Patent Application
20080308094
This invention concerns solar energy concentrators. More particularly, it concerns trough reflectors which are used to concentrate solar energy, and solar energy collectors which include such trough reflectors.
In this specification, including the claims, “directional” terms (such as “top”, “bottom”, “side”, “underside”, “upper”, “lower”, “front”, “back”, “above”, “upwards”, “below”, “Vertical”, “horizontal” and the like) will be used in the sense that these terms would have with reference to an embodiment of the invention positioned as shown in
Trough reflectors or trough concentrators of solar energy have been known for a considerable time. For example, a trough reflector is described in the paper by Frank Shuman, presented to the Manchester Association of Engineers on 14 Mar. 1914, and published in the Proceedings of the Manchester Association of Engineers, (Discussion Session 1913-1914), No. 9, page 405 et seq. More recently, trough reflectors have been described in the specifications of US patents Nos.:
U.S. Pat. No. 4,071,017 (1978; to John L Russell, Jr. and Robert Edward Potthof);
U.S. Pat. No. 4,106,484 (1978; to Richard E Dame);
U.S. Pat. No. 4,119,365 (1978; to Roger Andrew Powell);
U.S. Pat. No. 4,390,241 (1983; to John M Trihey);
U.S. Pat. No. 4,559,926 (1985; to Barry L Butler);
U.S. Pat. No. 4,571,812 (1986; to Randall C Gee); and
U.S. Pat. No. 4,820,033 (1989; to Erwin Sick).
A trough reflector (collector) comprises a trough-like reflecting surface which has a transverse cross-sectional profile that is parabolic, or approximately parabolic (other acceptable profiles can include a profile which is an arc of a circle or a hyperbolic profile). Trough reflectors are used to focus radiation from the sun onto a linear receiver, which is, typically, an absorber tube. The most efficient trough reflectors are mounted in such a manner that they are moved, during the course of a day, to present a maximum area to the sun at all times (that is, they “track” the sun). To do this, the reflector can be rotated about one axis or two axes. Single axis tracking of the sun is generally effected by rotation of the collector system about a horizontal axis, while two-axes tracking can be achieved by simultaneous rotation about both a horizontal axis and a vertical axis.
To be most cost effective, a solar energy collector which incorporates a trough reflector should be a structurally rigid, easily assembled and easily serviced collector which is economical to construct and relatively lightweight.
Trough reflectors having these characteristics have been constructed by mounting a rectangular, highly reflective surface on a series of curved ribs. The top surfaces of these ribs define, or approximately define—together with the top surfaces of several linear beams that also support the reflective surface—the envelope shape of the trough reflector. Other examples of trough reflectors for concentrators of solar energy which have a concave, preferably parabolic, reflective surface are described in each of the specifications of the US patents mentioned above (in the case of the specification of U.S. Pat. No. 4,119,365 only with reference to
The reflective surface of all these examples of trough reflectors is usually constituted by a single reflective sheet. However, the reflective surface may comprise a number of individual highly reflective panels.
If reflective panels are used, they should each be a rectangular panel (although, the panels may have other perimetric shapes). Each reflective panel will have a surface which is highly reflective of solar energy, or will have a surface to which such a highly reflective surface is bonded. In general, a panel will have a significant thickness and will be stiff enough to maintain a desired shape with minimal, or with no, support from supporting members or elements.
If a single reflective sheet is used, it will generally be a rectangular sheet (although it may have a perimetric shape which is not rectangular) of a thin material which has a surface that is highly reflective to solar energy, or to which such a reflective surface is bonded. Such a sheet is generally flexible and will not maintain a specific profile unless it is constrained to have that profile, for example, by supports on which the sheet is placed. The same problem exists when a number of thin reflective sheets are placed on the top surfaces of a large (in area) array of curved ribs.
Accordingly, almost all trough reflectors that are constructed using thin, highly reflective sheets have linear rails or supports for the sheets at the straight side edges of the trough reflector. (Some trough reflectors have additional supports that are orthogonal to the elongate direction of the ribs, between the edge rails.)
