Photovoltaics - Will they stand the test of time?

Tushar Nandwana
March 1, 2011

What are Photovoltaics?

Photovoltaics (PV) or solar cells are a type of semiconductor material that generates a DC electric current when sunlight strikes the cell and a portion of the light is absorbed by the cell. Cells are assembled and connected together to form a solar module or panel. Panels are grouped together to form an array. An array would also include other electrical components such as an inverter which converts the direct current (DC) to alternating current (AC). A transformer would step the voltage up/down before the current is directed into the power grid or other application.
There are several types of PVs in use today including Crystalline (single and poly) Silicon (cSi), Thin Film, and Concentrated PV (CPV). Power generation efficiencies for the average systems range from 20% for single crystalline cSi, 13-14% for polycrystalline cSi, 8-12% for thin film and 25-38% for CPV 12. Efficiency is a measure of how much of the sunlight striking the cell is converted into electrical energy versus being dissipated as heat. Technological advances such as Sharp Corporation’s use of a tri-layer cSi, have increased efficiencies to 35.8%3. This involves the use of indium-gallium-arsenide layers to convert light energy at multiple wavelengths into electrical current. For CPV based systems, Boeing’s Spectralab has developed tri-layer cells with efficiencies of 41.6%4.

Performance Degradation

So what can affect or degrade PV systems’ power generation performance over time? We can characterize degradation modes as either reversible or irreversible.

Reversible Modes

Reversible conditions can be readily corrected and once corrected, should result in normal power generation. A common condition is the accumulation of dust or soiling (bird droppings, water spots) onto the front surface of the PV array resulting in up to 10% estimated power reduction.

Inverter failure would be a short term effect on the PV array system’s output capability. Inverters are a critical electrical component that converts the PV array’s direct current into alternating current, which can then be fed into the power grid. An inverter can succumb to total electrical failure requiring the replacement of the inverter. Inverters can also sustain temporary downtime if they trip and go offline due to external grid effect factors, such as voltage or frequency spikes in the external power grid. Resetting the inverter rectifies this downtime condition and allows power generation to resume.

Irreversible Modes

Irreversible conditions play a far greater role in a PV system’s long-term power generation performance. As such, Sandia Labs in Albuquerque, New Mexico has conducted a variety of analyses on the reliability and availability of field-based PV systems. This research is based on data from a field-based PV system, extrapolated modeling of the field data and computer simulations. The focus was mainly on cSi systems rather than thin film and CPV systems as cSi systems have been in the field for a longer period of time.

Common irreversible, degradation modes that were observed in field-aged PV modules include degradation of packaging materials, adhesional loss, degradation of interconnects, degradation due to moisture intrusion, and semiconductor device degradation5. Let’s explore these degradation modes further:

  • Packaging – The protective glass on the panel can crack due to environmental factors, or bow because of poor installation. The anti-reflective coating can be destroyed due to use of unapproved cleaning chemicals, and sodium formations can occur on the glass surface due to chemical leaching that obscures the transmissivity (how much light gets through to the cell below) of the glass. UV absorbers in the encapsulant or packaging materials can degrade over time due to leaching or diffusion and this can cause the layer to turn brown. All of these negatively affect the amount of sunlight entering the cell and the resultant current production. The term encapsulant refers to the EVA resins that are converted to form a protective sheet around the silicon wafers and module circuitry.
  • Adhesional or Delamination – This is defined as the breakdown of the bonds between material layers that form a module laminate6. Delamination can be a latent manufacturing defect or be environmentally induced, although it is less likely for the newer generation of cSi cells. This can occur at both the front and rear side of the PV module. Front-side results in the optical decoupling of the material layers that transmit the sunlight down to the cell layer, and this greatly reduces the operating efficiency of the cell. Delamination also affects how efficiently the cell dissipates heat. Higher operating temperatures degrade the PV’s performance.
  • Interconnect – Environmentally induced thermal expansion and contractions can affect the solder interconnects between the cells on a module. Additionally, metallurgical factors within the solder material may affect the integrity of these joints over time. Failure of the interconnect joints may affect the flow and amount of current due to increased resistance or cell isolation. This can lead to temperature increases and localized hotspots that would degrade the performance of the PV module.
  • Moisture Intrusion – Failure of the seals/bonds between the front and backsheet of the modules would allow moisture penetration. Over time, this can result in delamination of the encapsulant and separation of the silicon/circuitry from the module. Moisture penetration at this level can corrode the circuitry as well as the metallized layers of the silicon resulting in loss of electrical power performance. It can also results in shorts and current leakage that can pose an electrical shock hazard.
  • Semiconductor – Failure within the silicon or another variant (Indium Gallium Arsenide) layer would seem to be a likely cause of performance degradation, but field experience indicates this actually isn’t the case. All cSi devices undergo a semiconductor based, permanent degradation of 1% to 5% called LID or Light Induced Degradation. It occurs within the first few hours of exposure to sunlight but actually stabilizes thereafter. LID is generally not a major factor in a PV’s performance because it would go through this permanent degradation during the manufacturer’s testing phase. The other potential degradation mode is the diffusion of dopants (phosphorus or other) from within the cell to the cell’s surface.

Based on the degradation modes noted above, field experience indicates module performance losses of 1%-2% a year in systems tested over a 10-year period from mid-1980s to mid-1990s. Another field-based study shows a loss of 0.5% to 0.7% per year7. A more recent study estimated the loss to be 0.5% per year after the first five years of installation8. Due to the lack of sufficient long-term data, Sandia conducted a 30-year projected computer simulation to predict the cumulative energy output of a cSi PV system over time with respect to several factors. These included solar irradiance (amount of sunlight falling on the system), PV module performance, and equipment availability (six components including the inverter)9. They varied the parameters through 11 separate computer simulations. Based on degradation rates of 0.5% (baseline), 1.0% and 1.5% per year, the following was developed10.

