WO1996029745A1

WO1996029745A1 - Solar collection system

Info

Publication number: WO1996029745A1
Application number: PCT/US1996/003699
Authority: WO
Inventors: Eldad Dagan
Original assignee: Solel Solar Systems Ltd; Helfgott & Karas Pc
Current assignee: Siemens Concentrated Solar Power Ltd; Helfgott & Karas Pc
Priority date: 1995-03-23
Filing date: 1996-03-19
Publication date: 1996-09-26
Anticipated expiration: 1997-09-23
Also published as: IL113098A; AU5524396A; IL113098A0

Abstract

A solar collector assembly comprising one or more solar collector units (9) of the one axis tracking type, each of which comprises an elongated solar collector device (12, 14) mounted on a support structure. The collector unit is mounted on the support structure (30, 33, 34) so that the tracking axis of the collector unit is inclined at an initial angle α with respect to the horizon and the longitudinal axis of the collector unit is inclined at an auxiliary angle β with respect to the tracking axis, whereby when the solar collector device faces the zenith the longitudinal axis of the collector unit is inclined at a superimposed angle ζ equal to α + β, and at a predetermined stow position, the longitudinal axis is substantially parallel to the horizon.

Description

SOLAR COLLECTION SYSTEM

FIELD OF THE INVENTION

The present invention is in the field of solar radiation collection systems ("solar systems") and more specifically it is concerned with an improved one axis tracking (OAT) solar collector.

BACKGROUND OF THE INVENTION

The rapid exploitation of natural energy resources together with the search for environmental friendly energy resources and the need to provide energy to remote settlements or plants, raises an increasing interest in solar energy systems and in improvements therein.

One way of increasing the efficiency of OAT solar collectors is through improving the angular radiation efficiency which is defined as the ratio between the solar radiation intensity received at a surface and between the radiation intensity received at the surface when it faces exactly in the sun's direction. Accordingly, in case of a parabolic reflector or a Fresnel's concentrator, when the sun's rays strike parallel to an optical axis of the reflecting or concentrating surface, then, the angular radiation efficiency is maximal.

11 has long been known that in order to improve the angular radiation efficiency of a solar system, tracking means are to be provided for tracking the sun as it progresses in the sky. One kind of tracking systems known is the two axis tracking systems, However, due to complexity of the two-axes tracking systems, they arc usually applicable to parabolic dishes supported by a substantially tall single leg. Rotating a parabolic trough-like collector in two axes is practical only for short and narrow collectors, since in large collectors there may be severe problems such as torsion of the reflector giving rise to poor optic performances and various torsion forces which may even cause damage to the heat collection element (HCE).

In order to use reflectors having substantially large areas, i.e. 500 square meters and more, single-axes tracking systems are typically used, having their longitudinal axes parallel with the meridian (north-south) and tilted above horizon at an optimal angle, depending on the latitude in which the system is positioned. As a typical example, at the latitude range of between 20 to 35° which is the so-called "sun -belt", in which some of the best radiation areas lie (e.g. Sahara Desert, California, New Mexico, etc.) a reflector tilted at approximately 8° with respect to the horizon was found to increase the annual solar radiation collection efficiency by about 6% as compared with a 0° tilt axis.

However, an inclined collection unit of the single axis tracking type involves several difficulties. First, a solar system unit typically having a reflecting area of about 500 square meters, e.g. being 50 meters long and having a span of about 10 mctcis, requires at least three supporting legs; for obtaining an inclination of approximately 8° the lowest leg should be approximately 6 meters high and the tallest leg should be about 13 meters high.

As a result of the legs' height, the reflector and the entire system arc prone to damage b strong winds, in particular in portions adjacent the higher supporting leg, such as deflection of the reflectors or even breakage of the HCE. Even when such a parabolic solar system is in its stow position, i.e., rotated to an angle in which the optical axis is about 30° below the horizon, in order to reduce the overall height of the solar system, the wind remains substantially strong and may inflict damage. Furthermore, OAT solar systems should be periodically serviced, e.g. regularly rinsing the reflectors and HCE and other maintenance procedures For the performance of such operations, the collector unit is rotated about its longitudinal axis to the stow position. However, in solar systems known to date when the longitudinal axes is inclined, in the stow position one end of the collector is several meters higher than the other end, and servicing the reflector and the HCE require elevating means.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a single axis tracking solar system with the above-referred to disadvantages significantly reduced while the annual angular radiation efficiency is increased.

