Field of the Invention

[0001] The invention relates to a system, method and apparatus for concentrating incident radiation from a source onto a target. In particular, the invention relates to a system for heliostat tracking of the Sun in concentrated solar power generators.

Background of the Invention

[0002] Reflecting or redirecting a distant source of incident radiation onto a target has a broad range of applications. This is a requirement in systems such as line of sight communications (optical or radio frequency) and synthetic aperture antennas for radio astronomy or point to point optical or radio power transmission. One prominent application is the use of a heliostat array to reflect sunlight onto the solar radiation receiver of a concentrated solar power (CSP) generator. Typically a field of heliostats reflect the sun's rays onto a receiver mounted on a tower to heat a working fluid circulating therein. Each heliostat has a reflective surface (mirror) moveably mounted to track the daily transit of the sun across the sky and keep the reflected sunlight shining on the receiver. This concentrated solar radiation heats a working fluid such as water/steam to drive a turbine. The heat generated can also be stored in a molten salt facility for later use.

[0003] The heliostat array may have several hundred, if not thousands of individual heliostats, some of which may be positioned up to a kilometre away from the receiver. This requires extremely accurate positioning of the heliostats to ensure the reflected sunlight remains on the receiver. For example, to achieve ±10% accuracy for the position on the reflected sun spot on the receiver, the heliostat mirror needs to be within 1 milliradian of the required angle. Currently, these levels of accuracy are achieved using substantial structures with high strength and stability, and therefore high construction and maintenance costs. Some tracking systems also incorporate complex feedback systems to correct errors. These may involve redirecting the reflected sunlight from the receiver to a calibration target where position errors are corrected before returning to the receiver. This is a time consuming and relatively complex operation for each heliostat to complete individually. If each individual heliostat is calibrated sequentially, calibration of the entire array can take many days. Furthermore, redirecting heliostats to a calibration target will reduce the flux at the receiver which is detrimental to the efficiency of the CSP generator.

[0004] The heliostat control system described in US 2012/0174909 A1 is an example of a calibration target (or at most, a few calibration targets) fitted to the tower at a known displacement from the receiver. The calibration target is itself a camera and each heliostat periodically directs the reflected sunlight away from the receiver onto the camera where the image data is analysed and position errors corrected. This is an inherently very slow process for large fields of heliostats, and ultimately will only provide an indirect measure of the positioning accuracy. Any inaccuracy when redirecting the heliostat back towards the receiver is not accounted for. Furthermore, there are significant dynamic range issues for the cameras used for calibration.

[0005] The system shown in US 2009/00250052 B2 provides a more direct measure of heliostat positioning by using an array of diffuse (not specular) reflectors spread across the target area (receiver). The image data from the camera is processed to determine if the energy flux onto the receiver is uniform, and if not, adjusts the heliostats until the diffuse reflected image from the reflectors is optimal. However, there is no direct measure of the positioning for individual heliostats. The camera is unable to distinguish between diffuse light reflected from different heliostats which makes this system unsuitable for large heliostat arrays.

[0006] The feedback system described in US 2011/0155119 A1 does seek to simultaneously align individual heliostats. The heliostat has a digital camera to capture an image of the sun and the receiver as two very bright spots. As the heliostat is a specular reflector (i.e. the angle of incidence is equal and opposite the angle of reflection), the system positions the heliostat reflector such that the normal vector extending from the surface of the mirror, bisects the line joining the image of the sun and the image of the receiver. This system will generally keep the reflected sunlight directed onto the receiver but there is still a degree of inaccuracy in the alignment between camera and heliostat that requires a secondary calibration system. Any such inaccuracy can result in overlapping spots of reflected sunlight on the receiver. As a result the energy flux onto the receiver is non-uniform which can cause extreme hot spots in localised areas. This is detrimental to the operation of the receiver and may exceed the specified temperature thresholds for the receiver structures.

[0007] With the above issues in mind, there is an ongoing imperative to improve the tracking accuracy of heliostats with a cost effective system to provide a relatively uniform flux of reflected radiation across the receiver surface.

[0008] Any reference herein to a patent document or other matter which is given as prior art is not to be taken as an admission that that document or matter was known or that the information it contains was part of the common general knowledge as at the priority date of any of the claims.

[0009] Throughout the description and claims of the specification, the word "comprise" and variations of the word, such as "comprising" and "comprises", is not intended to exclude other additives, components, integers or steps.

