MXPA97002452A

MXPA97002452A - Ac storage device

Info

Publication number: MXPA97002452A
Application number: MXPA/A/1997/002452A
Authority: MX
Inventors: Rycus Avigail
Original assignee: Yeda Research And Development Co Ltd
Priority date: 1994-10-04
Filing date: 1995-04-10
Publication date: 1997-06-01

Abstract

The present invention relates to a two layer heat storage system comprising two layers in contact with one another, wherein one layer is a heat storage layer and the other layer is an intermediate phase layer used to extract heat, wherein the heat storage layer is a phase change layer, in which a solid form of the phase change layer has a lower density than a liquid form of the phase change layer, and the intermediate phase layer is a layer of liquid metal that has a greater density than both phases of the change layer of fa

Description

HEAT STORAGE DEVICE FIELD OF THE INVENTION The present invention relates to an improved heat storage device, particularly for use in a solar energy system.

BACKGROUND OF THE INVENTION The storage of energy in the form of heat is very important in the management of many energy systems, but it is of greater importance for solar energy systems. The most mature systems are those that rely on systems of sensible heat or phase change. The advantage of phase change systems over sensitive heat systems is their greater heat capacity. For example, most storage materials based on sensible heat can provide less than one calorie per degree centigrade, while the phase change from solid to liquid can provide 160 calories per gram without any change in temperature. However, these systems are more difficult to use, since the solid material tends to precipitate in the heat exchanger, thus avoiding an efficient transfer of heat. Many solar receivers are tubular receivers and the precipitation of solid material inside the tubes can be fatal to the system.

