Patent Application
20100181054
The present invention relates to energy conversion in general, and, more particularly, to heat exchangers.
Patent application Ser. No. 12/484,542, filed Jun. 5, 2009 (Attorney Docket: 711-231US)
Large, robust heat exchangers are needed to build a closed-cycle Ocean Thermal Energy Conversion (OTEC) plant. OTEC heat exchangers must withstand prolonged exposure to a primary working fluid, typically ammonia, as well as a large secondary flow of seawater. Further, it is highly desirable, if not necessary, that such heat exchangers provide high overall heat transfer coefficients, exhibit minimal mechanical pumping losses, and are light weight. It is also important that the materials and fabrication costs of these heat exchangers are not excessive.
In the prior art, heat exchangers are typically either “shell and tube heat exchangers” or “plate-frame heat exchangers.”
Shell and tube heat exchangers commonly comprise a plurality of tubes enclosed within an open volume of a surrounding shell. Typically, the tubes and shell comprise conventional metallic materials. In some cases, shell and tube heat exchangers comprise shells made of more exotic materials, such as composites, graphite foam, etc. Such materials enable a reduction in the weight of a heat exchanger.
Shell and tube heat exchangers are widely used for liquid to liquid, liquid to vapor, evaporator and condenser applications. In operation, a primary fluid or vapor flows through the tubes. A secondary fluid, typically air, flows around and through the space that surrounds the tubes inside the shell. Heat is exchanged between the fluids/vapors through the walls of the tubes.
Although shell and tube style heat exchangers are often used for ship/submarine service, they are typically characterized by relatively low heat transfer coefficients. As a result, shell and tube heat exchangers are not well-suited for OTEC applications since they would require undesirably large surface areas. This drawback is further exacerbated by the fact that there is normally only a small difference between seawater and process fluid temperatures.
Plate-frame heat exchangers are typically constructed of thin metal plates joined together to form a thermal transfer path between the primary and secondary fluid streams. The plates are normally joined using welding, brazing, epoxy, or mechanical attachment. Plate-frame heat exchangers find use in a wide range of liquid to liquid, liquid to vapor, evaporator and condenser applications throughout the petro-chemical, pharmaceutical and beverage processing industries. In some commercial plate-frame style heat exchangers, the process fluid flows between alternate plate pairs, and the secondary fluid flows in the intervening plate pairs. Heat transfer is enhanced via chevron or other turbulence-inducing stamped patterns. Plate-frame heat exchangers have a high capital cost. For OTEC systems, wherein the heat exchangers are in contact with sea water, materials such as titanium or high alloy steel are normally required. As a result, the cost associated with such heat exchangers would be increased further.
Brazed aluminum plate frame heat exchangers are widely used in gas processing and cryogenic applications such as nitrogen and natural gas liquefaction and for LNG re-gassification. In cryogenic and gas processing heat exchangers, brazed aluminum fins are used on both the warm and cold fluid sides of the heat exchanger. Such heat exchangers are not well-suited for OTEC applications, however, since seawater is corrosive to brazed fin joints. Further, these heat exchangers are typically characterized by small passages that are prone to clogging due to biofouling.
In an attempt to overcome their corrosion problem in OTEC applications, brazed aluminum fin heat exchangers have been developed wherein aluminum extrusions are used in place of brazed fins on their seawater sides. The brazed fins are retained on the process fluid (e.g., ammonia) side of the heat exchanger, however. Unfortunately, these modified brazed fin/extruded aluminum fin heat exchangers require large braze furnaces for their fabrication. As a result, they are also quite expensive to produce.
The present invention provides a heat exchanger that overcomes some of the limitations and drawbacks of the prior art. Embodiments of the present invention enable thermal coupling between a primary fluid, such as a working fluid, and a secondary fluid, such as seawater. Embodiments of the present invention are particularly well-suited for OTEC applications.
An embodiment of the present invention comprises a plate-frame heat exchanger that includes graphite-foam-based plates, each of which comprises a plurality of conduits for conveying the primary fluid through the heat exchanger. The conduits are formed in the graphite foam itself. The plates are arranged in a frame such that spaced between the plates define channels for conveying the secondary fluid through the heat exchanger.
