JP2014513260A - Particularly suitable for heat storage equipment - Google Patents

Abstract

A heat storage facility for storing thermal energy at an elevated temperature is provided. The facility transfers heat from the enclosure for holding a heat storage medium, a first heat transfer surface for transferring heat from a circulating heat transfer fluid, and a steam pipe associated with the power generation facility from the heat storage medium. A second heat transfer surface. The heat storage medium is a metal phase change material, preferably has a melting temperature exceeding 500 ° C., and is disposed between the first heat transfer surface and the second heat transfer surface. The metal heat transfer fluid is preferably in a liquid state in a wide temperature range from room temperature to a maximum operating temperature exceeding the melting point of the metal phase change material. The metal heat transfer fluid is preferably selected from molten alkali metals and alkali metal alloys. The invention also extends to a solar power plant comprising one or more such heat storage facilities.
[Selection] Figure 3

Description

  The present invention relates to a heat storage facility that is particularly suitable for storing thermal energy, and is not limited thereto, but is particularly suitable for thermal energy obtained from the sun using a concentrating solar thermal device.

  One advantage of concentrating solar heat (CSP) using a solar receiver on the tower is that it allows thermal energy storage. In order to buffer the power cycle on a clear day so that the solar thermal power plant can provide electrical energy at night or in bad weather, it is necessary to store heat.

  Many conventional heat storage systems operate in a sensible heat system and store thermal energy in some molten salt or store heat in a solid object such as concrete. The use of molten salt has the disadvantage that the receiver loop may solidify at night, so it can be traced to keep the salt in a liquid state during non-operations, such as at night, or in auxiliary pipes. Heating is required, which can lead to maintenance problems.

  Other phase change concepts were also considered, but all are based on low conductivity salts, and a wide range of heat transfer improvements are needed to improve thermal conductivity.

  The output of solid storage materials such as concrete bodies is limited and can also be influenced by thermal stresses that limit the useful life of such media. It is expensive to modify a solid storage element.

  In other storage systems, it is also possible to use oil as storage material and heat transfer fluid. The main problem with this is that the maximum use temperature of the oil is about 400 ° C. This fact greatly limits the maximum receiver temperature and limits the maximum efficiency of the power plant.

  To the best of the applicant's knowledge, all conventional heat storage methods have limitations. There are generally two limitations with regard to thermal energy storage, one is temperature limitation, and the other is a high melting temperature heat transfer fluid that causes inherent plugging problems.

  Accordingly, there is a need for a new thermal energy storage system that eliminates the need for trace heating of the heat transfer fluid and provides high power thermal energy storage at higher temperatures than previously practiced.

  In a first aspect of the present invention, a heat storage facility is provided for storing thermal energy at an elevated temperature, said facility being for an enclosure for holding a heat storage medium and for transferring heat from a circulating heat transfer fluid. A first heat transfer surface; and a second heat transfer surface for transferring heat from the heat storage medium to a steam pipe associated with the power generation facility, the first heat transfer surface and the second heat transfer surface. The heat storage medium disposed between the surface and the surface is a metal phase change material (PCM), and the heat transfer fluid is a liquid metal.

  As a further feature of the invention, the metal phase change material has a high thermal conductivity, a high latent heat of fusion, and a melting temperature, preferably above about 500 ° C., and a high operating temperature. One candidate material is a metal phase change material that is an alloy of aluminum and silicon, preferably an alloy containing 87.76% Al and 12.24% Si (also called AlSi12, LM6 casting alloy). There are commercially available alloys.

  The present invention further provides a metal heat transfer fluid that is liquid in a wide temperature range from room temperature to a maximum operating temperature that exceeds the melting point of the metal phase change material, wherein the metal heat transfer fluid is mainly a molten alkali metal or An alkali metal alloy, in particular a sodium and potassium alloy known as NaK, and the heat transfer surface is generally a generally parallel pipe extending through the enclosure or a generally parallel pipe adjacent to the enclosure The heat transfer surface.

  The invention further provides a part of a unit selected from a heat storage facility selected from a preheater, boiler, superheater, reheater, or combination unit designed to perform the function of two or more units of any one thereof. A heat storage facility as defined above is provided.

  In one form of the invention, in general, a part of a relatively low capacity solar thermal device is constituted by one heliostat field with an associated solar receiver mounted on the tower, in this case the heat storage facility defined above May be built into the tower.

  In another form of the invention, the solar thermal device may include a plurality of heliostat fields with associated solar receivers mounted on the tower, each of the solar receivers having one or more shared thermal storage facilities as described above. It is connected to the. In such a case, the heat storage facility may be implemented in one or more units selected from preheaters, boilers, superheaters, and reheaters, generally implemented in all of these, each of the facilities The flow of heat transfer fluid to is controlled to achieve the relevant objectives of the equipment.

