IMPROVEMENTS IN AND RELATING TO HELIOSTATS ANDSOLAR FURNACESThis invention relates to improvements in and relating to heliostats, especially but not exclusively for use in solar furnaces.
Solar furnaces have found many uses in research due to the high temperature achieved by focusing of sunlight onto a target. The associated heat, which enables temperature of up to 4,000"C or higher to be achieved, can be exploited to provide the high temperature needed for research into advanced materials, and can also be used to provide electrical energy, or to dispose of chemical waste.
The key to the success of any solar furnace is the design of the heliostats which collect and focus the sunlight onto the target or onto the parabolic mirror. The temperature and the amount of heat generated are dependent upon the degree of focusing (concentration) that can be achieved and the area over which sunlight is collected.
The design of a heliostat is made complicated by the movement of the sun across the sky, and in order to maintain the reflected ray at the target, the heliostat must be able to track the sun, for example, by following the solar time.
In the past, the solar furnaces have employed a two-stage collection method in which a set of heliostats and a parabolic mirror are used.
Tracking is achieved by moving the heliostat which was usually a flat reflector , whilst focusing is achieved by the parabolic mirror. This twostage process has many disadvantages, including high cost and a complicated and time-consuming alignment procedure.
One area of use of the heliostat of the present invention is in a solar furnace. It has been long been known that the high temperature solar furnace provides many unique commercial and scientific applications in material science, chemical treatment, water dissociation etc. However, the high cost of the device has hindered the wide use.
The primary objective of at least one aspect of the invention is to provide a device that can achieve one stage reflection of solar ray collection without involving the use of a secondary parabolic mirror by providing a variable focusing heliostat to reduce the cost in building and alignment.
The high temperature of a solar furnace in accordance with one aspect of the invention is obtained by high concentration (about 5000) using a variable focusing heliostat.
According to a first aspect of the invention, we provide a heliostat comprising a plurality of mirrors mounted on a support member, each mirror being adapted to move angularly about a single axis relative to the support member.
Preferably the heliostat is adapted to focus the sun's rays in use in a single stage reflection onto a target.
Preferably the support member is adapted to move angularly about a second axis that is different from the first axis or axes about which the mirrors are capable of moving relative to the support member.
Preferably a plurality of the mirrors may be angularly movable relative to the support member about a common axis (or a substantially common axis). A plurality of mirrors may be angularly movable relative to the support member about axes that are substantially parallel to each other.
According to a second aspect of the invention we provide a heliostat comprising a support member, or support frame, that is angularly movable about a first axis, and a plurality of mirrors mounted on the support member and angularly movable relative to the support member about respective second axes.
Preferably a plurality of mirrors are angularly movable about a common, or substantially common, second axis. Preferably there may be a plurality of mirrors movable about a third axis which is preferably substantially parallel to the second axis. There may be mirrors angularly movable about fourth and subsequent axes substantially parallel to the second axis.
The mirrors are preferably provided in an array (or matrix), preferably an n x m array. The mirrors in a line of an array may be angularly movable relative to the support member (or frame) about a common axis (second axis). A single movement means may be provided to move the mirrors of that line, preferably together.
Control means may be provided adapted to move angularly mirrors about the second axis. The control means may control the movement of the support means about the first axis. The control means preferably controls the movement of the mirrors of a first group to be inclined relative to the support member by a different amount than mirrors of a second group. The mirrors of the first and/or second group may extend in a line, for example a straight line. The control means provides automatic control of the mirrors.
There may be a mirror, or group of mirrors, that is/are not movable angularly relative to the support means. These "master" mirrors are preferably adapted to be aligned with a target. A group of mirrors may be moved by common movement means, which may be driven by a common motive means (e.g. motor). The group of mirrors may be mounted on a common mounting member, which may be moved.
According to a third aspect of the invention we provide a heliostat-target combination having a heliostat according to the first or second aspect of the invention, and a target, the arrangement being such that the first axis of the heliostat extends towards the target, and preferably passes through the target.
