US20100314081A1

US20100314081A1 - High Temperature Graphite Heat Exchanger

Info

Publication number: US20100314081A1
Application number: US12/723,651
Authority: US
Inventors: Bradley E. Reis; Mark Segger; James T. Petroski; Robert Anderson Reynolds, III; Julian Norley; Marco Napolitano; Martin David Smalc; Kim E. Fledderman; Andrew Justin Francis; Victor William Leight; David M. Kaschak
Original assignee: Individual
Current assignee: Neograf Solutions LLC
Priority date: 2009-06-12
Filing date: 2010-03-14
Publication date: 2010-12-16
Also published as: KR200475201Y1; WO2010144841A1; JP3176537U; EP2440873A1; CN203053283U; EP2440873A4; KR20120002258U

Abstract

A graphite-based heat exchanger, especially for use as a solar energy receptor as part of the thermal process in a solar power system, including an energy collection panel, a heat spreader, and a thermal element, wherein the heat spreader is formed of flexible graphite having a density of at least about 0.6 g/cc and a thickness of less than about 10 mm, and the heat spreader further has a first side and a second side, wherein the heat spreader is in a thermal transfer relationship with the thermal element, and wherein the energy collection panel includes at least one sheet or block of graphite.

Description

This application claims the benefit of the following patent application: U.S. Provisional Application No. 61/186,678, filed Jun. 12, 2009.

BACKGROUND

1. Technical Field
The present disclosure relates to an improved high temperature heat exchanger, specifically a graphite energy receptor for use in the solar thermal process, which provides for greater and more uniform heat transfer as an element of a solar power system. The graphite heat exchanger includes a thermal element, which contains a heat transfer fluid, and provides for efficient heat transfer to the transfer fluid for solar power generation. The heat exchanger may further comprise a heat spreader, which comprises at least one sheet of compressed particles of exfoliated graphite in thermal relationship with the thermal element to improve heat transfer thereto.
2. Background Art
As concerns over the environment, the deterioration of fuel sources, and energy efficiency continue to increase, solar power plants have become the subject of worldwide attention. The conventional solar power plant has a tower-shaped configuration in which a field of heliostats reflects sunlight onto a solar receptor mounted on the tower structure. The concentrated solar energy on the receptor, or heat exchanger, heats a fluid therein, such as hydrogen, helium, oil or molten salt, to high temperatures. For instance, in a solar power tower, “cold” molten-salt at approximately 300° C. is heated to 565° C. and then pumped to a tank for storage.
To generate power, the heat transfer fluid, such as molten salt or helium, is pumped to a steam generating system to produce steam for a conventional Rankine-cycle turbine system, which, in turn, produces electricity. A similar process is employed when oil is used as the heat transfer fluid. To maximize efficiency of the solar tower, materials which are highly thermally conductive, such as graphite, are beneficial in aiding capture, storage, and transfer of the sun's energy throughout the power generation process.
Graphites are made up of layer planes of hexagonal arrays or networks of carbon atoms. These layer planes of hexagonally arranged carbon atoms are substantially flat and are oriented or ordered so as to be substantially parallel and equidistant to one another. The substantially flat, parallel equidistant sheets or layers of carbon atoms, usually referred to as graphene layers or basal planes, are linked or bonded together and groups thereof are arranged in crystallites. Highly ordered graphites consist of crystallites of considerable size, the crystallites being highly aligned or oriented with respect to each other and having well ordered carbon layers. In other words, highly ordered graphites have a high degree of preferred crystallite orientation. It should be noted that graphites possess anisotropic structures and thus exhibit or possess many properties that are highly directional such as thermal and electrical conductivity.
In the practice of the present disclosure, flexible graphite sheet materials can be employed. Such flexible graphite sheet material exhibits an appreciable degree of anisotropy due to the alignment of graphite particles parallel to the major opposed, parallel surfaces of the sheet, with the degree of anisotropy increasing upon compression of the sheet material to increase orientation. In compressed anisotropic sheet material, the thickness, i.e. the direction perpendicular to the opposed, parallel sheet surfaces comprises the “c” direction and the directions ranging along the length and width, i.e. along or parallel to the opposed, major surfaces comprises the “a” directions and the thermal and electrical properties of the sheet are very different, by orders of magnitude, for the “c” and “a” directions.
Sheets of compressed particles of exfoliated graphite are generally produced by a process familiar with the skilled artisan using natural graphite flake, which is intercalated with an intercalant solution of, for instance, a combination of nitric and sulfuric acids. The intercalated flakes are exfoliated, or expanded, by exposure to high temperatures and then compressed into sheet form. Synthetic graphite is manufactured in a different process. In the production of synthetic graphite, particles of coke are mixed with a binder, such as pitch, and heated such that the pitch softens. The mixture is then formed into shape, referred to as a “green body,” such as by molding or extrusion, depending on the desired graphite characteristics. The green body is then baked to drive off volatiles and carbonize the pitch. Additional pitch can then be impregnated into the voids created by the bake process, followed by an additional baking step. Once the desired density is achieved, the body is then graphitized by exposing to a temperature in excess of 2600° C. for a time sufficient to cause alignment of the crystals and thereby form graphite.
Accordingly, what is desired is a heat exchanger, i.e. a solar receptor, for improving the uniformity of heat transfer provided to a thermal element containing a heat transfer fluid, as well as for improving the heat flux obtained from the environmental energy and transferred thereto, by making use of the anisotropic properties of one or more carbon and graphite products, specifically, for example, flexible graphite sheets such as sheets of compressed particles of exfoliated graphite. Ideally, such a solar energy receptor is resistant to both high temperatures and thermal cycling and is capable of maximizing use of solar energy while reducing heat losses that are attributable to convection and radiation.