Furthermore, it is an object of the present invention to provide a solar system in which the longitudinal axis of the collector is substantially parallel to the horizon when the collector is in the slow position. According to the present invention there is provided a solar collector assembly comprising one or more solar collector units of the one axis tracking type, each of which comprises an elongated solar collector device mounted on a support structure, the assembly is characterized in that the collector unit is mounted on the support structure so that the tracking axis of the collector unit is inclined al an initial angle α with respect to the horizon and the longitudinal axis of the collector unit is inclined at an auxiliary angle ? with respect to said tracking axis, whereby when the solar collector device faces the zenith the longitudinal axis of the collector unit is inclined at a superimposed angle φ equal to a+β, and at a predetermined stow position, the longitudinal axis is substantially parallel to the horizon.

According to one application of the present invention, the solar collection device is a concentrating collector wherein said collector unit is of the line-focus type and comprises a solar collector device having an elongated optical concentrator or reflector and a longitudinal receiver fixed at the focus of the concentrator or reflector and wherein the optical concentrator or reflector may for example be a trough-like parabolic reflector or a Frcsncl's lens concentrator or a compound parabolic conccn- trator.

According to another application of the present invention, said solar collector device is a non-concentrating device, which may for example comprise photo-voltaic elements or may be adapted for heating a heat absorbing fluid. According to one embodiment of the present invention, φ

(expressed in degrees) meets the following formula: φ < L+5° (I) where L is the latitude (expressed in degrees).

In a preferred embodiment of the present invention the, stow angle τ is about -30°±5°; the initial angle a is about 2.5°±1°; and the auxiliary angle β is about 7°±4°. In a particular preferred embodiment, β is about 2a.

According to one embodiment of the invention, the support structure comprises al least two supporting legs of different heights, each one of which bears one of two remote ends of the solar collector unit. Where the collector unit comprises a trough-like parabolic reflector, the unit is preferably mounted on a support structure so that the tracking axis intersects the trough's nadir, i.e. it is above the nadir al the lowermost end and below the nadir al the uppermost end. BRIEF DESCRIPTION OF THE DRAWINGS

For better understanding, the invention will now be described in a non-limiting way, with reference to the accompanying drawings in which: Figs, la-c are isometric views of a solar collector assembly according to an embodiment of the present invention, showing the reflector in its sky- up position, side facing position and stow position, respectively;

Figs. 2a and 2b arc side elevations of the solar collector assembly shown in Figs, l a and lc, respectively;

Figs. 3a-c are cross-sectional views (not showing the support legs) along lines Illa-Illa, Hlb-lllb and II Ic-lIlc, respectively in Fig. 2a;

Figs 4a-c are cross-sectional views along lines IVa-lVa, IVa-IVa and IVc-IVc, respectively in Figs. 2a, showing in dashed lines selected angular positions of the reflector;

Fig. 5a is an illustration of the sun's progress in the sky from east to west over a solar collector assembly according to an embodiment of the present invention having its longitudinal axis extending from north to south; Fig. 5b is a side elevation of the illustration in Fig. 5a; Figs. 6a-c are side views of an hydraulic angular activating mechanism attached to a central support leg (not shown) in three angular positions; Fig. 7 is an isometric view of part of a field of solar collectors according to the present invention;

Fig. 8 is an isometric view of a solar collector unit according to the present invention, in which a Fresnel's lens concentrator is used, the collector shown in its sky-up position; Fig. 9 is an isometric view of a solar collector unit according to the present invention in which the solar collector device is a compound parabolic collector; Fig. 10 is an isometric view of a solar collector unit according to the present invention in which the solar collector device is a different kind of a compound parabolic collector; and

Fig. 11 is an isometric view of a solar collector unit according to the present invention in which the solar collector device is a non-concentrating collector comprising photo-voltaic elements.

