Lecture 7 PV Performance UTI 111 Prof Park Essex County College PV performance At high noon on a cloudless day at the equator the power of the sun is about 1 kW m on the Earth s surface to a plane that is perpendicular to the sun s rays Typical solar panels have an average efficiency of 12 with the best commercially available panels at 20 A Performance Calculator for Grid Connected PV Systems DC rating DC to AC derate factor Array type Tilt angle Azimuth angle Electricity cost DC Rating The size of a photovoltaic PV system is its nameplate DC power rating This is determined by adding the PV module power listed on the nameplates of the PV modules in watts and then dividing the sum by 1 000 to convert it to kilowatts kW PV module power ratings are for standard test conditions STC of 1 000 W m2 solar irradiance and 25 C PV module temperature The default PV system size is 4 kW This corresponds to a PV array area of approximately 35 m2 377 ft2 DC to AC Derate Factor The PVWatts calculator multiplies the nameplate DC power rating by an overall DC to AC derate factor to determine the AC power rating at STC The overall DC to AC derate factor accounts for losses from the DC nameplate power rating and is the mathematical product of the derate factors for the components of the PV system The default component derate factors used by the PVWatts calculator and their ranges are listed in the table below Component Derate Factors Component Derate Factors PVWatts Default Range PV module nameplate DC rating 0 95 0 80 1 05 Inverter and transformer 0 92 0 88 0 96 Mismatch 0 98 0 97 0 995 Diodes and connections 0 995 0 99 0 997 DC wiring 0 98 0 97 0 99 AC wiring 0 99 0 98 0 993 Soiling 0 95 0 30 0 995 System availability 0 98 0 00 0 995 Shading 1 00 0 00 1 00 Sun tracking 1 00 0 95 1 00 Age 1 00 0 70 1 00 Overall DC to AC derate factor 0 77 0 96001 0 09999 Component Derate Factors PV module nameplate DC rating This accounts for the accuracy of the manufacturer s nameplate rating Field measurements of PV modules may show that they are different from their nameplate rating or that they experience light induced degradation upon exposure A derate factor of 0 95 indicates that testing yielded power measurements at STC that were 5 less than the manufacturer s nameplate rating Inverter and transformer This reflects the inverter s and transformer s combined efficiency in converting DC power to AC power A list of inverter efficiencies by manufacturer is available from the Consumer Energy Center The inverter efficiencies include transformer related losses when a transformer is used or required by the manufacturer Mismatch The derate factor for PV module mismatch accounts for manufacturing tolerances that yield PV modules with slightly different current voltage characteristics Consequently when connected together electrically they do not operate at their peak efficiencies The default value of 0 98 represents a loss of 2 because of mismatch Component Derate Factors Diodes and connections This derate factor accounts for losses from voltage drops across diodes used to block the reverse flow of current and from resistive losses in electrical connections 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 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 Component Derate Factors Soiling The derate factor for soiling accounts for dirt snow and other foreign matter on the surface of the PV module that prevent solar radiation from reaching the solar cells Dirt accumulation is location and weather dependent There are greater soiling losses up to 25 for some California locations in hightrafffic high pollution areas with infrequent rain For northern locations snow reduces the energy produced and the severity is a function of the amount of snow and how long it remains on the PV modules Snow remains longest when sub freezing temperatures prevail small PV array tilt angles prevent snow from sliding off the PV array is closely integrated into the roof and the roof or another structure in the vicinity facilitates snow drift onto the modules For a roof mounted PV system in Minnesota with a tilt angle of 23 snow reduced the energy production during winter by 70 a nearby roof mounted PV system with a tilt angle of 40 experienced a 40 reduction System availability The derate factor for system availability accounts for times when the system is off because of maintenance or inverter or utility outages The default value of 0 98 represents the system being off 2 of the year Component Derate Factors Shading The derate factor for shading accounts for situations in which PV modules are shaded by nearby buildings objects or other PV modules and arrays For the default value of 1 00 the PVWatts calculator assumes the PV modules are not shaded Tools such as Solar Pathfinder can determine a derate factor for shading by buildings and objects For PV arrays that consist of multiple rows of PV modules and array structures the shading derate factor should account for losses that occur when one row shades an adjacent row The figure below shows the shading derate factor as a function of the type of PV array fixed or tracking the ground cover ratio GCR defined as the ratio of the PV array area to the total ground area and the tilt angle for fixed PV arrays As shown in the figure spacing the rows further apart smaller GCR corresponds to a larger derate factor smaller shading loss For fixed PV arrays if the tilt angle is decreased the rows may be spaced closer together larger GCR to achieve the same shading derate factor For the same value of shading derate factor land area requirements are greatest for two axis tracking as indicated by its relatively low GCR values compared with those for fixed or one axis tracking If you know the GCR value for your PV array the figure may be used to estimate the appropriate shading derate factor Industry practice is to optimize the use of space by configuring the PV system for a GCR that corresponds to a shading derate factor of 0 975 or 2 5 loss Shading derate factor for multiple row PV arrays Component Derate Factors Sun tracking 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 For