The performance of photovoltaic modules is a very significant parameter, which largely determines the level of the LCOE and the project economic efficiency of a solar power plant. Therefore, solar manufacturers are striving for technological innovation to increase the competitiveness of their products. In a hurry to bring new technologies to the market, manufacturers are losing quality control. As a result, we are witnessing a revival of old failure mechanisms and new solar panels degradation methods.
Reasons for the solar modules performance decline
Solar photovoltaic modules are prone to many Failure Modes and Aging Mechanisms. Manufacturers must follow strictly established production procedures and use high-quality components for the stable performance of solar panels throughout the entire service life. A premature decrease of the PV-modules efficiency occurs when the stages of product quality control are skipped or low-quality materials are used.
Lack of long-term field data
Modern manufacturing technologies use components that were not there 25 years ago. Therefore, real data that confirm the long-term reliability of many modern PV production technologies do not exist today.
However, there are independent laboratories that test equipment for solar power stations. You can calculate PV-system reliability basing on the obtained data. For example, the PV Evolution laboratory conducts tests that demonstrate the suitability of solar equipment. You can use PVEL quantify reports to plan large-scale projects for the generation of solar energy.
Figure 1 shows a graph and a table for the most common solar cell defects that arise after their stress testing.
Fig. 1. A number of the most common defects that occur in solar modules during stress testing. Source: 2020 PV Module Reliability Scorecard, PVEL, 2019.
Potential Induced Degradation
Potential Induced Degradation (PID) is a reduction in the output power of PV modules over time. It is a very undesirable phenomenon caused by both internal and external causes.
Potential Induced Degradation of photovoltaic modules is the expected and normal process because any equipment fails eventually. But it is economically feasible to limit or eliminate the PID causes. This reduces the solar panels’ degradation rate and improves the project economy.
What causes and accelerates PID processes in solar modules?
- A potential difference between the solar cell and the grounded module frame;
- Humidity and temperature exposure;
- Manufacturing defects;
- The insufficient density of the insulating layer of the module.
A lifetime for 25-30 years of PV-modules defines еhe economics of solar power plant projects. Therefore, a significant decrease in productivity in the early years of the Solar Power Plant operation is simply a disaster from a technical and financial point of view.
PID can occur within a few weeks or even days after the PV-plant commissioning. This happens when there is no grounding. In this case, the voltage between the frame and the module cells can cause a “drift” of sodium ions from the glass to the cell surface.
The cell typically has an antireflection coating of silicon nitride (SiN). If the pinholes in this coating are large enough to allow sodium ions to enter the cell, then productivity can be irreparably reduced. In such a situation, the voltage can cause a build-up of static charge, which also negatively affects performance, although this effect is usually reversible.
How to minimize the likelihood of PID processes?
PVEL offers a PV-module testing procedure so that investors can trust one or another manufacturer of solar panels. It allows you to determine how solar cell is PID-resistant. If testing reveals unsatisfactory results of the solar battery resistance to degradation, the use of alternative solutions is advisable. You can use grounding configurations or distributed electronics or choose another PV module.
How is PV-module testing done?
Once the module is placed in an environmental chamber, the voltage bias equal to the maximum system voltage (MSV) rating of the module (-1000V or -1500V) is applied with 85°C and 85% relative humidity for two cycles of 96 hours. These temperature, moisture, and voltage bias conditions help PVEL evaluate possible degradation and failure mechanisms related to increased leakage currents (Fig. 2).
The results presented in the histograms show a decrease in the average power for various test samples of the same model. The histograms take into account the comparison of 2020 indicators with the historical PVEL dataset.
Fig.2. Deterioration of electrical parameters when testing a module. The electrical parameters in the graphs are defined as follows: maximum power (PMP), voltage at maximum power (VMP), open circuit voltage (VOC), short circuit current (ISC) and current at maximum power (IMP).
Innovative technological solutions for solar modules
Over the past few years, there have been many innovations in photovoltaic technology. Manufacturers are actively introducing new processes and new components.
PERC (Passivated Emitter Rear Cell) – an additional dielectric layer on the backside of the cell technology for mono and polycrystalline cells. This technology increases the degree of absorption of photons and the quantum efficiency of cells;
Bifacial is a double-sided single-crystal element. Bifacial cells absorb light from both sides of the panel and in the right location and conditions can produce up to 27% more energy than traditional monofacial panels;
Multi Busbar – multi ribbon and wire busbars. Busbars are thin wires or ribbons which run down each cell and carry the electrons (current) through the solar panel;
Split panels with half-cut Cells is using half-cut or half size cells rather than full size square cells, and moving the junction box to the center of the module. This effectively splits the solar panel into 2 smaller panels of 50% capacity, which work in parallel. Hight performance due to lower resistive losses through the bus bars.
Dual Glass is known as glass-glass, dual glass, or double glass solar panels. The rear glass replaces the traditional white EVA (plastic) back sheet and creates a glass-glass sandwich that is considered superior as glass is very stable, non-reactive and does not deteriorate over time or suffer from UV degradation.
Shingled Cells are an emerging technology that uses overlapping thin cell strips that can be assembled either horizontally or vertically across the panel.
IBC (Interdigitated Back Contact cells) are not only more efficient but also much stronger than conventional cells as the rear layers reinforce the whole cell and help prevent micro-cracking which can eventually lead to failure.
HJT (Heterojunction cells) use a base of common crystalline silicon with additional thin-film layers of amorphous silicon on either side of the cell forming what is known as the heterojunction. The multi-layer heterojunction cells have the potential to increase efficiency when combined with IBC technology.
Trends in the PV Modules Production
Over the past few years, the technology for the production of photovoltaic modules changed quite a lot. And now, buyers face a complex market with various parameters and properties of solar equipment. Manufacturers are actively introducing new processes and new components. Three important trends are taking place in new technologies for the production of solar panels. It is important to understand these trends in terms of finding opportunities to reduce risks in solar energy projects.
| Trends | Risks | Rewards |
| Large-scale adoption of PERC cell architectures. | Some PERC cells are susceptible to light and elevated temperature induced degradation (LeTID), which can reduce energy yield by as much as 10% in the field. Susceptibility to boron-oxygen destabilization may also be a concern. | PERC cells are higher efficiency and usually perform better in low-light and high-temperature conditions, and they can be produced at comparable costs to Al-BSF. |
| New cell architecture: more busbars, round interconnect wires, larger wafers, half or third-cut cells | Some new cell designs are more susceptible to microcracks and may require difficult to implement process changes on manufacturing lines that lead to increased defect rates. | New cell designs are driving higher efficiencies and nameplate power ratings in PV panels and leading to decreased costs. |
| New module architecture: thinner frames, glass-glass, bifacial, light-redirecting films (LRF). | Newer module form factors may be more susceptible to damage, and they may not be compatible with existing mounting systems. The industry lacks long-term field data for new components and designs. | Lighter modules are easier to transport and install. New designs and materials can increase nameplate power ratings. |