The Anatomy of a Photovoltaic System
Photovoltaic cells do just what the word implies (photo = light, voltaic = electricity) - they convert solar energy into electricity.
Photovoltaic cells are made up of several semiconductors of which crystalline silicon is the most commonly used material. The semiconductor is really a multi-layered structure that has a positive layer and a negative layer. When light, or more specifically, photons, hit these cells, some of it is absorbed by the semiconductors. The absorbed photons knock electrons loose generating a flow of electric current. This current is harnessed as DC (Direct Current) from the cell through small metal contacts (or conductors).
A solar module or solar panel is composed of several electrically connected photovoltaic cells mounted in a support structure or frame. These modules are designed to supply electricity at a certain voltage. The current produced is directly dependent on how much light strikes the module.
A solar array consists of multiple solar modules connected and mounted together. In general, the larger the area of the array, the more electricity it will produce. Arrays can be mounted in fixed positions - meaning they don't move. This is the most common for residential rooftops. However, they can be setup on special tracking devices that follow sun as it moves across the sky.
The Inverter's primary purpose it convert the DC or Direct Current electricity coming from the solar array into AC or Alternating Current electricity which is the type of electricity used in our homes and businesses.
If the solar array produces more energy than you use, is passes backwards through your utility meter and into the utility grid. Yes, your meter can actually run backwards. This is known as net-metering.
Optimum Tilt Angle and Orientation to Mount a Solar Array
- Since solar arrays generate electricity when they're exposed to sunlight, the more sunlight they're exposed to the more electricity they will generate. This means that title angle and orientation of the array is very important.
- Tilt Angle
- In the Northern Hemisphere the sun moves across the sky at a slight angle. Only at the equator does it move directly over-head. Thus the optimum angle of an array equals the location's Latitude. For example, the latitude of San Francisco, CA is about 37 degrees. That also happens to be the optimum tilt angle for the array. Similarly at the equator where the latitude is zero, so is the optimum tilt angle for an array - zero, or flat. The reason solar arrays lose performance at non-optimal tilt angles is because at greater angles to the sun, much of the light simply reflects off the glass surface instead of being absorbed by the silicon semiconductor cells.
On flat roofs, it can be cost efficient to mount the panels at the optimum tilt angle. This becomes less cost efficient on residential sloped roofs. - Orientation
- Orientation is the direction the solar array is facing. This also is based on location but in much simpler terms. Basically in the northern hemisphere the orientation should be as south-facing as possible and in the southern hemisphere, as north-facing as possible. That's not to say a west facing array wouldn't generate more power during the late afternoon. It would. But it would also generate very little during the morning to mid afternoon. So in general the more south facing, the better.