WO2009152574A1

WO2009152574A1 - Heliostat concentrator photovoltaic power generating and control system

Info

Publication number: WO2009152574A1
Application number: PCT/AU2009/000782
Authority: WO
Inventors: John Beavis Lasich; David Hoadley; Wolfgang Hertaeg; Xinyi Zou
Original assignee: Solar Systems Pty Ltd
Priority date: 2008-06-17
Filing date: 2009-06-17
Publication date: 2009-12-23

Abstract

A photovoltaic power generating system and method, the system comprising: a receiver comprising a dense array of photovoltaic devices; a plurality of mirrors (such as provided in heliostats), each adapted to reflect sunlight onto the receiver; and a control system for controlling the angular orientation of each of the mirrors as the sun moves so that the sunlight is reflected towards an illumination position on the receiver. The control system is adapted to vary the illumination positions of at least a subset of the mirrors over time to provide or maintain a desired total flux distribution over all or part of the dense array. A solar thermal power generating system and method and a hybrid photovoltaic/solar-thermal power generating system and method are also provided.

Description

HELIOSTAT CONCENTRATOR PHOTOVOLTAIC POWER GENERATING AND

CONTROL SYSTEM

RELATED APPLICATION This application is based on and claims the benefit of the filing date of US application no. 61/073 , 331 filed 17 June 2008, the content of which as filed is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to a solar power generating system (in particular a heliostat concentrator photovoltaic (HCPV) power generating system) , to a method of controlling a solar power generating system and to a method of generating electricity with a solar power generating system.

BACKGROUND OF THE INVENTION

In the design of concentrator photovoltaic or solar thermal systems, it has been appreciated that it is important to deliver light to the receiver efficiently. One existing method of designing a concentrator photovoltaic power generating system to. do so is disclosed in WO 02/082037. The method aligns the mirrors of a concentrator dish array by determining the preferred pattern of light reflection from the array, characterizing the shape of each of the array's mirrors, simulating the array and the light reflection based on such characterizations, comparing simulated light reflection with the preferred pattern of light reflection, and varying the simulated array until the simulated light reflection is within acceptable tolerances of the preferred pattern of light reflection. The array can then be constructed according to the ultimate simulated array.

In a dish system, since the dish always points at the sun, the pattern of light intensity (flux distribution) at the receiver is relatively constant.

One existing heliostat concentrator system comprises multiple independently steerable mirrors reflecting light onto a central receiver on a tower, the receiver essentially comprising a boiler in which water is heated by the action of the concentrated solar flux distribution at the receiver. The precise flux distribution may not be of great importance in this system.

SUMMARY OF THE INVENTION

According to a first broad aspect of the present invention, there is provided a photovoltaic power generating system, comprising: a receiver (which may be cooled) comprising a dense array of photovoltaic devices; a plurality of mirrors {such as in heliostats) , each mirror adapted to reflect sunlight onto the receiver; and a control system for controlling the angular orientation of each of the mirrors as the sun moves so that the sunlight is reflected towards an illumination position on the receiver; wherein the control system is adapted to vary the illumination positions of at least a subset of the mirrors over time to provide or maintain a desired total flux distribution over all or part of the dense array.

The inventors have realized that there are two reasons why concentrator PV systems are especially sensitive to flux distribution. Firstly, in arrays of photovoltaic devices - such as photovoltaic (PV) cells_/ the PV cells are connected in series- linked groups to provide an optimally high output voltage. The current output of PV cells linked in series is limited by the current from the least illuminated cells, so if the spatial variation of the solar flux in a region of the receiver comprising a group of series-linked PV cells is too large, output will be less than the string is able to produce for the same flux delivered with less spatial variation.

Secondly, in a dense array concentrator PV system, the photovoltaic cells experience flux intensities of the order of 500 suns or higher, requiring efficient cooling of the PV cell to prevent destruction and to optimize operating efficiency and lifetime, both of which tend to be inversely related to operating temperature. If one region of the receiver is hotter than another at any time during the day owing to uneven flux distribution across the receiver, the maximum total flux that can be safely delivered to the receiver will be limited by the requirement that the hottest region is kept within the desired operating range. Consequently the area of highest flux places a limit on receiver output, and areas of lower flux are underutilized.

The inventors have realized that the cost of photovoltaic devices such as triple junction cells or other high efficiency photovoltaic cells require not just an average high optical concentration of sunlight but stringent minimization of receiver area per total watt output, and, in combination with the above two separate limits on efficiency, output and lifetime.

The ^illumination position' of a respective mirror is essentially the centre of the distribution of light reflected from that mirror onto the receiver. The distribution may comprise an image of the mirror (possibly somewhat distorted) , and the illumination position the simple geometric centre of that image. However, the illumination position may alternatively be regarded as the 'centre of mass' or centroid of a flux distribution contour equal to 10% (for example) of the maximum light intensity due to a particular mirror, or the centroid of an flux distribution contour encompassing 90% of the total flux reflected by the respective mirror. Other feasible definitions are also possible, as will be appreciated by those skilled in the art, including indirect measures effectively providing the same function for the purpose of implementing the invention.

The subset of the mirrors may comprise, for example, one or more mirrors selected from the mirrors used to generate power. In one embodiment, at least some of the subset of the mirrors are provided solely for adjusting a total flux distribution on the receiver.

In a particular embodiment, the system comprises a plurality of heliostats, each of which comprises one or more of the mirrors. In another embodiment, the system comprises a concentrator dish, wherein the dish comprises the mirrors .

It should be noted that the mirrors may comprise compound mirrors .

A 'dense array' or 'dense region' of a dense array of photovoltaic devices is an array (commonly two dimensional) of photovoltaic devices arranged in close proximity so that gaps between the cells are kept low to provide a substantially continuous electricity generating surface for the purpose of optical concentrator design. The cells may be individually manufactured and placed on the array, or alternatively the cells or groups of the cells may be monόlithically manufactured.

The time variation may occur dynamically over the day in response to measurements, or at fixed time intervals.

The control system may utilize programmed time variations of the illumination positions to achieve the desired total flux distribution.

The control system may alternatively or in addition utilize measurements of actual flux distribution to provide feedback to achieve the desired total flux distribution .

