WO2009132394A1

WO2009132394A1 - Solar tracking apparatus and method

Info

Publication number: WO2009132394A1
Application number: PCT/AU2009/000541
Authority: WO
Inventors: Anthony John Howard
Original assignee: Kent Holdings International Limited
Priority date: 2008-04-30
Filing date: 2009-04-30
Publication date: 2009-11-05
Also published as: GB201013480D0; GB2470849B; GB2470849A; ZA201005937B; GB201014061D0; WO2009132393A1; GB2469963B; GB2469963A

Abstract

The invention provides apparatus (150, 200, 220) and associated methods for controlling sun tracking of solar energy collectors and the like. In a preferred form, multiple radiation sensors (160, 202, 222) are used, and these are kept partly in shadow even when the apparatus is aimed directly at the sun. Their electrical output is monitored and used to determine whether and what reorientation is necessary. By measuring changes in the sensor outputs as the degree of shading varies it is possible to reduce or eliminate a "dead zone" in which the apparatus is unresponsive to misorientation. Accurate tracking is thus achievable.

Description

SOLAR TRACKING APPARATUS AND METHOD

Field of the Invention

The present invention relates to apparatus and associated methods for aiming of solar collectors at the sun. The invention further relates to solar energy generating systems and methods in which such apparatus and methods are embodied.

Background to the Invention

The desire to decrease and ultimately eliminate dependence on fossil fuels has stimulated research into clean and renewable ways to produce electricity, as an alternative to the burning of fossil fuels. Solar power has the attraction that it is a clean form of energy production and there is a potentially limitless supply of solar radiation. It is estimated that the solar energy flux from the sun is approximately 2.7 megawatt-hours per square meter per year in certain parts of the world. With this tremendous amount of free and clean energy available, there is a strong demand for more cost-effective ways of harnessing it.

Most systems for generating electric power from solar radiation fall into two categories. Solar photovoltaic systems involve direct generation of electricity by arrays of photovoltaic (PV) cells and solar thermal systems involve the heating of fluid in a conduit, the fluid then operating a turbine or the like which in turn powers an electric generator to produce electricity. It is also known to apply solar radiation directly to heat fluids without the prime intention of generating electricity, for example as an adjunct to electric or fossil-fuel-fired domestic water heaters.

Both solar photovoltaic systems and those involving heating of fluids may be of tracking or fixed type. Where only small quantities of electricity are to be generated by a solar photovoltaic system, for example in remote-area telecommunications installations, remote sensing or cathodic protection of pipelines, an array of solar cells in the form of a fiat panel may be used, the panel being fixed in a direction and at a latitude-dependent inclination, such that the solar radiation falls approximately perpendicularly onto the panel at the time of day when solar radiation is at its greatest intensity. This is not ideal for much of the day, as movement of the sun causes the radiation to fall obliquely on the panel, reducing output, but the cost and complexity of tracking arrangements designed to make the panel remain perpendicular to a line between the panel and the sun for all or a large part of the day, are often not justified. Provision may be made for occasional manual adjustment of the panel's inclination according to the season. Similarly, in domestic hot water systems, tracking of the sun is often found unnecessary and unjustified on cost grounds.

In solar thermal systems for electric power generation however, the higher the temperature to which the working fluid is heated, the higher the potential efficiency of energy conversion. Accordingly, concentrating means such as mirrors or lenses may be provided to intercept radiation over a comparatively large area and concentrate it onto a comparatively small fluid conduit. Maintaining power output and efficiency for large parts of the day requires the concentrating means to be aligned with a line from the concentrating means to the sun, i.e. to track the sun, so that the fluid conduit always receives the largest possible radiation intensity.

In solar photovoltaic systems, there is increasing use of solar cells that operate best with radiation intensities exceeding the natural intensity of incident solar radiation. Optical concentration means, such as lenses or reflecting mirrors, are used in such systems to achieve higher radiation intensity on the cell surface. Cells of this type in general have small surface areas onto which the radiation must be directed, so that accurate tracking is needed to ensure that the radiation is concentrated precisely on the radiation-sensitive surface for as much of the day as possible. Further, cooling means (such as a heat exchanger through which a fluid flows) may be provided directly behind the radiation sensitive semiconductor wafers of such cells to ensure that they are not overheated and accurate tracking is essential to ensure that the concentrated radiation falls where its thermal heating effect can be offset by the cooling means to avoid damage or destruction of the cell.

