1. PID in Solar cells Shubham Mishra,M.Tech student Renewable Energy Infrastructure 12th April 2018

2. Introduction to Potential Induced Degradation Potential Induced Degradation(PID) is a phenomenon which affects some PV modules with crystalline Si cells and leads to Power loss in a module, by electrically modifying solar cell resulting in leakage current. Deterioration in performance, may be 30 percent or more in few months / years. It is a process which occurs only a few years after installation. PID causes an accelerated degradation in performance which expands exponentially. It has only recently become apparent that PID is a serious threat to the performance of many PV systems. ble Energy InfrastructureInfrastructure Shubham Mishra,M.Tech studentRenewa PID in Solar cells12th April 20182 / 43

3. How it occurs exactly? A solar panel consists of individual photovoltaic cells. Each of these cells generates electricity from the collected sunlight. Technically spoken, the combination of two semiconductor materials exchange charges, resulting in an internal electric field. The incident sunlight dislodges electrons. These electrons then flow along the contacts and generate (green) electricity. Figure:PID in Solar Cells ble Energy InfrastructureInfrastructure Shubham Mishra,M.Tech studentRenewa PID in Solar cells12th April 20183 / 43

4. Working (Contd..) A positive charge is built up on the anti-reflective coating at the surface of the cells the result is a temporary short circuit in the affected cells. This leads to a decrease in cell voltage and a drop in efficiency – an effect that is reinforced by high temperatures humidity. In an individual PV module, the PID effect is stronger in cells that are located at the periphery (closer to the aluminium frame). The PID effect is less strong in cells that are located in the centre of the module. Figure:Individual PV moduledule ble Energy InfrastructureInfrastructure Shubham Mishra,M.Tech studentRenewaPID in Solar cells12th April 20184 / 43

5. contd.. In a string of PV modules, the PID effect is stronger in panels that are closer to the negative side of the string. After all, the negative potential with respect to earth is the highest at that side. Figure:String PV module ble Energy InfrastructureInfrastructure Shubham Mishra,M.Tech studentRenewa PID in Solar cells 12th April 2018 5 / 43

6. Causes of PID PID is caused by the high potential difference with respect to earth. This occurs within the panels when the PV system is in operation. A typical PV module normally produces about 30 V. A daisy chain of panels can result in a multiple of this. Figure:Causes of PID ble Energy InfrastructureInfrastructure Shubham Mishra,M.Tech studentRenewa PID in Solar cells 12th April 2018 6 / 43

7. Impacts of PID Figure:V-I characteristics due to PID ble Energy InfrastructureInfrastructure Shubham Mishra,M.Tech studentRenewa PID in Solar cells 12th April 2018 7 / 43

8. Impacts contd.. Risk in achieving 25 years warranty life. Gets worse with high temperature and high humidity can lead to more overall losses with the passage of time At early stage, PID is reversible by reverse voltage bias.If allowed to progress, degradation becomes irreversible. ble Energy InfrastructureInfrastructure Shubham Mishra,M.Tech studentRenewa PID in Solar cells 12th April 2018 8 / 43

9. PID Testing As per IEC 62804 under drafting, the PID testing involves a test run of 100 hours with Voltage applied to Test panel’s corresponding to maximum system voltage (as per panel datasheet) and exposed to environment with temperature and humidity of 60C ( 2C) and 85% ( 5%) respectively in a chamber. PID testing procedures, in many labs involves comparison of visual inspection, I-V measurement and EL testing prior to and after the actual PID testing. After the test, Module must not show any major defects and should not have power loss more than 5%. ble Energy InfrastructureInfrastructure Shubham Mishra,M.Tech studentRenewa PID in Solar cells 12th April 2018 9 / 43

10. Comparison of EL images of Modules Figure:Noticeable effects on panels ble Energy InfrastructureInfrastructure Shubham Mishra,M.Tech studentRenewa PID in Solar cells 12th April 2018 10 / 43