Summary of the Invention

[0009] In light of the above, a first aspect of the present invention provides a system for collecting and concentrating radiant energy from a source onto a target, the system comprising: a plurality of reflectors, each having a reflective surface for reflecting the radiant energy onto the target, an actuator for adjustment of the reflective surface relative to the target, and a photodetector for detection of radiant energy; and, more retroreflectors fixed relative to the target; wherein, the photodetector is configured to maintain a fixed field of view of at least some of the one or more retroreflectors, such that, the actuator adjusts the reflective surface until the photodetector senses retro reflected radiant energy from the one or more retroreflectors, the retro reflected radiant energy being a retro reflection of the radiant energy reflected from the source by the reflective surface.

[0009a] Retroreflectors reflect light such that the reflected rays are parallel to the incident rays, largely independent of the orientation of the retroreflector. In contrast, specular reflectors (e.g. planar mirrors) reflect light at an equal but opposite angle to that of the incident light (relative to the normal at the point of incidence). Light from a heliostat shining on a retroreflector, is retroreflected directly back to that heliostat. In other words, a photodetector at that heliostat will only detect retroreflected light when that heliostat shines on the retroreflector. This is direct confirmation that the heliostat is reflecting sunlight onto the retroreflector, and allows each heliostat to independently check its alignment, simultaneously along with other heliostats in the array. In this way, multiple heliostats can be tracked or calibrated in real time. In addition, the retroreflector will give a much brighter signal than a diffusely reflecting target, allowing good contrast between the retroreflector and the receiver.

[0010] Preferably, the system is a concentrated solar power system, the radiation source is the Sun, the target is a receiver with a target area, and the plurality of reflectors is an array of heliostats for reflecting sunlight onto the target area. However, in some circumstances, the Moon may be used as the source of radiant energy in order to perform heliostat calibrations at night, thus not impacting daytime operations. This also allows workers to be close to the target area without risking intense heat from the concentrated solar energy.

[0011] Preferably, the photodetector is an image sensor for sensing an image generated by retro reflections from the or each retroreflector. Skilled workers will appreciate that image sensors, such as charge-couple device (CCD) arrays are an arrangement of individual photodetectors which collectively generate the pixel data for a captured image.

[0012] Preferably, the system has an array of the retroreflectors wherein the image sensed by the image sensor is generated by a pattern of retro reflections from the array of retroreflectors, such that the actuator adjusts the reflective surface until the image sufficiently corresponds to a desired image. [0013] The heliostats each use an image sensor with a constant gaze or fixed view of at least some of the retroreflectors. When light reflected by that heliostat is incident on retroreflectors, the image sensor sees a bright retro reflection of the sunlight. This is a direct indication of the position of the reflected sun spot. Controlling the actuator to adjust the mirror angle will move the position of the reflected Sun spot accordingly, and change the retroreflected image seen by the image sensor. Some retroreflectors within the field of view become brighter relative to the others as the sun spot moves. Particular retroreflected brightness patterns correspond to specific sun spot positons within the field of view. This can be used in a closed-loop system to keep the reflected sun spots from each heliostat trained on the target area in a pattern for efficient and optimised heating of the working fluid therein.

[0014] A field of view that only encompasses the retroreflectors (or even just some of the retroreflectors), gives much greater image resolution than systems with much wider fields of view to see both the sun and the receiver. These large fields of view are necessary in the prior art heliostats as the cameras move. The photodetectors of the present system have a constant gaze towards a fixed field of view which is restricted to just the retroreflectors needed for closed loop feedback control. This gives the system far greater image resolution and the ability to more precisely position the reflected spot within that field of view. Furthermore, less expensive image sensors may be used. The cheaper cameras still provide a close-up image of the target with better resolution than more expensive image sensors used in systems that need to simultaneously view the sun and the receiver. In a large heliostat array, the individual heliostats are simultaneously tracked and/or initially calibrated in far less time than previous methods.

[0015] According to a second aspect, the present invention provides a method for concentrating radiant energy from a radiation source onto a target area, the method comprising the steps of: providing an array of reflectors, each reflector having a reflective surface and an actuator to adjust the reflective surface relative to the target area and the source, and a photodetector for detection of radiant energy; providing one or more retroreflectors fixed relative to the target area; configuring the photodetector to maintain a fixed field of view of at least some of the retroreflectors; and, controlling the actuator to adjust the reflective surface until the photodetector senses retroreflected radiant energy from the one or more retroreflectors, the retroreflected radiant energy being a retro reflection of the radiant energy reflected by the reflective surface.

