AU2006348550A1

AU2006348550A1 - A heliostat support and drive mechanism

Info

Publication number: AU2006348550A1
Application number: AU2006348550A
Authority: AU
Inventors: David Glover Van Der Westhuyzen
Original assignee: Eskom Holdings Pty Ltd
Current assignee: Eskom Holdings Pty Ltd
Priority date: 2006-09-22
Filing date: 2006-09-22
Publication date: 2008-03-27
Anticipated expiration: 2026-09-22
Also published as: CN101553698A; US20100024802A1; MX2009002965A; EP2079966A1; AU2006348550B2; WO2008035132A1

Description

WO 2008/035132 PCT/IB2006/002628 -1 HELIOSTAT SUPPORT AND DRIVE MECHANISM BACKGROUND OF THE INVENTION THIS invention relates to a heliostat support and drive mechanism. Heliostat support and drive mechanisms typically use a centrally mounted gear mechanism coupled to a torque tube to obtain azimuth and elevation movement. A very high degree of accuracy is required in this gear mechanism at an associated high cost. Thus an improved heliostat support and drive mechanism is required. SUBSTITUTE SHEET (RULE 26) WO 2008/035132 PCT/IB2006/002628 -2 SUMMARY According to an example embodiment a heliostat support and drive mechanism includes: a fixed pedestal; a central joint attached to the pedestal and attachable to a mirror frame so that the mirror frame is able to rotate with respect to the central joint, the central joint being movable with respect to the pedestal with at least two degrees of freedom; at least two arms connected to the pedestal; a first drive mechanism connected to one of the arms and being movable with respect to the arm, the first drive mechanism having a mirror frame connector attached thereto to allow the first drive mechanism to connect to a mirror frame, the first drive mechanism being movable between an extended position in which the mirror frame connector is extended away from the arm and a withdrawn position in which the mirror frame connector is withdrawn towards the arm; and a second drive mechanism connected to the other one of the arms and being movable with respect to the arm, the second drive mechanism having a mirror frame connector attached thereto to allow the second drive mechanism to connect to a mirror frame, the second drive mechanism being movable between an extended position in which the mirror frame connector is extended away from the arm and a withdrawn position in, which the mirror frame connector is withdrawn towards the arm.

WO 2008/035132 PCT/IB2006/002628 -3 The mirror frame connector attached to each of the drive mechanism may be a universal joint. The fixed pedestal is typically a pole which is able to be inserted into the ground. In one example embodiment the drive mechanisms are worm-gears. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a perspective view of an example embodiment of a heliostat support; and Figure 2 is a perspective view of the heliostat support and drive mechanisms connected to a mirror support frame which is schematically illustrated. DESCRIPTION OF AN EMBODIMENT A heliostat is a device that tracks the movement of the sun or a function thereof. It is typically used to orient a mirror, throughout the day, to reflect sunlight in a consistent direction towards a collection point. When coupled together in sufficient quantities, the reflected sunlight from a number of heliostats can equate to an enormous amount of heat if all are oriented towards the same target. Heliostats have been used for sunlight-powered interior lighting, solar observatories and solar power generation.