DETAILED DESCRIPTION OF A SPECIFIC EMBODIMENT

Attention is first directed to Figs. 1 and 2 of the drawings showing a collector assembly 8 of a solar system, having a collector unit 9 comprising a parabolic trough-like reflector 10 consisting of two aligned reflectors 12 and 14 with a heal collection element (HCE) 16 fixed along the focus of the parabolic reflectors by means of brackets 18, the HCE defining a longitudinal axis of the collector unit. The reflectors 12 and 14 arc supported by end brackets 20, 22 and

26 which together with a truss (not shown) maintain the reflectors' shape and impart the systems rigidness. The collector unit 8 is carried by three support legs 30, 32 and 34 aligned wilh the meridians (north-south), the northern support leg 30 being higher than the central leg 32 which in turn is higher than the southern leg 34. The collector unit is mounted on the three support legs so as to be rotatablc about a tracking axis 11. The tracking axis 11 is at an initial angle a with respect to the horizon (sec Figs. 2a and 2b). The height different Δh between two legs separated from one another by distance D, is defined by the following formula (II) Δh = Dxtgα (II)

Each of the legs 30, 32 and 34 comprises a lop portion 30', 32' and 34', respectively, inclined at an angle substantially equal to the angle a, with respect to the longitudinal axis of the legs. Referring now also to Fig. 3 of the drawings, it is shown that the northern side bracket 20 comprises an arm 40 projecting in a direction outward from the collector and is provided with a pivot center 42 located below the nadir 44 of the trough-like collector. The central end brackets 22 (at the adjacent ends of the reflectors 14 and 16) have their pivot center 46 right under the trough's nadir 44, whereas the southern end bracket 26 comprises an arm 48 projecting upward, i.e. in a direction inward the collector and has a pivot center 50 above the trough's nadir 44.

The construction of the collectors is such that an angle β is formed between the tracking axis 1 1 passing along the pivoting centers 42, 46 and 50 and between the longitudinal axis of the collectors defined by the HCE (shown in Figs, l b and 2a).

When the collector is in a tracking position facing the zenith, i.e. at 90° (also referred to as "sky- p" position or the "solar noon ") as shown in Figs, la and 2a, the longitudinal axis of the collectors is inclined at a superimposed angle substantially equal to α + β with respect to the horizon and the arms 40 and 42 arc substantially vertical and in alignment with support legs 30 and 34.

As shown in Fig. l b, when the collector is at a tracking angle of 0°, i.e. in a position in which the collector faces the horizon, then the arms 40 and 42 are substantially horizontal, the longitudinal axis of the collector is inclined at an angle substantially equal to α with respect to the horizon (when viewed from the side) and diverts from the meridians at an angle substantially equal to β (when viewed from above). When the collectors arc to be maintained or repaired, or al strong winds (usually over 70 km./hr.) the collector unit is rotated into a stow position in which the stow angle τ may be calculated according to the following formula (II I): x- arcsin (a/β) (111) where a condition for obtaining a true solution is that a < β.

In the stow position the longitudinal axis of the collector as well as the HCE 16 and edges 54 of the reflectors 10 are substantially parallel to the horizon as can be seen in Figs, l c and 2b. For better understanding, further reference is made to Figs. 4a-c of the drawings in which selected tracking angular positions of the collector arc shown.

Fig. 4a is a cross-section taken adjacent the northern support leg 30. It is shown that in the sky-up position, i.e., at a tracking angle of 90° (illustrated by full lines) the reflector is supported at a height superim¬ posed of the height of the support leg 30 and the arm 40, with the HCE extending above the pivoting center 42. In the horizontal position, i.e., at a tracking angle of 0° (illustrated by dashed lines), the reflector is supported substantially al the end of the support leg 30 and the arm 40 is displaced at a substantially horizontal position, with the HCE 16 extending substantially at the same height as the pivoting center 42. In the slow position, i.e. at a tracking angle of about 30° below the horizon (illustrated by dashed-dotted lines) the reflector is facing downward at an angle of -30° and is supported below the top end of leg 30 with the HCE 16 and edges 54 of the reflectors extending substantially parallel with the ground and at a height lower than said pivoting center 42, said height designated in the figure as II.

