Transcript Template for e7 Presentations

7. PV System Sizing
Herb Wade
Consultant
Solar PV Design Implementation O& M
March 31- April 11, 2008
Marshall Islands
e8 / PPA Solar PV Design Implementation O&M
Marshall Islands March 31-April 11, 2008
7. PV System Sizing
• Contents
7-1. System Sizing
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e8 / PPA Solar PV Design Implementation O&M
7-1. System Sizing
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Marshall Islands March 31-April 11, 2008
Marshall Islands March 31-April 11, 2008
e8 / PPA Solar PV Design Implementation O&M
Sizing Process
• Determine the load to be served in Wh/day
• Determine the available solar energy on at least a month
by month basis
• Determine the types of equipment that will be used in the
system so losses can be estimated
• Calculate the size of panel that will be needed to meet
the required load under the worst month conditions.
• Calculate the size and type of battery that will be needed
to provide needed reliability of power
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What is System Sizing
• System sizing is the process used for determining the
minimum panel and battery size needed to deliver the
required electrical energy under the solar conditions
that exist at the system site.
• It balances the output from the system with the solar
input while taking into consideration losses in the
system
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We need to know:
• The solar energy in kWh/m2/day at the site for the
lowest solar energy month of the year.
• The average Wh/day required by the user to operate
the desired appliances and any special needs for
power that go much beyond the average.
• The losses that occur in the PV system that reduces
the energy available to the user
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Estimating the load
•
•
•
•
Determine the Watts required by each of the
appliances
Estimate the hours per day that each appliance will
be used.
For each appliance multiply the Watts times hours to
get Wh/day
Total the Wh/day for all appliances
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Problem
• 4 lights of 11 watts each are installed.
– 1 light will operate 4 hours per day
– 3 lights will operate 2 hours per day
• 1 night light of 1 watt is installed
– Nightlight operates 10 hours/day
• 1 Radio of 10 watts is installed
– Radio operates 9 hours per day
How many Wh/day will be needed by the appliances?
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The Solar Resource
• Actual measurements at the site are best but at least
one full year is needed and several years is preferred.
Measurements taken with instruments tilted at the same
angle as the solar panels are best but horizontal
“meteorological” measurements are ok.
• NASA satellite measurements are better than “sunshine
hours” recorded for the site
• “Sunshine hour” measurements indicate the solar
variation over the year but are not a good measure of
actual solar energy in kWh/m2/day but are better than
nothing.
• Choose the average value of solar for the lowest month
as the design basis
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Wh/day that needs to come from the
panel for systems with batteries
• Wiring and connection losses about 10%
• Losses in the battery about 20%
• Total losses around 30% so the panel will need to
produce enough Wh/day for the load plus enough to
cover the losses. So it will have to produce about 130%
of the energy required by the load
• To calculate the Wh/d needed from the panel, multiply
the load Wh/d times 1.3
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Calculating the panel generation
factor (1)
• The lowest month kWh/m2/day value is the
starting point. (Typically between about 5 and 6
kWh/m2/day)
• This is the same total energy as would come
from the sun shining at 1000 W/m2 each day
for the number of hours equal to the
kWh/m2/day figure.
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Calculating the panel generation
factor (2)
• Since the Wp of the panel is rated using a value of
1000 W/m2, the number of hours at 1000 W/m2 that we
calculated can be directly applied to the Wp of the
panel to get the Wh/day the panel would provide under
perfect conditions.
• Suppose the lowest month solar has a daily average of
5.2 kWh/m2/day. That is equivalent to 5.2 hours of 1000
W/m2 sunlight every day. Each Wp of the panel would
therefore deliver 5.2 Wh/day if all other conditions were
perfect. The conditions are not perfect so we have to
correct for the variations from standard conditions.
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Calculating the panel generation
factor (3)
• Corrections include:
– 15% for temperature above 25 C
– 5% for losses due to sunlight not striking the panel
straight on (caused by glass having increasing
reflectance at lower angles of incidence)
– 10% for losses due to not receiving energy at the
maximum power point (not present if there is a
MPPT controller)
– 5% allowance for dirt
– 10% allowance for the panel being below
specification and for ageing
• Total power = .85 X .95 X .90 X .95X .90 = .62 of the
original Wp rating.
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Calculating the panel generation
factor (4)
• To get the panel generation factor (Wh/day per
Wp capacity) multiply the daily sun hours times
0.62.
• For the example, that would be 5.2x0.62 = 3.22
Wh/Wp/day.
• That is, for every Wp capacity in the panel we
can expect to get an average of 3.22 Wh/day
during the lowest solar month
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Calculating the panel size needed
• Divide the Wh/day needed from the panel (1.3
times the load Wh/day) by the Generation
Factor in Wh/Wp/day. The result is the
minimum Wp of panel needed to meet the
design load for the lowest solar month after all
losses and corrections have been applied.
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Calculating the battery size (1)
• The load electricity is provided by the battery. So
determining the Ah/day needed by the load will
determine the battery capacity that has to be available
each day to operate the appliances.
• For a 12V system, Ah/day = Wh/day/12V
• Solar design methods usually choose a 20% daily
depth of discharge (DOD) for deep discharge batteries.
For the modified automotive battery used by AMORE,
longer life will be seen if that percentage is reduced to
15% DOD.
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Calculating the battery size (2)
• The rate of discharge is about 5 hours a day for
lights. That represents about a C20 discharge
rate if 15% of the battery capacity is used in 5
hours (the discharge rate in Amperes being the
capacity of the battery divided by the hours to
discharge).
• So the total battery capacity needs to be the
daily Ah at C20 divided by 0.15 if 15% is to be
the daily depth of discharge.
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Use of automotive batteries
• Automotive batteries are quite sensitive to deep
discharge so the average percentage of daily
discharge should be reduced to 10% to provide
longer life and even then the life probably will
be less than two years.
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Summary of Sizing calculation
1.
2.
3.
4.
5.
6.
7.
Estimate the Wh/day of the load
Multiply the load Wh/day times 1.3
Determine the kWh/m2/day of sunlight for the lowest
solar month
Multiply the kWh/m2/day times .62 to get the
generation factor Wh/d/Wp
Divide the result of (2) by the result of (4) to get
minimum panel Wp.
Divide (1) by the battery voltage (12V) to get Ah/day
Divide (6) by .2 to get the minimum Ah of the battery
at C20.
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Estimating the Wh/day that can be
used for a particular size of panel
• To determine the maximum appliance Wh/day that can
be served by a particular size of panel:
– Multiply the kWh/m2/day times .62 to get the local
generation factor
– Multiply the local generation factor times the Wp of
the panel. This will give the estimated Wh/day from
the panel
– Divide the estimated Wh/day from the panel by 1.3
to get the estimated appliance Wh/day that can be
served by that panel
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Example
• A 36 Wp panel is installed at a site having a low month
solar value of 5.2 kWh/m2/day. What is the maximum
Wh/day of appliance load that this panel can serve?
– Multiply 5.2 x .62 = 3.22
– Multiply 36 x 3.22 = 116
– Divide 116 by 1.3 = 89 Wh/day of appliance use is
possible
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