Each plate comprises a “sandwich” structure of a graphite-foam core that interposes a pair of barriers. The barriers are substantially impervious to both the primary and secondary fluids. As a result, the barriers inhibit cross-contamination of the two fluids.
In some embodiments, the barriers are made of a thermally enhanced polymer that is disposed on each of two opposing sides of the graphite-foam core. The barriers are formed on these surfaces in an injection molding process.
In some embodiments, the barriers are made of metal sheets that are attached to the two surfaces of the graphite-foam core using either an adhesive or by welding, soldering, or brazing the sheets to a thin metal film disposed on the two surfaces.
In some embodiments, the conduits formed in the graphite-foam core comprise open pores and capillaries that serve to enhance heat transfer through the graphite foam.
In contrast to prior-art heat exchangers, embodiments of the present invention provide:
An embodiment of the present invention comprises a heat exchanger for thermally coupling a first fluid and a second fluid, wherein the heat exchanger comprises: (1) a first frame, wherein the first frame comprises a first core, wherein the first core comprises graphite foam comprising first conduits for conveying the first fluid, and a first barrier for fluidically isolating the first fluid and the second fluid; and (2) a second frame, wherein the second frame comprises a second core, wherein the second core comprises graphite foam comprising second conduits for conveying the first fluid, and a second barrier for fluidically isolating the first fluid and the second fluid; wherein the first frame and the second frame collectively define a channel for conveying the second fluid.
Turbo-generator 104 is a conventional turbine-driven generator. Turbogenerator 104 is mounted on floating platform 102, which is a conventional floating energy-plant platform. Platform 102 is anchored to the ocean floor by mooring line 132 and anchor 134, which is embedded in the ocean floor. In some instances, platform 102 is not anchored to the ocean floor but is allowed to drift. Such a system is sometimes referred to as a “grazing plant.”
In typical operation, pump 114 pumps a primary fluid (i.e., working fluid 108), in liquid form, through closed-loop conduit 106 to heat exchanger 110-1. Ammonia is often used as working fluid 108 in OTEC systems; however, it will be clear to one skilled in the art that any fluid that evaporates at the temperature of the water in surface region 118 and condenses at the temperature of the water in deep water region 126 is suitable for use as working fluid 108 (subject to material compatibility requirements).
Heat exchanger 110-1 and 110-2 are configured for operation as an evaporator and condenser, respectively. One skilled in the art will recognize that the operation of a heat exchanger as evaporator or condenser is dependent upon the manner in which it is configured within system 100. Heat exchanger 110 is described in detail below and with respect to
In order to enable its operation as an evaporator, pump 116 draws warm secondary fluid (i.e., seawater from surface region 118) into heat exchanger 110-1 via conduit 120. In a typical OTEC deployment, the water in surface region 118 is at a substantially constant temperature of approximately 25 degrees centigrade (subject to weather and sunlight conditions). At heat exchanger 110-1 heat from the warm water is absorbed by working fluid 108, which induces working fluid 108 to vaporize. After passing through heat exchanger 110-1, the now slightly cooler water is ejected back into the body of water via conduit 122. The output of conduit 122 is typically located deeper than the depth of surface region 118 to avoid reducing the average water temperature in the surface region.
The expanding working fluid 108 vapor is forced through turbogenerator 104, thereby driving the turbogenerator to generate electrical energy. The generated electrical energy is provided on output cable 112. Once it has passed through turbogenerator 104, the vaporized working fluid enters heat exchanger 110-2.
At heat exchanger 110-2, pump 124 draws cold secondary fluid (i.e., seawater from deep water region 126) into heat exchanger 110-2 via conduit 128. Typically deep water region 126 is approximately 1000 meters below the surface of the body of water, at which depth water is at a substantially constant temperature of a few degrees centigrade. The cold water travels through heat exchanger 110-2 where it absorbs heat from the vaporized working fluid. As a result, working fluid 108 condenses back into liquid form. After passing through heat exchanger 110-2, the now slightly warmer water is ejected into the body of water via conduit 130. The output of conduit 130 is typically located at a shallower depth than that of deep-water region 126 to avoid increasing the average water temperature in the deep-water region.