Fig. 4b is a cross-section taken adjacent the central support leg 32. In this figure, it is shown that the reflector rotates directly around the pivoting center 46 without a linking arm so that in the sky-up position the reflector is supported at the top end of the support leg 32 whereas similarly as in Fig 4a, when the reflector is rotated to the stow position (dashed-dotted lines), the HCE extends substantially parallel with the ground al the same height II In Fig. 4c, which is a cross-sectional view taken adjacent the southern leg 34, when the reflector is in the sky-up position, it is supported below the top end of the support leg 34 with the HCE 16 extending below said pivoting center 50. However, at the horizontal tracking position (illustrated by dashed lines), the arm 48 is substantially horizontal and the HCE extends substantially at the same height as the pivoting center 50, whereas at the slow position (illustrated by dashed-dotted lines) the HCE is below the pivoting center and extends substantially parallel with the ground at the same height designated II. The outcome of the above arrangement is that in the stow position, the longitudinal axis of the collector and the edges thereof arc substantially horizontal when viewed from the side, whereas when viewed from above the longitudinal axis appears to be diverted from the meridians at an angle substantially equal to β .

Referring now to Fig. 5a of the drawings, the collector unit is shown in selected tracking positions as the sun 52 progresses in the sky. As shown, initially the reflector faces east towards the rising sun and it gradually rotates in a counter-clockwise direction around its longitudinal axis, as the sun progresses towards the west, passing at midday through the sky-up position, facing the horizon. Fig. 5b shows the collector unit of Fig. 5a in a side view in its sky-up position, the longitudinal axis thereof being inclined at the superimposed angle of a + β.

It should be obvious to a skilled person that the reflector is symmetrically disposed at symmetric angles below and over 90°, i.e. before and over the sky-up position.

With collector units being 50 meters long and having an aperture of 10.5 meters, best results were obtained with the stow angle t preferably in the range of -30°±5° with the initial angle a in the range of about 2.5°±1 ° and the auxiliary angle β in the range of about 7°±4° with β being preferably about 2a. The height of the support legs was substantially reduced whereby a northern support leg of 8.5 meters tall and a southern support leg of 6 mcetcr were found to be suitable.

Whereas in the description and drawings reference was made to a solar system positioned in the northern hemisphere of earth, it should be obvious to a person versed in the art that in the southern hemisphere a solar collecting unit according to the present invention is so positioned with its southern leg being the highest one and the northern one being the lowest, with the other components arranged accordingly. Attention is now directed to Figs. 6a-c of the drawings schemati¬ cally illustrating the rotating mechanism and the control unit.

In order to minimize torsion forces in the system, the rotating mechanism generally designated 55 is attached to the central leg 32 (not shown). The rotation mechanism comprises a first hydraulic piston 57 pivotally attached to the leg 32 with a piston rod 59 pivotally linked to an arm 61 rigidly attached to the end bracket 22 at the pivoting center 46. A second hydraulic piston 63 is pivotally attached together with the first hydraulic piston 57 to the leg 32 and has a second piston rod 65 pivotally linked to a second arm 67 angularly disposed with respect to the first rigid arm 61, which is also rigidly attached to the end bracket 22 at the pivoting center 46.

Each of the hydraulic pistons 57 and 63 comprises a pair of flexible tubes 65, 66 and 68, 69 for the ingress and egress of hydraulic fluid received or delivered from a hydraulic power unit 70 as may be the case. The construction of the rotation mechanism is such that upon simultaneous extraction of one piston rod and rotation of the other piston rod, the reflector rotates around its longitudinal axis.

In Fig. 6a the reflector is shown in the sky-up position in which both piston ιθds 59 and 65 arc symmetrically retracted within the pistons 57 and 63 respectively, whereas at the position in which the reflector is in a tracking angle of about 30° above the horizon of Fig. 6b the first piston rod 59 is about or halfway extracted and the second piston rod 65 is fully retracted. In the stow position shown in Fig. 6c of the drawings, the first piston rod 59 is fully extracted and the second piston rod 65 is about halfway retracted.

In a preferred embodiment, the system is controlled by a computerized control unit generally designated 75 comprises a processing unit 76 comprising an internal clock and is piogrammcd to yield an output signal to the hydraulic power unit 70 according to the predetermined annular progress of the sun in the sky. The control unit further comprises a wind sensor 77 for detecting the speed of the wind whereby at predetermined speeds (typically over 70 km/hr) the collector is automatically rotated to the stow position).