It has been known for some time that the side edge rails or supports ensure that there is no sagging of the edge of the reflective sheet in the space between adjacent ribs. More recently, it has been shown that when a thin reflective sheet is laid on ribs, each having an upper surface profile which is parabolic, to the ends of which a side edge support rail is connected, the reflective sheet has a generally parabolic profile, but the edge regions, adjacent to the side support rails, are not truly parabolic. Therefore, to achieve maximum efficiency of the reflective surface, with regard to the collection of solar energy, the side edge regions of the sheet must be altered so that they also conform to the overall parabolic profile. This control of the edge regions is effected by applying an edge correction moment to the edge regions.
One technique for applying edge correction moments is described in the specification of U.S. Pat. No. 4,106,484 (mentioned above). This specification uses controlled rotation of rectangular side edge supports to modify the shape of the reflective sheet of a trough reflector. More recently, a different approach to the control of the edge region of a reflective sheet has been described in WIPO Publication No. WO 03/022578, which is the specification of International patent application No. PCT/AU02/01243, filed by The Australian National University. Edge control moments could also be applied—in principle, at least—to the reflective sheet used in the trough reflector illustrated in the specification of U.S. Pat. No. 4,820,033, using the adjustment screw 41 of the clamping arrangement that is mounted on the edge supports (and clamping section carriers) 11 of that trough reflector. Both of these last two edge region profile correction arrangements are quite complex and, it appears, are not used in currently marketed solar energy collectors having trough reflectors.
The primary objective of the present invention to provide a simple and effective method of applying an edge correction moment to a thin reflective sheet that is mounted on an array of ribs, each rib having an upper surface profile which is a parabola.
A secondary objective of the present invention is the provision of a method of constructing a low cost, lightweight, yet rigid, trough reflector for a solar energy collector.
A tertiary objective of the present invention is the provision of a rigid, lightweight, easily assembled and easily serviced solar energy collector, which can be used to track the sun using either the single axis tracking technique or the dual axis tracking technique.
The primary objective is achieved by the use of a novel edge support rail for the metal sheet of a trough reflector that is mounted on the ends of the ribs of the trough reflector, and that can be used to apply an edge correction moment to the edge regions of the sheet.
The novel edge support rail has a cross-sectional shape that comprises
One commercially available product that can be used as an edge support rail (which has an additional lip member and two additional flanges at the edges of its lip members) is the Unistrut P3300 (“Unistrut” is a trade mark), or another suitable “Unistrut” rail product.
To construct a trough reflector using the novel side edge rail, a number of the ribs are positioned on a jig or on a support structure so that their concave, parabolic surfaces
In this position, the ribs form a trough-shaped support structure. The reflective sheet is laid on the upper, concave surfaces of the ribs. The sheet sags until it is supported on the top surfaces of the ribs. The reflective, trough-like surface formed by the sheet has two arcuate edges that are supported by the end ribs, and two linear edges that are adjacent to, but inwards of, the tips or ends of the parabolic ribs. Each end region of the ribs which is not overlaid by a reflective sheet has an aperture extending at least partially through the rib from the top surface thereof.
The edge support rails are then mounted on, but above and near the tips of, the arcuate ribs to support the linear edges of the reflective surface. Each edge support rail is a linear member—preferably of metal—which has the cross-sectional shape defined above. Each edge support rail is positioned with its lip member underneath a linear edge of the reflective surface and with the end of its inner flange member resting on the top of the reflective surface.
Each edge support rail is connected to each rib that it overlies by a bolt that passes through a clearance aperture in the web member of the edge support rail, then through a clearance aperture in the reflective surface, to enter and engage with an internally threaded cylindrical aperture which extends into the associated rib from its upper surface. The internally threaded cylindrical aperture may be provided by a captive nut in the top surface of the associated rib, or an equivalent component contained within or below the associated rib.
To enable the edge support rails to be so bolted to the arcuate ribs of the support frame, that part of the lip member which is above an arcuate rib (and, if necessary, a portion of the correspondingly located outer flange member) is removed, forming a series of cut-out sections in each edge support rail. This ensures that the reflective sheet is in contact with the top surface of the ribs when each edge support rail is positioned on the support frame and is bolted to the ribs. The cut-out sections in the edge support rails also allow the edge support rails to apply a twisting force, or moment, to the edge of the sheet reflector, to control the shape of the sheet reflector at its side edges.
