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damp heat, humidity-freeze, and mechanical load. These tests have been identified as important in assessing reliability [38]; however, they are not designed to estimate useful lifetime because they do not show a strong correlation with field performance and degradation. Instead, these tests are designed to ensure safety and identify infant mortality issues due to basic manufacturing quality [28] [21].
SunPowers current generation module has a rate of only 27 returns per million modules built. This includes all post-site-commissioning world-wide warranty returns of E-Series modules (Jan 2006 through July-2012, 6.5 million modules).
Various field studies have measured the degradation rate of conventional crystalline modules at between 0.6% per year to 1.5% per year, so a reasonable assessment is 1.0% per year [8], [9], [10], [11], [12] (Figure 4). These studies are discussed further in Appendix A.
In order to perform a more robust assessment, SunPower recently completed its own fleet-wide system level degradation study of 445 systems within the SunPower operating fleet. The study included 266 systems (86 MW) using the previous generation of SunPower modules as old as 3.5 years, and 179 systems (42 MW), using Conventional Modules as old as 6 years. Data spanning back to the site commissioning date were used to determine fleet-wide degradation rates, representing 3.2 million module-years of monitored data. The study [13], and a review by independent engineering firm Black and
Veatch, are available upon request.
A key result from this study is shown graphically below in Figure 5. The annual system power degradation rate (including inverter) for SunPower systems with the previous generation of modules was found to be -0.32 +0.32 % (95% confidence) per year, while non-SunPower conventional systems were found to degrade at -1.25 +0.25% (95% confidence) per year, and in both cases were shown to be linear with time.
SunPower design differences
Cell architecture and metallization
Conventional cells are made of various grades of monocrystalline or multicrystalline p-type silicon.
The front-surface is also an n-type emitter, typically doped with phosphorus; the back is typically a p-type emitter doped with boron. When the conventional cell is illuminated, electron-hole pairs are formed within the cell, and they are collected at these doped regions and transferred into metal conductors.
Cell-to-cell interconnects
To create a module, cells have to be interconnected. From a reliability perspective, these interconnects are crucial, since failure to maintain electrical contact between cells results in total failure of the module to perform, and in the worst-case scenario could potentially result in an arc-fault failure.
Conventional Module manufacturers typically rely on tin-coated copper ribbons, which are soldered along the length of the cell to printed grid lines (Figure 7). Soldering metal and crystalline materials together is considered state of the art and still leads to reliability challenges from manufacturing induced micro-cracks and stress from differences in thermal expansion [14]. The cells are connected by daisy chaining ribbons that alternate from the front of one cell to the back of the next. As modules heat and cool, the gaps between cells expand and contract, kneading these ribbons back and forth [15].
A recent NREL study [16] has shown that as a result of thermal expansion, they are much more likely to fail within 25 years if not properly strain relieved (in the tabbing ribbon where it traverses between cells, Figure 7).
In contrast, the SunPower cell interconnection is an engineered tab (Figure 8). Instead of bonding ribbons along the entire length of the cell, a stamped metal interconnect is soldered to the edges of the cell.
Secondly, they have cut-outs which allow expansion and contraction as the cells grow and shrink with temperature, providing strain relief.
Third, there are three solder pads on each side of the inter-connect, providing redundancy. In the case a solder joint ever fails, current is rerouted through the remaining pads onto the cell surface, which also has parallel bus-bars to distribute current as necessary.
Finally, when there is a (hot cell) due to shading or local soiling, the solder joint does not get as hot because the thick copper interconnect efficiently draws heat away from the hot cell [18], keeping the solder pads cooler.
The design does not look very different to the casual observer, both have cells encapsulated in a polymer encapsulant that is bonded both to the front side glass and a polymer backsheet. However, the materials and their quality can vary widely and their specific properties can have important impacts on performance.
Materials and suppliers for other laminate components, such as glass, encapsulant, and backsheet, vary between manufacturers, and their specific properties can have important ramifications for long-term reliability. It is beyond the scope of this white paper to do exhaustive comparisons, but SunPowers materials qualification processes have identified a wide variation in quality for these materials.
SunPower has produced high efficiency cells for decades. The original cell design was intended for use in concentrating applications; however, in the mid-2000s, non-concentrated flat plate modules came into widespread production.
The generations of these SunPower modules can be put into three categories:
- Previous generation: 2005-2011. These modules required positive grounding. One version:
- Gen 2 Maxeon cells. Module efficiencies up to 18%.
- Current generation: 2011 onward. No positive grounding required. Two versions:
- E series: Gen 2 Maxeon cells. Module efficiencies up to 20%.
- X series: Gen 3 Maxeon cells. Module efficiencies up to 22% and better shade tolerance.