11. Avoiding / Mitigating PID Location:for a new plant, within other limitations, a site with lower temperature and humidity should be selected. Note that a windy site will also keep the system cooler. Use PID Resistant Hardware:For a new set up, there are modules available which are resistant to PID. However, the cost will be higher due to use of more expensive encapsulating materials, anti-reflective coatings, and other materials. ble Energy InfrastructureInfrastructure Shubham Mishra,M.Tech studentRenewa PID in Solar cells 12th April 2018 11 / 43

12. Mitigation contd... Earthing:Use modules where there is no restriction imposed by the manufacturer on connecting the negative end of the string to system ground. Charge Equalizers:They are built into inverters. When the inverter is inactive at night, they apply an opposite bias to the panel which cancels out the reversible type of PID effect overnight. ble Energy InfrastructureInfrastructure Shubham Mishra,M.Tech studentRenewa PID in Solar cells 12th April 2018 12 / 43

13. Current Scenario in Industries Since, environmental factors can not be controlled. The Three level strategy with important factors is :- 1.Cell level – Anti reflective coating (ARC). 2. Panel level – Encapsulation Material, 3. System level – Negative Grounding or PV Offset Box. (Grounding is possible in Central Inverters and not in String Inverters, normally used for small capacity Plants) In all new plants mostly PV offset boxes are employed. Scientific research has shown that each solar panel suffers from PID. Especially PV systems that were installed after 2008, appeared to be very sensitive for PID. ble Energy InfrastructureInfrastructure Shubham Mishra,M.Tech studentRenewa PID in Solar cells 12th April 2018 13 / 43

14. Current Scenario in Industries contd.. This conclusion is confirmed by independent research of Solar Tester, which shows that more than two thirds of solar panels of 10 years or younger is suffering from efficiency losses greater than 20 In 2008, it became impossible to ground the cable that runs from the solar panels to the inverter (negative pole). This resulted in larger tensions, and therefore in more leakage current to the earth. Nevertheless, PV-systems installed before 2008 suffer from Potential Induced Degradation. So-called ‘PID-free’ solar panels do not exist! ble Energy InfrastructureInfrastructure Shubham Mishra,M.Tech studentRenewa PID in Solar cells 12th April 2018 14 / 43

15. Current Scenario in Industries contd.. Many module manufacturers have taken steps to produce PID resistant modules, which currently are nearly 4 to 5% costlier than Normal module. Early finalisation of IEC for testing the modules, is likely to help in proper certifications of “PID-resistant” modules. Design of Solar Plant with string based Inverters must analyse grounding requirements carefully and include PV Offset Box. ble Energy InfrastructureInfrastructure Shubham Mishra,M.Tech studentRenewa PID in Solar cells 12th April 2018 15 / 43

16. Current Scenario in Industries contd.. This conclusion is confirmed by independent research of Solar Tester, which shows that more than two thirds of solar panels of 10 years or younger is suffering from efficiency losses greater than 20 In 2008, it became impossible to ground the cable that runs from the solar panels to the inverter (negative pole). This resulted in larger tensions, and therefore in more leakage current to the earth. Nevertheless, PV-systems installed before 2008 suffer from Potential Induced Degradation. So-called ‘PID-free’ solar panels do not exist! ble Energy InfrastructureInfrastructure Shubham Mishra,M.Tech studentRenewa PID in Solar cells 12th April 2018 16 / 43

17. Updates in Last Week: PID usually occurs when modules are in strings operating at high voltages (near 1000 V, but not only), combined with very warm and humid weather. Dust and glass degradation (releasing Sodium ions) may catalyse the PID phenomenon. PID affected modules (often from the negative pole side of a string) usually have a pattern of “black” cells, i.e., cells that were shut down closer to the frame of the module. This pattern has to be with the flow of negative charges, due to the excess of positive charges. ble Energy InfrastructureInfrastructure Shubham Mishra,M.Tech studentRenewa PID in Solar cells 12th April 2018 17 / 43

18. Updates contd.. Figure:PID IV data ble Energy InfrastructureInfrastructure Shubham Mishra,M.Tech studentRenewa PID in Solar cells 12th April 2018 18 / 43

