US20100043777A1

US20100043777A1 - Solar collector system

Info

Publication number: US20100043777A1
Application number: US12/197,722
Authority: US
Inventors: Harry Zvi Tabor
Original assignee: Ormat Technologies Inc
Current assignee: Ormat Technologies Inc
Priority date: 2008-08-25
Filing date: 2008-08-25
Publication date: 2010-02-25
Also published as: WO2010023528A3; WO2010023528A2

Abstract

The present invention provides a solar collector apparatus, comprising a horizontally-mounted cylindrical reflector, divided along its length into two reflector segments, each having its own axis of rotation. Preferably, the two reflector segments are produced by dividing the horizontally-mounted cylindrical reflector down its centerline into two equal reflector segments. The two reflectors segments perform novel tracking with different rotation angles about their corresponding axes by use of a trapeze coupling or a mechanism that permits separate tracking. Since the two reflector segments are coupled, preferably only one tracking mechanism (trapezoid or other) is required since the reflector segments preferably have parallel axes.

The present invention also provides a stationary preferably flat plate receiver facing downwards, absorbing the solar energy reflected upwards. The receiver preferably comprises a metal absorbing plate, fluid-carrying tubes, thermal insulation and preferably a black material front for maximizing the absorption. A control is provided to direct radiation away from the receiver in the event of danger of overheating of the receiver or during periods of maintenance.

Description

FIELD OF THE INVENTION

The present invention relates to the field of solar energy and to the concentration of solar radiation for producing heat and/or electricity. More particularly, the present invention relates to a solar collecting system that concentrates solar radiation onto receivers using solar collectors.

BACKGROUND OF THE INVENTION

In solar collector systems known in the art, solar radiation is concentrated by a reflector or reflectors onto a receiver, which converts the solar radiation into heat or electricity. The reflector or reflectors follow the movement of the sun in order to reflect solar energy efficiently onto the receiver. This operation is termed “tracking”. Single-dimension tracking systems known in the art can perform seasonal tracking from north to south or daily tracking from east to west. The reflector used in the prior art can be of cylindrical form and the receiver is of the form known in the art as a “flat type receiver”, or a horizontal tube or photo-voltaic converter or similar. In the prior art, the receiver is usually coupled to the receiver and moves with it.
In order to facilitate the understanding of the description that follows, a number of terms and elements will be defined hereinafter:

- (1) A cylindrical reflector, as used in single-axis tracking systems, is a portion, of width or aperture W, of a cylinder, defined by its length L, radius of curvature r, which may or may not be constant. Width W is determined by its subtended ‘half-angle’ (α), as shown in FIG. 1A, ‘half-angle’ (α) being the angle at equinox noon between edges (2) and (3) of cylinder part (1) and center—line (4-7), which is the optical axis of the reflector. Axis (5-8), which is perpendicular to the plane of the circle of the cylinder's base, is the cylinder axis. An incident beam of light from the sun I_othat is parallel to the optical axis, is reflected towards focus (7), and profile orientation (2-4-3), is chosen to concentrate the beam so that a narrow line of concentrated solar radiation is produced at focal area (7) where a receiver would be advantageously placed. A light beam parallel to the optical axis is defined as a ‘paraxial beam’. A beam coming from a different direction is defined as a ‘non-paraxial’ beam and the angle between this beam and the optical axis in the plane of the circle of the cylinder is defined as the ‘paraxial departure’ (θ), see FIG. 1B.
- (2) Solar swing: if the cylindrical reflector is mounted so that its cylinder axis is horizontal, in the east-west direction, the reflector is considered as an ‘east-west’ reflector system in which the reflector tracks the seasonal rise and fall of the sun. If the axis is horizontal in the north-south direction, the reflector is defined as a ‘north-south’ reflector system and the reflector tracks the daily traverse of the sun across the sky.
- (3) Concentration factor (or power factor) Cp is defined as the aperture width W of the reflector divided by the width of the receiver that receives the reflected radiation from the reflector.