[0016] Preferably, the radiation source is the Sun and the reflectors are heliostats in a concentrated solar power generator for reflecting sunlight onto a receiver having the target area.

[0018] Preferably, the or each retroref lector is configured to reflect a narrow, well defined beam back to the heliostat, such that a reflected beam from the or each retroreflector is generally parallel to an incident beam from the heliostat. Preferably, an array of the retroreflectors is arranged in a predetermined distribution relative to the target area. The retroreflectors within the spot of sunlight reflected by the heliostat appear bright to the image sensor for that heliostat, whereas the retroreflectors at the periphery or just beyond are correspondingly less bright. This allows the feedback system to provide fine positioning of the reflected rays from each individual heliostat within the target area of the receiver. Fine positioning of the reflected spot from each of the heliostats avoids overlapping spots that cause 'hotspots' on the receiver.

[0019] In some forms of this system, the heliostat array also has heliostats that do not have one of the image sensors but are nearby one fitted with one of the image sensors. This is useful when economics do not permit a camera on each heliostat, or for heliostats at large distances from the receiver where the beam spread from the retroreflectors is not narrow enough to exclude adjacent heliostats. In this case, tracking and calibration can be performed in the heliostats with the image sensors, while the adjacent heliostats are tracked or calibrated in the same manner using a method like that described in paragraphs 60 and 61. [0020] Preferably the retroreflectors are corner cube mirrors or prisms. Preferably, the corner cubes have 90° surfaces and heat resistant material such as ceramic or glass or metal. Optionally, the retroreflectors are internally reflective spheres. Preferably, the internally reflective spheres are a multitude of transparent beads of material with a refractive index selected such that incident radiation is internally reflected within the bead and emerges as reflected radiation with a predetermined angular beam spread to the incident radiation (including an angular spread of near 0°).

[0021] Preferably the retroreflectors each have a projected surface area facing the heliostats of less than 0.01 m ² .

[0022] Preferably the array of retroreflectors obscures less than 5% of the target area when viewed from any of the heliostats.

[0023] Preferably the heliostat array forms a predetermined pattern of reflected sunlight across the target area of the receiver.

[0024] Preferably, the predetermined pattern is a uniform distribution across the target area.

[0025] Preferably, the array of retrereflectors is fitted on or in the target area.

[0026] Optionally, at least some of the retroreflectors are fitted at a position at a known offset from their respective regions to be illuminated on the receiver.

[0027] Optionally, the system further comprises a control processor for analysis of image data captured by the image sensors and feedback control of the actuators until the retroreflected images display the predetermined patterns associated with a specified distribution of reflected sunlight across the target area.

[0028] Preferably, the control processor stores reference data related to the position of the sun at the geolocation of the heliostats for an initial positioning of the heliostats prior to fine positioning using the retroreflectors and the image sensors. [0029] Preferably, the heliostats are individually mounted to the ground without any interconnecting structure or foundation. Preferably, the heliostats each have an on-board processor for operating the actuator in response to analysis of image data from the image sensor. Preferably the control processor comprises a central processor networked to the on board processors. Preferably, the on-board processors are wirelessly linked to the central processor. Preferably the heliostats each have photo voltaic (PV) cells to generate on-board power. Preferably, the PV cells are mounted to a lower edge of the reflector surface.

[0030] Optionally, the image senor is configured to move between a number of the heliostats, the image sensor having a different fixed field of view at each of said number of the heliostats, to sequentially adjust the reflector surfaces for calibration and/or tracking of said number of the heliostats.

[0031] According to a third aspect, the invention provides a heliostat for use in a concentrated thermal power system having a receiver with an array of retroreflectors, the heliostat comprising: a base for mounting the heliostat in a fixed position relative to the receiver; a reflective surface mounted to the base; an actuator to move the reflective surface relative to the base; and a photodetector for detection of radiant energy; wherein, the photodetector is configured to maintain a fixed field of view of at least some of the retroreflectors, such that, the actuator adjusts the reflective surface until the photodetector senses retroreflected radiant energy from the retroreflectors, the retroreflected radiant energy being a retro reflection of the radiant energy reflected by the reflective surface. [0032] According to a fourth aspect, the present invention provides a receiver for use in a solar thermal power facility with a heliostat array for reflecting sunlight onto a surface of the receiver, at least some of the heliostats having an image sensor, the receiver comprising: a retroreflector array fixed relative to the surface of the receiver, each of the retroreflectors configured to reflect a narrow, defined reflected beam parallel to the incident beam from the heliostat.

