Electricity from Light: How?
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.
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% and 15% conversion. The theoretical maximum efficiency of a silicon cell is about 21%. Using a more costly technology 31% conversion has been achieved.
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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. Full system design should include proper mounting and location of the modules, proper wiring and circuit protection, choosing regulators, and fuses to protect the battery, as well as a proper and safe installation procedure. Trained Solar dealers or distributors should be consulted for proper system design.
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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
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. |
By having your panels following the sun with a solar 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. The following table shows the best array angles for 28 different locations around Australia.
The solar array would either be facing north on a fixed frame (at an angle from the horizontal to get the best results); or on a polar axis tracking device (tracking the sun from sunrise to sunset). |
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Is Tracking Cost Effective?
Whether tracking is really worth the expense depends on a number of factors:
1. The cost of the Tracker.
2. 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.
3. 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 (refer to diagrams page 113). 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.
In fact, if you check against the figures on the tables (1 & 2) you will find that the horizontal (flat mounting) panels would frequently exhibit better performance patterns than the panels on fixed frames tilted to the north.
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.
4. 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)
5. 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.
6. 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.
Shadow throwing objects tend to have their greatest effect on a solar panel site when the sun is lowest in the sky.
Even the smallest amount of shading reduces panel output significantly. By referring to the table on page 114 you may get an idea of the advantage in tracking the sun 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 Sun Tracking gainst the columns labelled Best Average Performance.
7. 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. An automatic solar tracking system usually costs more than a 75W solar panel. Unless your loads are predominantly summer loads (eg fridge, freezer, space cooling, pumping) and you already have at least 8 solar panels, you may be better off with another solar panel. An extra solar panel gives greater benefit in mid winter when there is the greatest demand for night time lighting and entertainment. If you need more power in winter for
lighting and entertainment because of the shorter daylight hours then an extra solar panel may be money better spent than having a tracking device.
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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 refering to the table on page 114 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.
Use of Reflectors *
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 iecreases. 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.
Grounding
Although it is not essential for the satisfactory operation of your system, the manufacturers of solar panels recommend that solar panels be grounded. Grounding with respect to photovoltaic installations serves several purposes:
1. it bleeds off static electric charge built up from wind and rain;
2. it is an integral part of lightning protection;
3. it provides fault protection, whereby any shorts or faults in circuitry will conduct enough current to ground to trip circuit breakers or fuses and allow fault detection. It is recommended that the earth stake should have a resistance to ground of 25 ohm or less. For adequate lightning protection, between 1 ohm and 5 ohm resistance to ground gives fairly reliable lightning protection. You may need to find an electrician with a multimeter to set up a good grounding for your system. More detailed information may be obtained from the Rainbow Power Company. |
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The adverse effects of heat
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).
NOTE: In very hot conditions, such as in the interior of
Australia, it is recommended to use a 36 cell panel (our larger BP panels have 36 cells).
What to expect from a Solar Panel
A single 83 watt solar panel should produce about 5 amps under sunny conditions. Each day of reasonable sunshine you should expect about 23.6 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.
Self Regulating Panels
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.
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Wiring up the Solar Array
You will find throughout "Energy from Nature" and particularly in the wire section near the back of the book, that a lot of emphasis is placed on using a sufficiently large wire to carry the current. 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. The following tables are a guide. Although you may use a larger cable it is recommended not to use a wire of lesser size.
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).
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 page 105). |
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