Method and device determining soiling of a shield

A device comprises a housing, a detector for receiving solar irradiance and for providing a detector signal providing an indication of an amount of solar irradiance received by the detector and a shield transparent to at least part of the solar irradiance to be detected, the shield and the housing providing a detector space for housing at least part of the detector. The device further comprises a first light source for emitting light to the shield and a first light sensor arranged to receive light from the first light source, arranged to provide a first signal providing an indication for an amount of light received by the first light sensor. Particles will and reflect light back to the detector space. The reflected light is received by the light sensor. Hence, a signal generated by the sensor is an indication for pollution of the shield.

TECHNICAL FIELD

The various aspects and embodiments thereof relate to detection of pollution of a shield of a radiometer.

BACKGROUND

Radiometers, such as pyranometers and pyrheliometers, are used for determining incoming radiation from the sun. For proper operation and for determining the actual radiation at the location, originating from the sun, it is important that transparency of protective windows separating a detector from the outside is substantially continuous over time. However, pollutants in the air, either airborne, carried by means of precipitation or present in any other way, deposit on the protective windows. This affects transparency of the protective windows over time, in a not easy to predict way. This, in turn, affects accuracy of measurements. Cleaning is a good solution, but as the progress of soiling is very difficult to predict, cleaning is usually done rather too often than not. This is at a certain cost.

SUMMARY

It is preferred to determine an amount of soiling on the protective window locally.

A first aspect provides a device for detecting solar irradiance. The device comprises a housing, a detector for receiving solar irradiance and for providing a detector signal providing an indication of an amount of solar irradiance received by the detector and a shield transparent to at least part of the solar irradiance to be detected, a shield connection body for connecting the device to the shield and the housing providing a detector space for housing at least part of the detector. The device further comprises a first light source for emitting light to the shield and a first light sensor arranged to receive light from the first light source, arranged to provide a first signal providing an indication for an amount of light received by the first light sensor. The device is arranged to be coupled to a processing unit arranged to compare a value of the first signal to a reference sensor value and arranged to generate a first warning signal if a difference between the sensed value of the sensor signal and the reference sensor value is above a first predetermined threshold. Alternatively or additionally, the processing unit is arranged to determine, based on the first signal, a transmission value related to a transmission factor of the shield for a range of electromagnetic waves and output the transmission.

Usually, if the shield is clean, the shield and in particular the inner wall thereof will reflect only a very small part of the light emitted by the light source. Most of the light emitted by the light sources will travel through the shield, to the outside of the device. However, particles, either solid or liquid—or both—will scatter light and reflect light back in to the detector space. The reflected light may be received by the light sensor. Hence, the magnitude of a signal generated by the light sensor in response to receive light is an indication for pollution of the outer wall of the shield. Therefore, a signal is generated if the difference between the signal received from the sensor and a reference signal value is too high, to warn for pollution.

Alternatively or additionally to providing a warning signal, also another type of information signal based on the first signal may be provided. For example, a value indicating a loss of transmission due to pollution may be reported or a value indicating actual transmission of the shield. The value provided is determined based on the first signal and is provided for a frequency range of electromagnetic waves, in particular for the sunlight spectrum or a part thereof, including at least a part of visible light and, optionally, near infra-red and/or near ultra-violet. Alternatively or additionally, based on a calculated transmission loss due to pollution of the shield, an efficiency loss of a photovoltaic panel may be reported.

In an embodiment, the first light source is arranged for emitting a first light beam that coincides with the shield at a first angle relative to the shield and at a first incidence area on the shield, resulting in a first reflected beam, the first light sensor is arranged for sensing light originating from the first incidence area; and the first light sensor is provided out of the path of the first light beam and the path of the first reflected first light beam.

While measuring the scattered light and reducing or even preventing incidence of direct light or a directly reflected beam, pollution of the shield may be better determined. Reason for this is that in this setup, the first light sensor will predominantly receive light from the first light source that is scattered by pollutants on the outer side of the shield.

Another embodiment of the first aspect comprises a second light sensor arranged for sensing light from the first beam or the first reflected beam. The value from the second sensor may be used to filter out any degradation of the first sensor, the second sensor and the light source.

