Photovoltaic, or solar, cells are semiconductor materials that, when struck by sufficient amounts of light, cause electrons to flow. Materials with photovoltaic properties include silicon and more complex composites. PV cells convert light energy to electricity as follows:
When light strikes the cell, its energy is absorbed by the semiconductor and knocks loose electrons. The PV cells are designed with layers of slightly different materials that make the loose electrons flow in a specific direction. This is what creates an electric current. Metal contacts are placed at the edges of each cell allowing this current to be drawn off for external use.
For most uses, multiple PV cells are connected and combined into a module that contains dozens of cells held within a frame. The module is then covered with glass. Positive and negative terminals are connected to the contacts from outside the frame to direct the electron flow. Multiple modules can be combined into an array to obtain the amount of power desired at a specific installation.
The cheapest way of mounting the arrays is to use a fixed mount on which the array does not move. The downside to this is that the array will not be oriented to the sun and thus will not receive maximum sunlight since the sun moves throughout the day.
The alternative is to build a tracking system that lets the array follow the movement of the sun. Tracking systems may be one-axis, which follow the sun’s movement from east to west across the day, or two-axis, which track daily movement in addition to the sun’s movement north to south on a seasonal basis.
PV systems connected to the transmission system are called utility-scale. For utility-scale systems the power coming off the arrays must be conditioned and converted from direct current (DC) to alternating current (AC). Also required is a step-up transformer to raise the voltage to that of the transmission line.
PVs located at customer facilities or connected to the distribution system are called “distributed.” Distributed PVs can be used as the sole source of power in a building, without a connection to the utility distribution grid. This is called off-grid. In this case, a battery system is required so that power is available when the sun is not shining. More common is the grid-connected system that allows the building to use solar power when it is available and to draw power from the utility grid when solar power is not available. In many cases excess solar power can be fed back into the utility grid for use by other customers. These systems are typically connected to the grid on the distribution system.