Thermal energy storage apparatus

Abstract

The present invention provides a thermal energy storage (TES) apparatus having a fast response to heat, which is also simple, lightweight, compact, and inexpensive to fabricate. In one preferred embodiment of the present invention, phase change material (PCM) is enclosed in a tubular container. A brush-like heat spreading element having radial bristles is installed in the tube within the PCM. The bristles are made of wires or fibers having good thermal conductivity and their ends are in a good thermal contact with the interior wall of the tube. Heat from external source is deposited into the tube exterior and conducted through the wall, transferred to the bristles, and conducted through the bristles in a generally radial direction substantially through out the PCM volume. The TES may be used in buildings to stabilize temperature and thus reduce the cost of building climate control such as heating and air conditioning.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from the U.S. provisional patent application U.S. Ser. No. 61/201,885, filed on Dec. 16, 2008, entitled “THERMAL ENERGY STORAGE APPARATUS.”

GOVERNMENT RIGHTS IN THIS INVENTION

This invention was made with U.S. government support under contract number FA8650-08-M-5026. The U.S. government may have certain rights in this invention.

FIELD OF THE INVENTION

This invention relates generally to apparatus and methods for conduction of heat and more specifically to to apparatus and methods for storage of thermal energy.

BACKGROUND OF THE INVENTION

The invention is for a thermal energy storage (TES) apparatus and method. TES is often used in devices and systems to temporarily store heat. For example, a TES apparatus may be used to average out a temperature in a heating system in a building or a vehicle with the objective of reducing the cost of energy expended in heating. In particular, TES can reduce the cost of heating a building during winter season by absorbing heat (possibly with the air of a solar panel) during warm daytime hours and releasing stored heat during colder nighttime hours. Similarly, a TES apparatus may be used to average out a temperature in an air conditioning system in a building or a vehicle with the objective of reducing the cost of energy expended in heating or air conditioning. Another application of TES is for averaging out of waste heat load in an automotive engine as disclosed by the Applicant in U.S. Pat. No. 7,464,672, entitled “Engine cooling system with overload handling capability,” issued to the Applicant on Dec. 16, 2008, which is hereby incorporated by reference in its entirety. A TES apparatus may use a phase change material (PCM).

Phase Change Materials: For the purposes of this invention, a material that changes in heat content upon undergoing a reversible solid-liquid phase transformation is defined as a phase change material (PCM). PCMs, synonymously known as latent thermal energy storage materials, are used for thermal energy storage. The absorption of the necessary quantity of energy by the solid PCM results in melting. The energy absorbed by the PCM to change phase at its characteristic melting temperature is known as the latent heat of fusion. The latent heat of fusion stored in the liquid state is released upon resolidification. Thus the PCM may absorb thermal energy from a body at a higher temperature than the PCM, until the PCM undergoes a reversible melt. A molten PCM may transfer thermal energy to a body at a lower temperature than the PCM and it may thereby undergo a reversible solidification (freeze).

Efficient PCMs have several desirable thermo-chemical properties including high latent heat of fusion, high thermal conductivity, low supercooling, and the ability to cycle thermally from solid to liquid and back to solid many times without degradation. The term “supercooling” refers to a discrepancy between the temperature at which solidification (freezing) initiates and the melting temperature of a given PCM when cooled and heated under quiescent conditions. A significant amount of PCM research is devoted to finding nucleating agents additives that will suppress supercooling. The term “additives” includes, in addition to nucleating agents, precursors of such additives which are non-detrimental to the function of the phase change materials. Considerations for selection of suitable PCMs may also include melting temperature, density, packaging, toxicity and cost. Many suitable PCM have a very low density, generally less than 2 grams per cubic centimeter and, in many cases, less than 1 gram per cubic centimeter.

Many PCM capable of storing significant amount of heat are also very poor thermal conductors. This makes it very challenging to transport heat into and out of the PCM, and limits the usefulness of such PCM in a TES. Heat spreading elements may be used to transport heat through out PCM volume, thereby improving the volumetric (bulk) thermal conductivity of PCM. Use of such heat spreading elements can significantly increase the rates at which heat is transported to or from the TES. As a result, TES has a faster temporal response to applied heat, see, for example, U.S. Pat. No. 7,106,777 granted to A. Delgado, Jr et al. on Sep. 12, 2006, which is hereby expressly incorporated by reference in its entirety. Such heat spreading elements disclosed in prior art may be formed as foams or structures. However, heat spreading elements of prior art are heavy, bulky, expensive to construct, and expensive to install into the TES. In view of the limitations of the prior art, there is a need for an improved TES apparatus capable of rapidly absorbing applied heat, and capable of rapidly releasing absorbed heat.