An embodiment of the device comprises a second light source for emitting light to the shield and a second light sensor arranged to receive light from the second light source, arranged to provide a second sensor signal providing an indication for an amount of light received by the second light sensor. The device according to this embodiment works best in conjunction with the processing unit that is further arranged to compare the first sensor signal to the second sensor signal and generate a second warning signal if a sensor signal difference between the first sensor signal and the second sensor signal is above a second predetermined threshold.

Different sensors and different light sources may be positioned at different locations around the detector. Hence, signals received by the different sensors provide an indication of pollution at different locations of the shield. A difference in values of the first sensor and the second sensor above a particular threshold indicates a difference in pollution at different areas of the shield. If signal values are substantially equal, pollution is homogeneous.

A further embodiment of the device works advantageously if the processing unit is further arranged to generate a third warning signal if a sensor signal difference between the first sensor signal and the second sensor signal is below a second predetermined threshold if the difference between the first sensor signal or the second sensor signal on one side and the reference sensor value on another side is above the first predetermined threshold.

This processing unit generates a signal in case of substantially homogeneous pollution of the shield. Signal distortion due to homogeneous pollution requires less processing power to compensate for.

In a further embodiment, the processing unit is arranged to obtain colour correction data related to colour characteristics of particles of surroundings of the device and the processing unit is arranged to adjust a value of the first signal or to adjust the reference value based on the colour information obtained. Different colour particles scatter light in a different way and may absorb a certain amount of light. Pollution of white particles an pollution of black particles provide different intensities of scattered light, even though they reduce transmission of light through the shield with substantially the same amount. This is because black particles usually absorb more light than white particles. this embodiment allows to compensate for the different signals and provide a signal that provides an uniform signal substantially the same for the amount of blocked light from outside, irrespective from the colour of the pollutants.

A second aspect provides a system for determining soiling of a shield for covering a detector for detecting solar irradiance, comprising the device according to any of the preceding claims and the processing unit.

A third aspect provides a solar panel. The solar panel comprises a laminate comprising a transparent shielding layer and a photovoltaic layer arranged for receiving solar irradiance transmitted through the shielding layer; and the device according to the first aspect. The device is provided such to receive solar irradiance transmitted through the shielding layer.

A fourth aspect provides a method of determining soiling of a shield for covering a detector for detecting solar irradiance in a device. The device comprises a housing, a detector for receiving solar irradiance and for providing a detector signal providing an indication of an amount of solar irradiance received by the detector, the shield, the shield and the housing providing a detector space for housing at least part of the detector. The device further comprises a first light source for emitting light to the shield and a first light sensor for providing a first signal providing an indication for an amount of light received by the first light sensor. The method comprises receiving the first signal, comparing a value of the first signal to a reference sensor value and generating a first warning signal if a difference between the sensed value of the sensor signal and the reference sensor value is above a first predetermined threshold. Alternatively or additionally to comparing the method comprises determining, based on the first signal, a transmission value related to a transmission factor of the shield for a range of electromagnetic waves and output the transmission value.

A fifth aspect provides a computer programme product comprising computer executable instructions for programming a processing unit to enable the processing unit to carry out the method according to the fourth aspect.

DETAILED DESCRIPTION

FIG. 1shows a pyranometer100as a radiometer. The pyranometer100comprises a device housing110provided with holder cup112for receiving a detector housing module120which forms part of the device housing110. The holder cup120may be a through hole in the housing110. The device housing110further comprises an optional circular depression114for receiving a rim of a dome130. At the bottom of the circular depression114, an O-ring may be provided for providing a substantially watertight closure. The dome130is provided for protection of electrical and electronic elements held by the detector housing module120. The dome130acts as a shield to any pollution, yet it is transparent or at least largely transparent for a spectrum of solar irradiation the pyranometer100is intended to detect.

The detector housing module120comprises a detector122for receiving solar irradiation to detect. The detector122is arranged to generate a signal upon receiving solar irradiation. Preferably, the detector122comprises a thermocouple, though other types of detectors may be envisaged as well. The detector122is provided in the centre of the detector housing module120and in the centre of the dome130.

The detector housing module120further holds multiple LEDs124, indicated as circles, as light sources. Alternatively to LEDs, also other light sources may be used, including, but not limited to laser and laser diodes in particular, incandescent or fluorescent light sources, other, or a combination thereof. The LEDs124are preferably blue light LEDs, with a spectrum peak between 400 nm and 500 nm. An advantage of such light emitting diodes is that their operation is only to a small extent affected by temperature.

