Coiled heat exchanger with beam spreader especially for use with solar-powered gas processors

Abstract

An apparatus and method for solar energy powered gas processing. There is disclosed a processor assembly for use in heating a process or feed gas by means of a convective heat exchanger which receives solar energy from a focused solar energy collector. A specialized heat exchanger is described, which includes a conical envelope into which a metal tube is spirally wound, such that the tube is coiled into a conical spiral corresponding generally to the envelope. A reflective beam spreader is situated near the vertex of the envelope cone to re-direct incident solar radiation toward the exchanger tube. The apparatus thus has a cavity into which solar energy is focused to be incident upon the exchanger tube to superheat a gas or gas mixture flowing through the tube. The heated gas can be used in various gas reaction processes, such as steam methane reformation, a reverse water-gas shift reaction, or to drive a heat engine. Various auxiliary features are disclosed for increasing the efficiency of the solar heat transfer into the flowing process gas.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of the filing of U.S. Provisional Patent Application Ser. No. 61/127,034 entitled Low Entropy Heat Exchanger, Especially for Use with Solar Gas Processors, and Beam Spreader Useable Therewith, filed on May 10, 2008 and the entire specification thereof is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention (Technical Field)

The present invention relates generally to systems for solar heating and processing of gases, particularly to heat exchangers for use in such systems, and more specifically to a heat exchanger for use in association with a solar energy collector, such as a focusing dish, to convert solar energy into thermal energy to process gases or to drive, for example, a Brayton Engine.

2. Background Art

One of the most critical challenges confronting mankind is that of dwindling sources of non-renewable energy. Consequently, a wide variety of increasingly sophisticated and promising efforts have been, and are, made in the field of solar energy. Devices and methods for exploiting solar energy as a renewable resource fall into at least two general categories: those attempting to convert solar energy directly into end-use energy (such as photovoltaic electricity generation, and passive thermal heating of dwellings), and those seeking to harness solar energy as an intermediate energy source for processing feedstock into end-use fuels (e.g., methane).

There are known apparatuses and methods for exploiting solar energy to process feedstock gasses to generate directly useable energy and/or derivative fuels. Two examples which serve as background to the present disclosure are the systems and methods of U.S. Pat. No. 6,066,187 to Jensen, et al., entitled “Solar Reduction of CO₂,” and U.S. Pat. No. 7,140,181 to Jensen, et al., entitled “Reactor for Solar Processing of Slightly-absorbing or Transparent Gases,” both of which name a co-inventor in common with the present application. The disclosures and teachings of these two patents are incorporated herein in by reference.

In the former '087 patent to Jensen, et al., the red shift of the absorption spectrum of CO₂ with increasing temperature permits the use of sunlight to photolyze CO₂ to CO. The disclosed processes of the '087 patent to Jensen, et al., include: preheating CO₂ to near 1800 K; exposing the preheated CO₂ to sunlight, whereby CO, O₂ and O are produced; and cooling the hot product mix by rapid admixture with room temperature CO₂. The excess thermal energy may be used to produce electricity, and to heat additional CO₂ for subsequent process steps. The product CO may be used to generate H₂ by the shift reaction or to synthesize methanol.

In the latter '181 patent to Jensen, et al., there is disclosed a solar-powered reactor for processing of slightly absorbing and transparent gases to providing storable, renewable, energy through solar dissociation of gas molecules. The dissociation products are the precursors readily useable and-use liquid and gaseous fuels, such as hydrogen and methanol/ethanol. An apparatus and method using a solar concentrator (such as a focusing trough or dish) directed at the receiving end of a reactor are disclosed. A range of designs of reactors for the dissociation of gases, both those that absorb slightly in the visible spectrum and those that are transparent in the visible and only absorb in the infrared, are described.

The methods and apparatuses of the foregoing two patents, however, involve the heating of the process gases to over 2,000 degrees C., complicating the design, and increasing construction costs, for functional reactor systems. It would be desirable to provide a solar-energy base system for generating useable energy, particularly derivative storable fuels such as methane, but which does not involve such relatively high operating temperatures. More specifically, lower operating temperatures (e.g., around 800-1100° C.) might be coupled with higher gas through-put, but at comparatively lower entropies, to permit the generation of directly exploitable energy, or for the reformation of methane as an end-use storable/portable fuel, or other hot gas processing.

Against the foregoing background, the present apparatus and method were conceived and reduced to practice.

