A PAN file is a file about photovoltaic modules and other parts that can be opened by special database software like PVsyst. The name PAN comes from “Panneaux photovoltaiques” which is the French for photovoltaic module. Why French, because the software PVsyst was originated in French in 1992. PV PAN files are not to be confused with”.pan” files of Panorama X software by ProVUE Development, although PAN files and “.pan” files are both database files. However, PAN files we are concerned primarily with photovoltaic module specifications. PVsyst and other such programs can read PAN files of different modules in your computer and do a lot of simulation for your photovoltaic energy system planning.
Like all database systems, the PAN files also have a specified format and terminology. It is important to understand the meaning of each term used in the file if one wants to get the maximum advantage from it. In the paragraphs below we will try to give the reader a good explanation of all terms that need explanation. But first, a little about the PVsyst software.
The PVsyst software is popular and comprehensive software for PV system planners. It contains a number of databases about photovoltaic products as listed below. The software has modules which help in designing PV systems for widely varying purposes. The data bases help you choose and specify system components and simulate their performance to help you finalize a design. The component databases (as listed on the PVsyst site) are:
Regulators for stand-alone,
Regulators for pumping,
a list of Manufacturers and Retailers.
and user defined prices for components.
A PVsyst file will invariably mention the software version with which it was created. As an example, a file marked with “PVSYST V6.79” will be opened by any later version of the PVsyst software, but the software designers do not guarantee a backward compatibility. As a solace, all previous versions of the software are available at their website free to download by any license holder. Further, more than one version of the software can coexist in the same computer. Hence, users should face no difficulty due to this restriction.
Defining a Module in PVsyst
(Please refer https://www.pvsyst.com/help/)
The PV module parameters, properties and their behavior are defined and displayed under six tabs in the software:
Basic Data, ie, model type along with model parameters, and main STC parameters. Also, the tools for display of results.
Additional data including secondary or optional module data and tools for elaboration.
Model parameters additional parameters for "one-diode" model and calculating the model unknowns.
Size and Technology ie, module and cells sizes, other specifications.
Commercial data eg, name of manufacturer, module availability, prices, etc.
Graphs a visualization tool for the PV-model behavior over widely varying operating conditions.
For best utilization, the user is expected to know a few terms used therein. The next few paragraphs will explain some of these terms as relevant to solar cells and modules so that the reader can get the best out of the software.
The PVsyst software represents a module having many cells with a one diode Shockley model which should strictly be used to represent a single cell. However, the software PVsyst uses the one diode model assuming all cells are identical. The benefits of simplicity thus provided far outweigh any inaccuracy.
A two-diode model would define a module with much better accuracy. However, in view of the fact that often the limited accuracy of the data from manufacturers does not really justify the complexity of a two-diode model a one diode model with a diode quality factor “Gamma” can be set between 1 and 2 to cater for a situation in-between the two models.
Equivalent Circuit One-Diode Model
A solar cell can be quite well represented by a semiconductor parallel combination of a diode with a current source and a shunt resistance Rsh together in series with another resistance Rs. The shunt resistance is the usual shunt resistance associated with a practical current source as studied in basic electrical circuits. The diode is connected such that it gets forward biased by the current source. The source current IPH is the one generated by the radiation. It will split into three possible paths:
Through the diode
Through the shunt resistance, and
To the external load via the series resistance.
The current equation can be written as:
Where, IPH is the source current generated by the light photons. Some people refer to it as IL.
I is the current flowing out to the external circuit
The second term on the right is the current shunted through the diode according to the usual semiconductor diode equation. Io is the diode reverse saturation current.
The third term on the right is the current shunted through the parallel resistance Rsh.
V is the voltage across the parallel combination, but not necessarily the output voltage.
Obviously, the current through the shunt resistance and the diode are both loss currents.
Rsh should ideally be very high. A low value of shunt resistance means power losses in solar cells because it shunts out part of the light-generated current. That is due somewhat to the reverse saturation current of the diode junction and largely to partial by-passing of the junction caused by forming of pinholes during manufacture. Rsh also has an exponential behavior with light level.
Another loss factor is the series resistance Rs through which output current must flow out to the external circuit. It is the sum of three effects. One is the bulk resistance of the diode semiconductors. Another is the contact resistance at the metal-silicon contact. The third is the resistance of the current collecting structures at the top and bottom of the cell. The series resistance causes a reduction of the fill factor FF (see later), and if it is very high it may affect the short circuit current.