Many solar collecting systems are known in the art. They generally comprise one or more reflectors or collectors that reflect the solar radiation onto one or more receivers, facing the reflectors. Some collector systems known in the art have reflectors and receivers rigidly mounted together, so that tracking of such collector systems is performed as one complete unit. The frame of this type of collecting system is very expensive and flexible piping needs to be used to convey the fluid heated in the receiver, the piping of which may develop leaks due to the constant movement of the system.
Another difficulty found in prior art collectors is related to the paraxial errors, which reduce the sharpness of the solar radiation focused onto the receiver and thereby bring about a reduction in the degree of concentration that can be obtained. The paraxial errors occur when the solar beam is not paraxial with the optical axis of the concentrator. If an incoming beam of light is parallel to the optical axis of a reflector, it can be brought to a sharp focus, when the reflector has the correct parabolic profile. A non-paraxial beam may come to a sharp focus only for one particular paraxial departure angle for which the profile was designed, but will not come to a sharp focus for all other paraxial departure angles. Consequently, a serious deficiency in the performance of the concentrator is produced since the paraxial departure angle varies from hour to hour and from day to day.
Two types of paraxial errors exist:

- (1) ‘aberration’—named so due to its similarity to the spherical aberration suffered by simple lenses or mirrors and namely, the fact that a peripheral region of the reflector does not deliver a non-paraxial beam to the same focus as the central region of the reflector.
- (2) ‘de-focusing’—the image of a non-paraxial beam does not lie in the same plane as the image of the paraxial beam, but closer to the reflector. Large paraxial departure means a closer plane to the reflector.

The non paraxial ray image is wider than the paraxial ray image, thus reducing the concentration factor. The de-focusing error behaves substantially in a similar manner to the aberration error. The aberration error is approximately proportional to the parallax departure angle and to the aperture angle squared. Thus, if the reflector aperture angle were halved, the aberration error would be reduced by a quarter. However, the aperture dimension is reduced by half, so that the gain in concentration is a factor of two. Since the aberration error and the defocusing error can partially or completely overlap, the larger of these errors determines the maximum concentration factor obtainable with this being further limited by the angular size of the solar disc.
Further difficulties that occur in prior art systems are the thermal heat losses due to the increase in operating temperatures above the ambient temperature. For example, U.S. Pat. No. 4,159,710 discloses two or more reflectors performing single-axis tracking, reflecting solar radiation onto a horizontally stationed receiver facing downwards. However, the apparatus can only perform simultaneous tracking for each coupled reflector, and does not enable different tracking for each of the reflectors.
In another example described in U.S. Pat. No. 4,192,287, a system with parabolic reflectors is depicted performing rotational movement in order to perform sun tracking and to reflect the solar radiation onto a flat plate reflector. However, the system performs two axis movement and uses a plurality of reflector segments, making it expensive and complex to manufacture.
It is therefore an object of the present invention to provide a solar collector system that obviates the disadvantages of prior art systems while providing a system that is relatively simple and inexpensive to construct and operate.
Other objects and advantages of present invention will become more apparent as description proceeds.

SUMMARY OF THE INVENTION

The present invention provides a solar collector apparatus, comprising a horizontally-mounted cylindrical reflector, divided along its length into two reflector segments, each having its own axis of rotation. Preferably, the two reflector segments are produced by dividing the horizontally-mounted cylindrical reflector down its centerline into two equal reflector segments. The two reflectors segments perform novel tracking with different rotation angles about their corresponding axes by use of a trapeze coupling or a mechanism that permits separate tracking. Since the two reflector segments are coupled, preferably only one tracking mechanism (trapezoid or other) is required since the reflector segments preferably have parallel axes.
The present invention also provides a stationary preferably flat plate receiver facing downwards, absorbing the solar energy reflected upwards. The receiver preferably comprises a metal absorbing plate, fluid-carrying tubes, thermal insulation and preferably a black material front for maximizing the absorption. A control is provided to direct radiation away from the receiver in the event of danger of overheating of the receiver or during periods of maintenance.
In addition, the present invention comprises a method of operating a solar collector, wherein solar radiation from the sun is tracked by preferably a single axis tracking mechanism adapted for performing daily (east to west) or seasonal (north to south) tracking, wherein the tracking mechanism enables two reflector segments, produced by dividing a horizontally-mounted cylindrical reflector along its length into two reflector segments, to rotate about their axes at different angular rates for reflecting the sunlight onto the receiver in order to ensure that the reflected radiation falls onto the receiver. The concentration is significantly increased because there are two reflector segments in place of one and the paraxial end de-focusing errors are reduced because each mirror segment has a smaller half-angle. Preferably, according to the present method, the two reflector segments used are produced by dividing the horizontally-mounted cylindrical reflector down its centerline into two equal reflector segments,
The solar collector system of the present invention can be used for converting solar radiation into heat for thermal collectors (e.g. hot water collectors) or for producing electricity using a system that converts heat into electricity or another system utilizing e.g. photo-voltaic cells.
In a preferred embodiment of the invention, the tracking mechanism is a coupling using a trapezoidal linkage for rigidly operating each half reflector.
In another preferred embodiment of the invention, the tracking is performed by two mechanisms for separate tracking of each half reflector.
The one-dimensional tracking can be seasonal single-axis tracking the sun elevation, or daily single-axis tracking following the sun from east to west.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other characteristics and advantages of the invention will be more readily apparent through the following non-limiting examples, and with reference to the appended drawings, wherein:

FIG. 1 is a schematic general view of the cylindrical reflector according to one embodiment of the apparatus;

FIG. 1A is a schematic front view of the cylindrical reflector according to one embodiment of the apparatus showing a paraxial solar beam;

FIG. 1B is a further schematic front view of the cylindrical reflector according to one embodiment of the apparatus showing a non-paraxial solar beam;

FIG. 1C is an additional schematic front view of the cylindrical reflector according to one embodiment of the apparatus also showing a non-paraxial solar beam;

FIG. 2 is a schematic cross-sectional view of the receiver according to one preferred embodiment of the invention used to harness the heat of the sun;

FIG. 2A is a schematic cross-sectional view of the receiver according to another preferred embodiment of the invention used to harness the heat of the sun;

FIG. 2B is a schematic elevation view of the receiver according to a further preferred embodiment of the invention used to harness the heat of the sun;

FIG. 2B′ is a schematic end view of the receiver shown in FIG. 2B used to harness the heat of the sun;

FIG. 2C is a schematic cross-sectional view of the receiver according to an additional preferred embodiment of the invention used to harness the heat of the sun;

FIG. 2D is a schematic cross-sectional view of the receiver according to a further preferred embodiment of the invention used to harness the heat of the sun;

FIG. 2D′ is a schematic cross-sectional view of the receiver according to a still further preferred embodiment of the invention used to harness the heat of the sun;

FIG. 3A and FIG. 3B are schematic side views of the reflectors orientation according to a preferred embodiment of the invention;

FIG. 3C′ and FIG. 3C″ are schematic side views of the half-reflectors orientation according to other preferred embodiments of the invention;

FIG. 3D is a schematic side view of the half-reflectors orientation according to a preferred embodiments of the invention;

FIG. 4 is a schematic side view of the trapezoidal linkage according to another preferred embodiment of the invention;

FIG. 5 and FIG. 5A are schematic side views of the overlapping of the image rays from the half-reflectors according to a preferred embodiment of the invention;

FIG. 6 is a schematic side view of the solar collecting system according to one preferred embodiment of the invention;

FIG. 7 is a schematic side view of supporting mechanism of the solar system according to a preferred embodiment of the invention; and

FIG. 8 is a schematic diagram showing an example of a preferred embodiment of the present invention.

Similar reference numerals and symbols refer to similar components.