The illumination positions may be varied among a discrete set of alternative illumination positions, or continuously whereby the illumination positions of mirrors in the subset are adapted to be varied over a locus or region on the receiver. The region may be the entire receiver, or the entire dense array, or a sub-area of the receiver or . dense array.

The illumination positions of the mirrors in the subset may be varied individually or coordinated to vary in groups by a group displacement. Membership of coordinated groups may be dynamically altered.

The illumination positions of some of the subset of mirrors may be varied at some times to.be outside the area of the receiver to achieve the desired flux distribution.

The subset of mirrors may be substantially the entire set of mirrors, or substantially less than 50% of the entire set.

The subset may be statically assigned as an appropriate subset for achieving a desired level of adjustment of the flux distribution. The subset may include mirrors positioned so as to be able to project an image over any part of the receiver. The subset may include mirrors close to the receiver projecting a relatively small image on the receiver. The subset may alternatively or in addition include mirrors far from the receiver projecting a relatively large image on the receiver. In one embodiment/ the dense array comprises a plurality of separate dense regions interposed with secondary optics .

In one embodiment/ the size of the flux distribution due to each, of the mirrors on the dense array is less than 50% of the area of the dense array. In another embodiment, the size of the flux distribution due to each of the mirrors on the dense array is less than 25% of the area of the dense array. In particular embodiments, the size of the flux distribution due to each of the mirrors on the dense array is less than 15% of the area of the dense array .

In a certain embodiment, each of the mirrors comprises a small mirror or multiple small canted mirrors in fixed interrelation focussing together on the receiver. Alternatively or in addition, each small mirror or small canted mirror can have a curved surface focusing on the receiver area to provide an image of desired size.

In one embodiment, the receiver is adapted to be cooled with a fluid coolant and the system comprises a thermal converter for receiving heated coolant from the- receiver and extracting energy from the heated coolant.

Thus, in this embodiment the system is, in effect, a hybrid photovoltaic/solar thermal system, as energy is extracted both by the receiver and the thermal converter.

In a particular embodiment, the system comprises controllers for controlling the mirrors, the controllers being controlled by the control system, and a timing synchronization module for transmitting a timing signal to the controllers adapted to provide the controllers with accurate time information. Thus, the controllers are thereby able to control the mirrors to perform programmed or other tasks at the correct time (and hence in a coordinated manner where necessary or desired) .

Each of the controllers may control a respective one of the mirrors / or - in other embodiments - a plurality of the mirrors .

According to a second broad aspect of the invention, there is provided a method of controlling a photovoltaic power generating system, comprising: reflecting sunlight with a plurality of mirrors onto a receiver (which may be cooled) , the receiver comprising a dense array of photovoltaic devices; controlling the angular orientation of each of the mirrors as the sun moves so that the sunlight is reflected towards an illumination position on the receiver; and varying the illumination positions of at least a subset of the mirrors over time to provide or maintain a desired total flux distribution over all or part of the dense array,

The method may include varying the illumination positions over time dynamically over the day in response to measurements of flux distribution, or at fixed time intervals.

The method may include varying the illumination positions at predefined or programmed times .

The method may alternatively or in addition include making measurements of actual flux distribution and utilizing the measurements to provide feedback to achieve the desired total flux distribution. in one embodiment , the method includes varying the illumination positions among a discrete set of alternative illumination positions, or continuously whereby the illumination positions of mirrors in the subset are varied over a locus or region on the receiver. The region may be the entire receiver, or the entire dense array, or a sub- area of the receiver or dense array.

In another embodiment, the method includes varying the illumination positions of the mirrors in the subset individually. In. still another embodiment, the method includes varying the illumination positions of the mirrors in coordinated groups so as to vary the groups by respective group displacements. Membership of the coordinated groups may be dynamically altered.

In some embodiments, the method includes varying the illumination positions of some of the subset of mirrors at some times to be outside the area of the receiver-

The method may include statically assigning the subset as an appropriate subset for achieving a desired level of adjustment of the flux distribution. The subset may include mirrors positioned so as to be able to project an image over any part of the receiver. The subset may include mirrors close to the receiver projecting a relatively small image on the receiver. The subset may alternatively or in addition include mirrors far from the receiver projecting a relatively large image on the receiver.

In one embodiment, the dense array comprises a plurality of separate dense regions interposed with secondary optics .

In. one embodiment, the size of the flux distribution due to each of the mirrors on the dense array is less than 50% of the area of the dense array. In another embodiment, the size of the flux distribution due to each of the mirrors on the dense array is less than 25% of the area of the dense array, In particular embodiments, the size of the flux distribution due to each of the mirrors on the dense array is less than 15% of the area of the dense array.

In one embodiment, the method includes controlling the mirrors with controllers, and transmitting a timing signal to the controllers adapted to provide the controllers with accurate time information.

In a certain embodiment, the mirrors are provided in heliostats.

According to a third broad aspect of the present invention, there is provided a solar thermal power generating system, comprising: a solar thermal collector for collecting sunlight (and which, in some embodiments, contains a thermal fluid - such as water - to be heated by the sunlight and in other embodiments thermo-electric devices to produce voltage and current directly) ; a plurality of mirrors (such as in heliostats) , each mirror adapted to reflect sunlight onto the solar thermal collector and heat the thermal fluid; and a control system for controlling the angular orientation of each of the mirrors as the sun moves so 5 that the sunlight is reflected towards an illumination position on the solar thermal collector; wherein the control system is adapted to vary the illumination positions of at least a subset of the mirrors over time to provide or maintain a desired total flux 10 distribution over all or part of the solar thermal collector.

In one embodiment, the system comprises a receiver (which may be cooled) comprising an array (such as a dense array) 15 of photovoltaic devices arranged to receive radiation re- radiated from the solar thermal collector and converting the radiation into electricity.

The solar thermal collector may be of any suitable form, 20 including a vessel - such as of steel, copper, brass, titanium or otherwise - containing the thermal fluid (whether a gas or liquid) or pipes through which the thermal fluid is passed.

25 The subset of the mirrors may comprise, for example, one or more mirrors selected from the mirrors used to generate power. In one embodiment, at least some of the subset of the mirrors are provided solely for adjusting a total flux distribution on the collector.

30.

