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1991-11-17
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INFO for the TECHNICALLY ADVANCED USER of PV COMPARE
Author: April Lorenzen
(401) 423-3939 voice
or (717) 853-3599 BBS
or Further Research
POB 293
Jamestown RI 02835
U.S.A.
NOTE: This program is not intended to tell you how to design a PV system or
to definitely specify which components to use in it. It is a
tool for answering what-if PV system design questions and making comparisons
between various system design options. It is as accurate as the data fed
into it, but the value of the results is largely dependent on the skills of
the person analyzing those results.
TOPIC 1: I-V CURVE DATA FILES ( *.MOD )
PV COMPARE includes a few module I-V curve data files. If you have I-V curve
data, you can create *.MOD files for other modules. Use any "DOS text",
ASCII text, etc. editor or word processor. Here's the format:
Manufacturer's model name/number (any characters)
Price (numeric only )
Array temperature in Celsius (numeric only )
Irradiance in watts per meter squared (numeric only )
Short circuit current in amperes (numeric only )
This is followed by a list of module voltages and corresponding current at
each voltage. Must start with the lowest voltage and progress to the highest.
Voltage and current are side by side on each line with at least one space
separating them, like this:
12.1 3.2267 (numeric only, of course)
12.2 3.221
The list must include an I-V pair at the end where current is zero. You can
use any voltage or current increment between pairs, or the increment can vary
randomly. The list can be as long as 200 voltage and current pairs. If you
need more than this, call me and I will increase the size for you in a
special version.
Before the release of PV COMPARE, I did not get much cooperation from PV
module manufacturers on I-V curve data. The *.MOD files included with PV
COMPARE were derived from the ARCO PV module owners manual printed curves.
I enlarged the printed curves and painstakingly measured to find the
current levels at each whole volt, and/or between two points connected
by a straight line. I then used the straight line data regression
feature in a spreadsheet to fill in the values to every tenth of a volt.
The accuracy of I-V curves found on manufacturer's spec sheets is question-
able. I have heard everything from "we played with the data until the
curve looked good" to "we measured thousands of modules and averaged the
results". Different manufacturers claim +-3% to +-10 for the expected
variance from the spec sheet power.
I hope that all PV module manufacturers will offer accurate I-V
curve data for use with this program in the near future. Alternatively,
curve tests can be done independently by universities, laboratories, or
other interested parties. I'm sure the curve data could be derived from
knowing short circuit current, open circuit voltage, and the maximum
power point current and voltage. I'll add this option to the program
when I have the formula to do it.
To simulate a module at a particular irradiance or temperature, you must create
a *.MOD file containing the I-V curve data taken at that specific irradiance
and temperature. The relationship of irradiance-to-current and
temperature-to-voltage is said by many to be essentially linear. I have
elected not to include a program feature to compensate I-V curve data for
temperature and irradiance because I believe that the above mentioned
relationships are not "linear enough" (definitely not in amorphous modules)
and I believe that accuracy of the simulation would be significantly affected.
Name your I-V curve data files with a MOD extension.
TOPIC 2: WIRE DATA FILES ( *.WIR )
The DEFAULT.WIR file included with PV COMPARE contains per foot prices for
marine grade stranded wire from a discount mail order distributor: Jamestown
Distributors 1-800-423-0030. This is not necessarily the most appropriate
wire for your particular application. You may want to edit the prices shown
to match those from your supplier or you can create your own *.WIR data
files in the following format:
Conductor Material (such as copper, aluminum, etc)
Construction of cable (insulation type or designator)
Weight of conductor per circular mil foot in pounds
Ohms per circular mil foot at 0 degrees Celsius
Temperature co-efficient of conductor material
Temperature of conductor in degrees Celsius
Gauge Name, Circular Mils, Ampacity, Cost per foot
(Repeat until all gauges are listed)
You can list up to 30 wire gauges - you must begin with the largest physical
wire gauge and proceed to the smallest. Use the DEFAULT.WIR file as an
example or template.
Name your wire data files with a WIR extension.
TOPIC 3: IMPORTING DATA TO A SPREADSHEET
The F7 key allows you to dump the system description and results for all wire
gauges to a *.TXT disk file. You can import this into a spreadsheet because
it contains commas as delimiters. Use the IMPORT feature of your spreadsheet,
do not try to read the *.TXT file in as a worksheet file. This is handy for
graphing and data manipulation. You can elect to print to paper instead of
disk file, but I have set up the printer output not to exceed 80 columns -
thus some decimal places are truncated.
TOPIC 4: CHANGING THE DEFAULT FILES
You can set any files you want as the DEFAULT.USR (system description),
DEFAULT.MOD (I-V curve data), and DEFAULT.WIR (wire data). Just rename
your preferred files with the default file names, or better yet, COPY your
files to the default file names. However, please only distribute PV COMPARE
to others with the original DEFAULT.* files.
TOPIC 5: USING SINGLE CELL I-V CURVES TO SIMULATE MODULE PERFORMANCE
Cell interconnect resistance and other important factors are not taken into
consideration by PV COMPARE.
END