WO2015162322A1

WO2015162322A1 - A dirt sensor and method for detecting the amount of dirt on a surface

Info

Publication number: WO2015162322A1
Application number: PCT/ES2015/070308
Authority: WO
Inventors: Roberto CALVO OSTIATEGUI; David CANTERO BURGOS
Original assignee: Fundación Tekniker
Priority date: 2014-04-24
Filing date: 2015-04-17
Publication date: 2015-10-29
Also published as: ES2549395B1; ES2549395A1

Abstract

The invention relates to a sensor (20) for detecting the amount of dirt on a surface, said sensor comprising: a transparent surface (21) exposed to dirt; a light emitter (22) for emitting a beam towards the transparent surface; a first light receiver (23) and a second light receiver (25); means (24) disposed between the light emitter (22) and the transparent surface (21) and configured to allow part of the light from the emitter to pass towards the transparent surface (21) and to re-direct the rest of the light towards the second receiver (25); acquisition and processing means (26). The first light receiver (23) is configured to receive light diffused upon impact with dirt on the external face (21e) of the transparent surface (21). The acquisition and processing means (26) are configured to calculate a percentage measurement of the degree of dirt on said transparent surface (21).

Description

A DIRT SENSOR AND METHOD FOR DETECTING THE AMOUNT OF DIRT ON A SURFACE

Field of the Invention

The present invention pertains to the field of dirt or dust sensors, and more specifically, to the field of dirt sensors deposited on surfaces, whether flat, curved, rough or of other characteristics. An application of the dirt sensor of the invention is the measurement of dirt in concentrating mirrors of solar thermal plants.

Background of the invention

There is currently a great interest in the manufacture of large solar plants to generate energy. One of the most advanced technologies is based on tower technology plants. The technique uses a set of mirrors or reflective surfaces (called heliostats) that follow the sun and concentrate solar radiation on one or more receivers that are located on top of a tower. From the receiver or receivers the heat is transferred to a heat transfer fluid, for example water, which will be the source of steam to in turn produce electricity. This technology is very efficient due to the high temperatures at which it can be operated. Also noteworthy are the parabolic trough technology plants, based on a set of parabolic trough mirrors (parabolic trough concentrators, CCP) that are placed on a structure so that they can follow the movement of the sun. The captured solar radiation is concentrated on a receiver tube that carries a fluid capable of absorbing heat. The fluid thus reaches high temperatures and transmits thermal energy to water vapor, which will be the source of steam to generate electricity. Especially in the case of reflective surfaces intended for use in solar plants to generate energy, it is interesting that the mirror is 100% used. In this application, heliostats are designed to be practically flat. Possible defects that may occur are edge or corner defects, for example.

A key element in these plants is the reflective surfaces that concentrate the solar radiation, also called solar thermal concentrators. One aspect to control in these reflective surfaces lies in the geometry, since for the surfaces to concentrate solar radiation in the area of interest, their shape must be adequate. Another fundamental aspect is its cleaning, because any dust particle impairs the solar performance of the concentrator.

To determine the dirt in mirrors (in general, on reflective surfaces of the type mentioned in the previous paragraph), commercial equipment called reflectometers is known, which allow to measure directly the loss of reflectivity of a mirror with great precision. Their main problem is that they are very expensive, partially interfere with the light that you want to reflect in the mirror, require a precise mechanical adjustment with respect to the surface and are susceptible to their optics picking up dirt, which detracts from the measurement. US patent US8323421 B2 describes an automatic cleaning system for solar panels comprising, among other elements, a detection device capable of determining if the solar panels need to be cleaned.

The international patent application WO2012 / 089485A1 describes a photovoltaic module that generates electrical energy from solar energy, in which the surface dirt can be detected. The photovoltaic module consists of a solar panel and detection means that indicate the amount of dust and dirt accumulated on the solar panel. Detection occurs through a receptacle with a surface parallel to the solar cell, a light source outside the receptacle, a set of sensors inside the receptacle and a control unit to monitor the amount of light that reaches the sensors and generate a distorted signal when the amount of light is below a certain threshold.