Pump 114 pumps the condensed working fluid 108 back into heat exchanger 110-1 where it is again vaporized; thereby continuing the Rankine cycle that drives turbogenerator 104.
Conduit 202 is a metal-based fluidic conduit for conveying a primary fluid, such as ammonia, from inlet 208 to outlet 210. Conduit 202 is typically folded into a serpentine shape to substantially maximize the amount of its surface area that is in contact with shell 204.
Shell 204 is a form of thermally conductive graphite foam. Shell 204 includes recess 206 for locating conduit 204. Shell 204 further includes pass-throughs that facilitate fluidic connection to inlet 208 and outlet 210. Shell 204 typically comprises two halves that fit together in a clam shell configuration to completely surround and sandwich conduit 202. Shell 204 also seals conduit 202 from direct contact with secondary fluid during operation of the heat exchanger.
In operation as an evaporator, relatively cooler primary fluid flows into inlet 208 in liquid form. Heat exchanger 200 is placed into a configuration wherein relatively warmer secondary fluid flows along the outside surfaces of shell 204. Heat from the secondary fluid is conducted through the graphite foam to the top and bottom surfaces of conduit 202. The conducted heat causes the primary fluid within conduit 202 to vaporize. Vaporized primary fluid exits heat exchanger 200 at outlet 210.
In operation as a condenser, relatively warmer primary fluid flows into inlet 208 in vapor form. Heat exchanger 200 is placed into a configuration wherein relatively cooler secondary fluid flows along the outside surfaces of shell 204. Heat from the primary fluid is conducted through the graphite foam from the top and bottom surfaces of conduit 202. This heat is conveyed into the secondary fluid, which acts as a heat sink. The loss of heat causes the primary fluid within conduit 202 to condense into a liquid. Primary fluid exits heat exchanger 200 at outlet 210 in liquid form.
One skilled in the art will recognize that heat exchanger 200 has several limitations, particularly with respect to OTEC applications. For example, the efficiency of heat exchange between conduit 202 and the graphite foam of shell 204 is a function of the contact area between these elements. Since only the top and bottom surfaces of conduit 202 are in direct contact with graphite foam, the heat transfer coefficient heat exchanger 200 is low.
Further, for operation in an OTEC application, the relatively small difference between the temperatures of the primary and secondary fluids would require that heat exchanger 200 be inordinately large in order to affect sufficient heat exchange.
Still further, conduit 202 is typically a heavy metallic tube. As a result, the weight of conduit 202 is a significant contributor to the overall weight of heat exchanger 200.
Heat exchanger core 302 (hereinafter referred to as “core 302”) comprises a plurality of plates that are joined to a frame to form a complete plate-frame heat exchanger core. Core 302 comprises conduits through which working fluid 108 can flow. Core 302 also includes channels through which seawater can flow. As they are conveyed through core 302, heat is transferred between the two fluids. Core 302 is described in more detail below and with respect to
Manifolds 304-1 and 304-2 (referred to, collectively, as manifolds 304) provide working fluid 108 and seawater to the channels of core 302. Each of manifolds 304 comprises distributor 306 and ports 308. Manifolds 304-1 and 304-2 are described in detail below and with respect to
Core 602 is a graphite-foam extrusion that comprises a plurality of conduits 608 for conveying working fluid through plate 402. Conduits 608 are depicted as having a circular cross-section; however, one skilled in the art will recognize that channels 608 can have any suitable cross-sectional shape (e.g., triangular, square, rectangular, asymmetric, etc.). Conduits 608 comprise graphite wall surfaces that comprise conduits 610. Conduits 610 are open pores and/or capillaries that extend into the thickness of the graphite foam. Conduits 610 enable evaporation and condensation to occur over a much larger surface area than comparable conventional heat exchangers. Heat exchangers in accordance with the present invention, therefore, can be characterized by an improved ratio of heat transfer surface density to heat exchanger volume, as compared to prior-art heat exchangers. As a result, for a given heat transfer duty, heat exchangers in accordance with the present invention can be smaller than prior art shell and tube or plate-frame heat exchangers. In some embodiments, conduits 610 are not present or are too small to significantly enhance heat transfer through core 602.