By way of example of a tracking device, US Patent No 4302710 (Menser, 1981) describes a tracking system useful for solar collectors of the type in which a focusing mirror is mounted on a base and rotatable about both vertical and horizontal axes for tracking of the sun. The disclosed device is to be arranged to move with the mirror. It has four radiation detecting devices arranged adjacent to a shadow-throwing formation, each device being arranged to operate a relay when the device is sufficiently far into the shadow. The relays in turn are arranged to actuate motor drives to rotate the collector about its axes. This arrangement is believed to have the difficulty that only a limited degree of tracking precision can be achieved.

US Patent No 4187123 (Diggs, 1980) discloses a solar collector in which an array of individual photovoltaic collector modules is mounted to a frame with each module being rotatable about two axes for sun tracking. Groups of modules are ganged together so as to move in unison. Arranged to move with the modules are tracking devices in each of which thermocouples are suitably arrayed near a shadow throwing body so that if the devices are not aligned to face the sun differences in outputs from the thermocouples are detected and arranged to operate a drive system and rotate the tracking devices and the collector modules. The Diggs' tracking device is also believed to lack precision of tracking and may be slow to respond to tracking error.

What is needed are more accurate and reliable systems for tracking of solar collectors, and the present invention addresses this need.

Summary of the Invention

According to the invention there is provided, in a first aspect, a sun tracking apparatus for use in controlling drive means of a solar collector or other solar device intended to be kept aimed at the sun by said drive means and comprising: a sensor adapted to provide an electrical signal when a proportion of said sensor is exposed to direct solar radiation said signal being variable with said proportion; and a formation arranged to cast a shadow on said sensor that varies said proportion of said sensor when said sun tracking apparatus deviates in a specified direction from direct aiming at the sun.

Preferably, said sensor is partially in shadow cast by said formation when said sun tracking apparatus is aimed directly at the sun, whereby any deviation in said specified direction from direct aiming at the sun varies said output signal of said sensor, hi this way, it is believed possible to eliminate or substantially avoid any "dead zone" in which the sun tracking apparatus does not respond to deviations from perfect aim at the sun.

In a particularly preferred embodiment, said sensor is one of a plurality of such sensors each associated with a said formation and a said specified direction, whereby deviations of said sun tracking apparatus from direct aiming at the sun in a plurality of directions are detectable by said sun tracking apparatus. This enables control of orientation about two separate axes, for example, it is possible to control the azimuth (direction faced) and elevation of a solar collector.

Conveniently although not essentially, one shadow casting means may comprise each said formation.

In one such arrangement, said one shadow casting means comprises an elongate member having a free end that when the apparatus is aimed at the sun is faces the sun and wherein said sensors are positioned remotely from said free end.

Said sensors may be partially received in a groove extending peripherally around said elongate member.

In another arrangement, said one shadow casting means comprises a cover means that partially contains said sensors. Preferably, the said specified direction associated with each sensor differs from the specified direction of the other sensors.

It is particularly preferred that the specified directions are spaced apart around an arc of approximately 360 degrees.

Preferably, the sun tracking apparatus includes means for reading the sensor output signals and determining therefrom a suitable direction for reorientation of said sun tracking apparatus so as to more closely aim said sun tracking apparatus at the sun.

Various types of sensors may be used. Suitably, a sensor may comprise a light emitting diode (LED). It has been found that although normally used in light emitting applications, such devices can also act as useful sensors for radiation. Preferred characteristics are mentioned herein. Where reliance is placed on a varying (i.e. analogue as opposed to off-on) output from a sensor used in the invention, the use of sensors that are normally thought of as "off-on" types is not necessarily precluded. For example, many diodes conduct to a degree that varies smoothly with applied voltage over a specific range.

In a further aspect, there is provided a sun tracking apparatus for use in controlling drive means of a solar collector or other solar device intended to be kept aimed at the sun by said drive means and comprising: a plurality of sensors each adapted to provide an electrical signal in response to solar radiation impinging thereon; at least one shadow casting means so arranged that when said sun tracking apparatus deviates in a specified direction from direct aiming at the sun at least one sensor experiences a change in the degree to which it is in a shadow of said shadow casting means; and means for monitoring output signals of said sensors and determining from changes thereto a need for and a preferred direction of a reorientation of said sun tracking apparatus.

Preferably, outputs of or derived from said sensors are measured periodically under control of a programmed computing means. This may be a microcontroller or microprocessor.

Said programmed computing means is preferably programmed to determine from changes to data from such periodic measuring said need for and said preferred direction of reorientation of said sun tracking apparatus.

Preferably, means are provided for connection of said programmed computing means to a data bus whereby said programmed computing means can communicate with and preferably also perform operations in support of other components of a solar energy collecting means. Such operations may for example include computing functions associated with control and management of PV cell-based solar collector modules. Further the programmed computing means may provide a bus master facility for the said data bus.