According to a fourth aspect of the invention we provide a method of tracking the sun in a heliostat comprising providing an array of mirrors, angularly movable about a first axis, aligning the first axis with a target, providing mirrors angularly movable about a second axis (or respective axes) that is/are transverse to the first axis and moving the mirrors about the second axis (or axes) about the first axis so as to direct sunlight from each mirror onto the target, the angles of the mirrors relative to the first and second axes changing as the sun moves across the sky.
In accordance with a fifth aspect of the invention, a variable focusing heliostat is provided comprising: a support means; a frame supported by the support means; an array of mirrors connected to the frame through one or more supporting assemblies, the mirrors comprising at least one master mirror and one or more slave mirrors; and in which said master mirror is fixed relative to said frame, and said slave mirrors are movable relative to said master mirror, said slave mirrors being limited in their movement so that when the heliostat is in its position of use a parallel beam of light from a source S incident upon said mirrors is reflected from said mirrors to focus at one or more points on a line contained within a focal plane, and in which each of one or more of said slave mirrors can be moved angularly relative to said master mirror about a respective single axis R-R' so as to focus substantially all of the light from said source which is reflected from the mirrors into a single spot on said line in said focal plane, said spot corresponding to a target T.
Preferably, a control means is provided which is adapted to control movement of said one or more of the mirrors relative to the frame to focus the parallel light into said spot in the focal plane. The mirror may be movable with at least one degree of freedom relative to the support means in order to facilitate tracking of the source of parallel light. The frame may be movable about two degrees of freedom.
Preferably, the frame is adapted to move angularly about a first axis containing a point substantially at the centre of the frame and a point substantially corresponding to the target at which, in use, light incident upon the heliostat from the source is to be focused. Preferably, the first axis passes through the centre of master reflecting element. Of course, the first axis need not pass through a point substantially at the centre of the frame, and could pass through an off-centre point or even a point in the outside of the frame. This first axis need not be orthogonal to the master mirror or plane of the frame. The first axis preferably passes through the target.
Preferably, the frame is further adapted to be angularly movable about a further axis (second axis), preferably orthogonal to the first axis, the further axis preferably passing substantially through the centre of the frame. This further axis is preferably substantially horizontal.
Preferably, the second axis passes through the centre of the master mirror and lies in the plane of the surface of the master mirror. The heliostat frame may be substantially symmetrical about the further axis. The frame may be angularly movable about the further axis only during initial set up/orientation of the heliostat and thereafter locked fixed relative to the further axis.
The frame may also be adapted to rotate about a still further, preferably vertical and/or horizontal direction. The support means may comprise a vertical column, in which case the still further axis will be the long axis of the column. Either the frame may rotate or be angularly movable relative to the support means about the still further axis, or the support means may be provided on a base and the frame and support means may be adapted to rotate about the still further axis relative to the base. Preferably, the frame only moves about the still further axis during initial set-up, and in use is restrained from moving about this axis.
Rotation about the first and second axes allows the source to be tracked so that light incident upon the master mirror is always reflected to the target.
Preferably, the master mirror and slave mirror(s) are arranged in an array substantially in one plane which may be parallel to the plane of the frame. For example, nine mirrors may be arranged in an array of three parallel rows and three parallel columns (3 x 3), with one master mirror at the centre of the array surrounded by eight slave mirrors. Other arrangements include five rows by five columns (5 x 5) and three rows by five columns (3 x 5). Of course, many other arrays of mirrors are possible. The master mirror is preferably at the centre of the frame and may be on the cross point of first axis of rotation and/or the second axis of rotation, and is preferably fixed relative to the frame. The slave mirrors may be each adapted to move relative to the master mirror.
Preferably, the long axis of a row is parallel or coincident to the second axis of rotation of the frame, and may be orthogonal to a normal to the surface of the master mirror. Preferably, the long axis of a column is orthogonal to the long axis of the rows.
Preferably, all the mirrors in a row are pre-adjusted and then fixed in relation to one another. The mirrors may be adapted to rotate, preferably synchronously, about the long axis of the row relative to the plane of the frame, and hence relative to the mirrors in other rows. The row containing the master mirror can, however, be pre-adjusted and then fixed relative to the frame. Thus, each row (and each slave mirror) has only one degree of freedom in relation to the frame. Preferably, each row is connected to the frame through a respective support assembly, movement of the support assembly enabling the whole row of mirrors to rotate relative to the frame at the same time during sun-tracking or similar operation.