The detector housing module120also comprises photo sensors126, arranged for detecting light and for generating a signal, of which signal the value relates to an amount of light received. The photo sensors126are at least sensitive to a spectrum emitted by the LEDs124.

The LEDs124and the photo sensors126are preferably arranged such that light emitted by any LED124and directly reflected by the inner wall of the dome130cannot reach a photo sensor126. Yet, light scattered by any particle, either solid or liquid, present on the inner wall or the outer wall of the dome130, may be received by any photo sensor126. Therefore, viewed from the top of the pyranometer100, the LEDs124and the photo sensors126are preferably not aligned on one line with the centre of the detector housing module120or with the detector122.

FIG. 2provides a schematic representation of a system200for determining soiling of the pyranometer100. The system200comprises the pyranometer100with the components discussed above. Furthermore,FIG. 2shows the pyranometer100to comprise an optional pyranometer signal processor128. The pyranometer signal processor128receives signals from the detector122and the photo sensors126, processes the signals and transmits the processed signals to a processing module210. The processing may include noise reduction, digitalisation, compression, amplification, filtering, other, or a combination thereof. The pyranometer signal processor128may also be arranged for controlling operation of the LEDs124.

The processing module210comprises a communication unit216for receiving signals from the pyranometer100, either processed or unprocessed. The received signals are provided to a general processing unit212for assessment of the signals. The processing module210may further comprises a storage module214for storing of data, including a computer programme product, like firmware, for programming the general processing unit212for executing operations as discussed above and below. The processing module210may be a separate entity or comprised by the pyranometer100.

The operation of the system200will now be discussed in further detail in conjunction withFIG. 2and a flowchart300provided byFIG. 3. The various parts of the flowchart are briefly summarised in the table below:

302start procedure304daylight operation?306light first LED308light second LED310receive first signal first photo sensor312receive second signal second photo sensor314compare first signal to first threshold316first signal below threshold?318compare second signal to first threshold320second signal below threshold?322compare first signal to second signal324difference below threshold?326determine compensation required328compensate detector signal330end342switch to AC mode344set first warning signal346set second warning signal348set third warning signal

The procedure starts in a terminator302and proceeds by checking for daylight conditions in step304. If daylight condition is determined, the procedure branches to step342for switching to AC—alternating current—mode and proceeds to step306. If no daylight condition is determined, the procedure does not branch to step342and proceeds to step306in DC mode.

In step306, a first LED is lit and in step308, a second LED is lit. Alternatively, more or less LEDs124are lit, yet in this embodiment, two LEDs124are lit. In DC mode, the LEDs124are preferably lit continuously for a pre-determined amount of time. In AC mode, the LEDs124are preferably lit intermittently for a pre-determined amount of time. As background light like daylight and also other regular light during night time, such as moonlight and street lighting, has a substantially continuous nature, intermittently emitted LED light may be differentiated from background light.

In step310, a first signal is received from a first photo sensor and a second signal is received from a second photo sensor126in step312. The first LED124and the second LED124may be activated over the same period or over different periods, either overlapping or complementary, in DC as well as in AC mode. So the second photo sensor126may receive light from the first LED124as well as from the second LED124. This applies to the first photo sensor126as well.

The signals received from the photo sensors126are generated by the photo sensors126in response to receiving light. That light may originate from the LEDs124, but also from background light such as sun, moon, street lighting, other, or a combination thereof. The AC mode is devised to compensate for light not originating from the LEDs124.

In the AC mode, the receiving of the signals includes determining an alternating component in the received signal, preferably a component alternating at the same frequency as at which the LEDS124intermittently emit their light. More in particular, an amplitude of the alternating component is determined as a signal value and a signal magnitude in particular for each of the signals provided by the photo sensors126.

In step314, the first signal and in particular a magnitude of the first signal is compared to a pre-defined threshold. Over normal operation of the pyranometer100, only a small amount of the light emitted by the LEDs124will be reflected by the inner wall of the dome130and most of the light will pass through the dome130. If the outer wall of the dome130is soiled, light passing through the dome130will be scattered and reflected towards the space inside the dome130. The scattered light is received by the photo sensors126. Hence, an increased amount of light received by the photo sensors126and an increased signal magnitude provide an indication of soiling of the outer wall of the dome130.

