Lightweight, low cost solar energy collector

Abstract

A lightweight solar concentrator of the reflecting parabolic or trough type is realized via a thin reflecting film, an inflatable structural housing and tensioned fibers. The reflector element itself is a thin, flexible, specularly-reflecting sheet or film. The film is maintained in the parabolic trough shape by means of a plurality of tensioned fibers arranged to be parallel to the longitudinal axis of the parabola. Fiber ends are terminated in two spaced anchorplates, each containing a plurality of holes, which lie on a desired parabolic contour. In a preferred embodiment, these fibers are arrayed in pairs with one fiber contacting the front side of the reflecting film and the other contacting the back side of the reflecting film. The reflective surface is thereby slidably captured between arrays of fibers, which control the shape, and position of the reflective film. Gas pressure in the inflatable housing generates fiber tension to achieve a truer parabolic shape. A plurality of bridges and or retention clips may be employed in certain embodiments to maintain the position of the reflective surface relative to the fibers.

Description

CROSS-REFERENCE TO CORRESPONDING APPLICATIONS

This application takes priority from provisional patent application Ser. No. 60/412,518 filed on Sep. 20, 2002 and from utility patent application Ser. No. 10/601,923 filed on Jun. 19, 2003.

ORIGIN OF THE INVENTION

The invention described herein was made in the performance of work under a NASA contract, and is subject to the provisions of public law 96-517 (35 USC 202) in which the contractor has elected to retain title.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates generally to the field of solar collectors having a parabolic trough to collect and concentrate solar energy. The invention pertains more specifically to an extremely lightweight and low cost parabolic trough solar collector.

2. Background Art

Parabolic trough technology is currently the most advanced solar thermal electric generating technology. This is primarily due to nine large commercial-scale solar power plants, the first of which has been operating in the California Mojave Desert since 1984. These plants, which continue to operate on a daily basis, range in size from 14 to 80 MW and represent a total of 354 MW of installed electric generating capacity. These plants, which were all built with government support, have explored the lifetime costs of operating large solar energy collection systems. A dominant finding from building and operating these solar power plants is that commercial viability has not yet been attained and that commercial viability depends upon reducing the per wattage capital required to build the solar collectors. Reducing the per wattage capital required to build a solar collector is a prime motivator for this invention.

The following issued U.S. patents appear to constitute relevant prior art:

U.S. PATENT NO.	PATENT DATE	INVENTOR
4,173,397	Nov. 6, 1979	Simpson
4,432,342	Feb. 21, 1984	Lucas
4,051,834	Oct. 4, 1977	Fletcher
4,318,394	Mar. 9, 1982	Alexander
4,071,017	Jan. 31, 1978	Russell
4,920,033	Apr. 11, 1989	Sick
4,243,019	Jan. 6, 1981	Severson
4,454,371	Jun. 12, 1984	Folino
4,077,392	Mar. 7, 1978	Garner
4,515,148	May 7, 1985	Boy-Marcotte
4,359,041	Nov. 16, 1982	Snodgrass
4,293,192	Oct. 6, 1981	Bronstein
4,291,677	Sep. 29, 1981	Monk

Of the foregoing prior art patents, the patent to Russell (U.S. Pat. No. 4,071,017) and to Simpson (U.S. Pat. No. 4,137,397) appear to be the most relevant.

Russell discloses a nonparabolic discontinuous (stepped) flat faceted mirror, which is supported by tensioned cables that are fixed to the ground via concrete and steel anchors. An embodiment uses a flexible reflective film, which is weaved through the cables and tensioned to generate a discontinuous flat faceted mirror. Being that the mirror is fixed to the ground, the receiver moves to maintain coincidence with the focus over the course of a day.

Simpson discloses a parabolic reflector sheet that is placed in tension against a plurality of tensioned wires to form a continuous, stepless, flat faceted parabolic mirror. The tensioned wires are supported by a pair of arcuate base members, which are mounted to ground or other structure such as a roof via suitable supports. The sheet is attached to bars at both ends. Torsion applied to one of the bars generates tension in the sheet.

