Download RETScreen - Photovoltaic Project Model

RETScreen® Photovoltaic Project Model
operating temperature. Most cell types exhibit a decrease in efficiency as their temperature
increases. The PV temperature coefficient, μ (expressed in % / ºC), is defined in the RETScreen
PV model as:
η(Τ) = η(Τref)(1−μ(Τ−Τref))
where
η(Τ) = the efficiency of the solar cell at temperature T,
η(Τref) = the efficiency of the cell at temperature Τref,
T = the operating temperature of the module, and
Τref = the reference temperature (usually 25ºC).
The value of μ depends primarily on the type of cell used. It is often fairly difficult to find this
value on manufacturer datasheets, therefore in the absence of this information the default values
found in the Nominal Efficiencies of PV Modules table may be used.
PV array controller
The user selects from the drop-down list the type of PV array controller that is used to interface
the PV array to the rest of the system: "MPPT" and "Clamped." A Maximum Power Point
Tracker (MPPT) is an electronic device used to maintain the operating voltage of the array at a
value that maximises array output, regardless of changes in load impedance or changes in array
operating conditions due to variations in temperature or insolation. If the user selects "MPPT,"
the efficiency of the array will be optimal. A "Clamped" PV array controller is a direct
connection between the array and the batteries; in this configuration the array operates at the
voltage set by the battery. This may not be the optimal voltage for the array and therefore its
efficiency will be lower.
The MPPT is often combined with other electronic devices such as the charge controller for offgrid systems (on-grid systems include a MPPT as part of the inverter). When this is the case, the
efficiency of the device in question should include the efficiency of the MPPT. In cases where the
MPPT is not combined with another device, its efficiency should be accounted for separately, for
example in the "Miscellaneous power conditioning losses."
Miscellaneous PV array losses
The user enters array losses (%) from miscellaneous sources not taken into account elsewhere in
the model. This includes, for example, losses due to the presence of dirt or snow on the modules.
Typical values range from zero to a few percent. In some exceptional circumstances (e.g. very
harsh environment) this value could be as high as 20%.
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