DE102011082954B4 - Method for determining the intensity of circumsolar radiation - Google Patents

Abstract

Method for determining the intensity of circumsolar radiation in any position of the sun using the following steps: measuring the intensity of the solar radiation with a light sensor (1) over a predetermined period of time, guiding a shadow band (3) through the sensor area of the light sensor (1) in the given period of time wherein the shadow band (3) at least once shadows the direct radiation of the sun, - analyzing the resulting by the shadowing of the direct radiation signal waveform, the course and the intensity of the signal when approaching the shadow band (3) to the sun's position (5) is determined, and in each case an intensity is determined before and after the range of shading of the direct radiation, - determination of the intensity of the circumsolar radiation from the course and the intensity of the signal when the shadow band (3) approaches the position of the sun (5) and the determined instability values.

Description

The present invention relates to a method for determining the intensity of circumsolar radiation in any position of the sun.

In solar thermal power plants, the solar radiance profile is imaged by optical concentrators on an absorber structure and thus used to generate usable for thermal energy processes temperature levels. Due to the scattering of sunlight in the earth's atmosphere, the solar radiance profile can vary widely. A broad beam density profile corresponds to a broad image of the sun in the absorber plane, so that parts of the radiation miss the actual absorber structure.

The proportion of radiation coming from the area directly around the solar disk, the circumsolar radiation, on the direct radiation is the circumsolar ratio. The circumsolar ratio may temporarily reach a level relevant to solar thermal power plants. Conventional measurements of direct radiation include not only the light incident from the solar disk but also the circumsolar radiation up to an angular distance about ten times greater than the solar disk. With conventional measuring methods of direct radiation, it is not possible to make any statements about the solar radiance profile so that the radiation supply that can be used for different types of power plant can only be determined with reduced accuracy.

A measurement of the solar radiance profile is also of interest for concentrating photovoltaics and for the determination of atmospheric properties.

In a known method, the circumsolar ratio is precisely determined by additional measurements, which is included in the yield analysis of solar thermal power plants.

There are so-called Rotating Shadowband Irradiance sensors known, which are used for the measurement of horizontal global radiation and horizontal diffuse radiation. Such instruments are often used to design solar thermal power plants. They are also known by the names Rotating Shadowband Radiometer and Rotating Shadowband Pyranometer. Such a sensor is in US 6,849,842 B2 described.

Various instruments have been developed for the measurement of the circumsolar ratio. The instruments for measuring the circumsolar ratio usually have a complicated structure and are costly, so that it is desirable to determine the circumsolar ratio with existing already for the planning of solar thermal power plants sensors. A scientific method for determining the circumsolar ratio, which is quite complicated and costly, is from Neumann, A .; of the Au, B .; Heller, P .: Measurements of circumsolar radiation at the plataforma solar (Spain) and at DLR (Germany). In: Solar engineering 1998: proceedings of the International Solar Energy Conference; presented at Renewable Energy for the Americas, June 14 - 17, 1998, Albuquerque, New Mexico. New York, NY: American Soc. Of Mechianical Engineers, 1998, pp. 429-438 , - ISBN 0-7918-1856-X known. Here, a system with a camera for measuring the radiation density is used. The radiation density is angularly resolved. From the image taken by means of the camera, the radiation density is determined by evaluating the corresponding gray values of the image. These values can then be integrated over the angle from the center of the sun in order to obtain the irradiance. Due to the image enhancement, this process is quite complicated.

It is therefore an object of the present invention to provide a method for determining the intensity of circumsolar radiation in any position of the sun available, which requires only a small device complexity and thus is inexpensive.

The invention is defined by the method according to claim 1.

• - Messen der Intensität der Solarstrahlung mit einem Lichtsensor über einen vorgegebenen Zeitraum;
• - Führen eines Schattenbandes in dem vorgegebenen Zeitraum durch den Sensorbereich des Lichtsensors, wobei das Schattenband mindestens einmal die Direktstrahlung der Sonne abschattet;
• - Analysieren des durch die Abschattung der Direktstrahlung entstehenden Signalverlaufs, wobei der Verlauf und die Intensität des Signals bei Annäherung des Schattenbandes an den Sonnenstand bestimmt wird;
• - Bestimmung der Intensität der Zirkumsolarstrahlung aus dem Verlauf und der Intensität des Signals bei Annäherung des Schattenbandes an den Sonnenstand.

