WO2013136171A2

WO2013136171A2 - Device for measuring and controlling the alignment of solar rays incident on a photovoltaic module

Info

Publication number: WO2013136171A2
Application number: PCT/IB2013/000487
Authority: WO
Inventors: Alessandro MINUTO; Gianluca TIMO'
Original assignee: RICERCA SUL SISTEMA ENERGETICO - RSE SpA
Current assignee: RICERCA SUL SISTEMA ENERGETICO - RSE SpA
Priority date: 2012-03-16
Filing date: 2013-03-05
Publication date: 2013-09-19
Anticipated expiration: 2014-09-16
Also published as: WO2013136171A3; ITMI20120409A1

Description

DEVICE FOR MEASURING AND CONTROLLING THE ALIGNMENT OF SOLAR RAYS INCIDENT ON A PHOTOVOLTAIC MODULE

The present invention relates to a device for the measurement and control of the alignment of solar rays incident on a photovoltaic module.

In particular, the present invention is advantageously used in solar tracking devices equipped with concentration photovoltaic modules, i.e. equipped with focalization optics to concentrate solar light incident on solar cells, to which the following description will make explicit reference, at the same time maintaining its generic characteristics, in order to measure and control the correct alignment of incident solar rays with respect to the normal to the plane of photovoltaic modules.

In general, a solar concentration apparatus is defined by a solar tracker and a concentration photovoltaic module (CPV) characterized by focalization optics which concentrate solar rays on receivers consisting of single or multi-junction solar cells.

The movement of the tracker with respect to possible rotation axes (azimuth and elevation) guarantees the alignment between the normal to the plane of the photovoltaic module and the direction of the solar rays incident on its surface. In general, the precision required for tracking solar rays, depends on the acceptance angle of the module which, in turn, is in relation to the concentration factor of the above-mentioned focalization optics.

With an increase in the concentration factor, the acceptance angle decreases and consequently a greater accuracy is required for the solar tracking.

It is currently known that a solar tracker of the type described above, uses the following alternative techniques for controlling solar tracking:

Open cycle control, based on the calculation of the astronomic position of the Sun in relation to the hour, date and geographical coordinates (longitude and latitude) of the site where the photovoltaic module is installed. This control allows solar tracking but, for systems operating at a high concentration with acceptation angles lower than 0.5°, the tracking accuracy should be considered insufficient;

Closed cycle control, based on the use of a tracking sensor which allows a considerable improvement in the tracking precision also for modules operating at a high concentration. In this closed cycle control technique, the accuracy of the solar tracker depends on the precision of the above-mentioned tracking sensor. In this respect, at present there are mainly three types of tracking sensors on the market :

- Photodiode sensor:

This is defined by a parallelepiped positioned vertically with respect to the plane of the tracker. Under a condition of misalignment of the solar rays, its shade is projected on the surroundings of its own base, where four photodiodes are positioned. If the electric currents generated by the photodiodes are different from each other, the controller moves the solar tracker until the position that balances the four currents has been found. This sensor is generally not capable of providing the degree of angular misalignment.

- PSD sensor:

This is a photodiode defined by a surface resistance uniformly distributed over a square- shaped area. The device is characterized by five terminals: four anodic electrodes positioned on four sides of the sensor and a single cathode. It requires an optical system which allows a light spot to be projected onto its surface. The current generated by the spot branches out towards the four anodes encountering various resistive routes with respect to the position of the spot; the four currents therefore change in relation to the position of the spot. When the spot is perfectly in the centre of the sensor, the four currents are the same and the perfect alignment condition is then revealed. The coordinates of the barycentre of the spot can in any case be obtained from the value of the four currents and on the basis of the characteristics of the optics, they can be translated into angular deviations;

CCD sensor:

This consists of an integrated circuit composed of a grid of semiconductor elements (of which each element is a pixel) capable of accumulating an electrical charge proportional to the intensity of the solar radiation striking it. This analogical information is converted to digital format by an AD converter which quantizes it (it transforms it into discrete levels) and associates a certain number of bits, which depends on the number of quantization levels, with each pixel. Like the PSD, it requires an optical system that projects a light spot on its surface. From a calculation of the position of the barycentre of the spot and on the basis of the optical characteristics the angular deviations of the tracker can be detected.

Among the three types of sensors indicated above, PSD sensors and CCD sensors have the highest resolution and precision characteristics.

In the case of the PSD sensor, the spatial resolution is linked to the smallest current variation that can be read by the electronic card which acquires the four anodic currents, whereas in the case of the CCD sensor, it depends on the number of pixels present in the dial. In order to obtain high precisions, the PSD sensor therefore requires high-precision analogical electronics capable of acquiring signals minimizing any possible environmental noise effects.

