Solar Electricity: How does it work?

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Light striking certain substances causes the surface of the material to emit electrons. It is as if light somehow kicks electrons right out of atoms. Light striking other substances causes the material to accept electrons. It is the combination of these two substances that can be made use of to cause electrons to flow through a conductor.

This is the so called photo-electric effect. Photovoltaic means sunlight converted into a flow of electrons (electricity). Photovoltaic devices, or solar cells, are like generators that work in sunlight. They make electricity without waste, noise or pollution. They produce electricity without combustion. A solar cell is a solid state device in which there are no moving parts (except for photons and electrons) so nothing wears out.

The fuel is "photons". These can be thought of as "packets of sunlight" that carry a phenomenal amount of energy to earth at a prodigious rate. The Solar Panels of today make use of this abundant energy by using silicon crystals with small amounts of impurity added. This process of adding minute amounts of different elements into an otherwise pure crystal is called "doping". By having two thin layers of doped material bonded against one another, an electric current can be induced when exposed to light.

Photo_Crystals doped with Boron

Photo_Crystals doped with Phosphorous

Energy Content of Sunlight

Sunlight has an energy content of 1 kW (1,000 watts) per square metre. The typical Solar Panel today achieves between 10% (amorphous solar panels) and 18% (poly/mono-crystalline solar panels) conversion. The theoretical maximum efficiency of a silicon cell is about 21%. Using a more costly technology 31% conversion has been achieved.

What to expect from a Solar Panel

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A single 190 watt solar panel (24V) should produce about 5.5 amps under sunny conditions. Each day of reasonable sunshine you should expect about 26 amp-hours from one such panel (based on solar radiation data for north coast NSW). You need to take into account the number of consecutive days when you may not see much sun, and allow for this by having a large enough solar array and battery bank to tide you over through such periods.

See our range of Solar Panels.

Solar System Design

The following information is provided to give you an idea of what is involved in overall system design in relation to a photovoltaic charging source.

Solar system design should include proper mounting and location of the panels, correct wiring and circuit protection, regulators and fuses to protect the battery, as well as a safe installation procedure. Trained Solar dealers or distributors should be consulted for proper system design.

Solar Power as an Energy Source

Solar Power has become a popular and dependable power source inrural Australia with the development and continuous improvement of Photovoltaic (solar electric power) over the last few decades.

A Solar electric system has a very distinct advantage in that it is relatively quick and easy to install with a minimal requirement for site preparation.

System Design

It is important to pick the best site for your solar modules. In order to get the most power, they need maximum exposure to direct sunlight for the longest possible time.

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Mounting Solar Panels and Frames

If you are not using a solar tracker, solar panels need to face the midday sun at an angle roughly equal to the latitude of your location. The angle that you choose would depend on the time of year that you need the most power. In the southern hemisphere you would of course face your panels to the north but whether you use magnetic north or true north is not critical.

A variation of up to 15° will not make a great deal of difference in the performance of the panels. You should never have stationary solar panels placed at less than 5° from the horizontal so that they don't collect too much dirt etc and they will automatically wash clean whenever it rains.

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Seasonal Adjustments

The seasonal variations in the sun's angle are 23°15' added to latitude at the winter solstice (either 21 or 22 June in the southern hemisphere) and 23°15' subtracted from latitude at the summer solstice (either 21 or 22 December). By referring to solar radiation figures you may get an idea of the advantage in seasonally adjusting (in this case month by month) the solar array for your area by using the figures for a location of similar latitude and similar weather conditions to yours. You simply compare the columns labelled Seasonally Adjusted against the columns labelled Best Average Performance. A simple system where you manually change the angle a few times per year would not involve much cost or effort but also gives you less gain than an automatic solar tracker.

Solar Tracking Devices

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By having your panels following the sun with a solar tracking device (polar axis tracking device) you can gain greater benefits in summer than in winter. This is due to the difference in the arc that the sun sweeps across the sky which is more than 180° in summer and less than 180° in winter.

The degree of variation depends on latitude and weather patterns with the greatest gain coinciding with clear skies and summer.

Is Tracking Cost Effective?

Whether tracking is really worth the expense depends on a number of factors:

1. The cost of the solar tracker and installation (concrete!).

2. Maintenance cost for the tracker (moving parts).

3. The extra energy gained by tracking. It is not enough to say that the panel(s) may put out twice as much power under given circumstances. It is the accumulated amp-hours (amps times hours) over the course of the day that determines your daily gain.

4. The gain is not consistent throughout the year. The greatest gain is usually in summer when the hours between sunrise and sunset are the longest and the sun sweeps its greatest arc across the sky (see diagram). If this potential for an increased gain in summer also coincides with a wet season or consistently overcast weather then the actual gain may be very little or nothing at all.

