17 March 2011
The broad global demand for installed solar energy is unprecedented. The rapid market growth has occurred in front of global standardization of module performance, most notably safety standards. The challenge for module manufacturers is to develop panels to meet safety and performance requirements that are not only unclear but vary by market region. Each of the three major consuming regions (Asia, North America, and Europe) has formed their own regional safety requirements. This evolution of safety requirements challenges the PV module manufacturer, who typically produces from a single plant location but supplies globally.
The real industrial cost for the undefined standards is, however, one level deeper.
Test protocols and the interpretation of the results are far from ideal.
Over the last several years, numerous new polymeric materials have emerged for PV applications. This is a very positive phenomenon, as new materials allow for more choices and the potential to reach lower cost and higher performance products. However, both the test protocols and the interpretation of the results are far from ideal, and it is difficult to ensure rigorous performance when PV module manufacturers are challenged to make informed safety decisions.
Material leaders, with long vested interest in the market, have done what they can to be proactive in this area. This includes, aggressively testing and certifying their backsheet products to exceed all current standards. In the interim, tests are conducted in anticipation of how standards will evolve. The testing is long, expensive, and absolutely necessary to ensure material use with confidence in the longevity of product safety and performance.
Currently the International Electrotechnical Commission (IEC), Underwriters Laboratories (UL) and other certifying groups’ standards outline testing procedures for polymeric materials for PV applications and are used as guidelines. Some of the specific shortcomings in these standards are outlined below.
- Time required for testing completion: Current procedures such as Damp Heat, UV Exposure and other tests require 2-3 months for completion. Even though current standards necessitate only 1000 hours of exposure to Damp Heat, backsheet and module manufacturers extend this procedure to 2000 hours or longer to ensure that the material performs adequately upon aging. Such lengthy procedures contribute to the delayed introduction of materials to the PV market. Most companies, in an attempt to shorten accelerated aging, utilize alternative test procedures. For example, HAST (highly accelerated stress test) has become popular among PV module component and PV modules manufacturers. However, the HAST procedure needs to be standardized and correlated with Damp Heat and outdoor performance.
- Interpretation of the requirements: Some standards do not clearly define the performance expectations of polymeric materials upon completion of the tests. For example, backsheet manufacturers perform damp heat testing, then measure and report adhesion values between the layers of the backsheet or between the backsheet and the encapsulant as a function of exposure to damp heat. Currently, the damp heat test protocol does not indicate what results are expected at the end of the test.
Unity is needed about interpretation of the requirements described in the IEC and UL standards.
Unity is needed between different parties, (including certifying agencies) about interpretation of the requirements described in the IEC and UL standards. For example, UL 746 C states that only materials directly exposed to sunlight have to be tested with respect to UV stability. This statement raises a lot of questions. Is this a requisite applicable to backsheets? How does the test need to be performed? Should it be done by a certifying agency or will the internal test results suffice?
The Relative Thermal Index (RTI) value requirement is even more complicated. RTI is a measure of the thermal stability of the polymeric material and determines whether the material is suitable for continuous use at certain temperatures. Usually, backsheets are multilayer structures; currently, the standard states that the RTI value of one layer can be assigned to the full construction. It is unclear, however, which value should be assigned to the full construction in the case where individual layers possess different RTI values. Certifying agencies offer different opinions on this matter.
Flammability is certainly a key performance concern for solar modules and requires clear standards and testing protocols. For example, does the testing require the backsheets and components alike to meet flammability standards vs. testing the module as a whole? How will the standards be set when flammability requirements vary between regions? For example in North America the housing and building materials are wood based and tend to be more flammable. Additional requirements need to be taken into consideration, such as insulation properties that prevent arching, in the case where arching may occur, how does the material perform to resists ignition? This is a challenge for the industry to identify and set clear standards.
Speed, or lack thereof
Speed is another challenge in the PV module market. There are material answers to the ever-growing demands and niche needs of the PV Industry. Most of these materials are polymer-based and are subjected to the same slew of certification tests for each region. The certifying agencies that reside in various regions all have their own set of requirements and are not unified. All of this leads to confusion and delay in the approval process.
Until these standards and regulations are defined, the growth of the market and research and development of new products will face delays.
Lastly, performance standards need to respond to module evolution. The PV community is growing beyond module concept for small homes or communities. It is evolving into a differentiated portfolio of performance materials with the legs to help solve the complexity of reaching grid parity. Certain new polymeric materials can provide the path to achieve this goal, but multiple certification processes will greatly delay their use.
The PV community realizes these limitations and is working on new IEC documentation to address these and other concerns. UL, a certifying body responsible for setting safety standards, is diligently working with various International standard groups to develop and initiate a unified requirements set. Doing this will ensure that solar panels meet all the needs of a global market. However, the timetable for the new standards that will bring relief to the highlighted challenges is a lengthy process. Until these standards and regulations are defined, the growth of the market and research and development of new products will face delays. The certification process and compliance will continue to be lengthy and costly. In the meantime, the current exponential growth of the PV industry is intensifying the strain on PV module and component manufacturers alike. To continue supporting the important, innovative efforts of the PV industry, a short-term solution is required to screen and qualify much needed materials.
About the Author
Marina Temchenko received her M.Sci. in Polymer Chemistry from Moscow State University in Russia and is a Senior Scientist at Madico, Inc. in Woburn, Massachusetts.
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