When you are buying a product, the two basic questions which come to your mind are ‘how much does this product cost?’ and ‘how long will this product last?’. When it comes to solar panels, many of us are aware of the fact that the cost of solar power is getting cheaper with time and the solar modules in the market come with at least 20 years of warranty. Unlike many other products, what makes the solar panel last so long? Presented here are a few technical insights which will answer this question of long-term reliability.
It all started with Siemens Solar industry extending its module’s warranty from 10 years to 25 years in the year 1997. It has been the industry standard for lifetime since then.
The first and foremost step towards delivering the industry-standard reliability of over 25 years starts with the design for reliability process. Failure Modes Effect Analysis (FMEA) and Qualification testing are the integral parts forming the foundation of this process.
To develop an FMEA, the concept design should be built initially and all the functional specifications of the design should be listed in detail. All possible ways in which the product will fail has to be determined accurately through a thorough brain-storming by experts in the field. Each failure will be ranked to create a prioritized list based on the following factors: Severity indicates the potential economic and safety impacts due to failure, Frequency of failure is the probability of its occurrence and finally detectability is the measure of how easily the failure mode can be detected before field deployment. Risk Priority Number will be calculated based on the above three factors to prioritize the failure modes for which the testing has to be performed in order to qualify the product.
The three primary approaches employed in FMEA to determine the design failure modes and their associated stresses are field testing, Highly Accelerated Life testing (HALT) and theoretical understanding. Field experience over years gives an idea of all possible failure modes that can occur in a solar panel. Failure analysis will give a fair idea of mechanism behind the failure and the qualification tests could be designed in such a way to check for those failures in future products. Highly Accelerated Life Testing is the method of accelerating the module degradation by applying higher stress levels of temperature, humidity, UV light exposure etc. individually or in combinations in order to discover weakness in the product design. Theoretical understanding is critical in identifying a slow-occurring failure mode. Physics behind the failure can be modeled by lab testing with a strong theoretical understanding.
Once the possible failure modes are identified, proper qualification tests should be derived in order to check for the failures in the products. The qualification tests usually include accelerated stress testing and sometimes field testing. If the products pass the tests, then it is assumed those failures will never occur in the modules throughout its warranty period. The PV modules verified and validated by qualification testing should come out through quality manufacturing in order to be reliable.
Since the complete design for reliability methodology has been followed for years now, it is no wonder that modules are degrading at 0.5 % per year indicating that modules perform at 88 % of its initial capacity even after a period of 25 years as studied by NREL a few years ago.