Typically, the graphite-foam composition used for core 602 has a specific gravity within the range of 0.6-0.7, and high tensile strength. As a result, core 602 is both lightweight and strong enough to withstand the fluid pressures and flow dynamics that develop within heat exchanger 110. Heat exchangers in accordance with the present invention, therefore, can be much lighter than comparable conventional metal-based heat exchangers.
Further, graphite foam has a bulk thermal conductivity of approximately 180 W/M Deg C. range. This thermal conductivity is as high as pure bulk aluminum, for example, and much higher than the effective conductivity of most aluminum fin constructions.
At sub-operation 702, a thermally enhanced polymer is injected into the mold to coat each of surfaces 606. The thermally enhanced polymer material is selected such that it is substantially impervious to both working fluid 108 and seawater once the polymer is cured. For the purposes of this Specification, including the appended claims, a thermally enhanced polymer is defined as a polymer composition that comprises a thermally conductive filler material, such as boron nitride, diamond composites, metals, aluminum nitride, and the like. It will be clear to one skilled in the art, after reading this specification, how to specify, make, and use a thermally enhanced polymer suitable for forming barriers 604. In some embodiments, a non-thermally enhanced polymer is used to coat each of surfaces 606; however, such embodiments are typically characterized by a lower thermal transfer rate between working fluid 108 and the seawater in the heat exchanger. In some embodiments, the surface of the graphite foam comprises graphite-foam fibers that protrude into the thickness of barriers 604. In such embodiments, the thermal transfer rate between the working fluid and secondary fluid is enhanced. It should be noted, however, that the integrity of the barrier, vis-à-vis the mitigation of fluid cross-contamination, is retained.
At sub-operation 703, the thermally enhanced polymer is cured to form barriers 604. As a result, barriers 604 are layers that are substantially impervious to both working fluid 108 and seawater. Barriers 604 are disposed on opposing surfaces 606 of core 602. Barriers 604, therefore, inhibit cross-contamination of the two fluids during operation of heat exchanger 110. In embodiments wherein barriers 604 comprise a non-thermally enhanced polymer, the thickness of the layers is typically reduced to reduce their thermal resistance.
Since graphite foam is typically quite porous, the polymer flows into the pores and other surface structure of surfaces 606. As a result, the polymer forms a strong mechanical bond between core 602 and barriers 604. In addition, this increases the surface area of the interface between the graphite foam and the polymer, which results in a highly thermally conductive interface between core 602 and barriers 604.
Upon completion of the injection molding process, plate 402 emerges from the mold as a single unit.
At operation 502, plates 402-1 and 402-2 are mounted in seats 408 of frame 404, which comprises sides 406, bottom 412, and top 414.
Sides 406 are rigid aluminum alloy members that comprise seats 410. Seats 410 are recessed regions of sides 406 for accepting and locating plates 402-1 and 402-2.
Each of bottom 412 and top 414 is a rigid aluminum-alloy plate that mates with sides 406 to complete frame 404.
Sides 406 and bottom 412 and top 414 are joined with substantially galvanic corrosion-free joints 416 to mitigate the effects of corrosion due to exposure to seawater.
In some embodiments, joints 416 are formed using friction-stir welding. Friction-stir welding employs a rotating probe, wherein a force is applied to the probe perpendicular to the weld surface to join similar metals or alloys together. The immense friction between the probe and materials causes material in the immediate vicinity of the probe to heat up to temperatures below its melting point. This softens the adjoining sections, but because the material remains in a solid state, its original material properties are retained. Movement of the probe along the weld line forces the softened material from the two pieces towards the trailing edge causing the adjacent regions to fuse, hence forming a weld.
As opposed to other common joining techniques, including other methods that produce galvanic corrosion-free joints, friction-stir welding has several performance advantages. In particular, the resultant weld is comprised of the same material as the joined sections. As a result, galvanic corrosion due to contact between dissimilar metals at the joint is reduced or eliminated. Furthermore, the resultant weld retains the material properties of the material of the joined sections. Friction-stir welding is described in more detail in U.S. patent application Ser. No. 12/484,542, filed Jun. 5, 2009 (Attorney Docket: 711-231US), which is included by reference herein.