The individual mirrors may be either flat-surfaced or concavesurfaced and may be mounted in relation to the support assemblies through an adjustable mounting. The mirrors may be deformable. In use, the adjustable mounting may be adapted to prevent movement of each of the mirrors relative to the respective support assemblies, but can be adjusted during "setting up" to allow some degree of movement of the mirrors relative to the support assembly, for example orthogonal to the axis of rotation of each row to allow the reflected ray to point to the target.
An advantage of the present invention is that by providing a heliostat as described hereinbefore, light can be focused from a parallel source (such as the sun or the moon) onto a target using only a single reflection stage. The first and second axes of rotation of the frame allow solar tracking or similar operation to be achieved, whilst the movement of the slave mirrors relative to the frame by moving the support assemblies allows the focusing to a fixed target without involving the second reflection stage (for example, the parabolic mirror).
In accordance with a still further aspect, we provide a method of controlling a heliostat in accordance with the first or the fifth aspect to direct light from a source onto a target, comprising the steps of: (a) moving the frame so that light from said source is reflected from said master mirror onto said target; and (b) rotating each of one or more of said slave mirrors about a respective single axis associated with each of said one or more of said slave mirrors to focus substantially all of the light from source which is incident upon said mirrors onto said target.
The advantage of this method over the prior art, as will become apparent, is that only one stage reflection is required to concentrate the light from a moving source onto a fixed target. To make this possible with the previous method, the slave mirrors have to be moved with two degrees of freedom which makes the mechanical design complicated. The rotating method described in the above via the first and second axis provided an advantage that a single degree of freedom for the slave mirrors is required relative to the frame. The reduction in the degrees of freedom needed is made possible in one embodiment because of the ability of the frame to rotate about the first axis as described hereinbefore, e.g. an axis containing a point in the normal plane of the master mirror (i.e. centre of the frame) and a point at the target - not unlike the rotation of the sails of a windmill.
The method may comprise, during the first step (a), performing the following sub steps: (c) rotating the frame about a first axis until a plane containing the source, the master mirror and the target is aligned with a plane containing a line normal to the master mirror and a line substantially orthogonal to the single axis of rotation of each of said one or more slave mirrors; and (d) rotating the frame about a second axis until the light from the source incident upon the master mirror is reflected onto the target.
Preferably, this second axis is orthogonal to said first axis, and parallel to said axes a rotation of each of said ones of said mirrors.
Preferably, said first axis contains a point at the master mirror and a point at the target.
Thus, solar tracking is achieved by rotation of the frame and hence the master mirror in two directions. Performing steps (c) and (d) ensures that only one degree of freedom of movement of the slave mirrors is needed to correctly adjust the focusing, as will become apparent.
The second step (b) may comprise the sub-steps of (e) recording an angle of incidence for light from the source upon the master mirror relative to an axis normal to the plane of the master mirror and an axis orthogonal to the axis of rotation of said ones of said mirrors, the incident angle being substantially entirely dependent upon the solar time in the case of sun-tracking and (f) calculating the required angle of rotation 6 of each of the slave mirrors about the single axis of rotation of the slave mirrors, 6 being dependent upon the value of the incidence angle 6 so that light from the source incident upon each slave mirror is directed to a spot at the target. The slave mirrors may be arranged in rows parallel to the second axis of rotation. In such a case, a whole row can be rotated through the angle 6.
The value of the angle 6 may be calculated from the angle 0 using the equation:
where L is the distance between the master mirror and the target, and H is the distance of the axis of rotation of each (row of) slave mirror(s) (i.e. axis of rotation of the support assemblies) from the centre of the master mirror.
Since the values of H and L can be fixed for a given design of heliostat and target, the values of 6 corresponding to o for each row of slave mirrors can be held in a look-up table (or calculated by computer) and step (f) may comprise the step of interrogating the look-up table to obtain the required value of 6 for a given value of 0.