WO 2008/035132 PCT/IB2006/002628 -4 Thus, heliostats include mirrors that track the sun and reflect the sunlight onto a central receiving point. Most heliostat solar power systems consist of arrays of heliostats. The simplest heliostat devices use a clockwork mechanism to turn the mirror in synchronisation with the rotation of the Earth. More complex devices need to compensate for the changing elevation of the Sun throughout a Solar year. Even more advanced heliostats track the sun directly by sensing its position throughout the day. If the heliostat is used to reflect the sun onto a stationary point, then the heliostat does not actually track the sun itself, but rather it tracks a point in the sky midway between the sun and the stationary receiver point. Referring to Figure 1, a heliostat support 10 includes a fixed support pedestal 12. This is usually inserted into the ground and supports the moving parts and reflectors of the heliostat. At least two arms 14 are connected to the support pedestal 12. In the illustrated embodiment the arms 14 cannot move with respect to the support pedestal 12. At the end of each arm is a drive mechanism support 34. The drive mechanism supports 34 can rotate with respect to their respective arm 14 about axis "B". Connected to each of the at least two arms 14 is a drive mechanism referred to hereafter as a first drive mechanism 16 and a second drive mechanism 18. Each drive mechanism is connected to one of the arms 14 by way of connectors 14a and 14b which form part of the drive mechanism supports 34 WO 2008/035132 PCT/IB2006/002628 and which are movable with respect to the remainder of the drive mechanism support as they are able to rotate about axis "A". This means that each drive mechanism 16 and 18 shown in Figure 2 is effectively moveable with respect to the arms of the pedestal with two degrees of freedom. Each drive mechanism 16 and 18 has a mirror frame connector 20 attached thereto to allow the drive mechanism to connect to a mirror frame 28 which is shown in Figure 2. The connectors 20 in the illustrated embodiment are a universal type joint allowing movement in a plurality of directions. The drive mechanisms are movable between an extended position in which the mirror frame connector 20 is extended away from the arm and a withdrawn position in which the mirror frame connector 20 is withdrawn towards the arm. The drive mechanisms are independently movable. Thus they could both be extended or withdrawn to the same position or one could be totally or partially extended while the other could be simultaneously totally or partially withdrawn. In this way the drive mechanism are able to be independently controlled to point the mirror frame to the desired position. In the illustrated embodiment the drive mechanisms are worm gears. The worm gears are driven'by motors incorporated in the drive mechanisms. The motors are able to receive control signals to move the worm gears by a predetermined amount in a predetermined direction. A central joint 22 is connected to the fixed support pedestal 12 for connecting to the centre of the frame 28 for supporting the reflector (mirror) element of the heliostat. This can best be seen in Figure 2 which shows a portion of the frame 28 but does not illustrate any reflector attached to the frame.

WO 2008/035132 PCT/IB2006/002628 -6 The central joint 22 is movably connectable to the fixed support pedestal 12 so that it can rotate around the fixed support pedestal 12. In addition, the central joint 22 includes a horizontal axis joint 24 around which the central joint 22 can rotate. Thus, the central joint 22 is attached to the pedestal 12 and attachable to the mirror frame 28, the central joint being movable with respect to the pedestal in two degrees of freedom. In this manner, a face 26 of the central support can be pointed to any point in the sky perpendicularly. The mirror frame 28 is connected, to the central joint through opening 36 and can rotate with respect to the central joint. The mirror frame 28 is thereby movable with respect to the pedestal with at least three degrees of freedom Traditional telescopes have to be able to align to all points in a semi-sphere, 360* in azimuth and 900 in altitude. This premise has been carried over into heliostat applications. However, a heliostat does not have to have such a large scope of motion as the heliostat is placed in a fixed position relative to the receiver which is also fixed. Thus, the maximum required range of motion of a heliostat is approximately 90* in azimuth and 45* in altitude. Obviously, the actual amplitudes depend on the relative location in the heliostat field and the degree of latitude of the plant. This reduction in required range of motion allows the replacement of the central azimuth gear in prior art applications with the extending arm WO 2008/035132 PCT/IB2006/002628 -7 mechanism illustrated. The system thus has two extending arm mechanisms 16 and 18 placed to orientate the supporting frame. The arms 16 and 18 are supported at a suitable distance from the pedestal to provide sufficient moment to overcome the forces and moments acting on the heliostat. In operation, each of the arms 30 of the drive mechanisms 16 and 18 is extended or withdrawn by an appropriate amount to cause the mirror frame 28 to be moved with respect to the pedestal and thereby to point in the desired direction. To allow motion of the frame 28 for supporting the reflector (mirror) element of the heliostat, more dimensional freedoms are required when compared to the standard design. The frame has to be able to rotate about its centre in addition to the azimuth and altitude degrees of freedom. The illustrated embodiment allows for approximately 1100 movement around the azimuth and 90* around the vertical axis. This is a sufficient range of motion based on the requirements specified for a heliostat. As the azimuth range is less than 360*, the heliostat azimuth orientation is sensitive to its relative position in a heliostat field and each heliostat deployed has to be orientated individually. As the position of the frame is controlled by two extending arms 16 and 18, a relation has to be derived to determine the angular position relative to the two distances the arms are extended. Depending on the arrangement of the two arms relative to the pedestal and mirror frame, this will result in a non linear relation between the azimuth and altitude degrees and distance the arms are extended.