Initially, the bolts used to mount the edge support rails on the ribs of the trough reflector are not fully tightened, but are screwed into their associated captive nuts or internally threaded apertures only until the edge support rails are held in position with
Further tightening of the mounting bolts now has the effect of
Thus, according to the present invention, a method of constructing a trough reflector for a solar energy collector comprises the sequential steps of
The invention also encompasses a trough reflector made by this method.
It is known that it is possible to produce a parabolic profile from metal ribs that have been rolled to have a circular arcuate profile. Since the production of rolled metal ribs with a circular arcuate profile is a relatively low cost procedure, the present inventor has extended the application of the novel edge support rail to achieve the secondary objective of the present invention with a trough reflector having a transversely parabolic reflecting surface which comprises an assembly of
To realise this second aspect of the present invention, the arcuate ribs of the trough reflector will normally be lengths of a flexible and resilient material, typically (but not necessarily) a metal which have been formed (rolled in the case of a metal) so that the uppermost surface of each rib is concave and has a profile that is an arc of a circle. Each end region of the ribs which is not overlaid by a reflective sheet has an aperture extending at least partially through the rib from the top surface thereof.
An array of the arcuate ribs are positioned on a jig or on a support structure so that their concave surfaces
In this position, the ribs form a trough-shaped support structure. The reflective sheet or the reflective panel elements are laid on the upper, concave surfaces of the ribs so that the ribs are mostly covered by the reflective sheet or panels but the end regions of the ribs are not covered. Each end region of the ribs which is not overlaid by a reflective sheet or panel has an aperture extending at least partially through the rib from, the top surface thereof.
If rectangular reflective panel elements are used,
If a reflective sheet is used, the sheet sags until it is supported on the arcuate top surfaces of the ribs. The reflective, trough-like surface formed by the sheet or the panels has two arcuate edges that are supported by the end ribs, and two linear edges that are adjacent to, but inwards of, the tips or ends of the arcuate ribs.
Edge support rails are then mounted above, and near the tips of, the arcuate ribs to support the linear edges of the reflective surface. Each edge support rail is a linear member—preferably of metal—which has the cross-sectional shape defined above, namely, it comprises
Each edge support rail is positioned with its lip member underneath a linear edge of the reflective surface and with the end of its inner flange member resting on the top of the reflective surface. Each edge support rail is connected to each rib that it overlies by a bolt that passes through a clearance aperture in the web member of the edge support rail, then through a clearance aperture in the reflective surface, to enter one of said apertures in said ribs and engage with threads which are formed in the aperture in the rib or are provided by a nut. The nut may be a captive nut in the top surface of the rib, or a nut held captive in an interior cavity of the rib, or it may be a separate nut which is engaged by the bolt after passing though an aperture that extends completely through an end of the rib.
To enable the edge support rails to be so bolted to the arcuate ribs of the support frame, that part of the lip member which is above an arcuate rib (and, if necessary, a portion of the correspondingly located outer flange member) is removed, forming a series of cut-out sections in each edge support rail. This ensures that the metal sheet is in contact with the top surface of the ribs when each edge support rail is positioned on the support frame and is bolted to the ribs. The cut-out sections in the edge support rails also allow the edge support rails to apply a twisting force, or moment, to the edge of the sheet reflector, to control the shape of the sheet reflector at its side edges.
Initially, the bolts used to mount the edge support rails on the ribs of the trough reflector are not fully tightened, but are screwed into their associated captive nuts or internally threaded apertures only until the edge support rails are held in position. Tensioning devices are then used to pull the opposed tips of each arcuate rib towards each other. The known consequence of this action is to cause the arcuate upper profile of each rib to adopt, after a predictable movement of the tips of the rib, a parabolic or essentially parabolic shape. Such modification of a resilient rib profile has been described in the aforementioned specification of U.S. Pat. No. 4,571,812 to Randall C Gee.