Description of the invention

The present invention provides a device and method capable of determining the amount of dust or dirt deposited on its sensitive surface.

In a first aspect of the invention, a sensor is provided to detect the amount of dirt on a surface. The sensor comprises: a transparent surface that is exposed to dirt on its outer face; a light emitter configured to emit a beam of light in a certain frequency range towards the transparent surface whose dirt on its outer face is to be measured; a first light receiver configured to detect light in said particular frequency range; a second light receiver configured to detect light in said particular frequency range; means located between the light emitter and the transparent surface, these means being configured to pass a part of the light from the light emitter towards the transparent surface and to redirect the rest of the light coming from the light emitter to the second receiver of light; means of acquisition and processing. The first light receiver is configured to receive, from said part of the light coming from the light emitter that reaches the transparent surface, the light diffused upon hitting the dirt on the outer face of the transparent surface. The acquisition and processing means are configured to calculate, from the light detected by the second light receiver and the light detected by the first light receiver, a percentage measurement of the degree of dirt of the transparent surface.

Preferably, the means located between the light emitter and the transparent surface are a beam splitter.

Preferably, the light emitter is located in a plane that forms an angle of α degrees with the plane in which the transparent surface is located, where this angle α varies between 30 and 60 ^e .

Preferably the first light receiver comprises a matrix of photodiodes.

Preferably, the means located between the light emitter and the transparent surface are located in a plane that forms an angle of β degrees with the plane in which the light emitter is located, where said angle β varies between 30 and 60 ^e .

Preferably the light emitter is configured to emit a light beam in the infrared light range and the first and second light receivers are configured to detect light in the infrared light range. Preferably, the acquisition and processing means comprise means for adjusting the measurement as a function of temperature. Preferably the transparent surface is a crystal.

In a possible embodiment, a light dimmer is arranged in front of the second receiver. In another possible embodiment, the use of the sensor described above is provided to detect the amount of dirt from a thermal energy concentrating mirror.

In another possible embodiment, a method is provided to detect the amount of dirt on a surface. The procedure has the steps of: emitting a beam of light in a certain frequency range towards a transparent surface that is exposed to dirt on its outer face; to capture in a first light receiver the diffused light when it hits the dirt of the transparent surface; divert a part of the emitted light, before it reaches the transparent surface, to capture it in a second light receiver, to take said deflected light as a reference; calculate, from the light detected by the second light receiver and the light detected by the first light receiver, a percentage measure of the degree of dirtiness of the transparent surface.

Finally, a computer program is provided that comprises computer program code instructions for performing the above method.

Additional advantages and features of the invention will be apparent from the following detailed description and will be pointed out in particular in the appended claims.

Brief description of the figures

To complement the description and in order to help a better understanding of the features of the invention, according to an example of practical embodiment It is accompanied as an integral part of the description, a set of figures in which the following has been represented by way of illustration and not limitation: Figure 1 illustrates the principle of operation of the sensor of the invention.

Figure 2A illustrates a scheme of the elements that form a device according to a possible embodiment of the invention. Figure 2B shows a front exterior view of the device according to a possible embodiment of the invention. Figure 2C shows a detail of the scheme of Figure 2A, in which it is observed how the angle of reflection of the light reflected by the beam splitter is equal to the angle of incidence of the incident beam in the beam splitter.

Figure 3 shows a block diagram of the acquisition and processing electronics according to a possible embodiment of the invention.

Figure 4 shows in detail the processing block of the signals provided by the measurement and reference receivers.