Seats 410 are arranged so that bottom 412 and plate 402-1 collectively define channel 408-1, plates 402-1 and 402-1 collectively define channel 408-2, and plate 402-2 and top 414 collectively define channel 408-3 (Channels 408-1 through 408-3 are collectively referred to as channels 408). Channels 408 are suitable for conveying seawater through heat exchanger 110.
At operation 503, seals 418 are formed along each intersection of a plate 402 and its respective seat 410. Seals 418 are lines of polymer adhesive applied to seal the spaces between plates 402 and seats 410 so as to inhibit the flow of either working fluid 108 or seawater through them. Seals also serve to inhibit motion of plates 402 relative to frame 404.
At operation 504, manifolds 304-1 and 304-2 are joined with core 302 using a substantially galvanic corrosion-free joining technology, such as friction-stir welding.
During operation as an inlet manifold, distributor 306 receives working fluid from ports 308 and distributes it into conduits 608 of core 302. During operation as an outlet manifold, conduits 804 receive working fluid from conduits 608 of core 302 and provide it to ports 308.
Distributor 306 comprises region 802 which physically expands the flow of primary fluid in order to introduce it into conduits 804 with substantially uniform flow velocity. The number of conduits 804 is based on the number of plates 402 in core 302. Conduits 804 are interposed by access ports 806. Access ports 806 enable access for seawater to channels 408-1 through 408-3 of core 302. In some alternative embodiments, manifold 304 comprises a second plurality of conduits for fluidically coupling seawater and channels 408-1 through 408-3. Although the illustrative embodiment of the present invention comprises a heat exchanger through which primary and secondary fluids flow in a substantially parallel manner, it will be clear to one skilled in the art, after reading this specification, how to specify, make, and use alternative embodiments of the present invention in which the primary and secondary fluids flow in a non-parallel manner, such as a cross-flow arrangement.
It should be noted that conduits 804 are tapered from wide end 708 to narrow end 710. This tapering facilitates the flow of primary fluid through conduits 608 with substantially uniform flow velocity.
Although the illustrative embodiment comprises manifolds 304 that are made of the same material as frame 404, it will be clear to one skilled in the art, after reading this specification, how to specify, make, and use alternative embodiments that comprises manifolds consisting of a different corrosion-resistant material (e.g., a fiberglass composite), which can be joined to frame 404 with a substantially galvanic corrosion-free joint.
Plates 902 are thin, thermally conductive sheets affixed to surfaces 606 of core 602 by means of adhesive 904. Core 602 and plates 902 collectively define a “sandwich” structure, wherein plates 902 inhibit cross-contamination between working fluid 108 and seawater during heat exchanger operation. In some embodiments, plates 902 are solid, mesh, or perforated sheets that augment the mechanical strength of core 602 to enable plate 900 to withstand greater pressures and forces inside heat exchanger 300. In embodiments wherein plates 902 are not solid, however, an additional barrier layer, such as barrier 604, is provided to inhibit fluidic cross-contamination. In some embodiments, plates 902 are made of aluminum, aluminum alloy, or another metal that is suitable for joining plate 900 to frame 404 by means of a galvanic corrosion-free joining technology, such as friction-stir welding. In such embodiments, therefore, joints 418 are substantially galvanic corrosion-free joints.
In some embodiments, plates 902 are affixed to core 602 by brazing or welding them to thin metal layers deposited on each of surfaces 606. In such embodiments, adhesive 904 is not required.
It is to be understood that the disclosure teaches just one example of the illustrative embodiment and that many variations of the invention can easily be devised by those skilled in the art after reading this disclosure and that the scope of the present invention is to be determined by the following claims.
This case claims priority of U.S. Provisional Patent Application U.S. 61/145,996, which was filed on Jan. 21, 2009 (Attorney Docket: 711-246US), and which is incorporated herein by reference. In addition, the underlying concepts, but not necessarily the language, of the following case is incorporated by reference: U.S. patent application Ser. No. 12/484,542, filed Jun. 5, 2009 (Attorney Docket: 711-231US). If there are any contradictions or inconsistencies in language between this application and one or more of the cases that have been incorporated by reference that might affect the interpretation of the claims in this case, the claims in this case should be interpreted to be consistent with the language in this case.
| Number | Date | Country | |
|---|---|---|---|
| 61145996 | Jan 2009 | US |
