SYSTEM FOR MEASURING THE COMPONENTS OF SOLAR RADIATION

Description

[0001] The present invention relates to the field of solar radiation and atmospheric properties.

[0002] The invention relates more particularly to a system for measuring solar radiation.

[0003] For some applications, it is necessary to know the different components of solar radiation. In particular, we want to know the following components: The direct component of solar radiation. When solar radiation that does not interact with the Earth's atmosphere as it passes through is measured at normal incidence, it is referred to as direct normal irradiance or DNI. DNI is expressed in Wm -2< . To measure DNI, a pyrheliometer is generally used, which is in the form of a tube whose dimensions define the opening angle and at the bottom of which is placed a thermopile. The pyrheliometer must be aligned with the Sun's axis, which is why a solar tracker is necessary if DNI acquisition is to be automated. The cost of a pyrheliometer is approximately €2.5k, that of a solar tracker being close to 10kE. In addition, to function optimally, this instrument must be maintained daily. Faced with these cost and maintenance constraints, less expensive and more robust instruments have been developed.Among them, the most widely used instrument is the rotating strip pyranometer (see below). However, the pyranometer has the disadvantage of requiring precise spectral calibration of the photodiode. This calibration is complex, producing a higher uncertainty on the DNI than for a pyrheliometer. The diffuse component of solar radiation. W hen solar radiation that has interacted with the Earth's atmosphere during its passage is measured on a horizontal surface, we speak of diffuse horizontal irradiance or DHI ("Diffuse Horizontal Irradiance"). DHI is expressed in Wm -2< . Most often, to measure DHI, a pyranometer and a solar tracker equipped with an element that masks the Sun (thus allowing the suppression of the direct component of solar radiation) are used together. For example, a rotating strip periodically obscures the photodiode in order to measure DHI.Global solar radiation, defined as the sum of direct and indirect radiation received on the ground. If this radiation is measured horizontally, we speak of global horizontal illumination or GHI (“Global Horizontal Irradiance”).

[0004] The GHI is expressed in Wm -2< .

[0005] To measure GHI, a pyranometer is usually used, which includes a horizontally mounted photodiode.

[0006] From the measurements of GHI and DHI and knowledge of the position of the Sun, the DNI can be determined.

[0007] In fact, the GHI, the DNI and the DHI are linked by the following relationship: GHI= cos(SZA) ⋅ DNI+DHI where SZA is the angle between the Sun and the zenith.

[0008] In some cases, we also seek to know the duration of sunshine.

[0009] The sunshine duration of a geographic area is the duration during which this geographic area is illuminated by the Sun.

[0010] The World Meteorological Organization (WMO) defines sunshine duration as the duration during which direct solar radiation exceeds the threshold of 120 Wm -2< .

[0011] In some cases, the atmospheric disturbance is also sought.

[0012] Atmospheric disturbance relates to the attenuation of solar radiation as it passes through the Earth's atmosphere. This attenuation is mainly due to the phenomena of diffusion by aerosols and absorption by various atmospheric components (ozone, water vapor, oxygen, etc.).

[0013] Atmospheric haze provides a reliable indication of the quality of the solar resource or atmosphere for a given site.

[0014] For example, an estimate of Atmospheric Disturbance (AD) is made using the relationship: TA = 11,1 ln b I 0 I cc m + 1 avec b = 0,664 + 0,163 ⋅ exp − h 8000 where h is the altitude of the site considered, I 0 is the extraterrestrial solar illumination (Wm -2< ), I cc is the DNI under clear skies (Wm -2< ) and m is the relative optical air mass.

[0015] In some cases, we also seek to know the solar profile, i.e. the distribution of luminance L in the circumsolar zone, i.e. the zone between the outside of the solar disk (approximately 0.26°) and the classic half-opening angle of DNI measuring instruments. In practice, the circumsolar zone is therefore defined by the condition 0.26° < SPA < β, where SPA is the angle between the pixel and the Sun and β is between 1° and 5°.