In summary, prior art does not teach a TES capable of fast response to applied heat that is also simple, lightweight, compact, and inexpensive to fabricate. It is against this background that the significant improvements and advancements of the present invention have taken place.

SUMMARY OF THE INVENTION

The present invention provides a TES apparatus having a fast response to heat, which is also simple, lightweight, compact, and inexpensive to fabricate.

In one preferred embodiment of the present invention, TES is configured as a tubular container (tube) substantially filled with PCM. A heat spreading element (HSE) having radial bristles is installed in the tube within the PCM. The HSE may be formed as a brush, having bristles extending radially from a central core. Brushes having bristles made of appropriate material and extending radially from a central core have been known in prior art as “pipe brushes”, see FIG. 1. Pipe brushes are typically used for cleaning of pipes, types, bottles, and laboratory test tubes. In the HSE of the subject invention, the bristles are made of wires or fibers having good thermal conductivity. Ends of the bristles are arranged to be in a good thermal contact with the interior wall of the tubular container. Heat from an external medium, which may be liquid, gas, solid, or environment is deposited into the exterior surface of the tubular container, conducted through the container wall, transferred to the bristles, and conducted through the bristles in a generally radial direction, and substantially transferred into the PCM. Conversely, heat may be released by the PCM, deposited substantially into the HSE bristles, conducted through the bristles and into the interior wall of the tubular container, conducted through the the container wall, and transferred from the exterior surface of the tubular container into a liquid, gas, solid, or environment. In either case, conduction of heat in radial directly inside the TES tubular contained takes place substantially in the radial bristles, thereby greatly enhancing the speed at which heat can be transferred to and from the volume of the PCM.

Accordingly, it is an object of the present invention to provide a TES apparatus having the capability to rapidly absorb large amounts of heat, which is also simple, lightweight, compact, and inexpensive to fabricate.

It is another object of the invention to provide a TES apparatus having the capability to rapidly release large amounts of heat, which is also simple, lightweight, compact, and inexpensive to fabricate.

It is yet another object of the invention to provide a heat spreading element which is also simple, lightweight, compact, and inexpensive to fabricate.

It is still another object of the invention to enhance radial conduction of heat in a PCM.

These and other objects of the present invention will become apparent upon a reading of the following specification and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a pipe brush for cleaning of tubes, pipes, and bottles (Prior Art).

FIG. 2 shows a cross-sectional view through the inventive TES apparatus in a plane parallel to the longitudinal axis.

FIG. 3 shows a a cross-sectional view through the inventive TES apparatus in a plane perpendicular to the longitudinal axis.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Selected embodiments of the present invention will now be explained with reference to drawings. In the drawings, identical components are provided with identical reference symbols. It will be apparent to those skilled in the art from this disclosure that the following descriptions of the embodiments of the present invention are merely exemplary in nature and are in no way intended to limit the invention, its application, or uses.

Referring now to FIG. 2, there is shown a cross-sectional view through the TES apparatus 100. The TES apparatus 100 comprises a tubular container 102, heat spreading element (HSE) 104, and PCM 106. The tubular container 102 is preferably made of material having good thermal conductivity, such as, but not limited to copper, copper alloys, aluminum, and aluminum alloys. The tubular container 102 has an external surface 110 adapted for communicating heat to and from a medium 112, such as gas, air, liquid, solid, or an environment. In some variants of the subject invention, the tubular element 102 may be substantially immersed in the medium 112. In addition, the tubular container 102 has an internal surface 114. The interior volume of the tubular container 102 is substantially filled with PCM 106. The internal surface 114 may be adapted for communicating heat to and from PCM 106. For example, the internal surface 114 may be equipped with groves and ridges, or other forms of surface extensions. The HSE 104 has radially extending bristles 108 in a good thermal contact with the internal surface 114. The bristles 108 are formed as wires, fiber, or filament, and are made of material having good thermal conductivity. For example, the bristles 108 may be made of metal such as copper, copper alloys, aluminum, or aluminum alloys. As another example, the bristles 108 may be made from carbon fiber or from graphite fiber. In particular, the bristles 108 may be made from Thornel K-1100 graphite fiber available from Cytec Industries Inc., Woodland Park, N.J. The Thornel K-1100 graphite fiber has a reported longitudinal thermal conductivity around 1,000 W/m-degree K, which greatly exceeds the thermal conductivity of known metals. In one variant of the invention, the HSE 104 may be generally formed as a pipe brush having bristles 108 that radiate from a central core 116. The central core 116 may be formed by twisting together two (or more) lengths of wire 118a and 118b. In particular, the of wires 118a and 118b are twisted so that the bristles 108 are trapped between them, thereby forming a pipe brush-like structure. As already noted, pipe brushes equipped with bristles of appropriate material are used for cleaning bottles, tubes, and pipes. It is important to make the brush diameter slightly larger than the diameter of the internal surface 114, so that upon insertion, the bristle ends are forced into a good thermal contact with the internal surface 114, see FIG. 3. Preferably, the HSE 104 is installed into the tubular container 102 first, and the PCM in a molten form is poured in afterwards.