When the tips of the ribs have been displaced by most, but not all, of the distance required for the rib profile to become parabolic, the bolts mounting the edge support rails on the ribs are tightened
The tensioning devices are then used to move the rips of the ribs further towards each other, until the upper surfaces of the ribs have achieved their final, parabolic profile. The final movement of the tips of the ribs causes the outer flange of the edge support rail to move slightly and apply a transverse force to the reflective surface. The application of this transverse force makes the reflective surface conform closely to the parabolic shape of the upper surface of the ribs, and ensures that, unless the mounting bolt is unscrewed, removal of the reflective surface from the ribs can only be achieved by the use of a significant force.
A respective “chordal strap”, which is preferably a thin metal rod, is then connected between the ends of each rib, to hold each rib in the position in which its upper, concave surface has a parabolic profile (or an essentially parabolic profile), and the rib-tensioning devices are relaxed and removed.
The trough reflector so formed is a rigid, lightweight reflector that can be removed from its jig and incorporated into a solar energy collector.
Thus, according to the second aspect of the present invention, a method of constructing a trough reflector for a solar energy collector comprises the sequential steps of:
These steps can also be undertaken with metal sheets or panels that do not have a reflective upper surface. In this situation, to produce a trough reflector, a reflective material will be applied to the upper surface of the metal sheet or the panels after the steps recited in the last preceding paragraph have been completed.
The present invention also encompasses a trough reflector constructed in accordance with the second aspect of the present invention.
An elongate trough reflector may be constructed by bolting together adjacent end ribs of two or more aligned trough reflectors, constructed in accordance with the present invention.
To achieve the objective of the tertiary aspect of the present invention, the trough reflector formed by the first or second aspect of the present invention, as described above, is mounted on a space frame that is supported by at least two hoop members.
In one form of this aspect of the invention, each hoop member rests on a respective guide and support wheel and on the top of a respective pinch roller and the pinch rollers are connected together by a torque tube.
In another form of this aspect of the invention, one hoop member rests on a respective guide and support wheel, and on top of a pinch roller, while the second (and any subsequent) hoop members are supported on guide and support wheels only. No torque tube is required for this form of the invention.
The axes of rotation of the hoop members are co-linear, so rotation of the pinch rollers causes the hoop members to be rotated simultaneously about a horizontal axis, and consequently the trough reflector is also rotated about a horizontal axis. Such rotation of the trough reflector may be used for single axis tracking the sun. For two-axes tracking of the sun, the entire assembly of trough reflector, space frame and rotatable hoops may be mounted on a base frame that is rotatable about a vertical axis.
Embodiments of the present invention will now be described, by way of example only. In the following description, reference will be made to the accompanying drawings.
a) depicts one end of one form of a tensioning device for applying a transverse force to the ends of the ribs of a trough reflector, prior to the attachment of a chordal strap, while
a) and
A trough reflector constructed in accordance with the first aspect of the present invention has ribs which have been pre-formed to have a parabolic upper surface profile. Such ribs may comprise ribs that have been worked to have a parabolic upper surface, or they may be constructed from thick or thin panels of metal, of a suitable plastics material, or of another suitable material. (A similar “non-rod” rib construction is shown in the aforementioned specifications of U.S. Pat. Nos. 4,390,241 and 4,820,033.) A rib which supports a reflective surface may also be formed by pressing tabs (which follow a parabolic curve) from thin metal panels, and supporting the reflective surface between the tabs, (Such a pressed-tab construction is illustrated in the aforementioned WIPO Publication No, WO 03/022578.) Alternatively, the tabs may be small pieces of a suitable material attached to a panel (such small pieces in this specification will be termed ‘tabs’).
At least one reflecting sheet is supported on the upper, parabolic surfaces of the ribs of the first aspect of the invention. The edge regions of the sheet or sheets are made truly parabolic by the application of a turning moment, using the novel form of side edge support rail, which is bolted by the ribs. This step produces the trough reflector.
This application of a turning moment using the side edge support rail, is also a feature of the second aspect of the present invention. Since an embodiment of the second aspect of the invention is described in detail below, additional description of an embodiment of the first aspect of the invention is not necessary.