SUMMARY OF THE INVENTION

An apparatus and method for solar energy powered gas processing. There is disclosed a processor assembly for use in heating a process or feed gas by means of a convective heat exchanger which receives solar energy from a focused solar energy collector. A specialized heat exchanger is described, which includes a conical envelope into which a metal tube is spirally wound, such that the tube is coiled into a conical spiral corresponding generally to the envelope. A reflective beam spreader is situated near the vertex of the envelope cone to re-direct incident solar radiation toward the exchanger tube. The apparatus thus has a cavity into which solar energy is focused to be incident upon the exchanger tube to superheat a gas or gas mixture flowing through the tube. The heated gas can be used in various gas reaction processes, such as steam methane reformation, a reverse water-gas shift reaction, or to drive a heat engine. Various auxiliary features are disclosed for increasing the efficiency of the solar heat transfer into the flowing process gas.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated into and form a part of the specification, illustrate several embodiments of the present invention and, together with the description, serve to explain the principles of the invention. The drawings are only for the purpose of illustrating a preferred embodiment of the invention and are not to be construed as limiting the invention. In the drawings:

FIG. 1 is a perspective view of a solar collector and gas processor assembly according to the apparatus of the present disclosure;

FIG. 2 is a perspective view, from the front, of a heat exchanger processor assembly according to the present disclosure, with the shroud removed to show the spirally wound exchanger tube and beam spreader;

FIG. 3 is a side sectional view of a portion of the processor assembly according to the present disclosure, showing (in section) the conical envelope and annular shroud, with the heat exchanger tube situated on the inside of the envelope and the conical beam spreader disposed near the vertex of the envelope;

FIG. 4 is an axial front view of the assembly shown in FIG. 3, with the supporting housing or frame visible;

FIG. 5 is an enlarged side sectional view of a limited portion of the processor assembly seen in FIG. 3, showing a section through three windings of the tube, the tube on the envelope, and a convective insert within the tube; and

FIGS. 6A and 6B are perspective views of short sections of un-wound tube according to the present apparatus, FIG. 6 being partially exploded view and FIG. 6B being in partial break-away view, to illustrate the installation and use of a absorptive and convective insert in the tube of the apparatus.

Like numbers are used to denote like elements and components throughout the various drawing figure views.

DESCRIPTION OF THE PREFERRED EMBODIMENTS (BEST MODES FOR CARRYING OUT THE INVENTION)

The present disclosure pertains to an apparatus and method for solar heating gases, such as to heat a feedstock gas to drive an engine, or to heat a gas for processing, for example for methane reformation or to drive a reverse water-gas shift. There is provided by the disclosure a processor by which a feed or process gas, which can be a gas relatively transparent to much of the solar spectrum, can be effectively heated by solar energy so that the hot gas can be harnessed to drive an engine, or for use in the production of useable fuels.

Succinctly, the presently disclosed apparatus and method allow for collected solar energy to be transferred by convection into a flowing feed or process gas. Convective heating of a flowing liquid is, of course, well-known in the art of heat exchangers generally, but the present invention enables the efficient exploitation of solar energy to heat a flowing process gas. Among advantages of the present invention is its capability to provide a very high heat exchanger surface area to maximize the transfer of solar power to a flowing gas. The apparatus maximizes the area A factor in the convective heat transfer equation:

P=K A ΔT

where P is the thermal power transferred, K is the convective heat transfer constant, and ΔT is the temperature difference between the gas temperature and the temperature of the surface. The area A is maximized, yet in a manner which permits solar energy directly to heat that surface area by direct impingement.

The apparatus and method can be harnessed for driving a heat engine, for example a Brayton Engine (e.g., in the form of a Capstone 30 engine), with the energy received from a solar energy focusing dish. There is disclosed hereby a gas processor apparatus including a heat exchanger capable of transferring solar energy into a relatively high flow gas, to drive for example a Brayton Engine. The processor assembly, including a beneficial coiled heat exchanger with beam spreader, is depicted generally in the drawing figures and will be described. It is contemplated that one embodiment of the solar-powered apparatus and method according to the present disclosure is capable, for example, of heating substantial volumes of gas approximately 500 degrees C. in a single pass. The single-pass methodology nevertheless can result in final temperatures of about 900° C. Such a gas temperature range is not only excellent for Brayton engines, but also for promoting hot gas process reactions, for example, methane reforming. By operation of the apparatus and method, the process gas can readily be heated by the solar radiation to a temperature of around 700° C., which permits for example feedstock gasses CH₄ and H₂O to be processed into desirable product gasses H₂ and CO.