SunPowers patented back-contact design is substantially different from the designs used by Conventional Module manufacturers.
Part of SunPowers design qualification includes a dynamic load test (DLT). In this test, a force of 2400Pa is repeatedly applied to the front and back of the module, deflecting it back and forth.
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.
A side-by-side comparison of a conventional multi-crystalline silicon module and a SunPower module in this dynamic load test is shown in Figure 13. After 1000 cycles, the standard efficiency module shows several broken cells in the center, and a power loss of nearly 4%. The shunt resistance of this module has dropped by more than 20%, which results in parasitic yield losses at lower irradiance levels [23]. Low shunt resistance can also push cells into reverse bias which leads to more frequent diode activation and yield loss. If the shunt resistance is low enough, or if the diode fails, the cell may form a catastrophic hotspot [24].
2400Pa of stress corresponds to extreme winds (130 mph, 209 kph) or snow loads (about
3m deep, assuming 80 kg/m3 snow density) that are unlikely to be observed in real life at most, but not all, installations. Nonetheless a basic tenet of design qualification testing is that larger safety factors are generally better, since real-world stresses can come from unexpected events. For example, stresses occur during shipping and installation. An installer weighing 80 kg (175 lbs) stepping on a module with a boot that has a contact area of roughly 3inch x 10inch (0.019 m2) induces local normal stress on the surface of the glass of about 41,000 Pa. Fortunately, the glass spreads this stress over a larger area (it bows relatively smoothly), reducing the strain on the cells; but, it is not as forgiving as a uniform pressure applied over the entire surface.
Results for Partial Shading and Reverse Bias Stress
Solar cells in a module are essentially current sources connected in series. When their current flow is not 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.
In agricultural areas, airborne dust settles on modules and sticks due to the morning dew; if the dew and dust preferentially collects at one end or corner of the module, the partial shading can also cause reverse bias.
Finally, cell manufacturing defects can also push cells into permanent reverse bias.
SunPowers back contact design performs differently than a conventional cell, due to fundamental design differences. In the conventional cell, heavily doped layers (regions rich with charge carriers) are separated by bulk silicon, which is lightly doped, creating space between heavily p-doped and n-doped areas on the front and back (see Figure 6, left).
SunPowers back contact design has steep doping profiles on the backside of the cell, which can be seen where the p-doped and n-doped areas are immediately adjacent (Figure 6, right). These regions are rich in charge carriers, so when a cell is in reverse bias, current flows more easily, resulting in a lower reverse-bias voltage.
SunPowers current generation module has a rate of only 27 returns per million modules built. This includes all post-site-commissioning world-wide warranty returns of E-Series modules (Jan 2006 through July-2012, 6.5 million modules).
Various field studies have measured the degradation rate of conventional crystalline modules at between 0.6% per year to 1.5% per year, so a reasonable assessment is 1.0% per year [8], [9], [10], [11], [12] (Figure 4). These studies are discussed further in Appendix A.
In order to perform a more robust assessment, SunPower recently completed its own fleet-wide system level degradation study of 445 systems within the SunPower operating fleet. The study included 266 systems (86 MW) using the previous generation of SunPower modules as old as 3.5 years, and 179 systems (42 MW), using Conventional Modules as old as 6 years. Data spanning back to the site commissioning date were used to determine fleet-wide degradation rates, representing 3.2 million module-years of monitored data. The study [13], and a review by independent engineering firm Black and
Veatch, are available upon request.
A key result from this study is shown graphically below in Figure 5. The annual system power degradation rate (including inverter) for SunPower systems with the previous generation of modules was found to be -0.32 +0.32 % (95% confidence) per year, while non-SunPower conventional systems were found to degrade at -1.25 +0.25% (95% confidence) per year, and in both cases were shown to be linear with time.
SunPower design differences
Cell architecture and metallization
Conventional cells are made of various grades of monocrystalline or multicrystalline p-type silicon.
The front-surface is also an n-type emitter, typically doped with phosphorus; the back is typically a p-type emitter doped with boron. When the conventional cell is illuminated, electron-hole pairs are formed within the cell, and they are collected at these doped regions and transferred into metal conductors.
Cell-to-cell interconnects
To create a module, cells have to be interconnected. From a reliability perspective, these interconnects are crucial, since failure to maintain electrical contact between cells results in total failure of the module to perform, and in the worst-case scenario could potentially result in an arc-fault failure.
Conventional Module manufacturers typically rely on tin-coated copper ribbons, which are soldered along the length of the cell to printed grid lines (Figure 7). Soldering metal and crystalline materials together is considered state of the art and still leads to reliability challenges from manufacturing induced micro-cracks and stress from differences in thermal expansion [14]. The cells are connected by daisy chaining ribbons that alternate from the front of one cell to the back of the next. As modules heat and cool, the gaps between cells expand and contract, kneading these ribbons back and forth [15].