DETAILED DESCRIPTION

As used herein the term “tracking” refers to a process in which the reflector follow the motion of the sun. The present invention can be better understood by reference to FIG. 1A, FIG. 1B and FIG. 1C which cover the basic physics which is advantageously exploited in the present invention.
FIG. 1A shows mirror 2-3 of width W, which concentrates a beam of solar radiation onto a receiver (not shown) in the focal region F. The beam is paraxial and if the profile is parabolic, the concentrated beam forms a thin line of energy on the receiver. However, if the beam is not paraxial, but tilted at an angle (θ), an image of concentrated light is formed at F₂(see FIG. 1B). Whatever the paraxial departure (θ) is, the image always falls on the cc, the circumscribing circle, that passes through the mirror extremities 2 and 3 and the focus F. The center of this circle is at C, C being a geometrical property of the mirror so that if the mirror moves, C moves with it. The central area of the mirror reflects to a point F′, not quite at F. F F′ is the spherical aberration already referred to.
Since the receiver for utilizing the concentrated solar energy will usually be placed in the region F, the image at F₂can be brought to the region of F by rotating the mirror about a horizontal axis h which, generally, is just below the center of the mirror (see FIG. 1C). In FIG. 1C, the position before rotation, see FIG. 1B, is shown in dotted lines while the position after rotation is shown in full lines. From FIG. 1C it can be seen that F₃(previously F₂) is closer to the mirror than the image F. The distance F F₃is the defocusing error or distance. Note that I₃is the incoming solar beam that is focused at F. Wherever the image plane is placed to receive the concentrated solar radiation, the image size will be larger than the image at F due to the angular spread of the rays from the mirror—and the minimum image size occurs when the pane or receiver is approximately in the middle of the defocusing distance.
In the present invention, the mirror of width W is preferably divided into two halves of width W/2 each, with the images of both parts overlapping. In such a situation, each half behaves as described herein with substantially reduced parallax and defocusing errors, because of the smaller angular size, resulting in a smaller final image, thereby providing higher concentration and leading to an improved solar collector system.
FIG. 2 is a cross-sectional view of the receiver according to a preferred embodiment of the present invention used to harness the heat of the sun. Receiver (14) is a ‘flat type receiver’ comprising a metal absorbing plate (9) to which one or more fluid-transporting pipes (11) are connected. The receiver can be a metal structure including fluid-carrying tubes, or may be a single elliptical fluid-carrying tube 11B, as shown in FIG. 2B (an end view presented in FIG. 2B′) or may be a PV panel or other means of converting solar heat to electricity. In FIG. 2B, reference numeral 13B designates connecting pipes connected to pipe 11B, the end plates of which are designated as 9B (see FIG. 2B′). Reference numeral 12 designates insulation material present at the back of tube 11B while the front has the insulation of the air in gap 15B between tube 11B and cover window 10B, preferably curved. Examples of fluids that can be used in fluid-transporting pipes (11) are water, thermal heat transfer fluid e.g. thermal oil, motive fluids e.g. n-pentane, iso-pentane, butane, propane, hexane, Terminol LT, Dowtherm J, dodecane, etc. Note that Therminol LT is the commercial name for the alkyl substituted aromatic fluid of the Solutia Company having a center in Belgium. Dowtherm J, on the other hand, is the commercial name for a mixture of isomers of an alkylated aromatic fluid of the Dow Chemical Company being centered in the U.S.A. The pipes can be connected to metal absorbing plate (9) so that pipes (11) protrude below the level of absorbing plate (9) as shown. However, if preferred, the pipes can be located above or behind metal absorbing plate (9) by connecting pipes (11) to the rear or upper side of absorbing plate (11) so that heat is transferred to the fluid flowing in pipes (11) by heat conduction from metal absorbing plate (9) to the walls of pipes (11)—see FIG. 2B. Heat is extracted from receiver (14) by supplying suitable a fluid through tubes (11). The front side of the absorbing plate receives the solar radiation and is separated from the outside atmosphere by a transparent window (10), spaced, in this example, about two cms from the plate. The rear side of the plate and the edges are covered with thermally-insulating material (12). All the above is encased in a water-resistant box (14) referred to herein as the ‘receiver box’.
The front side of plate (9) is preferably coated with black material to maximize the absorption of impinging solar radiation. The receiver is preferably coated with a low emittance black coating, known in the art as a ‘selective surface coating’, to reduce thermal radiation heat loss from the absorbing plate. Consequently, the dominant heat loss is air convection in the air volume between plate 9 and window 10. Also, as known in the art, in low temperature (near ambient) applications, such as water heating, the black material may comprise any heat resistant paint with a thermal emissivity of about 0.9, whereas, in such a case, most of the heat loss is by thermal radiation from the absorbing plate.
Unlike the receivers in water heating systems, box (14A) is mounted substantially horizontally, facing downwards, and solar radiation coming from a concentrating reflector (not shown) enters window (10) of box (14A). By mounting the box facing downwards, the convection loss in the air space is substantially reduced. Due to the combination of preferred use of a selective surface coating and box (14A) facing downwards, the thermal heat loss at any specific temperature (above the ambient temperature) is reduced considerably compared to that in conventional solar water heating systems. Consequently, in the present invention, the thermal efficiency is increased so that the system can operate at higher temperatures. Furthermore, receiver box (14A) can be smaller in size, reducing possible shading and capital costs.