The present apparatus and method also are well-suited for chemical gas processing, such as reforming of methane. That utility was as a secondary use in prior U.S. Pat. No. 7,140,181, but it has been determined that the design of that disclosure is “overkill” for reforming methane. The presently disclosed simpler, higher through-put, lower entropy production design is more advantageous for heating at to around 700° C. Also, the corrugated foil coil hot body exchanger disclosed hereby lends itself to catalytic reforming.

Attention is invited to FIG. 1. The apparatus and method include the use of a solar collector 10 configured to “gather” solar energy. Many known types of solar energy collectors, or others devised hereafter, may be adapted for use according to the present apparatus and method. The solar collector 10 depicted in FIG. 1 is a mirrored dish type collector, in which a plurality of mirrors are mounted according to convention for redirecting gathered solar energy toward a focal point. Mirrored dish-type collectors are well-suited to the practice of the invention; however, other types of solar energy collectors, such as a field of heliostats of known or hereafter-developed configurations, may also be adapted for use in the present apparatus and method.

Referring still to FIG. 1, the present apparatus also has a gas processor assembly 20 which is used with the solar collector 10. The gas processor assembly 20, which is described further herein, is disposed with the solar collector 10 in a manner such that solar energy collected by the solar collector 10 is reflectively re-directed toward the gas processor assembly 20. Thus, the processor is mounted to be at or near the “focal point” of the solar collector 10 in use.

As suggested by FIG. 1, the processor assembly 20 is situated such that its axis is coincident with the focal axis of the collector 10, so that that focused solar energy is directed into an open mouth or aperture of the processor assembly 20. Referring generally to FIGS. 2-4, the processor assembly 20 includes a conical metal envelope 24 with one or more heat-absorbing tubes 30 spirally coiled against the interior surface 26 of the envelope 24. The tightening spiral of the installed tube 30 itself thus defines a cone. The interior surface 26 of the envelope is reflective, so that incoming radiation that is not absorbed by the tube 30 in a first pass is reflected back toward the tube 30 for absorption on the second or subsequent pass. In a preferred embodiment, the feed gas is introduced into a port 32 at one end of the tube 30, such as near the open mouth of the envelope 24, and is then conducted in the spiral path toward a second port 34 nearer the vertex of the cone.

The processor assembly 20 thus functions to receive the incoming beam of focused solar energy, and to transfer the energy into the feed or process gas flowing through the tube 30, as the walls of the tube 30 allow solar energy to be conducted into the gas flowing in the tube. The surface area of the tube 30 available for heat transfer is increased by the effect of spiraling the tube within the envelope 24, thus multiplying the length of the tube to maximize the area into which the gas can come in contact (factor A in the convective heat transfer equation), as well as maximizing the amount of tube area exposed to incident solar radiation. A marked advantage of the disclosed apparatus is that no enclosing “window pane” (e.g., of quartz) need be placed across the mouth of the processor assembly 20 as often is the case with other solar powered heat exchangers for heating gasses. Such sealing windows can reduce by 10% or more the amount of received radiation, as solar rays are reflected or absorbed by the quartz pane or other window material. Rather, the incoming rays enter the present apparatus through an open aperture 27.

During operation of the apparatus, the highest intensity of solar energy tends to impinge near the vertex of the conical shell or envelope 24, both because of the focused nature of the incoming energy beams, and because the conical “hat” shape tends to reflectively direct any unabsorbed radiation further down the envelope 24 towards its vertex. Consequently, there is a propensity for the concentrated solar energy destructively to burn the processor assembly 20, particularly the coil 30, in the vicinity of the envelope's vertex. This phenomenon can be mitigated by the disposition of a highly reflective (e.g., a shiny metal) beam spreader 50 on the optical axis, and within the greater conical envelope 24. The beam spreader 50 preferably is a smaller cone positioned coaxially with the conical envelope 24, with the beam spreader's vertex directed outwardly toward the mouth of the envelope and toward the center of the incoming solar beam.

A conical beam spreader 50 evenly spread any incident solar ray into a sector of a circle that diverges as it moves away from the spreader cone. That is, an incoming ray of solar energy becomes a “pie shaped” beam as it reflectively departs from the surface of the spreader 50. This disperses or spreads the beam in an organized way. By selective choice of the spreader cone vertex angle, it is possible to direct the beams reflected there from toward the front (preferred) of the envelope 24, or toward the back (i.e., nearer the envelope's vertex).

Reference to FIGS. 2-4 is continued, which show the heat exchanger processor assembly 20 useable in the presently disclosed system. When capturing the solar heat from a solar concentrator or collector 10, one must accommodate the fact that the concentrated “spot” of energy is not of even intensity, but has a Gaussian distribution of intensity in all directions across the spot. This distribution complicates the task of collecting the heat at high temperature. It is preferable to have a controlled heat distribution over the heat exchanger system. The presently disclosed apparatus satisfies many of the demands in this regard.

