SolarCity Photovoltaic Modules With 35 Year Useful Life
Andreas Meisel1, Alex Mayer1, Sam Beyene1,
Jon Hewlett1, Karen Natoli Maxwell1, Nate Coleman1,
Frederic Dross2, Chris Bordonaro3, Jenya Meydbray2, Elizabeth Mayo2
1) SolarCity, 161 Mitchell Blvd, Suite 104, San Rafael, CA 94903
2) DNV GL, 1360 Fifth Street, Berkeley, CA 94710
3) DNV GL – Renewables Advisory, 155 Grand Ave, Oakland, CA 94612
SolarCity believes that the Useful Life of the photo-voltaic (PV) modules that are being installed on its residential and commercial systems is 35 years or longer. Per this definition, 95 % of all modules in-stalled are expected to have an annual average deg-radation rate of less than ~0.5 % and produce at least 80 % of their power after 35 years of service. Experimental data from accelerated stress tests ac-cording to the industry standard IEC 61215, which were performed by DNV GL (the leading US certified 3rd party), demonstrates that the median power deg-radation of modules supplied by seven key SolarCity approved module manufacturers for all tests and all module suppliers combined is as low as -1.1 % and as much as 35 % lower than for a comparable indus-try-wide selection of non-SolarCity modules meas-ured at DNV GL. Furthermore, data from DNV GL demonstrates that after extending accelerated test-ing to more than 3x beyond the conditions of IEC 61215, the modules produced for SolarCity show only 1 to 2 % median degradation and outperform non-SolarCity modules, which are typically warrant-ed for 25 years.
The reason for this advantage is SolarCity’s imple-mentation of a stringent and industry-leading Total Quality Program, which adopted its features from the Automotive Industry and was implemented by SolarCity in early 2014. Following this program, So-larCity strategically chooses to engage with a select group of Tier-1 suppliers only. In order to be quali-fied as a SolarCity supplier, manufacturers need to have effective Quality Assurance programs and re-fined manufacturing processes in place, and steady product and manufacturing quality must be demon-strated. Rigorous tests need to be passed on an on-going basis, performed by a qualified 3rd party lab. Furthermore, we require that factory controls and in-line testing are in place to ensure quality is sus-tained over time and deviations are rapidly detected, so the deployment of faulty products in the field is prevented. Additional work is underway to demon-strate that the degradation rate from SolarCity mod-ules in the field is lower than industry-standard. Last-ly, the development and implementation of state-of-the-art accelerated testing methods will enable So-larCity to probe degradation modes that are not de-tectable with the current industry-standard suite of testing and to more reliably predict real-life perfor-mance in the field.
The most comprehensive meta-study of Field Degra-dation rates to date, where more than 11,000 annual degradation rates have been aggregated and ana-lyzed, observed a near-linear degradation behavior for the majority of crystalline-Silicon (Si) modules and established a median degradation rate for Si modules of around 0.5 % per year [1, 2]. The data in this study was analyzed and filtered by DNV GL ana-lysts, and the annual median degradation rate for crystalline-Si modules was confirmed to be ~0.50 % per year, while the corresponding value determined for systems is 0.77 % per year .
The data presented in the following supports the assumption that SolarCity’s PV modules, as a result of its Total Quality Program and advancements in Materials Science, manufacturing, and quality con-trol, perform at least similar, if not better than the median of all crystalline-Si modules observed in the study above. Therefore, an annual module degrada-tion rate of 0.5-0.6% per year is a realistic assump-tion, which warrants a postulation of Useful Life of 35 years with a power output of 80 to 82.5 % there-after.
Useful Life – Introduction
SolarCity defines 35 year Useful Life as 95% of mod-ules producing at least 80% of their power after 35 years in their use environment . ‘Use environ-ment’ is defined as all geographic and meteorologi-cal conditions that the PV modules will experience during their lifetime. Site environmental conditions, installation, and handling are included in use-environment considerations. This definition of Useful Life postulates a higher threshold for the remaining power output than the value of 70 % that has been assumed elsewhere .
Industry analysists have been getting more comfort-able with the idea of Useful Life beyond 30 years . The Useful Life of a PV module is determined by wear-out failures, which occur at the end of the working lifetime of the module. SolarCity defines the end of a PV module’s Useful Life if a safety problem occurs or if the module power drops below 80 % of the initial power rating. Long-term studies that have investigated wear-out failures  found that the predominant End-of-Life failures led to a median power loss of only 10 % (between 0 % and 20 %), and that nearly all of these PV modules were still functional and met the manufacturer’s power war-ranty. Another literature meta-study summarizing ~400 reports on degradation rates of silicon modules confirms that modules are usually observed to de-grade slowly in the field . The degradation most often is dominated by a gradual loss of short-circuit current, which is mostly associated with discolora-tion and/or delamination of the encapsulant materi-al. In other words, the most critical module failures have been observed to occur relatively fast, whereas modules that do not show early failures are likely to reach the wear-out portion of the ‘bathtub’ product reliability curve, where the power declines in a grad-ual and slow manner rather than showing abrupt failure [9, 10].
There are numerous examples of installations that have delivered stable performance for well over 25 years . In 1984, Sweden’s first grid-connected photovoltaic system was built in Stockholm. Since its installation, the 2.1 kW system has been continuous-ly and reliably producing energy – with less than 3 % change since the system was installed 31 years ago. Another system installed in 1984 is at Kyocera’s Sa-kura Solar Energy Center near Tokyo. The 43 kW array continues to generate a stable amount of elec-tricity today 32 years later .
Given the drastic advancements in terms of Materi-als Science, manufacturing processes, quality control and standards, and theoretical understanding over the last 30+ years, it is considered reasonable to as-sume that the quality and reliability of modules fab-ricated over the last few years can have Useful Life well beyond 35 years, provided that adequate quali-ty assurance measures, such as SolarCity’s Qualifica-tion Program, have indeed been implemented.
The most comprehensive study of Field Degradation rates to date, where more than 11,000 annual deg-radation rates have been aggregated and analyzed, established a median degradation rate for crystal-line-Silicon (Si) modules of consistently around 0.5 % per year (Figure 1) [1-3]. The data presented in the following supports the assumption that SolarCity’s PV systems, as a result of its Total Quality Program and industry-wide advancements in Materials Sci-ence, manufacturing, and quality control, perform at least similar, if not better than the median of all crystalline-Si systems observed in the study above. Therefore, an annual module degradation rate of 0.5 % per year is a realistic assumption, which war-rants a postulation of Useful Life of 35 years with a power output of 82.5 % thereafter.
Figure 1 [Taken from Ref 1]: Histograms of all data, high quality data