Photovoltaic Systems

As single PV cells have a working voltage of about 0.5 V, they are usually connected together in series (positive to negative) to provide larger voltages. Panels are made in a wide range of sizes for different purposes. They generally fall into one of three basic categories:

Low voltage/low power panels are made by connecting between 3 and 12 small segments of amorphous silicon PV with a total area of a few square centimeters for voltages between 1.5 and 6 V and outputs of a few mW. Although each of these panels is very small, the total production is large. They are used mainly in watches, clocks and calculators, cameras and devices for sensing light and dark, such as night lights.
Small panels of 1 - 10 W and 3 - 12 V, with areas from 100 to 1000 cm² are made by either cutting 100cm2 single or polycrystalline cells into pieces and joining them in series , or by using amorphous silicon panels. The main uses are for radios, toys, small pumps, electric fences and trickle charging of batteries.
Large panels, ranging from 10 to 60 W, and generally either 6 or 12 V, with areas of 1000 cm² to 5000cm2 are usually made by connecting from 10 to 36 full-sized cells in series. They are used either separately for small pumps and caravan power (lights and refrigeration) or in arrays to provide power for houses, communications pumping and remote area power supplies (RAPS).

Arrays and Systems

If an application requires more power than can be provided by a single panel, larger systems can be made by linking a number of panels together.

However, an added complexity arises in that the power is often required to be in greater quantities and voltage, and at a time and level of uniformity than can be provided directly from the panels. In these cases, complex PV systems are used with the following parts:

A PV panel array, ranging from two to many hundreds of panels
A control panel, to regulate the power from the panels
A power storage system, generally comprising of a number of specially designed batteries
An inverter, for converting the DC to AC power (e.g. 240 V AC)
Backup power supplies such as diesel startup generators (optional)
1. framework and housing for the system
2. trackers and sensors (optional)

Arrays generally run the panels in series/parallel with each other, so that the output voltage is limited to between 12 and 50 volts, but with higher amperage (current). This is both for safety and to minimize power losses. Arrays of panels are being increasingly used in building construction where they serve the dual purpose of providing a wall or roof as well as providing electric power for the building. Eventually as the prices of solar cells fall, building integrated solar cells may become a major new source of electric power.

The daily energy output from PV panels will vary depending on the orientation, location, daily weather and season. On average, in summer, a panel will produce about five times its rated power output in watt hours per day, and in winter about two times that amount. For example, in summer a 50 W panel will produce an average of 250 Wh of energy, and in winter about 100 Wh. Trackers can be used to keep PV panels directly facing the sun, thereby increasing the output from the panels. Trackers can nearly double the output of an array. Careful analysis is required to determine whether the increased cost and mechanical complexity of using a tracker is cost effective in particular circumstances.

Energy storage is often necessary when power is required when the sun is not shining - either at night or in cloudy periods - or in quantities greater than can be supplied directly from the array. Specially designed "deep-cycle" lead acid batteries are generally used. Unlike normal batteries, they can discharge about half of their stored energy several thousand times before they deteriorate. Each battery is usually 2 V, and the total battery bank usually has many batteries in series and parallel to give the required power rating. Battery banks need to be individually sized to suit the particular applications, depending on total daily solar radiation, total load, peak load and the number of days storage required. Inverters transform low voltage DC power (e.g. 12V, 24V, 32 or 48V from batteries) into high voltage AC (generally 230 V in Australia). Inverters are necessary if mains-voltage appliances are to be used. In assessing the cost of the total system, it may be more economical to purchase an inverter and mass produced consumer appliances than to use low voltage DC appliances which may be more expensive. (reference 18)