The method may further comprise an initial step of bending or rotating one or more of the slave mirrors, possibly about an axis substantially orthogonal to the rows of mirrors to pre-focus the array during initial "set-up" of the heliostat. The correct focusing alignment of the mirrors during set-up may be achieved by trial and error or by calculating the value of 6 for each mirror in two orthogonal directions (i.e. parallel to an orthogonal to each row) using the above equation. The method steps (a), (c) and (d) align the master mirror and the frame so that in use, 0 in one of the directions is always constant zero. Thus, 6 remains constant in this direction (hence the need for only a single axis of rotation of the mirrors in use).
In accordance with a further aspect of the invention, we provide a method of aligning a heliostat having a number of mirrors on a frame, comprising the steps of providing a video camera (or other image-producing means such as a monitor) so that an image of the mirrors is observed by the monitor, which may be located near the heliostat, and adjusting the alignment of one or more of the mirrors in relation to the frame until an image of the source is formed in the centre of the image of each mirror seen by the monitor.
Preferably the method further comprises providing a partialreflector between the target and the camera or other image-forming means to reduce the intensity of the light entering the aperture of the camera.
Preferably, one of said mirrors is a master mirror in a fixed position, and the heliostat is moved until the image of the source in the master mirror is seen by the camera.
Preferably, the partial-reflector is a planar object, and preferably the reflector has a low reflectivity so that only a small amount of the light incident on the object is reflected onto the camera, the rest being transmitted .
Preferably the set-up method may be automated using image recognition to identify when the mirrors are correctly aligned.
In accordance with a still further aspect, we provide a solar furnace comprising a target and one or more heliostats in accordance with any of the preceding aspects of the invention.
There will now be described, by way of example only, one embodiment of the present invention with reference to the accompanying drawings, in which:Figure 1 is a perspective view of a heliostat;Figure 2 is an illustration of the arrangement of the mirrors on theframe of the heliostat of Figure 1;Figure 3 shows the various axes of rotation of the mirrors relativeto the frame;Figure 4 shows the axes defining the degrees of movement of theheliostat referred to herein;Figures 5(a) to (c) illustrate a method of aligning the focusing ofthe heliostat; Figure 6 shows the image seen by a video camera during thealignment procedure Figure 7 shows the direction of the light rays incident upon andreflected from the heliostat of Figure 1 viewed along the axis A-A'axis in the plane of the frame after "set up";Figure 8 shows how, when the heliostat is aligned to track the sun,the angle o of the light upon the master mirror is always zero whenviewed along the C-0-Cl axis;Figure 9 shows various values of 6 produced using the equationpresented hereinafter corresponding to different incident angles 0 and distance of the slave mirror from the master mirror;Figure 10 shows a sketch very similar to Figure 4;Figure 11 shows an arrangement for initial set-up of the apparatusto ensure the first rotation angle pointing to the target;Figure 12 shows a rear view of the heliostat.
Figure 1 is a perspective view of a heliostat which may, for example, be used with only one stage of reflection to form a solar furnace. A number of similar heliostats may be arranged in an array such as a matrix or an array of a circle to focus sunlight onto a receiver, producing ultra-high temperature which is otherwise very difficult to produce by other means in the laboratory. In the context of this description, only a single heliostat will be discussed, as the construction and operation of any other heliostats in the array will be understood to be similar.
The heliostat 1 comprises a rigid square support frame fabricated from steel box section. The rigid support frame acts as a support for twenty-five mirrors arranged in an array of five mirrors by five mirrors, i.e. five parallel rows of the mirrors, or, looked at another way, five parallel columns of five mirrors. To support the frame above the ground, a tubular support column 4 mounted on a base 5 is attached to the frame through a rotatable joint 6 which allows the frame to rotate about a central point 0 relates to an axis connecting the centre of the base and the target (not shown). The frame may also rotate about a central axis passing through the point 0 as shown in figure 4.
The mirrors 3 are arranged on the frame in five rows of five as described hereinbefore. The mirrors 3 in each row R1, R2, R3, R4, R5 are connected to a support structure 7 on the rear side which in turn is connected to the frame. The support structure 7 is adapted so that the individual rows of mirrors may be tilted about their longitudinal axis.
The angle of tilt of each row is controlled by a respective motor (not shown) which acts upon the support structure 7 for each row of mirrors.
