The Physics of Solar Arrays ( aka Photovoltaic Arrays )

This section describes the important aspects of how solar panels work. I've tried to provide a quick overview of each aspect, but if you would like more in-depth information visit "How Solar Cells Work" or visit the links section.

Sunlight Intensity

The sun provides a maximum intensity of 1000 Watts per square metre (1000 W/m²). This intensity is significant, since it could power ten 100 Watt lightbulbs if 100% of the energy could be converted to electricity. But solar panels are only 10-15% efficient, so in reality it could only light one or two bulbs. Also, 1000 W/m² is the MAXIMUM intensity. This only occurs at noon at the equator once a year! Usually the sun's intensity will be between 300 - 600 W/m², due to cloud cover. Also, the number of hours of sunlight per day has a huge effect on the amount of electricity that can be generated by a solar panel.

What Solar Panels are Made of

Solar cells are made of silicon wafers and belong to the class of semiconductors. Solar cells actually consist of two different types of semiconductors that are placed in layers. The first type is N-type doped silicon. N-type semiconductors have the property that they conduct electricity much better than pure silicon does because they are doped with impurities that have an extra electron. These electrons can be knocked free from the impurity atoms and can flow as a current if they have someplace to go. Unfortunately, with only a single layer of N-type doped silicon, there isn't anywhere for the free electrons to go. That's why there is a second layer of semiconductor material, called P-type doped silicon, that is put in contact with the N-type semiconductor. P-type semiconductors have the property that they attract electrons because they are doped with impurities that are missing one electron. These impurity atoms are called "holes" and these holes can steal electrons from other atoms, making it look like the holes are moving. Also, since the holes are from a shortage of one electron, they can be considered as positive charges. Thus, when P-type semiconductors are put in contact with N-type semiconductors, an electric current will flow if an external force of energy is applied that is energetic enough to eject electrons from the N-type semiconductor (such as light photons).

Solar panels are constructed by connecting together individual solar cells in series or parallel. Each solar cell has a voltage of approximately 0.5 Volts and the current is dependent on the size of the cell, but is normally around 100-150 milliAmps. To increase the voltage, cells are connected in series and to increase the current, cells are connected in parallel. Normally, 28-40 cells are connected together to make a solar panel with a DC electricity output of about 12 Volts and 4 Amps.

How Light is Converted to Electricity

Solar cells take advantage of the Photoelectric Effect to convert light to electricity. When light of a certain energy collides with the atoms in the P-type semiconductor material, the energy of the light photons is absorbed, which excites the atom. This increase in energy can cause the excess electron to be ejected from the atom. If there is also a hole created in the N-type semiconductor material near the P-N boundary, the electron and hole will flow across in opposite directions. Since the exchange of a hole and an electron disrupts the electrical neutrality of the cell, the electron wants to return to its original side of the semiconductor. If an electrical circuit is connected to the solar cell, the electron will try to return to its original side (P-type semiconductor) in order to restore the neutrality. This electron flow is what creates the current of the cell and the force on the electrons across the P-N boundary is what creates the voltage of the cell.

Using a Solar Array

Solar arrays are constructed by connecting many solar panels together in series or parallel to get the required voltage and current. A solar array should be oriented towards the sun to the South, so that for the majority of the day sunlight hits it at a 90 degree angle (or as close as possible to 90 degrees). This maximizes the absorption of the light, since some proportion of the light hitting at other angles will be reflected off the glass covering of the solar panels. The amount of light absorbed can be increased by using a solar tracker, which is a device that keeps the solar array oriented towards the sun so that the light is always hitting at 90 degrees.

The solar array also needs to be connected to a bank of storage batteries if the electricity is to be used when there is no sun (i.e. at night). The battery bank must have enough storage capacity so that the batteries will not be completely drained when being discharged at a pre-determined rate. Usually, this means having a large bank of batteries, which adds a significant cost to the setup of a solar power system. As well, unless the users of the solar electricity have equipment that can use direct-current (DC), an alternating-current (AC) inverter and other electronic equipment will be required, further increasing the initial investment.