The performance of a PV module will decrease over time. The degradation rate is typically higher in the first year upon initial exposure to light and then stabilizes. Factors affecting the degree of degradation include the quality of materials used in manufacture, the manufacturing process, the quality of assembly and packaging of the cells into the module, as well as maintenance levels employed at the site. Regular maintenance and cleaning regimes may reduce degradation rates but the main impact is specific to the characteristics of the module being used. It is, therefore, important that reputable module manufacturers are chosen and power warranties are carefully reviewed.
The extent and nature of degradation varies among module technologies. For crystalline modules, the cells may suffer from irreversible light-induced degradation. This can be caused by the presence of boron, oxygen or other chemicals left behind by the screen printing or etching process of cell production. The initial degradation occurs due to defects that are activated on initial exposure to light.
Amorphous silicon cells degrade through a process called the Staebler-Wronski Effect. This degradation can cause reductions of 10-30% in the power output of the module in the first six months of exposure to light. Thereafter, the degradation stabilizes and continues at a much slower rate. Amorphous silicon modules are generally marketed at their stabilized performance levels. Interestingly, Sdegradation in amorphous silicon modules is partially reversible with temperature. In other words, the performance of the modules may tend to recover during the summer months, and drop again in the colder winter months.
Additional degradation for both amorphous and crystalline technologies occurs at the module level and may be caused by:
• Effect of the environment on the surface of the module (say, of pollution).
• Discoloration or haze of the encapsulant or glass.
• Lamination defects.
• Mechanical stress and humidity on the contacts.
• Cell contact breakdown.
• Wiring degradation.
PV modules may have a long term power output degradation rate of between 0.3% and 1% per annum. For crystalline modules, a generic degradation rate of 0.5% per annum is often considered applicable. Banks often assume a flat rate of degradation rate of 0.5% per annum.
The conversion efficiency is a measure of the losses experienced during the conversion from DC to AC. These losses are due to multiple factors: the presence of a transformer and the associated magnetic and copper losses, inverter self-consumption, and losses in the power electronics. Conversion efficiency is defined as the ratio of the fundamental component of the AC power output from the inverter, divided by the DC power input:
Conversion efficiency = PAC/PDC
The conversion efficiency is not constant, but depends on the DC power input, the operating voltage, and the weather conditions including ambient temperature and irradiance. The variance in irradiance during a day causes fluctuations in the power output and maximum power point (MPP) of a PV array. As a result, the inverter is continuously subjected to different loads, leading to varying efficiency. The voltage at which inverters reach their maximum efficiency is an important design variable, as it allows system planners to optimize system wiring. Due to the dynamic nature of inverter efficiency, it is better depicted through diagrams than by uniform numeric values. An example depicting the dependency of the inverter efficiency on the inverter load is given in the figure here.
The European Efficiency is an accepted method of measuring inverter efficiency. It is a calculated efficiency averaged over a power distribution corresponding to operating climatic conditions of a central European location. As a useful means of comparing inverter efficiencies, the efficiency standard also attempts to capture the fact that in central Europe most energy is generated near the middle of a PV module’s power range. Another method of comparing efficiencies is using the Californian Efficiency. While the standard is based on the same reasoning as the European efficiency, it is calibrated for locations with higher average irradiance.
Inverters can have a typical European Efficiency of 95% and peak efficiencies of up to 98%. Most inverters employ MPPT algorithms to adjust the load impedance and maximize the power from the PV array. The highest efficiencies are reached by transformer-less inverters.
The inverter market is dominated by SMA Solar Technology AG, which has a higher market share than the combined share of the next four largest vendors (Power-One, Kaco, Fronius, and Siemens). Other inverter manufacturers hold the remaining 18% share of the global market. Over the past year, a number of major industry players have started to enter the inverter market. These include GE, ABB, and Schneider Electric (through the acquisition of Xantrex). In 2010, the growth in the solar PV market and delays in production (due to scarcity of key electronic components) led to a global shortage of inverters.
According to a recent report of IMS Research, the 2012 global PV inverter supplier rankings are as follows:
- Advanced Energy
- Enphase Energy
- Danfoss Solar Inverters
- Omron Corporation
- Schneider Electric
Quantifying Plant Performance
The performance of a PV power plant is expected to fall during its lifetime, especially in the second and third decade of its life as modules continue to degrade and plant components age. In addition to the quality of the initial installation, a high degree of responsibility for the performance of a PV plant lies with the O&M contractor.
Capacity Utilization Factor
The capacity factor of a PV power plant (usually expressed as a percentage) is the ratio of the actual output over a period of one year and its output if it had operated at nominal power the entire year, as described by the formula:
CUF = Energy generated per annum / (8760 hrs per annum X installed capacity, kWp)
The capacity factor of a fixed tilt PV plant in southern Spain will typically be in the region of 16%. This means that a 5 MWp plant will generate the equivalent energy of a continuously operating 0.8 MW plant. Plants in India operating within a reliable grid network are expected to have a similar capacity factor.