Durability & Reliability Challenges for Photovoltaics

Issue 11 and Volume 112.

By: Kurt P. Scott, Director of Research and Development, Atlas Material Testing Technology LLC

The increase in activity in the clean energy sector leads one to believe that this time around the excitement may actually amount to something.

Despite declining production costs and increasing cell conversion efficiencies, cost remains a significant issue for so-called clean energy technologies. Economic pressure to attain parity with current, conventional electricity generation increases as government incentives for renewables are under threat, will decrease over time and eventually will be withdrawn. Soon each renewable technology will be required to stand on its own economic merit.

Photovoltaic (PV) modules and other solar conversion technologies are most effective when deployed in direct sunlight, an inherently harsh service environment for any material, as damaging solar radiation in combination with heat and humidity are the bane of all man-made materials. Industry experts say that economic viability of the still expensive technologies depends on their ability to function effectively for 25 to 30 years. Over this period of time their high initial costs would be amortized to approach the costs per kilowatt of current, conventional electricity production.

Laboratory testing must be used in concert with natural exposure. Click here to enlarge image

So the question becomes: How do PV manufacturers build systems that will be durable and reliable for up to 30 years? Since durability and reliability are often confused, let’s start with their definitions.

Generically, reliability is the measure of unanticipated interruptions during intended use of a product. Its measurements typically include failure rates (time until failure or between failures), cumulative failures, component lifetimes and estimates of product lifetimes or service life. Analytical techniques are drawn mainly from probability statistics and the theory of stochastic processes. Typically, large sample populations of production items are required for good, statistically sound calculations.

Durability is the ability of a material, component or product to resist wear or decay under stress over time. Measurements may include changes to chemical, physical or appearance properties, loss of performance, rate of property change with time or stress or time to unacceptable change.

The definitions indicate interdependency between these two product aspects. At their interface, durability in some cases may be seen as a subset of reliability. After all, durability changes may affect reliability at any level. In general terms, reliability primarily is concerned with discrete, absolute failure of the overall system.

Durability often involves understanding the route to failure (mechanisms) and the rate of property change (kinetics). These individual changes may not lead to loss of reliability, but may lead to declining performance and shortened service lifetimes.

Within the larger context of durability lies the ability of a material, component or product to resist degradation caused by stresses of the service environment.

For photovoltaics, the “service environment” means terrestrial outdoor exposure. However, testing PV long-term durability is a big challenge. PV’s inherent stability would necessitate extremely long outdoor exposure tests, which are not ideal in business environments where development-to-production cycles must be kept to a minimum. The industry has a pressing need to develop reliable accelerated laboratory tests that may be used in concert with “natural exposures.”

Some possible objectives of weatherability testing include validating design, characterizing performance, supporting warranty claims and conducting competitive analyses.

The inadequacy of existing standards to appropriately test for either durability or reliability is widely cited in industry literature. As PV production moves from research projects to business concerns, good service life prediction must be part of the economic viability calculus. The renewable technology industry must build upon the existing standards that have served the PV community for years, and develop even more meaningful accelerated weathering tests that will yield reliable service life estimations in a relatively short period. This will provide a more scientific basis for warrantees and financial risk assessment.

Although electrical efficacy is a fundamental performance criterion, others such as safety and appearance attributes are also critical. The variety of possible failure modes dictates that comprehensive testing of photovoltaics must be carried out at both the component level and the complete system (or module) level.

The most widely used PV durability and performance test methods are governed by the International Electrotechnical Commission, the Institute of Electrical and Electronics Engineers, ASTM International and Underwriters Laboratory.

Engineers should anticipate growing demand from PV business managers who will look for sound durability and reliability analyses. Speed and accuracy will be paramount: Speed, to facilitate a significantly compressed development-to-production business cycle, essential for this fast-paced sector, and accuracy, to mitigate any risks associated with product durability and reliability.

Engineers and materials scientists can prepare themselves for these demands in a number of ways. For one thing, become active in the development of consensus standards to contribute to more relevant test methods. The slow process of this route may, however, render it unviable for business entities that need quick results. In such cases, companies may have no other option but to take responsibility for developing their own company-specific methodologies. For another, conduct a clear-eyed assessment of in-house resources and capabilities. Determine whether it is more feasible to enhance in-house capabilities or purchase the services of experts on an as-needed basis. And last, but certainly not least: start now.