While the tubular container 102 shown in FIG. 3 has a circular cross-section, the invention may be practiced with tubular containers having elliptical, square, rectangular, hexagonal, polygonal, and other suitable cross-sections. The HSE 104 also may be formed to have other than round cross-section. The internal wall 114 may be equipped with groves to designed to receive the ends of bristles 108 and thus enhance heat transfer between the tubular container 102 and the bristles 108. Suitable groves may be radial or longitudinal with respect to the tubular container 102. The external surface 110 may also include suitable surface extensions such as groves, ridges or fins. The PCM 106 may be a wax, paraffin, or other suitable material.

In operation, heat is transferred from medium 112 to the external surface 110 of the tubular container 102, conducted through the wall to the internal surface 114, transferred to the bristles 108, conducted through the bristles 108, and transferred to the PCM 106. As a result of received heat, the PCM 106 may substantially melt, thereby temporarily storing the heat. Conversely, the PCM 106 may substantially solidify and release heat. Released heat is conducted to the bristles 108, and through the bristles 108 to the internal surface 114 of the tubular container 102, conducted through the wall to the external surface 110, and transferred therefrom to the medium 112.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” and “includes” and/or “including” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

The terms of degree such as “substantially”, “about” and “approximately” as used herein mean a reasonable amount of deviation of the modified term such that the end result is not significantly changed. For example, these terms can be construed as including a deviation of at least ±5% of the modified term if this deviation would not negate the meaning of the word it modifies.

Moreover, terms that are expressed as “means-plus function” in the claims should include any structure that can be utilized to carry out the function of that part of the present invention. In addition, the term “configured” as used herein to describe a component, section or part of a device includes hardware and/or software that is constructed and/or programmed to carry out the desired function.

While only selected embodiments have been chosen to illustrate the present invention, it will be apparent to those skilled in the art from this disclosure that various changes and modifications can be made herein without departing from the scope of the present invention as defined in the appended claims. Furthermore, the foregoing description of the embodiments according to the present invention are provided for illustration only, and not for the purpose of limiting the present invention as defined by the appended claims and their equivalents. Thus, the scope of the present invention is not limited to the disclosed embodiments.

Claims

1. A thermal energy storage (TES) apparatus comprising:

a) a tubular container having an external surface, an internal surface, and internal volume;

b) a phase change material (PCM) substantially filling said interior volume; and

c) a heat spreading element (HES) inside said interior volume;

said HES having radially extending bristles;

said bristles being substantially immersed in said PCM;

said bristles having end tips;

said end tips being in a good thermal contact with said interior surface of said tubular container.

2. The TES of claim 1 further comprising groves on said interior surface.

3. The TES of claim 1 further comprising surface extensions on said interior surface.

4. The TES of claim 1 wherein said external surface is substantially immersed in a medium selected from the family consisting of liquid, gas, solid, and environment.

5. The TES of claim 1 wherein said external surface is substantially immersed in a medium selected from the family consisting of liquid, gas, solid, and environment.

6. The TES of claim 1 wherein said bristles are made of material selected from the family consisting copper, copper alloys, aluminum, aluminum alloys, graphite fiber, carbon fiber, and Thornel K-1100 graphite fiber.

7. The TES of claim 1 wherein said bristles are configured as wires.

8. The TES of claim 1 wherein said tubular container is made of material selected from the family consisting copper, copper alloys, aluminum, and aluminum alloys.