19. Updates contd.. There is an IEC standard (yet in Draft stage) that aims to test the resistance of PV modules to withstand PID phenomenon. Basically one of methods consist is subjecting PV modules to 1000 V (DC) inside a climate chamber at 85% rH and 60 o C, for 96h. The graph shows the Pmpp and Electroluminescence images, before and after the 96h test. Figure:PID stress test ble Energy InfrastructureInfrastructure Shubham Mishra,M.Tech studentRenewa PID in Solar cells 12th April 2018 19 / 43

20. Updates contd.. The result showed above means a power loss about 25% after the 96 hours PID stress test. The “Pass” criteria defined at IEC PID standard states that power loss cannot be above 5% between initial and final measurements. Although this result is relatively to 1 module, it’s representative of dozens of PID tests done by the same laboratory to different modules. ble Energy InfrastructureInfrastructure Shubham Mishra,M.Tech studentRenewa PID in Solar cells 12th April 2018 20 / 43

21. 10 MW PV plant Capital Losses – case study for UK, US, India and South Africa Picking up the same assumptions and performing a different assessment but more realistic; simulation results will be closer to the reality. So, for the next scenario these are the assumptions: 1.Two simulations, one based on real/ measured data, and the other, based on manufacturer data 2.For the “real/ measured data” simulation, the assumption is that from the beginning there’s a LID loss of 5.5% from the beginning, a yearly degradation of 1% and PID will affect modules at the “Year 3” by 12% loss of power relatively to the previous year 3.For the “manufacturer data”, no LID loss was considered, except a power degradation of 0.7%/year, and no PID effect; 4.Fit or PPA rate assumed are the closest possible as the last auction s in each country. ble Energy InfrastructureInfrastructure Shubham Mishra,M.Tech studentRenewa PID in Solar cells 12th April 2018 21 / 43

22. losses contd.. Figure:Ahmedabad ble Energy InfrastructureInfrastructure Shubham Mishra,M.Tech studentRenewa PID in Solar cells 12th April 2018 22 / 43

23. losses contd.. Figure:Columbia ble Energy InfrastructureInfrastructure Shubham Mishra,M.Tech studentRenewa PID in Solar cells 12th April 2018 23 / 43

24. losses contd.. Figure:Kimberley ble Energy InfrastructureInfrastructure Shubham Mishra,M.Tech studentRenewa PID in Solar cells 12th April 2018 24 / 43

25. losses contd.. Figure:Exeter ble Energy InfrastructureInfrastructure Shubham Mishra,M.Tech studentRenewa PID in Solar cells 12th April 2018 25 / 43

26. losses contd.. The three simulations were done using 250 Wp rated modules, with 11 MW of installed capacity, limited to the grid at 10 MW. PID usually doesn’t occur at the first year of operation, although; let’s assume a total mismatch (due to the two phenomena) of 30.5% – the worst scenario. A very optimistic result is obtained by making use of manufacturer’s data. Although, replacing the PV module parameters (at “software” modules database) by the real values measured on a Solar Simulator The amount of energy injected into the grid is less 36.2% (comparing with the manufacturer’s data). For the case of simulation c), making use of “software” algoritm, and filling the LID and deviation relatively to manufacturer’s data (I used this one for PID effect), the amount of energy injected into the grid is effectively less 30.5% comparing to the manufacturer’s data simulation. ble Energy InfrastructureInfrastructure Shubham Mishra,M.Tech studentRenewa PID in Solar cells 12th April 2018 26 / 43

27. Inference One of the main goals of this text is to warn for two “issues” that can have a strong influence at the performance of a PV plant. Most of the project developers and investors are aware of Potential Induced Degradation (PID) and Light Induced Degradation (LID) mismatch, but how to quantify their effect on the phase of a PV plant project? Another goal is to alert for inaccurate energy forecasts. Taking into account that majority of softwares have already specific tools where the user can fill with the percentage of deviations relatively to manufacturer’s data, there’s nothing better than consider real/measured values such as Pmpp, Voc, Isc, Impp, Isc, Eff., etc... but also Temperature Coefficients play a crucial role, as well as others. ble Energy InfrastructureInfrastructure Shubham Mishra,M.Tech studentRenewa PID in Solar cells 12th April 2018 27 / 43