[0016] The energy luminance, noted L and expressed in Wm -2< .sr -1< , is the flux radiated per unit of solid angle and surface. The energy luminance L is linked to the illumination I by the relation: I = ∫ φ = 0 2 π ∫ θ = 0 π / 2 L θ φ cos θ ⋅ dΩ where dΩ is the elementary solid angle, θ and ϕ defining the direction of propagation of light.

[0017] For isotropic radiation, we have I = π·L.

[0018] The most accurate instrument for this measurement is the SAM (“Sun Aureole Measurement”) instrument. This instrument has the disadvantage of being very expensive (around €80,000).

[0019] Each of the above-mentioned instruments costs at least several thousand euros and requires regular maintenance, both for the solar tracker and for the sensors themselves. Measuring the various components of solar radiation is therefore an expensive and restrictive process.

[0020] The present invention aims to remedy these drawbacks.

[0021] The thesis "Evaluation of solar resources for the optimized management of CSP power plants" by Rémi Chauvin, 2016, XP055478458, presents an example of the state of the art.

[0022] The invention proposes a system for measuring the components of solar radiation more easily and at lower cost.

[0023] This goal is achieved by the fact that the system includes a camera equipped with a hemispherical lens and a sensor capable of capturing light to generate an image, and a processor capable of: Perform a geometric calibration of the camera to obtain the correspondence between the image pixel coordinate system and the camera coordinate system. Calculate the solid angle subtended by each pixel in the image, which will allow the luminance value to be weighted in subsequent calculations. Perform a camera calibration giving the position of the Sun in the sky so as to obtain the correspondence between the camera coordinate system and the cardinal points. Calculate the angle between each pixel and the zenith (PZA), the angle between each pixel and the azimuth (PAA), the angle between the Sun and the zenith (SZA) and the angle between the Sun and the azimuth (SAA), then the angle between each pixel and the Sun (SPA), using the relationship: cos(SPA) = cos(SZA) ⋅ cos(PZA) +sin(SZA) ⋅ sin(PZA) ⋅ cos(|SAA-PAA|) Obtain a high dynamic range (HDR) image of the sky. Calculate the global horizontal illuminance index (σ GHI ), the direct normal illuminance index (σ DNI ) and the diffuse horizontal illuminance index (σ DHI ), defined as the sum of the luminances measured by the image pixels belonging to a first region of interest (Λ 1 ), a second region of interest (Λ 2 ) and a third region of interest (Λ 3 ), respectively, by the following formulas, where Ω p , L p and PZA are the solid angle subtended by a pixel p, the luminance measured by this pixel and the angle between this pixel and the zenith, respectively: σ GHI = ∑ p ∈ ∧ 1 L p Ω p cos PZA ∑ p ∈ ∧ 1 Ω p σ DNI = ∑ p ∈ ∧ 2 L p Ω p ∑ p ∈ ∧ 2 Ω p σ DHI = ∑ p ∈ ∧ 3 L p Ω p cos PZA ∑ p ∈ ∧ 3 Ω p Convert the global horizontal illuminance index (σ GHI ), direct normal illuminance index (σ DNI ), and horizontal diffuse illuminance index (σ GDHI ) to global horizontal illuminance (GHI), direct normal illuminance (DNI), and horizontal diffuse illuminance (DHI), respectively.

[0024] Thanks to these provisions, an estimate of the different components of solar radiation (DNI, GHI, DHI) is obtained with a single camera, which advantageously replaces several measuring instruments. The system according to the invention is therefore less expensive to purchase and maintain and less complex.

[0025] In addition, the system according to the invention does not use a mask that obscures the Sun. This eliminates the operating and maintenance costs of a mask and the costs associated with a mechanical system that tracks the Sun's trajectory. Furthermore, since the Sun remains visible during measurements, the algorithms for estimating the various components are more efficient.

[0026] High dynamic range (HDR) imaging is achieved by acquiring multiple low dynamic range images taken with different exposure times and then combining these images into one HDR image, while maintaining the linearity of the camera sensor.