The processor assembly 20 includes a suitable frame or housing 22 for containing and supporting the functional components of the system. The complete processor assembly 20, with most its functional elements contained within the housing 22, may be (by example only) about 50 cm wide, 50 cm high, and about 65 cm in axial length. However, the apparatus is scalable to accommodate the particularly selected application. As mentioned, the housing 22 is so positioned and mounted that the processor assembly 20 is on the optical axis, and at or near the focal point, of the solar energy concentrated and re-directed by the collector 10 (FIG. 1). Appropriate and necessary delivery and removal tubes, pipes, and conduits are provided to and from the processor assembly 20 whereby gasses can be supplied to and removed; additionally, electrical cables can supply power for the controlled operation of any valves, meters, gauges, and the like, that may be included in the structure and function of the apparatus according to design choice.

A foundational component of the processor assembly 20 is the envelope 24. The envelope 24 is rigid, and preferably is fabricated from a durable metal or allow capable of withstanding elevated temperatures. Aluminum or alloys thereof may be used, or stainless steel. The envelope 24 is in the shape of a right cone. The “base” of the cone is open to define a mouth or aperture 27. The aperture 27 thus preferably is approximately circular, and faces toward the collector 10 (FIG. 1); preferably, the circular aperture is approximately concentric with the optical axis of the collector 10, upon which axis is the solar energy focal point. The envelope 24 is stably supported within the housing 22 in any suitable manner, such as by secure connections between the housing and the base and/or vertex of the envelope's cone.

Preferably, the interior surface 26 of the envelope is treated or covered so to be highly reflective of any solar energy incident thereon. The surface 26 may be highly polished, or may be covered or coated with an appropriate reflective substance or layer. The outside of the envelope 24 preferably is well-insulated with a layer of material 48 that minimizes the escape of heat energy radially outward from the envelope. Thus, an object of the apparatus is to “trap” solar heat energy within the cavity defined by the envelope 24 to maximize transmission to the gas flowing in the tube 24.

There preferably is provided an optical shroud 44 around the aperture 27. The shroud 44 is a lightweight annulus, secured to the periphery of the “base” of the envelope's cone and coaxial with the envelope 24. As suggested in FIGS. 3 and 4, the inside radius of the shroud 44 is less than the radius defining the base of the envelope's cone, so that the shroud modestly reduces the effective diameter of the processor aperture 27. The inside surface 45 of the shroud 44 preferably is reflective to solar energy, as by being highly polished or coated with reflective material. Any solar rays impinging on the shroud inside surface 45 accordingly are directed back into the interior of the conical envelope 24.

One or more spirally disposed tubes 30 serve to transfer the solar heat energy into the feed or process gas. There may be provided a single heat exchanger tube 30, or in alternative embodiments a plurality of tubes may be disposed parallel to each other inside the envelope 24. At or near each end of each tube 30 there is a port 32, 34 to permit gas to be inserted or removed from the tube. In multi-tube embodiments, the ports at respective ends of the tubes may be fluidly joined by a manifold, so that the plurality of tubes is connected in parallel fluid flow.

The tube 30 may be composed of copper or other alloys for lower-temperature applications, but more preferably is a nickel superalloy. A nickel superalloy is generally indicated, due to the need for a high ΔT in a spiral coil exchanger of the present type. An alloy such as Haynes® 214® alloy available from Haynes International Company may be selected for the tube composition. The tube 30 may have, for example, a diameter of ⅜ inch, and a wall thickness of from about 0.030 inch (30 mil) to about 0.090 inch (90 mil). The exterior of the tube 30 preferably but not necessarily is blackened, as by anodization, chemical treatment, or the like, so as to be more absorptive of incident radiation. Copper tube 30 tends to blacken automatically as a result of the temperature, but suitable black coloration may have to be applied in a layer to a tube crafted from nickel.

The tube 30 (or plurality of parallel tubes) is spirally wound to define a cone-shaped coil corresponding generally to the conic dimension of the interior surface 26 of the envelope 24. Such a winding operation may be performed, for example, upon a correspondingly sized slowly rotating mandrel. The tube 30 preferably is wound into a configuration which allows the coiled tube to be situated in the envelope 24 cavity such that most or all of the windings of the coil are closely adjacent to, or in contact with, the interior surface 26. A properly wound tube is disposed into proximate relationship with the envelope 24 as indicated in FIG. 3. The coiled tube 30 may be secured into place using appropriate brackets and/or straps such as those seen in FIGS. 2 and 4; any strap or other fastener situated within the cavity of the envelope 24 should be a high-temperature alloy.