Module Degradation Rate/YrModule System Performance on Year 30Cumulative Median Energy Output
(% of 0.5% baseline) in kWh
0.5% 85% 7,000,000 (100%)
1.0% 74% 6,500,000 (93%)
1.5% 64% 6,000,000 (85%)

 

At the end of 30 years, a system with 1.0% and 1.5% degradation per year generated 7% and 15% respectively less energy than the baseline 0.5% system. Their modeling shows that for the first 15 years, all three systems generated a similar amount of power but there is a dramatic difference thereafter. Lastly, on year 30 of the simulation, the systems with 0.5%, 1.0% and 1.5% degradation were respectively performing at 85%, 74% and 64% of maximum capacity.

The results suggest that variation in energy production is mainly dependent on degradation rate of the modules followed by the irradiance (sunlight per area) factor. Energy production was not strongly sensitive to variations in inverter and non-module equipment failure11. This makes sense because equipment is readily replaceable and if an adequate preventative and corrective maintenance program is in place, this does not become a major factor in the efficacy of a cSi PV system.

This is not to say that inverter failures are not likely. A separate analysis by Sandia on a PV system operated by Tucson Electric Power did indicate that inverters were the most unreliable component in system with an expected failure rate of 429 units in 20 years versus 38 PV module failures in the same time frame12. However, the impact of inverter failure is not as significant on the overall performance of the system because it is a component that can be readily replaced with relatively little downtime.

Standards

There are various UL and IEC (International Electrotechnical Commission) standards for solar based components and systems. These include IEC 61215, 61646, 61730, 6218013 and UL 1703, 174114. UL standards are followed in the US while IEC standards are followed mainly in Europe and Asia. These are focused primarily on product safety and performance, but not on long term reliability. To simulate long-term reliability, manufacturers conduct ALCT (accelerated life cycle testing) but there currently is no standard that provides guidance for this. They resort to internal experience, product knowledge and/or expand on existing performance standards as a basis for their ALCT. Many manufacturers use this as the basis for developing their long-term warranty periods that can range from 15 to 25 years.

Conclusion & Mitigation Strategies

It should be clear that the analysis in this paper is based on a limited number of studies and simulations, and may change as new data is developed. These indicate that cSi PV systems have an annual performance degradation factor that ranges from 0.5% to 1.0% per year; extending this out to 30 years, simulations show a power generation degradation of 15% and 26% respectively. Degradation rate is driven primarily by various failure modes attributable to the cell module. Many of these failure modes may become less significant with newer generations or types of PV modules due to technological improvements in the design and manufacture of the modules (solder interconnects, seals, etc.). This may likely decrease the degradation factor but to what degree remains unknown, until more long-term field data becomes available. Additionally, degradation factors are likely to differ for thin film and CPV systems. Inverter failure appears to be a likely failure mode but in long term simulations, it does not manifest itself as a significant performance degradation factor for PV systems. This is likely due to such equipment being readily available and replaceable. If manufacturers, installers or users of PV systems incorporate the applicable best practices noted below, they should expect to see some level of reduction in the annual performance degradation rate, which should result in better than expected, long-term system performance.:

  • Manufacturing PV components and systems that meet, or exceed, UL and/or IEC standards.
  • Focusing on the design, manufacture and QC to address critical, long-term failure points, such as solder welds, interconnects and seals.
  • Proactive monitoring of the PV system to detect problems in the infancy stage, and repair/replace components before they compromise the effectiveness of the system.
  • Scheduled preventative maintenance and replacement programs for critical components such as inverters once the systems are installed in the field.
  • Ongoing cleaning of PV arrays to manufacturer’s specifications to ensure normal performance levels.

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References:
1 U.S. Energy Information Administration, Solar Photovoltaic Cell/Module Manufacturing Activities 2009, January 2011, p 2
2 S. Kurtz, Opportunity & Challenges for Development of a Mature Concentrating Photovoltaic Power Industry, Technical Report NREL/TP 250-43208, Revised November 2009, p 6
3 Sharp Corporation news release as of October 2009
4 Boeing Spectralabs website FAQ, accessed January 2010
5 M. A. Quintana, D. L King, T. J. McMahon, C. R. Osterwald, Commonly Observed Degradation in Field-Aged Photovoltaic Modules; Sandia National Laboratories & National Renewable Energy Laboratory, April 2003, p. 1
6 Ibid. at p. 2
7 Ibid. at p. 1
8 E. Collins, S. Miller, M. Mundt, J. Stein, R Soresnsen, J Granata, M. Quintana, A Reliability and Availability Sensitivity Study of a Large Photovoltaic System; Sandia National Laboratories, July 2010, p. 4
9 Ibid. p. 1
10 Ibid. p. 4 & 5
11 Ibid. p. 6
12 Elmer Collins, Michael Dvorack, Jeff Mahn, Michael Mundt, Michael Quintana, Reliability and Availability Analysis of a Fielded Photovoltaic System; Sandia National Laboratories, June 2009, p. 6
13 IEC 61215 – Crystalline silicon terrestrial photovoltaic (PV) modules - Design qualification and type approval; IEC 61646 –Thin-film terrestrial photovoltaic (PV) modules - Design qualification and type approval; IEC 61730 - Photovoltaic (PV) module safety qualification - Part 1 & 2: Requirements for construction & Testing; IEC 62180 – Concentrating PV modules - Concentrator photovoltaic (CPV) modules and assemblies - Design qualification and type approval
14 UL 1703 - Standard for Flat-Plate Photovoltaic Modules and Panels; UL 1741 - Standard for Inverters, Converters, and Controllers for Use in Independent Power Systems.