[0027] The first region of interest (Λ 1 ) is the set of image pixels verifying PZA < 90°, the second region of interest (Λ 2 ) is the set of image pixels verifying SPA < β, where β is the angle defined as the limit between the circumsolar zone and the rest of the sky, and the third region of interest (Λ 3 ) is the set of image pixels verifying {SPA > β} ∩ {PZA < 90°}.

[0028] This way, reliable values ​​are obtained for the various components of solar radiation.

[0029] Advantageously, the measuring system includes a waterproof and dustproof protective case in which the camera is housed, and which includes a transparent window located in front of the camera lens.

[0030] This protects the camera from bad weather (rain and wind) and foreign objects.

[0031] Advantageously, the protective box is equipped with a temperature regulator capable of regulating the temperature inside the box.

[0032] This makes the system suitable for operation in hostile environments, for example on the site of a solar power plant, where temperatures are sometimes very high.

[0033] Advantageously, the system comprises at least one filter which is capable of attenuating or modulating the light entering the camera through the lens.

[0034] The invention also relates to a method for measuring solar radiation.

[0035] According to the invention, this method for measuring solar radiation uses a camera equipped with a hemispherical lens and comprising a sensor capable of capturing light to generate an image, and a processor, the camera being oriented along a Z axis which points from the ground towards the zenith, and this method comprises the following steps: (a) The camera is geometrically calibrated to obtain the correspondence between the pixel coordinate system of the image provided by the camera and the camera coordinate system. (b) The solid angle subtended by each pixel of the image is calculated, in order to weight the luminance value in subsequent calculations. (c) The camera is calibrated a second time by comparing the theoretical position of the Sun with its position obtained on the image. In this way, the correspondence between the camera coordinate system and the cardinal points is obtained. (d) The angle between each pixel and the zenith (PZA), the angle between each pixel and the azimuth (PAA), the angle between the Sun and the zenith (SZA) and the angle between the Sun and the azimuth (SAA) are calculated, then the angle between each pixel and the Sun (SPA), using the relationship: cos(SPA) = cos(SZA) ⋅ cos(PZA) +sin(SZA) ⋅ sin(PZA) ⋅ cos(|SAA-PAA|) (e) A high dynamic range (HDR) image of the sky is obtained, said high dynamic range image is obtained by acquiring several low dynamic range images taken with different exposure times and then combining these images into a high dynamic range image, while maintaining the linearity of the camera sensor.(f) The global horizontal illumination index (σ GHI ), the direct normal illumination index (σ DNI ) and the diffuse horizontal illumination index (σ DHI ) are calculated, where Ω p , L p and PZA are the solid angle subtended by a pixel p, the luminance measured by this pixel and the angle between this pixel and the zenith, respectively, and where said first region of interest (Λ 1 ) is the set of pixels (125) of the image verifying PZA < 90°, said second region of interest (Λ 2 ) is the set of pixels (125) of the image verifying SPA < β, β being the angle defined as the limit between the circumsolar zone and the rest of the sky, and said third region of interest (Λ 3 ) is the set of pixels (125) of the image verifying {SPA > β} ∩ {PZA < 90°} : . σ GHI = ∑ p ∈ ∧ 1 L p Ω p cos PZA ∑ p ∈ ∧ 1 Ω p σ DNI = ∑ p ∈ ∧ 2 L p Ω p ∑ p ∈ ∧ 2 Ω p σ DHI = ∑ p ∈ ∧ 3 L p Ω p cos PZA ∑ p ∈ ∧ 3 Ω p (g) A conversion is performed between the global horizontal illuminance index (σ GHI ), the direct normal illuminance index (σ DNI ) and the diffuse horizontal illuminance index (σ DHI ), and the global horizontal illuminance (GHI), the direct normal illuminance (DNI) and the diffuse horizontal illuminance (DHI), respectively.

[0036] Advantageously, the method according to the invention further comprises the following step: (h) The solar profile is calculated by measuring the direct normal illumination (DNI) along a line of pixels passing through the center of the Sun on the image, then this measurement is repeated for a plurality of angles, so as to obtain the radial distribution of the luminance around the Sun.

[0037] This avoids the use of expensive additional equipment, and provides a more accurate measurement of the solar profile because it takes into account the anisotropy of the luminance around the Sun.