An optical sensor 78 ensures that the reflector is facing the sun at an optimal angle, thus ensuring maximal radiation whereas an angular displacement sensor 79 continuously updates the controller 76 as of the angular displacement of the reflector according to which the controller 76 yields a signal to the hydraulic power unit 70, typically rotating the reflector at increments of milli— radians.

Fig. 7 of the drawings illustrates a field of solar collector units 8 according to the present invention, connected to one another by a flexible connecting means 80 as known per se. As shown in the figure, all the collectors are rotated at the stow position in which the longitudinal axis of all the collectors arc cqui-lcvcllcd and arc parallel with the horizon, whereby in a top view the longitudinal axis of the reflectors divert from one another at said angle β constituting a so-called "saw-tooth " pattern. This arrangement enables accessibility and easy maintenance of the solar system. The example of Fig. 7 illustrates how a vehicle 82 travels along the collector units emitting a fluid jet for rinsing the reflectors.

In the specific description and drawings, reference was made to a solar collector device of the concentrating type, comprising a trough-like reflector, however it should be obvious to a person versed in the art that the invention may well be used with other types of solar collector devices e,g. with an optical concentrator of the Fresnel's lens 85 type as illustrated in Fig. 8 of the drawings, or with a compound parabolic concentrator (CPC) 87 with an HCE 16', or a compound parabolic concentrator 88 with a flat collecting zone 89, as illuslraled in Figs 9 and 10 respectively. Fig. 11 illustrates a collector unit in which the solar collector device is a non- concentrating collector 90 which consists of photo-voltaic elements 92 or compartments for heating a heal absorbing fluid, all as known per-se in the art.

Claims

96/29745- 13 -CLAIMS:

1. A solar collector assembly comprising one or more solar collector units of the one axis tracking type, each of which comprises an elongated solar collector device mounted on a support structure, the assembly is characterized in that the collector unit is mounted on the support structure so that the tracking axis of the collector unit is inclined al an initial angle ru with respect to the horizon and Ihc longitudinal axis of the collector unit is inclined al an auxiliary an«lc β with respect lo said tracking axis, whereby when the solar collector device faces the zenith the longitudinal axis of the collector unit is inclined at a superimposed angle φ equal to a+β, and at a predetermined stow position, the longitudinal axis is substantially parallel to the horizon.

2. A solar collector according to claim 1, wherein said solar collector device is a concentrating collector.

3. A solar collector assembly according to claim 2, wherein said collector unit is of the line-focus type and comprises a solar collector device having an elongated optical concentrator or reflector and a longitudinal receiver fixed at the focus of the concentrator or reflector.

4. A solar collector assembly according to claim 3, comprising a trough-like parabolic reflector.

5. A solar collector assembly according to claim 3, comprising a Fresnel's lens concentrator.

6. A solar collector assembly according to claim 2, wherein said solar collector device is a compound parabolic concentrator.

7. A solar collector assembly according to claim 1, wherein said solar collector device is a non-concentrating collector.

8. A solar collector assembly according to claim 7, wherein said solar collector device comprises photo-voltaic elements.

9. A solar collector assembly according to claim 7, wherein said solar collector device is adapted for heating a heat absorbing fluid.

10. A solar collector assembly according to claim 1 , wherein φ, at a latitude L meets the following formula (I).

Φ ≤ L+5° (I) φ and L expressed in degrees.

11. A solar collector assembly according to claim 1 wherein the stow angle τ which is the angle about the tracking axis in which the solar collector device rotates from the horizon to reach the stow position, is defined by the following formula (II): τ=arcsiιι (a/β) (II) wherein a < β.

12. A solar collector assembly according to claim 11 , wherein τ is about -30°±5°, α is about 2.5°±1°, and β is about 7°±4°.

13. A solar collector assembly according to claim 12, wherein β is about 2a.

14. A solar collector according to claim 1 , wherein the initial angle a is about 0° and the auxliary angle /? is other than 0° and wherein the stow angle r is about 0°.

15. A solar collector assembly according to claim 3, wherein said support means comprises at least two legs of a different height, each of which is positioned adjacent one remote end of said collector unit and baring the collector unit at said initial angle α, and wherein at a lowermost end of the collector unit said axis of rotation is above the trough's nadir and at the highermost end of the collector, said axis of rotation is below the trough's nadir.