Therefore, if the magnitude of the first signal is below a first pre-determined threshold, the soiling of the outer wall of the dome130is determined to be below a per-determined threshold. This is implemented in step316: if the first signal is below the first pre-determined threshold, the process proceeds to step318in which the magnitude of the second signal is compared to a second pre-determined threshold. If the first signal is above the first pre-determined threshold, the process branches to step344in which a first warning signal is set, to indicate the outer wall of the dome130is soiled above a particular level—and action may need to be taken.

Alternatively or additionally to providing the first warning signal, the first value of the first signal may provided to a user, for example on a display. Alternatively, the first signal is processed to provide other useful information to a user. Such information may be a loss of transparency of the shield, an indication of a level of pollution of the shield, a level of production loss of a photovoltaic panel in the vicinity of the pyranometer, other, or a combination thereof.

After step318, the process continues to step320. In this step, if the magnitude or another value of the second signal is above the second pre-determined threshold, the process branches off to step346in which a second warning signal is set. If the magnitude or another value of the second signal is below the second pre-determined threshold, the process continues to step322. Also after step346, the process continues to step322. The second pre-determined threshold may be the same as the first pre-determined threshold—or different.

In step322, values of the first signal and the second signal are compared. Preferably, magnitudes of the signals are compared. As discussed above, photo sensors126and LEDs124are distributed along the detector and preferably at regularly distances. It is noted that preferably, a photo sensor126, the detector122and an LED124are not provided on one line. So each photo sensor receives light shattered by another particle at the outer wall of the dome130or shattered by multiple particles at particular areas of the outer wall of the dome130.

This means that, if the signal values compared are substantially equal—or differ by not more than a third pre-determined threshold, the soiling of the outer wall of the dome130is substantially homogeneous. If the signal values compared differ by more than the third pre-determined threshold, the soiling of the outer wall of the dome130is determined to be homogeneous.

If the soiling of the outer wall of the dome130is determined to be homogeneous, difference between the first signal and the first pre-determined threshold and/or difference between the second signal and the second pre-determined threshold may provide an indication for compensation of a detector signal generated by the detector122upon receiving irradiation. Homogeneous soiling affects the general sensitivity of the pyranometer100in general, which allows for determining a compensation. This will be discussed below in further detail.

Compensation may also be possible in case of inhomogeneous soiling, though this will be more difficult as it is difficult to determine the distribution of the soiling. In particular if the level of soiling is distributed randomly over the outer wall of the dome130, determining a way of compensating the detector signal to take the soiling into account is a tedious task. Whereas it may be possible, this embodiment will only compensate for soiling if substantial homogeneous soiling is determined.

Additionally or alternatively, providing compensation may comprise providing an accuracy estimator. The accuracy estimator may be provided as a percentage of a measurement value or a value of a signal provided by the detector122. Alternatively, it may be provided as an absolute value, to be added to or subtracted from a detected value. A radiometer like the pyranometer100has an accuracy of about 1%, out of manufacturing. However, soiling of the dome130of the pyranometer100may seriously affect the accuracy of the pyranometer100as not all light incident to the pyranometer100reaches the detector122. This may even be the case if homogeneous soiling has been determined. A decreased accuracy may be caused by randomness of soiling: even when homogeneous is detected, the soiling will have a random character. This random character means transparency of the dome130to radiation to be detected will also be affected in a random way.

The value with which accuracy is to be corrected due to soiling may be determined based on experimental data. Experiments may provide a link between a level of soiling, a level of homogeneity of the soiling, a signal level of a signal generated by the detector122, another factor or a combination thereof on one hand and the accuracy of the detector122under particular circumstances on the other hand. The corrected accuracy value may be used for correcting a signal level. Alternatively or additionally, the corrected accuracy level may be provided as such to an observer of the system200as shown byFIG. 2.

In step324, the process branches to step348if the difference between the first signal and the second signal is above the third pre-determined threshold. In step348, a third warning signal is issued and the process branches to a terminator330and the process ends.

The process branches to step326from step324if the difference between the first signal and the second signal is below the third pre-determined threshold—and substantially homogeneous soiling is determined. In step326, a particular amount and, in case desired and available, a particular method of compensation is determined. It may be determined the detector value is only to be multiplied by a fixed amount. Alternatively or additionally, compensation may be made dependent on the time of the day.