[0038] The invention will be better understood and its advantages will appear better, on reading the detailed description which follows, of an embodiment shown by way of non-limiting example. The description refers to the appended drawings in which: there figure 1 is a schematic view of a measuring system according to the invention; the figure 2 is a diagram illustrating the correspondence between a pixel of the camera sensor of the system according to the invention and a solid angle; the figure 3 is a diagram showing the different angles used to define the position of the Sun and a pixel of the camera of the system according to the invention relative to a reference frame.

[0039] The system according to the invention comprises a camera 10, provided with a hemispherical lens 20 (also called a “fisheye” lens), that is to say a wide-angle lens, with a field of vision greater than 180°. The camera 10 comprises a sensor 12 capable of capturing light to generate an image, for example a CMOS (“Complementary Metal Oxide Semiconductor”) sensor or a CCD (“Charge Coupled Device”) sensor. The sensor 12 comprises on its surface a certain number of pixels (125) on which the light falls to form an image.

[0040] Advantageously, the camera 10 is located on a horizontal platform, and is located in a place where there is no shading (for example high up and away from buildings and vegetation). Thus, an image of the entire or almost the entire sky is obtained, without obstacles hiding parts of the sky.

[0041] Camera 10 is oriented along a Z axis that points from the ground towards the zenith.

[0042] Advantageously, the measuring system comprises a waterproof and dustproof protective housing 40 in which the camera 10 is housed.

[0043] The box 40 comprises a transparent window 42, for example a transparent dome as shown in the figure 1 , which is located in front of the lens 20 of the camera 10 and which protects this lens 20. Thus, the transparent window 42 is located between the lens 20 and the Sun.

[0044] This protects the 10 camera from water and dust, while still allowing sunlight to reach the 20 lens.

[0045] Advantageously, the protective box 40 is equipped with a temperature regulator 50 capable of regulating the temperature inside the box 40.

[0046] Thus, condensation inside the box 40 is reduced or prevented. Consequently, the images taken by the camera 10 are not polluted by humidity.

[0047] Advantageously, the system comprises one or more filters 60 which are capable of attenuating or modulating the light entering the camera 10.

[0048] The filter(s) 60 are positioned between the Sun and the camera 10. Thus, the filter(s) 60 are located either between the camera 10 and the lens 20, or in the lens 20, or in front of the lens 20, or a combination of these positions.

[0049] For example, the inner face of the transparent window 42 is covered with a filter 60, as shown in the figure 1 in case window 42 is a dome.

[0050] Advantageously, the transparent window 42 is tinted in order to attenuate parasitic reflections (which could, erroneously, be taken for clouds by the developed algorithms).

[0051] Alternatively, the transparent window 42 is not tinted. In this case, the processor 30 (see below) includes an algorithm that removes these stray reflections.

[0052] The system according to the invention comprises a processor 30 connected to the camera 10.

[0053] This processor 30 is capable of performing the following operations: Carry out a first calibration of the camera 10 in order to obtain the correspondence between the coordinate system of the pixels 125 of the image produced by the sensor 12 and the coordinate system of the camera 10. This geometric calibration is carried out, in a known manner, for example using a checkerboard and an algorithm. Calculate, in a known manner, the solid angle Ω p subtended by each pixel (125, p) of the sensor 12 of the camera 10 recording the images, in order to weight the value of the luminance L in subsequent calculations. figure 2 illustrates a solid angle Ω p and shows the correspondence between this solid angle Ω p and a pixel (125, p) of the sensor 12. Carry out a second calibration of the camera 10 by comparing the theoretical position of the Sun with its position obtained on the image. In this way, the correspondence between the coordinate system of the camera 10 and the cardinal points (X, Y, Z coordinate system) is obtained. The theoretical position of the Sun is obtained in a known manner, for example using an algorithm such as SPA (“Solar Position Algorithm”) or SG (“Solar Geometry”). Calculate the angle between each pixel (125, p) and the zenith (PZA), the angle between each pixel and the azimuth (PAA), the angle between the Sun and the zenith (SZA) and the angle between the Sun and the azimuth (SAA), then the angle between each pixel (125, p) and the Sun (SPA), by the relation: cos(SPA) = cos(SZA) ⋅ cos(PZA) +sin(SZA) ⋅ sin(PZA) ⋅ cos(|SAA-PAA|)

[0054] There figure 3 shows the relationship between these different angles, the Sun S, and the zenith Z, as well as the reference direction north (N, cardinal points).