BRIEF DESCRIPTION

In one embodiment of the present disclosure, a heat exchanger system comprises a thermal element, such as a conduit or passageway containing transfer fluid, is provided, and a heat spreader wherein the heat spreader comprises at least one flexible graphite sheet, such as at least one sheet of compressed particles of exfoliated graphite.
In another embodiment of the disclosure, the heat spreader is in thermal contact with the thermal element to maximize heat flux between the thermal element and an energy collection panel with which thermal transfer is to occur.
In yet another embodiment, the heat spreader is in contact with the “underside” of the thermal element (underside is used with respect to the surface to be heated; in other words, underside refers to the surface opposite to the surface to be exposed to an energy source), to maximize heat flux to the thermal element from the energy collection panel.
Yet another embodiment provides a heat spreader which improves the heat flux to the thermal element, and which thereby enables the absorption of more thermal energy by the thermal element.
In still another embodiment of the disclosure, the heat spreader, which comprises at least one sheet of compressed particles of exfoliated graphite having a density of at least about 0.6 grams per cubic centimeter (g/cc), is disposed in thermal contact with both the thermal element and the surface of the heat exchanger system at which thermal transfer is to occur, such as the energy collection panel.
In another embodiment, the heat spreader has a density of at least about 1.1 g/cc, and most preferably at least about 1.5 g/cc, and/or a thickness of less than about 10 mm.
In yet another embodiment, the heat spreader, which comprises at least one sheet of compressed particles of exfoliated graphite having a thermal conductivity parallel to its major surfaces of at least about 140 watts per meter-Kelvin (W/m-K), is disposed in thermal contact with the thermal element, and it may also be in contact with an energy collection panel, which is ordinarily heated by the sun.
Still another embodiment of the disclosure comprises a heat spreader having a thermal conductivity of at least about 220 W/m-K, and more preferably at least about 300 W/m-K.
In another embodiment of the disclosure, the thermal element(s) of a heat exchanger for a solar power system is disposed in grooves or slots formed between the energy collection panel and a synthetic graphite substrate, wherein the heat spreader is positioned between the energy collection panel and the synthetic graphite substrate.
In yet another embodiment of the heat exchanger, the heat spreader comprises at least two components, a first component and a second component, wherein the first component of the heat spreader is positioned between the thermal element and the graphite substrate. The second component of the heat spreader may be positioned elsewhere.
In some embodiments, the graphite substrate comprises a recess dimensioned to accommodate the thermal element. The heat spreader may cooperate with the substrate to form a substrate spreader recess.
In other embodiments, the heat exchanger system may include layers of graphite with a thermal element positioned between the layers. Such embodiments may also include a heat spreader positioned in thermal transfer relationship with the various layers of graphite and the thermal element.
In another embodiment, the heat exchanger system may comprise a layer of graphite which is traversed by a thermal passageway for containing heat transfer fluid therein. This embodiment may also include a heat spreader positioned in thermal transfer relationship with the graphite and the thermal passageway.
Each embodiment of the heat exchanger may comprise a solar energy receptor.
In one embodiment, the heat exchanger system can also include a substrate, preferably formed of synthetic graphite, having a recess dimensioned to accommodate the thermal element, wherein the substrate is disposed adjacent the second side of the heat spreader, such that the heat spreader is positioned between the thermal element and the substrate, and wherein the substrate has a thermal conductivity of greater than about 120 W/m-K. Moreover, in some embodiments, the heat spreader can comprise two components, a first component and a second component, where the first component of the heat spreader is positioned between the thermal element and the substrate. In certain embodiments, the second component of the heat spreader extends across the recess such that the second component of the heat spreader is positioned between the thermal element and the energy collection panel at the recess.
Another aspect of the disclosure relates to a heat exchanger system having a substrate comprising a recess; a heat spreader comprising at least one sheet of compressed particles of exfoliated graphite with a density of at least about 0.6 g/cc and a thickness of less than about 10 mm, wherein the heat spreader extends into the recess of the substrate to form a substrate/spreader recess which is dimensioned to accommodate a thermal element. In other words, the heat spreader is within the recess of the substrate and, therefore, a recess is formed by the heat spreader as it sits within the recess of the substrate, to form what is called the substrate/spreader recess. The heat spreader can be formed of two elements, a first element and a second element, where the first element of the heat spreader cooperates with the substrate to form the substrate spreader recess.
In another aspect, the present disclosure relates to a heat exchanger system having a structural element having a first surface and a second surface; a protective layer having a first surface and a second surface; a thermal element positioned proximate to the second surface of the structural element and having a portion disposed toward the second surface of the structural element and a portion disposed away from the structural element, with respect to each other; a heat spreader comprising at least one sheet of compressed particles of exfoliated graphite, wherein the heat spreader is positioned in a thermal transfer relationship with both the second surface of the structural element and the thermal element, and further wherein the heat spreader is positioned in thermal transfer relationship with the portion of the thermal element disposed away from the second surface of the structural element.
Another aspect of the disclosure involves providing a heat exchanger system, comprising (a) a structural element having a first surface and a second surface; (b) a thermal element positioned adjacent the second surface of the structural element and having a portion disposed toward the second surface of the structural element and a portion disposed away from the structural element, with respect to each other; and (c) a heat spreader comprising at least one sheet of compressed particles of exfoliated graphite, wherein the heat spreader is positioned in a thermal transfer relationship with both the second surface of the structural element and the thermal element, and further wherein the heat spreader is positioned in thermal transfer relationship with a portion of the thermal element disposed away from the second surface of the structural element.
In one embodiment of the heat exchanger, the heat spreader comprises at least one sheet of compressed particles of exfoliated graphite, one sheet of which is in thermal transfer relationship with the entire surface of the thermal element. Preferably the at least one sheet of compressed particles of exfoliated graphite has a density of at least about 0.6 g/cc, more preferably at least about 1.1 g/cc, or even 1.5 g/cc. In addition, the at least one sheet of compressed particles of exfoliated graphite may have an in-plane thermal conductivity of at least about 140 W/m-K, more preferably at least about 220 W/m-K, or even as high as 300 W/m-K or higher.
The thermal transfer system can also include a substrate disposed proximate to the second surface of the structural element such that the heat spreader is positioned between the substrate and the structural element, wherein the substrate is highly conductive, that is, it has a thermal conductivity in the in-plane direction of greater than about 120 W/m-K, more preferably greater than about 150 W/m-K.
Another aspect of the heat exchanger involves providing a heat exchanger system for a solar tower, comprising: (a) an energy collection panel having a structural element with a first surface and a second surface; (b) a protective coating positioned adjacent the first surface of the structural element and having a portion disposed toward the first surface of the structural element and a portion disposed away from the structural element, with respect to each other; (c) a heat spreader comprising at least one sheet of compressed particles of exfoliated graphite having a density of at least about 0.6 g/cc and an in-plane thermal conductivity of at least about 140 W/m-K, wherein the heat spreader is positioned in a thermal transfer relationship with the second surface of the structural element, and further wherein the heat spreader is positioned in thermal transfer relationship with a thermal element.
These embodiments and others, which will be apparent to the skilled artisan upon reading the following description, can be achieved by providing a heat exchanger system, which includes a thermal element comprising a surface; and a flexible graphite heat spreader, especially a heat spreader comprising at least one sheet of compressed particles of exfoliated graphite having a density of at least about 0.6 g/cc, preferably it has a density of at least about 1.1 g/cc, and a thickness of less than about 10 mm, and further comprising a first side and a second side, wherein the heat spreader is positioned relative to the thermal element so that the heat spreader is at least partially wrapped around the thermal element such that the first side of the heat spreader is in a thermal transfer relationship with a portion of the thermal element surface.
It is to be understood that both the foregoing general description and the following detailed description present embodiments of the invention and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention and together with the description serve to explain the principles and operations of the invention. Other and further features and advantages of the present invention will be readily apparent to those skilled in the art upon a reading of the following disclosure when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of one embodiment of the heat exchanger in accordance with the disclosure.

