Types of Solar Cells
A solar cell is the basic photovoltaic device which generates electricity when exposed to sunlight. A solar cell is a photo-diode which conducts current in one direction only. It is formed by creating a p-n junction using p-type and n-type silicon.
Monocrystalline Solar Cells
Semiconductor grade silicon is melted and, through a cystal seeding process, a large single silicon crystal is formed. Crystals with diameters around 12cm are not uncommon.
Once the crystal has been formed, it is sliced into wafers about 0.2 to 0.4mm thick. A phosphorous impurity is introduced into the surface layers of the wafer and metal grids are attached to the front and back of the wafer to facilitate the collection of electrons.
Monocrystalline cells tested in the laboratory have given efficiencies of over 23%. In this case, various inefficiencies such as reflection and grid coverage have been reduced. When encapsulated in a solar module, the working efficiency of solar cells drops by around 3%.
Polycrystalline Solar Cells
Polycrystalline silicon is a block of material which, instead of being a single crystal, is made up of many tiny crystals pieced together. Manufacturers have pioneered processes for mass-producing inexpensive poly crystalline cells, taking advantage of the fact that it is inherently easier to grow little crystals than big ones.
A disadvantage of polycrystalline cells is that the boundaries between the tiny crystals tend to impede the flow of holes and electrons through the material. The cell efficiency suffers, though several techniques for coping with this problem have proven successful enough for 18% efficient polycrystalline cells to be produced. Again, when encapsulated in a solar module, the working efficiency of solar cells drops by around 3%.
Amorphous Solar Cells
Techniques such as the condensation of gaseous silicon are used to make cells with a thickness that can be measured in the number of atomic layers. The atoms in such thin films of silicon are arranged in a completely random fashion and the cell is called an amorphous thin-film cell.
Though these cells are inexpensive, abandoning the crystal structure wreaks havoc on their efficiency. About 12% is the best that has ever been achieved, with average efficiencies of around 5%.
Types of Solar Panels
Standard Solar Panels
When solar cells are joined physically and electrically and placed into a frame, they form a solar panel or PV module. Solar panels joined together form a solar array.
Most commercially available panels are configured to produce an open circuit voltage of around 20 Volts and a nominal voltage of around 14 Volts to make them suitable for charging a 12 Volt battery. They are generally made up of 36 cells in series and referred to as 12 Volt panels.
Self Regulating Solar Panels
A self-regulating module has a limited number of cells connected in series, normally 30 or 32. This limited number allows the module to only produce a maximum of 14.5 Volts, thus making it difficult to overcharge the battery.
Using a self-regulating module does not automatically assure that a PV system will be self-regulating. Battery capacity, use of the loads and temperature must be considered. Generally, self-regulating modules can be safely used when the battery capacity is large. If the capacity is small, there is still the possibility of overcharging the battery.
Note: Most modules used in RAPS systems are not self-regulating.
Battery State of Charge
A good estimate of a battery’s state of charge can be made by measuring the voltage across the battery terminals with the battery at rest (i.e. no energy input, no energy output) for at least three hours. These readings are best taken in the early morning or in late evening. Take the reading while all loads are off and no charging sources are producing power. Connect a voltmeter across the positive and negative terminals of the battery or battery bank.
The following table will allow conversion of the readings obtained to an estimate of state of charge. The table is good for batteries at 25·C that have been at rest for 3 hours or more. If the batteries are at a lower temperature you can expect lower voltage readings.
Battery State of Charge Voltage Table
| % of Full Charge | 12 Volt DC System | 24 Volt DC System | 48 Volts DC System |
100% | 12.7 | 25.4 | 50.8 |
| 90% | 12.6 | 25.2 | 50.4 |
| 80% | 12.5 | 25.0 | 50.0 |
| 70% | 12.3 | 24.6 | 49.2 |
| 60% | 12.2 | 24.4 | 48.8 |
| 50% | 12.1 | 24.2 | 48.4 |
| 40% | 12.0 | 24.0 | 48.0 |
| 30% | 11.8 | 23.6 | 47.2 |
| 20% | 11.7 | 23.4 | 46.8 |
| 10% | 11.6 | 23.2 | 46.4 |
| 0% | <11.6 | <23.2 | <46.4 |
Sinewave and Modified Sinewave Notes
When we talk about Sinewave and Modified Sinewave inverters, we are referring to the shape of the AC waveform output by the inverter. The three most common outputs for inverters are Squarewave, Modified Sinewave (sometimes called a Quasi Sinewave), and Sinewave outputs.
Pure Sine Wave Inverters
Pure or true sinewave inverters supply power of better quality than grid power and will work correctly with any appliance that you would normally run on grid power. Their output waveform is a smooth sine wave.
Modified Sine Wave Inverter
Modified Sinewave is a sales term used for a Modified Squarewave type of AC power which is not quite the same as grid electricity. Modified Sinewave inverters are cheaper to produce than Sinewave inverters, slightly more efficient, and almost all appliances work fine with them.
Microwave ovens will work on a Modified Sinewave inverter, but they hum and buzz a little louder when in operation and do not cook as quickly.
Items that we do not recommend using with a Modified Sinewave inverter include: Photocopiers, laser printers, cordless tool rechargers, electric tools with variable speed control (lose speed control), equipment containing SCR’s (Silicon Controlled Rectifiers) or Triac’s and any sensitive electronic equipment.
In general, AC appliances are designed and manufactured to run on true Sinewave power. Most of them can run on Modified Sinewave power, but if you are concerned about the reliability and longevity of the appliance, you should use a Sinewave inverter.
Handy Hints and Tips
- When sizing an inverter for a microwave oven, note that the advertised rating is the cooking power, not the amount of power that it consumes. e.g. An 800W microwave will actually consume around 1150W when cooking. Check for an appliance rating plate on the back of the oven and remember this when sizing inverters, etc.
- Ensure that the cables between your battery/s and inverter are suitably sized to carry the required current when the inverter is running at full load. The current on the DC side of the inverter is much greater than the current on the AC side. A rough approximation of the maximum current on the DC side can be found by dividing the continuous rating of the inverter by the DC input voltage. e.g. 1500Watt / 12V = 125Amps!