According to a fourth broad aspect of the invention, there is provided a method of controlling a solar thermal . power generating system, comprising j reflecting sunlight with a plurality of mirrors

35 onto a solar thermal collector (which, in some embodiments, contains a thermal fluid - such as water - to be heated by the sunlight) ; controlling the angular orientation of each of the mirrors as the sun moves so that the sunlight is reflected towards an illumination position on the solar thermal collector; and varying the illumination positions of at least a subset of the mirrors over time to provide or maintain a desired total flux distribution over all or part of the solar thermal collector.

In one embodiment, the method comprises arranging a receiver (which may be cooled) comprising an array (such as a dense array} of photovoltaic devices to receive radiation re-radiated (whether reflected or emitted) from the solar thermal collector and to convert the radiation into electricity.

According to fifth broad aspect, the invention provides a solar power generating system, comprising: a plurality of mirrors (such as in heliσstats) _r each mirror adapted to reflect sunlight onto a photovoltaic receiver or solar thermal collector; controllers for controlling the angular orientation of each of the mirrors as the sun moves so that the sunlight is reflected towards an illumination position on the receiver or collector; and a control system for controlling the controllers, comprising a timing synchronization module for transmitting a timing signal to the controllers, the signal being adapted to provide the controllers with accurate time information.

According to sixth broad aspect, the invention provides a method of controlling a solar power generation system, comprising: reflecting sunlight with a plurality of mirrors

(such as in heliostats) onto a photovoltaic receiver or solar thermal collector; controlling the angular orientation of each of the mirrors with controllers as the sun moves so that the sunlight is reflected towards an illumination position on the receiver or collector; and transmitting a timing signal to the controllers, the signal being adapted to provide the controllers with accurate time information.

According to seventh broad aspect, the invention provides a solar power generating system, comprising: a plurality of solar power generating units, each comprising: a receiver (which may be cooled) comprising a dense array of photovoltaic devices; and a concentrator dish (typically comprising a plurality of mirrors) adapted to reflect sunlight onto the respective receiver; controllers for controlling the angular orientation of the dishes as the sun moves so that the sunlight is reflected by each of the respective dishes towards the respective receiver; and a control system for controlling the controllers, comprising a timing synchronization module for transmitting a timing signal to the controllers, the signal being adapted to provide the controllers with accurate time information.

Thus, a system comprising a field of dish concentrator units may be controlled in a synchronized manner.

According to eighth broad aspect, the invention provides a method of controlling a solar power generation system, comprising: reflecting sunlight with a plurality of concentrator dishes (each typically comprising a plurality of mirrors) onto a plurality of respective photovoltaic receivers; controlling the angular orientation of each of the dishes with controllers as the sun moves so that the sunlight is reflected by each of the respective dishes towards the respective receiver; and transmitting a timing signal to the controllers, the signal being adapted to provide the controllers with accurate time information.

According to ninth., tenth, eleventh and twelve broad aspects of the invention, there is provided a method of producing electricity comprising operating the power generating systems of the first, third, fifth or seventh aspects, respectively.

According to thirteenth, fourteenth, fifteenth and sixteenth broad aspects of the invention, there is provided an electrical product comprising a quantity of electrical power produced by the methods of the ninth, tenth, eleventh and twelve aspects, respectively.

It will be understood that features of the above aspects can be employed in respective ones of the other aspects.

BRIEF DESCRIPTION OF THE DRAWING In order that the invention may be more clearly ascertained, embodiments will now be described, by way of example, with reference to the accompanying drawing, in which:

Figure 1 is a schematic view of a heliostat concentrator photovoltaic (HCPV) power generating system according to an embodiment of the present invention;

Figure 2 is a schematic view of a receiver of the system of figure 1;

Figure 3 is a schematic view of a receiver according to an alternative embodiment of the present invention for a heliostat of the system of figure 1; Figure 4 is a schematic view of the control system of the system of figure 1;

Figure 5 is a schematic view of a receiver of a heliostat of the system of figure 1/ indicating exemplary illumination positions; Figure 6 is a schematic view of the control system, of a HCPV power generating system according to another embodiment of the present invention;

Figure 7 is a schematic view of a solar thermal power generating system according to another embodiment of the present invention; and

Figure 8 is a schematic view of the solar thermal collector of the system of figure 7.

DETAILED DESCRIPTION OF THE EMBODIMENTS A heliostat concentrator photovoltaic (HCPV) power generating system according to an embodiment of the present invention is shown schematically at 100 in figure 1, with the sun 102. System 100 includes a heliostat field 104 of heliostats 106, a cooled HCPV receiver 108, a supporting tower 110 that supports receiver 108, a heat exchanger 112 next to tower 110 for dispersing waste heat from receiver 108, and a control centre 114. The height of tower 110 is selected to be sufficiently great to substantially prevent shadowing of heliostats 106 by each other.

System 100 also includes a control system (not shown) , located in control centre 114, and connected by data cables (also not shown) or by wireless transmission to each of heliostats 106/ as is described in greater detail below. Power to drive the heliostats and the control system is provided from a mains or other power supply.

Referring to figure 2, receiver 108 comprises a dense array 116 of PV cells and a solar flux modifier 118.

Dense array 116 comprises a single contiguous arrangement of densely packed photovoltaic cells inside a. boundary occupied by a solar flux modifier 118. Solar flux modifier 118 borders dense array 116, and comprises four cooled reflective panels for reflecting at least some reflected light that would otherwise fail to fall on receiver 108 towards receiver 108. Indeed, receiver 108 and dense array 116 are comparable though larger to the cooled receiver and dense array taught by WO 02/080286 (incorporated herein by reference) ; alternatively, dense array 116 may be constructed out of a plurality of the receivers taught therein, and system 100 also includes the cooling systems (not shown) for the reflective panels of solar flux modifier 118 and dense array 116 such as are taught by WO 02/080286. Such reflective panels constitute one example of secondary optics that redirect light from helioεtats 106 falling immediately outside dense array 116 onto dense array 116, and other secondary optics may be arranged around dense array 116 - as will be appreciated by those skilled in the art - or secondary optics may be omitted entirely in a less efficient system. Another cooling system is disclosed in WO 2005/022652.

While the embodiment is described in relation to photovoltaic cells, .other embodiments may employ other photovoltaic devices such as monolithically integrated cell modules.