FIG. 2 is a partial cross-sectional view of the heat exchanger of FIG. 1.

FIG. 3 is a cross-sectional view of the heat exchanger of FIG. 2.

FIG. 4 is a partial cross-sectional view of the heat exchanger of FIG. 3.

FIGS. 5-9 are a partial cross-sectional views of a graphite heat exchanger in accordance with various embodiments described herein.

FIG. 10 is a partial cross-sectional view of one embodiment of the graphite layer.

FIG. 11 is a partial cross-sectional view of one embodiment of the graphite layer including the heat spreader.

FIG. 12 is a partial cross-sectional view of one embodiment of the graphite layer including the heat spreader and thermal highway.

FIG. 13 is an exploded partial cross-sectional view of an embodiment of the graphite layer(s) included in the heat exchanger.

FIG. 14 is a partial cross-sectional view of one embodiment of the graphite layer(s) included in the heat exchanger along with the heat spreader.

FIG. 15 is an exploded partial cross-sectional view of the offset graphite layer(s) which may be included in the heat exchanger.

FIG. 16 is a partial cross-sectional view of one embodiment of the offset graphite layers which may be included in the heat exchanger along with the heat spreader.

DETAILED DESCRIPTION

As noted, the heat exchanger system is advantageously formed to include at least one sheet of flexible graphite, such as at least one sheet of compressed particles of exfoliated graphite as well as an energy collection panel, which may be comprised of graphite, and, in one embodiment, a substrate, which may be composed of synthetic graphite.
While this disclosure is written in terms of a high temperature solar heat exchanger system, it will be understood that it is meant to relate also to other types of heat exchanger systems, including radiant floor heating systems, such as wall or ceiling systems, resistance systems, under-floor staple up systems; and various cooling systems, for which the concepts taught herein will also apply.
The graphite starting materials used to provide the heat spreader in the present disclosure may contain non-graphite components so long as the crystal structure of the starting materials maintains the required degree of graphitization and they are capable of exfoliation. Generally, any carbon-containing material, the crystal structure of which possesses the required degree of graphitization and which can be exfoliated, is suitable for use with the heat exchanger. Such graphite preferably has a purity of at least about eighty weight percent. More preferably, the graphite employed for the heat spreader of the heat exchanger will have a purity of at least about 94%. In the most preferred embodiment, the graphite employed will have a purity of at least about 98%.
Compressed exfoliated graphite materials, such as graphite sheet and foil, are coherent, with good handling strength, and are suitably compressed, e.g. by roll pressing, to a thickness of about 0.05 mm to 3.75 mm and a typical density of about 0.4 to 2.0 g/cc or higher. Indeed, in order to be consider “sheet,” the graphite should have a density of at least about 0.6 g/cc, and to have the flexibility required for the present heat exchanger, it should have a density of at least about 1.1 g/cc, more preferably at least about 1.5 g/cc. While the term “sheet” is used herein, it is meant to also include continuous rolls of material, as opposed to individual sheets.
If desired, sheets of compressed particles of exfoliated graphite can be treated with resin and the absorbed resin, after curing, enhances the moisture resistance and handling strength, i.e. stiffness, of the graphite article as well as “fixing” the morphology of the article. Suitable resin content is preferably at least about 5% by weight, more preferably about 10 to 35% by weight, and suitably up to about 60% by weight. Resins found especially useful in the practice of the disclosed heat exchanger include acrylic-, epoxy- and phenolic-based resin systems, fluoro-based polymers, or mixtures thereof. Optionally, the flexible graphite may be impregnated with fibers and/or salts in addition to the resin or in place of the resin. Additionally, reactive or non-reactive additives may be employed with the resin system to modify properties (such as tack, material flow, hydrophobicity, etc.).
Unlike the heat spreader, which, in certain embodiments, comprises the above-described compressed particles of exfoliated graphite, formed in the preferred embodiment using natural graphite, the substrate and energy collection panel are preferably composed of synthetic, also called artificial or manufactured, graphite processed from particles, especially fine-grain particles, which are isostatically molded. In some embodiments, the synthetic graphite can also be formed by extrusion molding. The graphite used to form the substrate and energy collection panel is manufactured from carbon-base materials, rather than mined as a natural substance.
In manufacturing graphite, raw petroleum coke is first calcined in large rotary or shaft kilns to shrink it and to drive out the volatile content. It is then broken down by crushers and mills and sized into a series of carefully controlled fractions through screens. Selected size fractions are recombined to produce a dry aggregate. The proportion of fractions and the fraction size is varied, within limits, to control the properties of the end product.
Next, the crushed raw material is combined with a binder such as coal tar pitch to make a formable plastic mix, which, once mixed, is heated to assure homogeneity. Other binders include sugar solutions, or other materials which can be carbonized upon heating. Once heated, the mix is ready to be formed. As the graphite is formed, the long axes take a preferred, or with-grain, orientation parallel either to the direction of molding or to the direction of extrusion. The against-grain direction is perpendicular to this. The anisotropy of the graphite crystals causes differences in the with-grain and against-grain properties of the material.
However, isostatic molding produces near isotropic properties, and anisotropy of isostatic molded materials is less than other forming methods, such as press molding or extrusion. The sizing of the fractions in the dry aggregate and the ratio of the coarse fractions to the fine fractions also influence anisotropy.
