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Data Sheet No 3  -  Solar Panels

Solar panels are a silent, maintenance free, convenient and cheap way of keeping your glider batteries fully charged.   For practical reasons they are not usually fitted to the glider structure although you occasionally see them taped to turbo engine doors or on the top of the cockpit coaming.   However, a panel on the roof of your trailer can go a long way to keeping your glider batteries fully charged.

On a sunny day the sun radiates 1000 watts of energy into every 1 sq m on Earth.   If we were able to catch, store and use all this energy, it would be sufficient to meet the planet's power requirements many times over.   Unfortunately, solar panels are only 8 to 15% efficient and by the time thay are framed and mounted this has reduced to 6 to 10%.   Solar panels produce electricity by use of the photovoltaic effect in which solar radiation strikes a semi-conductor silicon panel, causing electrons to migrate.   When enough of these panels are coupled together, they produce a useable voltage that can be used to charge a storage battery.   There are two types of silicon photovoltaic panel; the silicon can be cut into thin wafers and mounted on a sub strata, known as crystalline cells;  or the silicon can be vaporised onto a surface to form what are know as amorphous cells.   Crystalline panels are expensive but do generate about 30 to 40% more power per unit area than the cheaper amorphous panels.

Each cell produces approximately 0.5 volts in sunlight and normally 30 cells are linked in a panel to provide an output voltage of 15 volts.   The amperage, or current capacity depends on the size of the panel; bigger is better.  A 5 watt panel will produce about 350 m/a of current when it is orientated normal to full sunlight, while laying the panel flat will reduce the current to about 200 m/a in full sunlight, falling to about 50m/a in cloudy conditions.   In UK, the average sunlight duration is about 3.5 hrs per day.   Thus in a week a flat 5 watt panel can provide sufficient power to fully charge a 5 A/H battery.   This is more than enough to keep two standard glider batteries fully charged.   Using a 5 watt panel, the output can be connected directly to the battery, ensuring that a blocking diode is included in the circuit so that the battery does not discharge back through the panel at night time.   No regulator is necessary.   However, if a larger panel is used, then a solar panel charging regulator is necessary to prevent the battery from being overcharged.   The regulator can either be an expensive commercial device, or for panels up to 10 watts, a simple 15 volt zener diode mounted to a heat sink and connected across the battery will be more than adequate.   

For charging a single glider battery, a 3 watt amorphous panel would be more than adequate and would cost about 30.  Generally, if you add a 'nought' to the wattage, then you have the order of cost of a panel.

Because of their low output when not mounted normal to the sun, a solar panel mounted in a glider cannot be expected to be very efficient and during an average 3 to 4 hour flight, very little power will be generated making their use in gliders not cost effective.    However, when a 3 to 5 watt panel is mounted on the roof of a trailer and coupled to the glider battery when the glider is not flying, a solar panel becomes a practical proposition and unless the glider is flown every day, will keep the battery charged and ready for flight.

August 2004

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