The PSD sensor also has a non-linearity when the spot is far from the centre and close to the electrodes .

The interface at the CCD sensor, on the other hand, requires digital electronics: the main requisite of this electronics is a scanning rate which is adequate for avoiding saturation of the charge accumulated in the sensitive elements (pixels) .

With an increase in the number of pixels of the CCD sensor, a digital electronic interface card must be used again with increasingly high performances in terms of scanning and processing rates. For this reason, a CCD sensor requires a rapid digital acquisition system which is not low-cost (such as a PC or FPGA) .

At present, one of the main disadvantages common to all of the tracking sensors described above is that they must be installed on a plane coplanar to the plane on which the photovoltaic modules rests.

More specifically, the coplanarity must be effected with a much lower error with respect to the tracking precision required.

If the two planes are not coplanar, in fact, the tracker would maximize the solar tracking of the sensor but not of the module.

During the first installation of the solar tracker, the operators must therefore make the plane of the sensor coplanar with that of the module by acting on the regulation screws.

With time, however, possible deforming effects due to thermal variations can alter the coplanarity between two planes, distorting the correction functioning of the tracking sensor, and consequently the functioning and efficiency of the same solar tracker on which the sensor is assembled.

An objective of the present invention is therefore to provide a device capable of overcoming the problems and drawbacks of the known art described above .

In particular, an objective of the present invention is therefore to provide a device suitable for measuring and controlling the incidence angle of solar rays, allowing the optimum alignment of the solar rays with respect to the normal to the plane of the photovoltaic module, and therefore allowing a correct and effective functioning of the solar tracker on which the module is assembled, directly integrating the device in the housing plane of the solar cells of the module .

Another objective of the present invention is to provide a high-efficiency device equipped with low-cost components .

The structural and functional characteristics of the present invention and its advantages with respect to the known art will appear more evident from the following claims, and in particular from the following description, referring to the enclosed drawings, which illustrate the schematization of a preferred but non- limiting embodiment of a device for measuring and controlling the alignment of solar rays incident on a photovoltaic module, wherein:

figure 1 illustrates a schematic view of a preferred embodiment of the measuring and control device object of the invention;

- figure 2 represents a perspective view on an enlarged scale of a scheme of a detail of the device of figure 1;

- figure 3 is a graph illustrating a comparison between a real sun path and a path measured by the device object of the invention; and

- figure 4 represents a view on an enlarged scale of a component of the device object of the invention.

With reference to figures 1 and 2, D indicates as a whole a device suitable for measuring and controlling the alignment of solar rays 7 with respect to the normal to the plane of a concentration photovoltaic module 3 assembled on a solar tracking apparatus 5, of which the device D forms an integrant part.

The solar tracking apparatus 5 is of the known type on which photovoltaic modules are assembled and provided with motors M (figure 2) , which are suitable for the rotational movement of the apparatus 5 itself with respect to two axes (azimuth and elevation) , and acting on a mechanical structure schematized in figure 2 with the reference SM.

The device D also comprises, in addition to the photovoltaic module 3 for concentrating the solar light coming from the rays 7 on the cells 4 by means of its focalization optics, a CCD tracking sensor 1, integrated in the structure of the photovoltaic module 3 itself, an optical collimator 6 suitable for defining, through its hole 9 through which the rays 7 pass, a light spot 10 to be captured by the surface of the CCD sensor 1, and an electronic control card 2.

The card 2 is a low-cost digital electronic interface card also integral with the module 3, and is suitable for exerting the following functions, when in use:

- it reads information coming from the sensor 1 in real time,

- it calculates the position of the light spot 10 and therefore the angular misalignment between the normal to the plane of the module 3 and direction

Z of the solar rays 7 incident on the module 3,

- it automatically controls the optimal integration time in relation to the solar light intensity,

- it controls the motors M of the solar tracker 5 in order to ensure optimum tracking of the module 3 with respect to the Sun with the precision required for the correct functioning of the same module 3,

- it also calculates the direct irradiance incident on the plane of the module, on the basis of the maximum intensity value revealed by the sensor and integration time established during the measurement .

The development of a communication protocol that can transmit the information revealed by the sensor 1 through a bus of the type RS485 onto a PC or onto an external controller of the solar tracker 5, is also an integrant part of the device D.

More specifically, according to what is better illustrated in figure 2, the collimator 6 is suitably equipped with an optical filter 8 suitable for attenuating the light intensity of the solar rays 7.

Once the solar rays 7 have been attenuated after passing through the optical filter 8, they are therefore suitable for passing through the hole 9 of the collimator 6 for defining the above-mentioned light spot 10 which reaches the sensor 1.