The next step in forming manufactured graphite is converting the carbon to graphite, called graphitizing, which requires a temperature in the 2600° C. to 3400° C. range. During graphitization, the graphite crystals rearrange in an ordered pattern of stacked parallel planes. The properties of any piece of graphite are directly dependent on the highest temperature it reached in graphitization; however, heat losses increase exponentially with increasing temperatures, which means that each temperature increment used is increasingly expensive. However, the graphite produced in this process is mechanically stronger than natural graphite. Additionally, as a result of the extreme thermal treatment, synthetic graphite products are high purity, typically 99% carbon or more.
As noted above, the current disclosure relates to a heat exchanger comprising a heat spreader which comprises flexible graphite, such as at least one sheet of compressed particles of exfoliated graphite. The heat spreader should have a density of at least about 0.6 g/cc, more preferably at least about 1.1 g/cc, most preferably at least about 1.5 g/cc. From a practical standpoint, the upper limit to the density of the graphite sheet heat spreader is about 2.0 g/cc. The heat spreader (even if made up of more than one sheet of compressed particles of exfoliated graphite) should be no more than about 10 mm in thickness, more preferably no more than about 2 mm and most preferably not more than about 1 mm in thickness.
In the practice of the present disclosure, a plurality of graphite sheets may be laminated into a unitary article for use as the heat spreader, provided the laminate meets the density and thickness requirements set forth hereinabove. The sheets of compressed particles of exfoliated graphite can be laminated with a suitable adhesive, such as pressure sensitive or thermally activated adhesive, therebetween. The adhesive chosen should balance bonding strength with minimizing thickness, and be capable of maintaining adequate bonding at the service temperature at which heat transfer is sought. Suitable adhesives would be known to the skilled artisan, and include acrylic and phenolic resins.
The graphite sheet(s) which make up the heat spreader should have a thermal conductivity parallel to the plane of the sheet (referred to as “in-plane thermal conductivity”) of at least about 140 W/m-K for effective use. More advantageously, the thermal conductivity parallel to the plane of the graphite sheet(s) is at least about 220 W/m-K, more advantageously at least about 300 W/m-K. In certain embodiments, the heat spreader has an in-plane thermal conductivity of at least about 500 W/m-K. In other embodiments, the in-plane thermal conductivity may be at least about 1500 W/m-K.
Of course, it will be recognized that the higher the in-plane thermal conductivity, the more effective the heat spreading characteristics of the heat spreader. From a practical standpoint, sheets of compressed particles of exfoliated graphite having an in-plane thermal conductivity of up to about 600 W/m-K are all that are necessary. The expressions “thermal conductivity parallel to the plane of the sheet” and “in-plane thermal conductivity” refer to the fact that a sheet of compressed particles of exfoliated graphite has two major surfaces, which can be referred to as forming the plane of the sheet; thus, “thermal conductivity parallel to the plane of the sheet” and “in-plane thermal conductivity” constitute the thermal conductivity along the major surfaces of the sheet of compressed particles of exfoliated graphite.
Other embodiments of the heat spreader may comprise graphite sheet which can comprise pyrolytic graphite or a composite of natural graphite and pyrolytic graphite, which may provide in-plane thermal conductivity of 600 W/m-K or higher; indeed, thermal conductivity as high as 1500 W/m-K can be obtained. In certain embodiments, “pyrolytic graphite” refers to graphite materials formed from carbonized polymer films. For instance, in the production of some embodiments of pyrolytic graphite a film such as a polyimide film is first cut and shaped to anticipate the subsequent shrinkage during the carbonization step. During carbonization a large amount of carbon monoxide typically evolves from the film, resulting in a substantial shrinkage of the film. The carbonization may take place as a two step process, the first step at a substantially lower temperature than the second step. During the first step of carbonizing a polyimide film, the weight loss is primarily due to the breakage at the carbonyl groups in the imide part of the polyimide film. Specifically, the ether oxygen appears to be lost at the end of the first step. In the second step of carbonization, nitrogen gas is released during the decomposition of the imide groups of the film. The graphitization process includes a high temperature heat treatment with the temperature of the heat treatment resulting in different alignment of the carbon atoms. The resulting pyrolytic graphite has a high degree of preferred crystallographic orientation of the c-axes perpendicular to the surface of the sheet and is, consequently, extremely anisotropic. In fact, any of the structural elements of the heat exchanger 100 may comprise pyrolytic graphite or a composite graphite containing pyrolytic graphite.
Likewise, the heat exchanger comprises a high temperature heat absorption plate (sometimes referred to as an energy collection plate) and a substrate, each of which may comprise synthetic graphite. In one embodiment, the synthetic graphite can be medium-grain graphite, which, for the purposes of this disclosure, is defined as having a coke grain size with a maximum of about 1.0 mm, and preferably between about 0.6 mm and about 0.9 mm; more preferably. In another embodiment, the synthetic graphite from which the heat absorption plate and the substrate are formed is fine-grain graphite, having a coke particle size of 0.024 mm or less on average. The synthetic graphite comprising the energy collection panel and graphite substrate has a thermal conductivity with the grain of at least about 120 W/m-K. Additionally, in some embodiments of the heat exchanger, the heat absorption plate and/or the substrate may comprise extruded graphite such that each may provide anisotropy and thus support efficient transfer of heat within the heat exchanger by directing heat to the heat spreader.
Although the heat exchanger of the present disclosure is described primarily in the context of a solar thermal power system, it will be understood that the principles hereof may be applied to heating or cooling systems embedded in any other similar heat exchanger systems.
Referring now to the drawings, FIG. 1 schematically illustrates a high temperature heat exchanger 100. The high temperature heat exchanger 100 includes an energy collection panel 112, which has a protective coating 110 through which heat is absorbed when the exchanger 100 is exposed to sunlight. A thermal element 114, which can be a heating or cooling element or, alternatively, any passageway for heat transfer fluid, depending on the specific application, is in heat transfer relationship with energy collection panel 112. Heat transfer relationship means that thermal energy is transferred from one article or entity to another.
