Optical Soiling Measurement Device for Photovoltaic Arrays

Abstract

A device comprising a transparent window, a photoemitter, a photodetector, and a measurement unit, wherein said photoemitter is configured to illuminate soiling particles accumulating on a surface of said transparent window, said photodetector is configured to generate a signal based on detection of light that passes through said transparent window and reflects and/or scatters from said soiling particles, and said measurement unit determines a soiling level of said transparent window based upon a measurement of said signal.

1 . A device comprising
 a transparent window,   a photoemitter,   a photodetector, and   a measurement unit,   wherein   said photoemitter is configured to emit light that illuminates soiling particles accumulating on a surface of said transparent window,   said photodetector is configured to generate a signal based at least upon detection of a portion of said light that passes through said transparent window and reflects and/or scatters from said soiling particles, and   said measurement unit is configured to determine a soiling level of said transparent window, wherein said soiling level corresponds to a reduction in transmission of sunlight due to said soiling particles, based at least upon said signal and upon calibration data, wherein said calibration data relates said signal to sunlight transmission loss due to said soiling particles.   
     
     
         2 . The device of  claim 1 , wherein said light emitted by said photoemitter is incident upon said transparent window at an angle less than the angle of total reflection. 
     
     
         3 . The device of  claim 1 , wherein said photoemitter is configured to emit said light at a wavelength that is not dominant within sunlight and wherein said photodetector is configured to substantially reject light at wavelengths other than said wavelength. 
     
     
         4 . The device of  claim 1 , wherein said photoemitter is configured to operate in a pulsed or modulated mode, and said signal from said photodetector is detected synchronously with said pulse or said modulation. 
     
     
         5 . The device of  claim 1 , further comprising a reference photodetector which is configured to receive a portion of said light from said photoemitter and to produce a signal therefrom, and wherein said soiling level is determined by comparison of said signals from said photodetector and said reference photodetector. 
     
     
         6 . The device of  claim 1 , further comprising an irradiance sensor upon which soiling particles accumulate, wherein said device is configured to correct readings of said irradiance sensor by said determinations of said soiling level to account for soiling particles obscuring said irradiance sensor, and/or to calibrate said soiling sensor by comparison with said irradiance sensor before and after cleaning. 
     
     
         7 . The device of  claim 1 , wherein said photoemitter and/or said photodetector are configured to be incident upon an edge face of said transparent window. 
     
     
         8 . A method comprising
 allowing soiling particles to accumulate on a surface of a transparent window,   illuminating said surface by emission of light from a photoemitter,   measuring by a photodetector a portion of said light that scatters or reflects from said soiling particles, and   determining a soiling level of said transparent window from a signal from said photodetector, wherein said soiling level corresponds to a reduction in transmission of sunlight due to said soiling particles, based at least upon said signal and upon calibration data, wherein said calibration data relates said signal to sunlight transmission loss due to said soiling particles.   
     
     
         9 . The method of  claim 8 , wherein said light is incident upon said transparent window at an angle less than the angle of total reflection. 
     
     
         10 . The method of  claim 8 , further comprising directing a portion of said light from said photoemitter to a reference photodetector, and determining said soiling level by comparing signals from said photodetector and said reference photodetector. 
     
     
         11 . The method of  claim 8 , further comprising operating said photoemitter in a pulsed or modulated mode and detecting signals from said photodetector synchronously with said pulse or said modulation.