In addition, the support column 4 is adapted to allow the whole frame assembly to be rotated around the longitudinal axis of the support column to enable tracking of the sun.
The attachment of the individual mirrors to the support assembly is shown in Figure 2 for two mirrors in one of the rows. As can be seen, each mirror can be moved relative to the support assembly through two degrees of freedom. Firstly, as described hereinbefore, the mirrors may be rotated about the longitudinal axis R-R' of the support structure for the row of mirrors which contains the individual mirror. This rotation may be achieved by rotating the whole support assembly relative to the frame, using, say, an electric motor or hydraulics.
Secondly, each of the mirrors 3 may be individually movable (or deformable) about a second axis orthogonal to the longitudinal axis of the support structure and in the plane of the mirror 3, or may be deformable so that in effect each part of the mirror may be tilted by a different amount about an axis orthogonal to the second axis. This movement is only needed during "set up" and there is no remotely controlled motor provided to move them - only manual adjustment. This is shown inFigure 3. The central mirror is the "master" mirror of the array and could be fixed, as this does not need to be moved relative to the frame (as will become apparent hereinafter), preferably in the plane of the frame.
For ease of reference, a point at the centre of the master mirror is hereinafter referred to as the origin 0.
As described above, it can be seen that the whole frame 2 carrying the array of mirrors 3 can move in two ways: it can rotate about the second axis A-Al which passes through the central mirror parallel to the rows of mirrors, and it can rotate about its centre - like the sails on a windmill about a first axis O-T connecting the centre of the "master" mirror to the target. As well as the movement of the frame, each individual slave mirror also has one degree of rotation. In use, the mirrors in an individual row can be rotated relative to the long axis R-R' of the row. Finally, each mirror may be rotated or curved about axis C-C1 orthogonal to the axis R-R' during "set-up".
All these axes are shown in Figure 4.
The operation of the heliostat 1 and a method of alignment will now be described with reference to the drawings.
Setting up of the heliostat is initially performed by aligning the frames so that an image of the source S, say the sun, is reflected by the central master mirror onto the chosen target T. With all the other slave mirrors, for example, being substantially flat and parallel to the plane of the frame (i.e. in the same plane as the central "master" mirror) the resultant image formed around the target T will be in the form of an unfocussed grid of light reflected from the mirrors with no effective focusing. This is shown in Figure 5a. For simplicity, a three by thtee array is shown, rather than five by five.
Next, the mirrors are individually displaced (or deformed) relative to the frame so that the light reaching the target area is focused into a "strip" of light as shown in Figure 5b along a line in the desired focal plane. This is achieved by individual movement of the slave mirrors or individual deformation of the slave mirrors. Finally, the mirrors in each row associated with the support structures 7 are rotated about the long axis of the row R-R' until a substantially focused spot of light at the target T is achieved as shown in Figure 5c. At this point, initial set-up is complete. Once set-up, the slave mirrors are restrained so that they can only rotate about one axis, being the axis by which the mirrors are tilted to connect the line in the focal plane into a focused spot.
In order to simplify the task of aligning the mirrors, a low reflectivity planar surface may be located at the target area and viewed by a video camera, as shown in Figure 6. When the mirrors are correctly aligned, the image viewed on the video camera will show an array of small images of the sun, each one in the centre of each image of the mirror. The low reflectivity of the surface ensures that the image observed by the camera does not damage the camera.
By providing an array of mirrors which can be focused onto a fixed single area while the sun is moving crossing the sky , the light is concentrated by a singe stage, rather than the dual stage designs commonly known in the prior art. This greatly reduces cost and increases the speed at which the heliostat can be set up on site.
The method of tracking the sun using the heliostat described inFigures 1 to 6 is as follows:Once initial set-up has been completed, the individual mirrors are fixed relative to each row by ensuring that they cannot move relative to the support assembly supporting that row. As stated hereinbefore, in this method the only degree of movement of the mirrors relative to the frame during use (after initial set-up) is rotation about the long axis R-R' of each of the rows. Tracking is achieved by rotating the frame about the axis 0T until the plane S-O-T defined by the source (i.e. the sun), the origin 0 at the master mirror and the target T is aligned with the plane containing an axis O-N orthogonal to the plane of the central mirror and an axis C-C' orthogonal to the axis O-N and the axis A-A' of rotation of the rows of mirrors as shown in the Figures. Next, the entire frame is rotated about the axis A-A' until the light from the sources which is incident upon the central mirror is reflected directly onto the target T. Finally, the individual rows of mirrors are rotated about the respective axes R-R' until all the light incident upon the mirrors is focused onto the target T.