SUMMARY OF THE INVENTION It is the object of this invention to provide a improved method for the storage of heat, based on two layers of different materials, one used for storage, and the other for the extraction of heat from the storage layer. hot. As an example, the heat storage phase of the system is a phase change material and it cooperates with another phase, which is a molten metal well above the melting point, and with a higher density (specific gravity) than that of the storage phase. After the heat is stored in the heat storage phase, it can be extracted by the liquid metal through heat exchange by direct contact with the storage layer, and then transferred to another medium for application in many different ways. : i. Liquid (molten) metal can be circulated to another environment where it can release heat. ii. The heat can then be transferred to submerged tubes in the liquid metal. The tubes can contain a liquid that circulates inside the tubes and removes the heat from the walls of the tubes. iii. The transfer of heat takes place through the walls of the tubes submerged in the liquid metal, but inside the tubes there is a material with high vapor pressure, and the boiling liquid inside the tubes is converted from liquid to vapor, stirring a large amount of heat, which is usually associated with the change of phase from liquid to vapor. Such a system is practically a "heat pipe". Then the steam can transfer the heat to another medium after condensing in the environment where it takes place. final heat transfer. Then the condensed liquid is returned to the source (preferably by gravitation) for recycling. iv. If the liquid metal is selected from metals: on a high vapor pressure, then the heat pipe can be formed by immersing a bell-shaped collector in the liquid metal, and the metal vapor is collected in tubes which are integrated in the upper part of the "bell". Alternatively, put. that the liquid metal with high vapor pressure must be confined in a container, the vapor can be collected in the free space above the liquid phase in that closed container. The advantage of this last mentioned system is the large contact area between the two phases that provides a very efficient transfer of heat, and the very high heat conductivity of the liquid metal. This last invented system is the most efficient since it includes a very efficient passage of heat transfer from liquid to vapor through a large area. Within each of the two phases of the previous systems, the heat is distributed efficiently by convection, using the large volume of each phase. A small inclination at the bottom of the container containing the lower phase can improve the free convection within that phase, at the interconnection between the layers, where the heat transfer from the storage layer to the conduction layer takes place. If the storage phase is a phase change system, then in the interconnection between the layers crystals will precipitate. If the storage phase is a material in which the solid has a lower density than the liquid, then it is preferred to choose the liquid metal of a material with a density higher than the storage phase. The liquid metal will remain below the storage phase, and the solid that forms in the interconnection will float to the top of the system, without interfering with heat transfer. If the storage phase of a material in which the solid phase has a higher density than the liquid phase is selected, then the liquid metal must be a metal with a density lower than the storage phase, such that the metal It will float in the storage phase, and the exchange of heat from the liquid metal to an external system can be carried out efficiently. The chips that are formed by the crystallization of the heat storage material will precipitate at the bottom of the container, without interfering with the heat exchange. If the storage system is part of a solar system, then the preferred structure is such that the solar receiver is the storage system, and the sun is directly illuminating the upper surface of the storage phase. It is preferred to select that phase of a material relatively transparent to sunlight, so that light can penetrate into the mass, and a very efficient convective heat transfer can take place. If the material is not transparent, then the metal layer may be a candidate to facilitate the absorption of light. The use of the metal layer for the absorption of sunlight is limited to metals with low reflectivity, and preferably the metal layer is used in the lower layer. In order to obtain an illumination of the upper layer in a large central receiving system (heliostatic field based on a plurality of sun-reflecting mirrors that reflect sunlight to a tower), a reflective tower system can be used, wherein a mirror placed in the tower reflects the light