In a further aspect, the invention provides a method for controlling sun tracking of a solar collector or other solar device intended to be kept aimed at the sun the method including the steps of: periodically monitoring outputs of a plurality of sensors comprised in a sun tracking apparatus each sensor arranged to respond to a deviation in a specified direction of said sun tracking apparatus from direct aiming at the sun; and determining from results of such monitoring whether there is a need for a reorientation of the sun tracking means and a solar collector or other solar device under tracking control of the sun tracking means and a preferred direction for such reorientation.

Preferably, the said specified directions are spaced apart around an arc of approximately 360 degrees.

Preferably, the sensors provide analogue outputs according to the degree of deviation from correct aiming in the sensors' specified directions and the step of determining a preferred direction of reorientation includes assessing relative magnitudes of deviations in a plurality of said specified directions.

The preferred direction of reorientation may be permitted to be either one of the specified directions or a direction intermediate between two of the specified directions and inferred using measurements made using a plurality of the sensors. Apart from providing more possible preferred reorientation directions than the number of sensors, i.e. better directional resolution, this approach assists in reducing the possible influence of differences among the sensors.

The method preferably includes a step of applying a specific criterion to a plurality of the sensors' specified directions to determine whether a single preferred direction of reorientation can be determined and if not then aborting the determination of that direction and substituting an estimate of a preferred reorientation obtained by a separate method. This is to allow for the problem of days where clouds or overcast make precise tracking difficult or impossible. Preferred alternative methods for making estimates are discussed below.

Generally, the invention includes in its scope a solar energy collecting apparatus comprising a sun tracking apparatus according to any one of the forms disclosed herein. Particularly preferred is solar energy collecting apparatus in which the sun tracking apparatus is connected via a data bus to tracking drives and preferably also to other components such as, for example, output management circuitry for individual photovoltaic cells. Further aspects and preferred features of the invention are set out in the remainder of this specification, are believed inventive in themselves.

Brief Description of the Drawings

The invention "will now be described in detail, by reference to the attached Figures.

Figure 1 is a perspective view of a solar power generating apparatus including a tracking device according to the invention;

Figure 2 is a schematic view of a first form of sun tracking device according to the invention; Figure 3 is a schematic view of a second form of sun tracking device according to the invention; Figure 4 is a schematic view of a third form of sun tracking device according to the invention; Figure 5 is an isometric view of a fourth form of sun tracking device according to the invention; Figure 6 is a perspective view of a further sun tracking device according to the invention; Figure 7 is a perspective view of the sun tracking device shown in Figure 6, now inverted;

Figure 8 is an exploded perspective view from above of a further sun tracking device according to the invention;

Figure 9 is an exploded perspective view from below of the tracking device shown in Figure 8;

Figure 10 is a schematic diagram of a management system for a solar power system based on a tracking device according to the invention;

Figure 11 is a schematic plan view of an array of radiation sensors as used in the tracking device shown in Figure 8.

Description of the Preferred Embodiment

Figure 1 shows a solar power generating apparatus 50. Apparatus 50 comprises a module 52 in which a Fresnel-type lens 54 is arranged to concentrate solar radiation on a radiation-sensitive surface of a photovoltaic (PV) solar cell (behind lens 54 and not visible) at or near the focus of the lens. Apparatus 50 works best when the plane of lens 54 is always kept perpendicular to the direction of incident solar radiation. This is to maximize the radiation intercepted and concentrated by the lens 54 and to ensure that the concentrated radiation is aimed accurately at the radiation sensitive surface of the PV cell, thus avoiding the potential for damage to surrounding components. Although not shown, the PV Cell is fluid-cooled, using a heat exchanger located behind it.

Since the sun appears to move in the heavens during the day, this orientation requirement means that module 52 must assume varying orientations through the day. That is, it must "track" the sun. To enable the necessary movement of module 50, it is gimbaled, i.e. mounted on a cradle 56 that can pivot on trunnions 58 about an axis 60, and trunnions 58 are themselves comprised in a second cradle 62 that can pivot about an axis 64 of a base frame 66. Axes 60 and 64 are perpendicular to each other and one will generally be oriented east- west. Motor drives 68 and 70 are provided to pivot cradles 56 and 62 respectively and are controlled by a tracking device 72 secured to module 52. Tracking device 72 provides electrical output signals that are used to operate drives 68 and 70 when it is not pointing directly at the sun, so as to correct any misalignment of module 52. Improvements in tracking devices such as device 72 are the subject of the present invention.