[0055] A high dynamic range (HDR) image is defined as an image in which the entire luminance of the sky is faithfully reproduced (i.e. without underexposure or overexposure), from the luminance L min of the darkest area to the luminance L max of the brightest area (Sun). Taking into account the entire luminance by a camera implies that the camera sensor has a dynamic range DR cam at least equal to the dynamic range of the sky DR sky with: DR cam = 20 log 10 L 2 L 1 et DR sky = 20 log 10 L max L min where L 1 is the minimum luminance measurable by the camera sensor and L 2 is the maximum luminance measurable by the camera sensor.

[0056] If this condition is not satisfied, that is to say if the sensor 12 of the camera 10 is capable of taking only low dynamic range (LDR) images, the high dynamic range image is obtained by acquiring several LDR images and then combining these images into an HDR image, while preserving the linearity of the sensor, in a known manner. An example of a method for obtaining an HDR image is described below.

[0057] Given several HDR images taken with different exposure times, the total acquisition time of the LDR image sequence must be short enough to ensure that the movement of the clouds between each LDR image is negligible, i.e. that the luminance of the scene is constant during the acquisition. In addition, it is important to correctly set the desired number of LDR images as well as the minimum t min and maximum t max exposure times so that the dynamic range that can theoretically be scanned by the HDR image satisfies the relationship: DR H = DR L + 20 log 10 t max t min > DR sky where DR H is the dynamic range of the HDR image and DR L is the dynamic range of the LDR image. The estimated value of each pixel in the generated HDR image is obtained from a weighted average of the pixel values ​​of each LDR image (considering only the pixels within the linearity range of the sensor).

[0058] Alternatively, the HDR image is obtained by using a 12 sensor with a dynamic range greater than the dynamic range of the DR sky.

[0059] According to the invention, the global horizontal illumination index (σ GHI ), the direct normal illumination index (σ DNI ) and the horizontal diffuse illumination index (σ DHI ) are then calculated, defined as the sum of the luminances measured by the pixels (125) of the image belonging to a first region of interest (Λ 1 ), to a second region of interest (Λ 2 ) and to a third region of interest (Λ 3 ), respectively, these sums being weighted by the solid angle of the region considered.

[0060] Thus, these illumination indices are obtained as follows: σ GHI = ∑ p ∈ ∧ 1 L p Ω p cos PZA ∑ p ∈ ∧ 1 Ω p σ DNI = ∑ p ∈ ∧ 2 L p Ω p ∑ p ∈ ∧ 2 Ω p σ DHI = ∑ p ∈ ∧ 3 L p Ω p cos PZA ∑ p ∈ ∧ 3 Ω p where Ω p is the solid angle subtended by a pixel p (see figure 2 ), L p is the luminance measured by this pixel and PZA is the angle between this pixel and the zenith (see figure 3 ).

[0061] Advantageously, the first region of interest (Λ 1 ) is the set of pixels (125) of the image verifying PZA < 90°, the second region of interest (Λ 2 ) is the set of pixels (125) of the image verifying SPA < β, where β is the angle defined as the limit between the circumsolar zone (as defined above) and the rest of the sky, and the third region of interest (Λ 3 ) is the set of pixels (125) of the image verifying {SPA > β} n {PZA < 90°}.

[0062] This provides a more accurate assessment of the illumination indices.

[0063] Then, the global horizontal illuminance index (σ GHI ), direct normal illuminance index (σ DNI ) and diffuse horizontal illuminance index (σ DHI ) are converted into global horizontal illuminance (GHI), direct normal illuminance (DNI) and diffuse horizontal illuminance (DHI), respectively, by conversion functions.