If PV collectors are used to harness the solar energy to produce electricity, PV cells are bonded to the face of the receiver plate in place of the selective surface and the thermal insulation material may be omitted. If needed, a heat-removal fluid is passed through pipes to cool the PV cells.
Alternatively, a secondary non-imaging concentrator (see e.g. FIGS. 2C, 2D and 2D′) can be used to concentrate the solar radiation concentrated already by the primary reflectors or collectors of the present invention. FIG. 2C shows a secondary compound elliptical concentrator (CEC) having secondary reflector surfaces 14C which can be used in the present invention to further concentrate the concentrated solar radiation onto the receiver. On the other band, FIGS. 2D and 2D′ show a secondary involute concentrator having secondary reflector surfaces 14D and 14D′ respectively (FIG. 2D showing e.g. a receiver having one pipe while FIG. 2D′ shows a receiver having e.g. two pipes) which can be used in the present invention to further concentrate the concentrated solar radiation onto the receiver.
FIG. 3A and FIG. 3B illustrates a side view of reflector (22) according to a preferred embodiment of the present invention. Reflector (22) is preferably divided down its centerline (4-7) into two equal parts (17) and (18), each having its own axis of rotation (20) and (21) for tracking. The receiver (not shown) is placed at focal point (7). Referring now to FIG. 3A, at the time of the equinox, the two reflector-halves (17) and (18) behave as the original reflector (22) and the parallax deviation is zero or close to zero i.e. there is no or very little parallax error.
FIG. 3B shows that each preferred half reflector (17) and (18) of reflector (22) tracks the sun. The rotation of reflector halves (17) and (18) is about their respective axes (20) and (21), located preferably under the center of each half reflector. However, if preferred, the axes of rotation 20 and 21 need not be located under the center of mirror elements 17 and 18. Rather, they may be preferably located, as shown e.g. in FIG. 3C′ and FIG. 3C″, toward the edge of mirror elements 17 and 18 either below or above respectively the mirror elements on lines passing through the focal point F and center C₁or C₂of circumscribing circles 40 and 41, these circumscribing circles shown for additional clarity in FIG. 3D. If preferred, axes of rotation 20 and 21 may be placed to coincide with the centers C₁or C₂of circumscribing circles 40 and 41 as shown in FIG. 3D. In such a case, the mirror elements can be suspended from the axes of rotation using arms 43. Tracking illustrated in FIG. 3B, is particularly useful during times when parallax errors start to occur (not at the time of the equinox), and, reference to the previous description will make it apparent to the skilled person in the art that the parallax error obtained by two half reflectors (17) and (18) is much smaller than the error produced by full-aperture reflector (22). Furthermore, because solar images of each half reflector (17,18) can overlap, the total parallax error is that of a half-aperture reflector and not twice its value. The net result is that the concentration factor that can be obtained is in excess of about 20.
Preferably, each preferred half reflector (17,18) carries out its tracking at a different rate since the reflector halves are wide, so that better overlap of the images is obtained when using different tracking rates. If the reflector halves (17), (18) were narrow, they would track at the same rate (similar to the rate of a Fresnel-type concentrator).
FIG. 4 illustrates a side view of trapezoidal linkage (24) according to a preferred embodiment of the present invention. Trapezoidal linkage (24) is rigidly attached to each preferred half-reflector (17) and (18) respectively, the half-reflectors being equal in length. Radial bars (15) and (16), equal in length, connect to half-reflector (17) and (18) respectively at their distal end, and are connected to coupling bar (19) via hinges (13) and (14), respectively, at their rear ends.
Bar (19) is preferably longer than the distance between rotation axes (20) and (21) of the half-reflectors, resulting in a different amount of rotation of half-reflectors (17,18). Starting from a symmetrical trapeze position, an anti-clockwise rotation of half-reflector (17) by x degrees results in an anti-clockwise rotation of half-reflector (18) by y degrees, whereas y is slightly bigger than x (Similarly, a clockwise rotation of half-reflector (18) from the symmetrical position by x degrees results in a clockwise rotation by y degrees of half-reflector (17)). The trapeze is defined by ‘trapeze-angle’ θ′ which represent the departure angle of symmetrical trapeze from a rectangle. If bar (15) is displaced by angle w₁, bar (16) will be displaced by angle w₂, whereas the ratio
$\frac{w_{2}}{w_{1}} \approx Const$
and is a function of θ′:
$\cot (θ^{'}) = \frac{2 - \cos (w_{2}) - \cos (w_{1})}{\sin (w_{2}) - \sin (w_{1})} .$
For example, for θ′=7.3°, the ratio is 1.05 and a 20° rotation of half-reflector (17) produces a 21° rotation of half-reflector (18). Any other ratio can be obtained by setting θ′ according to the above approximate relationship.
FIG. 5, and FIG. 5A (which is the top of FIG. 5), illustrate the overlapping of the image rays from preferred half-reflectors (17) and (18) and indicates plane XX, with the minimum image size xx. Referring to FIG. 5 a, (33)-(34) is the aberration of the LHS reflector (17). Point (33) lies on the circumscribing circle (37-38) of the LHS half-reflector (17) in the solstice position. Similarly, point (35) lies on the circumscribing circle (41-42) of the RHS half-reflector (18) (the aberration is two small to see). (D₁) is the defocusing error of the RHS half-reflector, while (D₂) is that of the LHS reflector. At the winter solstice most of these extreme rays appear concentrated on the LHS (17) of center line (45-46), and at the summer solstice they will appear in the RHS but within the same image area xx.