Other forms of solar power and heat generating apparatus also benefit from tracking of the sun and can utilize tracking devices according to the invention. Two possible types are shown in the US Patents of Menser and Diggs mentioned above, for example.

The basic mode of operation of tracking devices according to the invention in sunlight sufficient for objects to cast shadows will be described firstly by reference to devices adapted for rotating about one axis only. In general, and as shown by apparatus 50, rotation about two axes is required.

Figure 2 shows schematically a portion of a solar tracking device 100 adapted for use in maintaining a surface 102 of device 100 perpendicular to rays from the sun such as ray 104, by controlling rotation about an axis 106 of apparatus (not shown) to which device 100 is secured and with which it moves. Axis 106 extends in a direction perpendicular to the page. Extending from surface 102 is a wall 108 whose length also extends perpendicular to the page. Secured to surface 102 and closely adjacent to wall 108 are radiation sensors 110 and 112. These produce electrical output signals that vary according to the relative proportions of their respective radiation-sensitive surfaces 118, 120 that are in shade and exposed to the sun.

Surface 102 is shown as not perpendicular to ray 104, so that clockwise rotation about axis 106 is required. Since wall 108 casts a shadow on radiation-sensitive upper surface 118 of sensor 110, that sensor's output is less than the output from sensor 112. If the two signals are compared by some suitable means (for example a voltage comparator, not shown) a difference signal can be obtained whose polarity will indicate the direction (clockwise or anticlockwise) of rotation required to reduce misalignment of device 100. Such a signal can operate rotating means such as an electric motor drive (not shown). When the device 100 is correctly aligned, the outputs of sensors 110 and 112 are equal so that there is no longer a difference signal and rotation about axis 106 stops. Using this principle, an apparatus comprising device 100 can be made to rotate about one axis 106 as required to track apparent movement of the sun about that axis.

A drawback of the arrangement shown in Figure 2 is that practical radiation sensors such as may be used for sensors 110, 112 generally have radiation-sensitive areas that do not extend fully to a side edge or face of the sensor, so that a certain minimum angle of deviation from correct alignment must exist before a control signal can be generated. It is desirable to minimize such a "dead zone".

Figure 3 shows how this drawback can be overcome. The arrangement shown in Figure 3, except for one change, is generally the same as that in Figure 2 and elements having the same nature and function as corresponding elements in Figure 2 have the same item numbers except for a suffix "a". Wall 108a has grooves 116 adjacent to surface 102a, and sensors HOa and 112a are partially received in grooves 116. If rays such as ray 104a are perpendicular to surface 102a, each of sensors HOa and 112a is partially exposed to radiation and partially shaded. Any deviation from this correct alignment, however small, produces a difference in the output signals, so that a control signal can be generated. The "dead zone" referred to above can thus be eliminated.

Figure 4 shows one way in which the arrangement of Figure 3 can be changed to improve sensitivity. Elements having the same nature and function as corresponding elements in Figure 3 have the same item numbers except for a suffix "b" instead of suffix "a". Wall 108b extends further from surface 102b than wall 108a from surface 102a. Ray 104b is shown shading surface 118b to the same extent that ray 104a shades surface 118a. However, it will be seen that ray 104b is closer to being perpendicular to surface 102b than ray 104a is to being perpendicular to surface 102a. Thus, the arrangement in Figure 4 is more sensitive to small deviations from correct tracking than that in Figure 3.

Tracking devices 100, 100a and 100b can control a device which requires rotation about only one axis, such as axis 106, 106a or 106b. Although these single-axis devices have been used by way of example to explain concepts used in the present invention, they can also be used in practice where only single-axis tracking is required and to do so is within the scope of the invention. For example, apparatus 50 could be made with single-axis tracking devices on each of cradles 56 and 62.

However, the same operating principle can be extended to the control by one tracking device of apparatus whose tracking requires rotation about two axes, for example focusing mirror type collectors having motor drives to orient the mirror both in azimuth (i.e. about a vertical axis) and elevation above the horizontal. This will now be described.

Figure 5 is an isometric view of a tracking device 150 comprising a flat plate 152 from a surface 154 of which protrudes an elongate post 158 (which could alternatively be called a "stile" or "style") of circular cross-sectional shape. Longitudinal axis 156 of post 158 is perpendicular to surface 154 and is intended to point to the sun in use of tracking device 150. Secured on surface 154 is an array of identical radiation sensors 160. Devices 160 are arrayed in a ring around post 158 and are circumferentially equi-spaced and all at the same radius from axis 156. Post 158 has a peripheral groove 162 adjacent to surface 154 and sensors 160 although extending radially beyond post 158 are partially received in groove 162 so that if axis 156 points directly at the sun, each sensor 160 is partially shaded within the groove 162 and partially exposed to direct sunlight. The function of groove 162 is the same as that of groove 116, namely to enable detection of even very small deviations from the direction of the sun. Eight (8) sensors 160 (of which six are visible) are used in device 150, but any physically practicable number of sensors from three (3) upward may be used. In a most preferred embodiment, twelve (12) sensors 160 are used.