Neither of these patents discloses a continuous, stepless, unfaceted, parabolic sheet reflector. Also, neither such patent discloses a transparent tubular enclosure that is pressurized to generate the tension in the fibers. Additionally, neither patent discloses a structure, which is of comparable light weight or low cost.

SUMMARY OF THE INVENTION

A lightweight solar concentrator of the reflecting parabolic cylinder or trough type is realized via a unique combination of thin reflecting film, an inflatable structural element and tensioned fibers. The reflector element itself is a thin flexible, specularly-reflecting sheet or film. (Aluminized polyester sheet, for example). The reflector element is not self-supporting.

The film is maintained in the parabolic trough shape by means of a plurality of tensioned fibers (high strength carbon, for example) arranged to be parallel to the longitudinal axis of the parabola. Fiber ends are terminated in two spaced anchorplates, each containing a plurality of holes, which lie on the desired parabolic contour.

In the preferred embodiment, these fibers are arrayed in pairs with one fiber directly above the reflecting film and the other immediately behind the reflecting film. The reflective surface is thereby captured between arrays of fibers. The fibers might constrain the membrane by other arrangements. These fibers control shape and position of the reflective membrane.

The anchorplates are centrally fastened to circular endplates. These endplates also serve to seal the ends of a transparent thin film cylindrical enclosure tube, which functions as a housing. The enclosure tube may be seamless or may comprise one or more seams which enable the enclosure tube to be formed from a flat flexible sheet. Once sealed, raising the pressure of the gas (air) inside the enclosure tube increases the stiffness of the enclosure tube. With only a modest internal pressure the enclosure becomes structurally stable with the capability to provide a weather tight housing for the internal mirror, receiver and other components. In addition, the inflated enclosure is designed with endplates that impart a portion of their pressure load into the anchor plates and hence the reflector forming fibers. In this manner, tension is provided to the fibers without using additional costly structure.

Because of the tension, sag or deformation of the array of fibers can be minimized even in the presence of the gravitational load represented by the reflector sheet. As tension is increased, deformation of both fiber and reflector is reduced and the reflector is even further constrained to follow the specific parabolic contour defined by the array of fiber-locating holes.

Thus, the tension resulting from pressurization of the gas inside the cylindrical envelope forces the reflector sheet into the parabolic trough shape enabling a line focus to be created above the reflector. The location of this focal line is determined by the array of holes and the particular parabolic form they follow. In most embodiments the focal line is created inside the transparent cylindrical envelope, including being coincident with the axis of the cylindrical envelope, although it can otherwise be arranged to fall outside the cylinder.

A substantially line like receiver of the focused concentrated solar direct beam radiation is located at the line focus of the trough reflector. This receiver can be a conduit containing a flowing gas or liquid to which the radiant energy will be transferred and thereby be captured and utilized. Alternately, a photovoltaic receiver may be located at the position of this focal line for the purpose of converting the radiant energy directly into an electrical form. Alternatively, a hybrid receiver having both thermal and electrical outputs may be placed at this line focus.

Concentrators are fastened to the ground via brackets at the endplates only. The collector design allows a two axis polar mounting configuration to enable maximum energy collection over the day and the year in any location. Hourly or azimuth sun tracking is accomplished via rotation or the cylindrical collector about the cylindrical axis, while elevation tracking is accomplished via vertical tilting of the collector or array of collectors.

As used herein the terms “string”, “fiber” and “wire” are interchangeable and each refers to an elongated membrane support member.

As used herein, the terms “reflector sheet”, “reflector film”, “membrane” and “reflector” are interchangeable and each refers to an ultra-thin, ultra-light, non-self-supporting member having at least one highly reflective surface.

As used herein the terms “housing”, “enclosure”, “cylindrical tube”, “enclosure tube”, “envelope”, “transparent film”, are interchangeable and each refers to a transparent cylindrical tubular member that encloses and structurally supports the parabolic membrane.