It can be seen in FIG. 5A, that the minimum image size xx is approximately 1/20 the width of the full (double) reflector width W (shown in FIG. 1A) i.e. by placing a receiver at xx, a concentration factor of 20 is achievable (according to an example of a reflector having half-aperture of 34.5°.
FIG. 5 illustrates an example for a system with a reflector half-aperture angle of 34.5° i.e. 17.25° half-aperture for each reflector half. It was found that in order to track a 40° swing of the sun, from equinox to early or late hours of the solstice day (winter, for example,—a similar swing occurs on the summer solstice day), the lower half-reflector should be rotated approximately by 19.5° eastwards (northern hemisphere) while the upper half-reflector should be rotated approximately by 21.5°. For other reflector half-aperture angles, the rotation ratio will be slightly different and the angles and lengths in the trapezoidal linkage will be slightly different.
FIG. 5 also shows that, in some extreme positions, partial shadowing p-q, of one half-reflector by the other, may occur. This can be reduced by cutting off some parts of the reflectors in the central region, and if necessary, starting with a slightly larger reflector aperture angle. However, since most of the shading occurs in early morning or at night, only a small part (if any) of the half-reflectors need to be cut.
FIG. 6 and FIG. 7 illustrate a side view of the complete solar collecting system and its supporting pillar in detail, according to one preferred embodiment of the invention comprising two half-reflectors (17) and (18). The full line is the equinox position, while the dotted line is the winter solstice position, see FIG. 8. FIG. 8 is presented by way of example only, where each half mirror AM, MB has a total angle of 34.5°, having a width of approximately 1.5 m and a length of about 4 m. The reflector profile is part of a circle of radius 5 m or more precisely a parabolic profile of 2.5 m focal length. Receiver 14 facing downwards and set at an angle in the range 0-30° to the horizontal (dependent on the local altitude). In FIG. 8, the local latitude is 32—purely for illustration, but the system is in no way limited by this angle and reference numeral 60 designates the ground for this latitude. Receiver 14 is placed in the focal area 2.5. m distant from M and is held in place by crossbar and stabilizer 51, and is held in place by cross-bar and stabilizer (51) being connected to elements (44) and (49), that slide on pillar (48) and fixed in position by clamps (50), (52). Receiver plate (9) width is 15 cm (see FIG. 2), assuming a concentration factor of 20. Box (14A) has a width of 25 cm with transparent window (10) being 3 mm thick low-absorbing glass. The top and sides of box (14A may be curved to reduce shading.
The reflector surface of this example has a high-reflectivity coating on a metal base sheet of about 1 mm thickness which is held by five shaped elements 40, 1 m apart, mounted on a tubular support 41. The upper (RHS) half-reflector (18) performs tracking by means of a tracking motor pinion and segment (42). The 4 m long half-reflectors (17,18) are supported on axes (20) and (21) respectively, which rotate with cross bar (43). Cross-bar (43) is mounted on vertical support pillar(s) (48) that are 4 m apart in rows. The half-reflectors (17,18) elements may be coupled in order to make-up continuous long mirror elements (complete reflector). The overall height (H) is 3.20 m, including clearance h=40 cm from the ground. Similarly, the 4 m long receivers have their inlet and outlet pipes connected in series to form a single fluid flow system. Bars (15) and (16) are the coupling bars of the trapezoidal linkage (24), (19) is the link bar and (13) and (14) are pivots.
Pillar (48) has a basic element which is about 10 to 12 cm in diameter and can be made, e.g., of galvanized steel tube, standing on the ground. The upper portion is of a reduced diameter, in order to reduce shading on the mirrors. At the top of this column is a horizontal side arm (44) that supports the receiver (14). Horizontal cross beam (43) present at an intermediate level provides support for the half-reflector elements (17,18) via the bearings for the rotation axes (20,21). Cross-bar (43) is part of an element that slides on pillar (48) and is locked in position by clamps (47). The construction of this embodiment is designed to allow uniform heights of reflector elements (17,18) to be set (with the aid of a theodolite) so that long rows can be established without the need for precise leveling of the ground, bringing about a cost-saving improvement. Similarly, the height of receiver (14) is adjustable using clamp (47 c).
While the description refers to a solar collector comprising two equal reflector segments produced by dividing the horizontally-mounted cylindrical reflector down its centerline into two equal reflector segments, the present invention also considers that two reflector segments produced by dividing a horizontally-mounted cylindrical reflector along its length into two reflector segments even though the segments are not equal in size can also be used in the present invention.
Furthermore, while the present invention specifically mentions to use of two reflector segments as described herein, the present invention also contemplates the use of more than two reflector segments, e.g. 3, 4, etc. At present, it is considered that two reflector segments provide sufficiently improved performance at a reduced cost of the horizontally-mounted cylindrical reflector solar collector.
While some embodiments of the invention have been described by way of illustration, it will be apparent that the invention can be carried into practice with many notifications, variations and adaptations, and with the use of numerous equivalents or alternative solution that are within the scope of persons skilled in the art, without departing from the spirit of the invention or exceeding the scope of the claims.