The card 2 therefore reads the information coming from the CCD sensor 1, effects the calculation of the barycentre of the spot 10, i.e. the coordinates Χ',Υ' in figure 2, and subsequently activates the motors M of the tracker 5 in order to effect the correction of the orientation of the module with respect to the sun bringing the barycentre of the spot 10 to the point of origin of the reference system shown in figure 2. According to what is illustrated in figure 4, the above focalization optics is integrated inside the module 3 between the plane of the lens of the module 3 itself and the plane of the sensor 1.

It should be pointed out that the CCD sensor 1 is defined by 256 x 256 pixels over an area of 4 mm²; for the purposes of the present patent application, however, it is sufficient to have information on the light intensity profile along the two orthogonal Cartesian axes of the plane of the sensor. In this respect, the sensor is able to provide this information .

For this reason, the interface electronics that acquires the information of the sensor 1 does not require the acquisition of 256 x 256 = 65536 values (the single pixels) but simply 256 + 256 = 512 values (projections on the Cartesian axes) .

Each of the 512 values is characterized by information digitalized at lObit (1024 light quantization levels) .

As, in order to rapidly acquire a matrix of 65536 points with the known technique, it is necessary to use advanced electronics (and therefore costly) such as a computer of FPGA, for the acquisition of only 512 points, the use of a simple very low-cost 8bit microcontroller, was sufficient.

This microcontroller was programmed for reading the data of the sensor 1, calculating the angular misalignment, calculating the direct irradiance incident on the plane of the module, controlling the movement of the tracker and transmitting information onto a bus RS485. With respect to the quantity of light revealed by the sensor 1, moreover, it is possible to calculate the optimum scanning rate of the pixels, in order to exploit the whole useful quantization scale, at the same time avoiding saturation of the pixels.

More specifically, the above collimator 6 is preferably cylindrically-shaped, with the central hole 9 situated at a distance of 30 mm from the sensor 1, which projects the light spot 10 onto the surface of the sensor 1 allowing a maximum acceptance angle of +/- 2° and an angular resolution of 0.015° to be revealed.

Depending on the characteristics of the module 3 in which the sensor 1 is installed, the collimator 6 can be modified in relation to the angular resolution required. The spatial resolution of the sensor 1, on the other hand, is independent of the optics (and is equal to 7.8 micron).

It should finally be noted that tests revealed that the tracker 5 reached a tracking precision lower than 0.1°, as shown in the graph of Figure 3, which compares the real sun path (in terms of azimuth and elevation angles) with that measured by the sensor 1.

In short, the following advantages are obtained with the device D described above:

- a measurement procedure of the misalignment of solar rays with respect to the normal to the plane of a photovoltaic module, through a low-cost sensor of the CCD type, integrated inside the concentration photovoltaic module, on the same plane in which the solar cells are positioned;

- a control system and procedure which, through the measurement information indicated above, allows the alignment of the normal to the plane of the concentration photovoltaic module with respect to solar rays ;

- a measurement procedure of the direct radiation incident on the plane of the module.

Claims

1. A device (D) for the measurement of the direct irradiance on the plane of a photovoltaic module (3) and control of the alignment of solar rays (7) incident on said photovoltaic module (3), comprising tracking sensor means (1) of the CCD type, said sensor means (1) being integrated in the structure of said photovoltaic module (3), optical collimator means (6) suitable for the passage of said solar rays (7) for defining a light spot (10) which is captured by said sensors means (1) ; and electronic means (2) for acquiring information from said sensor means (1) , for calculating the angular alignment of said solar rays (7) with respect to a normal to the plane of said photovoltaic module (3), and transmitting the data calculated to control means.

2. The device according to claim 1, characterized in that said device (D) is integrated in the module (3) which in turn is installed and coupled on a solar tracking apparatus (5) , said apparatus (5) being equipped with rotational movement means (M, SM) with respect to two-axes of the tracking apparatus (5) itself; said electronic means (2) also being suitable for controlling said movement means (M, SM) to ensure the correct tracking of said module (3) with respect to the sun with a predetermined precision degree.

3. The device according to claim 1 or 2, characterized in that said collimator means (6) are provided with optical filtering means (8) suitable for attenuating the light intensity of said solar rays (7) .

4. The device according to any of the claims from 1 to 3, characterized in that said sensor means (1) are also capable of measuring the direct solar irradiance on the plane of the photovoltaic module (3) .

5. The device according to any of the claims from 1 to 4 , characterized in that said electronic means (2) comprise an electronic digital interface card (2) .

6. The device according to any of the previous claims 1 to 5, characterized in that said electronic means (2) comprise a communication protocol suitable for transmitting said calculated data through communication means of the bus RS485 type.

7. A solar tracking apparatus, characterized in that it comprises at least one device (D) according to one or more of the previous claims 1 to 6.