DESCRIPTION OF AN EMBODIMENT OF THE INVENTION Figure 1 illustrates the principle of operation of the sensor of the invention: when lighting with a light source 12 the dirt particles 1 1 deposited on a transparent surface 10, the light is diffused in different directions, increasing the amount of scattered light 13 in proportion to the amount of dust deposited on that surface. The light is emitted towards the inner part of the surface 10 (for example, a crystal), so that the light diffused 13 by the dirt particles 1 1 deposited on the outer part of the surface 10, is projected (scattered) towards an optical transducer (for example, a set of photodetectors) 14. The amount of light captured by these is proportional to the number of particles deposited 1 1. The optical transducer, controlled by a signal processing and processing unit (not illustrated), provides a measure corresponding to that amount.

The sensor of the invention, based on the above operating principle, manages to optimize a number of properties, namely: minimizing interference from light sources (natural or artificial); minimize dependence on measures with temperature; and maximize sensitivity.

Figure 2A illustrates a scheme of a device according to a possible embodiment of the invention. It is a dirt sensor 20 for detecting the amount of dirt (dust or other particles) of a transparent surface 21 that is exposed, by its outer face 21 e, to the dirt. The transparent surface 21 is preferably a transparent crystal. In the scheme of Figure 2, the transparent surface 21 is substantially flat, but it is not essential that it be so. On the contrary, the surface 21 can be curved, rough or of any other characteristic. Surface 21 acts as a dirt collector. In use of the device 20 as a dirt sensor, the device can be placed next to a surface of interest, whether flat, curved, rough, etc. The device 20 will indicate the amount of dust presumably deposited on that surface of interest, considering that it is next to the sensor (and therefore both will pick up a similar amount of dust or dirt). Therefore, the device 20 serves to indicate that in its sensitive area (outer face 21 e of the surface 21) it has captured a certain amount of dirt.

As Figure 2A generally shows, the dirt sensor 20 comprises a transparent surface 21 which is the sensitive area whose captured dirt is to be detected, emulating what happens on a surface whose dirt is to be controlled. A possible example of a surface whose dirt is to be emulated, for which the device 20 is located close to that surface, is an energy concentrating mirror. Thus, if dust is deposited in the mirror, substantially to the same extent dust will be deposited on the transparent surface 21 of the sensor 20. By measuring the powder on the surface 21 of the sensor 20, it is extrapolated that approximately the same amount will have been deposited in the concentrating mirror

The dirt sensor 20 also comprises a light emitter 22, preferably infrared. Alternatively, the light emitter 22 may emit at another frequency or frequencies other than the infrared spectral range, such as the visible spectrum or other frequencies of the non-visible spectrum. This emitter 22 emits a beam of light towards the inner surface 21 i of the transparent surface 21 whose dirt is to be measured. Preferably, the emitter 22 is placed in a plane that forms an angle of α degrees with respect to the plane of the sample (transparent surface 21). That is to say, preferably the emitter 22 does not emit from a plane parallel to the flat surface 21. Preferably the angle α varies between 30 and 60 ^e . In the implementation of the example, α = 45 ^e has been chosen. Alternatively, the emitter 22 can emit its light beam substantially perpendicular to the transparent surface 21. In this case, the emitter 22 is placed in a plane substantially parallel to that of the transparent surface 21 (implementation not shown).

The sensor 20 also comprises a light receiver or photodetector 23 configured to capture the light in the same spectral range of the emitter 22. That is, if the emitter 22 emits an infrared beam of light, the receiver 23 is an infrared light receiver. The light receiver 23 must be located in that place / position that allows to collect the maximum light diffused by the dust / dirt deposited on the outside of the transparent surface 21. In a possible embodiment, illustrated in Figure 2A, the light receiver 23 is placed in a plane substantially parallel to the sample (transparent surface 21), on the inner side 21 i thereof. The receiver 23 can alternatively be placed in other positions, as long as it can from them substantially capture the amount of light diffused by the powder according to the sensitivity required for the particular sensor 21. This receiver 23 is preferably formed by a set of photodiodes. In a specific implementation, receiver 23 is implemented with a matrix of four photodiodes. The inventors have observed that these four photodetectors, whose effects add up, distributed over part of the area towards which the diffused light is directed, provide high sensitivity for the concrete geometry tested. The sensor 20 also comprises a beam splitter 24 (in English, beam splitter), located between the emitter 22 and the transparent surface 21 whose dirt is to be measured. The beam splitter 24 must be placed in a position such that the beam of light emitted by the emitter 22 towards the surface 21 is in its path with the beam splitter 24.