The method according to the invention for determining the intensity of circumsolar radiation in any position of the sun provides the following steps:

• Measuring the intensity of solar radiation with a light sensor over a predetermined period of time;
• - Passing a shadow band in the given period through the sensor area of the light sensor, wherein the shadow band at least once shadows the direct radiation of the sun;
• - Analyzing the resulting by the shadowing of the direct radiation waveform, the course and the intensity of the signal is determined when approaching the shadow band to the sun's position;
• - Determination of the intensity of the circumsolar radiation from the course and the intensity of the signal as the shadow band approaches the position of the sun.

Sun position is understood in the context of this invention, the direct connection between the light sensor and the sun.

The inventive method has the advantage that the intensity of the circumsolar radiation can be determined in a very simple manner, wherein existing measuring devices, such as a rotating shadowband Irradiance sensor can be used. The method according to the invention even makes it possible to determine the intensity of the circumsolar radiation for existing measurements, so that no additional measuring operations have to be carried out.

The invention further provides that an intensity value is determined in each case before and after the region of the shadowing of the direct radiation during the analysis of the signal profile resulting from the shadowing of the direct radiation. The intensity value can be used to calculate the intensity of the circumsolar radiation.

It is preferably provided that in the method according to the invention, a rotating shadowband Irradiance sensor is used. This can be for example a Rotating Shadowband Radiometer or a Rotating Shadowband Pyranometer.

Such instruments are usually already available for the measurement of horizontal global radiation and horizontal diffuse radiation for the planning of solar thermal power plants, so that with existing equipment, and thus with low device complexity, the intensity of circumsolar radiation can be determined.

It can be provided that the shadow band can be performed continuously or discontinuously. When using a continuously guided shadow band, the light sensor should have a short response time, so that the intensity of the light is measured at a high frequency and a waveform from a plurality of measurement points is formed.

It may be provided that the penumbra function of the shadow band position belonging to the intensity values is used to determine the intensity of the circumsolar radiation from the intensity values.

For Rotating Shadowband Irradiance sensors, the penumbrafunction can be calculated, so that the intensity of the penumbrafunction can be used to determine the intensity of the circumsolar radiation.

In an embodiment of the invention, it is provided that the intensity values are the shoulder values which are corrected by a correction value dependent on the position of the sun.

The signal curve of the light intensity as the shadow band approaches the position of the sun causes the intensity value to fall sharply. This waveform is also called "burst". For a burst, the shoulder values that are before and after the minimum of the burst can be determined. The shoulder values can then be corrected to form the intensity values by a correction value dependent on the position of the sun. The use of corrected shoulder values makes it possible to determine the intensity values in a particularly simple manner.

The inventive method can provide that the direct radiation is determined, wherein from the previously determined circumsolar radiation and the direct radiation of the circumsolar ratio is formed.

The invention further provides that the solar radiation density profile can be determined from the circumsolar ratio.

The method according to the invention is explained in more detail below with reference to the following figures.

• 1 eine schematische Darstellung eines Schattenbandes im Verhältnis zu einem Lichtsensor und
• 2 eine beispielhafter Signalverlauf der Lichtintensität.

Show it:

• 1 a schematic representation of a shadow band in relation to a light sensor and
• 2 an exemplary waveform of the light intensity.

In the method according to the invention for determining the intensity of circumsolar radiation in any position of the sun, for example, rotating shadowband sensors can be used. Such sensors usually have a light sensor 1 to make a shadow band 3 rotates. The relationship between light sensor 1 and shadow band 3 is schematic in 1 shown. Upon reaching the position of the sun 5 by the direct connection between the light sensor 1 and the sun is given, produces the band of light 3 a shadow 7 standing on the light sensor 1 falls. If the shadow band 3 is outside the sensor range is from the sensor 1 the global radiation measured. The schematic representation in 1 is used only for clarification purposes and is shown in greatly simplified form.