Referring to figure 3, in an alternative embodiment, system 100 may comprise a modified receiver 108' with a dense array that comprises a plurality of dense regions 300 (in this example, 12) of densely packed photovoltaic cells. Dense regions 300 are internally contiguous and separated by gaps occupied by elements of a solar flux modifier 118' (which, in the embodiment of figure 3, has correspondingly more elements) . Regions 300 are arranged in a 2 -dimensional grid, composed of repeatable separate subunits easily manufactured and serviced, in an analogous manner to the receivers taught by WO 02/080286. Each of heliostats 106 comprises a mirror 120, a support pole 122, a drive system (not shown) for changing the orientation of the respective mirror 120 in two axes, and an encoder (not shown) both for controlling the drive system to orient the respective heliostat 106 as desired (that is, in response to orientation instructions received from the control system of system 100) and to return data indicative of the orientation of the respective heliostat 106 to the control system. The drive system is thus controlled according to a prescribed encoder position under the command of the control system, so as to correctly orient respective mirror 120 throughout the day. The correct orientation of each mirror 120 is, in broad ^■. terms, that which causes light 124 from the sun 102 to be reflected by respective mirror 120 towards receiver 108.

Thus, in use light 124 from the sun 102 is reflected by each of heliostats 106 towards receiver 108, which - through the response of its array 116 of PC cells - outputs electricity. Each heliostat 106 reflects light - which may comprise essentially an image of respective heliostat 106 - onto receiver 108; the deposited light from each heliostat 106 can be characterized with a position (termed an * illumination position') . Referring to figure 2, the illumination positions 200a, 200b, 200c of three exemplary heliostats 106 are shown. Illumination positions 200a and 200b coincide with dense array 116, but illumination position 200c - whether by design or accident - coincides with flux modifier 118. The flux distribution

202a, 202b, 202c deposited on dense array 116 by heliostats 106 will generally surround the respective illumination positions 200a, 200b, 200c, but when an illumination position lies near or on flux modifier 118, at least a portion of the flux distribution on dense array 116 will arise from reflection from flux modifier 118 (as is the case with the heliostat with exemplary illumination position 200c) .

The control system of system 100 performs several functions/ and is illustrated schematically at 400 in figure 4. Control system 400 includes the following components (though simpler versions of the components adapted to provided coarser control are also envisaged) ; these components include a processor, memory and software or firmware as necessary, though may optionally share such elements where suitable.

Thus, control system 400 includes a solar position determiner 402, a helioεtat orienter 404, a translation sub-controller 406, an energy distribution detector 408, a heliostat characteristic memory 410, a .heliostat " flux modelling sub-controller 412, a total flux modelling sub- controller 414, a desired total flux distribution controller 416, a heliostat illumination position determiner 418, a group heliostat illumination position determiner 420 and a heliostat characteristic determiner 422.

Solar position determiner 402 determines the position of the sun at any required time. In this embodiment, solar position determiner 402 determines the sun's position by calculation, with inputs being the location of system 100, the date and the time. Optionally, system 100 may include a mechanism for determining the sun's position empirically (such as is known, for example, from DE 4 118 894) , and solar position determiner 402 may be configured to employ empirical data generated thereby, or to employ a combination of both calculation and empirical data.

Heliostat orienter 404 receives solar position data from solar position determiner 402, and sends command signals to the encoders of respective heliostats 106 to control the drive systems of respective heliostats 106 to orient respective mirrors 120 to reflect the sun's light towards dense array 116.

Translation sub-controller 406 is adapted to determine a predicted illumination position of the respective heliostat 106 on receiver 108, from data on the positions of the respective encoders (indicative of the angular orientation of each heliostat 106) and solar position data received from solar position determiner 402.

Energy distribution detector 408 is adapted to provide data indicative of the actual flux distribution over all or part of the dense array. The indicative data may include any one or a combination of (1) electrical output data from photovoltaic cell modules forming dense array

116, (2) temperature sensor data indicative of temperature distribution across dense array 116 (in which case energy distribution detector 408 includes temperature sensors located at each photovoltaic cell module) , and/or (3) light intensity data (in which case energy distribution detector 408 includes one or more suitable cameras - which may be, for example, sensitive to visible and/or infrared radiation) positioned to collect and image light reflected from dense array 116.

Heliostat characteristic memory 410 stores data for each heliostat 106, from which a predicted spatial distribution of light reflected by a respective heliostat 106 around an illumination position on receiver 108 may be derived. The data may be stored in the form of an empirically collected spatial distribution of the light measured for one particular position of the sun, transformable into predicted spatial distribution for other positions of the sun. As optical characteristics of individual mirrors may vary, it may be advantageous to store actual spatial distributions, but it is also envisaged that mirror characterisation data - determined such as is taught by WO 02/082037 - could be stored in heliostat characteristic memory 410 and employed in determining such spatial distributions of reflected light.

Heliostat flux modelling sub-controller 412 predicts the contribution of each heliostat 106 to the total flux distribution over dense array 116, using heliostat characteristic memory 410 and the sun's position from solar position determiner 402, for a given illumination position of a respective heliostat 106 (as determined by translation sub- controller 406} . Heliostat flux modelling sub-controller 412 also takes into account the effect of solar flux modifier 118 (and of any other secondary optics provided around dense array 116) .

Total flux modelling sub-controller 414 predicts the total flux distribution over dense array 116, using the output of heliostat flux modelling sub -controller 412, for a given set of heliostat illumination positions .

Desired total flux distribution controller 416 is adapted to determine the desired total flux distribution over dense array 116. When dense array 116 is fully functioning, the. desired total flux distribution is ideally an even distribution with maximal power. However, partial system underperformances or operating parameters such as high temperature or grid levelling requirements may require that the desired total flux distribution be adjusted away from an even distribution. Thus, desired total flux distribution controller 416 has inputs that include system performance data, detailed receiver performance in terms of either temperature or efficiency of each module of photovoltaic cells in the receiver, other operating parameters and, where applicable, grid levelling requirements.