Sensors 160 are shown as having rectangular shapes, but such a shape is not essential. AU sensors 160 are oriented in the same way relative to the tangential direction of their ring-shaped array.

If the direction of axis 156 deviates from a direction that points directly to the sun, some of sensors 160 will be shaded to a greater extent and others to a lesser extent, and due to the circular ring array of sensors 160, this will be so irrespective of the direction of deviation. Thus, output signals from the sensors 160 in combination contain enough information to allow the most direct direction of re-orientation to be determined. For example, the directions and necessary relative proportions of movement about two perpendicular axes such as 164 and 166 can be determined.

Various types of solar radiation detecting devices may be used for components 160, 110 (including HOa and 110b) and 112 (including 112a and 112b). Suitable ones include light emitting diodes (LEDs), photo-diodes, solar cells, photo-transistors, camera sensors, light- dependent resistors, although this list is not intended to preclude the use of other sensors found to be suitable. These sensors may be of types used primarily for detection of radiation in any part of the visible, infra-red or ultraviolet parts of the spectrum, but sensitivity to the green part of the visible spectrum is preferred for accuracy and sensitivity and for alternate use as a method of visual feedback of tracker operation. LEDs are intended for the generation of light, but also can develop electrical power usable for instrumentation purposes when radiation falls upon their diode chip surface. It has been found in testing of a range of commercially available LEDs that their performance in terms of output voltage and power in use to detect solar radiation was correlated with both the wavelength and intensity of light produced in normal (light-emitting) operation. Noting that solar radiation is not uniformly spread across the visible spectrum, LEDs with a characteristic light wavelength in the range from about 570nm to about 590nm were found to be satisfactory and are preferred.

LEDs are commercially available in various packages and as unpackaged semiconductor chips. It is important to note that references herein to partial shading of radiation detecting devices mean, in the case of LEDs and other sensors based on semiconductors, partial shading of the actual semiconductor component.

Figures 6 and 7 show a practical tracking device 200 that is in substance of the type shown in Figure 5, although twelve (12) LED-type radiation sensors 202 are used. Corresponding to plate 152 of tracking device 150 is a printed circuit board (PCB) 204 to which components of the tracking device's signal conditioning and processing, drive control, communications and power management circuitry are mounted. The handling of the LED signals is described below. Also mounted to PCB 204 is a post 206 equivalent to post 158 of tracking device 150. One of the components mounted to the PCB 204 is a capacitor 207 for storing energy to operate the tracking device 200 in a low-power mode overnight, and particularly to enable it to detect morning light without additional power input and initiate normal operation of the tracking device 200 itself and other components of the system being controlled by the tracking device 200, as described below. A rechargeable battery (not shown) may be used as an alternative to the capacitor.

Figures 8 and 9 show, in exploded form, a yet further tracking device 220 according to the invention. This differs from tracking device 200 in the way its radiation sensors 222 are mounted and partially shaded, but is functionally equivalent to device 200. A PCB 224 has mounted on it components of the device's signal conditioning and processing, drive control, communications and power management circuitry and radiation sensors 222, which may be LEDs of the type having the actual semiconductor chip in a cylindrical external plastics package. A hat-shaped flanged cover 226 is in use secured to PCB 224, and has openings 228 equi-spaced around its periphery in which sensors 222 are received. Cover 226 partially shades the sensors 222 and generates shadows over their exposed parts when the cover's upper peripheral edge 230 is not in a plane perpendicular to the direction of the sun (not shown), and is therefore functionally equivalent to the post 206 of tracking device 200. An advantage of the use of cover 226 is that its openings 228 can be made close fitting on the sensors 222₅ so that they are held in precisely controlled positions, which is desirable for ease of assembly and uniformity of response among the sensors 222. (By comparison, sensor 200 requires that sensors 202 be precisely positioned when being secured in place.) Cover 226 also provides protection for components mounted on one side of the PCB 224.

Note that the shadow generating means (post 206 in tracking device 200, cover 226 in tracking device 220) need not be a single formation or assembly. For example, the shading function of cover 226 in tracking device 220 could in principle be performed by separate covers on the individual sensors 222 or even by applying an opaque coating to the radially inwardly-facing surfaces and upper surfaces of the packages of sensors 222.

