AT1-QT5, respectively. The cooling efficiency in the cells 3a-3e is not significantly lowered insofar as the flow rate of fluid 1 is kept constant. It is then possible to fast cool the fluid 1 and maintain the temperature of the fluid exiting the tank outlet 2b at T5. Since melting of heat storage media proceeds substantially at the same time in all the cells 3a-3e, the latent heat stored in the media can be effectively taken out. Changes in the temperature and/or flow rate of the fluid 1 entering the tank 2 do not affect the effective take-out of the latent heat stored in the media because melting of the heat storage media proceeds substantially simultaneously in all the cells. The temperature reduction in each cell may be controlled by a proper choice of the quantity of heat transfer between fluid and heat storage medium which in turn, depends on a particular type of heat storage medium and heat pipe. If each cell can accomplish only a small temperature reduction, the fluid may be finally cooled to the desired low temperature by increasing the number of cells. The apparatus in which heat storage media have melted may be regenerated by passing a fluid at a temperature lower than the freezing point of the heat storage media in the reverse direction to cause the media to be frozen again through a reverse process to the above-mentioned process. Also effective freezing of heat storage media can be accomplished in the regeneration procedure. The apparatus of the present invention may be modified by providing it with an additional inlet or outlet port between the serially disposed cells so that fluid may be passed across any desired number of cells. Then the incoming fluid having a wide range of temperature may be fed or the outgoing fluid may have a wide range of tempeature. The heat storage medium-containing cell used in the present apparatus may be of vertical type wherein heat pipes are vertically (as viewed in FIG. 2) extended through the cell. Since the temperature difference between a fluid to be cooled and a heat storage medium serving as a heat sink is not significantly reduced with the progress of fluid cooling, the apparatus of the present invention as illustrated above provides an improved cooling efficiency per unit time or thermal response and is thus successful in fast cooling the fluid. Since the heat storage media in the cells concurrently melt to substantially the same extent while receiving heat from the fluid, the cooling efficiency is not significantly lowered with the lapse of time and the latent heat stored in the media is effectively utilized substantially 100%. Thus, it is not necessary to contain an extra amount of heat storage medium, which also contributes to size reduction. While there has been described a preferred form of the invention, obviously modifications and variations are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

Claims: CLAIM 1. A latent heat storage apparatus intended for cooling a fluid, comprising a cooling tank through which the fluid flows in a given direction, and a series of cells disposed in said tank, each containing a heat storage medium for storing heat in the form of latent heat associated with its phase change and capable of heat exchange with said fluid, said cells being disposed in such an order that the phase change temperature of the heat storage medium is lowered by degrees in the flow direction of said fluid. 2. The latent heat storage apparatus of claim I wherein said heat storage medium is comprised of polyethylene glycol. 3. The latent heat storage apparatus of claim 2 wherein said cooling tank has an inlet and an outlet for the passage of the fluid, and the average molecular weight of polyethylene glycol in the cells is reduced in sequence from the cell on the inlet side to the cell on the outlet side. 4. The latent heat storage apparatus of claim 1 wherein said heat storage medium is comprised of an aqueous solution of a salt wherein the cells are filled with the solutions of varying concentrations to provide different phase change temperatures. 5. The latent heat storage apparatus of claim 1 wherein said heat storage medium is comprised of n-paraffins. 6. The latent heat storage apparatus of claim 1 wherein each said cell comprises a container having the heat storage medium charged therein, and a heat pipe extending throughout the container.

Inventors: HASEGAWA, YUTAKA, ITO, MASAHIKO, MOCHIZUKI, MASATAKA, MOTAI, TSUNEAKI, TAKAGI, HIROTAKA
Applicants: FUJIKURA LTD., THE CHUBU ELECTRIC POWER COMPANY, INC.
Application year: 1986
Main classification: F28D20
IPC codes: F28D20