[0033] Preferably, the retroreflectors are corner cube retroreflectors distributed across a target area on the receiver.

Brief Description of the Drawings

[0034] Preferred embodiments of the invention will now be described by way of example only, by reference to the accompanying drawings, in which:

[0035] Figure 1 is a schematic representation of a concentrated solar power generation system according to the present invention.

[0036] Figure 2 is a schematic representation of two adjacent heliostats positioned to reflect sunlight onto two adjacent retroreflectors.

[0037] Figure 3 shows an image sensor floating within a transparent enclosure using a magnet to maintain a constant gaze toward the retroreflectors.

[0038] Figure 4 is a diagrammatic representation of a corner cube retroreflector, reflecting light from two incident angles.

[0039] Figure 5 is a diagrammatic representation of the field of view of an image sensor seeing a retro reflection of the sun.

[0040] Figure 6 shows an energy distribution along the X axis derived from the captured image data. [0041] Figure 7 is another diagrammatic representation of the field of view of the image sensor in which the retro reflection of the sun is offset to account for known offset between the retroreflector and the receiver.

[0042] Figure 8 is a schematic representation of an embodiment in which several heliostats see retroreflected light from a single heliostat.

[0043] Figure 9 is a schematic representation of one of the heliostats within the array.

[0044] Figure 10 is a flow chart of the steps followed in the method for feedback control of the heliostats according to the present invention.

Detailed Description of the Preferred Embodiments

[0045] Figure 1 , shows a concentrated solar power system 1 according to the invention. As with all concentrated solar power systems, an array of heliostats 2 reflect light from the Sun 3 onto a receiver 4. The energy of concentrated solar radiation heats a working fluid circulating through the receiver 4 mounted on the tower 11. The heated working fluid (such as steam) drives a steam turbine to generate electricity, or the heat energy is stored in a molten salt facility. As the Sun 3 moves across the sky, actuators 9 attached to the base 8 of each heliostat 6 adjust the position of the reflective surface 7 so that the reflected rays of the Sun stay trained on the receiver 4. This is commonly referred to as heliostat tracking and requires very accurate angular positioning of the reflector surface.

[0046] To this end, the system shown in Figure 1 mounts, a photodetector in the form of an image sensor, such as a camera 10, on the base 8 of each heliostat 6. Each camera 10 maintains a fixed field of view that encompasses at least some of the retroreflector array 5. The individual retroreflectors 12 may be mounted on or near the receiver 4. The retroref lectors each have a narrow, well-defined beam spread. In one convenient form, the retroreflectors are corner cube mirrors or internally reflective prisms configured such that incident light is reflected back at the same angle. [0047] As best seen in Figures 1 and 2, the reflected beams from the corner cube retroref lectors 12 are parallel to the incident beams, regardless of the incident angle. Likewise, the reflected beam spread is small. This allows the light from one individual heliostat to be retroref lected back to that heliostat, but not any neighbouring heliostats. Hence the camera 10 sees an image of the retroreflectors 12 reflecting only that light which launched from that heliostat. The brightness from each retroreflector will vary depending on whether it is within the reflected spot of sunlight from that heliostat. The strong brightness of retroreflector 43 indicates it is within the reflected spot while the lower brightness of retroreflector 44 indicates it may be at the periphery of the spot. This variation in brightness creates a pattern 45 within the camera's field of view 15 which is corresponds to a particular positon of the reflected spot from that heliostat. The control processor 42 analyses the pattern 45 and in particular the relative brightness of individual retroreflectors. By comparing the pattern 45 seen by the camera 10 with a reference pattern corresponding to the desired position of the reflected spot, the control processor 42 operates the actuators 9 to adjust the angle of the reflective surface 7 until the patterns match (or match to the required degree). Fine angular adjustments of the reflective surface 7 about two axes require precision actuators such as a pair of linear screw actuators individually driven by a pair of DC motors.

