esent.
In utility-interactive, grid-direct systems, as shown in Fig. 1 (click here to see Fig. 1), there is no energy storage, and the PV modules are connected to an inverter, which inverts the DC power produced by the PV modules into AC power that matches the grid power. These systems have the advantage of higher efficiencies and are more simplistic in their design/installation compared to their backup counterparts. Because the grid-direct systems do not employ a method of storing energy, when the electric utility grid fails, they automatically shut down until the grid is back up. On the other hand, utility-interactive, backup systems, as shown in Fig. 2 (click here to see Fig. 2), do employ energy storage, typically in the form of a battery bank. These systems can operate specific loads that have been isolated from the main distribution panel and are connected to a backup load center. It is common for these systems to only run a few loads off the backup system, not the whole house. If a whole house backup is required, doing so with a battery bank and inverter system can be difficult.
All utility-interactive systems use a safety feature known as “anti-islanding” to prevent the solar array from remaining connected to the electric utility when the grid is down. The inverters used in utility interactive systems are required by UL1741 listing to disconnect themselves from the grid when the voltage or frequency of the electric utility falls out of a specific range. The inverters will automatically reconnect to the electric utility system only after the voltage and frequency values are within the acceptable range for five continuous minutes. This is a safety feature to protect electric utility line workers from being injured by PV systems that are pushing power back on to otherwise de-energized power lines.
Now that you have a better feel for the major types of systems being designed and installed today, let’s go over the major system components used in these systems. This list will not be all inclusive, but rather a good primer on the major components and the roles they play in the entire system.
Modules — The heart of the PV system is the module, an individual unit that contains a number of PV cells, typically encapsulated behind a sheet of glass with an aluminum frame protecting the edges. A number of PV modules are wired together to produce a desired amount of voltage and current that matches the requirements of the power electronics connected on the other end of the PV array. In grid-direct PV systems, a number of modules are typically connected in series to create a string of modules that can operate at elevated voltages (i.e., up to 600VDC in some situations). Battery-based systems, either stand-alone or utility-interactive, will generally operate at lower voltage levels but can still exceed 150VDC within the string. These PV strings are then connected in parallel to create the PV array.
PV modules are offered in a variety of technologies; however, crystalline silicone modules are the most commonly used. Thin film modules, such as amorphous crystalline or cadmium telluride, are another category of PV modules you may come across.
Racking — Traditional PV modules require a support structure to hold the array in place. There are modules known as building integrated photovoltaics (BIPV) that actually replace the outer shell of a building, but these systems are not as common as PV modules mounted to the exterior of a building or ones mounted at ground level. PV modules can be installed in a variety of locations, and there are racking systems for every conceivable location. The racking systems generally fall into one of three categories: roof mount, ground mount, or top-of-pole mount. Within each of these general classifications, there are multiple subcategories.
Regardless of the mounting classification, racking systems are commonly made of extruded aluminum rails that support the PV modules at four points along the module’s aluminum frame. The exact form the rack’s extrusion takes on is a function of the racking manufactur