[0064] These conversion functions consist of correlations between the illumination indices (σ GHI , σ DNI and σ DHI ) and the different components of solar radiation (GHI, DNI and DHI).

[0065] A simple linear regression, or even multiple when several illumination indices and, possibly, their histories are taken into account, should make it possible to approximate each conversion function.

[0066] Alternatively, it is possible to approximate these functions using artificial intelligence tools, for example an artificial neural network or a neuro-fuzzy system (such a system combines the connectionist structure of a neural network and the writing of fuzzy rules; fuzzy logic is an extension of Boolean logic). A digital learning phase allows, from a base of examples (the learning is supervised here), to determine the topology of the tool considered (its structure) and to identify its parameters.

[0067] The invention also relates to a method for measuring solar radiation using a camera 10 provided with a hemispherical lens 20 and comprising a sensor capable of capturing light to generate an image, and a processor 30, which comprises the following steps, already described above: (a) Geometric calibration to obtain the correspondence between the pixel coordinate system of the image provided by the camera and the camera coordinate system. (b) Calculation of the solid angle subtended by each pixel of the image, in order to weight the luminance value in subsequent calculations. (c) Second calibration based on the comparison between the theoretical position of the Sun and its position obtained on the image, in order to obtain the correspondence between the camera coordinate system and the cardinal points. (d) Calculation of the angle between each pixel (125, p) and the zenith (PZA), the angle between each pixel and the azimuth (PAA), the angle between the Sun and the zenith (SZA) and the angle between the Sun and the azimuth (SAA), then the angle between each pixel (125, p) and the Sun (SPA), by the relation: cos(SPA) = cos(SZA) ⋅ cos(PZA) +sin(SZA) ⋅ sin(PZA) ⋅ cos(|SAA-PAA|) (e) Obtaining an HDR image of the sky. (f) Calculation of the global horizontal illumination index (σ GHI ), the direct normal illumination index (σ DNI ) and the diffuse horizontal illumination index (σ DHI ), where Ω p , L p and PZA are the solid angle subtended by a pixel p, the luminance measured by this pixel and the angle between this pixel and the zenith, respectively: σ GHI = ∑ p ∈ ∧ 1 L p Ω p cos PZA ∑ p ∈ ∧ 1 Ω p σ DNI = ∑ p ∈ ∧ 2 L p Ω p ∑ p ∈ ∧ 2 Ω p σ DHI = ∑ p ∈ ∧ 3 L p Ω p cos PZA ∑ p ∈ ∧ 3 Ω p (g) Conversion between σ GHI , σ DNI and σ DHI and GHI, DNI and DHI.

[0068] Advantageously, the method according to the invention further comprises the following step: (i) Calculation of the duration of sunshine from direct solar radiation.

[0069] The method according to the invention then comprises the provision of a time counter which measures the time when the duration of sunshine is greater than the threshold specified above, and which stops when the duration of sunshine is less than this threshold. The duration of sunshine over a chosen period, for example a day, a month, or a year, is then known.

[0070] Advantageously, the method according to the invention further comprises the following step: (j) Calculation of the atmospheric disturbance from the determination of the “clear sky” instants and the DNI.

[0071] Atmospheric disturbance relates to the attenuation of solar radiation as it passes through the Earth's atmosphere. This attenuation is mainly due to the phenomena of diffusion by aerosols and absorption by various atmospheric components (ozone, water vapor, oxygen, etc.).

[0072] Atmospheric haze gives a good indication of the quality of the solar resource or atmosphere for a given site.

[0073] From the images acquired by the camera 10, the system according to the invention is capable of determining the evolution of the atmospheric disturbance on the site studied.

[0074] To do this, the system uses an algorithm to detect when the Sun is not obscured by a cloud. This is known as a "clear sky" situation.

[0075] A "clear sky" instant detection algorithm therefore acquires one or more successive images taken with different exposure times, in a relatively short period of time (for example, a few milliseconds). The system then analyzes the distribution of luminance in the circumsolar zone and, if necessary, performs a contour detection of the Sun, in a known manner.