In the method according to the invention is first with the light sensor 1 measured the intensity of solar radiation over a given period of time. This is the shadow band 3 through the sensor area of the light sensor 1 led, with the shadow band 3 at least once the direct radiation of the sun shades off. The shading of the direct radiation of the sun is in 1 schematically through the shadow 7 shown.

Subsequently, the signal profile resulting from the shading of the direct radiation is analyzed. A corresponding signal curve is shown schematically in FIG 2 shown. The course becomes the intensity of the signal as the shadow band approaches 3 to the position of the sun 5 certainly. Subsequently, the intensity of the circumsolar radiation is determined from the course and the intensity of the signal when approaching the shadow band 3 to the position of the sun 5 certainly.

For a more detailed explanation, the method according to the invention will be described by way of example with reference to FIG 2 illustrated waveform explained. The Indian 2 shown waveform shows the means of the light sensor 1 measured light intensity, plotted against the number of the measuring point, which equates to a time course. The waveform is in the range of the measuring point 700 a strong drop in intensity conspicuous by the shading of the light sensor 1 through the shadow band 3 is produced. This signal is called a burst, which is a minimum 11 has. Through a curve analysis different characteristic values can be determined. The top line shows the horizontal global radiation 13 , The lower line marks the horizontal diffuse radiation 15 , Furthermore, a left shouldervalue 17 and a right shouldervalue 19 marked. The line between the two shouldervalues 17 . 19 is their mean. The distance between the mean of the shouldervalues 17 . 19 and horizontal global radiation 13 becomes the minimum of the burst 11 added to the horizontal diffuse radiation 15 to obtain.

To determine the shoulder values 17 . 19 the width of the burst of the burst is determined. The width of the signal break is defined as the measuring point number between minimum and maximum of the gradient of the burst. Further, the center of the signal dip is defined as the measurement point number in the middle between minimum and maximum of the gradient of the burst. The shoulder values 17 . 19 Thus, the intensity values for measuring points at the measuring point distance are equal to the width of the signal break to the left and to the right of the minimum 11 the signal collapse.

The method according to the invention can provide that intensity values which lie before and after the region of the shading of the direct radiation are determined during the analysis of the signal profile produced by the shadowing of the direct radiation. For this purpose, the shoulder values can be used, wherein preferably the shoulder values are corrected by a correction value dependent on the position of the sun. In one embodiment of the method according to the invention, the shoulder values are reduced on average by 0.2%.

In determining the intensity of the circumsolar radiation from the course of the intensity of the signal when approaching the shadow band 3 to the position of the sun 5 In a first sub-step, the difference Δ between the horizontal global radiation 13 and the intensity values (reduced shoulder values). These values represent the total intensity of the light, that of the shadow band 3 is caught before the light is the light sensor 1 reached. In this value, parts of the circumsolar radiation are also part of the horizontal diffuse radiation 15 contain. The reduction of the shoulder values is due to the fact that the shoulder values are too far outside the sun disc.

The rotating shadow band Irradiance sensor used for the method according to the invention has different penumbra functions for different shadow band positions.

The by the shadow band 3 blocked diffuse radiation is subtracted from Δ. The shadow band covers from the center of the light sensor 1 about +/- 2.5 ° off. Blocked diffuse radiation is the radiation that is not incident directly from the sun or from the circum-solar area and that comes from points in the sky for which the penumbra- tional function at the shadow band position assumes values greater than zero during the measurement of the shoulder values.

wobei DHI die horizontale Diffusstrahlung 15 und SZA den Zenithwinkel der Sonne in Grad bezeichnet. Der Zenithwinkel SZA ist in der obigen Gleichung enthalten, da die Zirkumsolarstrahlung stärker zur blockierten Diffusstrahlung beiträgt, wenn sie in einem steileren Winkel auf den Sensor fällt.Since the ray of the sky is highest near the sun, the subtraction of the diffuse radiation is performed as follows: $$\mathrm{\Delta }\text{'}=\mathrm{\Delta }-DHI\times \frac{2\times 2.5\text{°}}{180\text{°}}\times \left(1+\text{sin}\left(90\text{° -}SZA\right)\right)$$

where DHI is the horizontal diffuse radiation 15 and SZA denotes the zenith angle of the sun in degrees. The zenith angle SZA is included in the above equation because the circumsolar radiation contributes more to the blocked diffused radiation when it falls on the sensor at a steeper angle.