The sensor 20 also comprises a reference receiver (photodetector) 25. Optionally, the sensor 20 may have a light dimmer 27 located in front of the reference receiver 25 and preferably substantially parallel thereto. The dimmer 27 is used if it is necessary to avoid saturating the reference photodetector 25. The sensor 20 also has acquisition and processing means 26, to interpret and process the measurements, which are detailed in relation to Figures 3 and 4.

The operation of the sensor 20 is as follows: the emitter 22 projects the light it emits on the inner face 21 i (protected with respect to the external dirt) of the transparent surface 21 whose dirt is to be measured. The light emitted by the emitter 22 is in its path with the beam splitter 24, which must therefore be arranged between the emitter 22 and the transparent surface 21, so that the beam of light emitted by the emitter 22 reaches the beam splitter 24. The splitter 24 allows a percentage of the emitted power to pass to the flat surface, while redirecting (deflecting) the rest of the emitted power. Figure 2C shows a detail of the scheme of Figure 2A, in which it is observed, as an expert knows, how the angle of incidence p-with respect to the perpendicular of the divider 24- of the incident beam from the emitter 22 is equal to angle of reflection p of the light reflected or deflected by the beam splitter 24. This deflected power is captured by the reference photodetector 25. Therefore, the reference photodetector 25 can be placed in any position, provided it is capable of capturing the radiation deflected by the beam splitter 24. It is thus possible to obtain a sample of the light signal emitted by the emitter 22. The amount (percentage) of light that passes through without diverting the beam splitter 24 towards the transparent surface 21 and the amount (percentage) of light that passes through the beam splitter 24 to the transparent surface 21 depends on the geometries of the beams and the specific elements used in the sensor. For example, the sensor 20 can be designed so that the percentage of light that passes through without deflecting the beam splitter 24 towards the transparent surface 21 is between 50% and 90%, while the percentage of light that deflects towards the reference photodetector 25 is between 50% and 10%. The reference photodetector 25 thus provides a measure of the actual intensity of the light generated by the emitter 22.

The light (the percentage of light that has passed, without deviating, the beam splitter 24) received on the transparent surface 21 (on its inner face 21 i) crosses said surface 21. If there is dirt on its outer surface 21 e, the light, when it hits the dirt, diffuses in the direction of the inner face 21 i, until it reaches the array of photodetectors or measurement receiver 23. For this reason, the measurement receiver 23 must be placed in any position that allows it to capture diffused radiation when it hits With the dirt. In the ideal case that there was no dirt on the outer face 21 e, there would be no diffusion of the light and, therefore, all the light that reached the flat surface 21 would pass through it to the outside. However, in practice, even assuming that the outer face 21 e was completely clean, the inner face 21 i of the transparent surface would reflect a small part of the light towards the inside of the sensor, behaving like a mirror. The amount of light reflected by this effect depends on the type of surface. The sensor 20 of the invention eliminates this effect by means of a parameter that is obtained in the sensor calibration process. The arrangement of the emitter 22, beam splitter 24, sample surface 21 and reference receiver 25 should be chosen such that a sufficient amount of power from the emitter 22 reaches the sample surface 21 (for example, between 50% and 90%), that a sufficient amount of power from the emitter 22 arrives, redirected by the beam splitter 24, to the reference receiver 25 (for example, between 50% and 10%) and that interference between the elements is minimized internal to the sensor 20. In a preferred embodiment, the beam splitter 24 is preferably located in a plane that forms an angle of β degrees with respect to the plane in which the light emitter is located 22. Preferably the angle β varies between 30 and 60 ^e . In the implementation of the example, β = 45 ^e has been chosen. Taking into account that, in the implementation of the example, the reference receiver 25 is located at 90 ^e with respect to the main beam emitted by the emitter 22, this β = 45 ^and minimizes interference between the sensor elements.