28. Inference contd.. Figure:data ble Energy InfrastructureInfrastructure Shubham Mishra,M.Tech studentRenewa PID in Solar cells 12th April 2018 28 / 43

29. Inference contd.. Last, but not the least, and regarding an accurate/ inaccurate PV project, there is a huge difference between considering the Yearly Power degradation, and Yearly Energy degradation. The effective Pmpp difference at the beginning of operation is arround 4.8%, which leads to a difference of energy production arround 5.7% and 6.5%. It shall not be considered that Yearly Power degradation has an equal percentage of Yearly Energy degradation. ble Energy InfrastructureInfrastructure Shubham Mishra,M.Tech studentRenewa PID in Solar cells 12th April 2018 29 / 43

30. References 1.Solar Power Europe, Global market outlook for solar power 2016–2020, 2016. 2.IEA, Snapshot of Global PV 2015, OECD/IEA, Paris, 2016. 3.IEA, Technology roadmap: solar photovoltaic energy, OECD/IEA, Paris, 2014. 4.D. C. Jordan and S. R. Kurtz, Prog. Photovoltaics, 2013, 21,12– 29. 5.Pidbull.com/pidcauses 6.ilumen.be/piduse 7. http://sinovoltaics.com/quality-control/pid-lid-devastating- phenomena-pv-plants/http://sinovoltaics.com/quality-control/pid-lid-devastating- ble Energy InfrastructureInfrastructure Shubham Mishra,M.Tech studentRenewa PID in Solar cells 12th April 2018 30 / 43

31. Methods for Detection and Reduction of PID PID existence can be detected by monitoring Vmpp (maximum power point voltage) periodically. Difference in such Voltage among modules increasing along the string is a sign of PID being present. Detection is more likely on a day of good irradiation. There are few methods to detect PID in solar farms as follows: 1.IV curve Tracing The typical test to know if a PV-module is free of PID is to measure its IV (current-voltage) characteristic with an electronic tracer. I-V curve tracers have two unique advantages.First, I-V curve tracers make it easier and safer to take I s c measurements. Second, curve tracers can reveal what happens to an array under load. The four key measurements that can be performed with a digital multimeter (Voc, Vmp, Imp, Isc) are inadequate if we want to get a picture of how the PV system responds to a varying load, ble Energy InfrastructureInfrastructure Shubham Mishra,M.Tech studentRenewa PID in Solar cells 12th April 2018 31 / 43

32. IV tracing Figure:IV curve ( Courtesypidbull. com ) ble Energy InfrastructureInfrastructure Shubham Mishra,M.Tech studentRenewa PID in Solar cells 12th April 2018 32 / 43

33. EL Imaging Electroluminescence (EL) is an optical and electrical phenomenon in which a material emits light in response to an electric current/ PID affected modules will appear darker. In the daytime, we also measured different open-circuit voltage figures for the two parts of the string. ble Energy InfrastructureInfrastructure Shubham Mishra,M.Tech studentRenewa PID in Solar cells 12th April 2018 33 / 43

34. EL Imaging contd.. As you can see in figure 1, there are a few different options for tracking down this phenomenon. Such Images are obtained from: https : // www. photovoltaikbuero. de / en / product / inspecting − photovoltaic − systems − with − pvserve / Figure:EL imaging ble Energy InfrastructureInfrastructure Shubham Mishra,M.Tech studentRenewa PID in Solar cells 12th April 2018 34 / 43

