ANNOUNCEMENTS
The performance of the PV modules mainly depends on environmental conditions in the field. The modules should perform in the field reliably in terms of their safety and performance during their entire lifetime. In Indian climatic conditions, the module degradation rates are higher compared to that in moderate climatic zones. It has been observed that the performance of some of the PV modules deployed in the different climatic zones in India is not as per the claim of manufacturer. Some of the modules show different failure modes in the field, even for the modules qualified as per IEC 61215.
For selection of reliable PV module technology for different climatic zones, it is imperative to understand the performance and safety of the modules in the particular climatic conditions and thereby perform the site-specific accelerated tests for the module technology before installation. The present thesis attempts to solve some of these field specific issues. The objectives of the thesis are given as follows:
1. Deriving a methodology for estimation of energy rating of the PV modules.
2. Quantification of reliability of PV modules exhibiting different failure modes.
3. Study of two prominent failure modes observed in the field deployed PV modules - Potential Induced Degradation (PID) and UV radiation induced degradation.
The energy rating of a PV module is defined as the energy produced by one kWp of PV array of specific technology per annum in a particular climate zone. In this study, a methodology for estimation of energy rating of three types of PV module technologies was derived for six different climatic conditions. The performance of different PV technologies is estimated using the energy rating value and compared with each other. This methodology was applied to three module technologies viz. amorphous silicon (a-Si), multi crystalline silicon (mc- Si), and Heterojunction intrinsic thin film (HIT) to estimate the energy ratings for different climatic zones of India. The basic differences between the methodology used for energy rating estimation in this thesis and the procedure mentioned in IEC 61853, are the incorporation of degradation rates, data sets for effective climatic conditions and uncertainty analysis of estimated energy rating.
A method to estimate the most frequent conditions (MFC) in terms of irradiance and temperature is also reported under this objective. The MFC of the three PV technologies studied, varies with different climatic zones, which lead to a different energy rating. The energy rating of PV modules studied is high in cold & sunny zones in India. The uncertainty is high with the in-plane irradiance as compared to effective irradiance for all the three module technologies. The uncertainty in estimation of energy rating is further reduced by incorporation of degradation rate.
For the reliable operation of the PV modules in the field, the failure modes affecting the safety and performance degradation of the modules, should be negligible. Before installation of PV modules in field, their failure modes need to be identified through accelerated tests to find out their suitability for a particular climatic zone.
Analysis and quantification of the reliability of the PV module is required to be done as per the field operating conditions. Risk Priority Number (RPN) was used to quantify the reliability by analyzing the failure modes of PV modules and results are reported in this thesis.
The variety of failure modes were considerably low in Cold & Cloudy climatic zone than the other climatic zones. For modules with more than five years of exposure, solder bond fatigue is one of the most common defects in all the climatic zones except Cold & Cloudy.
In the Hot climatic zones, the prevalent performance failure modes are problem in internal circuitry, glass discoloration, hot spots, solder bond failure and in the non-hot climatic zones, the problem in junction box, backsheet, fractured cells, permanent soiling, are observed. The dominant common safety issues observed in the hot & non-hot climatic zones are the problem in backsheet insulation, glass breakage, frame cracking, ground wires/strips corrosion, compromise in junction box & wire insulation.
It has been observed in the field that although modules are qualified as per IEC standards, they still failed in the field. So, it is necessary to design stress tests as per the prevalent climatic conditions of a country/ region.
In this thesis, two major failure modes of PV modules are observed in India, viz., the effect of UV radiation exposure & Potential induced degradation, have been studied and their test conditions have been analyzed.
The effect of UV radiation has been analyzed through observation of various defects in 180 modules which were field exposed for 20 years, and supplied by nine different manufactures. A method for estimating UV radiation for different climatic zones has been proposed, and UV radiation for representative sites has been estimated. Based on this, the required testing conditions for assessing the effect of UV radiation on PV module has been reported as a function of module temperature, UV irradiance and relative humidity.
A study on the potential induced degradation of the PV module has been done in a controlled environment. The effect of temperature, humidity, and voltage on leakage current, as well as PID, has been analyzed for mc-Si and HIT module technologies. With the increase in bias voltage, the severity of degradation is found to be increasing. The relation between applied voltage and degradation rate is found to be non-linear. It has been observed that module leakage current increases with an increase in applied voltage with other fixed operating conditions. A procedure has been developed to design accelerated test conditions for PID stress under different climatic conditions. A procedure for mitigating the PID in the PV modules after installation in the field is also reported in the thesis.
To sum up, the severe problems of the PV industry in India are high module degradation rates, resulting in low power output, and eventually unsafe field operations. In the present thesis an attempt is made to provide possible remedial measures to mitigate these problems.