downwards. A convex mirror can be placed below the focal point (or target point) of the heliostat, and a concave mirror above that point. In a plate system, a secondary mirror is placed near the focal point of preference, to reflect the light to the main axis of the tracking system, thus maintaining a receiver at a fixed point, and the collection system will be placed in that location. point. In this way, the heavy collection system is stationary, while the main mirror is tracking. Alternatively, the secondary mirror is not kept stationary, but is controlled by computer to reflect light to a previously selected station, where the collection system is parked. The new solar systems, according to the invention, include different thermal machines operated by the sun, which convert heat energy to mechanical energy. These can be gas turbines (Brayton cycles), steam turbines, machines made of sterling silver and any other thermal machine. Usually gas turbines and steam turbines are specific to large systems, while plate systems are often used for small machines made of sterling silver. It is important to note that steam from the heat pipe can be used as part of the thermal machine, for example, in the case of machines made of sterling silver, where sodium vapor can be used as an internal component in the machine, and in such arrangement sodium is selected as the liquid metal in the storage and transfer system. hot. The system of the invention can also be used to transfer solar energy to solar chemical reactors such as solar reformers or solar gasification systems, or to store solar energy for process heat. Molten salts can be used as the heat storage materials, both for sensible heat storage, and for phase change material. In accordance with this invention, it is particularly convenient to select pairs of metals and salts of the same metal as the binary system. This selection or combination minimizes the possibilities of chemical reactions between the layers. Alkaline metals have low melting points and relatively high vapor pressure and low specific gravity, therefore, these are especially useful for applications in these systems. Many fluorides have a high melting heat value, are relatively stable at high temperature and can be used in the inventive heat transfer and storage system. Magnesium can be used in conjugation with its salts, 6: 1 magnesium has a high vapor pressure, low density, and is generally safer for use than alkali metals. Aluminum has a low melting point, but a very high boiling point, and can be used when safety is important and a heat pipe application is not needed. Zinc is a relatively heavy metal and can be used as a lower metal layer. The relatively high vapor pressure of zinc allows its use for heat pipe applications. Lead and tin can be used in the invention, and are examples of heavy liquid metals with low vapor pressure. Alkali metal salts such as sodium chloride or fluoride, potassium chloride, calcium chlorides and magnesium chlorides are examples of salts that can be used in the invention. Its molten form can be used in different applications of the invention including heat storage. Also useful are eutectic mixtures or mixed salts in the invention. These examples of specific materials show that there is a large selection of salts and metals that can be used for different conditions and applications of the present invention. When a heavy metal is used in the system, then a large hydraulic pressure is formed in the lower part of the system. This pressure can compensate the internal pressure in a tubular heat exchanger associated with high pressure systems such as gas turbines. This effect is very important because the mechanical strength of many materials degrades at high temperature. Blowing the working gas through the heavy liquid layer is another important possibility in the present invention because the direct contact heat exchange of the two phases is much more efficient than the heat exchange through the two phases. walls of tubes or other containers. In the invention, heat transfer by free convection within each of the phases of the system is the preferred way of carrying out the invention. However, in some cases, as in the case where the heating is done in the upper part of one of the layers, or when the heat extraction is made from the lower layer, the convection can be used forced as an alternative, or as a supplement to achieve the desired heat transfer. Forced convection can be achieved by mechanical systems such as circulation pumps, or mixers, or by boiling an inert gas through one of the liquid layers of each of them separately. Other objects and advantages of the present invention will become apparent from the following detailed description of the preferred embodiments when taken in conjunction with the drawings.