The following description primarily refers to a heating element as thermal element 114. It will be understood that thermal element 114 could also comprise a cooling element. Thermal element 114 could more generally be referred to as a heat transfer element which can either heat or cool. In the heat exchanger 100, the thermal element 114 is heated by its surroundings. Further, thermal element 114 may be comprise recess, cavity, or passageway for conveying a heat transfer fluid that is defined as the available space between various other structural features of the heat exchanger 100.
Thermal element 114 may also be any available type of heat transfer element, including but not limited to a conduit or tubing network for carrying heat transfer fluids or gases. The phrase “heat transfer fluid” shall include all fluids, gases, gels and/or phase change media suitable for transferring thermal energy.
Protective coating 110 can be any conventional material used for oxidation protection of a type suitable for use within the solar thermal system. Suitable thermal elements 114 and panels 112 are described in further detail below.
A heat spreader 116, which comprises at least one sheet of compressed particles of exfoliated graphite, is in heat transfer relationship with energy collection panel 112, and thus, thermally engages energy collection panel 112. It should be noted that the phrase “thermally engages” can include a conductive, convective, or radiative relationship (the latter two relationships including those embodiments wherein the heat spreader 116 need not be in physical contact with energy collection panel 112 as described further below). A synthetic graphite substrate 118, described in more detail herein below, lies below energy collection panel 112, with heat spreader 116 positioned between graphite substrate 118 and energy collection panel 112.
It will be appreciated that the energy collection panel 112 need not directly engage heat spreader 116 and may be separated therefrom by various layers of graphite or other materials. Thus when one layer of material within the heat exchanger 100 is described as overlying another, that does not require that they physically touch each other, unless further specific language so states. Indeed, any of the components of the heat exchanger 100 may be in thermal transfer relationship or heat exchange relationship with one another without physically touching one another.
As the energy collection panel 112 engages in photothermal conversion of solar energy, it is heated to extremely high temperatures, and these high temperatures promote oxidation. To increase the lifetime of the energy collection panel 112, a protective coating 110 is provided to reduce the rate of oxidation. The protective coating 110 may be any conventional material that will increase the service life of graphite at high temperatures by protecting the graphite against oxidation, such as quartz or a material having similar properties to quartz.
Heat spreader 116 is also in heat transfer relationship with thermal element 114. Thermal element 114 may be, for example, a transfer material or device typically found in a conventional heat exchanger system. For example, thermal element 114 may be electrical wiring elements or of the type comprising a tubing network for carrying a heat transfer fluid such as molten salt or helium. Such tubing systems in solar power towers generally use either titanium or stainless steel tubing. Such systems may also use other thermally-conductive tubing materials which resist corrosion.
The tubing of the thermal element 114 may also comprise a series of impermeable, semi-permeable, or permeable graphite tubes that can conduct helium or another phase change medium or a heat-carrying inert gas through the solar receptor. Such graphite tubes may be made impermeable or semi-impermeable with a series of resin impregnations. Using graphite tubing as thermal element 114 may alleviate any mismatch of the respective coefficients of thermal expansion of various materials used in constructing the high-temperature heat exchanger 100 according to the disclosure.
If the tubes comprising thermal element 114 are not impermeable, such as is the case with some graphite tubes, then the heat exchanger 100 can be positively pressurized with helium or other heat transfer gas in order to minimize gas loss from the permeable or semi-permeable tubing, i.e. the permeable or semi-permeable thermal element 114. Moreover, while generally round in cross-section, the thermal element 114 can also assume other cross-sectional shapes, such as oval, square, rectangular, etc.
The heat spreader 116 may comprise graphite with a polished fine surface finish, i.e. a small particle size, in order to improve thermal absorptivity. A better surface finish allows for smaller diameter tubes and higher heat transfer fluid velocities, which, in turn, improves thermal flow through the heat spreader 100 in general.
In some embodiments of heat exchanger 100, substrate 118, as shown in FIGS. 1, 3 and 4-9, comprises a synthetic graphite material, such as a medium-grain graphite with high strength properties that excels in higher temperature applications like SLX® Graphite produced by GrafTech International Holdings Inc. The thermal conductivity of substrate 118, when a manufactured or synthetic graphite material is used, should be at least about 120 W/m-K, more preferably at least about 150 W/m-K in the in-plane direction. Preferably, but not necessarily, a fine-grain graphite may also be used. As such, graphite substrate 118 can help ensure that as much thermal energy as possible is transferred from energy collection panel 112 to thermal element 114.
As noted, heat spreader 116 comprises at least one sheet of compressed particles of exfoliated graphite and may be positioned between substrate 118 and energy collection panel 112. As such, since heat spreader 116 is in heat transfer relationship with both thermal element 114 and energy collection panel 112, heat spreader 116 will spread the thermal energy (be it through heating or cooling) to or from thermal element 114 more uniformly from the surface of energy collection panel 112.
