A METHOD FOR AMPLIFYING THE EXERGY OF THERMOCLINES

Abstract

The present invention describes a method for enhancing the exergy level of a Thermal Energy Storage (TES) single-tank unit to a level that is nearly equal to that of a two-tank (hot and cold tank) TES system. The present method applies to single-tank TES and may be used in domestic hot-water cylinders, solar water heaters, buffer tanks for hot or chilled fluid storage or in Concentrated Solar Power (CSP) plants. It can be applied at the manufacturing stage of the TES or, while in operation.

Claims

1. A method for maximizing the exergy of a Thermal energy storage (TES) tank, said method comprising the use of a thermally insulating suspension wherein said thermally insulating suspension comprises a matrix made by a plurality of overlapping floating elements, thus, providing an improved performance with respect to spherical pellets or, in general, elements that do not feature a flat face.

2. The method according to claim 1, wherein said overlapping floating elements comprise at least one of plates and wafers.

3. The method according to claim 1, wherein said overlapping floating elements are smaller than the diameter of a port hole of the TES tank and are suitable to be introduced through said port hole during normal operation activity or installed at the manufacturing stage.

4. The method according to claim 1, wherein said overlapping floating elements have a form comprising circular, rectangular elliptical or any other shape that best suits the form of the TES internal walls.

5. The method according to claim 1, wherein said overlapping floating elements comprise at least a first core material of relatively higher density and a second thermally insulating material of relatively lower density, encapsulating said first core material.

6. The method according to claim 1, wherein the TES tank contains hot and cold fluids and wherein a resultant density of the suspension matrix is designed to be of intermediate value between the densities of the hot and cold fluids of the TES.

7. The method according to claim 1, wherein said suspension matrix comprises a plurality of floating elements of mixed shape and size.

8. The method according to claim 1, wherein said TES tank is configured to work with higher temperature storage fluids including molten salts, said molten salts comprising molten nitrate, molten chlorides, a molten halides salt, or molten metals.

9. The method according to claim 5, wherein said first core material comprises copper or stainless steel and said second thermally insulating material comprises alumina paper to operate with temperature above 450? C.

10. The method according to claim 5, wherein a ceramic-to-metal binder is used to bond said first and said second materials.

11. The method according to claim 5, wherein said first core material comprises AISI-316 stainless steel and said second thermally insulating material, wrapping said first core material, comprises mechanically-needled ceramic blanket layers for being used with molten salt, said blanket layers being stitched together around the edge by alumina thread, and said thermally insulating material being sprayed with a pore-sealing coating layer of Boron Nitride (BN), said coating layer being applied and cured on the outer surface of said thermally insulating material.

12. The method according to claim 5, wherein said first core material comprises AISI-304 or AISI-316 stainless steel and said second thermally insulating material, wrapping said first core material, comprises mechanically needled alumina-silicate blanket layers, said floating element being encapsulated in a refractory metal, comprising Tantalum, having the purpose of providing a 30-year lifespan in said molten halide salts.

13. The method according to claim 1, wherein said TES tank is a single tank storage system being able to deliver the exergy levels of a two-tank storage system.

14. The method according to claim 1, wherein said TES tank belongs to a domestic hot-water cylinder, solar water heaters, buffer tanks or Concentrated Solar Power (CSP).

15. The method according to claim 5, wherein said overlapping floating elements comprise a cast of said first core material and said second thermally insulating material.

16. The method according to claim 5, wherein the overlapping floating elements comprise of a single material of a shell of any shape made out of metallic or ceramic materials.

17. The method according to claim 7, wherein said mixed shape comprises at least one of spherical and ellipsoidal.

Description

SHORT DESCRIPTION OF THE DRAWINGS

[0032] The figures given herein provide an embodiment and the gist of the invention. Any variations are intended to be within and not as a departure from the scope of the invention.

[0033] FIG. 1 shows a floating element design for domestic hot-water cylinders according to an embodiment of the present invention.

[0034] FIG. 2(a) is cross-sectional view of a vertical domestic hot-water cylinder carrying a thermally insulating wafer matrix according to an embodiment of the present method. The figure also presents Computational Fluid Dynamics results CFD with and without the use of the wafer matrix.

[0035] FIG. 2(b) is an illustration of the wafer matrix function to sink beneath the heat source in the presence of a rising plume of heated fluid according to an embodiment of the present method.