Means and methods by which a tracking device according to the invention can be used in a solar power generating system will now be described. Tracking device 150 will be used purely as an example, with its radiation sensors 160 being assumed to be LEDs.

Broadly, in use, outputs from the LEDs 160 are regularly polled under the control of a programmed computing means such as (in preferred embodiments) a microcontroller which in turn determines the need for and preferred direction of any movement of the tracking device to align it with the sun, and sends drive signals to driving means (for example, electric motor drives) to effect that movement, hi a typical application, the driving means will be arranged to move a solar collector apparatus to which the tracking device 150 is secured, for example, in azimuth and elevation.

Figure 10 shows a preferred approach to realization of this approach. There is shown generally at 250 a management system in which tracking device 150 comprises the microcontroller 252 and acts as bus master for a data bus 254 which in turn enables communication between the tracking device 150 and other parts of a solar power generating system. These include drives 256 for tracking the collector(s) and conveniently may include systems 257 incorporated in individual collector(s) 258 for management of power, PV cell cooling or the like. Using a single microcontroller or microprocessor in an arrangement such as that of Figure 10 can help contain overall system costs.

The application of outputs from sensors 160 to determination of tracking drive operation will now be described.

Output signals from the sensors 160, preferably first buffered and filtered to limit electrical noise, are polled by an analogue to digital converter (ADC) under control of the microcontroller 252. The signals, reflecting differences in shading of the various sensors 160 are then compared to estimate the direction in which any tracking movement needs to be made. Because each sensor 160 is always at least partially in shadow, there is no minimum angle that has to be detected as in prior art systems. The limitation on precision will generally be noise and measurement resolution. It is not strictly necessary that the sensors be in a circular ring arrangement, but doing so simplifies the computing code and is preferred.

It is found that with the partial coverage of sensors 160 by shadow, the tracking device 150 is sensitive to differences amongst them caused for example by manufacturing tolerances. However, by using more than the minimum number of sensors and by employing certain methods set out below in the shadow detection algorithm in the microcontroller 252 this error can be limited.

For 2-axis control, a tracking device according to the invention must have at least three sensors although as previously stated more are preferred, device 150 having twelve sensors 160. The number of sensors 160 and the algorithm used to monitor them determines the precision with which a preferred direction of movement can be estimated. Generally, the more precisely this direction is known the less power is required during the day to accurately follow the sun, as less total movement is required. If only three sensors 160 are used, only three or six possible movement directions can be determined, depending on whether it is possible for two out of the three sensors 160 to be obscured concurrently. This means that there is a significant chance that at least the initial indicated direction of movement will not be a direct one, potentially leading to slow and inaccurate responses to tracking errors, wasted energy in the drive means, some sacrifice of power generated and in concentrating PV systems even thermal damage to components. A larger number of sensors 160 allows for more possible movement directions, hence lesser losses from these effects.

A suitable algorithm for handling sensor 160 outputs can be summarized as follows:

Step 1. Regularly poll the (preferably buffered and filtered) output signal of each sensor 160 and place in a rolling buffer containing previous values, dropping the oldest value. Average the values in the buffer. In this way, measurement error due to electrical, optical and ADC noise can be reduced, at a cost of the system taking longer to react to rapid changes in light level. Using a faster sample rate of the sensors however can offset this increased reaction delay.

Step 2. (This step applies if more than the minimum number of sensors 160 is used.) For each member of a set of circumferentially spaced apart positions including the sensor positions, and if applicable for intermediate positions between the sensors, derive a smoothed signal value estimate from the results of step 1. Do this by taking the mean of the signal values from step 1 for members of a group of sensors at and/or clustered around each such position. This step is further explained below.

Step 3. Detect which of the positions referred to in Step 2 are in shadow. The maximum and minimum sensor levels for the ring are found, and used to normalize all the sensors in the ring. This allows the system to account for varying absolute levels due to local conditions. An arbitrary threshold between the minimum and maximum levels is chosen, preferably half way between them, and any level below this threshold is taken to be in shadow, and any sensor level above it taken to be in light. If there is not enough difference between the minimum and maximum levels, or if no sensors are found to be in shadow, then the shadow detection routine is aborted as there is no shadow to detect any misalignment.

Step 4. Check all positions (of the type mentioned in steps 2 and 3 to determine whether there is one continuous arc of such positions in shadow, with the rest of the ring in light. If there are discontinuities in the shadow or lit sections, it is taken as multiple shadows being detected, with no single light source to track, and so abort this algorithm without returning a movement-directing signal.