This is because on mildly overcast days the sunlight is scattered and the best results on such days are often when the solar panels are facing straight up and getting maximum benefit of the diffused light rather than attempting to pick up the direct sunlight.

5. The gain is also dependent on latitude. At increased latitudes the sun's arc across the sky in summer is also increased but in winter it is decreased. The following table shows the daylight hours (between sunrise and sunset)

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6. Solar energy usually has its greatest strength in the middle of the day and often the greatest cloud cover is in the mornings and evenings. The energy from the sun has to penetrate through the greatest depth of atmosphere at the horizon.

7. Your immediate environment and your geographic location may play a major role in the hours of direct sunlight (without shading) that your panels may receive. Nearby mountains, hills, trees, tall buildings etc may considerably reduce the number of hours of direct sunshine that your panels receive.

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Shadow throwing objects tend to have their greatest effect on a solar panel site when the sun is lowest in the sky.

Conclusion

After having determined how much gain you could expect in mid summer and mid winter you may find that you get the most benefit when you least need it (see point 4). ALso, an automatic solar tracking system usually costs more than 1.5kW worth of solar panels. Hence in most cases extra panels would be more efficient than a tracker. At this point RPC only recommends solar trackers if you don't have enough roof space and have to use ground-mount frames anyway.

More information on solar trackers.

The adverse effects of heat

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Contrary to what you may expect, when photovoltaic solar panels become hot, their output is reduced. It is therefore advisable to install panels at a distance from hot tin roofs. This is to allow ventilation around the panels which helps to reduce the temperature. Unlike Solar Collectors (eg to heat water), photovoltaic panels depend on light (mostly visible light) to produce electricity and not on heat. The effect on current due to increased temperature of a solar panel is not as drastic as the effect on voltage (assuming that the voltage is still high enough in order to charge the battery).

Use of Reflectors *

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By having reflectors to increase the amount of light falling onto the panels, you may be able to increase the output of a solar panel. Unfortunately this approach may have the undesirable effect of increasing the temperature of the solar panel. As temperature increases above 25°C the nominal voltage of the panel increases. If the temperature of the panel is increased to 50°C the open circuit voltage (OCV) may be decreased by as much as 2 volts (for a 12 volt panel). If the panel happens to be a self-regulating panel such a voltage drop may have the undesirable effect of the panel ceasing to charge the battery altogether. You must also be careful that these reflectors don't have the reverse effect by shading the panel at any time. This would best be insured by combining both tracker and reflectors (and hoping that the tracker doesn't fail). In this instance seasonal adjustments a few times per year should be considered. This idea should only be contemplated if the potential for a temperature increase could be kept under control.

* Warranty on solar panels is voided if reflectors are used.

Self Regulating Panels

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A self regulating panel has fewer cells so that the voltage produced is always less than with a standard panel. This means that the panel puts less and less power into the battery bank as it is charging and increasing in voltage as a result. This tail-off of charging rate starts at around 50% of battery charge and in the 70% to 100% range, where we recommend you operate, the differences are dramatic. Under overcast conditions the self regulating panel may cease to charge where a standard panel may still be able to produce a reasonable charging rate. The wattage rating of a self-regulating panel in itself may thus be quite misleading. It is recommended that you use standard (not self regulating) panels in conjunction with a regulator in a home power situation. A fully fledged 36 cell panel will give better performance when the weather is overcast. Once a month or so, it may be advisable to over-ride the regulator for a day to give your battery bank a boost charge.

NOTE: NiCad batteries have an OCV of 1.25 V per cell. Ten cells make up a 12 volt battery. The voltage of a NiCad battery rises higher when approaching 100% charge than a lead-acid battery. For this reason it is recommended to use a 36 cell solar panel and not a panel with less cells. Even a 33 cell solar panel behaves like a self regulating panel with a 12v Nicad battery bank.

Wiring up the Solar Array

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Choosing the optimum size cable is often a compromise between minimising the voltage drop in the cable on the one hand and financial constraints on the other.

How much voltage drop is allowable in different circumstances depends very much on the difference in voltage between the battery voltage at full charge and the open circuit voltage of the solar panels. Because Nicad batteries will charge up to a higher voltage, there is less voltage difference between the panels and the batteries. To compensate for this, it is recommended to use the next larger size of cable than is presented in the tables below.

All the figures are based on the use of 36 cell solar panels. The wire sizes (in the body of the tables) are a measure of conductor cross sectional area and are given in square millimetres (mm²).

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NOTE: The route length is the point to point distance and it includes both positive and negative conductors (ie half the total conductor length). These figures assume a maximum 10% transmission loss.

The wire sizes specified will give more than 90% efficiency in energy transfer. For a higher level of efficiency, use the next larger wire size. Where wire runs become prohibitively expensive or the distance too great, the use of a Maximizer is recommended.

See our range of Wires / Cables.