[0036] FIG. 2(c) is a cross-sectional view of a horizontal domestic hot-water cylinder, further highlighting the floating element's ability to sink beneath the heat source (for example the back-up electric heater and coil) according to an embodiment of the present method.

[0037] FIG. 3(a) is a simulated temperature field in a solar water heater tank that uses the present method, after 3 hours of solar-thermal charging.

[0038] FIG. 3(b) is a simulated temperature field in a solar water heater tank that does not use the present method, after 3 hours of solar-thermal charging.

[0039] FIG. 3(c) shows the calculated absorbed thermal power of the solar water heater with and without the use of the present method.

[0040] FIG. 4 is a top and cross-sectional view of a floating element suited for application in a solar salt TES according to an embodiment of the invention.

[0041] FIG. 5(a) is a top and cross-sectional view of a floating element design suitable for TES using molten chlorides, according to another embodiment of the present invention. In this example, the floating element comprises materials that permit a 30-year lifespan at a temperature of 1050? C.

[0042] FIG. 5(b) is a side-view of the floating element design for use in molten chlorides.

[0043] FIGS. 6(a) and 6(b) are typical cross-sectional views of a CSP TES utilising the floating elements matrix, according to an embodiment of the present method. The schematics exemplify the function of the matrix system to avoid the dead volume that results from the use of the divider plate concept.

[0044] FIG. 7 is evidence of the relatively better heat blocking effect of the present invention of using a plurality of elements that feature at least a flat face as opposed to the concept of using elements having only curved surfaces such as balls.

[0045] FIG. 8 is an axial water temperature profile in a tank having initially a hot water mass at 45? C. and a cold mass at 10? C., while (a) is the profile that results 2 hours after, when using circular wafers to block heat diffusion from the hot to the cold mass and (b) is the corresponding profile that results from using balls as the heat blocking matrix.

DETAILED DESCRIPTION

[0046] The following discussion is presented to enable a person skilled in the art to make and use the invention. Various modifications to the embodiments will be readily apparent to those skilled in the art, without departing from the scope of the present invention as claimed. Thus, the present invention is not intended to be limited to the embodiments described therein, but it must be accorded the widest scope consistent with the principles and features disclosed herein and defined in the appended claims.

[0047] The invention described herein is based on the method of using a matrix made by a plurality of overlapping floating elements, said floating elements having at least one flat face, such as plates or wafers, that form a thermally insulating suspension, having a density intermediate to that of the hot and cold fluid regime of a TES, for both domestic hot water and concentrated solar energy (CSP) applications. The flat face and the element overlapping features permit sealing of any gaps in between the elements, thereby, minimizing heat diffusion compared to three-dimensional shapes that do not feature at least one flat face or consist of a layer of elements.

[0048] Said plates or wafers, which form said plurality of overlapping floating elements of the suspension matrix, may have different size and shape, said shape comprising circular, elliptical or any other shape that best suits the intended application or the TES shape: for example, floating elements of rectangular shape provide a better contact with the internal wall of a TES of rectangular shape.

[0049] In one embodiment of the present invention, said suspension matrix may comprise overlapping floating elements having one and the same shape or more than one shape and one and the same size or more than one size, in this way offering a more effective heat barrier, as required by the tank geometry. For example, a plurality of square and circular wafers may be deployed in a rectangular tank with rounded corners, as the former provides a relatively better seal against the tank flat walls. Similarly, the latter performs relatively better at the rounded corners of the tank.

[0050] According to an embodiment, said suspension matrix may comprise more than one layer to form a matrix of overlapping layers, each layer comprising a plurality of overlapping floating elements, thereby, reducing the thermal diffusion from the hot to the cold fluid regions of the TES, which may take place though gaps formed between adjacent floating elements in the matrix.

[0051] According to a preferred embodiment of the present method, the material composition of plates or wafers, which form said plurality of overlapping floating elements having at least one flat face, may comprise at least two materials: one relatively denser core material, which is encapsulated in a thermally insulating material of relatively lower density.

[0052] Referring to FIG. 1, a floating element, which is suitable to be used in hot-water storage according to an embodiment of the present method, is showed in top-down and in cross-sectional (B-B) views. The floating element, which specifically consists of a wafer (1), comprises a core material made of stainless-steel sheet (2), which is interposed between two layers of flexible ceramic paper (alumino-silicate) (3). A suitable binder (4) is applied on either side of the metal sheet, according to the manufacturer's instructions.