Most advantageously, heat spreader 116 is in heat transfer relationship with the entire outer surface of thermal element 114. However, in another embodiment, heat spreader 116 may also be in heat transfer relationship with the portion of thermal element 114 which is furthest from energy collection panel 112. In other words, when viewed in the orientation of FIGS. 1-4, heat spreader 116 should be at least partially wrapped around thermal element 114 and thus be in heat transfer relationship (most preferably in actual physical contact) with at least a portion of the surface of thermal element 114, preferably the entire surface of thermal element 114, but alternatively only a portion of thermal element 114.
In this way, heat spreader 116 improves the heat flux to thermal element 114 by providing a pathway for thermal energy from the surfaces or portions of thermal element 114 which are in the remotest heat transfer relationship (i.e., the most physically removed) from energy collection panel 112. Moreover, the flexibility and conformability of heat spreader 116 can improve the thermal transfer with energy collection panel 112, which is an important advantage from an efficiency standpoint. In addition, since heat spreader 116 has a relatively uniform cross-sectional thickness and density, the advantageous physical properties of heat spreader 116 are uniform across its entire area.
In one embodiment of the heat exchanger 100, illustrated in FIG. 5, given the flexible nature of the sheets of compressed particles of exfoliated graphite used to form heat spreader 116, heat spreader 116 can be positioned between substrate 118 and energy collection panel 112, and extend under thermal element 114 (it will be recognized that the term “under” refers to that portion of thermal element 114 facing away from the energy collection panel 112). Alternatively, heat spreader 116 can be formed of two discrete components, first heat spreader component 116 a and second heat spreader component 116 b, as illustrated in FIGS. 2 and 3. First heat spreader component, 116 a, comprises at least one sheet of compressed particles of exfoliated graphite, as described above, and is positioned between substrate 118 and energy collection panel 112, but does not extend under thermal element 114. Rather, first heat spreader component 116 a does not extend into the area in which thermal element 114 is positioned, as shown in FIG. 6; or, first heat spreader component 116 a extends completely across the upper surface of thermal element 114 and, thus, is in good thermal contact with the upper portion of thermal element 114.
Second heat spreader component 116 b is a discrete component which thermally (and, advantageously physically) contacts and is at least partially wrapped around thermal element 114 or surfaces thereof, including portions of the underside or sides of thermal element 114, and thermally contacts (most preferably physically contacts) first heat spreader component 116 a, as shown in both FIGS. 2 and 3. Second heat spreader component 116 b can be formed of at least one sheet of compressed particles of exfoliated graphite, or it can be a different material, such as an isotropic material like a metal like aluminum. Second heat spreader component 116 b may have a different density and/or a lower through-plane conductivity than first heat spreader component 116 a. In some embodiments, second heat spreader component 116 b will have a lower through-plane conductivity than first heat spreader component 116 a in order to reduce heat loss. For example, in some embodiments, 116 b may provide some shielding to minimize heat loss to the substrate 118.
In some arrangements it may also be advantageous to have second heat spreader 116 b only partially surrounding the sides of thermal element 114 (not shown), allowing thermal element 114 to be installed and/or attached to second heat spreader component 116 b, which is in turn installed or otherwise attached to first heat spreader component 116 a.
In yet another embodiment, illustrated in FIG. 8, second heat spreader component 116 b can completely envelope, or extend about, thermal element 114, provided second heat spreader element 116 b remains in heat transfer relationship (and, most advantageously, actual physical contact) with first heat spreader component 116 a.
In another embodiment, substrate 118 or energy collection panel 112 comprises a series of stacked highly thermally conductive graphite plates. The graphite plates may comprise, for example, stacked natural graphite such as GrafTech® SS500 having a thermal conductivity of 500 W/m-K. Alternatively, a series of stacked synthetic graphite plates may be used. Ideally, graphite with high thermal conductivity is used to form the stacked plates in order to quickly move heat from the surface of the heat spreader 100 to the thermal element 114.
In yet another embodiment, substrate 118 or energy collection panel 112 comprises porous carbon or conductive graphitic foam block, which acts as a thermal receptor. In some embodiments, graphitic foam having thermal conductivity of at least about 120 W/m-K, preferably at least about 200 W/m-K, thermal conductivity at room temperature is used as the substrate. Heat transfer fluid may be directed through the porous substrate or through pathways cut through the substrate. Orientation and number of passages through the substrate depend on the required pressure drop across the receptor. A series of conduits, which may be arranged normal to the gas flow, could be cut in the substrate to improve porosity and to reduce the pressure drop across the substrate. In one embodiment, the conductive graphitic foam block would be sealed from the atmosphere.
The energy collection panel 112 may also comprise non-porous carbon or graphite block. The substrate may comprise inexpensive graphite from anode grade cokes with thermal conductivity of 100 W/m-K or higher, even thermal conductivity of 150 W/m-K or higher, at room temperature.
The use of the heat spreader 116 in the heat exchanger 100, with its greater thermal contact with a heating element, can significantly improve the heat flux from an energy collection panel 112 to a thermal element 114. Accordingly, it is possible to array the heating elements for the heating system closer together, and/or increase the temperature of the transfer fluid flowing through the tubing, with resultant significant improvement in energy efficiency.
In one embodiment of the heat exchanger 100, as illustrated in FIG. 9, thermal element 114 may be defined by a series of graphite machined blocks 120 that are either directly or indirectly cemented or fastened together to create gas or fluid flow passages 122 in between the blocks 120, wherein the passages 122 are equivalent to thermal element 114. In one embodiment, the heat transfer fluid, such as helium, would be allowed to flow unrestricted through the passages 122 to transfer heat to or from the heat exchanger 100. In FIG. 9, thermal element 114 is defined as the passages 122 between the graphite blocks 120.