The method according to the invention can then provide that, to determine the intensity of the circumsolar radiation in the horizontal light sensor plane, the difference Δ 'is first multiplied by a temporally constant factor of 19.92. This factor compensates for the partial suppression of circumsolar radiation. The shadow band 3 covered in the measurement of shoulder values 17 . 19 not the entire circular circumolar area, but only a strip of the circumsolar area on one side only. The factor 19.92 is obtained from the average of the penumbrafunction for different shadow band positions (sunrise to solar noon). Of course, it is possible that the time constant factor in another Penumbrafunktion takes on a different value.

The calculated intensity of the circumsolar radiation in the light sensor plane is now determined by the difference of the horizontal global radiation determined from the signal course 13 and the horizontal diffuse radiation 15 divided to obtain a provisional circumsolar ratio.

The circumsolar ratio can now be determined even more accurately by correcting the provisional circumsolar ratios by the respective associated air mass. The Airmass is proportional to the light path of the direct radiation in the atmosphere to the ground and 1, when the sun is in the zenith. The variation of the penumbra function at different shadow band positions causes the burst signal in the burst 11 at low sun levels 5 wider than at high sun levels 5 , This includes the shoulder values 17 . 19 at midday less circumsolar radiation than shoulder values 17 . 19 that are measured in the evening or in the morning. The method according to the invention thus makes it possible to determine the intensity of the circumsolar radiation as well as the circumsolar ratio and the solar radiation density profile by means of a measurement by means of a rotating shadowband Irradiance sensor. The measurement can thus be carried out with instruments already present in a solar power plant or for its planning, wherein the determination of the desired values can even take place on measurements already taken. The method according to the invention can use different measuring instruments. For example, Rotating Shadowband Irradiance sensors can be used, namely both Rotating Shadowband Pyranometers and Rotating Shadowband Radiometers. The shadow band 3 can rotate continuously or discontinuously. In an instrument with non-continuously rotating shadow band, at least three intensities are measured. A measurement is made with the shadow band outside the field of view of the light sensor, another intensity is measured with the shadow band centrally in front of the sun and a third intensity is determined when the shadow band is right next to the sun. At the last measuring point, the sky radiation blocked by the shadow band is approximately determined.

Claims (7)

Method for determining the intensity of circumsolar radiation in any position of the sun, comprising the following steps: - Measuring the intensity of the solar radiation with a light sensor (1) over a predetermined period of time; - Guide a shadow band (3) in the given period by the sensor area of the light sensor (1), wherein the shadow band (3) at least once shadows the direct radiation of the sun; - Analyzing the resulting by the shadowing of the direct radiation signal waveform, the course and the intensity of the signal when approaching the shadow band (3) to the sun's position (5) is determined and wherein before and after the range of shading of the direct radiation in each case an intensity is determined ; Determining the intensity of the circumsolar radiation from the course and the intensity of the signal when the shadow band (3) approaches the position of the sun (5) and the determined instability values. Method according to Claim 1 , characterized in that a Rotating Shadowband Irradiance sensor is used. Method according to Claim 1 or 2 , characterized in that the shadow band is guided continuously or discontinuously. Method according to one of Claims 1 to 3 Characterized in that the Penumbrafunktion of the intensity values in each case belonging shade band positions is used to determine the intensity of circumsolar from the intensity values. Method according to one of Claims 1 to 4 , characterized in that the intensity values are shoulder values which are corrected by a correction value dependent on the position of the sun (5). Method according to one of Claims 1 to 5 , characterized in that the direct radiation is determined, wherein the circumsolar ratio is formed from the determined circumsolar radiation and the direct radiation. Method according to Claim 6 , characterized in that the solar radiation density profile is determined from the circumsolar ratio.

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