 in PV systems primarily are used to invert DC energy into AC that can be used within the building to run loads or be pushed back onto the grid. The inverters used in stand-alone and utility-interactive battery backup applications are very similar in their appearance and function, whereas the inverters used in grid-direct systems are very different from the battery-based units.
Battery-based inverters are modular units, meaning multiple small units can be connected together to create a single large power source. The most commonly used battery-based inverters are actually inverter/chargers, meaning they have the ability to take the energy from the battery bank to run loads as well as connect to an AC source and recharge the batteries as necessary. A stand-alone inverter will commonly have a traditional generator connected to it to provide battery charging as needed. The generator is not a requirement in these systems, but it gives users greater flexibility in their use of energy.
A utility-interactive, battery-based inverter will connect to the electric utility instead of a generator for its AC source of power, allowing the user to send energy back to the grid and receive credits for that energy when the PV array is able to produce more energy than is being consumed. The major difference between a stand-alone inverter and a battery-based backup inverter is the ability to push current back toward the AC source. In stand-alone systems, pushing current back into the generator can quickly damage the generator and serves no good purpose. In utility interactive systems, pushing current toward the grid is a desirable situation and results in a financial benefit to the system owner.
Grid-direct inverters come in a wide variety of power output sizes — from hundreds of watts (micro-inverters) to kilowatts (string-inverters) to thousands of kilowatts (central inverters). These inverters are simply a way to invert the DC energy produced by the PV array and connect it directly to the utility grid. Multiple grid-direct inverters can be combined to increase the overall power output as required. It is important to remember that these systems offer no form of energy backup, so when the electric utility goes down, the inverter will shut down and not come back online until the grid is back up.
The individual components listed here should give you a good idea of the major components used with most PV systems. Additional components required for a complete installation will be covered in future articles.
Mayfield is a principal with Renewable Energy Associates, Corvallis, Ore. He can be reached at: email@example.com.