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Solar
electric system design is not rocket science. However,
to make sure that a system will provide the years of
potential service of which it is capable, there are
a few basic factors which must be considered. Southwest
PV Systems offers complete engineering services for
your solar electric application, no matter the scale,
and will insure that you will be getting a system that
will meet your expectations. If you would like to calculate
your system's requirements, please see the following
explanation and chart which will allow you to determine
those requirements precisely. If you would rather contact
us directly, the minimum information you will need prior
to contacting us will be:
- The
power consumption of your load, in Watts or Amps.
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The intended geographical location of your site.
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The specific Voltage or Voltages of your load.
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The duration of time for which the loads will be activated.
- Doing
a Load Analysis
- Load
Chart
- Worldwide
solar
insolation maps
 Acrobat
Reader is required to view worldwide insolation
maps. Download it for free by clicking on the
Acrobat Reader icon. Once the main map image
displays, click on the region of interest inside
the red block for a more detailed display. |
Regardless
of your approach, we strongly recommend you read through
the following information to orient you with the basic
and simple nature of solar electric system design.
1.
Determining the total load current and operating
time requirements in ampere-hours.
Devide
the wattage of the appliance by its operating voltage,
i.e, 12V, 24V. Multiply this number by the total amount
of hours per day the appliance will be used.
EXAMPLE:
In a PV system with three electrical devices :
- Device
A: 60 watts /12 volts = 5 amps for 24 hrs = 120 amp-hrs.
- Device
B: 6 watts/12 volts = 5 amps for 24 hrs = 12 amp-hrs.
- Device
C: 12 watts/12 volts = 1.0 amp for 8 hrs = 8 amp-hrs.
Total:
140 amp-hrs
For
AC devices the DC AMP-HR consumption is determined by
dividing the AC energy consumption (Wh), by the inverter
efficiency (typically .85 for worst case scenario) to
give the DC energy consumption. This is then divided
by the nominal system voltage to give the DC AMP-HRS
consumed.
EXAMPLE:
An AC television drawing 175 watts operated for 6 hrs
per day.
- 175watts
x 6 hrs = 1050 WH.
-
DC consumption = 1050 WH / .85 (inverter efficiency)
= 1235 WH.
-
DC current consumption = 1235 WH / 12V (DC system
voltage) = 103 amp-hrs/day.
2.
Taking care of system losses and safety factors
For
solar electric systems of 1000 watts or less, a factor
of 20% should be added to the loading to account for
system losses and to include a reasonable safety factor.
Therefore, the amp-hr loading determined in STEP 1 is
multiplied by 1.20 to include the system losses and
safety factor.
EXAMPLE:
TOTAL LOADING + LOSSES = 140 amp-hrs x 1.20 = 168 amp-hrs.
3.
Determining the worst case (wintertime) equivalent sun
hours (ESH)
The
maps
provide the worst case (usually wintertime) ESH based
on the optimum tilt angle for the solar array.
Locate your system site on the map
and determine the ESH by extrapolating (if necessary)
between nearest equivalent sun hour lines Using
the worst case scenario sun hours, ensures the system
will operate even under the worst conditions.
EXAMPLE: Systems site is New York City.
From the map of North America, the ESH = 2.5 hours.
4.
Determining total solar array current requirements
The
total solar array current required is determined by
dividing the total loading plus losses and safety factor
(calculated in STEP 2) by the equivalent sun hours (ESH)
as determined in STEP 3.
EXAMPLE:
- Loading
+ safety, etc. = 168 amp-hrs.
- ESH
for New York City = 2.5 hours.
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Total Array Current Required = 168/2.5 = 67 amps.
5.
Determining Optimum Module Arrangement for Solar Array
BP
Solar manufactures an entire series of solar electric
modules for PV energy systems. The optimum arrangement
is one that will provide the total array current requirements
with the minimum number of modules.
Determine the optimum module configuration by finding
the minimum number of modules that will provide the
required array current value as determined in STEP 4.
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The number of modules in series is determined by dividing
the nominal system voltage by the nominal module voltage.
- The
total number of modules is the product of the number
required in series and the number required in parallel.
EXAMPLE: For a total array current requirements
of 67 amps, an array consisting of 20 MSX 60 modules
in parallel will provide the load requirement for this
nominal 12 volt system application, i.e. 67/3.5 = 20
MSX 60 modules.
6.
Determining Battery Size Recommended Reserve Time.
The
majority of PV solar electric systems include storage
batteries to provide load operation at night or in combination
with the PV modules during periods of limited sunlight.
The recommended reserve time capacities vary with the
latitude of the installation site and are as follows:
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Latitude
of Installation Site
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Recommended
Reserve Time *
|
|
0-30 degree
(N or S)
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5 to 7days
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30-50 degree
(N or S)
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7 to 10days
|
|
50-60 degree
(Nor S)
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10 to 15days
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*Based
on 80% depth of discharge
The
Ampere Hour capacity of the battery bank is calculated
by multiplying the total load plus safety factor (as
determined in STEP 2) by the number of days of recommended
reserve time.
EXAMPLE:
-
In STEP 2, the Total Load + Safety, etc. = 168 Amp
hrs/day. The installation Site (N.Y.C.) latitude
= 40 degrees. Recommended Reserve time for the latitude
= 5days
- Storage
capacity = 168 x 6 = 1008AH
- Batteries
should not be discharge more than 80% of their total
capacity. Therefore, minimum required battery capacity
is, 1008 / 0.8 = 1260 A
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