Because it has no moving parts (the major source of reliability issues in other types of electrical generating systems), a PV module's operating life may be determined by the stability and resistance to corrosion of the materials from which it is constructed. Manufacturer's guarantees of up to 40 or 50 years indicate the quality of bulk silicon PV modules currently being produced. Nevertheless, there are several failure modes and degradation mechanisms which may reduce the power output or cause the module to fail. Nearly all of these mechanisms are related to water ingress or temperature stress.
Reversible Reductions in Output Power
A PV module may be producing reduced output for reversible reasons. It may be subject to shading, for example, by a tree which has grown in front of it. The front surface may be soiled (PV modules will generally experience up to 10% loss of output due to front surface soiling). One module may have failed, or the interconnects between modules may have changed the operating point of the array. However, these reductions in power are all reversible, provided that the original cause is rectified.
Degradation and Failure of PV Modules
Degradation mechanisms may involve either a gradual reduction in the output power of a PV module over time or an overall reduction in power due to failure of an individual solar cell in the module.
Solar Cell Degradation
A gradual degradation in module performance can be caused by:
- increases in RS due to decreased adherence of contacts or corrosion (usually caused by water vapor);
- decreases in RSH due to metal migration through the p-n junction; or
- antireflection coating deterioration.
Short circuiting can occur at cell interconnections, as illustrated below. This is also a common failure mode for thin film cells since top and rear contacts are much closer together and stand more chance of being shorted together by pin-holes or regions of corroded or damaged cell material.
This is a common failure mode, although redundant contact points plus "interconnect-busbars" allow the cell to continue functioning. Cell cracking can be caused by:
- thermal stress;
- hail; or
- damage during processing and assembly, resulting in "latent cracks", which are not detectable on manufacturing inspection, but appear sometime later.
Fatigue due to cyclic thermal stress and wind loading leads to interconnect open circuit failures.
Open circuit failures also occur in the module structure, typically in the bus wiring or junction box.
Although each module is tested before sale, module short circuits are often the result of manufacturing defects. They occur due to insulation degradation with weathering, resulting in delamination, cracking or electrochemical corrosion.
Module Glass Breakage
Shattering of the top glass surface can occur due to vandalism, thermal stress, handling, wind or hail.
A common failure mode in early generations of modules, module delamination is now less of a problem. It is usually caused by reductions in bond strength, either environmentally induced by moisture or photothermal aging and stress which is induced by differential thermal and humidity expansion.
Mismatched, cracked or shaded cells can lead to hot-spot failures, as discussed previously in Hot Spot Heating.
By-Pass Diode Failure
By-pass diodes, used to overcome cell mismatching problems, can themselves fail, usually due to overheating, often due to undersizing 1. The problem is minimised if junction temperatures are kept below 128°C.
UV absorbers and other encapsulant stabilizers ensure a long life for module encapsulating materials. However, slow depletion, by leaching and diffusion does occur and, once concentrations fall below a critical level, rapid degradation of the encapsulant materials occurs. In particular, browning of the EVA layer, accompanied by a build-up of acetic acid, has caused gradual reductions in the output of some arrays, especially those in concentrating systems 2
- 1. , «Attaining Thirty-Year Photovoltaic System Lifetime», Progress in Photovoltaics: Research and Applications, 1994.
- 2. , «Decline of the Carrisa Plains PV Power Plant: The Impact of Concentrating Sunlight on Flat Plates», 22nd IEEE Photovoltaic Specialists Conference. Las Vegas, USA, с. 586-592, 1991.