  In the construction of the heat storage facility described above, the presence of a metal phase change material between the molten high temperature alkali metal alloy and water or steam prevents contact between the alkali metal and water or steam that would otherwise have disastrous consequences. As far as it is understood, it is possible to use a suitable molten alkali metal alloy as the heat transfer fluid. This configuration makes it possible to usefully apply the unique properties of materials that have not been used in related situations so far, and to take advantage of specific properties. Thus, the present invention allows a water or steam heater or generator to be combined with a heat storage facility.

  Thus, in use, heat is transferred from the receiver on the tower to the metal phase change material by a molten heat transfer fluid, typically a molten alkali metal alloy such as NaK (eutectic or hypoeutectic), and the metal phase change. The material melts to absorb the heat energy due to the latent heat of fusion of this material, and the heat is stored and, if necessary, to generate steam from water and to superheat the steam for use in the power plant And transferred to the second heat transfer surface.

  In order that the above and other features of the present invention may be fully understood, two different embodiments will now be described in detail with reference to the drawings.

1 is a schematic illustration of an embodiment of a solar thermal device with a facility according to the invention in which a single solar receiver is provided at the top of the tower. A plurality of solar receivers are provided on top of a plurality of towers, the heat transfer fluid is supplied to a plurality of heat storage facilities, and the heat storage facilities are used to heat water or steam as needed. FIG. 2 is a view similar to FIG. 1 of another type of concentrating power equipment including equipment according to FIG. 1 is a schematic cross-sectional view of an embodiment of a storage container according to the present invention showing first and second heat transfer pipes distributed in the metal phase change material.

  In the embodiment of the invention shown in FIG. 1, the concentrating solar thermal apparatus comprises a substantially conventional heliostat (1) with a solar receiver (2) mounted on the tower (3). Solar receivers in the tower are used to heat the heat transfer fluid in the form of alkali metal NaK.

  In particular, in this embodiment, the heat storage unit, the steam generator, and the circulation system are all included in the tower. Such a configuration is expected to be applicable only to certain sizes of concentrating solar devices and is more likely to be more similar to the embodiment of the invention described with reference to FIG. Is not expected to apply to

  Returning to the embodiment of the invention shown in FIG. 1, the heat transfer fluid is circulated by the pump (4) through the metal phase change material (5) contained in the receiver and the enclosure (6) to form a series of heat transfer pipes (7). To substantially fill the tower through. The heat transfer fluid, ie the molten NaK heat transfer liquid, is thus heated in the heat receiver and circulates through a series of heat transfer pipes (7) constituting the first heat transfer surface.

  The steam pipe (8) constituting the second heat transfer surface is also in contact with the metal phase change material, but is disposed with a gap with respect to the heat transfer pipe (7). The steam pipe may extend from the lower tank (9) to the upper steam chamber (10) or there may be an external steam drum.

  The remainder of the concentrating solar thermal device is substantially conventional, with the usual steam treatment devices, high pressure, medium pressure, and low pressure turbines (11, 12, 13), generally numbered (14) And the condensing recycling apparatus shown.

  It should be further noted that in this case, a reheater (15) can also be constructed for heating the steam output from the high pressure turbine (11) before entering the intermediate pressure turbine (12). It is possible that this reheater has its own heliostat field (16), receiver, tower, and heat storage equipment.

  Next, with respect to the embodiment of the present invention shown in FIG. 2, in a larger concentrating solar thermal apparatus, a plurality of heliostat fields (21), in this case four heliostat fields, are each provided in the tower (23). With their own receiver (22), the output of these heat transfer fluids provides a separate configuration of heat storage, where the entire solar receiver assembly forms part of the water or steam heater and generator Connected to each other so that you can.

  In particular, in this embodiment, the heat storage section and the water and steam heating and generator are a preheater (24), boiler (25) disposed between a high pressure turbine (28) and a medium pressure turbine (29). ), A superheater (26), and a reheater (27). In each case, the appropriate unit has its own enclosure, which is not shown separately because the enclosure is configured substantially similar to that described below with reference to FIG. Includes pipes and steam pipes.

  As shown in FIG. 3, the interior of the containment enclosure (33) filled with a metal phase change material in a suitable pattern, such as a series of concentric circles, etc., in each case where a heat storage configuration according to the present invention is used. Distributing the pipes therein provides a preferred general configuration of steam pipes (31) and heat transfer fluid pipes (32).

  Preferred properties of the metal heat transfer fluid are a low melting point (preferably below room temperature), high thermal conductivity, high density, and high specific heat capacity. The low melting point is important for a reliable concentrating solar thermal device because the receiver is cold for at least half of its lifetime. In the receiver, the heat transfer fluid having a high melting point reduces the reliability of the apparatus because solidification of the heat transfer fluid causes a serious problem.