FIG. 5 is an enlarged axial sectional view of a portion of the envelope 24, tube 30 and envelope insulation 48. It is seen that in a preferred embodiment, the tube 30 is wound so that each winding of the tube is close to or in contact with the adjacent winding. The tube wall 36 may actually contact itself on each side of each coiled winding, reducing the ability of solar rays to pass between adjacent winding to impinge the underlying envelope 24. Also as seen in FIG. 5, the tube 24 optionally may be coiled and placed such that some or all the windings contact the interior surface 26 of the envelope 24. Alternatively, the tube 30 may be mounted to be spaced away a modest distance from the interior surface 26. Also depicted in FIG. 5 is the absorptive and convective insert 38. The insert 38, which shall be described further herein, absorbs radiant energy from the tube wall 36 and convectively transfers it into the gas flowing through the tube 30. The layer of insulation 48 is any of a number of known materials that may be placed at or on the outside of the envelope 24 to reduce the passage of heat radially outward through the envelope.

Referring more particularly to FIGS. 2-3, the coiled tube 30 preferably is configured and situated relative to the envelope 24 such that a first outer one 32 of the ports is located at or near the mouth of the envelope 24 proximate to the processor aperture 27. The second or inner port 34 is located between the first port 32 and the vertex of the envelope cone, more preferably substantially proximate to the vertex as indicated in FIG. 3.

The preferred embodiment of the processor assembly 20 features a beam spreader 50 on the axis of the envelope 24 near its vertex. The beam spreader 50 preferably is a rigid metal cone, as best seen in FIGS. 2-4. The cone of the beam spreader 50 has its base in proximate confronting relation with the vertex of the envelope cone, that is, the two cones “face” in opposite directions. The spreader 50 can be mounted, for example, on a beam spreader stem 56 secured to extend through the vertex of the envelope 24, as seen in FIGS. 2 and 3. The vertex of the beam spreader cone is “aimed” toward the center of the incoming beam of solar energy. Thus, the cone of the beam spreader 50 and the cone of the envelope 24 are mutually coaxial and with the optical axis of the solar collector 10.

The beam spreader 50 has a very shiny exterior surface, so to maximally reflect solar energy incident thereon, and direct it radially outward toward the tube 30. The cone of the beam spreader 50 preferably is made of, or covered with, a highly polished nickel superalloy. A well-polished and maintained surface allows the beam spreader to reflect up to about 50% of incident solar radiation.

As mentioned, the tendency is for the inner, shorter-radius portions of the tube 30, closer to the vertex of the envelope 24, to overheat because the solar collector is focus to deliver the concentrated beam or spot on the central axis of the apparatus. The cone for the beam spreader 50 therefore should be configured to reflect as much of the incident energy as possible toward the outer-most portions of the tube 30, that is, out toward the portions of the tube bearer the processor aperture 27. It has been determined that a shorter cone for the spreader 50 is desirable, that is, a spreader cone having a relatively large vertex angle. The included angle of the spreader's cone preferably is at least about 40 degrees, and could be as large as, for example, approximately 110 degrees. The cone height for the spreader 50 correspondingly may be comparatively foreshortened, and may be less than the diameter of the base of the spreader's cone. In some preferred embodiments, the vertex tip of the beam spreader 50 is situated at a point on the apparatus' axis less than about 33% of the distance from the envelope's vertex toward the aperture defined by the peripheral edge of the envelope's base.

Due to the very high temperatures to which the beam spreader 50 is subjected, normally is strongly indicated that it be actively cooled. FIG. 3 illustrates how the beam spreader 50 defines therein a cavity through which a coolant, such as water or a water-glycol mix, can flow to absorb and remove heat from the surface of the spreader. Coolant is circulated through the beam spreader cavity by means of coolant lines 53, 54, which are in operative communication with an appropriate pump. As depicted in FIG. 3, the spreader coolant lines 53, 54 can be disposed within the spreader's supporting stem 56 which runs through the vertex of the envelope 24.

An optional but highly beneficial aspect of the apparatus is the use of the radiation absorbing insert 38 shown in FIGS. 5, 6A and 6B. In this embodiment, a strip of material is inserted into the tube 30, preferably for at least a majority of the tube's length. Placement of the insert 38 is more readily accomplished prior to the winding of the tube 30 into the spiral coil. FIGS. 5A and 5B show a length of tube 30 (shortened in the figures for simplicity of illustration) into which insert material is pushed and/or pulled into position within the tube.