DESCRIPTION OF THE FIGURES OF THE DRAWING Figures ad show a two-layer system where the upper layer is the heat extraction layer and the lower layer is the heat storage with the Figure showing the layers one on top of the other in the For example, Figure Ib shows the circulation of the extracycling layer of heat used to transfer the heat, the Figure showing submerged tubes in the phase of heat extraction which contains a fluid that circulates inside tubes to transfer heat to an external system, and Figure Id showing an external heat pipe submerged in the upper layer used for heat transfer. The figure shows you two configurations of heat pipes based on the volatility of the heat extraction phase. Figure lg shows the heat pipe equipped with micrometer tubes inside the receiving systems to facilitate the transfer of heat instead of the metal fins} as shown in Figures Id, 12 and lf Figures 2a, b, c, d, and e show similar systems, wherein the metal layer is the bottom layer. Figure 2f shows a system where the vessel is provided with a slope in the bottom to improve J | to free convection of the lower level. Figures 3a ad show solar systems with Figure 3a showing a transparent heat storage system and a top liquid metal layer, Figure 3b showing a top liquid metal layer and the metal layer used to absorb sunlight, 3c showing the metallic layer as the layer of the fondci and the absorption of light in the upper layer, and Figure 3d showing the lower metallic layer used pa_j * to absorb the light. Figure 4 shows a solar system where sunlight is collected by means of a heliostatic field and a mirror is placed over the tower and the light is reflected to the storage system. Figure 5 shows a solar plate with the storage system placed on the axis of rotation so that the light recollected by the plate is reflected by means of a mirror secondary to the storage system. Figure 5b shows a solar plate with the recollection system placed on the ground with the light reaching the storage system from the plate that is braking to the sun continuously through a secondary mirror which moves in a responsive manner to a separate control that continuously positions the secondary mirror in such a way that the light is reflected to the storage system.

DETAILED DESCRIPTION OF THE PREFERRED MODALITIES Referring now to the drawings in detail, the Figure shows a two-layer system contained in a container 20. The upper layer 22 is liquid metal (heat extractor) and the lower layer 24 is liquid salt ( heat collector). The materials are selected from those to which reference was made and which were described in detail above. Heat exchange occurs at the interconnection between the two layers. Figure Ib shows the pipes 26 which open towards the layer 22 and which lead out of the container 20 to remove the liquid metal towards the point of use or heat transfer and return the liquid metal to the container 20. The figure shows the pipes 28 submerged in the upper layer 22. A suitable fluid flows through the pipes 28 to extract the heat from the upper layer of liquid metal 22. The pipes 28 extend outward from the container 20 and provide a circulation system. which can be closed or opened as required. Figure Id shows an external pipe 30 submerged in the upper layer 22. The liquid metal of the upper layer 22 can flow down the pipe 30. The upper part of the pipe 30 is adjusted with the fins 32 and the upper part and the fins are contained in the container 34 through which a luid may flow to extract heat via the fins 32. In Figure 11, the pipe 30 is not submerged in the upper layer 22., but rather is separated from it, opening within the vapor space 36 on the layer 22. In this embodiment, the upper layer of liquid metal 22 is sufficiently volatile for the hot vapors to enter the pipe 30. and transfer the heat to the fluid flowing in the container 34 via the fins 32. In Figure lf, the pipe 30 is fitted with a larger nozzle bell 38 to facilitate the collection of the liquid metal or vapor from the upper layer 22. In Figure 1, the upper end of the tube 30 is assembled with miniature tubes 40 that open inside the tube 30 to improve the heat transfer of the vapor or liquid. Figures 2a-e show systems in which the liquid metal layer is below the liquid salt layer. Similar parts have been designated with similar reference numerals. Note in Figures 2c and d, the pipe 30 should not extend through the liquid salt layer 24 to be submerged in the liquid metal layer 22. In Figure 2e, the liquid metal layer 22 extends laterally beyond of the liquid salt layer to provide a lateral extension 50 accessible from the outside. In this case, the pipe 30 is immersed with a nozzle bell 38 at its lower end in the layer 22 in the lateral extension 50. In Figure 2f, the container 20 is provided with a tapered bottom to 'improve the convection in the lower layer 22. Figures 3a-d show the novel heat storage and transfer system incorporated into a solar system. Figure 3a shows a transparent heat storage system using the upper liquid metal layer 22 separated from the lower liquid salt by a baffle 60 to allow the solar energy 62 to fall directly and be absorbed by the exposed liquid salt. In Figure 3b, the upper liquid metal is transparent. Figure 3c: shows the absorption of solar energy 62 by the now upper layer 24 of liquid salt. The container 20 has a tapered bottom 21 and the heat transfer pipes 28 are submerged in the liquid metal layer 22. The solar energy enters the container 20 through the transparent section 64. In Figure 3d, the container is provided. 20 with a side extension 50 having a transparent top to allow penetration of solar energy 62 and absorption by the lower layer of liquid metal 22. Figure 4 shows a solar energy system where sunlight is collected 62 by means of a heliostatic field 70 consisting of mirrors 72 on mounts 74 supported on the ground on a suitable platform 76. The mirrors 72 are appropriately directed towards the convex mirror r'8 mounted on a tower (not shown). The mirror 78 focuses the reflected solar energy on the novel heat storage device 80 of the invention (in a previously described manner). Figure 5a shows a solar dish 90 reflecting the solar energy 62 for storing the device 80. The dish 80 rotates around the axis 92 and reflects the solar energy in all positions (two positions are shown) to the secondary mirror 94 which is focuses on the device 80. In Figure 5b, the solar plate 90 rotates about the axis 92. The device 80 is positioned on the ground. The secondary mirror 94 is actuated by a motor and articulation suitable to constantly reflect the light of the mirror 90 to the device 80. A detector detects the reflection of the solar energy of the mirror 90 and a controller in the control 96 drives the articulation of the motor 98 and the secondary mirror 94 for directing the solar energy reflected from 90 to the device 80. Although the invention has been shown and described with reference to the preferred embodiments, changes and modifications that do not depart from the teachings will become apparent to those skilled in the art. of the present invention. It is considered that such changes and modifications fall within the substance and scope of the invention as claimed.