[0048] Referring to Figure 2, the system allows individual heliostats 13 and 14 to calibrate and/or track simultaneously. The pattern 45 seen by heliostat 13 will differ from that of heliostat 14, and the light reflected by heliostat 13 will not affect the pattern 45 seen by heliostat 14 (and vice versa). There may be cases where some 'cross talk' between adjacent heliostats occurs, particularly when positioned a long distance from the receiver 4, and these situations are discussed further below. Notwithstanding this, skilled workers in this field will readily understand that simultaneous calibration and/or closed loop tracking of individual heliostats provides substantial time efficiencies and can maintain an optimum distribution of reflected sunlight across the receiver.

[0049] If the retroreflector array 5 is not mounted on or in the receiver 4 but rather adjacent the receiver, the processor 42 periodically aligns the heliostat to the retroreflector array 5 and corrects any errors, then determines the appropriate angular adjustment to offset the reflected sunlight spot back onto the surface of receiver 4. This semi-open loop technique will still ensure the reflected light from the heliostat array is uniformly distributed across the surface of the receiver 4 to avoid any 'hot spots' that may be detrimental to operation. This may be advantageous if a retroreflector array 5 on or in the receiver surface, would occlude too much sunlight from the receiver. The retroreflector 5 should obscure less than 5% of the receiver 4 from the point of view of each heliostat. Shading the receiver 4 too much will negate the improvements from accurate positioning of the reflected sunlight. To avoid this, the retroreflectors are kept small in terms of their projected surface area facing their corresponding heliostat. For most concentrated solar power facilities with large heliostat arrays, some up to 1000m from the receiver, retroreflectors with projected surface areas of less than 0.01 m ² will be sufficiently visible to the image sensors.

[0050] Similarly the camera 10 should shade the reflective surface 7 as little as possible. However, it is advantageous to position the camera 10 so that it is at or near the centre (or centroid) of the reflective surface 7 when viewed from the retroreflector array 5. It is feasible to mount the camera 10 at or near the periphery of the reflective surface 7 but this increases the potential for detrimental 'cross talk' between neighbouring heliostats. That is, the camera 10 is more likely to 'see' retroreflected light from the neighbouring heliostat which will interfere with the retro reflection captured from its own heliostat.

[0051] With this in mind, there are multiple options for mounting the camera 10. As shown in Figure 1 , the camera 10 can be mounted to the base 8 such that it extends in the front of the reflective surface 7. Mounting the camera 10, to the base 8 provides the necessary fixed field of view on the retroreflector array 5. Similarly, the heliostats 13 and 14 shown in Figure 2 are fixed to the base 8 to view the retroreflector array 5 through a small central window 47 in the reflective surface 7. The camera 10 may also have a separate mounting structure in front of the reflective surface 7 or (as discussed in more detail below) a structure linking several heliostats so that the camera 10 can move between each to perform the necessary calibration and/or tracking.

[0052] Figure 3 shows a further embodiment that allows the camera 10 to be mounted directly to the reflective surface 7 even though this surface moves as it tracks the sun across the sky. The image sensor assembly 46 has a transparent casing 50 containing a clear liquid 56 such as water or oil. A domed float 54 suspends a camera 10 within the liquid 56. A bar magnet 58 provides ballast and aligns with an external magnetic field such as the Earth's magnetic field. With precise relative positioning of the magnet 58, the camera 10 and the float 54, the field of view 48 remains constant regardless of some angular adjustment to the casing 50. To keep the whole assembly 46 fully enclosed and self-contained, the float 54 is fitted with a photo voltaic cell 52 to power the camera 10 and wirelessly transmit the image data to an onboard processor fitted to the heliostat (discussed below).

[0053] The skilled worker will understand that the 'fixed' field of view of the camera, is not absolutely fixed in the sense that no movement whatsoever is contemplated. The small perturbations arising from actuator wear, ground subsidence, wind load and so on will result in small shifts in the field of view. However, it is these shifts that the tracking and calibration system of the present invention detects and corrects.

