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Photovoltaics or solar electricity |
Continuing our example, at a battery charge voltage of 13.5 volts the SQ-75 will generate about 4.8 amps and produce 65 watts (13.5 volts x 4.8 amps) to charge the battery. Always use the module's current rating when sizing an array and not the rated peak power.
Some PV modules that look like good buys when comparing rated power turn out to be not so great when comparing battery charging power. When you are shopping for modules to charge batteries, compare the amp rating and not the rated watts! Modern Pulse Width Modulation (PWM) controllers convert excessive voltage into current - see the controller section for more details on this.
| "The amount of solar energy that hits the surface of the Earth
every minute is greater than the total amount of energy that the worldºs human
population consumes in a year."
- US National Renewable Energy Laboratory |
Choosing a Module
Choosing the type of photovoltaic module for your application is an important
decision. Each cell in a PV module produces approximately 0.4 to 0.5 volts at
rated power. This means a module with 33 cells will produce approximately 16
volts at rated power.
PV modules can be divided into three categories depending on the number of cells or the peak power voltage (Vpp). Low voltage modules, sometimes referred to as self regulating produce about 14.5 volts. Self regulating modules do not have a built in regulator. Medium voltage modules produce about 16 volts and high voltage modules produce about 17.5 volts or more. Each type of module is suited to different applications.
Low Voltage
Low voltage modules are ideal for year-round use in Canada when the
greatest need is during cold, clear winter months (due to lack of sunshine or
higher winter loads). These modules are also suited for use in small systems
such as cabins or recreational vehicles because sometimes they can be used
without a regulator. Low voltage modules may not adequately charge a battery in
hot climates.
Medium Voltage
Medium voltage modules are also ideal for year-round use in the Canadian
climate. During the summer, when cell temperature rises, the voltage is
sufficiently high to fully charge batteries. These modules are a good choice
for larger systems that necessitate long wire runs that cause voltage losses in
the transmission wire.
High Voltage
High voltage modules are suitable for hot climate battery charging or for
direct connection to motor loads such as water pumps. High voltage modules are
also good for long distance transmission when used with a linear current
booster. See the section on water pumping for more information about linear
current boosters.
Controllers
Most PV systems require a charge controller or regulator. A controller is
essential if a system is to remain unused for long periods of time.
A charge controller will protect your battery from over charging, which leads to water loss and decreased battery life. A charge controller is an inexpensive device for preventing accidental damage to your expensive batteries.
Modules may not require a controller when the rated peak charge current (amps) of the PV module is less than 1.5% of the battery capacity in amp hours. For example, a module rated at 1.5 amps will not overcharge a 100 amp hour battery at normal operating temperatures.
Low voltage modules may be used without a regulator under certain circumstances. In hot climates the maximum voltage will drop due to the higher temperatures. This is what is meant by self regulating.
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| Design & Resources |
| System Design Tools |
- Spreadsheets
- Voltage Drop Calculators
- Atmospheric Carbon Calculator
- Water Heating Calculator
- What is a Watt?
- Conversion Tools
- Reports