A recent NREL study [16] has shown that as a result of thermal expansion, they are much more likely to fail within 25 years if not properly strain relieved (in the tabbing ribbon where it traverses between cells, Figure 7).
In contrast, the SunPower cell interconnection is an engineered tab (Figure 8). Instead of bonding ribbons along the entire length of the cell, a stamped metal interconnect is soldered to the edges of the cell.
Secondly, they have cut-outs which allow expansion and contraction as the cells grow and shrink with temperature, providing strain relief.
Third, there are three solder pads on each side of the inter-connect, providing redundancy. In the case a solder joint ever fails, current is rerouted through the remaining pads onto the cell surface, which also has parallel bus-bars to distribute current as necessary.
Finally, when there is a (hot cell) due to shading or local soiling, the solder joint does not get as hot because the thick copper interconnect efficiently draws heat away from the hot cell [18], keeping the solder pads cooler.
The design does not look very different to the casual observer, both have cells encapsulated in a polymer encapsulant that is bonded both to the front side glass and a polymer backsheet. However, the materials and their quality can vary widely and their specific properties can have important impacts on performance.
Materials and suppliers for other laminate components, such as glass, encapsulant, and backsheet, vary between manufacturers, and their specific properties can have important ramifications for long-term reliability. It is beyond the scope of this white paper to do exhaustive comparisons, but SunPowers materials qualification processes have identified a wide variation in quality for these materials.
SunPower has produced high efficiency cells for decades. The original cell design was intended for use in concentrating applications; however, in the mid-2000s, non-concentrated flat plate modules came into widespread production.
The generations of these SunPower modules can be put into three categories:
- Previous generation: 2005-2011. These modules required positive grounding. One version:
- Gen 2 Maxeon cells. Module efficiencies up to 18%.
- Current generation: 2011 onward. No positive grounding required. Two versions:
- E series: Gen 2 Maxeon cells. Module efficiencies up to 20%.
- X series: Gen 3 Maxeon cells. Module efficiencies up to 22% and better shade tolerance.
SunPowers patented back-contact design is substantially different from the designs used by Conventional Module manufacturers.
Part of SunPowers design qualification includes a dynamic load test (DLT). In this test, a force of 2400Pa is repeatedly applied to the front and back of the module, deflecting it back and forth.
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.
A side-by-side comparison of a conventional multi-crystalline silicon module and a SunPower module in this dynamic load test is shown in Figure 13. After 1000 cycles, the standard efficiency module shows several broken cells in the center, and a power loss of nearly 4%. The shunt resistance of this module has dropped by more than 20%, which results in parasitic yield losses at lower irradiance levels [23]. Low shunt resistance can also push cells into reverse bias which leads to more frequent diode activation and yield loss. If the shunt resistance is low enough, or if the diode fails, the cell may form a catastrophic hotspot [24].
2400Pa of stress corresponds to extreme winds (130 mph, 209 kph) or snow loads (about
3m deep, assuming 80 kg/m3 snow density) that are unlikely to be observed in real life at most, but not all, installations. Nonetheless a basic tenet of design qualification testing is that larger safety factors are generally better, since real-world stresses can come from unexpected events. For example, stresses occur during shipping and installation. An installer weighing 80 kg (175 lbs) stepping on a module with a boot that has a contact area of roughly 3inch x 10inch (0.019 m2) induces local normal stress on the surface of the glass of about 41,000 Pa. Fortunately, the glass spreads this stress over a larger area (it bows relatively smoothly), reducing the strain on the cells; but, it is not as forgiving as a uniform pressure applied over the entire surface.
Results for Partial Shading and Reverse Bias Stress
Solar cells in a module are essentially current sources connected in series. When their current flow is not 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.
In agricultural areas, airborne dust settles on modules and sticks due to the morning dew; if the dew and dust preferentially collects at one end or corner of the module, the partial shading can also cause reverse bias.
Finally, cell manufacturing defects can also push cells into permanent reverse bias.
SunPowers back contact design performs differently than a conventional cell, due to fundamental design differences. In the conventional cell, heavily doped layers (regions rich with charge carriers) are separated by bulk silicon, which is lightly doped, creating space between heavily p-doped and n-doped areas on the front and back (see Figure 6, left).
SunPowers back contact design has steep doping profiles on the backside of the cell, which can be seen where the p-doped and n-doped areas are immediately adjacent (Figure 6, right). These regions are rich in charge carriers, so when a cell is in reverse bias, current flows more easily, resulting in a lower reverse-bias voltage.
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