PVWATTS DERATING FACTORS FOR SOLARBRIDGE PANTHEON MICROINVERTERS AND ACPV SYSTEMS

Transcription

1 PVWATTS DERATING FACTORS FOR SOLARBRIDGE PANTHEON MICROINVERTERS AND ACPV SYSTEMS AUTHOR Vincent Bartlett Senior Member of Technical Staff Version 1.5 March 22, 2013 SolarBridge Technologies 1

2 INTRODUCTION The purpose of this white paper is to provide updated derating factors for PVWatts that reflect the improvements gained by using AC modules with integrated SolarBridge Technologies Pantheon microinverters over DC modules with string inverters. This paper will discuss each parameter listed on the DC to AC Derate page of PVWatts and describe how these parameter values are enhanced with an ACPV system powered by SolarBridge. Figure 1 shows the default for the DC to AC derate factors with an overall rating of or 76.9%. The overall DC to AC derate factor is calculated by multiplying the component derate factors. For the PVWatts default values the calculation is as follows: Overall DC to AC derate = 0.95 x 0.92 x 0.98 x x 0.98 x 0.99 x 0.95 x 0.98 x 1.00 x 1.00 x 1.00 = % Figure1: Default DC to AC Derating Factors for String Inverter Systems While these values are suitable for a typical string-inverter system where solar panel mismatch, diode conduction and long DC wiring come into play, with an ACPV system with an integrated Pantheon microinverter these values must be recalculated and updated: The Pantheon microinverter adapts to the solar panel eliminating much of the energy lost by string-inverter systems. The minimal DC wiring in an ACPV system results in almost no DC losses. In ACPV, system availability is not an issue since the Pantheon microinverter is matched and guaranteed for the life of the solar panel, typically 25 or 30 years, depending on the specific AC module you buy. 2

3 PVWATTS DC TO AC DERATE FACTORS The following section discusses each parameter in the PVWatts DC to AC Derate Factor page and how that parameter is affected by the Pantheon microinverter used in an ACPV system. PV Module Nameplate DC Rating: The derate factor for this parameter accounts for the accuracy of the manufacturer s nameplate rating. The default derate factor of 0.95 indicates that standard test conditions (STC) yielded power measurements that were 5% less than the manufacturer s nameplate rating. SolarBridge leaves this parameter set to the default value. Inverter and Transformer: The derate factor for this parameter is the combined efficiency of the inverter and transformer converting DC power to AC power. The SolarBridge P250 microinverter s CEC efficiency is 95.0% or SolarBridge Technologies recommends setting the Inverter and Transformer value at Pantheon II (P250) The SolarBridge P235 microinverter s CEC efficiency is 94.5% or SolarBridge Technologies recommends setting the Inverter and Transformer value at Pantheon (P235) Mismatch: For a string inverter the mismatch derate factor accounts for manufacturing tolerances when stringing together PV modules with slightly different current-voltage characteristics. Consequently, when connected together electrically, they do not operate at their peak efficiencies but are affected by the lowest performing panel in the string. The default value of 0.98 represents a 2% loss due to mismatch. ACPV systems have no mismatch loss. Each solar panel has its own microinverter converting all of the power the solar panel can supply. SolarBridge recommends the highest setting allowed for this parameter, which is

4 Diodes and Connections: In a string inverter the derate factor for diodes and connections accounts for losses from voltage drops across the diodes used to block the reverse flow of current and from resistive losses in panel to panel electrical connections. For Pantheon microinverters factory-assembled onto a solar panel, there are no reverse diode losses, and without interconnection between the panels, there are no connection losses. SolarBridge recommends the highest setting allowed for this parameter, which is DC Wiring: The derate factor for DC wiring accounts for resistive losses in the wiring between modules and the wiring connecting the PV array to the inverter. For AC modules with Pantheon microinverters, total DC wiring is typically less than 2 feet (0.61 meters) from junction box to the microinverter, which results in almost no DC wiring loss. SolarBridge recommends setting the DC Wiring parameter to the highest setting, which is AC Wiring: The derate factor for AC wiring accounts for resistive losses in the wiring between the inverter and the connection to the local utility service. This is a system-level parameter that applies to all inverter technologies. The PVWatts default ratings apply. Soiling: The derate factor for soiling accounts for dirt, snow or other foreign matter on the front surface of the PV module that reduces the amount of solar radiation reaching the solar cells. Dirt accumulation on the PV module surface is location and weather dependent, with greater soiling losses (up to 25% for some California locations) for high-traffic, high-pollution areas with infrequent rain. Since this is a system-level parameter that is specific to the area where the PV array is installed, the PVWatts default of 0.95 applies. Of note, however, is that in string-inverter systems this parameter affects the entire string where a single dirty panel affects overall performance. In ACPV systems, the performance of one soiled panel does not affect the performance of other panels in the system. This parameter will be different for each solar panel in an ACPV system. SolarBridge leaves this parameter set to the default value. System Availability: The derate factor for system availability accounts for times when the system is off due to maintenance or inverter and utility outages. The default value of 0.98 represents a system being off for 2% of the year. SolarBridge feels that 2% down time is far too much for urban environments, but may be correct for some heavy weather or rural locations. However, given the reliability of ACPV systems this factor should be used for power outages only and not for maintenance of ACPV. SolarBridge assumes an urban environment and reminds the user that there is no need for scheduled maintenance and that system-level outages are very rare, which leads to the highest PVWatts value of Even with a value of the AC power grid would have to be down 21.9hrs/year (during sunlight hours), which might be overly pessimistic for some locations. Shading: This parameter accounts for situations when PV modules are shaded by nearby buildings, objects or other PV modules and array structure. For the default value of 1.00, PVWatts assumes the PV modules are not shaded. Tools such as Solar Pathfinder can be used to determine a derate factor for shading by buildings and objects. Since this is a system-level parameter that is specific for the area where the PV array is installed, SolarBridge leaves this parameter set to the default value of 1.0 4

5 Suntracking: The derate factor for sun-tracking accounts for losses for one- and two-axis tracking systems when the tracking mechanisms do not keep the PV arrays at the optimum orientation with respect to the sun s position. For the default value of 1.00, PVWatts assumes that the PV arrays of tracking systems are always positioned at their optimum orientation and performance is not adversely affected. Since this is a system-level parameter that is specific for the area where the PV array is installed, SolarBridge leaves this parameter set to the default value of 1.0. Age: The derate factor for this parameter accounts for losses in performance over time due primarily to weathering of the PV modules. The loss in performance is typically 1% per year. For the default value of 1.00, PVWATTS assumes that the PV system is in its first year of operation. For the 11th year of operation, a derate factor of 0.90 would be appropriate. Since this is a system-level parameter that is specific to the area where the PV array is installed, SolarBridge leaves this parameter set to the default value of 1.0. SOLARBRIDGE ACPV PVWATTS DC TO AC DERATE FACTORS To summarize, the following table provides both the default values for PVWatts derate factors and the values that SolarBridge recommends for ACPV systems using the Pantheon microinverter. Figure2: SolarBridge Recommended DC to AC Derating Factors for ACPV Systems Figure2: SolarBridge Recommended DC to AC Derating Factors for ACPV Systems Even with the limitations imposed by PVWatts, the overall DC to AC derate factor increases from in the case of a string-inverter system to for a SolarBridge ACPV system. The third column above shows the desired derate factors in the absence of the constraints of PVWatts. Applying the derate factors of an ACPV system produces an overall DC to AC derate factor of , which further highlights the advantages of a SolarBridge-powered ACPV system over a string-inverter system. 5