Step 5. Find the angle in the centre of any detected arc of shadow, as the angle required to be moved in to correct the alignment error. This may be at a sensor position or at an intermediate position estimated using Step 2.

Further explanation of Step 2 will now be provided, by reference to Figure 11, which shows schematically a ring-shaped array of twelve radiation sensors 300 such as for example sensors 202 or 222. The sensors 300 are labeled "A" to "L". To reduce any effects of variations between sensors 300 in subsequent steps, the measured signal value for each sensor 300 can be replaced by a value obtained by averaging the values derived in Step 1 for a chosen group of sensors including the one in question and neighbouring sensors. Such a group should not include sensors spaced around more than 180 degrees of arc. For example, the "smoothed" signal estimate for sensor "A", corresponding to radial direction 302 could be based on the average of measured values for any of the following groups of sensors:

A, L, B;

K, L₅ A, B, C; and

J, K, L, A, B, C, D.

Further, it is possible with this process to arrive at estimates for radial directions between pairs of adjacent sensors 300, thereby doubling the number of directions in which movement can be commanded. For example, radial direction 304 is halfway between radial directions 302 and 306 and can be estimated by averaging measured vales for any of the following groups of sensors:

A₅ B;

L₅ A, B₅ C; and

K₅ L, A₅ B₅ C₅ D.

Thus, a total of 24 possible directions for movement in response to a tracking error can be arrived at using twelve sensors 300. This is believed to give better directional control than for example the comparatively coarse control obtainable with the devices disclosed in US Patent No 4302710 of Menser.

In trials using the above described physical arrangements and processing algorithm, total misalignment angles as low as ±0.06 degrees over the period of a full sunlit day have been achieved in continuous tracking, with an average short term jitter (over a one minute window) in the range ±0.013 degrees.

It will be appreciated that the algorithm above may be modified if required. For example, the quantity calculated in Step 1 is a simple moving average and other methods known in the field of time series analysis may be substituted. For further example, the actual effect of a movement commanded by the tracking device can be seen in subsequent changes to the sensor signals, and it is possible to check periodically from these that the command did lead to an improvement, i.e. reduced shading of the most shaded sensor/s and if necessary adjust the direction command. This approach may also be quite robust against variations in performance among the sensors. For still further example, a weighted average rather than a simple mean could be used in Step 2.

The tracking devices (including 150) and the methods described above are only effective when direct sun is available to cast a reasonably well defined shadow, and provision is needed for situations where the sun is wholly or partially obscured (for example by cloud/s. What is required to limit wastage of power generating capacity is to ensure that during periods of direct sun absence appropriate tracking movements occur so the tracking device is already close to optimum alignment when direct sunlight returns.

Two preferred methods for providing such tracking will be described, which can be used together or separately in the absence of direct sunlight.

The first method requires provision of a real time clock (or means to acquire a real time signal). Preferably, a microcontroller (such as 252) is used with an inbuilt real time clock. Using the clock, a theoretical sun path can be calculated based on the time of day, and for best accuracy the longitude, latitude and physical angle the mechanism is mounted on. This calculated path can then be used in the absence of visible sun. Having a time signal available has the added advantage of allowing for other scheduled events, such as a return to home position during the night. Including any data beyond the time of day increases setup complexity when the unit is installed, but a rough path can be calculated simply from the time of day.

In the second method, positional feedback from the drive mechanism is recorded at intervals during the day and stored in memory, for example based on an absolute position from encoder/s, or based on the number of steps taken by stepper motor/s. In the event of no sun being visible, the saved data from earlier in the day and/or previous day/s can be used to calculate an estimated future path requirement, either by simply moving to the saved point closest to the current time, or preferably by calculating a more accurate position between saved points either linearly or more preferably by calculating an elliptical regression line that matches the saved points. It has been found that quite small samples of data can suffice to obtain a useful result.

It is to be recognized that various alterations and equivalent forms may be provided without departing from the spirit and scope of the present invention. This includes modifications within the scope of the appended claims along with all modifications, alternative constructions and equivalents.

In this specification, including in the appended claims, where the terms "comprise", "comprises", comprised", "comprising", "including" or "having" are used, they are to be interpreted as specifying the presence of the stated features, integers, steps or components referred to, but not to preclude the presence or addition of one or more other feature, integer, step, component or group thereof.

Background material on the art as discussed in the present specification is intended to explain the context of the present invention, and is not to be taken as an admission that the material forms part of the prior art base or relevant general knowledge in any particular country or region.