A reason for doing so is that soiling of the outer wall of the dome130results in scattering of sunlight. This means that if the sun is very low, for example at an angle of less than five degrees with earth's surface, yet more light will be received by the detector122due to the scattering of particles. And if the sun is at a high position, around noon, the detector122will receive less radiation as certain radiation will be reflected away from the detector122, depending on distribution of particles or any other type of soiling.

The compensation may be made more accurate by providing a compensation curve comprising a compensation factor as a function of the time of day. Further accuracy may be obtained by providing calibrated curves per specific location. A reason for providing location dependent curves is that the nature of soiling particles and dust particles in particular vary per geographical location. The compensation value may be used for correcting the signal generated by the detector122or by correcting another value representing sun radiance intensity at a further step of processing the signal provided by the detector122.

In step328, the compensation determined in step326is applied to the detector signal. Subsequently, the process ends in the terminator330.

The processing of the signals, from the photo sensors126and from the detector122, is handled by the general processing unit212. In the embodiments discussed above, the general processing unit212is comprised by the processing module210. Alternatively, the general processing unit212is comprised by the housing110of the pyranometer120.

FIG. 1andFIG. 2show a particular configuration of the pyranometer100and the dome130, the LEDs124and the photo sensors126in particular. In the embodiments shown byFIG. 1andFIG. 2, the LEDs124and the photo sensors126are provided in a single plane, with the LEDs124and the photo sensors126provided in a concentric circle around the detector122. Furthermore, the LEDs124and the photo sensors126are provided in one space, defined by the housing110and the detector housing module120in particular on one side and the dome130on the other hand.

FIG. 4A shows another embodiment, in which the LED124is arranged to couple light into the material of the dome130. At another position at the rim of the dome130, the photo sensor126is provided for receiving—more or less—light emitted by the LED124. Soiling of the outer wall of the dome130provides particles at the dome having a particular refractive index. This may result in light emitted by the LED124to be coupled out of the dome130. And, in turn, this results in less light received by the photo sensor126, resulting in a change of signal strength provided by the photo sensor126.

FIG. 4B shows a further embodiment of the pyranometer100.

Pyranometers are not uncommonly provided with two domes.FIG. 4B shows the pyranometer having two domes, an outer dome130and an inner dome132. Within the inner dome132, the detector122is provided. In space defined by the housing110, the outside of the inner dome132and the inner wall of the outer dome130, the LEDs124and the photo sensors126are provided. The pyranometer100as shown byFIG. 4B has a working principle equivalent to that of the pyranometer shown byFIG. 1.

FIG. 4C shows a yet another embodiment, a radiometer100having a flat window130as a shield for shielding the detector122. The radiometer100is not designed as a common pyranometer, though it may be used for other measurement with respect to intensity and/or radiation of the sun. For example, the radiometer100as depicted byFIG. 4C may be embodied as a pyrheliometer. Also the radiometer100shown byFIG. 4C comprises a housing110and a space defined between the housing110and the shield130for housing the detector122, one or more LEDs124and one or more photo sensors126.

In yet another alternative, the LED124and the photo sensor are provided at opposite sides of the dome130or the shield130.FIG. 4D shows a detector device400as again another embodiment. The detector device400shown byFIG. 4D may have the same construction as the radiometer100shown byFIG. 4C, though without the detector122. The detector device400is primarily intended for detecting soiling of the shield130. The shield130may be directly comprised by the detector device. Alternatively or additionally, the shield of which the soiling is to be detected is a window of a building, like a greenhouse, or a cover of a photovoltaic laminate. Hence, the detector device400may be provided without shield of its own; a transparent part of the building, like a window, fulfils the function of the shield. In such embodiment, the detector device400comprises a shield connection member for connecting the detector device400to a panel of which the soiling is to be determined. The shield connection member may be embodied as a ridge as shown byFIG. 4D, at the outer perimeter of the detector device400. Alternatively or additionally, adhesive elements may be provided, like glue, suction cups, other, or a combination thereof. Yet, providing the detector device400with some shielding is preferred for protection of the components. Such shield of the device itself does not play a role in detection of soiling of a window or other transparent panel of the building

The soiling detection is not limited to the shield comprised by the detector device, it may also be used for detecting soiling of a further shield like a window. In that case, the shield130of the detector device400is place in close vicinity or even in contact with the window. Between the window and the shield130, a substance may be provided for adaptation of refractive indexes to prevent unwanted reflections at interfaces of shield, window and air.