BRIEF DESCRIPTION OF THE DRAWINGS

The aforementioned objects and advantages of the present invention, as well as additional objects and advantages thereof, will be more fully understood herein after as a result of a detailed description of a preferred embodiment when taken in conjunction with the following drawings in which:

FIG. 1 is a three-dimensional view of a preferred embodiment of the present invention;

FIG. 2 is an enlarged view of an anchorplate illustrating the string anchoring technique used therein;

FIG. 2a is a view of the bridges and reflector film retention clips;

FIG. 3 is an enlarged view of the spring-based interface between the string anchorplate and the hub;

FIG. 4 is a view of a bolted interface between the string anchorplate and the hub;

FIG. 5 is a view of the hub from outside the enclosure;

FIG. 6 is a cross section view of the hub and endplates;

FIG. 7 is an enlarged view showing the retention of the strings into the anchorplate;

FIG. 8 is a simplified illustration of the preferred string pair film support system;

FIGS. 9-11 illustrate a first alternative film supporting technique;

FIGS. 12 and 13 illustrate a second alternative film supporting technique; and

FIGS. 13-17 illustrate the manner in which the enclosure tube is secured to the endplates.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to the accompanying figures and initially FIGS. 1 and 2 in particular, it will be seen that a lightweight parabolic trough solar concentrator 10 is shown. Concentrator 10 comprises an inflatable transparent enclosure tube 12 terminating at its axial ends in a pair of circular endplates 18 each supported at its center by a hub 33. Within enclosure tube 12 is a string-supported reflector 14 configured by tensioned strings 15 to form a parabolic shape having a line focus. The reflector 14 may comprise a single reflector sheet or a plurality of adjacent reflector sheet sections.

A receiver 16 is positioned along the line focus of the parabolic reflector and may be configured as a pipe carrying a liquid to be heated by the concentrated sunlight or may be configured as a surface supporting a line array of photovoltaic cells. The ends of the strings 15 terminate in and are secured by an anchorplate 20 at each axial end of the concentrator 10. FIG. 1 shows two concentrators 10 ganged together for a joint elevation tracking as well as azimuth sun tracking.

Extending internally along a radius of each end plate 18 is a pipe member 23 connecting receiver 16 to a central hub 33. A counterweight 24 compensates for the weight of receiver 16. Gas pressure within enclosure tube 12 causes the endplates 18 to separate the anchorplates and place the strings under tension. The array of holes 31 in each anchorplate 20 follows the desired parabolic form thereby causing the strings 15 and reflector 14 to form the same parabolic shape. As the gas pressure in the tubular housing 12 increases, the strings become more taut and thus more precisely conform to the desired shape along their entire lengths.

In certain embodiments one or more bridges 80 are located at intervals along the length of the tensioned strings 15 to provide means to maintain the tensioned strings 15 in a desired parabolic shape. Shown in FIG. 2a, a bridge 80 is shown in combination with a retention clip 81. One or more retention clips 81 provide means to ensure the reflector sheets do not slide out of the tensioned strings 15 and provide a connection between two adjacent reflector sheets with a bridge 80 caught in between. A clearance 82, is provided between the clip 81 and the bridge 80. This clearance ensures the reflector 14 can slidably adjust its position relative to the strings and maintain a smooth undistorted surface regardless of thermal growth or other disparity. If a reflector 14 attempts to slide too far relative to the tensioned strings 15, the retention clip 81 will contact the bridge and prevent the reflector sheet from exiting the tensioned strings 15.

FIG. 2 illustrates an anchorplate 20 in an enlarged view. As shown therein, anchorplate 20 comprises a bent rectangular tube having a plurality of through-holes 31. The holes 31 are arranged along a substantially parabolic curve to receive and secure strings 15. A cross bar 32 is bolted to the anchorplate at two locations and is integral to a hub faceplate 34, which is secured to a central hub 33. Rotation of the endplate 18 will rotate the hub 33, the anchorplate 20, the pipe 23 and the counter-weight 24 along with the receiver 16. The reflector 14 will also rotate so that its focal line remains coincident with receiver 16.

FIGS. 3 and 4 illustrate two embodiments used to secure anchorplate 20 to the hub 33. The first embodiment, shown in FIG. 3 utilizes springs to enhance axial compliance between the reflector assembly and transparent enclosure tube assembly. As shown therein, four symmetrically located shoulder bolts 28 extend through a pair of spaced anchorplate hubs 34 which are welded to the anchorplate crossbar 32. Each shoulder bolt 28 supports a corresponding helical spring 25 between hub 34 and a retainer 30. This arrangement precisely positions the anchorplate 20 relative to the hub 33 in all directions and rotations except along the hub axis. In the direction of the hub axis, the compliance of the helical springs 25 allow the anchorplate 20 to attain an optimal position relative to the hub 33 for maintaining string tension under a variety of the pressure and thermal loadings.