Heliostat illumination position determiner 418 is adapted to determine the actual illumination position of a respective heliostat 106 by (1) using energy distribution detector 408 to obtain data indicative of the flux distribution a first time, (2) instructing heliostat orienter 404 to command the encoder of the respective heliostat 106 to move the respective illumination position of that respective heliostat 106, (3) using energy distribution detector 408 to obtain data indicative of the flux distribution a second time, and (4) outputting a measure of actual heliostat illumination position of the respective heliostat 106 by comparing the data indicative of the flux distributions at the first and second times. The first and second times may be sufficiently close, and the movement of the illumination position sufficiently large, that the actual heliostat illumination position is calculable by simple subtraction of the flux distribution data of the first time from that of the second time, followed by computation of the position by centroid, centre of mass or other suitable measure. Alternatively the movement of the illumination position can be small and simple subtraction of the two images forms a spatial derivative which is then followed by transformation to the absolute value, thus allowing calculation of the centroid of transformed subtraction to find the illumination position.

Noise in the measurement of the flux distribution data each time may be reduced by data averaging, such as by making multiple repeat measurements over a short interval and obtaining an average over the multiple repeat measurements .

Group heliostat illumination position .determiner 420 is similar to heliostat illumination position determiner 418, but is operable to move a group of heliostats (such as group 126) in unison to determine an aggregate illumination position of the group, if less fine control is needed or rapid action is needed. Group heliostat illumination position determiner 420 employs heliostat illumination position determiner 418.

Heliostat characteristic determiner 422 is a subprogram of heliostat illumination position determiner 418, and determines the heliostat characteristic by the simple subtraction described above. Heliostat characteristic memory 410 can be updated by heliostat characteristic determiner 422.

Initial setup and calibration of each of heliostats 106 can be effected by any known method to provide start values for the parameters in translation sub- controller 406.

In an off-line calibration mode, heliostat position determiner can be used to orient individual ones or subsets of heliostat at the receiver when other heliostats are not illuminating the receiver. In this case, one of the flux distributions to compare can be a background, un- illuminated distribution. This calibration may be provided for a plurality of sun positions to provide a plurality of calibration points relating, directly or indirectly, heliostat orientation readings and/or sun positions with flux distribution due to the heliostat on the receiver. In power generating operation, control system 400 commands heliostats 106 to position their respective encoders to values corresponding to a set of programmed illumination positions previously calculated to provide the desired flux distribution over part or all of dense array 116 at the given time and date. This previous calculation may have been provided off line by total flux modelling sub-controller 414.

Control system 400 may periodically use feedback of the actual flux distribution using energy distribution detector 408 or some cruder measure. If comparison of the actual with the desired flux distribution indicates that a global displacement of the illumination positions will improve the match, as may occur if a strong wind distorts the position of receiver 108, or if the prediction of illumination positions is globally inaccurate for any other reason, such displacement is added to the desired set of illumination positions and the orientations of heliostats 106 are adjusted. If comparison indicates that a local change is appropriate, a subset of the illumination positions are adjusted by amounts predicted to improve the match. This subset may be a fixed subset of heliostats 106 designated as trim heliostats, or a dynamic subset assigned by control system 400 on the basis of a predictive calculation using the total flux modelling sub-controller 414. The heliostat illumination position determiner 418 or group heliostat illumination position determiner 422 may also be used to resolve any uncertainty about the actual illumination positions of relevant heliostats 106 to assist in the determination of the appropriate subset. The amounts of adjustment of each illumination position may also be determined by a predictive calculation using the total flux modelling sub- controller 414, by cruder but faster heuristic methods, or by trial and error.

Control system 400, also works in a calibration update' mode without interrupting power generation to periodically update the settings of translation sub-controller 406 and heliostat characteristic memory 410 for each heliostat

106, using heliostat illumination position determiner 418 during power generation and comparing with the intended illumination position predicted from the translation sub- controller 406. This capability allows actual illumination position data to drive an on-the-fly calibration capability which may be particularly important where cost minimizations in the design have mandated the use of drive components, mirror mounting components, heliostat placement components and receiver fixing components with a propensity to drift in time due to wear and tear or the actions of the natural elements. This calibration may be provided for a plurality of sun positions to provide a plurality of calibration points relating, directly or indirectly, heliostat orientation readings and/or sun positions with flux distribution due to the heliostat on the receiver.

In order to take full advantage of the opportunities to achieve the desired flux distribution afforded by the present embodiment, it has been found that the size of the image projected by most of the heliostats 106 should optimally be smaller than the area of dense array 116, for a substantial part of the day. This enables more freedom in variation of the illumination positions to allow collection of substantially all of the reflected light at receiver 108. However, the total flux distribution should still be adequately controllable if at least a subset of heliostats 106 are positioned - or have mirrors of a suitable size - such that this subset produces image sizes on dense array 116 smaller - and preferably substantially smaller - than the area of dense array 116? this subset may then be used as trim mirrors, that is, for trimming the total flux distribution.

The size of the image produced by a reflective object such as a heliostat on dense array 116 is defined for the purposes of this description as an area of the flux distribution produced by the object shining sunlight onto the array within an equal intensity flux contour enclosing 90% of the total flux reflected by the object.

Thus, it is preferred that the size of the image produced by heliostats 106 (or at least the aforementioned subset) be less than 50%, more preferably less than 25% of the area of dense array 116. Even more preferably, the size of the image should be less than 15% of the area of dense array 116. An example of illumination positions according to these criteria is illustrated in figure 5, which is a schematic view of receiver 108 with illumination positions 500. It will again be noted that, while most illumination positions 500 are on dense array 116 {for example, illumination position 500a) , some are on flux modifier 118 (for example, illumination position 500b) . The flux distribution (not shown) of each heliostat 106 generally surrounds the corresponding illumination position 500 and overlap with those of its neighbours, but nonetheless is each significantly smaller in size (by the above 90% criterion) than 15% of the area of dense array 116.

The optimal small image size may be produced by providing respective mirror 120 of heliostat 106 as a single small mirror or as multiple small canted mirrors in fixed interrelation focusing on receiver 108. Alternatively or in addition,- each mirror 120 may have a. curved surface focusing on receiver 108 to provide the small image size.

Small heliostat mirrors 120 may offer a further advantage in the performance of this embodiment _e in providing more degrees of freedom in the individual adjustment of each of a larger number of heliostats 106 to provide the desired flux distribution at different times of the day and year.

