How Reliable a Solar PV Module is?
12th December 2017
The first thing that comes to our mind when we think of solar modules is "Can they generate power for their lifetime?" The current commercial solar technologies (Multi and Mono) have been installed before a few years.However none of these installations have still reached the end of solar module's life time (i.e. 25 years).This makes it a little difficult to predict whether the module would reach its claimed lifetime. In order to offset such doubts, a solar manufacturer gives product warranty and power output warranty (also known as performance warranty) for 10 and 25 years respectively. Performance warranty as the name suggests is the warranted time period in which the module would perform at or above the said power. This is required so that the buyer or the customer is assured that the module is reliable and would not fail in its lifetime. Additionally, it would also ensure that the module (and hence the power plant) would generate the expected amount of power.While the warranty is in place, it is important to understand the factors that cause degradation of solar modules which are discussed below:
- Light Induced Degradation: Light Induced Degradation or more commonly known as LID occurs in solar module during its initial exposure to sunlight. This effect is seen in crystalline module due to the presence of oxygen during manufacturing of Silicon Ingots (Czochralski process). While doping the solar cell with Boron the oxygen reacts with it leading to reduced movement of electrons (leading to loss in power production).This effect causes a reduction in power output upto 3% in initial days and or monthsand is not seen then.
- Repeated Thermal Cycling: Solar modules as we know are expected to stay outdoors for a period of 25 years. In this period they are exposed to repeated and reversed temperature cycle ranging (depending on site) froma minimumof 10 °C to a maximum of 50 °C (under average Indian conditions). This would cause the module to operate well above (and below) the standard operating conditions. As shown in figure 1 below, the power output of the module varies with respect to temperature.Such repeated thermal cycle would also generate high reversed stress in the solar cells and other materials in module, which degrades them in long run, resulting in reduced power output.
Figure 1: Variation of power output with cell temperature (Source: Waaree Energies)
- Static and Dynamic loads: The solar module in its lifetime deals with both static and dynamic loads. Such loads if outside its permissible stress limit can cause heavy (and many times irreversible) damage to the module/entire plant (Figure 2). Static loads are those which may be continuously applied on the module without any (significant) impact speed (say force due to accumulated hails on the module). Dynamic loads on the other hand are such forces which are applied on the module with significant impact speed (say wind loads at high speed) which could uproot/rupture the entire plant. Both these kind of loads are indiscernible in nature reducing the power output to almost nothing if its effect is on larger scale.
Figure 2: Destroyed solar module due to hails/stone (left) destroyed solar plant due to wind load (right)
- Climatic conditions: An unseen yet an important factor affecting the power output of the module are the climatic conditions. The solar module is exposed to varying levels of irradiance and humidity. Both these factors affect the power output of the module. Seeping of moisture into the module primarily degrades the welded joints and/or other materials. Such degradation would (fully or partially) loosen the interconnection in the module.With a prolonged exposure to irradiance (and hence UV light) and when moisture seeps in through the back sheet into the panel, the acetic acid from EVA is released. This causes discoloration of EVA leading to reduced transmission of light through it and hence reduced power output.Additionally moisture may further lead to corrosion of the solar cell which would further lead to reduced power output. While dependant on the location, it is seen that a maximum of 40% reduction in power output can be seen. This number may further shoot up failing the module (sometimes the string) making the plant feasibility vulnerable.
As evident from above, there are a few factors which can adversely affect the performance of the module. It is hence necessary to test all the modules before deploying them on site. International Electro-Technical Commission (IEC) and various other certifying (national and international) agencies have formed testing standards in order to realize the reliability of the modules. A few of those tests are as mentioned below:
- Thermal cycling test: The purpose of this test is to determine the ability of the PV module to withstand the effect (of material fatigue, temperature stress, etc.) for fast changes of temperature. The test puts strain on PV modules so that subsequent effect of thermal coefficient shows up hidden defects like poor soldering, cracked cell, performance reduction, insulation resistance, etc.Tested according to IEC standards, the module is subjected at a temperature from - 40 °C to +85 °C for 200 numbers of cycle swings and the power degradation from such module cannot be more than 6% of the rated power.
Figure 3: Thermal cycling test (Source: Google images)
- Dynamic loading test: According to IEC 61215, Dynamic or Mechanical load test is done with a load being applied on the module for a specified period of time (say 1 hour). The load is applied such that the corresponding pressure on the module is 5400 Pa. A similar test may be applied on the back of the module. The module (and hence the material and cell) under the test would be subjected to alternate stress and strain which may cause crack/ breakage if the module is not made correctly.This test would assure that the module is capable of withstanding heavy loads/ wind loads of specified pressure when installed at site.
Figure 4: A typical dynamic loading test
- Damp heat test: Humidity as we mentioned above has worst effects on the solar modules. The purpose of this test is to check the reliability of PV modules to withstand the effect of long term penetration of humidity. As per the standards, the module is kept in a chamber at a temperature of 85 °C at aRelative Humidity (RH) of 85% for a specified number of cycles (1000). This would enable the end users to understand their products long term reliability.
- Climate chamber for UV & PID testing: UV irradiance as we mentioned above has adverse affects on the solar module. Also in our previous blog "Common problems in PV plant - Part 2" we had mentioned about PID and its effect on the performance of module. PID has been recently discovered (comparing it to other defects).The standards for both UV and PID testing were made after continuous consultation with the industry. The UV testing is done in a temperature and RH controlled chamber (Figure 5) where the module is exposed to a specified amount of UV light (UVA + UVB +UVAB) maintaining the module at 60 - 5 °C. After exposure of the module for specified number of cycles, the module is tested for its power output to determine the effect of UV on solar module. The PID testing is done in a similar where the module is tested by simulating on site situations (i.e. module connected to high negative voltage) and tested for PID. An Electro luminance (EL) image is taken before and after the test to confirm if the module has been affected due to PID or not.
Figure 5: Climate chamber for performing UV & PID test (Source: Google images)
Waaree is committed to delivering Avant grade products and services to its consumers. All our modules are tested stringently and are certified by various bodies to extract maximum power out of it under any conditions. Over and above the IEC testing standards, our modules are tested for extended number of cycles so that we can ensure the long term reliability in our modules. Our modules are manufactured in lines with international standards which make them both nationally and internationally reliable for any on site conditions.
Let us all pledge to make solar energy the primary source of energy in the near future.
RAHE ROSHAN HAMARA NATION