Whereas it is preferred that the LED124and the photo sensor126are provided within one and the same housing110, embodiments may be envisaged in which the photo sensor126and the LED124are placed on either side of a transparent surface of which soiling is to be detected. The detector device400may be embodied in various ways, of which examples are provided below.FIG. 5shows the detector device400provided in a photovoltaic panel500. The photovoltaic panel500comprises a transparent front layer530, a photovoltaic active layer520and a support layer510. The transparent front layer530may be provided in glass, an organic polymer, other, or a combination thereof and is transparent for at least a part of the spectrum of electromagnetic radiation to which the active layer520is sensitive. The transparent front layer530may be provided with an anti-reflective coating.

The photovoltaic active layer520preferably comprises a semiconductor material like silicon, germanium, gallium arsenide, other, or a combination thereof. The active layer520comprising one or more junctions between areas that have opposite conductivity types. The support layer510comprises a material suitable to provide rigidity to the photovoltaic panel500.

The detector device400is in this embodiment integrated in the photovoltaic panel500. For integration in the photovoltaic panel500, the active layer520and the support layer510are locally omitted or removed for accommodating the detector device400. Whereas this constitutes a preferred embodiment, the detector devices400presented in the various examples may also be provided as stand-alone devices as presented byFIG. 4D. Otherwise, the detector device may be provided at the inside of a glass panel of a greenhouse or another building.

In this embodiment, the detector device comprises a first light emitting diode—LED—124as a light source. The first LED124has a focussed beam. The focussing may be enabled by providing the first LED124with a lens, a collimator, another optical beam forming element, or a combination thereof. The beam of the first LED124is directed towards the front layer530of the photovoltaic panel500, which front layer530acts as a shield for the detector device400. More in particular, the beam of the first LED124is directed to the front layer530under a first angle and projects the beam on a first area of incidence532. The area of incidence is in this embodiment defined at the outer side of the front layer530.

The detector device further comprises a first light sensor126that may be embodied as a photodiode or any other suitable device or devices. The first light sensor126is arranged in the detector device400to detect light in a small area, thus detected within a relatively narrow first sensor beam indicated by the dotted line inFIG. 5. The first sensor beam coincides with the outer side of the front layer530at a first detection area534.

The first detection area534is in this embodiment provided within the first area of incidence532. In another embodiment the two areas may have the same size, one may be larger than the other or the other way around; most important is that the two areas at least partially overlap. In yet another embodiment, the first light sensor is arranged to sense light at a wide angle, resulting in a large detection area.

Particles on the outside of the front layer530—dust, sand, pollen, soot, other, or a combination thereof—reflect light emitted by the first LED126in a scattered fashion. The amount of light scattered provides an indication of an amount of pollution on the outside of the front layer530. To properly determine an amount of scattered light, it is advantageous that the first light sensor126does not receive any direct light emitted by the first LED or light within the reflected first beam. The reflected first beam may be a beam reflected by the inside of the front layer530, the outside of the front layer or a beam provided by both reflections. The reflected first beam extends from the front layer under the same angle as under which the first beam is incident to the front layer.

To prevent or at least reduce incidence of direct light of the first beam or the first reflected beam, the first light sensor126may be provided such that it is located out of the light path of the first beam or the first reflected beam. Alternatively or additionally, the first light sensor126is provided with a lens, a collimator or another optical element for reducing a detection angle of the first light sensor.

As depicted byFIG. 5, the first sensor beam is provided under an angle different from the angle under which the first beam is incident to the front layer530. Either the angle under which the first sensor beam is provided, the location of the first light sensor126within the detector device400or both parameters may both be tweaked to ensure in particular light scattered by potential particles on top of the front layer510arrives at the first light sensor126. In the same way, a minimum amount or at most a very small amount of light of the first beam or the reflected first beam arrives at the first light sensor126. This allows for accurate determination of scattered light. And if a proper position is picked, the angle under which the first light sensor126is provided may be the same as the angle under which the first LED124is provided.

The scattered light detected by the first light sensor126is processed by means of a signal processor128which is arranged to amplify, filter, encode, decrypt, compress or digitise the signal or provide a combination of these and/or other processing. The processed or unprocessed signal is provided to the a processing module210. The processing module210comprises a communication unit216for communicating with the detector device400.

