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Configuring a solar system

wellsee / 2011-05-24
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Configuring a solar system


How to build a solar system? We know we need solar charger controller,solar inverter, solar panel and solar battery. How big of them we should choose? Here is a basic design for building an off-grid solar system. The material presented here can be used on a wide variety of solar systems, but is geared to off-grid systems in the 10 Amp range.


The formulas:



Power is equal to Voltage times Current where

P= power in watts,

E=Voltage in Volts,

I= Current in Amps.


The letter E, short for energy potential is used for voltage and the unit for energy potential is Volts abbreviated as V.


Using simple algebra, the formula can be rearranged to find Voltage when the power and current are known.



Or we can find current when the voltage and power are known.



Another formula: E=I X R (Voltage is equal to Current times Resistance )

E=Voltage in Volts

R=Resistance in Ohms.

This can also be rearranged to find current or resistance I=E/R or R=E/I


Sizing a solar system

Sizing a solar system requires specifying three components, the photovoltaic solar panel, the solar controller (charger regulator, charge controller) and battery (or batteries). All of the charge controllers supplied by Wuhan Wellsee New Energy Industry Co., Ltd. can be used in one battery of 12V or two batteries of 24V system. We refer to 12 Volt or 24 Volt systems based on the number of batteries even though the battery voltage ranges from around 11.8 to 14.4 Volts (or twice that for 24 Volt systems)


Some systems will be configured based on an available solar panel or panels, and others on the load current required.


For example, we will size a solar lighting system. There are a number of 12 Volts LED lamps available, and they are specified in terms of power I.E. 3 Watts. Using the formula above, we can determine the current needed for one 6 watt LED lamp.

I =P/E=6/12=0.5 Amps

Multiplying by the number of lamps will give us the total current required. Suppose we wanted 24 of these LED lamps. We would have a load current of 12 Amps. We would need a 15 Amp controller.


Note: When selecting a controller, use care not to exceed around 80% of solar controller capacity. For example, we recommend a 10 Amp solar controller for up to 8 Amps and a 20 Amp solar charge controller for up about 16 or 17 Amps. This is especially important when driving motors which draw more current if there is a mechanical problem such as a pump being clogged with mud or debris.


Solar Panel Size

The most expensive component in most systems is the solar panel. The larger, the more expensive. For picking the size of thesolar panel, and by size we actually mean power output or Watts. All solar panels manufacturers also specify a peak current rating, and that is the maximum current that will be available to charge a battery. The rule of thumb is to oversize the solar panel. Buy the biggest panel possible. There are many factors that affect how much current will be available from a panel, and that includes weather, geographic location, shading from natural objects such as trees and mountains. Two more variables to consider are, the number of days that there will be no sunlight, and how critical the application is . It is obviously more important to have lights on in a hospital parking lot that it is to have a backyard fountain operate after three straight days of clouds.


There are a lot of applications for solar systems, but we can classify three distinctly different types.


Lighting Systems

The solar panel charges the battery during the day.

The load is energized for some part of the night.

General calculations can be made based on the number of lamps and the current draw of the lamps.


Pumping / aeration / fountains

These systems are characterized by the need to energize a load for as long as possible during periods of sunlight.

The charging current available from the solar panel(s) should be greater than the load current.


Intermittent duty

These systems are characterized by having a load or loads that are energized for different amounts of time.

An example would be a marine system that charges a battery used for lighting, radio communication and possibly a bilge pump.

For accurate sizing, the load must be defined both in terms of load currents and time that the loads are energized.



Battery size

To calculate the size of battery, the key specification is Amp-hours. Amp-hours are a rough indication of how much energy a battery can store. A battery’s Amp-hour rating indicates the total amount of energy it will deliver at a constant rate of discharge. For example a 50 Amp-hour battery theoretically could deliver 50 Amps for one hour or 1 Amp for 50 hours. This is not true in practice. At different rates of discharge, a battery’s actual Amp-hour rating will change. In order to standardize, manufactures now specify the Amp-hour rating over a period of 20 hours before it reaches a voltage at which it is discharged. As an example, a 100 Amp-hour battery can deliver 5 Amps for 20 hours. (100 Amp-hour divided by 20 hours=5 Amps.) A 12-volt battery, the most common nominal voltage, is fully discharged at about 11 Volts. A 100 Amp-hour, 12 Volt battery will run a 5 Amp load for 20 hours, and a 200 Amp-hour battery will run a constant 10-amp (120 watt) load for 20 hours. In general, when a 100 Amp-hour battery is discharged at a rate greater than 5 Amps, it will not deliver all the advertised Amp-hours before it goes dead. On the other hand, if you discharge it at a steady rate of less than 5 amps, you’ll get slightly more Amp-hours than the manufacturer’s rating shows. That principle applies to most lead-acid batteries, the faster the discharge rate, the fewer Amp-hours delivered. The slower energy is taken from a battery, the longer it will last.


We can calculate the desired Amp-hour rating for a battery based on the current needed for a specific application. As an example, suppose we were designing a lighting system that drew 8 Amps and we needed the lights on for 6 hours each night. The calculation would be (6 hours/ 20 hours) X 8 Amps = 2.4 Amps which would indicate a 50 Amp-hour battery. A 50 Amp-hour battery would work for most conditions. The battery would charge fully during the day when there was sunlight, then run to depletion in 6 hours each night. All of our controllers will shut off the load(lights) when the battery approaches full discharge in order to preserve battery life. Since we can’t depend on having sun each day, it would be better to oversize the battery, so that there is additional capacity. We would recommend an 85 or 100 Amp-hour battery. This can be scaled for other systems. For a similar system that drew 12 amps, we would recommend a 200 to 240 Amp-hour battery.


Determine time to charge a battery

Start with the Peak Current specification (in Amps) for your selected solar panel. The information is usually given as part of the specifications for the solar panel.


Divide your battery Ah by the Peak Current spec from the previous step. This will give you the ideal maximum charging time for the battery, in hours. Multiply the result by 1.2 to adjust for the average inefficiency of the charging process. The result is the estimated maximum real charging time for the battery. This time represents the number of hours of sunlight needed to fully charge a fully discharged battery.


A note of caution is necessary here; batteries have a limitation as to how fast they can be charged. One tenth of the Ah rating is always safe. Most batteries can be charged at one fifth of the Ah rating (0.2 X Ah). This is a good starting point for a solar system. Using the above formulas, charging at 20% of the Ah rating will fully charge a battery in about 6hours. A good system designs will also not fully discharge the battery during normal operation.


Once a battery reaches about 85% of full charge, our solar controllers will slow down the rate of charge to prolong the life of the battery.


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