Solar Electric Modules
Solar Modules Power Characteristics:
The current and power output of photovoltaic modules are approximately
proportional to sunlight intensity. At a given intensity, a module's
output current and operating voltage are determined by the
characteristics of the load. If that load is a battery, the battery's
internal resistance will dictate the module's operating voltage.
A module which is rated at 17 volts will put out less than its
rated power when used in a battery system. This is because the working
voltage will be between 12 and 15 volts. As wattage (power) is the
product of volts times amps, the module output will be reduced. For
example: a 50 watt module working at 13.0 volts will produce 39.0 watts
(13.0 volts x 3.0 amps = 39.0 watts). This is important to remember
when sizing a PV system.
An
I-V curve as illustrated to the right is simply all of a module's
possible operating points, (voltage/current combinations) at a given
cell temperature and light intensity. Increases in cell temperature
increase current slightly, but drastically decrease voltage.
Maximum power is derived at the knee of the curve. Check the
amperage generated by the solar array at your battery's present
operating voltage to better calculate the actual power developed at
your voltages and temperatures.
Shading: PV modules are very sensitive to shading. Unlike a
solar thermal panel which can tolerate some shading, many brands of PV
modules cannot even be shaded by the branch of a leafless tree.
Shading obstructions can be defined as soft or hard sources.
If a tree branch, roof vent, chimney or other item is shading from a
distance, the shadow is diffuse or dispersed. These soft sources
significantly reduce the amount of light reaching the cell(s) of a
module. Hard sources are defined as those that stop light from reaching
the cell(s), such as a blanket, tree branch, bird dropping, or the
like, sitting directly on top of the glass. If even one full cell is
hard shaded the voltage of that module will drop to half of its
unshaded value in order to protect itself. If enough cells are hard
shaded, the module will not convert any energy and will, in fact,
become a tiny drain of energy on the entire system.
Partial-shading even one cell of a 36-cell module, such as the
KC120, will reduce its power output. Because all cells are connected in
a series string, the weakest cell will bring the
others down to its reduced power level. Therefore, whether ½ of one
cell is shaded, or ½ a row of cells is shaded as shown above, the power
decrease will be the same and proportional to the percentage of area
shaded, in this case 50%.
When a full cell is shaded, it can act as a consumer of
energy produced by the remainder of the cells, and trigger
the module to protect itself .The module will route the
power around that series string. If even one full cell in a
series string is shaded, as seen on the right, it will likely
cause the module to reduce its power level to ½ of its full
available value. If a row of cells at the bottom of a module
is fully shaded the power output may
drop to zero. The best way to avoid a drop in output power
is to avoid shading whenever possible.
Tilt Angle: To capture the maximum amount of solar radiation
over a year, the solar array should be tilted at an angle approximately
equal to a site's latitude, and facing within 15º of due south. To
optimize winter performance, the solar array can be tilted 15º more
than the latitude angle, and to optimize summer performance, 15º less
than the latitude angle. At any given instant, the array will output
maximum available power when pointed directly at the sun.
To compare the energy output of your array to the optimum value, you
will need to know the site's latitude, and the actual tilt angle of
your array-which may be the slope of your roof if your array is
flush-mounted. If your solar array tilt is within 15º of the latitude
angle, you can expect a reduction of 5% or less in your system's annual
energy production. If your solar array tilt is greater than 15º off the
latitude angle, the reduction in your system's annual energy production
may fall by as much as 15% from its peak available value. During winter
months at higher latitudes, the reduction will be greater.
Azimuth Angle and Magnetic Declination: If a south-facing
roof is unavailable, or the total solar array is larger than the area
of a south-facing roof section, an east or west-facing surface is the
next best option. Be aware that solar power output decreases
proportionally with a horizontal angle, or "azimuth," greater than 15º
from due south. The decrease in annual power output from a
latitude-tilted east or west-facing array may be as much as 15% or more
in the lower latitudes or as much as 25% or more in the higher
latitudes of the United States. Avoid directing your tilted solar
panels northwest, north or northeast, as you'll get little power
output.
Magnetic declination, the angle difference between magnetic
south and true solar south, must also be taken into account when
determining proper solar array orientation. If a magnetic compass alone
is used to determine where to point the array, you may not capture the
maximum amount of solar radiation. For a general view of the magnetic
declination field lines in North America, see the map below.
If you wish to gain in-depth information about magnetic declination, visit the following website: http://www.ngdc.noaa.gov/seg/geomag/declination.shtml .
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Article Links
Shading
Tilt Angle
Azimuth Angle
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