Today, solar energy generation has become one of the commonly used methods to fulfill the energy requirements. The drawbacks of conventional sources of energy have pushed the generation towards non-conventional field. Indian continent has 250-300 sunny days giving rise to approximately 5x1015kWh energy. When solar radiations fall on the surface of solar panels, they are absorbed by solar cells and due to photoelectric effect, these solar radiations are converted into useful electrical energy. The performance of solar panels decreases due to rise in temperature. The main reason behind the temperature rise is dust, dirt, shading of solar panels due to tree leaves, bird and cement deposits. This loss of power is in heat form and not visible by naked human eye. Therefore, a thermal imaging camera is used to visualize the emitted heat from the heated solar panel surface. The thermal imaging camera captures infrared radiations emitted by heated object and produce thermal images, which can be used to show the temperature of due to these deposits.
Solar Panels
Solar panels are composed of several solar cells, which are arranged in a well-designed combination. Solar cells are the smallest component of solar panels which are responsible for converting solar radiations into electrical energy. The basic equivalent circuit diagram of a solar cell is shown in the figure 1. Where Ipv and Vpv are output photo-voltaic current and output photo-voltaic voltage of the solar PV cell respectively, Iph is the photo-generated current, ID is the diode current, Rsh and Rse are the shunt and series resistances respectively, Ish is the shunt field current.
Figure 1: Equivalent circuit diagram of a solar cell
There are different types of solar photo-voltaic technologies used which can be listed as follows:
• Crystalline silicon type- they are further divided into Mono-Crystalline and Poly-Crystalline PV Cells.
• Thin film type- They are divided into Cd Te (Cadmium Telluride), CIGS (Copper Indium Gallium Selenide), and a-Si (Amorphous Silicon).
Basics of Thermal Imaging
Thermal imaging is a technique to measure the infrared radiations emitted by any object whose temperature is above absolute zero (-273°C). The device used to capture these infrared radiations is known as thermal imaging camera and the captured image is called as thermogram or thermal image. These thermograms show colourful thermal images on the screen of thermal camera with a temperature scale mentioning the temperature range of the captured object. The dark colour shows hot areas of temperature while bright colour represents the colder portions in thermal image.
Temperature Measurement by Thermal Imaging
When solar panels are exposed to direct contact of open atmosphere the dirt and dust are the main factors to increase the temperature. The rise in temperature of solar panel components causes heating and loss of power in both heat form and power form. The deposition of bird beat and cement deposits cause a local shading on the deposited surface area as a result heating due to rise in temperature starts. When the heating due to these deposits exceed, a power loss in the generated solar energy occur which finally leads to decrease the performance.
Normal visualization of solar panels through naked eye does not identify the heating due to these deposition effects. Thermal images are obtained by thermal imaging camera which shows the temperatures of these deposits with the help of a temperature scale. Therefore, thermal imaging is a good technique for measuring the temperature of the heated solar panels.
Figure 2: Kusam Meco Thermal Imaging Camera
Work Done to Study Temperature Measurement of Bird and Cement Deposits on Solar Panels
Bird & cement deposits were found on solar string consisting of 18 solar panels connected in series together. So, thermal and electrical measurements were performed with the help of thermal imaging camera and clamp meter respectively. A Kusam Meco Thermal imaging camera Model TE-P shown in fig. 2 was used in the experiment to obtain thermal images for bird and cement deposits.
The table 1 shows normal photographs and thermal images for bird and cement deposits noticed on the surface of solar string at regular interval of one hour from 11.30 am to 4.30 pm on 26.07.17. The weather changed from clear sky to cloudy sky with small rain.
The table 2 shows thermal and electrical parameters obtained through thermal images and clamp meter for bird and cement deposits found on solar panel surface from 11.30 am to 4.30 pm on 26.07.17.
The fig 4 shows the temperature variation of bird deposit, cement deposits and ambient temperature with time. Temperature of bird beat and cement deposit is obtained from thermal images and ambient temperature is noticed from thermocouple. The fig. 5, fig. 6 and fig. 7 show variation of output voltage, output current and output power of solar string with time.
Figure 4: Temperature variation for bird deposit, cement deposits and ambient temperature with time
Figure 5: Variation of output voltage for bird deposit and cement deposits with time
Figure 6: Variation of output current ad output power for bird deposit and cement deposits with time
Figure 7: Flowchart digital image processing based contact-less temperature measurement for bird and cement deposits
The thermal images show that at 2.30 pm the temperature of the bird and cement deposited area is more compared to temperature of rest points on solar panel surface. Therefore for measuring the maximum temperature using digital image processing the techniques of image segmentation is used. The captured thermal image is first processed through matlab digital image processing to segment the desired bird and cement deposits. The marker based watershed transform algorithm is used to obtain the coloured segmented image for bird and cement deposits. Finally, the 3D temperature plots representing the maximum temperature points on both bird and cement deposits are obtained. The following flowchart shown in fig. 7 represents the procedure of the work dines for this section.
The fig. 8(a) and fig. 8(b) are thermal images of bird and cement deposits. Fig. 8(c) and fig. 8(d) are the segmented coloured watershed transform images of bird and cement deposits obtained after processing thermal images in Matlab R2014a .
The 3D temperature plots for bird and cement deposits are obtained for maximum temperature as 60°C at 2.30 pm on 26.07.17. The dark colour represents high temperature and bright colour shows low temperature in 3D image as indicated by temperature scale on right of image.
Conclusion
The experimental result of thermal measurement shows a brief study about the temperature effect of bird deposit and cement deposits on surface of solar panel string. The heat generated by bird and cement deposits are measured in terms of temperature by thermal images. The variation of output voltage, output current and output power with time is shown with the help of graphs. The results show that as the temperature of deposits on the surface of solar string increase the output voltage of solar string falls as shown in fig. 4 and fig. 5 respectively. The output current of solar string rises with temperature as shown in fig. 4 and fig. 6. This variation of output voltage and output current affect output power of solar string as shown in fig. 6. The figure 8 (c) and fig. 8(d) are the segmented images for bird and cement deposits using marker based watershed transform. Fig. 8 (e) and fig. 8 (f) are 3D temperature plots showing high temperature regions in bird and cement deposits.
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