In the implementation of the example, a substantially flat transparent surface 21 has been chosen, but another type of surface could alternatively be chosen. In addition, the transparent surface is, in this example, rectangular in shape, as seen in Figure 2B. Also in the implementation of the example, as a transparent surface 21 a substrate has been chosen which is used to manufacture parabolic concentrators, because a prominent application of the sensor of the invention is the detection of dirt in parabolic concentrators. The same substrate has been chosen to have the same level of adhesion.

The acquisition and processing means (acquisition and processing electronics) 26 first control the light emission (from the emitter 22) by means of a controlled sinusoidal current signal that is digitally generated by a software component (for example, but not limited to, executed at 4 kHz). As detailed in relation to Figures 3 and 4, the acquisition and processing electronics acquire the signals of the measurement photodetectors 23 and reference 25 and process them in a software component (which in an example, but not limitatively, it also runs at a frequency of 4kHz).

As indicated above, sensor 20 minimizes interference from light sources (natural or artificial); minimizes the dependence of the measurements with the temperature; and maximizes sensitivity.

The interference of the external light sources to the sensor (for example, from the sun) is minimized as explained below: First, the light source 22 is chosen within the spectrum so that its intensity exceeds the amount of light received from the sun in the same spectrum band. Preferably, an infrared light source is chosen. Secondly, the photodetectors 23 25 incorporate selective filters (infrared in the event that the emitter emits infrared light). Finally, the light source 22 is modulated with a relatively high frequency (for example, between 80 and 150 Hz). In one example, a signal modulated at 135 Hz is chosen. This frequency is chosen so that it is high enough to reject possible oscillating natural or artificial light sources, but avoiding in any case the 50Hz and 60Hz harmonics of the networks of feeding, and so that it is generated with a high resolution taking into account the sampling frequency of the generator. Since the light source 22 is modulated at a specific frequency, the part of that light diffused by the dirt also oscillates at that same frequency. For this reason, the detection means 23 comprise a selective bandpass filter that allows collecting only the diffused light caused by the emitter 22 and not by other light sources. The measure of diffused light is thus always provided to the light itself emitted by the emitter 22 of the sensor 20.

With respect to the effect on the measures of the temperature variation, this affects the sensor 20 in multiple aspects, which include both the electronics itself and the opto-mechanical structure of the assembly and the infrared emitting and receiving devices. Therefore, the sensor 20 comprises means and techniques to perform thermal compensation In particular, the sensor 20 comprises means for performing thermal compensations in the infrared emitting and receiving devices, in which these thermal effects are especially relevant. The infrared emitters have a high variation with the temperature of the relationship between the intensity of light emitted and the current applied (gain variation around 0.5% / ^e C or higher). This variation, in addition, is different between different units of the same series of devices with the same product reference. Also the photodetectors show variation of gain with the temperature, although to a lesser extent, and this is also different between different units of the same component. In addition, photodetectors have a signal level or offset in the dark that varies with temperature. This "offset" is also different between different units of the same component.

Preferably, the sensor 20 minimizes the effect of the offset signal of the photodetectors 23 25 by the aforementioned modulation technique and by means of a demodulation and bandpass filtering that eliminates all contributions of continuous light reception in the signal received in all photodetectors 23 25.

In addition, the effect of the gain variation on the light emission source 22 is preferably minimized by a differential photodetection technique: the emitted light is continuously measured (on the emitter 22) and the received diffused light is weighted (on the receiver 23) with respect to the emitted light. This technique also eliminates the common gain variation component of the two receivers (reference 25 and measurement 23), but does not eliminate the difference in gain between the different photodetector components. Preferably, the effect of the gain difference between the photodetectors is minimized by a temperature transducer and a compensation algorithm.