In some embodiments, the graphite used to form the blocks 120 may be resin impregnated in order to reduce permeability of the blocks. High temperature graphite cements could be used to cement the graphite blocks 120 together. Moreover, the entire heat exchanger 100 may be positively pressurized with helium, or alternatively with another heat transfer fluid, in order to minimize gas loss from the passages 122. Pressurizing the exchanger 100 is particularly beneficial if the graphite forming the blocks 120 and passages 122 is not impermeable.
When a portion of the heat exchanger 100 is pressurized, helium is often utilized as the heat transfer fluid. Helium is useful due to its high heat capacity as an inert gas; however, another phase change medium could alternatively be used to pressurize the heat exchanger 100.
In another embodiment, shown in FIGS. 10-12, thermal element 114 may be formed by at least one graphite block 124 having a series of premachined passages 126 cut from it forming the graphite block plenum. The graphite block 124 may also be referred to as a “layer of graphite,” “sheet of graphite,” or “piece of graphite.” The premachined passages 126 as shown in FIG. 10 define thermal element 114 in some embodiments of the heat exchanger 100.
As illustrated by FIG. 11, the passages 126, and thus the thermal element 114, may be lined with a layer of graphite, which functions as heat spreader 116. Indeed, the passages 126 may be wholly or partially lined with at least one sheet of compressed particles of exfoliated graphite, i.e. heat spreader 116, which is in thermal transfer relationship with the thermal element to improve heat transfer thereto.
Furthermore, as shown in FIG. 12, at least one sheet of compressed particles of exfoliated graphite, thermal highway 117, may traverse the graphite block 124, be contained within graphite block 124, or run between the passages 126 to facilitate uniform and efficient heat transfer thereto. FIG. 12 shows one embodiment including thermal highway 117 in a particular orientation; however, thermal highway 117 may be positioned differently based upon changing geometry of the passages 126 and may even be positioned such that it runs through the passages 126.
Thermal highway 117 comprises either synthetic or natural graphite material having a thermal conductivity of at least about 50 W/m-K; in some embodiments, the thermal conductivity is at least about 150 W/m-K or even higher. Each embodiment of the heat exchanger 100 that includes the graphite block may be pressurized to facilitate heat transfer and to prevent heat loss.
In another embodiment, shown in FIG. 13, thermal element 114 may be formed by connecting a first graphite block 128 and a second graphite block 130, each having at least one half slot cut from at least one block 128, 130. The one half slot may be semi-cylindrical in shape. Thus, when the first graphite block 128 and the second graphite block 130 are connected together, the two blocks 128 and 130 form a series of passages 122 for the helium or other heat transferring fluid to flow through the heat exchanger 100, as shown in FIG. 14. The blocks may be bolted together, cemented together using high temperature graphite cements or otherwise connected by any means known in the art. The resulting passages 126, which are analogous to thermal element 114, may be defined between any number of graphite blocks 128, 130.
Graphite block 128, 130 thickness may be minimized to reduce the thermal path length between the protective layer 110 and the thermal element 114. Also, as illustrated in FIG. 14, a heat spreader 116 comprised of natural or synthetic graphite may be used to line the inside of the passages 126 to ensure maximum heat flow around their respective perimeters. Heat spreader 116 may comprise, for example, at least one sheet of compressed particles of exfoliated graphite. Heat spreader 116 may either partially or completely line the passages 126. Heat spreader 116 may expand upon heating in order to offset any expansion of the contained heat transfer fluid.
The size of passages 126 may increase between the heat transfer fluid inlet of the heat exchanger 100 and the heat transfer fluid outlet. This increase in passage size may be progressive along the pathway of the heat transfer fluid through the heat exchanger 100. As the heat transfer fluid moves through the heat exchanger 100 and is heated, passage 126 size increases. The increase in passage size helps to reduce pressure drop across the heat exchanger, which results, for example, when the heat transfer fluid, such as helium, heats up and expands as it travels through the passages 126 of the heat exchanger 100. Use of graphite provides structurally sound pathways for the heat transfer fluid to travel through the heat spreader 100 even at high temperatures.
Moreover, the number of passages 126 may vary. For example, a number of smaller diameter passages may be bored through a thin graphite block instead of a few larger holes. Smaller holes would have more surface area per unit volume to maximize heat flow from the graphite to the heat transfer fluid. Pressure drop would increase with smaller holes, but conventional modeling may be used to optimize the number of holes versus the inherent strength of the block.
Additionally, as illustrated in FIG. 15, a multilayered series of passages 126 may be used to maximize heat transfer. Multiple graphite blocks 128 and 130 can be layered to form passages 126, and the passages 126 created by the stacked blocks 128 and 130 may be offset from one another to further facilitate heat transfer. Regardless of the offset of the passages 126, the terminal ends of each block 128 and 130 may line up and be flush with one another.
The various layers of passages 126 may be offset from each other by row, as shown if FIG. 16, in order to minimize graphite thickness and to decrease the thermal pathway from the external surface of the heat spreader 100 to the thermal element 144, i.e. the passages 126, and consequently to the heat transfer fluid. Here, again, as shown in FIG. 16, a heat spreader 116 may either partially or fully line the passages 126 in order to minimize thermal gradients.
All cited patents and publications referred to in this application are incorporated herein by reference.
The invention thus being described, it will be apparent that it may be varied in many ways. All such modifications and variations may be practiced in any suitable combination. Such modification and variations are not to be regarded as a departure from the spirit and scope of the present invention and all such modifications and variations as would be apparent to one skilled in the art are intended to be included in the scope of the following claims.