Claims

The Claims defining the Invention are as follows:

1. A sun tracking apparatus for use in controlling drive means of a solar collector or other solar device intended to be kept aimed at the sun by said drive means and comprising: a sensor adapted to provide an electrical signal when a proportion of said sensor is exposed to direct solar radiation said signal being variable with said proportion; and a formation arranged to cast a shadow on said sensor that varies said proportion of said sensor when said sun tracking apparatus deviates in a specified direction from direct aiming at the sun.

2. A sun tracking apparatus according to claim 1, wherein said sensor is partially in shadow cast by said formation when said sun tracking apparatus is aimed directly at the sun, whereby any deviation in said specified direction from direct aiming at the sun varies said output signal of said sensor.

3. A sun tracking apparatus according to claim 1 or claim 2, wherein said sensor is one of a plurality of such sensors each associated with a said formation and a said specified direction, whereby deviations of said sun tracking apparatus from direct aiming at the sun in a plurality of directions are detectable by said sun tracking apparatus.

4. A sun tracking apparatus according to claim 3, wherein one shadow casting means comprises each said formation.

5. A sun tracking apparatus according to claim 4, wherein said one shadow casting means comprises an elongate member having a free end that when the apparatus is aimed at the sun is faces the sun and wherein said sensors are positioned remotely from said free end.

6. A sun tracking apparatus according to claim 5, wherein said sensors are partially received in a groove extending peripherally around said elongate member.

7. A sun tracking apparatus according to claim 4, wherein said one shadow casting means comprises a cover means that partially contains said sensors.

8. A sun tracking apparatus according to any one of claims 3 to 7, wherein the said specified direction associated with each sensor differs from the specified direction of the other sensors.

9. A sun tracking apparatus according to claim 8, wherein the specified directions are spaced apart around an arc of approximately 360 degrees.

10. A sun tracking apparatus according to any one of claims 3 to 9, having means for reading the sensor output signals and determining therefrom a suitable direction for reorientation of said sun tracking apparatus so as to more closely aim said sun tracking apparatus at the sun.

11. A sun tracking apparatus according to any one of claims 1 to 10, wherein a sensor comprises a light emitting diode (LED).

12. A sun tracking apparatus for use in controlling drive means of a solar collector or other solar device intended to be kept aimed at the sun by said drive means and comprising: a plurality of sensors each adapted to provide an electrical signal in response to solar radiation impinging thereon; at least one shadow casting means so arranged that when said sun tracking apparatus deviates in a specified direction from direct aiming at the sun at least one sensor experiences a change in the degree to which it is in a shadow of said shadow casting means; and means for monitoring output signals of said sensors and determining from changes thereto a need for and a preferred direction of a reorientation of said sun tracking apparatus.

13. A sun tracking apparatus according to claim 12, wherein outputs of or derived from said sensors are measured periodically under control of a programmed computing means.

14. A sun tracking apparatus according to claim 13, wherein said programmed computing means is programmed to determine from changes to data from such periodic measuring said need for and said preferred direction of reorientation of said sun tracking apparatus.

15. A sun tracking apparatus according to claim 13 or claim 14, comprising means for connection of said programmed computing means to a data bus whereby said programmed computing means can communicate with and preferably also perform operations in support of other components of a solar energy collecting means.

16. A method for controlling sun tracking of a solar collector or other solar device intended to be kept aimed at the sun the method including the steps of: periodically monitoring outputs of a plurality of sensors comprised in a sun tracking apparatus each sensor arranged to respond to a deviation in a specified direction of said sun tracking apparatus from direct aiming at the sun; and determining from results of such monitoring whether there is a need for a reorientation of the sun tracking means and a solar collector or other solar device under tracking control of the sun tracking means and a preferred direction for such reorientation.

17. A method according to claim 16, wherein the said specified directions are spaced apart around an arc of approximately 360 degrees.

18. A method according to claim 16 or claim 17, wherein the sensors provide analogue outputs according to the degree of deviation from correct aiming in the sensors' specified directions and wherein the step of determining a preferred direction of reorientation includes assessing relative magnitudes of deviations in a plurality of said specified directions.

19. A method according to claim 18, wherein the preferred direction of reorientation is permitted to be either one of the specified directions or a direction intermediate between two of the specified directions and inferred using measurements made using a plurality of the sensors.

20. A method according to any one of claims 16 to 19, including a step of applying a specific criterion to a plurality of the sensors' specified directions to determine whether a single preferred direction of reorientation can be determined and if not then aborting the determination of that direction and substituting an estimate of a preferred reorientation obtained by a separate method.

21. A solar energy collecting apparatus comprising a sun tracking apparatus according to any one of claims 1 to 15.