Finally, the sensitivity is preferably maximized thanks to the selection and control of a light source 22, preferably infrared, of high intensity and photodetection through a matrix of 4 photodetectors that add the current obtained in each of them and are also exposed to different areas of detection of diffused light, thus adding the collection of light energy in an area greater than a single photodetector. Figure 3 shows a block diagram of a possible implementation of the acquisition and processing electronics 26. A Calib_Gain parameter allows to adjust the measuring range of the sensor. This parameter is obtained through the calibration process. Another Calib_Bias parameter allows to eliminate the diffusion effects of the emitted light generated by the internal walls of the sensor 20 itself and the reflection effect of the internal face 21 i of the surface 21. This parameter is also obtained through the calibration process. A processing block 261 performs the demodulation of the two signals provided by the measurement photodiodes 23 (signal 2623) and reference 25 (signal 2625) and provides the weighted information equivalent to the measured / reference ratio out_261. A block 262 converts the values from both by 1 and by 100 using the Calib_Gain parameter. A block 263 allows the measurement to be modified as a function of temperature by means of an interpolation algorithm. It allows therefore to correct the difference in gain with the temperature between photodetectors and other thermal drifts, for which a subsystem 265 intervenes. Finally, block 264 performs statistical calculations of the measurement in a given period of time (averaged measurement, maximum value, minimum value and standard deviation).

Figure 4 shows in detail a part of the processing block 261 of the signals provided by the measurement 23 and reference receivers 25. On the two signals 2623 2625 (from the two receivers 23 25) the same processing is performed: First The received signal 2623 2625 is treated by a zero-order sampler or sampler 261 1. The resulting signal is treated by a bandpass filter 2612, preferably of order 8, and with cut-off frequency centered on a frequency that preferably varies between 80 and 150 Hz (for example, 135 Hz). The signal thus obtained is rectified 2613 and subsequently treated by a low-pass filter 2614 preferably of the second order (for example 2 Hz cut-off frequency). The result of this processing is the value of the amplitude of the measurement signals 2614m and reference 2614r. By dividing 2615 between the two, the measured / reference ratio out_261 is obtained, which therefore corresponds to a differential measurement (in which the thermal influences indicated above disappear). The processing block 261 further comprises other elements intended for Detect if the measurement is incorrect. For simplicity, these additional blocks are not shown in the scheme of Figure 4.

As can be seen, the dirt sensor 20 provides a percentage measure of the degree of cleanliness (or dirt) of the exposed sample surface 21. This measure thus has a value of 0 when the dirt is maximum and a value of 100 when the dirt is minimal. The terms maximum dirt- minimum dirt are established in the device calibration process, considering that the minimum dirt is reached after cleaning the surface 21 as much as possible, while for different dirt different patterns of dirt or diffusion of the surface can be considered. light. To detect the dirt of any surface, for example a concentrating mirror of a solar thermal power plant, the sensor 20 must be placed next to the surface of interest. Considering that, due to the proximity, the sample surface 21 of the sensor 20 will capture substantially the same amount of dirt as the surface of interest, the device 20 will indicate the amount of dust presumably deposited on that surface of interest.

The measurement process establishes an initial period of programmable stabilization after which the equipment measures cyclically or periodically. The period between measurements is also programmable and is limited by the minimum acquisition and processing time of the measurement data. In a possible embodiment, this minimum period has been established in 10 seconds (optimized after experimental tests), while the result of the sensor measurement corresponds to the average of the measurements calculated in the stationary time. Preferably, the result of the sensor measurement corresponds to the average of the measurements calculated in the second half of the acquisition period, which is when the demodulated signals reach stability.

Preferably, the calibration process requires two measurements that define the dynamic range of the sensor. With the substrate (transparent surface 21) completely clean, the first reference measurement is made that defines the maximum degree of cleanliness identified with the 100% value. For the second measure, which is used as a reference for the minimum cleaning, the degree of dirt that is identified with the value 0% is deposited on the substrate. The final result of the process It is a relative measurement of dirt with a range between 0 and 100 so that the maximum value corresponds to the maximum relative cleanliness and the minimum value to the minimum relative cleanliness. To simplify access to measurements and calibration, a computer application has been developed that allows real-time monitoring of the measurement process and modification of the device's internal configuration parameters. The application also has support mechanisms that simplify the calibration process.

