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PV Module qualification & Reliability

Accurate measurements of the energy yield and power delivery of PV modules under different climatic locations play an important role for the economic operation of PV systems and thus for the growth of the PV industry and market. Two major factors on the performance of the PV Modules are the efficiency with which the irradiation of the Sun is converted into electric power and how this efficiency is decreasing through time, which is known as degradation rate. Better understanding of the degradation mechanisms are not only important for the financial analysis but they are very important from the technically point of view because eventually they may result to failure [1-3].

The performance of the PV modules must be evaluated under real outdoor operating conditions rather than based on data sheet information given by the manufacturer at Standard Test Conditions (STC)[4,5]. Recently, research labs [6-10] have developed long-term outdoor test facilities in various locations, in order to investigate the performance of the PV Modules under real climate conditions and to study the outcome results of the energy yield of different PV technologies.

Standard testing procedures for PV Modules suggest that the output power of a PV Module is rated at Standard Testing Conditions, which are temperature of PV Module at 25°C, Solar Irradiance at 1000 W/m² and Air Mass of 1.5 of solar spectrum. Outdoor conditions hardly correspond to these standard values. Therefore, in depth investigations in the performance of the PV modules is required under real climate operating conditions, by performing outdoor measurements over longer periods of time.

PV modules performance under long-term outdoor testing is vary due to spectral variations, irradiance and temperature responses and their metastable nature. Thus, in depth investigation and better understanding of these effects is required.

This understanding can be achieved through a first-principles approach, through empirical models resulting from measured data, or from a combination of both.

Unlike indoor measurements [11,12], long-term outdoor measurement are strangle from the lack of reproducibility, but they allow a direct comparison between the energy yields of different module types under varying climate and weather conditions. In addition these measurements are partly performed in accordance with internal procedures that each lab has developed, due to lack of international standardization.

There are many challenges associated with accurately measuring degradation rates, particularly over a short time period. Module Pmpp degradation rates of between 0.02%/year and 4%/year have been measured. Most modules fall in the 0.5%/year to 1.5%/year range. Even with high-accuracy measurement equipment, it is a challenge to determine degradation rates over periods of less than three years. Even with high-accuracy measurement equipment, it is a challenge to determine degradation rates over periods of less than three years [13].

In order, to fulfill the requirements of an international standardization for long term energy yield measurements more work must be done. In this direction, the European Distributed Energy Resources Laboratories’ (DERlab) approach to filling the gap of international standardization has led to the development of a basic protocol that complies with European and international standards, while providing specific common guidelines and procedures for measuring the energy yield of PV modules for at least one year in outdoor conditions [14].

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