Now we can start discussing some of the terms used in PAN files.
“Pnom” is the queen parameter in specification of a solar panel or module. The term stands for “nominal power output”. Engineers working with electric power and machinery prefer to use the term “name-plate power” etc. because most heavy electrical machinery is fitted with a plate carrying the name, rated power, and other necessary specification of such equipment. In the solar context, Pnom means the maximum power one could expect from the module “under standard conditions”
Standard Test Conditions (STC)
Standard test conditions (STC) for measuring Pnom of solar devices are given in various standards like IEC 61215, IEC 61646 and UL 1703. In short, they specify a light intensity equal to 1 KW/m2 with spectrum being similar to AM1.5, cell temperature kept at 25º C. Here Am1.5 stands for airmass 1.5, or atmospheric conditions resembling those in summer periods at latitude 35º North at a level 500 ft above sea level.. This airmass specification is necessary because the spectrum of the solar energy changes while traverses through different layers of the atmosphere. Since different technologies of solar cells show different spectral responses, this specification is necessary.
The Unit of Power
The SI unit for power is watt. But you will see that solar modules use the symbols WP or Wp. This suffix P is used to limit your expectations from the device. If a module is specified for Pnom = 1000 WP, you should expect no more than 1000 w output from the panel with normal incidence of solar irradiance conforming to the specified conditions as discussed above. It is obvious from physics principles that slant radiation delivers much less power per unit area than it carries. By the way, that is why people need to use trackers with solar systems to keep their panels always facing the sun normally.
Rshunt used in the PAN files is the same as Rsh discussed above. It is expressed in ohms. A low value would mean poor quality of the module. The value in case of a specific module also varies exponentially with light level.
Rseries has also been discussed above as Rs. It should have a low value.
The symbol Tref is pretty obvious and hopefully, does not require any explanation. It stands for reference temperature for measurement of various parameters as specified by the manufacturer of the device. It is the temperature at which the specifications of the device under discussion are valid. Usually it should be 25º C as in “standard Test Conditions” (STC).
Gref- Reference Irradiance
The symbol Gref stands for reference irradiation This is the level of irradiance at which to expect the name-plate power Pnom from the device. The manufacturer can specify the test conditions but if standard test conditions are used, this should be 1000 W/m² as discussed above.
Module Area, and Cell Area
You may also see terms “Amodule”, and “Acells”. Amodule simply stands for the area of the module. This is roughly the area of the module for layout design purposes. Acells means the area of the cells which stands for the active area. This represents the sensitive area of the module and is less than the module area. For example, in one instance, Amodule is given as 2.00 m2, while Acells is specified as 1.72 m2.
Open Circuit Voltage
Open Circuit Voltage with symbol Voc is an important performance parameter. You are aware that as you draw from sources- sources of any kind in this world including philanthropists dishing out charity- their will to give starts decreasing. This is the reason why your car radio will momentarily go out as you turn the start key. The starter draws a huge current from the battery and the battery voltage dips for that small duration returning back to the normal value after the starter has thrown.
Voc is the voltage offered at the terminals of a source when you are not drawing any current from it. It has the same meaning here as in case of all electrical and electronic devices. This is also called no-load voltage in case of electrical power systems. In the context of solar modules and panels it means the output at the terminals when no external load is connected to the device. Open circuit voltage depends on the number of cells connected in a series string by the manufacturer, with each cell contributing slightly about 0.65 V. Open circuit voltage together with the short circuit current gives vital information about the device power output.
Short Circuit Current Isc
The short circuit current is another parameter which is a very significant performance indicator. It is well known that a source will deliver more current when the load resistance is less. The least load resistance we can apply to a source is zero ohm or a short circuit. A source will give its maximum current into a short circuit. This maximum current is called the short circuit current and abbreviated as Isc is a definitive parameter of the source. Together with the open circuit voltage Voc, the short circuit current Isc is a good indicator of the maximum power available from a solar device.
Just as Voc is dependent on temperature, the short circuit current is also. MuIsc (μIsc) is the gradient of the short circuit current with change of temperature. It is expressed in percent /º C. The one-diode model adopted in the PVsyst software assumes muISC parameter of the order of +0.05 % / °C for silicon technology.
