Definition and calculation formula of photovoltaic module degradation rate
Definition of photovoltaic module degradation rate
Photovoltaic module degradation rate refers to the ratio of the maximum output power after a period of operation to the initial maximum output power under standard test conditions (AM1.5, temperature 25°C, irradiance 1000W / ㎡)
Determination of the degradation rate of photovoltaic modules
Normally the accelerated aging test method, and the comparison of module on the same field verification method are adopted to determine the degradation rate of photovoltaic modules. The accelerated aging test method is to use an environmental test room to simulate the environmental conditions such as irradiance, temperature, and humidity during outdoor actual operation, and to double or restrict related parameters to achieve the purpose of accelerating module aging and decay in a short period of time. After the accelerated aging test is completed, a power test is implemented on the test module under standard test conditions. The degradation rate of the photovoltaic module's power generation performance is determined according to the degradation rate formula.
The comparison of module on the same field verification method is that firstly a sufficient number of module are taken as samples according to the installed capacity of the project from the date of module starting operation, then the third-party testing laboratory certified by CMA shall test the modules in accordance with the method specified in the GB/T 6495.1 standard. After the initial maximum output power is tested, the tested modules are installed in the same environment with other modules produced in the same batch to generate electricity. And their maximum output power is measured again one year after the date of operation. The maximum output power at beginning and after a year are compared, and the degradation rate of the photovoltaic module's power generation performance is determined according to the degradation rate calculation formula.
Analysis of photovoltaic module degradation rate
In practice, photovoltaic modules have been in a degradation state after being manufactured. When they are in the no light package, the degradation is very slow. Once modules are irradiated by solar radiation，the degradation increases rapidly, and the degradation will gradually stabilize after a certain percentage of degradation. Schematic diagram of the first year degradation curve model is shown in the figure below:
The 2.5% total degradation of the first year in the figure above is taken from the 25-year degradation warranty of Sunrise's photovoltaic module. The 25-year degradation warranty is shown in Figure below.
Figure2. Degradation curve of photovoltaic module
It can be seen from the above figure that the maximum degradation value of photovoltaic modules in the first year is 2.5%, and the annual degradation value in the next 24 years is 0.7%.