Figure 7 shows the direction of the light rays reflected from the heliostat to the target when correctly focused. The sunlight incident upon the central mirror forms an angle 0 with the normal to the mirror surface.
A similar angle 0 is defined between the normal line and the reflected light directed to the target from the central master mirror. From a knowledge of the angle 0, the required angular rotation 6 of a respective one of the slave mirrors can be calculated from the following expression:
where H is the distance between the origin 0 on the axis of rotation of the frame which is directly below the central master mirror and the axis of rotation of the row containing the one of the slave mirrors, and L is the horizontal distance from the surface of the central master mirror to the target area. Since H and L are fixed values for a given design of heliostat and target, there is only one variable and so a look-up table can be used to calculate the correct value of 8 for a given value of incident angle 0. Angle 0 is known from a knowledge of the position of the sun in the sky or from the solar time. Hence, once the central master mirror is correctly aligned and the frame is rotated so that the plane S-O-T is aligned with the plane N-O-F, the value 6 of rotation of the individual rows to complete the focusing is readily achieved by use of a look-up table. Several values of 6 for different values of o and sizes of heliostat are shown in Figure 9 of the accompanying drawings.
It will be understood tha initially. This is possible because of the facility to rotate the frame about the first axis O-T.
In order to verify the principle of the variable focusing heliostat described hereinbefore, a prototype has been built consisting of twentyfour slave mirrors and one master mirror. The frame supporting all the mirrors can rotate around the O-T axis and the A-A' axis to achieve sun tracking. The slave mirrors were driven by clock motors (one per row) to achieve the variable focusing. A PC was used to generate the signal for global tracking and the displacement of the slave mirrors according to solar time, latitude, distance to target and orientation of the heliostat.
Excellent results were achieved from the prototype, and as expected, once initial alignment was completed, only a single degree of movement of the mirrors was required through all the period of use of the prototype.
Some further points we wish to make include:From a study of the formula, with reference to Figure 7, of
where L is the horizontal distance between the pivot point of the central master mirror (in the case of Figure 1, it is mirror 3) to the target point Q, H is the distance between the pivot point of the subset slave mirror in question (in the case of Figure 1, it is mirror 3) to the pivot point of the master mirror, 0 is the incident angle of the sun ray, we conclude that there are two important features.
Firstly, in practice, the parameters H,L are all fixed, the angle 0 will have its definite value at any time of the day as it is solely relative to three factors: the solar time (including the longitudinal location and the hour angle), the latitudinal location and the orientation of the heliostat. T he formula to calculate the incident angle can be found in the literature and related text books.
Therefore, provided the master mirror is under sun tracking, in order to concentrate all the rays to the target point the mirrors will move by an amount 6 which can be obtained by instant computing or by retrieving from a database storage. In the case of calculation, given preset parameters L and H, one calculation by the Pentium processor only takes CPU time of less than 0.491 ms. A PC or a microprocessor, therefore, can easily fulfil the job for more than 100 subset mirrors if a multi-plexing process is adopted.
The small movement, and in consequence of the linear relationship between the angle value and the linear movement of the displacement device, permit the design of heliostat to use a low cost and durable devices to realise the motion to achieve 6. In practice, for example, a clock motor or piezoelectric ceramic can be employed.
With the above principle of variable focusing heliostat, the rays reflected by each subset of mirrors will be concentrated at the target.
Therefore, for a reasonable distance target, the concentration will be increased in proportion to the number of mirrors multiplied by the number of heliostats. A high concentration of more than 5000 can be achieved simply by one stage reflection.