The communication unit216may be arranged for processing the signal in accordance with processing performed by the signal processor128; the received signal may for example be decompressed. The signal processed by the communication unit216is provided to the processing unit212. The processing unit212may provide further processing of the signal, for example determining an average value, a derivative of the signal value over time, other, or a combination thereof.

The processing unit212is further arranged to compare the processed signal value, the instantaneous signal value or a combination thereof to one or more pre-determined values stored in a storage module214comprised by the processing module210. Based on that comparison, a warning signal may be presented if the amount of scattered light sensed is too high, as this may be an indication of pollution of the front layer530. And if the front layer530is polluted, less light will reach the active layer520and less energy may be generated.

Alternatively or additionally to providing the first warning signal, the first value of the first signal may provided to a user, for example on a display. Alternatively, the first signal is processed to provide other useful information to a user. Such information may be a loss of transparency of the shield, an indication of a level of pollution of the shield, a level of production loss of a photovoltaic panel in the vicinity of the detector device400or in which the detector device400is provided, other, or a combination thereof. Performance of LEDs is known to degrade over time. Some LEDs have a performance degradation already in the first active hour, typical LEDs lose a few percent of their performance after 10.000 to 20.000 hours and even half their performance after approximately 100.000 hours. Degradation of the performance of the first LED124results the processing module210being able to determine an amount of pollution in a less accurate way. If the first LED124produces less light, less light will be reflected by the same amount of particles on the outside of the front layer530and the signal value generated by the first light sensor126will be less, with the same amount of pollution. Without correction for the degradation, the processing module210will report less pollution than is present in reality.

FIG. 6provides the same photovoltaic panel500with the detector device400as shown byFIG. 5. In addition to the detector device shown byFIG. 5,FIG. 6shows the detector device400comprising a second light sensor626. Other than the first light sensor126, the second light sensor626is arranged to receive light directly from the first beam or from the reflected first beam. Receiving light directly from the first beam is preferred over measurement on the reflected first beam, as the intensity of the reflected beam may be influenced by scattering by particles at the outside of the shield. The signal generated by the second light sensor626in response to receiving the direct light is processed and subsequently evaluated for the signal value by the processing module210.

By assessing pollution of the front layer530based on a quotient of a first value of the first signal provided by the first light sensor126on one hand and a second value of the second signal provided by the second light sensor626, the factor of the degradation of the first LED124is filtered out as a factor that may influence the determined amount of pollution. It is noted that should there be question of degradation of the light sensors, such degradation is also filtered out. Optionally, the first value of the first signal provided by the first light sensor126is divided by the first value plus the second value. This is particularly preferred if the directly reflected signal is measured by the second light sensor626, rather than the direct beam provided by the first LED124.

The amount of light scattered by polluting particles depends on characteristics of the particles. Colour of the particles is a relevant factor in this aspect, apart from other aspects like state of the particle—liquid or solid—material of the particle, smoothness of the particles (rounded vs. sharp edges), other, or a combination thereof. The colour of a particle is an important factor in the amount of light scattered. With the same amount of polluting particles or with the same amount of the front layer530covered with polluting particles, white pollution and highly reflective pollution will provide more scattered light than black pollution or mat pollution. It is preferred to correct a signal value received from the first sensor to adjust for this effect.

Information on adjustment may be provided by storing once or periodically, with updates, correction information comprising a correction factor, in the storage module214. Additionally or alternatively, the colour of the pollution may be determined automatically. The configuration shown byFIG. 7is arrange to provide a correction factor to compensate to at least some extent for differences in colour of pollution.

In the configuration shown byFIG. 7, the detector device comprises a second LED724as a second light source. The second LED724emits light in a second spectrum, that is different from the first spectrum at which the first LED124emits light. More in particular, the first spectrum as a part that has no overlap with the second spectrum and the second spectrum has a part that does not overlap with the first spectrum. For example, the first spectrum covers red to yellow and the second spectrum covers yellow to blue. And the first light sensor126is arranged to be sensitive to the total of the first spectrum and the second spectrum—or at least the largest part of the combined spectrum, including the non-overlapping parts.