The second embodiment, shown in FIG. 4 depends upon an endplate 18 and or strings 15 and or enclosure tube 12 to provide axial compliance between the reflector assembly and tube assembly. As shown therein, the anchorplate 20 is attached to the hub 33 via a pinned and bolted joint. The pins 27 precisely position the anchorplate 20 relative to the hub 33 in all directions and rotations. The bolts 29 transfer loads from the anchorplate 20 to the hub 33.

FIGS. 5 and 6 illustrate the manner in which the hub 33 is attached and sealed from air leakage to the endplate 18. FIG. 5 provides a cross section view of the hub 33 to endcap 18 interface. As shown therein, the hub 33, is reduced in diameter to provide a shoulder 41 for axial positioning and sealing against the endcap 18. A gasket 39 is provided to ensure the seal and provide a soft interface with the endcap 18. A bolt ring 20 and gasket 42 are located on the outside of the collector enclosure. Bolts 43 secure the bolt ring 40, gaskets 39 and 42 and endplate 18 and against the shoulder 41 of the hub 33 and generate an airtight seal. FIG. 6 provides an isometric view of the hub 33 operatively connected to an endplate 18 interface.

FIG. 7 illustrates the manner in which each pair of strings 15 in anchored to anchorplate 20. As shown therein, each such string pair is terminated by a ferrule 36, which is received in a split collet 38 having an internal retaining shoulder 22. A portion of the split collet 38 is tapered to be received and retained in a corresponding tapered hole 31 in the anchorplate. Tapered hole 31 has a flat 44, which in conjunction with a flat 37 on the split collet 38 controls the rotational orientation of the strings. The collet also includes an external shoulder feature for limiting the depth of penetration of the collet into its corresponding tapered hole 31.

The cross-section view of FIG. 8 illustrates the preferred reflector/string interface wherein string pairs 15 support the reflector 14 between the strings 15. The string pairs 15 are spaced apart by a gap slightly greater than the thickness of the reflector 14. The reflector 14, which is untensioned and allowed to slide relative to the strings 15, is guided by the strings to take a parabolic shape. By allowing sliding relative to the strings 15, the reflector 14 is not impacted by thermal growth differences or other disturbance that will negatively impact the parabolic mirror shape. In addition, by allowing sliding relative to the strings 15, the reflector 14 will avoid faceting and form a surface that is much closer to a parabola, which has the desired optical performance.

Another embodiment 50 of a reflector/string interface is shown in FIGS. 4-6. As shown therein, the reflector 50 comprises a plurality of reflector segments 52, each of which is welded along an edge to a tubular hinge piece 54, which is hingedly attached to a single fiber or string 56. The fibers 56 serve the same purpose as the strings 15 of FIG. 2, namely to locate and shape the reflective surface.

Still another membrane embodiment 60 is shown in FIGS. 12 and 13 wherein a reflective membrane 62 employs an integral backside sleeve 64 through which a single fiber 66 is threaded. Sleeve 64 may be integrally formed by welding the membrane surfaces. In one such embodiment, reflective membrane 62 is about 0.001 inches thick and sleeve 63 is about 0.010 to 0.030 inches in diameter. However, because in this embodiment the sleeves do not obstruct the reflective surface of the membrane, the sleeve diameter can be virtually any practical size.

FIGS. 14-18 illustrate the manner in which the tubular housing 12 shown in FIG. 16 is secured to each endplate 18 shown in FIG. 14. As shown therein, the circumferential edge 70 of each endplate has a regular convoluted shape. This edge is surrounded by a ring assembly 72 (see FIG. 15), which comprises a clamping ring 74, a plurality of shoes 76 and a clamp 78. As seen best in FIGS. 17 and 18, the end of tube 12 is positioned and hermetically bound to the endplate edge 70. This is done by tightening clamps 78 until the plurality of shoes 76 engage the tube end and endplate edge. The convolutions cause the tube to be circumferentially stretched to ensure a wrinkle-free and hermetic assembly.