The solar rays in the above configuration for each heliostat will converge to a target point, not reflect parallel to a parabolic mirror as in the traditional configuration. The optical alignment will be much simpler as the receiving area is small enough to accommodate a commercially-available opto-electronic device or a video camera to assist the alignment.
In the illustration of Figure 7, o is the incident angle. The two components of the incident ray on the azimuth and elevation directions would require two dimensional movements if a conventional heliostat is used to carry out the invention. Usually, for a conventional heliostat the frame of the mirror is turned to track the sun by rotations around the horizontal and vertical axes.
A new method of rotational mode is used in the. The purpose of this new rotation mode is to keep the incident plane in a certain way that the slave mirrors only need one dimensional movement relative to the frame to achieve the variable focusing. This will form the best mode to carry out the invention.
To understand this method, we have to define the following axes (see Figure 10): assume the central point 0 of the master mirror as the origin of the axes., O-T is the axis towards the fixed target; O-N is the axis of the master mirror surface normal; O-S is the axis towards the sun; O-R is the local axis on the heliostat along the array of the mirrors; O-F is the local axis on the heliostat along the other array of mirrors (perpendicular to O-R). To track the solar ray so that it falls on the fixed target, we may use two freedoms of rotation so as to keep the O-F axis and the O-N axis in Figure 3 in the same plane as that formed by the O-S axis and O-T axis (named as the first axis rotation) and O-R axis (named as the second axis rotation).
In this solar tracking mode, the displacements of the slave mirrors will fall into two categories: one is to fall into the normal plane where the incident angle 0 varies with the hour angle; and the other is perpendicular to the normal plane where the projection component of the incident angle 0 is zero. Therefore, this new sun tracking method will ensure that the subset slave mirrors require only one dimensional focusing movements as determined by the previous equation. The scheme will be able to greatly save the cost of the design of the variable focusing heliostat.
With reference to Figure 8, to focus the solar ray onto the target, the displacement angle,8, for the slave mirror in the direction perpendicular to the normal plane (top view of the heliostat, shown inFigure 8) can be adjusted in a preset manner by manual manipulation, since, as we mentioned in the above, the projection components for the incident angle 0 in this direction is zero. This displacement value is fixed and is determined by substituting 0 with zero in the 8 equation.
Either open loop or closed loop control algorithms can achieve the tracking in this rotation mode.
Mirror AlignmentIn the conventional two-stage solar collator, optical alignment is one of the most costly and time-consuming engineering works. The feature of focusing each sun's rays reflected from the variable focusing heliostat in this invention permits the development of a new technique by using a simple vision system to realise the mirror alignment. For example, a camera located at the focusing point can capture all the sun images on the mirrors after a rough visual alignment and can transfer them by TV monitor to the person who is doing the alignment. In this way, the person can easily adjust the positioning screws on the frame to displace the slave mirrors. The best alignment result can be achieved when the full image of the sun is reflected from each subset mirror of the heliostat as shown in Figure 6. Figure 6 illustrates schematically the layout of this new mirror alignment technique, where the camera will capture the sun's images reflected by a low reflectivity glass so as to protect the camera from strong sunlight. This method of alignment has greatly reduced the time and manpower needed to focus the heliostat.
Structure AlignmentAs required by the rotation mode described earlier, the rotating shaft of the heliostat must be in line with the O-T axis. In order to achieve this the structure of the rotating frame must be designed to allow the rotation axis to point at the target. Therefore, a structure alignment is required to make all the heliostat to be in line. A telescope or a tube containing a laser can be employed, mounted along the central axis of the shaft, to achieve the structure alignment. After the adjustment is completed, the rotation of the shaft will be tightened by the locking bolt.
This is shown in Figure 11.
We have appreciated that to align the heliostat using the sun can be dangerous (as we approach focus, the temperature at the target can be around 4000"C). We have appreciated that we can align the heliostat using the moon (program the heliostat to track the moon whilst we are aligning the mirrors). We then simply re-program the heliostat to track the sun after the alignment is done - the geometry of heliostat-target relationship remaining the same and so the optical alignment procedure is the same. Working with the moon is far less hazardous/uncomfortable.
According to a further aspect of the invention, we provide a method of alignment of a heliostat comprising using the moon instead of the sun as a source of parallel light rays.