The first LED124and the second LED724are operated one after the other. Depending on a first signal value with the first LED124switched on and a second signal value with the second LED724switched on, the colour of the scattered light, collected by the first light sensor, may be determined. If the first signal value is higher with the first LED124switched on compared to the second signal value with the second LED724switched on, it is likely the colour of the particles is reddish, this may indicate the particles comprise sand. If it is the other way around, the particles may be greenish, indicating presence of algae. The processing of the signals received, as well as the method for controlling the LEDs may be executed by the processing module210and the processing unit212in particular.

FIG. 8shows another embodiment, in which the detector device400comprises the first LED124, the first light sensor126and a third light sensor826. Like the first light sensor126, the third light sensor826is arranged to capture light scattered by particles at the outer side of the front layer530, rather than a beam of light provided by the first LED124.

Whereas the embodiment discussed in conjunction withFIG. 7makes use of a wide band sensor and two narrow band light sources, the embodiment shown byFIG. 8makes use of a wide band light source and two narrow band light sensors. More in particular, the first LED124is in this embodiment arranged to emit light over a large part of the visible spectrum. To this purpose, the first LED124may be an LED module, comprising LEDs emitting light at different narrow spectra. Alternatively or additionally, the first LED124is a white LED, i.e. a blue light LED covered with a layer breaking down blue light into light with multiple longer wavelengths or lower frequencies.

Likewise, the first light sensor126is arranged to cover a lower part of the spectrum emitted by the first LED124and the third light sensor826is arranged to be sensitive to a higher part of the spectrum emitted by the first LED124. For obtaining at least some colour information on particles on the outer side of the front layer530, the first LED124is switched on and signal values of signals provided by the light sensors are compared. If a first signal value provided by the first light sensor126is higher than a third signal value provided by the third light sensor826, the particles probably have a reddish colour and if the first signal value is lower than the third signal value, the particles probably have a greenish colour.

The embodiments discussed in conjunction withFIG. 7andFIG. 8may be extended with more LEDs, light sensors, or both for more accurate detection of the colours of particles polluting the front layer530. An issue that may not be directly addressed at this point is an amount of absorption of the particles; pure black particles will provide the same frequency response on sensors are pure white particles. However, pure white particles will provide higher signal values as they reflect more light than black particles. Black particles will absorb more light.

To address this issue, the intensity of light directly emitted by the light sources may be measured and the intensity of the light of light in the beam reflected by the front layer530may be measured and both values are compared. These values, optionally together with a value indicating an amount of received scattered light, may provide an indication of an amount of light absorbed by the particles. And the amount of absorption may provide a further indication on characteristics of the particles, which, in turn, may contribute to providing a correction factor.

FIG. 9shows another embodiment for determining colour of particles on the front layer530or in the surroundings of the photovoltaic panel500. The configuration shown byFIG. 9is based on the configuration discussed in conjunction withFIG. 5. In addition to the configuration discussed in conjunction withFIG. 5, at least one of a first colour sensor912and a second colour sensor922may be added to the system. The first colour sensor912is provided in the detector device400, preferably close to or in contact with the front layer530. This allows the first colour sensor912to gather information on colour or colours of particles present at the outside of the front layer530. This information may be used to look up a correction factor in a correction factor database218stored on the storage module214.

The second colour sensor922is provided apart from the photovoltaic panel500. This allows the second colour sensor922to collect information on colour within the surroundings of the photovoltaic panel. This information may be used to look up a correction factor in a correction factor database218stored on the storage module214.

Expressions such as “comprise”, “include”, “incorporate”, “contain”, “is” and “have” are to be construed in a non-exclusive manner when interpreting the description and its associated claims, namely construed to allow for other items or components which are not explicitly defined also to be present. Reference to the singular is also to be construed in be a reference to the plural and vice versa.

In the description above, it will be understood that when an element such as layer, region or substrate is referred to as being “on” or “onto” another element, the element is either directly on the other element, or intervening elements may also be present.

Furthermore, the invention may also be embodied with less components than provided in the embodiments described here, wherein one component carries out multiple functions. Just as well may the invention be embodied using more elements than depicted in the Figures, wherein functions carried out by one component in the embodiment provided are distributed over multiple components.

A person skilled in the art will readily appreciate that various parameters disclosed in the description may be modified and that various embodiments disclosed and/or claimed may be combined without departing from the scope of the invention.