[0076] From these parameters obtained, the system determines using this algorithm whether or not the moment corresponds to a so-called “clear sky” situation.

[0077] Optionally, temporal filtering, based on several successive images, can be used to ensure that this situation is correctly detected.

[0078] Each time a “clear sky” instant is determined by the algorithm, an estimate of the Atmospheric Disturbance (AT) is made.

[0079] For example, this estimation is performed using the following relationship: TA = 11,1 ln b I 0 I cc m + 1 avec b = 0,664 + 0,163 ⋅ exp − h 8000 where h is the altitude of the site considered, I 0 is the extraterrestrial solar illumination (Wm -2< ), I cc is the DNI under clear skies (Wm -2< ) and m is the relative optical air mass.

[0080] Advantageously, the method according to the invention further comprises the following step: (h) Calculation of the solar profile from the analysis of the radial distribution of the luminance.

[0081] According to the invention, to obtain the solar profile, the DNI is measured along a line of pixels passing through the center of the Sun on the image. By repeating this measurement for a plurality of angles, a radial distribution of the luminance L is obtained. This advantageously avoids the use of the SAM instrument (see above), which is very expensive.

[0082] Furthermore, the measurement of the solar profile according to the invention is more precise than a measurement with the SAM instrument because the measurement according to the invention takes into account the anisotropy of the luminance around the Sun (whereas the SAM instrument wrongly assumes a radial symmetry of the solar radiation).

Claims

1. Solar radiation measuring system including a camera (10) provided with a hemispherical objective (20) and comprising a sensor (12) adapted to receive light to generate an image, and a processor (30) able to: - perform a geometric calibration of the camera (10) in order to obtain the correspondence between the system of coordinates of pixels (125) of the image and the system of coordinates of said camera (10), - calculate the solid angle subtended by each pixel (125, p) of said image, in order to weight the luminance value in the subsequent calculations, - perform a second calibration of the camera (10) by comparing the theoretical position of the Sun and its position on the image, so as to obtain the correspondence between the system of coordinates of said camera (10) and the cardinal points, - calculate the angle between each pixel (125) and the zenith (PZA), the angle between each pixel (125) and the azimuth (PAA), the angle between the Sun and the zenith (SZA) and the angle between the Sun and the azimuth (SAA), then the angle between each pixel (125) and the Sun (SPA), by the relation: cos(SPA) = cos(SZA) ⋅ cos(PZA) +sin(SZA) ⋅ sin(PZA) ⋅ cos(|SAA-PAA|) , - obtain a high dynamic range (HDR) image of the sky, said high dynamic range image is obtained by the acquisition of several low dynamic range images taken with different exposure times then the combination of these images into a high dynamic range image, while maintaining the linearity of the camera sensor, characterized in that the processor (30) is able to: - calculate the global horizontal irradiance index (σGHI), the direct normal irradiance index (σDNI) and the diffuse horizontal irradiance index (σDHI), defined as the sum of the luminances measured by the pixels (125) of said image belonging to a first region of interest (Λ1), to a second region of interest (Λ2) and to a third region of interest (Λ3), respectively, where Ωp, LP and PZA are the solid angle subtended by a pixel p, the luminance measured by this pixel and the angle between this pixel and the zenith, respectively, and where said first region of interest (Λ1) is the set of pixels (125) of the image verifying PZA < 90° said second region of interest (Λ2) is the set of pixels (125) of the image verifying SPA < β, β being the angle defined as the limit between the circumsolar zone and the rest of the sky, and said third region of interest (Λ3) is the set of pixels (125) of the image verifying {SPA > β} ∩ {PZA < 90°}: σ GHI = ∑ p ∈ ∧ 1 L p Ω p cos PZA ∑ p ∈ ∧ 1 Ω p σ DNI = ∑ p ∈ Λ 2 L p Ω p ∑ p ∈ Λ 2 Ω p σ DHI = ∑ p ∈ Λ 3 L p Ω p cos PZA ∑ p ∈ Λ 3 Ω p - convert the global horizontal irradiance index (σGHI), the direct normal irradiance index (σDNI) and the diffuse horizontal irradiance index (σDHI) into global horizontal irradiance (GHI), direct normal irradiance (DNI) and diffuse horizontal irradiance (DHI), respectively.