Turning now to the illustrated embodiment of the second aspect of the present invention,
Beams with other cross-sections may be used to construct the ribs 10 of the trough reflector. Materials other than metal (for example, some plastics materials) may be used for the arcuate ribs. If the ribs are rods constructed of a plastic material, they will normally be moulded (and not rolled) so that the upper surface of the ribs has the profile of an arc of a circle.
Each rib shown in
The upper surface of each sheet 12 (and of each panel if panels are used) is highly reflective of solar energy. The reflective nature of the upper surface may be established by using a highly polished metal sheet or panel (to produce, for example, a highly polished sheet or panel of aluminium or of stainless steel). Alternatively, the reflective nature of the upper surface may be established by bonding thin silver-backed or aluminum-backed glass mirrors to a sheet metal substrate (such as a sheet or panel of plain steel, of an aluminium alloy, of galvanised steel, or of painted steel). Such laminates can be bonded together using mechanically applied or sprayed on liquid adhesives, contact or pressure sensitive adhesives, or fusible film type adhesives. (These alternative laminate constructions are not exhaustive.) Such laminates are preferably made before the trough reflector is constructed.
As will be seen in
Edge support rails 20 are now mounted on the ribs 10. One long edge support rail may be used on each side of the trough reflector, or a number of shorter edge support rails 20 may be used. Each edge support rail 20, when hi place,
Each edge support rail (as already noted above) has a cross-sectional shape which, as shown in
It will be appreciated that this cross-sectional shape is essentially a “C” section with the arms of the “C” establishing the flange members 22 and 23, and with the lip member 24 formed integrally with, or by an attachment to (for example, by welding if the edge support rail 20 is of steel) the edge of the outer flange member 23. The edge support rail 20 may be formed as a rigid extrusion (for example, an extrusion of steel or an aluminium alloy, or other rigid material).
The edge support rail 20 shown in
Three alternative cross-sectional shapes of an edge support rail are shown in
Each edge support rail 20 has a portion of its lip member 24 and a portion of its outer flange 23 removed (or cut out) from the support rail where the edge support rail passes over a rib of the support frame, as shown in
When in position, the edge support rails 20 are lightly bolted to the ends of the ribs 10 of the support frame, using bolts 30. Each bolt 30 passes through a clearance aperture 31 in the web member 21 of the edge support rail, then through a clearance aperture 32 (see
When each edge support rail 20 has been lightly bolted to the ribs 10 of the support frame, a tensioning device is used to progressively pull the opposed tips of each arcuate rib towards each other.
a) shows one end of one such tensioning device. (The other end is similarly configured.) A pair of metal side arms 16 are bolted to a spacer block 17 which has an aperture 18 passing through it. The aperture 18 is a clearance aperture for a rod 37 that has at least its end regions threaded. The spacer block 17 has dimensions such that it separates the side arms 16 by a distance which
A respective nut 36 is threaded onto each end of the rod 37. Tightening the nuts 36 causes the distance between the two spacer blocks 17 of the device to be reduced, and thus a force to be applied, via the side arms 16, to the ends of the ribs.
Other types of tensioning device may be used instead of the device illustrated in
As noted above, it is known that the consequence of this progressive application of a tensioning force to the ribs is to cause a perturbation of the arcuate upper profile of each rib until it becomes, after a predictable movement of the tips or ends of the rib, a profile that is parabolic or is essentially parabolic in shape.
When the tips of the ribs have been displaced by most, but not all, of the distance required for each rib profile to become parabolic, the progressive movement of the ends of the ribs is temporarily halted and the bolts 30, mounting the edge support rails 20 on the ribs 10, are tightened to fix and clamp the linear edges of the reflective surface firmly between the edge support rails 20 and the ribs 10. Tightening the bolts 30 also applies a turning moment to each linear edge region of the (or each) metal sheet 12. (The application of this turning moment has been described previously in this specification.)