In embodiments in which energy distribution detector 408 includes one or more cameras or sensors, the data collected by energy distribution detector 408 may be subjected to image smoothing to eliminate effects caused by fluctuations in sunlight intensity (such as due to cloud) , so that data from a day with such fluctuations may validly be compared with data from days without.

In some embodiments, it may be found that the expected illumination position of a heliostat 106 (derived from the heliostat' s geometry and orientation) does not correspond to the actual illumination position (such as illumination position 200a) of the heliostat. This illumination position is generally formed on receiver 108 (and in particular on dense array 116} or on an alignment target (not shown) . In effect, this means that the actual illumination position may be found not to correspond to the point on dense array 116 at which heliostat 106 is aimed.

This may be due to imperfections in the heliostat' s mirrors or in the alignment of those, mirrors (possibly arising after manufacture or instalment) , or from the effect of the changing angle of incidence of sunlight. As will, be appreciated by those skilled in the art, the distance between heliostat 106 and HCPV receiver 108 is typically sufficiently great that minor imperfections in heliostat 106 can result in a significant difference between the theoretical and actual illumination position on dense array 116. The shape of the resulting flux distribution (such as flux distribution 202a) may also change throughout the day, owing to the evolving angle of incidence of sunlight. Furthermore, an alignment target is typically located below receiver 108, so the illumination position and the flux distribution on receiver 108 are different from those on the alignment target.

Thus, according to the present invention, compensation is made for this difference between expected and actual illumination position of an individual heliostat 106 (and between expected and actual flux distribution of that individual heliostat) .

This may be done based on calculated or measured values of the difference between the expected and actual illumination positions and of the expected and actual flux distributions. These values may be calculated by ray tracing techniques using accurate determinations of the geometry and orientation of the heliostat, taking into account the sun's precise apparent position as a function of date and time.

These values of the difference between the expected and actual illumination positions and of the expected and actual flux distributions may be based on measured values of these differences. Measurements of these differences may be made over a representative period (typically a year) at regular intervals (such as daily) ; the measuredvalues can then be used over subsequent periods. Alternatively, a plurality of measurements of these differences may be made and then used to characterize the performance of the heliostat (in terms of actual illumination position and actual flux distribution) under various angles of incidence of sunlight, from which future values may be deduced by, for example, interpolation.

According to another embodiment of the present invention, a HCPV power generating system is provided generally identical with system 100 of figure 1. However, according to this embodiment, the control system includes a timing synchronization module that periodically transmits a time synchronization signal to respective programmable logic controllers (PLCs) (not shown) that are typically located logically in or between control system 100 and the respective encoders of the heliostats (either with one PLC per heliostat and hence encoder, or one PLC per group of heliostats and hence encoders) , so that the real time clocks of the PLCs can be synchronized. This can be particularly advantageous where each PCL is programmed with a heliostat movement pattern that is to be executed_? in such cases, the PLCs need an accurate and substantially identical notion of time in order to execute the movement pattern at the correct time,

As will be appreciated by the skilled person, this approach may also be employed with a solar power generating system comprising a control system and one or more individual solar energy collectors (each with a dish concentrator and photovoltaic receiver) . A PLC is typically provided for each dish or for a respective group of dishes, to control that dish or group of dishes under the control of the control system. In an installation having a large number of dishes or heliostats these may be connected to the control system 400 via a network which may, depending on the size of the installation and number of dishes or heliostats, include a number of sub-networks each connected to the control system 400 via an intermediate group controller. In such a system data transmitted between the control system and the dishes or Heliostats of a sub-network are relayed via the subnetwork group controller,

According to this embodiment, in such systems every heliostat (or dish) needs a reasonably accurate clock, so that it can accurately predict the sun's position. Hence, the real time clock of each PLC (of heliostat or dish) has to be synchronized to an external time reference source. It is convenient to have this synchronisation take place automatically via the Communications network used by control system 400- Networks connected via the internet have a standard protocol (the Network Time Protocol (NTP) or its simplified relation (SNTP)) for time synchronisation. However, NTP and SNTP are complex . because synchronising time over a communications network must normally take into account the time delays that can take place sending the data over that network. When the network is complex, these time delays are not constant, so it is not possible simply to broadcast the current time over such a network: such a message will arrive at different points at different times. For this reason, NTP and, to a lesser extent SNTP, must try to determine what the various network delays are, making these protocols difficult to implement on an industrial PLC such as might be used for heliostat (or dish) control.

NTP also offers greater accuracy than typically required in such systems. For adequate tracking, the heliostats (or dishes) need only be synchronised to within a second or two of true time. Hence, according to this embodiment, a scheme is employed in which control system 400 and any group controller for a group of heliostats or dishes periodically synchronises its clock using the version of SNTP supplied with Microsoft Windows (trade mark) , and then periodically transmits the current time to the PLCs controlling the corresponding encoder or encoders of the corresponding heliostats (or dishes) .

Even if this transmission entails broadcasting the current time to a group of PLCs, this will typically constitute only a local broadcast as each group controller usually broadcasts to no more than 40 PLCs, all of which are connected on the same network segment. Hence, the time delays are short and relatively uniform. The error in time at each encoder is thus expected to be only a few milliseconds ♦

Figure 6 is a schematic view of the control system 600 of this embodiment . Control system 600 is generally identical with control system 400 of system 100 (see figure 4) , and like reference numerals have been used to identify like elements. In addition, however, control system 600 includes a host computer 602 with timing synchronization module 604. Timing synchronization module 604 comprises a synchronisation application that is adapted to run as a Windows (trade mark) service (termed the ¹SS Time Sync Service') on host computer 602, and is implemented as a UDP server installed on host computer 602 hooked on a subnet of the network with, which control system 600 communicates with the heliostats (or dishes) - The timing synchronization module is configured to provide time synchronisation only to heliostats (or dishes) that are connected on the same subnet of the network, not to any heliostats (or dishes) further afield.

When the timing synchronization module is started/ it opens the UDP port 2688 over the subnet. Through this port, the module broadcasts 606 packets containing timestarnp information collected from the host computer onto the sub-network, every 20 minutes. As a result, every node on the subnet, vi2. the PLCs, can receive the broadcast UDP packet and synchronise its own Real -Time Clock with the packed timestamp every 20 minutes.