35. EL imaging contd.. The easiest and quickest option, though, is outdoor electroluminescence, since it shows all of a string’s panels in one image and lets you immediately see the different panels’ varying output levels. Using the electroluminescence process, We can obtain more detailed information about solar arrays. Here too, a specific current is fed into the solar module. Unlike thermography, this technique can also be used in total darkness. As soon as a reverse current is applied to the module it begins to glow in the infrared region. This glowing can be captured with a special camera capable of producing very detailed images. This method can even reveal very fine hairline cracks in the solar cells. You can carry out the process as part of the final acceptance/testing phase of a solar installation, during regular maintenance, or for troubleshooting specific problems. ble Energy InfrastructureInfrastructure Shubham Mishra,M.Tech studentRenewa PID in Solar cells 12th April 2018 35 / 43

36. Infrared (IR) or Thermography This method uses the fact that PID affected cells have higher temperatures than the neighbor cells free of PID.Thermography has proved to be particularly useful in identifying problems in solar power systems. This process relies on converting the heat emission of the solar module into an optical image. The thermal image immediately pinpoints any solar cell that is warmer than its immediate environment. This method can also be used to locate faulty connections and sockets. ble Energy InfrastructureInfrastructure Shubham Mishra,M.Tech studentRenewa PID in Solar cells 12th April 2018 36 / 43

37. Thermography Contd.. The prescribed standards state that you should only carry out thermal imaging when there is a minimum irradiation level of 400W/ m 2 because only a relatively large current through the solar cells will cause a noticeable temperature difference. With new reverse current thermography technique, however, you can now perform these tests at lower solar irradiation levels, and even at night. To do this, you send a current through the solar cells in a reverse direction. This current is less than the solar cell’s rated current and thus perfectly harmless to the cells under normal circumstances.Such interpretation can be concluded from figure ble Energy InfrastructureInfrastructure Shubham Mishra,M.Tech studentRenewa PID in Solar cells 12th April 2018 37 / 43

38. Thermography Contd.. Such Images are obtained from: https : // www. photovoltaikbuero. de / en / product / inspecting − photovoltaic − systems − with − pvserve / Figure:Thermography ble Energy InfrastructureInfrastructure Shubham Mishra,M.Tech studentRenewa PID in Solar cells 12th April 2018 38 / 43

39. Checking Operating Voltage The working voltage of the panel can also be measured by means of a simple voltmeter. This parameter gives more information about a solar panel than the open circuit voltage. Solar panels affected by PID have a lower operating voltage than panels not affected by PID. This is the reason why solar panels with PID have higher leakage currents and consequently have to deliver the same power than the other solar panels in de string. As a consequence, these solar panels work at a lower operating voltage. ble Energy InfrastructureInfrastructure Shubham Mishra,M.Tech studentRenewa PID in Solar cells 12th April 2018 39 / 43

40. Checking Operating Voltage This can easily defined by next formula P = U · I If the power (P) decreases due to PID, and the current (I) remains the same by the other panels in series, the voltage (U) is to fall. Moreover the fall of the operating voltage is proportional to the PID-losses. ble Energy InfrastructureInfrastructure Shubham Mishra,M.Tech studentRenewa PID in Solar cells 12th April 2018 40 / 43

41. Infrared Inspection by Drones In order to detect possible defects or problems of solar panels, an inspection is necessary on a regular base. An alternative way of inspection is an infrared inspection by drone.The figure here shows infrared images of a drone inspection at some industrial photovoltaic installations.Such Images are obtained from: https : // www. photovoltaikbuero. de / en / product / inspecting − photovoltaic − systems − with − pvserve / Figure:IR inspection by drones ble Energy InfrastructureInfrastructure Shubham Mishra,M.Tech studentRenewa PID in Solar cells 12th April 2018 41 / 43

42. IR inspection by drones An infrared inspection by drone makes it possible to detect efficiency losses caused by Potential Induced Degradation (PID). The yellow color of some solar cells indicates a higher temperature, which is an important indication for PID. ble Energy InfrastructureInfrastructure Shubham Mishra,M.Tech studentRenewa PID in Solar cells 12th April 2018 42 / 43

43. Comparison of Detection methods Figure:Comparison of Several Detection Methods ble Energy InfrastructureInfrastructure Shubham Mishra,M.Tech studentRenewa PID in Solar cells 12th April 2018 43 / 43