2. Solar radiation measuring system according to claim 1, characterized in that it includes a waterproof and dustproof protective casing (40) in which said camera (10) is housed and which comprises a transparent window (42) located in front of the objective (20) of said camera (10).

3. Solar radiation measuring system according to claim 2, characterized in that said casing (40) is equipped with a temperature regulator (50) capable of regulating the temperature inside said casing (40).

4. Solar radiation measuring system according to any one of claims 1 to 3, characterized in that it comprises at least one filter (60) which is adapted to attenuate or modulate the light entering through the objective (20) into said camera (10).

5. Method for measuring solar radiation characterized in that it uses a camera (10) provided with a hemispherical objective (20) and comprising a sensor (12) adapted to receive light to generate an image, and a processor (30), the camera being oriented along a Z axis which points from the ground towards the zenith, and in that it includes the following steps: (a) geometrically calibrating the camera (10) in order to obtain the correspondence between the system of coordinates of the pixels (125) of the image provided by said camera (10) and the system of coordinates of the camera (10), (b) calculating the solid angle subtended by each pixel (125, p) of said image, in order to weight the luminance value in subsequent calculations, (c) calibrating said camera (10) a second time by comparing the theoretical position of the Sun and its position obtained on the image, so as to obtain the correspondence between the system of coordinates of said camera (10) and the cardinal points, (d) calculating the angle between each pixel (125, p) and the zenith (PZA), the angle between each pixel (125, p) and the azimuth (PAA), the angle between the Sun and the zenith (SZA) and the angle between the Sun and the azimuth (SAA), then the angle between each pixel (125, p) and the Sun (SPA), thanks to the relationship: cos(SPA) = cos(SZA) ⋅ cos(PZA) +sin(SZA) ⋅ sin(PZA) ⋅ cos(|SAA-PAA|) (e) obtaining a high dynamic range (HDR) image of the sky, said high dynamic range image being obtained by acquiring several low dynamic range images taken with different exposure times then combining these images into a high dynamic range image, while maintaining the linearity of the camera sensor, (f) calculating the global horizontal irradiance index (σGHI), the direct normal irradiance index (σDNI) and the diffuse horizontal irradiance index (σDHI), defined as the sum of the luminances measured by the pixels (125) of said image belonging to a first region of interest (Λ1), to a second region of interest (Λ2) and to a third region of interest (Λ3), respectively, where Ωp, Lp and PZA are the solid angle subtended by a pixel p, the luminance measured by this pixel and the angle between this pixel and the zenith, respectively, and where said first region of interest (Λ1) is the set of pixels (125) of the image verifying PZA < 90° said second region of interest (Λ2) is the set of pixels (125) of the image verifying SPA < β, β being the angle defined as the limit between the circumsolar zone and the rest of the sky, and said third region of interest (Λ3) is the set of pixels (125) of the image verifying {SPA > β} ∩ {PZA < 90°}: σ GHI = ∑ p ∈ Λ 1 L p Ω p cos PZA ∑ p ∈ Λ 1 Ω p σ DNI = ∑ p ∈ Λ 2 L p Ω p ∑ p ∈ Λ 2 Ω p σ DHI = ∑ p ∈ Λ 3 L p Ω p cos PZA ∑ p ∈ Λ 3 Ω p (g) performing a conversion between the global horizontal irradiance index (σGHI), the direct normal irradiance index (σDNI) and the diffuse horizontal irradiance index (σDHI), and the global horizontal irradiance (GHI), the direct normal irradiance (DNI) and the diffuse horizontal irradiance (DHI), respectively.

6. Method for measuring solar radiation according to the preceding claim, characterized in that it further comprises the following step: (h) calculating the solar profile by measuring the direct normal irradiance (DNI) along a row of pixels passing through the center of the Sun in the image, then repeating this measurement for a plurality of angles, so as to obtain a radial distribution of the luminance around the Sun.