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The US provider of solar design and performance software Aurora Solar has published a guide to the main causes of energy loss in photovoltaic systems and how to avoid them.
When investing in solar energy, maximizing production is a common goal. Aurora Solar offers guidelines to get the most out of a solar installation avoiding losses.
KWh Analytics, a renewable energy risk management and weather insurance company, released its 2022 Solar Generation Index and reported that solar assets are generally performing below expectations. Systems installed since 2015 have performed between 7% and 15% less than expected, with some regional differences. How can this poor performance be avoided?
Aurora Solars definitive guide to photovoltaic system losses covers solar performance basics, such as the effect of tilt, orientation, and shading on production metrics. The guide explains how mismatched equipment can cause losses and discusses the effects of incidence angle modifiers, nominal module losses, etc.
Tilt and Orientation
The angle of the panels affects the amount of solar radiation that the system receives over the course of a year. Tilting the facility toward the equator will maximize incident irradiance and increase production, the Aurora report notes.
Making the most of the solar incidence angle is also important for production. The angle of incidence refers to the angle of the panel surface with respect to the suns rays. Angles of incidence affect the amount of sunlight that passes through the glass on the front of the panel.
According to Aurora, these losses, measured as the incidence angle modifier, typically range from 3% to 4.5%. The DeSoto model is used to understand the effects of the angle of incidence modifier.
Clogging
Dirt, or the accumulation of dust and other debris on the surface of the panel, is a major cause of power loss in some regions. In areas with long dry seasons, it can cause losses of 5%. In regions with frequent dust deposits, it can add 1-2% to that figure, and locations near high-traffic areas typically see another 1% loss. In regions where it rains all year round, dirt losses are usually around 2%, according to Aurora.
Performance parameters from the National Renewable Energy Laboratory (NREL) suggest that a 5% fouling loss is typical in the US.
Based on an NREL model, an annual cleaning on a system with a 1.9% dirt loss would reduce the loss to about 1.5%. Two cleanings per year could reduce the average loss to 1.3%, and three cleanings per year would further reduce it to an average annual loss of 1.2%. A local NREL analysis of the effects of dirt can be found here.
Birds and their droppings are another production problem. Bird droppings substantially block one or two cells and may not be washed away by rain. On modules without bypass diodes, total blocking of one or two cells can cause the entire module to stop working. Aurora advises a quick manual cleanup of bird droppings.
Snow loads are another mitigating factor. An NREL study calculated losses ranging from 10 to 30% for fixed-tilt systems. Snow factors can be difficult to accurately model on an annualized basis, so Aurora recommends measuring on a monthly format. Here you can consult a local study on the estimated snow losses based on the inclination of the panels.
Shading is another vitally important aspect of system performance. Aurora compares a shaded solar cell to a clogged pipe. When a cell is shaded, the current through the entire string of cells is reduced. The panels incorporate bypass diodes, which allow the assembly to “skip” the shaded cell, but at the cost of giving up output that could have been obtained from that cell. An analysis from Stanford University on the effects of shading can be found here.
Aurora suggests the use of Module Level Power Electronics (MLPE) or microinverters to avoid shading losses.
Environmental losses
The temperature coefficients are another factor to take into account in performance. A temperature coefficient is measured as the percentage decrease in power output for every 1 degree Celsius increase above the 25 degree Celsius benchmark.
Some cover materials absorb more heat than others, which affects their performance. Panel angles can alter the temperature, and according to Aurora, flat-mounted panels tend to get hotter. The type of panel also influences. Thin film panels typically have a lower temperature coefficient than monocrystalline or polycrystalline solar panels.
Modules
Modules in systems with misaligned or long strings can lose between 0.01% and 3% of total production. Aurora uses a 2% assumption in its models for this category of losses. Mismatched modules with tight power tolerances can cause another 1% loss in the system.
Light-induced degradation occurs when the electrical characteristics of crystalline silicon solar cells change when exposed to light. The losses oscillate between 0.5% and 1.5% and occur in the first hours of exposure of the new panel.
Module rated power losses represent the loss due to the difference between the modules declared power and its actual performance under standard test conditions. Aurora suggests that losses in this category do not occur for modern modules, as most accurately reflect standard test results.
However, some vendors may indicate a range of performance, called a "power tolerance." It is usually expressed as a percentage plus or minus. For example, a 250W panel with a power tolerance of +/- 5% can produce between 237.5W and 262.5W.
Cables
Cabling losses typically contribute another 2% to system losses. If the project uses thicker cables in short runs, those losses can approach 1%.
“Various components can cause a voltage drop in circuits, such as connections, fuses and resistors. Differences in cable length or size between parallel strings can also introduce voltage drop,” according to Aurora.
According to an NREL study, connection losses can contribute to another 0.5% of loss. Wiring connectors and bypass diodes have physical imperfections that cause resistance, resulting in small voltage drops.
Inverters
Inverter efficiency measures how effectively DC power is converted to AC power. Inverter manufacturers provide both a maximum efficiency rating for performance under ideal conditions, and a weighted efficiency rating for performance under a range of conditions.
“It is important to take into account the weighted efficiency because the efficiency of an inverter will change depending on the capacity it transports. Most inverters peak around 20% load and drop slightly as the load reaches the maximum input rating,” the Aurora report notes.
Inverter saturation typically occurs in systems on the sunniest days. When the DC output of the panels is greater than the amount of DC power the inverter can convert, a clipping loss occurs. Auroras NEC Validation Report can help properly size inverters.
Miscellaneous
The publicly available PVWatts system performance model uses a default value of 3% loss of system availability. Aurora claims that systems with O&M or fault alert systems configured can experience availability losses of only 0.5%. Availability includes inverter shutdowns or failures, grid outages, and other events that shut down the PV system.
Thermal expansion and contraction, ultraviolet light, and damage from windblown particles will reduce production over time. Production guarantees from solar panel manufacturers provide a conservative estimate of production if the panels degrade over time. |