The insert 38 boosts dramatically the effectiveness of the tube 30 as a heat exchanger. The insert functions to absorb heat energy that radiates inwardly through the tube wall 36 into the tube interior (FIG. 5). Once absorbed into the insert 38, the heat energy is available for convective transfer directly from the insert 38 into the gas flowing within the tube 30. So, the insert 38 offers the practical advantage of greatly increasing the surface area (factor A in the convective heat transfer equation) for convective exchange of thermal energy to the passing gas. The insert 38 is, effectively, a functional adjunct to the tube 30 for purposes of convective heat exchange into the gas.

The insert 38 may be fashioned from nearly any material that is thermally stable at the elevated temperatures obtained within the tube 30, and which absorbs radiant energy for convection into the gas. The insert does not need to be composed of a material which efficiently conducts thermal energy. Also, as suggested by FIGS. 6A and 6B, the insert preferably is in the form of a long thin strip having a lateral width slightly or somewhat less than the inside diameter of the tube 30. Because the insert 38 typically is pushed and/or pulled through the tube 30 it should manifest adequate axial tensile strength for the task. A thin strip of copper or of nickel superalloy performs well as an insert 38 composition. The strip may be, for example, about 0.002 inch to about 0.007 inch (2 to 5 mil) thick. It has been determined that the insert after installation need not have significant physical contact with the inside wall 36 of the tube 30 to perform well its functions of absorption and convection. Although such contact will occur at points along the lengths of tube 30 and insert 38, especially after the insert-containing tube is wound into its final spiral configuration, such contact does not need to be encouraged or avoided. Rather, the primary mode of energy transfer from tube wall 36 to insert 38 appears to be by radiation from the wall 36 to the insert 38.

The insert 38 does not need to be significantly thermally conductive or to be in contact with the tube 30 to enhance the heat transfer to the gas. It is supposed by applicant that solar thermal radiation passing though the tube wall 36 (as suggested by the parallel wavy directional arrows at the upper portion of FIG. 5) effectively is transmitted to the insert due to the high temperature of the wall 36 during operation.

The radiation emitted by a body is directly related to its temperature. If the body such as the insert 38 can be assumed to be a “black body” as the tube wall 36 may be assumed, the amount of radiation given off is proportional to the 4th power of its temperature as measured in Kelvin units. This natural phenomenon is described by the Stephan-Boltzmann Law. The following simple equation describes this law mathematically:

E=σ T⁴

where σ equals 5.67×10⁻⁸ Wm⁻²K⁻⁴, and T is the temperature of the radiant body (Kelvin). (Another factor accounting for the fact that that the boy may be a very imperfect emitter is omitted from this simplest form of the equation.) According to the Stephan-Boltzmann equation, a small increase in the temperature of a radiating body results in a large amount of additional radiation being emitted. Because the temperature of the tube wall 36 is extremely high during operation of the apparatus, the radiation from the wall to the insert 38 accordingly is high. The radiation is absorbed by the insert and transferred by convection into the gasses flowing inside the tube and in contact with the insert 38.

To increase further the effective surface area presented by the insert 38, the strip of insert material preferably is knurled or corrugated laterally prior to insertion within the tube 30 as suggested by FIG. 6A. The insert 38 thus knurled or “crinkled” manifests a zig-zag lateral profile, with alternating valleys to facilitate gas flow past the surfaces of the insert. An alternative possible mode of increasing the effective heat transfer surface area of the absorptive insert 38 is to twist the insert strip into a helix, as indicated in FIG. 6B, prior to disposing the strip through the tube 30. The presence of an insert, particularly the helically twisted insert 38 of FIG. 6B may inhibit somewhat the free flow of the gas axially through the tube 30, which flow already is the subject of dynamic flow losses attributable to the spirally coiled tube itself. However, the fluid dynamics losses are justified, it is believed, by the increased gas residence time and heat transfer enhancement provided by the insert 38; it has been determined that a processor assembly 20 featuring an insert 38 within its tube 30 has about twice the heat transfer to the gas, per unit length of tube, as an assembly lacking a tube insert.