The Concept of Maximum Power Point
Power output is the product of terminal voltage with the terminal current. As discussed earlier, the terminal voltage of any source drops as we draw current from it. In the case of pure resistors, the drop is proportional to the current. The VI curve for a resistor is a straight line of constant negative slope, the slope being proportional to the value of the resistance. In the case of solar cells and modules the voltage drop is very little and the terminal voltage remains quite constant for a good range of current values going close to the short circuit current.
Beyond that value of terminal current, the output voltage drops quite suddenly, creating a “knee” in the V-I curve. The power vs Current plot of a resistor rises to a maximum value at half of Voc. Not so in the case of the solar cells. The power output thus rises steadily until it peaks when the value of the output current is close to the short circuit current Isc. This point, ie values of V and I, is called the Maximum Power Point and abbreviated as MPP. The value of current is called the maximum power point current IMPP and corresponding voltage is known as the maximum power point voltage abbreviated as VMPP. Both VMPP and IMPP are specified at Gref and Tref. The product of VMPP and IMPP is the maximum power Pmax.
Pmax for a given device is a function of the irradiance and the temperature. The value of Pmax under the standard (or specified Gref,Tref)) test conditions is what we call Pnom discussed above.
The variation of the maximum available power with temperature at the specified irradiance is called μPMPP. Again, it is given in percentage per ºC.
Maximum Power available should be close to product of open circuit voltage Voc with short circuit current Isc id the diode were ideal. In that case the knee of the VI characteristic curve would be infinitely sharp. But in practical cells it is not. Pmax is slightly less than the product Voc.Isc. the ratio is called the fill factor and denoted as FF.
Additional Data Parameters (Optional)
Some optional parameters are discussed below.
LID Loss- LID Loss or light induced degradation loss may be optionally specified by the manufacturer. It can have a value of 1 to 3% or more for crystalline silicon modules and arises due to manufacturing processes.
Number of bypass diodes- This figure is useful shading calculations when designing a module layout. Normal practice is 3 diodes for 60 or 72 cell modules and 4 for the 96 cell modules.
BRev Reverse voltage rating. It has a negligible effect on shading calculations and is recommended to be left to default value.
Model parameters (one-diode model) have mostly been discussed in the above paragraphs. Some additional notes:
Rshunt-as discussed earlier is measured at the reference irradiation Gref. If a measured values is not specified PVsyst will propose a suitable value.
Rsh (G=0) the value of Rshunt at zero irradiation.
RshExp Exponent in estimating exponential behavior of Rsh. Best to leave it at the system proposed default value of 5.5.
Rseries, model Series resistance in the one-diode model used by PVsyst. For crystalline modules, the system will propose a value depending on the predefined gamma or low light specs.
Rserie app This is the apparent series resistance, as measured slope around Vco at (GRef, TRef).
Gamma Diode Quality Factor mentioned above.
muGamma Thermal correction factor on Gamma.
IoRef Diode saturation current, involved in the "one-diode" model.
MuVoc and Specified MuVoc- Open circuit voltage Voc is not an absolute value. It depends multiple factors, like irradiation level and cell temperature. MuVoc (μVoc) is the gradient of the cell open circuit voltage with changes in cell temperature. It is related to the cell series resistance. The units are % / ºC. PVsyst will derive the value from the model. However, the value proposed by the model is quite often incompatible with the manufacturer’s measured values. The latter may be specified against the term “Specified MuVoc”, and used in such cases.
muPmpp Gradient of power at max power point as varying with temperature. Normally generated by the model the user may forced to a specified value by varying the muGamma parameter.
“Sizes and Technology" Parameters
NCell seriesis the number of cells in series and is necessary for the determination of the model.
NCells parallis the number of cells in parallel.
Cell Area, Module Length and Module Width have the obvious meaning. Cell area is not a necessary parameter. The last two are used for calculation of module efficiency.
Apparent length and Apparent width apply only to building-integrated photovoltaics (BIPV) and if specified will be used instead of the module length and module width respectively
Max voltage IEC Maximum array open circuit voltage output under worst conditions at low temperature according to IEC standard (usually 1000 V).
Max voltage UL Same, but to US standards (usually 600 V).