It should be noted that, in order to provide accurate time ticking to each heliostat (or dish) on the subnet, the system clock of the host computer must be synchronised with a good time source that is external to the group subnet. This is obtained using SNTP, NTP or some other suitable external time reference.

The time is transmitted to each heliostat (or dish) in the following data packet format:

The timing synchronization module attempts to keep running when errors are detected. Error messages are logged to the Windows system error log. The message source appears in the log as "TimeSyncSrv" .

The timing synchronization module can be installed onto the Group Controller, so that all the heliostat (or dish) clocks within the group will be synchronised with the host computer's system clock.

This timing synchronization method has the advantage of reducing the amount of signalling overhead required to perform the timing synchronization. This can, in turn, reduce the required capacity of the data network that connects the heliostats and the required processing capacity of the heliostat PLCs and network server, resulting in cost reductions and reliability improvements. (Otherwise, periodic polling of each PLC and individual clock adjustment are typically required, which add significant network overhead.) Tracking is thus simplified: for example, if a heliostat (or dish) has a memory containing characterization data based on time and date for a given set of flux distribution patterns, then a simple instruction to follow a given pattern could be transmitted to the heliostat' s PLC. The heliostat' s PLC then simply operates its actuators to move the mirrors according to the present pattern and maintains synchronization using the broadcast timing signal.

It will be appreciated by the skilled person that the present invention may also be employed with a dish concentrator photovoltaic power generation system, a solar thermal power generation system or a hybrid photovoltaic- solar thermal system,, provided that such a system comprises a plurality of mirrors (whether provided as heliostats or otherwise) and a control system adapted to vary the illumination positions of at least a subset of the mirrors to provide or maintain a desired total flux distribution over all or part of the receiver (in the photovoltaic case) or solar collector (in the solar thermal case) .

For example, figure 7 is a schematic view of a solar thermal power generation system 700 according to another embodiment of the present invention. In some respects system 700 is similar to HCPV power generating system 100 of figure 1, and like reference numerals have been used to. identify like features.

Thus, system 700 includes a heliostat field 104 of heliostats ^* 106, a supporting tower 110 and a control centre 114. However, rather than a HCPV receiver system (cf. cooled HCPV receiver 108 of system 100), system 700 includes a solar thermal collector 702 supported by supporting tower 110. Solar thermal collector 702 contains a thermal fluid in the form of water that is heated by the sunlight directed by heliostats 106 against the exterior of collector 702 (as is well understood in this art) . System 700 includes a power generation station 704 for receiving steam from collector 702 and generating electrical power. This power generation station 704 may include a heat exchanger (cf.. heat exchanger 112 of system 100) .

Thus, in use light 124 from the sun 102 is reflected by each of heliostats 106 towards collector 702, which causes water in collector 702 to be converted to steam. The steam is piped to generation station 704, which outputs electricity. The light deposited by each heliostat 106 onto collector 702 can again be characterized -with an illumination position: referring to figure 8, the illumination positions 800a, 800b, 800c of three exemplary heliostats 106 are shown. The flux distribution 802a_r 802b, 802c deposited on collector 702 by heliostats 106 surround the respective illumination positions 800a, 800b, 800σ.

System 700 has a. control system essentially identical with control system 400 of HCPV power generating system 100, so that the illumination positions of some or all of heliostats 106 on collector 702 can be controlled. The control system can be used to vary the illumination positions to provide or maintain the desired total flux distribution over all or part of the collector.

Modifications within the scope of the invention may be readily effected by those skilled in the art. It is to be understood, therefore, that this invention is not limited to the particular embodiments described by way of example hereinabove .

In the claims that follow and in the preceding description of the invention, except where the context requires otherwise owing to express language or necessary implication, the word '"comprise" or variations such as ^■"comprises" or ^comprising" is used in an inclusive sense, that is, to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.

Further, any reference herein to prior art is not intended to imply that such prior art forms or formed a part of the common general knowledge in any country.

Claims

CLAIMS :

1. A photovoltaic power generating system, comprising: a receiver comprising a dense array of photovoltaic devices; a plurality of mirrors, each mirror adapted to reflect sunlight onto the receiver; and a control system for controlling the angular orientation of each of the mirrors as the sun moves so that the sunlight is reflected towards an illumination position on the receiver; wherein the control system is adapted to vary the illumination positions of at least a subset of the mirrors over time to provide or maintain a desired total flux distribution over all or part of the dense array.

2. A photovoltaic power generating system as claimed in claim 1, adapted to vary the illumination positions dynamically over the day.

3. A photovoltaic power generating system as claimed in claim 1 or claim 2, adapted to vary the illumination positions at defined time intervals.

4. A photovoltaic power generating system as claimed in any one of claims 1 to 3, wherein the illumination positions are be varied among a discrete set of alternative illumination positions, or continuously whereby the illumination positions of mirrors in the subset are adapted to be varied over a locus or region on the receiver.

5, A photovoltaic power generating system as claimed in any one of claims 1 to 4 , wherein the illumination positions of the mirrors in the subset are varied individually.

6. A photovoltaic power generating system as cJLalmec. m any one of claims 1 to 5, wherein the illumination positions of the mirrors in the subset are varied in groups by a group displacement.

7. A photovoltaic power generating system as claimed in claim 6, wherein the membership of groups is dynamically altered.

8. A photovoltaic power generating system as claimed in any one of claims 1 to 7, wherein the illumination position of the mirrors in the subset are varied such that the illumination positions of at least of the subset of mirrors may be varied at some times to be outside the area of the receiver to achieve the desired flux distribution.

9. A photovoltaic power generating system as claimed in any one of claims 1 to 8, wherein the subset is statically assigned as an appropriate subset for achieving a desired level of adjustment of the flux distribution.

10. A photovoltaic power generating system as claimed in any one of claims 1 to 9, wherein the subset includes mirrors positioned so as to be able to project an image over any part of the receiver.

11. A photovoltaic power generating system as claimed in any one of claims 1 to 10, wherein the subset includes

•mirrors close to the receiver projecting a relatively small image on the receiver.

12. A photovoltaic power generating system as claimed in any one of claims 1 to 11, wherein the subset includes mirrors far from the receiver projecting a relatively large image on the receiver.

13. A photovoltaic power generating system as claimed in any one of claims 1 to 12, wherein the dense array comprises a plurality of separate dense regions interposed with secondary optics.