[0054] Figure 4 is a diagram showing the operation of a corner cube retroreflector 22. The internal reflective surfaces of the corner cube 22 are mutually orthogonal so incident light beams 23 and 25 entering the retroreflector 22 are reflected at the same angles 24 and 26. As discussed above, a reflected beam from the retroreflector will only be seen by the cameras 10 on the heliostat responsible for the incident beam. Therefore the image data from the camera is unique to that heliostat and may be used for closed loop tracking or calibration. As discussed above, this direct, closed loop tracking allows multiple heliostats to calibrate simultaneously rather than sequentially. In CSP generators with thousands of heliostats, this is significantly more time efficient than individually calibrating each heliostat in succession. With the entire array tracking more accurately on the receiver, the flux of incident radiation is higher for greater operation efficiency.

[0055] Similarly, for the retroreflector 22, one or more of the internal reflective surfaces can be slightly curved or scattering such that its reflected beam spread encompasses a subset of the heliostats. This is advantageous in particularly large heliostat arrays, where it may be desirable to have one camera service several heliostats. Using retroreflectors 22 that reflect incident light from one heliostat back to one or more neighbouring heliostats (as well as the original heliostat) requires more complex image processing but is still beneficial when positioning the reflector surfaces 7 to provide an accurate or uniform flux across the receiver 4.

[0056] In other forms, the retroreflectors can be an array of beads (not shown) of transparent material with a refractive index selected such that incident light is internally reflected and emerges parallel to the angle of incidence, or with a defined beam spread. Another option is to add a lens to the retroreflector to achieve the desired beam spread. Retroreflectors of this type are also suited for use as initial calibration targets that may be spaced from the retroreflector array 5 at the receiver 4. To quickly calibrate large numbers of heliostats, several calibration targets positioned about the receiver tower 1 1 are used for a coarse initial or periodic calibration of associated sections of the heliostat array, and also provides information of beam shape and hence mirror quality. With each section coarsely calibrated to a target at a known offset from the receiver 4, the heliostats are then adjusted to see the retroref lectors at the tower 11 for more precise tracking and/or calibration.

[0057] Figure 5 is a diagrammatic representation of the field of view 27 for one of the cameras 10. The retroref lectors seen by the camera are mounted on (or in) the receiver 4. As discussed above, the control processor 42 will position the reflective surface 7 such that the reflected brightness pattern 45 sufficiently matches the desired pattern for that heliostat. The pattern 45 also indicates the geometry of the reflected sunlight spot 28 which will change during the day (and accounted for by the control processor). Ideally, the spot 28 is centred within the field of view 27. As discussed above, the camera's field of view 27 should be restricted to provide high image resolution, however the field of view should be marginally larger than the reflected spot 28, sufficient to see all the retroreflectors. By seeing the entire spot 28 centred in the field of view, the processor can flag potential issues discussed below.

[0058] Figure 6 shows the energy flux distribution spatially resolved across the x axis of the field of view. From the retroreflected image seen by the camera, the brightness or pixel intensities of the retroreflectors 12 in column 62 are processed to give the energy flux measure 162. Likewise, the retroreflected intensities from columns 60 to 72 give the energy flux measures 160 to 172. The resulting energy flux distribution is processed to find the centroid (at 160) and the X axis actuator is adjusted to place the centroid at the desired X axis position. The equivalent process finds the Y axis distribution to precisely position the sun spot 28 in the Y axis direction within the field of view 27.

[0059] In Figure 7, the reflected spot 28 is shown off centre in the camera field of view 27. This can be the result of small shifts in the heliostat due to actuator wear, mechanical slack, wind load or slight ground subsidence. In the case of wind load, the deviations 29 and 30 from the centre of the field of view 27 will be transient and the processor can compensate with fine adjustments via the actuators. In the case of ground subsidence or some other permanent misalignment, the deviation 29 and 30 will be constant and the processor 42 can flag and monitor the issue. If the misalignment worsens with time, corrective action can be taken before the sun spot 28 starts to move out of the camera field of view 27

[0060] Figure 8 shows an embodiment of the system in which several of the heliostats (31 , 32 and 33) have cameras 10 that 'see' retro reflections from neighbouring heliostats (crosstalk). As discussed above, this may be the case if there is a very large distance between the heliostats and the receiver 4. However, a combined brightness pattern 45 still provides sufficiently accurate alignment of the reflected sunlight 20 onto the receiver 4. In these arrangements, the finite beam spread 21 of the light reflected from the retroref lector 12 diverges enough to encompass the cameras 10 of heliostats 31 , 32 and 33. Providing the retroreflectors give a return beam with a finite spread, but one where the return intensity is ever reducing with angle, the effect of the crosstalk from nearby heliostats can be removed by a process of deconvolution using the known beam shape of each heliostat that is approximately the same for all mirrors in a localised area of the heliostat array.