Claims

CLAIMS 1. A two-layer heat storage system comprising two layers in contact with each other, wherein one layer is a heat storage layer, and the other layer is an intermediate phase layer that is used to extract heat, in which where the heat storage layer is a phase change layer, in which a solid form of the phase change layer has a lower density than a liquid form of the heat phase change layer, and the layer intermediate phase is a liquid metal layer that has a higher density than both phases of the phase change layer.
2. A system as in Claim 1, wherein the heat storage layer is a molten salt.
3. A system as in Claim 1, wherein the transfer of heat to an external system is achieved by the circulation of a heat transfer liquid.
4. A system as in Claim 1, wherein the transfer of heat to an external system is achieved by a fluid circulating in tubes that are immersed in a heat transfer fluid.
5. A system as in Claim 1, wherein the transfer of heat to an external system is achieved by an external heat pipe submerged in a heat transfer liquid. A system as in Claim 1, wherein the intermediate phase is vaporizable liquid metal, and the heat exchange to an external system is achieved by condensing the vapor from the liquid metal heat extraction layer, and returning the metal condensed to the heat extraction layer by gravitation or by pumping. 7. A system as in Claim 4, wherein the gravitational hydraulic pressure in the system is used to compensate the pressure used for the fluid in the external system. 8. A system as in Claim 7, wherein the heat transfer is achieved by direct contact of working gas with the liquid heat transfer layer. 9. A system as in Claim 1, wherein the heat storage system is a component in a solar system. 10. A system as in Claim 9, wherein the solar system includes a concentration system based on: a tracking of a sun axis. 11. A system as in Claim 9, wherein the solar system is a concentration system based on a two-axis tracking of the sun. 12. A system as in Claim 11, wherein the tracking system is a single-mirror or multi-mirror system. 13. A system as in Claim 12, wherein the storage system is placed near the focal point of the tracking mirror. A system as in Claim 12, wherein the sunlight is reflected to the storage system, which is placed on the tracking axis, and a fixed mirror is placed near the focal point of the tracking mirror, and reflects the sunlight to the storage system. 15. A system as in Claim 12, wherein the storage system is placed at a fixed point, and the lu ?: solar is pointing to the target storage system by a separate control of the orientation of a secondary mirror. 16. A system as in Claim 11, wherein the solar system is a heliostatic field that reflects sunlight to a high target, where it is directed to the storage system. 17. A system as in claim 13, wherein the solar light is reflected down to the storage system by a mirror placed in the solar tower. 18. A system as in Claim 1, wherein the liquid metal layer is separated from the heat storage layer by a baffle, whereby the heat storage layer directly receives and absorbs solar energy. 19. A system as in Claim 1, wherein the liquid metal layer has a transparent top to allow penetration of solar energy into the liquid metal layer. 20. A system as in Claim 1, where the liquid metal layer is placed under the heat storage layer. 21. A system as in Claim 20, wherein the heat storage layer, which is in an upper layer, directly receives the solar energy. 22. A system as in Claim 20, wherein the container is provided with a lateral extension having * a transparent upper part, whereby the liquid metal layer receives and absorbs the solar energy. 23. A two-layer heat storage system comprising two layers in contact with one another, wherein one layer is a heat storage layer, and the other layer is an intermediate phase layer that is used to extract heat , wherein the liquid metal layer is separated from the heat storage layer by a deflector, by which the heat storage layer directly receives and absorbs the solar energy. 24. A two-layer heat storage system comprising two layers in contact with each other, wherein one layer is a heat storage layer, and the other layer is an intermediate phase layer that is used to extract heat , wherein the liquid metal layer has a transparent top to allow the penetration of solar energy into the liquid metal layer. 25. A two-layer heat storage system comprising two layers in contact with each other, wherein one layer is a heat storage layer, and the other layer is an intermediate phase layer that is used to extract heat, where the intermediate phase layer is placed under; the heat storage layer. 26. A system as in claim 25, wherein the heat storage layer, which is a top layer, directly receives the solar energy. 27. A system as in Claim 25, wherein the container is provided with a lateral extension having a transparent top, whereby the liquid metal layer receives and absorbs the solar energy. 28. A system as in Claim 23, wherein the heat storage system is a component in a solar system. 29. A system as in Claim 28, wherein the solar system includes a concentration system based on a tracking of a sun axis. 30. A system as in Claim 28, wherein the solar system is a concentration system based on a tracking of two sun axes. 31. A system as in Claim 30, wherein the tracking system is a single-plate or multi-plate system. 32. A system as in Claim 31, wherein the storage system is positioned near the focal point of the tracking mirror. 33. A system as in claim 31, wherein the sunlight to the storage system, which is placed in the tracking axis, and a fixed mirror is placed near the focal point of the scan mirror reflects, and reflects the sunlight to the storage system. 34. A system as in claim 31, wherein the storage system is placed at a fixed point, and sunlight is pointing to the target storage system by a separate control of the orientation of a secondary mirror. 35. A system as in Claim 30, wherein the solar system is a heliostatic field that reflects sunlight to a high target, where it is directed to the storage system. 36. A system as in Claim 31, wherein the sunlight is reflected down to the storage system by means of a mirror placed in the solar tower. 37. A system as in Claim 24, wherein the heat storage system is a component in a solar system. 38. A system as in Claim 37, wherein the solar system includes a concentration system based on a tracking of a sun axis. 39. A system as in Claim 37, wherein the solar system is a concentration system based on a tracking of two axes of the sun. 40. A system as in Claim 39, wherein the tracking system is a single-plate or multi-plate system. 41. A system as in Claim 40, wherein the storage system is positioned near the focal point of the tracking mirror. 42. A system as in Claim 40, wherein the sunlight is reflected to the storage system, which is positioned on the tracking axis, and a fixed mirror is placed near the focal point of the tracking mirror, and reflects the sunlight to the storage system. 43. A system as in Claim 40, wherein the storage system is placed at a fixed point, 51. A system as in Claim 49, wherein the sunlight is reflected to the storage system, which is placed on the tracking axis, and a fixed mirror is placed near the focal point of the tracking mirror, and reflects the sunlight to the storage system. 52. A system as in Claim 49, wherein the storage system is placed at a fixed point, and sunlight is pointing to the target storage system by separate control of the orientation of a secondary mirror. 53. A system as in Claim 48, wherein the solar system is a heliostatic field that reflects sunlight to a high target, where it is directed to the storage system. 54. A system as in Claim 50, wherein sunlight is reflected down to the storage system by a mirror placed in the solar tower.