WO 2008/035132 PCT/IB2006/002628 -8 Thus it will be appreciated that the system does not have a central gearbox to orientate the mirror frame about the azimuth axis. As the mirror frame is supported at its centre no torque tube is required for rotation about the altitude axis. As no torque tube is required for the mirror frame the potential exists to significantly enhance the design of this structure, thus reducing the amount of material and mass and ease of assembly. Furthermore, the ball worm gears 14 and 16 allow much faster angular displacement than the central gearbox traditionally used and the mirrors may thus be moved much quicker from one position to another, based on plant requirements. It will also be appreciated that the azimuth gearbox currently used in heliostat technology has a very large gear ratio to provide the required forces and accuracy in positioning. Although the technology has good reliability it remains a very expensive component in both the design and maintenance facets of heliostat applications. The replacement of this component by means of an extending arm mechanism, such as a worm gear, or preferably a ball worm gear, greatly reduces both the initial capital and operating costs associated with heliostats. There are many variations possible on the support and drive mechanism described above. The base of the application is that two extending arms are used to position the mirror frame in the desired direction and the mirror frame support has the required degrees of freedom for alignment. The illustrated embodiment has the extending arm drive mechanisms placed orthogonally from each other. It would be possible to change these angles relative to each other and still satisfy the required operating conditions.

WO 2008/035132 PCT/IB2006/002628 Also, the loading profile evaluation in the prototype could depict which axes must be optimised. Thus the geometry of the structure (orientation of axes and lengths of arms) can be changed so as to optimise on material and other design factors. The loading profile refers to the forces the structure is subjected to when placed in the field. The drive mechanism may be changed from a ball screw to another linear actuator mechanism such as a hydraulic actuator. It will be appreciated that the joints in the illustrated embodiment could be changed to other types of joints which provide the same or more degrees of freedom. For example, roller bearings could be replaced with other mechanisms such as a ball joint. In addition, the following could also be changed without changing the principle of orienting philosophy: " relocation of an axis of rotation , symmetry in two drives could be changed - they do not have to be the same * the mirror frame does not have to be centralized. * the position where each drive is attached to the mirror frame . * the position where each drive is attached to its fixed reference point on the arms.

Claims

1. A heliostat support and drive mechanism including: a fixed pedestal; a central joint attached to the pedestal and attachable to a mirror frame so that the mirror frame is able to rotate with respect to the central joint, the central joint being movable with respect to the pedestal with at least two degrees of freedom; at least two arms connected to the pedestal; a first drive mechanism connected to one of the arms and being movable with respect to the arm, the first drive mechanism having a mirror frame connector attached thereto to allow the first drive mechanism to connect to a mirror frame, the first drive mechanism being movable between an extended position in which the mirror frame connector is extended away from the arm and a withdrawn position in which the mirror frame connector is withdrawn towards the arm; and a second drive mechanism connected to the other one of the arms and being movable with respect to the arm, the second drive mechanism having a mirror frame connector attached thereto to allow the second drive mechanism to connect to a mirror frame, the second drive mechanism being movable between an extended position in which the mirror frame connector is extended away from the arm and a withdrawn position in which the mirror frame connector is withdrawn towards the arm. WO 2008/035132 PCT/IB2006/002628 - 11

2. A heliostat support and drive mechanism according to claim 1 wherein the mirror frame connector attached to each of the drive mechanism is a universal joint.

3. A heliostat support and drive mechanism according to any preceding claim wherein the fixed pedestal is a pole which is able to be inserted into the ground.

4. A heliostat support and drive mechanism according to any preceding claim wherein the drive mechanisms are worm-gears.