[0061] Likewise in very large heliostat arrays, costs can be reduced by fitting a camera to a portion of the heliostats distributed throughout the array. For example, if cameras are only fitted to every third heliostat, then each heliostat in the array will be adjacent to at least one heliostat that has a camera and can align with the receiver 4. The adjacent heliostats without cameras can be aligned on the same basis described in the paragraph above. In the case of heliostats at a very long range, it can be difficult to restrict the reflected beam spread such that adjacent heliostats would only ever see reflections of their own incident beams.

[0062] In yet another option, the cameras move from heliostat to heliostat within sections of the array. Once the camera focusses on the field of view corresponding to one of the heliostats (and captures the retro reflection pattern 45), it moves to the next heliostat to focus on its corresponding field of view. The camera can be mounted to a carriage on a track or rail linking the relevant heliostats.

[0063] Figure 9 shows a single heliostat 6. A reflective surface 7 is mounted to a stem 8 extending upright from a base plate 37. The base plate 37 is anchored to the ground or a foundation using bolts or pins 40. The reflector surface 7 is mounted at the upper end of the stem 8 for rotation about two orthogonal axes using the actuator 9. The heliostat camera 10 is fixed to the stem 8 such that its field of view covers the retroreflectors (not shown) on or near the receiver 4. To allow this, the reflector surface 7 has a central aperture or viewing window for the camera 10. However, it will be appreciated that the camera may also be fixedly mounted elsewhere on the heliostat, in front of the reflective surface or closely adjacent the heliostat 6. However, as discussed above, mounting the camera at the centre of the reflective surface 7 has less risk of retro reflection cross talk between neighbouring heliostats.

[0064] The heliostat may have some processing capacity 38 for on-board analysis of image data captured by the camera 10. The on-board processor 38 may be powerful enough to analyse the image data and precisely control the actuators 9 or it may in turn communicate with a central processor linked to all the heliostats in the array. This connection may be hardwired or wireless 39.

[0065] Power for the actuators 9 and the on-board processor 38 may be provided by photovoltaic cells 41. These may be attached to the reflector surface 7 for good exposure to the sun. However when the sun is at a low angle, there is usually some shading of the reflector surfaces 7 by the adjacent heliostats. This is not a significant issue as photovoltaic cells do not require exposure to direct sunlight to generate power. Diffuse light is also sufficient to generate the necessary power required to align the reflector surface 7. Of course, the on-board processor has a rechargeable battery to power the actuators 9.

[0066] It will be appreciated that the closed loop alignment system described above allows the individual heliostats 6 to be mounted directly to the ground rather than large, expensive and finely toleranced structures or foundations. This has a significant impact on the initial cost of the concentrated solar thermal plant and provides added flexibility for placement of each heliostat.

[0067] Figure 10 is a flow chart illustrating the main steps involved in the closed loop alignment of the reflector surfaces 7 within the heliostat array. A theoretical position of the sun is derived from stored reference data for the sun's transit at the heliostat's geolocation at a particular time and date to calculate the initial alignment required. The initial alignment of the reflective surface 7 at the start of the day, or when the sun is not visible, is relatively low precision, achieved using either calibrated mechanical alignment or using accelerometers and magnetometers. The camera 10 captures an image and the processor 42 analyses the brightness pattern 45 of the retroref lectors within the field of view 15. If the pattern is not correct, the processor 42 determines the appropriate corrective adjustment and operates the actuators 9 accordingly. [0068] The camera again captures an image of the retroreflector brightness pattern 45 and if still not sufficiently close to a predetermined pattern for that heliostat, at that time and date, a further corrective adjustment is calculated. The process iterates until the spot of reflected sunlight from the heliostat 6 is at the desired position on the receiver 4. If the retroreflector array 5 is offset from the receiver (as discussed above) the processor then calculates the necessary adjustment to compensate for the offset. As the Sun moves across the sky, the heliostat array will require regular re-alignment at set periods, or by triggered when the image data indicates an unacceptable degree of misalignment. These recalibrations are individual and simultaneous, not sequential or 'scheduled'.

[0069] The invention has been described herein by way of example only. Skilled workers in this field of technology will readily recognise many variations and modifications that do not depart from the spirit and scope of the broad inventive concept.