The operation of the apparatus and the practice of the corresponding method are generally apparent from the foregoing. The collector 10 and processor assembly 20 are arranged in relation to each other so that the collected solar energy is reflected toward the processor aperture 27. The feed or process gas or gas mixture, according to the reaction to be driven of the process to by realized, is fed as by an appropriate pump into one of the ports, such as the outer tube port 32, as indicated in FIG. 3. The gas flowing inside the tube 30 follows the increasingly tighter spiral path of the tube 30. The solar energy beam is focused to a spot within the interior cavity of the conical envelope 24, the focal point preferably being at or near the vertex tip of the beam spreader 50. The incoming solar energy heats the tube 30, and the hot tube heats the gas flowing therein. Due to the large surface area of the tube 30 (which effectively a very long cylinder offering a surface area of up to πD, where D is the outside diameter of the tube), the gas residence time resulting from the lengthy spiral, and the high temperature generated by the solar beam, the gas in the tube is heated to effective processing temperatures of from 500° C. to 900° C., preferably about 700° C., by the time it is discharged from the second or inner port 34. Once emitted from the second port 34 the hot gas can be further processed or harnessed in a heat engine, depending on the particular design application for the apparatus.

The incoming solar rays are largely confined within the cavity defined by the envelope 24 and the shroud 44, although some losses back out through the aperture 27 are mostly inevitable. Some rays passing through the aperture 27 impinge directly upon the tube 30 to provide radiant heating. A large portion of the rays are incident upon the shiny surface of the beam spreader 50 and thus are reflected radially outward toward the tube 50, whereby they also heat the tube 30. Any rays passing between windings of the tube 30 are reflected by the reflective interior surface 26 of the envelope 24, and thus redirected toward the tube 30. Any solar energy rays reflecting from the tubes 30, envelope 24 or beam spreader 50 which happen to impinge upon the reflective inside surface 45 of the shroud 44 are reflected back into the cavity, and are thus again available to heat the tube 30. Leakage of heat energy from inside the envelope cavity is reduced by the insulating function of the insulting layer 48. Consequently, a substantial portion of the solar energy entering the cavity inside the processor assembly 20 initially or eventually impinges the tube wall 26. The tube 30, with its optionally contained insert 38 acts as a convective heat exchanger, and the gas flowing in the tube is heated to the desired process temperature.

There is disclosed, therefore, an apparatus for solar heating a feed gas, the apparatus including a heat exchanger 20 having: an envelope 24 substantially in the shape of a hollow cone having an vertex and a base, the base being open to a substantially circular aperture 27, and the envelope preferably having a reflective interior surface 26; and at least one tube 30 wound in a conical spiral within the envelope 24 and proximate to the interior surface 26. The apparatus has a collector 10 for reflectively directing solar energy toward the heat exchanger assembly 20 and through the aperture 27 to heat the one or more tubes 30, so that the feed gas flowing through the tube 30 is heated thereby. The at least one tube 30 preferably has or is in fluid communication with an outer port 32 at a first tube end and an inner port 34 at a second tube end, via which ports the feed gas enters and exits the tube 30. The outer port 32 may be disposed proximately to the aperture 27, and the inner port 34 may be disposed between the aperture 27 and the envelope vertex. There ordinarily is a pump 42 which moves gas in the tube 30 from the outer port 32 toward the inner port 34. Alternatively, the pump 42 may move gas in the tube 30 from the inner port 32 toward the outer port 34. The tube 30 ideally is a nickel superalloy. There optionally but preferably is a convective insert 38 disposed within the interior of the tube 30. In alternative embodiments there are a plurality of tubes 30, there being provided a manifold for providing parallel gas flow into the plurality of tubes.

The apparatus preferably includes a beam spreader 50, disposed proximately to the envelope vertex, for reflectively directing solar energy toward the tube 30. The beam spreader 50 preferably is substantially in the shape of a cone disposed coaxially with the envelope 24 and having its base in confronting relation to the envelope vertex. The beam spreader 50 or at least its reflective surface, preferably is composed of a nickel superalloy. There typically is provided some means 52, 53, 54 for cooling the beam spreader 50.

The preferred embodiment of the apparatus has a shroud 44 disposed around the assembly's aperture 27, the shroud having a reflective interior surface 45 for reflecting solar energy toward the one or more tubes 30.

The apparatus and method work well for solar applications because its design transfers concentrated heat (bright solar spot) into a feed or process gas by providing a large gas contact surface area. The use of a wound tube 30 of small (e.g. ½-inch to about 1 inch) diameter, wound numerous times (for example 35-60 windings) into a close spiral, provides a serviceable heat exchanger. The gas passing through the spirally coiled tube exchanger has copious surface for heat transfer. This is expressed by the convective heat transfer equation:

P=K A ΔT

where P is the thermal power transferred, K is the convective heat transfer constant, and ΔT is the temperature difference between the gas temperature and the temperature of the surface. The convective heat transfer constant K is a function of the type of gas and its velocity, and may be calculated using known concepts and formulae. The convective heat transfer rates typically are between 10 and 35 watts/m² per degree K, depending on the gas, the surface, and the temperature.