Claims

1. A heat exchanger system, comprising:

(a) a thermal element comprising a surface;

(b) a heat spreader comprising at least one sheet of compressed particles of exfoliated graphite having a density of at least about 0.6 g/cc and a thickness of less than about 10 mm, and further comprising a first surface and a second surface, wherein the heat spreader is positioned relative to the thermal element so that the heat spreader is at least partially wrapped around the thermal element such that the heat spreader is in a thermal transfer relationship with a portion of the thermal element surface; and

(c) a substrate comprising graphite, wherein the substrate is positioned relative to the heat spreader so that the substrate is in a thermal transfer relationship with the heat spreader.

2. The heat exchanger system of claim 1, wherein the substrate comprises a recess dimensioned to accommodate the thermal element, wherein the substrate is disposed adjacent to the heat spreader, such that the heat spreader is positioned between the thermal element and the substrate, and wherein the substrate has a thermal conductivity of greater than about 150 W/m-K.

3. The heat exchanger system of claim 1, wherein the heat spreader comprises two components, a first component and a second component, further wherein the first component of the heat spreader is positioned between the thermal element and the substrate.

4. The heat exchanger system of claim 1, which further comprises an energy collection panel.

5. The heat exchanger of claim 2, wherein the heat spreader comprises two components, a first component and a second component, further wherein the second component of the heat spreader extends across the recess such that the second component of the heat spreader is positioned between the thermal element and an energy collection panel.

6. The heat exchanger system of claim 1, wherein the at least one sheet of compressed particles of exfoliated graphite has a density of at least about 1.1 g/cc.

7. The heat exchanger system of claim 1, wherein the heat spreader comprises two components, a first component and a second component, further wherein the first component of the heat spreader cooperates with the substrate to form a substrate spreader recess.

8. The heat exchanger system of claim 1, further comprising a structural element comprising a surface that is in thermal transfer relationship with a portion of the thermal element surface and wherein the structural element comprises at least one layer of synthetic graphite.

9. The heat exchanger system of claim 1, wherein the substrate comprises at least one layer of synthetic graphite.

10. The heat exchanger system of claim 1 further comprising a protective layer having a first surface and a second surface wherein the first surface of the protective layer is located between an energy source and the thermal element.

11. The heat exchanger system of claim 1, which comprises a solar energy receptor.

12. A heat exchanger system comprising:

(a) a first layer of graphite having a first surface and a second surface;

(b) a second layer of graphite positioned below the second surface of the first layer of graphite;

(c) a thermal element positioned between the first layer of graphite and the second layer of graphite;

(d) a heat spreader comprising at least one sheet of compressed particles of exfoliated graphite having a density of at least about 0.6 g/cc and an in-plane thermal conductivity of at least about 140 W/m-K, wherein the heat spreader is positioned in thermal transfer relationship with the first layer of graphite, the second layer of graphite, and the thermal element.

13. The heat exchanger system of claim 12, wherein the at least one sheet of compressed particles of exfoliated graphite has an in-plane thermal conductivity of at least about 220 W/m-K.

14. The heat exchanger system of claim 12 wherein the first layer of graphite has an in-plane thermal conductivity of at least about 120 W/m-K.

15. The heat exchanger system of claim 12, which comprises a solar energy receptor.

16. A heat exchanger system, comprising:

(a) at least one layer of graphite;

(b) at least one thermal passageway for containing heat transfer fluid therein, wherein the passageway traverses the at least one layer of graphite;

(c) a heat spreader comprising at least one sheet of compressed particles of exfoliated graphite having a density of at least about 0.6 g/cc and an in-plane thermal conductivity of at least about 140 W/m-K, wherein the heat spreader is positioned in thermal transfer relationship with both the at least one layer of graphite and the thermal passageway.

17. The heat exchanger system of claim 16, wherein the at least one sheet of compressed particles of exfoliated graphite has an in-plane thermal conductivity of at least about 220 W/m-K.

18. The heat exchanger system of claim 16, wherein the at least one layer of graphite has an in-plane thermal conductivity of at least about 120 W/m-K.

19. The heat exchanger system of claim 16, which comprises a solar energy receptor.

20. The heat exchanger system of claim 16, further comprising a protective layer having a first surface and a second surface wherein the first surface of the protective layer is located between an energy source and the thermal element.