Various aspects of the disclosed embodiments have been omitted to avoid obfuscation of the more salient features. By way of example, it will be understood that the inflatable tubular assembly may have one or more sealed seams and a pressure valve. Furthermore, also not shown explicitly is a drive mechanism for slowly rotating the collector assembly to keep the direct beam solar radiation on the receiver as the Earth rotates. Moreover, the ancillary interfaces for the receiver are well known in the art and are also not shown.

Having thus described various embodiments of the present invention, it will now be evident that many modifications and additions are contemplated. Accordingly, the scope hereof is limited only by the appended claims and their equivalents.

Claims

1. A solar concentrator comprising:

a membrane reflector having a unitary line focus;

a transparent tubular housing enclosing said reflector; and

a plurality of strings extending within said tubular housing, wherein said plurality of strings shape said reflector to have a substantially stepless parabolic cross-section.

2. (canceled)

3. The concentrator recited in claim 1 wherein said strings are in a state of tension.

4. The concentrator recited in claim 1 wherein said strings are arranged in pairs, each such pair having a string on front and back surfaces of said reflector.

5. The concentrator recited in claim 4 wherein said string pairs are in a state of tension.

6. The concentrator recited in claim 1 wherein said housing is internally pressurized above external atmospheric pressure by a gas within said housing.

7.-8. (canceled)

9. The concentrator recited in claim 1 wherein said housing comprises opposed end plates, a gas in said housing being pressurized to cause said endplates to be extended further from one another; and wherein said reflector is shaped by a plurality of string pairs, each said pair supporting said reflector on front and back surfaces of said reflector, said string pairs being connected to said endplates and being subjected to tension depending on the separation between said endplates.

10. (canceled)

11. The concentrator recited in claim 1 further comprising a solar energy receiver extending along at least a portion of said line focus.

12. The concentrator recited in claim 1 further comprising means for rotating said housing to control the orientation of said reflector relative to incident sunlight.

13.-15. (canceled)

16. The concentrator recited in claim 1 wherein said membrane reflector is slidably received by said strings without any significant tension being applied to said membrane reflector.

17. The concentrator recited in claim 9 wherein said endplates each comprise an axially flexible material.

18. The concentrator recited in claim 1 further comprising at least one bridge operatively positioned at an interval along the length of the strings to provide means to maintain the string positions in a desired parabolic shape.

19. The concentrator recited in claim 1 wherein said membrane reflector comprises a plurality of adjacent membrane reflector sections.

20. The concentrator recited in claim 19 further comprising at least one retention clip operatively connected to said adjacent membrane reflector sections to operatively position said reflector sections relative to the strings.

21.-22. (canceled)

23. A parabolic trough solar energy concentrator comprising a stepless parabolic reflector shaped by a plurality of tensioned string pairs extending along said reflector, each said tensioned string pair having respective strings positioned on opposed surfaces of said reflector.

24. The concentrator recited in claim 23 further comprising a gas-tight tubular transparent housing enclosing said reflector.

25. The concentrator recited in claim 23 wherein said reflector comprises a film having a reflective surface and wherein said film is received between said pairs of strings without any significant tension being applied to said film.

26. The concentrator recited in claim 24 wherein said housing is hermetically sealed by a pair of opposed endplates, each such endplate comprising an axially flexible material.

27. (canceled)

28. The concentrator recited in claim 24 wherein a gas inside said housing is under pressure and wherein said pressure at least partially contributes to said tension of said strings.

29. (canceled)

30. The concentrator recited in claim 24 further comprising means for rotating said concentrator to control the orientation of said reflector and receiver relative to incident sunlight.

31. (canceled)

32. The concentrator recited in claim 23 wherein said reflector comprises a plurality of reflector sections.

33. The concentrator recited in claim 32 further comprising at least one retention clip operatively connected to two or more reflector sections to operatively position said reflector sections relative to the strings.

34. The concentrator recited in claim 32 further comprising at least one bridge operatively positioned at an interval along the length of the strings and between a pair of adjacent reflector sections to provide means to maintain the string positions in a desired parabolic shape.

35. (canceled)