The apparatus and method may be used, for example, to reform methane. The elevated temperatures obtained in the exchanger tube 30 are realized from solar energy, and are sufficiently high to drive the natural gas reformation step of the known steam methane reforming process. At high temperatures (700-1100° C.) and in the presence of a metal-based catalyst (e.g., nickel), steam reacts with methane to yield carbon monoxide and hydrogen. The appropriate feedstock gases are passed through the tube 30 according to the forgoing disclosure, and the reformation reactions thereby driven forward.

Some of the chemical reactions that can take place in the course of methane reformation are:

C_nH_m+n H₂O→n CO+(m/2+n)H₂

and

CO+H₂O→CO₂+H₂

The produced carbon monoxide can combine with more steam to produce further hydrogen via the water gas shift reaction. Of course, other reactions (some undesirable, like coke formation) can take place if local conditions are favorable. The first reaction is endothermic while, the second reaction is exothermic. Additional fundamentals regarding methane reformation are found in, for example, U.S. Pat. No. 7,087,651 to Lee-Tuffnell, et al., and U.S. Pat. No. 6,312,658 to Hufton, et al.

The disclosed apparatus and method may also be adapted to exploit the Reverse Water-Gas Shift Reaction (rWGSR). The rWGSR reaction is given by equation:

CO₂+H₂→CO+ΔH₂O H=+9 kcal/mole (38.9 kJ/mol)

The rWGS reaction accordingly may be exploited to generate CO from CO₂. The CO may then be used as feedstock for further processing into useable fuels. For example, produced CO can be feed directly into a known Fischer-Tropsch synthesis system to generate synthetic fuels. The Fischer-Tropsch synthesis is a relatively complex network of both parallel and series chemical reactions; it is a carbon-chain building process whereby CH₂ groups are attached to the carbon chain, and involves the catalytic reaction of H₂ and CO to form hydrocarbon chains of various lengths. A principle by-product of the Fischer-Tropsch process system is water.

This reaction is endothermic, and occurs at relatively reduced temperatures in the presence of certain catalysts. The present apparatus and method are capable, however, of sufficient heat transfer to eliminate the need for catalysis during the rWGSR; the use of expensive and sometimes unpredictable catalysts should be considered optional in the presently disclosed method.

Although the invention has been described in detail with particular reference to these preferred embodiments, other embodiments can achieve the same results. Variations and modifications of the present invention will be obvious to those skilled in the art and it is intended to cover in the appended claims all such modifications and equivalents. The entire disclosures of all patents cited above are hereby incorporated by reference.

Claims

1. An apparatus for solar heating a feed gas comprising: wherein a feed gas flows through the tube to be heated thereby.

a heat exchanger comprising: an envelope substantially in the shape of a hollow cone having a vertex and a base, the base being open to a substantially circular aperture, the envelope comprising a reflective interior surface; at least one tube wound in a conical spiral within the envelope and proximate to the interior surface; and

a collector for reflectively directing solar energy toward the heat exchanger and through the aperture to heat the at least one tube;

2. An apparatus according to claim 1 wherein the at least one tube comprises an outer port at a first tube end and an inner port at a second tube end, via which ports the feed gas enters and exits the tube.

3. An apparatus according to claim 2 wherein the outer port is disposed proximately to the aperture and the inner port is disposed between the aperture and the envelope vertex.

4. An apparatus according to claim 3 further comprising a pump which moves gas in the tube from the outer port toward the inner port.

5. An apparatus according to claim 3 further comprising a pump which moves gas in the tube from the inner port toward the outer port.

6. An apparatus according to claim 2 wherein the tube comprises a nickel superalloy.

7. An apparatus according to claim 2 further comprising a convective insert disposed within the tube.

8. An apparatus according to claim 2 wherein the at least one tube comprises a plurality of tubes, and further comprising a manifold for providing parallel gas flow into the plurality of tubes.

9. An apparatus according to claim 1 further comprising a beam spreader, disposed proximately to the envelope vertex, for reflectively directing solar energy toward the at least one tube.

10. An apparatus according to claim 9 wherein the beam spreader is substantially in the shape of a cone disposed coaxially with the envelope and having its base in confronting relation to the envelope vertex.

11. An apparatus according to claim 9 wherein the beam spreader comprises a nickel superalloy.

12. An apparatus according to claim 9 further comprising means for cooling the beam spreader.

13. An apparatus according to claim 1 further comprising a shroud disposed around the envelope aperture the shroud comprising a reflective interior surface for reflecting solar energy toward the at least one tube.