Patent Application
20090277442
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.
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.
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:
Like numbers are used to denote like elements and components throughout the various drawing figure views.
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
Referring still to
As suggested by
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
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 (
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 (
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
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
Referring more particularly to
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
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.
An optional but highly beneficial aspect of the apparatus is the use of the radiation absorbing insert 38 shown in
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 (
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
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
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
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
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:
CnHm+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.
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.
| Number | Date | Country | |
|---|---|---|---|
| 61127034 | May 2008 | US |