NaK is a eutectic mixture of sodium and potassium, and its eutectic composition is 78% potassium and 22% (mass%) sodium. It is liquid between minus 12.6 ° C and 785 ° C. A composition containing 40% to 90% potassium (mass%) is liquid at room temperature. Because sodium has a higher specific heat capacity than potassium, a mixture containing more sodium is preferred. The properties of NaK include high reactivity with water, and when stored in air, potassium superoxide is formed, which can ignite and has high reactivity with organic matter, This makes it dangerous to store in organic solvents and mineral oil. Eutectic NaK has a very high surface tension, a density of 0.855 g / mL at 100 ° C., and a thermal conductivity of 23.2 Wm ⁻¹ K ⁻¹ at 100 ° C.

  After years of research in the nuclear industry, a standard design and handling protocol for NaK has been established so that NaK can be used safely in concentrating solar devices.

  For metal phase change materials, two good candidates are alloys with a melting point of 557 ° C. and a heat of fusion of 498 J / g, including Al 87.76% and Si 12.24%, Al 83.14%, Si 11.7%, And an alloy having a melting point of 555 ° C. and a heat of fusion of 485 J / g, including 5.16% Mg. The former was selected because it is a general low-cost cast alloy and a high thermal conductivity alloy. Furthermore, it is non-toxic, readily available, and has the potential to improve latent heat of fusion through further alloying.

  The upper receiver temperature above 770 ° C. and the melting point of the AlSi alloy at 557 ° C. allow the use of a steam cycle. Superheated steam can therefore be generated directly by the heat stored in the metal phase change material.

  There are several ways to implement the heat transfer concept, but the final form of this concept depends on the specific design requirements. The storage enclosure need not be cylindrical, but if a tower is used, the cylindrical shape is reasonable.

  With respect to possible pipe layouts associated with the storage enclosure, more specific shapes may be appropriate for specific applications. One important factor to consider is the heat transferred through the first heat transfer surface, which is the surface of the heat transfer fluid pipe. Another factor is the distance between this heat transfer fluid pipe and the second heat transfer surface, the face of the steam pipe, to provide the necessary heat flux. Yet another requirement is the volume of metal phase change material present. Of course, the heat transfer requirements depend on the size.

  It will be appreciated that there are numerous possibilities and that the above-described embodiments of the present invention are merely representative of possible configurations.

  Accordingly, the present invention provides an efficient heat storage facility, and in particular, provides a heat storage facility suitable for combined use with a heliostat field receiver type facility.

Claims (11)

A heat storage facility for storing thermal energy at an elevated temperature,
An enclosure for holding a heat storage medium;
A first heat transfer surface for transferring heat from the circulating heat transfer fluid;
A second heat transfer surface for transferring heat from the heat storage medium to a steam pipe associated with the power generation facility,
The heat storage medium disposed between the first heat transfer surface and the second heat transfer surface is a metal phase change material;
The heat storage facility, wherein the heat transfer fluid is a liquid metal.   The heat storage facility according to claim 1, wherein the metal phase change material has high thermal conductivity, high melting latent heat, and high melting temperature.   The heat storage facility according to claim 2, wherein the melting temperature of the metal phase change material is a temperature exceeding 500 ° C.   The metal phase change material comprises an aluminum and silicon alloy containing 87.76% Al and 12.24% Si, 83.14% Al, 11.7% Si and 5.16% Mg. The heat storage facility according to claim 3, wherein the heat storage facility is selected from alloys to be configured.   The heat storage facility according to any one of claims 1 to 4, wherein the metal heat transfer fluid is in a liquid state in a wide temperature range from normal temperature to a maximum operating temperature exceeding the melting point of the metal phase change material.   The heat storage facility according to claim 5, wherein the metal heat transfer fluid is selected from a molten alkali metal or an alkali metal alloy.   The heat storage facility according to claim 6, wherein the metal heat transfer fluid is an alloy of sodium and potassium.   The heat storage facility according to any one of claims 1 to 7, wherein the heat transfer surface is a surface of a generally parallel pipe group extending through the enclosure or a surface of a generally parallel pipe group adjacent to the enclosure. .   The heat storage facility forms part of a unit selected from a preheater, boiler, superheater, reheater, or combination unit designed to perform the function of any two or more units thereof. Item 9. The heat storage facility according to any one of Items 1 to 8.   A solar thermal device comprising one heliostat field comprising an associated solar receiver mounted on a tower, wherein the thermal storage facility according to any one of claims 1 to 9 is incorporated in the tower . A solar thermal device having a plurality of heliostat fields, with an associated solar receiver mounted on the tower,
Each of the solar receivers is connected to one or more shared heat storage facilities according to any one of claims 1 to 9,
The heat storage facility is implemented in one or more units selected from a preheater, boiler, superheater, and reheater,
A solar thermal device, wherein the flow of heat transfer fluid to each of the facilities is controlled to achieve an associated purpose of the associated unit.

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