Claims

1. A solar collector apparatus, comprising a horizontally-mounted cylindrical reflector divided along its length into two reflector segments, which concentrate solar radiation to the same receiver area and each reflector segment is provided with its own axis of rotation for tracking.

2. The solar collector apparatus according to claim 1 wherein said two reflector segments are produced by dividing the horizontally-mounted cylindrical reflector down its centerline into two equal reflector segments, each having its own axis of rotation.

3. The solar collector apparatus according to claim 1, wherein the two reflector segments move at different angular rates about their corresponding axes.

4. The solar collector according to claim 3, wherein the two reflector segments are coupled and have parallel axes and the movement of the reflectors is performed by a trapeze coupling or like mechanism that permits separate tracking.

5. A method of operating a solar collector, wherein solar radiation from the sun is tracked by a single axis tracking mechanism adapted for performing daily (east to west) or seasonal (north to south) tracking, wherein the tracking mechanism enables two reflector segments, produced by dividing a horizontally-mounted cylindrical reflector along its length into two reflector segments, to rotate about their axis at different angular rates for reflecting the sunlight onto the receiver in order to ensure that the reflected solar radiation falls on the receiver, thereby significantly increasing the concentration ratio by reducing paraxial errors and defocusing errors.

6. The method according to claim 5 wherein said tracking performed by the tracking mechanism is performed by a tracking mechanism that enables two equal reflector segments produced by dividing the horizontally-mounted cylindrical reflector down its centerline into two equal reflectors, to rotate about their own axis of rotation at different Is angular rates for reflecting the sunlight onto the receiver.

7. The method according to claim 6, wherein the tracking performed by the tracking mechanism is performed by a tracking mechanism comprising a coupling using a trapezoidal linkage for rigidly supporting each half reflector.

8. A method according to claim 6, wherein the tracking performed by the tracking mechanism is performed by two mechanisms providing separate tracking of each half reflector.