The tensioning devices are then used to move the tips of the ribs further towards each other, until the upper surfaces of the ribs have a parabolic profile. The combination of the tightening of the bolts 30 and this final movement of the tips of the ribs applies a transverse force to the reflective surface, which forces the reflective surface down into the ribs and mates the reflective surface conform closely to the shape of the upper surface of the ribs. An additional consequence of this further application of a transverse force is that, unless the bolts 30 are loosened, the reflective surface can be removed from the ribs only by the application of a significant force (so that, in effect, the reflective surface is fixed against the top surfaces of the ribs).
When this final movement of the tips of the ribs has been effected and the arcuate profile of the upper surface of the ribs has been changed into a parabolic profile, a respective “chordal strap” 34 is connected between the ends of each rib, to hold each rib in that position (that is, with its upper, concave surface having a parabolic profile or an essentially parabolic profile) when the force applied by the tensioning devices is removed.
As shown in
Various forms of chordal strap may be used. A thin, metal rod is preferred because
A turnbuckle which connects wires attached to each end of a rib may be used, but this form of chordal strap is not preferred if the wires can stretch when they are under tension for a long period of time.
Another form of chordal strap is shown in
The trough reflector constructed by this technique is a relatively lightweight structure. It is also a rigid structure (both transversely and longitudinally).
A trough reflector constructed in accordance with the first or second aspect of the present invention may be used advantageously in a collector of solar energy if it is mounted on a suitable space frame with an absorber tube (or other solar energy collecting device) mounted above the reflecting surface, at the focus of the parabolic reflector.
The tertiary objective of the invention is realised if such a trough reflector is used as shown in
The backbone strut 40 and the edge support rails 20 are fixedly connected (typically, using bolts), at each end of the trough reflector, to respective hoop members 42 and 43. The hoop members 42 and 43 each have a horizontal axis and are formed from a strong material (they are preferably rolled from a single steel beam). The hoop members may have any one of a number of cross-sectional shapes. Those cross-sectional shapes for a hoop member include an “I” section, a composite “I” section (comprising two “back to back” “C” sections), a single “C” section, a rolled rectangular hollow section and flange, and a rectangular solid cross-section (This list is not exhaustive.) The important feature of the cross-section of the beam that is roiled to produce the hoop members 42 and 43 is that, when the hoop members are formed,
In principle, since the hoop members 42 and 43 of the depicted solar energy collector will need to be rotated about their co-linear horizontal axes to track the sun from the horizon where the sun rises to the horizon where the sun sets, the hoop members 42 and 43 need not be entirely circular. The upper portions of the hoop members could be omitted and be replaced by, for example, a rigid chordal strut. However, in practice, it is easier, and costs less, to form each hoop member as a complete, circular hoop.
Reverting to the embodiment illustrated in
The space frame structure shown in
As shown particularly in
A respective inner pinch bearing 57 is associated with each hoop member of the solar collector. Each pinch bearing 57 is mounted so that it applies pressure to its associated hoop member, to keep the outer surface of the hoop member in contact with its associated pinch roller 50.
The pinch rollers 50 can be connected together by a torque tube 55. The torque tube 55 is rotated by a motor and gearbox combination 56, which is also mounted on one of the wheel support beams 52. Using a torque tube 55 to connect the pinch rollers provides increased control over longitudinal distortion that may occur due to torsional flexibility in the trough structure.
It should be appreciated that although, for convenience, the pinch bearings have been shown in
For single axis tracking of the sun, the horizontal axes of rotation of the hoop members 42 and 43 can be aligned in any orientation at the location in which the solar energy collector is used. For two-axes tracking of the sun, the support blocks 52 at each end of the solar concentrator will be mounted on a base frame that is partially rotatable about a second (vertical) axis. A separate motor and gearbox will be used to rotate the base frame about its second axis.
A micro-processor will normally be used to control the (or each) motor, to move the solar energy concentrator to track the sun, and (with an appropriate warning mechanism) to “park” the solar energy concentrator in a suitable position when wind speeds above a predetermined value are experienced.
A solar energy concentrator or collector, which includes the trough reflector of the present invention, and is preferably constructed as shown in
This list is not exhaustive.
| Number | Date | Country | Kind |
|---|---|---|---|
| 20069200512 | Feb 2006 | AU | national |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/AU07/00095 | Feb 2007 | US |
| Child | 12194696 | US |