A prototype has been developed that is powered by a voltage between 5 and 24V and has a MODBUS RTU interface via RS485 through which the measurement result can be accessed and allows the configuration of operating parameters remotely.

The sensor described here has special application in the measurement and control of dirt in concentrating mirrors of solar thermal plants.

In this text, the word "comprises" and its variants (such as "understanding", etc.) should not be construed as excluding, that is, they do not exclude the possibility that what is described includes other elements, steps, etc.

On the other hand, the invention is not limited to the specific embodiments that have been described but also covers, for example, the variants that can be made by the average person skilled in the art (for example, in terms of the choice of materials, dimensions , components, configuration, etc.), within what follows from the claims.

Claims

1 .- A sensor (20) to detect the amount of dirt on a surface, characterized in that it comprises: a transparent surface (21) that is exposed to dirt on its outer face (21 e); a light emitter (22) configured to emit a beam of light in a certain frequency range towards said transparent surface (21) whose dirt on its outer face (21 e) is to be measured; a first light receiver (23) configured to detect light in said certain frequency range; a second light receiver (25) configured to detect light in said particular frequency range; means (24) located between said light emitter (22) and said transparent surface (21), said means (24) being configured to allow a portion of the light from said light emitter (22) to pass through said transparent surface ( 21) and to redirect the remaining light from said light emitter (22) to said second light receiver (25); means of acquisition and processing (26); said first light receiver (23) being configured to receive, from said part of the light coming from the light emitter (22) that reaches the transparent surface (21), the light diffused when hitting the dirt of the external face ( 21 e) of said transparent surface (21); and said acquisition and processing means (26) being configured to calculate, from the light detected by said second light receiver (25) and from the light detected by said first light receiver (23), a percentage measure of the degree of dirt from said transparent surface (21).

2. - The sensor (20) of claim 1, wherein said means (24) located between said light emitter (22) and said transparent surface (21) are a beam splitter.

3. The sensor of any of claims 1 or 2, wherein said light emitter (22) is located in a plane that forms an angle of α degrees with the plane in which the transparent surface (21) is located, where said angle α varies between 30 and 60 ^e .

4. The sensor of any of the preceding claims, wherein said first light receiver (23) comprises a photodiode array.

5. The sensor of any of the preceding claims, wherein said means (24) located between said light emitter (22) and said transparent surface (21) are located in a plane that forms an angle of β degrees with the plane in which is located the light emitter (22), where said angle β varies between 30 and 60 ^e .

6. The sensor of any of the preceding claims, wherein said light emitter (22) is configured to emit a beam of light in the infrared light range and said first (23) and second (25) light receivers are configured to detect light in the infrared light range.

7. The sensor of any of the preceding claims, wherein said acquisition and processing means (26) comprise means for adjusting the measurement as a function of temperature.

8. The sensor of any of the preceding claims, wherein said transparent surface (21) is a crystal.

9. The sensor of any of the preceding claims, further comprising a light dimmer (27) arranged in front of the second receiver (25).

10. - Use of the sensor of any of the preceding claims to detect the amount of dirt of a thermal energy concentrating mirror.

eleven . -A procedure to detect the amount of dirt on a surface, characterized by the stages of:

- emitting a beam of light in a certain frequency range towards a transparent surface (21) that is exposed to dirt on its outer face (21 e);

- capturing in a first light receiver (23) the light diffused when hitting the dirt of said transparent surface (21);

- diverting a part of the emitted light, before it reaches said transparent surface (21), to capture it in a second light receiver (25), to take said deflected light as a reference;

- calculating, from the light detected by said second light receiver (25) and from the light detected by said first light receiver (23), a percentage measurement of the degree of dirtiness of said transparent surface (21).

12. Computer program comprising computer program code instructions for performing the method of claim 1 1.