14. A photovoltaic power generating system as claimed in any one of claims 1 to 13, wherein the size of the flux distribution due to each of the mirrors on the dense array is less than 50% of the area of the dense array.

15. A photovoltaic power generating system as claimed in any one of claims 1 to 14, wherein the size of the flux distribution due to each of the mirrors on the dense array is less than 25% of the area of the dense array.

16. A photovoltaic power generating system as claimed in any one of claims 1 to 15, wherein the size of the flux distribution due to each of the mirrors on the dense array is less than 15% of the area of the dense array.

17. A photovoltaic power generating system as claimed in any one of claims 1 to 16, wherein each of the mirrors comprises a small mirror or multiple small canted mirrors in fixed interrelation focussing together on the receiver.

18. . A photovoltaic power generating system as claimed in claim 17, wherein each small mirror or small canted mirror has a curved surface focusing on the receiver area to provide an image of desired size.

19. A photovoltaic power generating system as claimed in claim 1, wherein the receiver is a cooled receiver.

20. A photovoltaic power generating system as claimed in claim 19, wherein the receiver is cooled with a fluid coolant and the system comprises a thermal converter for receiving heated coolant from the. receiver and extracting energy from the heated coolant .

21. K photovoltaic power generating system as claimed in claim 1, comprising controllers for controlling the mirrors, the controllers being controlled by the control system, and a timing synchronization module for transmitting a timing signal to the controllers adapted to provide the controllers with accurate time information.

22. A photovoltaic power generating system as claimed in claim 21, wherein each of the controllers controls a respective one of the mirrors.

23. A photovoltaic power generating system as claimed in claim 21, wherein each of the controllers controls a plurality of the mirrors.

24. A photovoltaic power generating system as claimed in any one of claims 1 to 23, comprising a plurality of helioεtatε, each comprising one or more of the mirrors.-

25. A photovoltaic power generating system as claimed in any one of claims 1 to 23, comprising a concentrator dish, wherein the dish comprises the mirrors.

26. A method of controlling a photovoltaic power generating system, comprising: reflecting sunlight with a plurality of mirrors onto a receiver, the receiver comprising a dense array of photovoltaic devices; controlling the angular orientation of each of the mirrors as the sun moves so that the sunlight is reflected towards an illumination position on the receiver; and varying the illumination positions of at least a subset of the mirrors over time to provide or maintain a desired total flux distribution over all or part of the dense array.

27. . A method as claimed in claim 26, wherein the receiver is a cooled receiver.

28. A method as claimed in claim 26, comprising controlling the mirrors with controllers, and transmitting a timing signal to the controllers adapted to provide the controllers with accurate time information.

29. A method as claimed in any one of claims 26 to

28, comprising providing the mirrors as heliostats, each comprising one or 'more of the mirrors.

30. A method as claimed in any one of claims 26 to 28, comprising providing the mirrors in a concentrator dish, wherein the dish comprises the mirrors.

31. A solar thermal power generating system, comprising: a solar thermal collector for collecting sunlight; a plurality of mirrors, each mirror adapted to reflect sunlight onto the solar thermal collector and heat the thermal fluid; and a control system for controlling the angular orientation of each of the mirrors as the sun moves so that the sunlight is reflected towards an illumination position on the solar thermal collector; wherein the control system is adapted to vary the illumination positions of at least a subset of the mirrors over time to provide or maintain, a desired total flux distribution over all or part of the solar thermal collector.

32. A system as claimed in claim 31, comprising a receiver comprising an array of photovoltaic devices arranged to receive radiation re-radiated from the solar thermal collector and converting the radiation into electricity.

33. A method of controlling a solar thermal power generating system, comprising: reflecting sunlight with a plurality of mirrors onto a solar thermal collector; controlling the angular orientation of each of the mirrors as the sun moves so that the sunlight is reflected towards an illumination position on the solar thermal collector; and varying the illumination positions of at least a subset of the mirrors over time to provide or maintain a desired total flux distribution over all or part of the solar thermal collector.

34. A method as claimed in claim 33, comprising arranging a receiver comprising an array of photovoltaic devices to receive radiation re-radiated from the solar thermal collector and to convert the radiation into electricity.

35. A method as claimed in claim 33, wherein the receiver is adapted to be cooled with a coolant.

36. A method as claimed in claim 35, comprising conducting heated coolant from the receiver to a thermal converter for extracting energy from the heated coolant.

37. A solar power generating system, comprising: a plurality of mirrors, each mirror adapted to reflect sunlight onto a photovoltaic receiver or solar thermal collector; controllers for controlling the angular orientation of each of the mirrors as the sun moves so that the sunlight is reflected towards an illumination position on the receiver or collector; and a control system for controlling the controllers, comprising a timing synchronization module for transmitting a timing signal to the controllers, the signal being adapted to provide the controllers with accurate time information.

38. A method of controlling a solar power generation system, comprising: reflecting sunlight with a plurality of mirrors onto a photovoltaic receiver or solar thermal collector; controlling the angular orientation of each of the mirrors with controllers as the sun moves so that the sunlight iε reflected towards an illumination position on the receiver or collector; and transmitting a timing signal to the controllers, the signal being adapted to provide the controllers with accurate time information.

39. A solar power generating system, comprising: a plurality of solar power generating units, each comprising: a receiver comprising a dense array of photovoltaic devices; and a concentrator dish adapted to reflect sunlight onto the respective receiver; controllers for controlling the angular orientation of the dishes as the sun moves so that the sunlight is reflected by each of the respective dishes towards the respective receiver? and a control system for controlling the controllers, comprising a timing synchronization module for transmitting a timing signal to the controllers, the Signal being adapted to provide the controllers with accurate time information.

40. A method of controlling a solar power generation system, comprising: reflecting sunlight with a plurality of concentrator dishes onto a plurality of respective photovoltaic receivers; controlling the angular orientation of each of the dishes with controllers as the sun moves so that the sunlight is reflected by each of the respective dishes towards the respective receiver; and transmitting a timing signal to the controllers, the εsignal being adapted to provide the controllers with accurate time information.

41. A method of producing electricity comprising operating the power generating system of any one of claims 1 to 25, 31, 32, 37 and 39.

42. An electrical product comprising a quantity of electrical power produced by the method of claim 41.