Photovoltaic power systems are generally classified according to their functional and operational requirements, their component configurations, and how the equipment is connected to other power sources and electrical loads.
Photovoltaic power systems are generally classified according to their functional and operational requirements, their component configurations, and how the equipment is connected to other power sources and electrical loads. The two principal classifications are grid-connected or utility-interactive systems and stand-alone systems. Photovoltaic systems can be designed to provide DC and/or AC power service, can operate interconnected with or independent of the utility grid, and can be connected with other energy sources and energy storage systems.
Grid-connected or utility-interactive PV systems are designed to operate in parallel with and interconnected with the electric utility grid. The primary component in grid-connected PV systems is the inverter, or power conditioning unit (PCU). The PCU converts the DC power produced by the PV array into AC power consistent with the voltage and power quality requirements of the utility grid, and automatically stops supplying power to the grid when the utility grid is not energized. A bi-directional interface is made between the PV system AC output circuits and the electric utility network, typically at an on-site distribution panel or service entrance. This allows the AC power produced by the PV system to either supply on-site electrical loads, or to back-feed the grid when the PV system output is greater than the on-site load demand. At night and during other periods when the electrical loads are greater than the PV system output, the balance of power required by the loads is received from the electric utility This safety feature is required in all grid-connected PV systems, and ensures that the PV system will not continue to operate and feed back into the utility grid when the grid is down for service or repair.
Photovoltaic system types can be broadly classified by answers to the following questions:
Will it be connected to the utility’s transmission grid?
Will it produce alternating current (AC) or direct current (DC) electricity, or both?
Will it have battery back-up?
Will it have back-up by a diesel, gasoline or propane generator set?
These systems generate the same quality of alternating current (AC) electricity as is provided by your utility. The energy generated by a grid-connected system is used first to power the AC electrical needs of the home or business. Any surplus power that is generated is fed or “pushed” onto the electric utility’s transmission grid. Any of the building’s power requirements that are not met by the PV system are powered by the transmission grid. In this way, the grid can be thought of as a virtual battery bank for the building.
Common System Types – Most new PV systems being installed in the United States are grid-connected residential systems without battery back-up. Many grid-connected AC systems are also being installed in commercial or public facilities.
The grid-connected systems we will be examining here are of two types, although others exist. These are:
Grid-connected AC system with no battery or generator back-up.
Grid-connected AC system with battery back-up.
Example configurations of systems with and without batteries are shown in Figures 1 and 2. Note there are common variations on the configurations shown, although the essential functions and general arrangement will be similar.
Is a Battery Bank Really Needed? – The simplest, most reliable, and least expensive configuration does not have battery back-up. Without batteries, a grid-connected PV system will shut down when a utility power outage occurs. Battery back-up maintains power to some or all of the electric equipment, such as lighting, refrigeration, or fans, even when a utility power outage occurs. A grid-connected system may also have generator back-up if the facility cannot tolerate power outages.
As an example of a variation on the configuration shown in Figure 1, the inverter has been shown in an interior location, while very often it is installed outside. If located outside, there may not be a need for a separate Array Disconnect and Inverter DC Disconnect, as a single DC Disconnect can serve both functions. In Figure 2, system meter functions are often included in charge controllers and so a separate System Meter may not be required.
With battery back-up, power outages may not even be noticed. However, adding batteries to a system comes with several disadvantages that must be weighed against the advantage of power back-up. These disadvantages are:
Batteries consume energy during charging and discharging, reducing the efficiency and output of the PV system by about 10 percent for lead-acid batteries.
Batteries increase the complexity of the system. Both first cost and installation costs are increased.
Most lower cost batteries require maintenance.
Batteries will usually need to be replaced before other parts of the system and at considerable expense.
Figure 1. One common configuration of a grid-connected AC photovoltaic system without battery back-up
Figure 2. One common configuration of a grid-connected AC photovoltaic system with battery back-up
- PV System: GRID-TIE (BATTERY FREE)
The simplest and most cost effective PV design for most sites is the "Grid-Tie" (sometimes referred to as intertied or utility-interactive) system. This system does not provide backup power during a power outage (even if the sun is shining) but for sites with reliable grid power, this is usually the logical system choice.
- PV System: GRID-TIE WITH BATTERY BACKUP
The Grid-Tie With Battery Backup system can also push excess electricity produced to the electric utility grid but has the added feature of batteries in order to power some selected backup loads when the grid is down. With this benefit comes increased complexity, cost and maintenance requirements.
- PV System: STAND-ALONE
The Off-Grid or Stand-Alone PV System incorporates large amounts of battery storage to provide power for a certain number of days (and nights) in a row when sun is not available. The array of solar panels must be large enough to power all energy needs at the site and recharge the batteries at the same time. Most Off-Grid systems benefit from the installation of more than one renewable energy generator and may include Wind and/or Hydro power. A gas generator is often employed for emergency backup power. You may have seen mini versions of the stand-alone system on remote road signs and radio towers.
- PV System: PV DIRECT
PV Direct systems are usually very simple systems where the photovoltaic panel is connected directly to a motor or pump which matches the voltage and amperage output of the panel. When the sun shines and the PV panel produces electricity, the device runs--when the sun is not available, the device stops. This system is often used for livestock where a well-pump lifts water out of the ground to a watering trough in remote locations. Other applications include solar powered attic fans, irrigation systems and small day-time garden waterfalls or fountains.