Solar power generation is one of the fastest growing industries in the world. This year, solar module shipments will exceed 7.8 gigawatts globally. Taking an average output of 200 Watts per module, this represents 39,000,000 modules per year with an annual growth rate of 30 percent expected over the next several years. Solar Engineering and Manufacturing Association president Matthew Holzmann discusses how module reliability is becoming a crucial factor in photovoltaics.
Germany and Japan have recently announced plans to exit nuclear power generation, and photovoltaic installations are one of the most attractive solutions to fill this gap. But there is a problem: solar power users expect modules to last between 25 and 30 years. However, there is little data to support this expectation and there are no recognized test standards in place to validate these assumptions. The current standards do not come close to the 25-year benchmark.
Companies have internal test methodologies, but there are no generally recognized standards. Warranties for manufacturing defects are typically five to 10 years, while efficiency warranties are between 10 to 25 years. Almost all of these warranties are based upon internal testing, which is held as “company secret” for competitive advantage. Additionally, there is little traceability. So, what happens in five or 15 years if a module goes bad? Solar installations are multi-generational. Some of the companies manufacturing and installing solar power now might not be around in 25 years. What then?
Most solar modules generate DC electricity, which is fed through a junction box to an inverter, where that electricity is converted to AC and then fed into a home or business, or directly to the grid. A typical home array is feeding approximately 2.5 kilowatts (kW) constantly while the sun is shining. There is no way to turn it off except to cover the array.
In the wide range of applications for global solar power, solar modules can be exposed to temperatures exceeding 65 °C (149 °F) and down to as low as -60 °C (-76 °F). Additionally, humidity has been identified as a significant contributor to module failures in tropical climates. Additional failure mechanisms based on the location of an installation include atmospheric salt exposure, corrosion from pollution, extreme weather conditions and exposure to ammonia in rural installations where livestock are kept.
Quality assurance and reliability
A two-day conference was recently held in conjunction with the Intersolar conference and exhibition in San Francisco. Organized by Japan’s Institute of Advanced Industrial Science & Technology (AIST), the National Renewable Energy Laboratory (NREL), SEMI and PVTECH, the Japanese solar power research association, more than 170 industry participants met to discuss the issues related to quality assurance and long-term reliability.
The packed house included representatives from the major test laboratories, national and international research institutes, module manufacturers, suppliers, the insurance industry, system operators, and other industry stakeholders. Michio Kondo, representing AIST, outlined that organization’s seven-year test methodology and a failure rate of 0 percent for some manufacturers, and up to six percent for others. Later, a representative from one of the world’s largest solar investment firms documented a 10 percent failure rate for inverters within the first seven years of operation.
Field failures in solar power installations include cell cracking, junction box delamination, module delamination, diode failure, junction box and gasket cracking, glass breakage, and soldering defects leading to circuit failure. One utility scale user has reported six fires in seven years in 50 megawatts of modules. Solar power generation requires the same safety factors as other forms of power generation. That same user expressed his concern that a major fire might occur within the next few years if this issue is not addressed.
The IEC test standards used and those from UL and TÜV are primarily fire safety standards and simulate approximately five years of harsh environment usage. None of these tests are performed while the device is under power, but rather are static tests. Many manufacturers perform additional tests and also benchmark their products versus their competitors. However, no national or international standard, or long-term test methodologies yet exist. The test specifications and standards for the materials and components are similar to those for modules.
Another factor affecting long-term reliability is the drive to reduce the cost per watt. In the crystalline module market, the price of cells represents 70 percent of the total cost of the manufactured product and profit margins are thin. Module prices have fallen by as much as 25 percent in 2011, and manufacturers are desperate to reduce costs of materials while improving manufacturing efficiency. This has resulted in efforts to utilize new, less expensive materials that in some cases may meet the current standards, but are not as effective in the long run. A large number of field failures have been traced directly to materials and manufacturing processes.
Chain of responsibility
The solar power supply chain is a long one and the chain of responsibility is complex. Governments require safety and code compliance. Insurance companies and manufacturers provide warranties, liability and other forms of insurance. In 10, 20 or even 30 years from now, repair and maintenance providers will demand a safe, reliable product upon which to work.
With 40,000,000 new modules entering usage every year, it is incumbent upon all of the key stakeholders to act rapidly to develop and implement the methodologies and test methods to simulate long-term exposure under harsh conditions.
Matthew Holzmann is the president of Christopher Associates Inc., a supplier to the solar power industry and president of the Solar Engineering & Manufacturing Association.