[0053] According to the present method, the density of the suspension matrix is designed to be of intermediate value between the density of the hot and cold fluid regime of the TES tank.

[0054] The suspension matrix density may be calculated from the following equation:

[00001]${?}_T=\frac{1}{V_T}{.Math.}_{i=1}^N{?}_i{V}_i$

where ?.sub.T, V.sub.T are respectively the floating element density and volume, i is the constituent material index and ?.sub.i, V.sub.i are the respective density and volume of each of the N materials composing the floating element. Depending on the hot water outlet temperature of the boiler, heat pump or solar system, and the temperature of the relatively cooler layer, the material thicknesses can be designed to achieve the required intermediate density value. For example, using the above equation, a flexible aluminosilicate sheet or ceramic paper (?=100 kg/m.sup.3) and stainless-steel sheet (?=7650 kg/m.sup.3) having a thickness of 4 mm and 0.5 mm respectively, yields a wafer density of 986 kg/m.sup.3. This value of density leads to a matrix of elements suspended beneath a hot-water layer at 70? C. and above a relatively colder layer at 30? C.

[0055] The metal sheet thickness (2) (for example, stainless-steel or copper-sheet), should be less than 1 mm to ensure adequate wafer flexibility against any thermal contraction of the hot-water cylinder or thermal expansion of the floating elements.

[0056] Referring to FIG. 2(a), a floating element matrix (5) according to an embodiment of the present method, used in a vertical hot-water cylinder (6), is shown. The floating element matrix (5), for example a wafer matrix, essentially limits the thermal diffusion from the hot-(7) to the cold-fluid (8). Numerical simulation results (9, 10) obtained using the Navier-Stokes equations coupled with the energy equation indicate a drop in the hot water temperature from 90? C. (for example, the hot-water temperature output of a central heating oil or gas burner) to 70? C. over a period of 3 hours, as a result of the thermal diffusion. Similarly, a hot water temperature of 65? C. (for example, the hot-water temperature output of a typical domestic heat pump) reduces to 56? C.

[0057] For both scenarios, if the cold water is supplied at a temperature between 5? C. and 10? C. and is mixed with hot water at a shower head or a tap to yield water at 45? C., then the use of the floating element matrix (5), which helps to attain a relatively higher water temperature in the tank, implies a reduced consumption of hot water by a factor of 1.8. In other words, by keeping the temperature of the hot water (7) in the cylinder (6) at a relatively higher temperature, the supply of hot water from the cylinder (6) is about half of that without using the suspension matrix (5) of floating elements.

[0058] Referring to FIG. 2(b), it is shown that in the presence of an ascending jet of heated fluid (13) generated by the electric heater (12) or by the boiler/heat-pump coil (11), the floating element matrix (5) sinks beneath the heat source (11 or 12). The floating element is specifically designed to have a length, Lw, which is about a third of the spacing, S, between the cylinder wall (6) and the coil (11), in this way preventing the element from getting jammed between the cylinder wall (6) and coil (11). As also shown in FIG. 2(b), the relatively small dimensions of the floating elements allow the suspension matrix (5) to reside along the curved walls (14) of the cylinder (6), thus overcoming any dead space effects, that is any unused space left between the barrier and the tank wall, as in the case of the divider plate method.

[0059] Referring to FIG. 2(c), a floating matrix according to another embodiment of the present method is shown. According to this embodiment, a specific floating element matrix suitable for utilization in TES systems including hardware and having curved walls, is developed. The floating element length, in this case, is specifically designed to be a third of the coil pitch, thereby preventing the element from getting jammed between the coil loops.

[0060] With reference to FIG. 3(a), the simulated temperature field inside a solar water heater tank that utilizes the present method after 3 hours of solar-thermal charging is shown. Similarly, FIG. 3(b) illustrates the corresponding contour for the same tank, but this time without using the present method. The relatively higher temperature achieved in the former case is evident. FIG. 3(c) compares the calculated thermal power of the solar water heater with and without the use of the present method. The power of the solar water heater has increased by a factor of 1.5. The present method, which minimizes the heat diffusion from the hot to the colder water layer and, thereafter, the re-entry to and loss of this heat from the solar collector, has helped to nearly double the collection of heat by the solar collector.

[0061] According to different embodiments of the present method, the matrix of overlapping floating elements may be applicable to other TES fluids and working temperature. For instance, the method may be applied for molten salt storage fluids, comprising molten nitrate or higher temperature storage fluids, like, for example, molten chloride salts for a working temperature above 600? C. The material composition of the overlapping floating element used in this application may include, for example, a core material of copper or stainless-steel sheet, sandwiched between alumina paper to be suitable for hot-water energy storage TES.

