5 Ways Solar Panels Degrade and How SunPower Resists Them
Photovoltaic solarpanel technology has come a long way since it's beginnings. Innovations in technology by top solar manufacturers have made it so that panels are more powerful, durable, and reliable. Specifically, innovations in durability have made it so that the lifespan of the average solarpanel has increased significantly over the years.
Despite these innovations, there are still environmental factors that can cause solar panel efficiency can degrade significantly over time. Those factors include:
1. Damp Heat, or Humidity
Humidity and moisture significantly increase metal corrosion and breakdown of plastics in solarpanels. When water vapor seeps through a typical module’s backsheet and into the panel, acetic acid is formed. This acid weakens the bonds of the front-side silver contacts and rear-side aluminum metallization, which decreases the ability to carry charge and decreases performance. Humidity can also promote corrosion of the metal, which can also hinder efficiency. Also, although water doesn't generally mix with silicon, it can create local interstitial defects.
The standard test in the solar industry for panel humidity resistance is referred to as the DH3000 (Damp Heat 3000, in which the 3,000 refers to 3,000 hours of dampness). This test puts panels through humidity and dampness conditions that are supposed to simulate if the panels were in a humid climate similar to Miami for twenty years. The results of these tests reveal that conventional modules have 28% power loss after 3000 hours, which is evidence of water vapor diffusion through the backsheet and around the edges of the solar cells. This weakens the bonds and corrodes the silver grid lines, resulting in a substantial loss in the ability to carry current.
SunPower Dampness Resistance
In a similar test that more than doubles the number of hours of dampness than the DH3000, the DH7750 (7,750 hours of dampness), the SunPower module only experiences 2.7% power loss. That drastic difference in degradation is due to the fact that the current carrying copper layer in SunPower panels is much more substantial than required to carry the current, proving a large design safety factor. It is also coated with tin plating, providing a high level of corrosion resistance.
2. Dynamic Loading (Weight or Wind)
Dynamic loading refers to any force on the panel, whether it's weight from installers walking on them, heavy snow on top of the panels, or high winds blowing against the panels. The danger in all of these scenarios is that the panels will flex, which can cause the brittle silicon wafers to crack. This can impact production in three ways:
A. If the crack breaks electrical connections within the cell, then the disconnected portion cannot pass charge carriers out of the cell, causing a “dead zone”
B. A crack can reduce shunt resistance, a measure of the resistance between the front and the back of a cell. Low shunt resistance provides an alternate current path for the carriers, reducing the amount flowing to the emitters and out of the cell.
C. Cracks create traps and defects throughout the thickness of the cell, increasing recombination rates and lowering efficiency.
How a cell fractures and what it takes to fracture a cell depend on how the cell is built. In conventional solar cells, the silicon crystal itself provides the mechanical strength of the cell. The metallization paste is comprised of metallic powders carried in volatile solvents. Wafer firing burns off the solvents, leaving a porous metallic layer which exists for electrical purposes and is not intended to provide structural support.
The standard solar industry test for panel strength is called the DLT (dynamic loading test). Approximately 50 pounds of pressure, the equivalent to 90 mph of wind, is applied back and forth on both sides of the cell in cycles. This test is designed to ensure that a product can withstand a lifetime of shipping, installation, and environmental stresses and that there are no unfavorable characteristics inherent in the design.
In a side-by-side comparison between SunPower and conventional panels,1000 cycles of DLT left the conventional panel with several broken cells in the center and a power loss of 4%. The SunPower panel, on the other hand, suffered no broken cells and 0% power loss. This is attributable to the SunPower Maxeon cells, which have thick ductile copper metallization on the back of each cell that provides both high electrical and thermal conductivity, and structural support. The rear of the silicon crystal is plated with solid copper, rather than the porous metal paste applied by conventional manufacturers. SunPower’s electroplating process yields consistent, strong, and low stress bonding, and therefore strength.
Thermal Cycling is the alternate heating and cooling of a material. Due to differences in temperature between day and night, Thermal cycling occurs at least one time per day in the life of a module. In cloudy areas where irradiance can vary dramatically throughout the day, a module can experience tens of thousands of thermal cycles throughout its lifetime. As materials grow and shrink by different amounts, areas where they are bonded to each other become stressed. This stress can cause cracks and as a result, lower production
Solar panel resistance to thermal cycling stress is generally tested through the IEC Thermal Cycling test, which subjects modules to periodic cycles from -40°F to 185°F. The current IEC standard certification of solar modules requires that panels cannot degrade more than 6% after 200 cycles of temperature swings (TC200). However, the acceleration factor for thermal cycling is difficult to correlate directly to years of performance, so it's more accurate to look at a longer period, such as TC800.
When compared to TC tests performed by conventional panel manufacturers on their own panels, SunPower panels prove to be much more resistant. While results vary greatly across different brands, some panels were down over 25% at TC800 (800 cycles). SunPower panels, however, only experience 2% degradation at TC2000 (2,000 cycles), which is ten times that of the industry standard.
SunPower panels use a fundamentally different cell connection system than that of conventional panels: a stamped plated copper interconnect with three cell connections per side and integrated strain relief between cells . Multiple solder junctions provide redundancy if there is ever an issue with a solder joint, while the strain relief provides robust resistance to thermal cycling.
4. Partial Shading and Reverse Bias Stress
Solar cells in a pv module are essentially current sources connected in series. When their current flow isn’t perfectly matched, mismatch losses occur and the “weakest” cells can operate in reverse bias. When a cell is in reverse bias it essentially consumes power from neighboring cells and converts it into heat, as opposed to absorbing light and converting it to electricity. This causes wasted power and potentially damaging heat dissipation in the affected cell.
On conventional cells the breakdown, and subsequent heating, occurs non-uniformly at the weakest points of the cells. These points occur in areas with uneven doping, crystalline defects, trace processing contaminants, etch sites, or edge effects. The heat dissipated through these points can reach temperatures high enough to destroy the module
In contrast, the SunPower Maxeon cell has a stable reverse-biased breakdown that happens uniformly across the back of the cell, so the additional energy is dissipated evenly across the full area of the cell and temperatures remain relatively low. This keeps the panel from degrading, or being destroyed by overheating, so some shading won't put the life of the panel in Jeopardy.
5. Light-induced Degradation
Believe it or not, excessive light actually degradessolar cells as well. While it is less dramatic than other forms of solarpaneldegradation, it still can significantly hinder efficiency over a number of years.
The good news is that if you know own SunPower solar panels, you don't have to worry about light-induced degradation. This type of degradation occurs only in p-type, and specifically boron-doped silicon, which is common in conventional panels. SunPower Maxeon cells however, are n-doped both on the front surface and in the bulk, are fully resistant to light.
Why do SunPower modules have a degradation rate that’s so much lower than Conventional Modules? SunPower’s back-contact Maxeon cells have important design differences from conventional cells: the key differences in the thick tin-plated high-density copper foundation on the backside of the cell and the use of strain-relieved interconnects offer much higher corrosion resistance and a dramatic reduction of mechanical stress and fatigue at the cell interconnects. This design has been refined through SunPower’s stringent internal qualification criteria, which require passing much longer durations than those prescribed for the IEC standard certification test, as well as additional tests not prescribed by IEC.
So if you've been wondering about solarpaneldegradation, know that it is real, and significantly hinder the ROI on your system if you get the wrong panels. That's why you should consider going with a solar company that installs panels that are resistant to the guaranteed elements of nature. All we ask is to take a look at SunPower panels, so that you're solar investment will be paying you back for the long run.