Photovoltaic System Configurations
Photovoltaic (PV) solar power systems can be put together in a number of ways, ranging from simple to complex, depending on their use. The systems can produce either direct-current (DC) just like a battery, or can convert that current into alternating current (AC), as is used in most households. The main configurations are as follows:
DC Direct-Drive PV Systems — For off-grid DC loads that can use intermittent power
DC PV Systems with Battery Storage — For DC loads that need a steady power supply
Off-Grid AC PV Systems — For off-grid households and other AC applications. These systems also incorporate batteries.
Utility-Connected AC PV Systems — For displacing power that would otherwise be purchased from a utility, using the power grid as a backup source, or selling power to the grid.
DC Direct-Drive PV Systems
Some applications, such as water pumping for cattle or irrigation, can run on DC power and achieve their purpose while operating only part of the time. These applications use a PV array connected directly to a pump, which feeds a water tank or trough. The system must be sized to ensure that the water in storage doesn't run dry during extended periods of cloudy weather (unless the need for water is also reduced during cloudy weather). A simple controller can regulate the voltage supply to the pump and shut the pump off when the water storage tank is full.
DC PV Systems with Battery Storage
Many applications can use DC power, but require a steady power supply. Lighting applications are a good example—these systems are often used for flashing warning lights. The systems require a controller to govern the flow of electricity to and from the batteries while maintaining a steady flow of power to the application. Note that using energy-efficient lighting will greatly reduce the cost of the PV system.
A good example of a DC PV system with battery storage is found outside the Prince Jonah Kuhio Kalanianiole (PJKK) federal building in Hawaii. DC PV systems are installed on top of the parking lot light poles, using two 48-watt solar panels per lamp and a 90 amp-hour battery to provide 12 hours of power per night to two 30-watt fluorescent lamps that produce 2,500 lumens each.
Small individual DC systems have many applications, such as providing power for home systems, public area lighting, schools, health clinics, pumping water and water purification, as well as rural telephony and micro-enterprise development.
Off-Grid AC PV Systems
Many electrical appliances require AC power. To power a typical off-grid household, most people prefer to use a standard AC wiring system and AC appliances, which means that the power system must produce AC power. For PV systems, that means that an inverter must be used to convert the DC power into AC. A typical off-grid AC PV system includes the PV modules, a bank of batteries, a controller, and an inverter.
Grid-Connected PV System
In most buildings that have access to the electrical grid, the preferred configuration is to connect the PV system directly into the building wiring on the customer's side of the meter. In this configuration, the PV system can be used to supplement the grid during the day while the grid meets the building's power needs at night. And if the PV system produces more power during the day than is needed, the excess power can be fed back into the power grid, turning the meter backwards! In many states, the building owner can earn credit on the power bill for any power fed back into the grid—a concept known as net metering.
Grid-connected systems save money by eliminating the use of battery banks. Instead, the inverter controls the flow of power between the PV system, the building (or other AC load), and the power grid. These units typically include safety features to disconnect the system from the grid in the event of a power failure on the gird, in order to avoid powering lines on which utility crews are working. However, some utilities also require outside disconnect switches, extra meters, and other equipment. The disadvantage of removing the batteries is if the utility goes down, so does your own power system.
The Presidio Thoreau Center is an excellent example of a utility-connected PV system and has integrated a 1.25-kilowatt PV array into the skylights over the building's atrium. Spaces between the PV cells allow daylight into the atrium.