Renewable Hydrogen Production Technologies


By: Jennifer Gangi, Program Director, Fuel Cells 2000

Hydrogen’s potential lies in its partnership with fuel cells, electrochemical devices that combine hydrogen and oxygen to produce electricity, with water and heat as the by-products. Since the conversion of the fuel to energy takes place via an electrochemical process, not combustion, the process is clean, quiet and highly efficient—two to three times more efficient than fuel burning. But it’s how the hydrogen is produced that’s of interest in terms of renewable energy.

For the most part, hydrogen generation is produced by reforming natural gas, which still produces fewer emissions than other forms of power generation (think ounces not tons). But many researchers and companies are working on cleaner, more renewable ways to generate hydrogen.

Hunts Point Water Pollution Control Plant in the Bronx, N.Y. Three UTC 200 kW fuel cells use the plant’s digester gas to provide grid-parallel power. Photo courtesy Fuel Cells 2000. Click here to enlarge image

Hydrogen is produced from a wide variety of domestic energy sources, using an array of process technologies. Hydrocarbon fuels—methanol, ethanol, natural gas, petroleum distillates, liquid propane and gasified coal—can yield hydrogen in a process called “reforming.” Hydrogen can be extracted from landfill gas or anaerobic digester gas from wastewater treatment plants, from biomass technologies or from hydrogen compounds containing no carbon, such as ammonia or borohydride.

Hydrogen can also be produced from water via electrolysis. And if the process is powered with emissions-free, renewably generated energy, it becomes a truly renewable fuel. Dozens of demonstrations and research projects are underway at energy labs, universities and technology companies to develop sources of hydrogen with solar, wind, biomass and even microorganisms like green algae and cyanobacteria. One of the most promising processes uses the hydrogen found in anaerobic digester gas (ADG) and methane.

ADG is an organic waste product, so in many states it is considered a renewable fuel and eligible for tax incentives. Several fuel cell manufacturers are installing large systems at wastewater treatment plants, landfills and even breweries, capturing the waste gas that would normally be released into the atmosphere. Once captured, the gas is used for power and space heating (combined heat and power—CHP—or cogeneration.)

More than a decade ago, the New York Power Authority (NYPA) installed a fuel cell power plant manufactured by UTC Power at the Westchester County Wastewater Treatment Plant in Yonkers. This was one of the first commercial fuel cells to use ADG to produce electricity. The plant generated 17,400 square cubic feet (scf) of ADG a day. About 70 percent of that was used in boilers and engines, while the other 30 percent (6,000 scf/hr) was flared into the air. The fuel cell captured 3,000 scf/hr of flared ADG for power generation. The U.S. Environmental Protection Agency measures the emissions of the fuel cell as carbon monoxide, less than 1 parts per million (ppm); sulfur oxide, less than 1 ppm; and nitrous oxides, less than 0.37 ppm.

Because of the success of the Yonkers plant, NYPA bought eight more fuel cell power plants from UTC Power in 2001 and installed them at four New York City Department of Environmental Protection (NYC DEP) wastewater treatment facilities in Brooklyn, Staten Island, the Bronx and Queens. FuelCell Energy, a Danbury, Conn.-based fuel cell manufacturer, has 13 systems running off of ADG, most of which are in California.

UTC Power and FuelCell Energy also have fuel cells installed at breweries in Japan, Nevada and California, running off the ADG produced from brewing processes. Untreated brewery effluent can undergo anaerobic digestion, which breaks down organic compounds to generate methane. In the CHP system, the fuel cell maximizes the energy available in methane and can achieve 85 percent efficiency. According to UTC Power estimates, fuel cells running on ADG release 72 pounds of emissions into the environment, compared to more than 41,000 pounds from the average coal- or oil-fired plant.

Fuel cells are currently being demonstrated in cars, buses and other vehicles such as forklifts, planes, boats and trains. Thousands are installed around the world in stationary applications. Fuel cells provide both primary and backup power, clean, reliable, energy with low to zero emissions—and if the fuel that powers them is produced renewably, they could make a substantial contribution to our energy portfolio.