[0062] Any industrial ceramic-to-metal binder that conforms to environmental and health and safety guidelines can be used for bonding the layers within the matrix. Other shapes may be adopted, for example, a thermally insulating compressible material casted into spheres having a denser core of, for example, metal.

[0063] Referring to FIG. 4, a top-down and cross-sectional view of a floating element suitable to be used in a molten salt TES are shown. According to this embodiment, the floating element consists of a plate comprising an AISI-316 steel core (16) wrapped in mechanically needled ceramic (alumina-silicate) blanket (17). The layers are bonded using a ceramic-to-metal binder (18). The plate assembly is completed by stitching the ceramic layers around the circumference using alumina thread (19). A pore-sealing Boron Nitride (BN) coating (20) is applied on the outer layer of the plate. The BN layer also helps to reduce friction, thereby, facilitating the movement of one plate with respect to another.

[0064] To achieve a density value which is intermediate to that of the hot (560? C., ?=1734 kg/m.sup.3) and cold (300? C., ?=1899 kg/m.sup.3) solar salt, a metal sheet thickness of 4 mm and diameter of 250 mm must be used as core part of the floating element and an alumino-silicate blanket thickness and diameter of 25 mm and 300 mm respectively must be used as thermal insulation layer. These values yield a plate density of 1814 kg/m.sup.3, which exactly an intermediate value between the hot and cold solar salt densities. These dimensions are given as an example only and clearly other dimensions, number of layers and materials can be considered more suitable for the intended application.

[0065] According to another embodiment of the present method, the matrix of floating elements is suited for relatively high temperature fluids comprising molten halide salts like: [0066] NaClMgCl.sub.2 (63-37 mol %) for a temperature up to 700? C., [0067] NaClKCl for a temperature up to 750? C., [0068] LiFNdF.sub.3NdO (27-63-10 wt %) for a temperature up to 1050? C.

[0069] Referring to FIG. 5a, the floating element consists of a core material (16) of AISI-316 or AISI-304 steel wrapped in mechanically needled ceramic blanket (17). A suitable ceramic-to-metal binder (19) is used to bond the materials, as this can facilitate the handling of the metal plate during the encapsulation. Tantalum sheet is used as the encapsulation shell (21), since this refractory metal has a corrosion rate of less than 0.03 mm/30 years and extending the lifetime to more than 30 years in application comprising molten halide salts.

[0070] The Tantalum sheet thickness must be in the range 0.07-0.125 mm. to allow for flexibility, preventing any thermal ratcheting effects of the TES. Laser welding may be used to create leak-tight Tantalum sheet joints (22), indicated in FIGS. 5(a) and 5(b). In this embodiment, the TES storage fluid should be under an Argon atmosphere to prevent corrosion of the Tantalum encapsulation.

[0071] According to a preferred embodiment of the present method, said overlapping floating elements have a size compatible with the introduction through a TES port hole during normal operating conditions.

[0072] According to another embodiment, said overlapping floating elements are simply introduced into the TES at the manufacturing stage.

[0073] Referring to FIG. 6(a), a typical schematic of a CSP TES (23), with back-up electric heaters (24), is shown. The TES fluid (25) includes a floating element matrix (26). As already noted, the floating elements allow for introduction into an operating CSP TES through a port hole (27). The floating element diameter or size is customizable, as required by the available port hole diameter, therefore the present method is applicable to a TES which is already in operation, and it is not required for the floating element matrix to be installed during the TES manufacturing stage.

[0074] In FIG. 6(b), the capacity of the floating matrix method to bypass the dead energy volume that results through the use of a divider plate, is illustrated. In fact, the floating elements will sink beneath the heat source when the cold fluid is heated.

[0075] FIG. 7 shows simulation results of thermal diffusion across (a) overlapping square wafers (elements with flat face) and (b) overlapping balls that help to demonstrate the improved heat blocking performance of the former over the latter. As per these illustrative simulations, FIG. 8 shows the relatively higher temperature that exists in the TES using the present method of overlapping elements that feature flat faces with respect to elements featuring no flat faces, such as spheres.

[0076] Finally, numerous modifications and variations can be made to the present invention, all of which fall within the scope of protection of the invention as defined in the appended claims.