PV Systems with Battery Storage
This 4-kilowatt solar electric system, dubbed "Solar Independence," is the largest mobile power unit ever built. The flag's field of blue consists of photovoltaic panels made of silicon; these panels generate enough electricity to provide power to one or two homes. Workhorse batteries that can store up to 51 kilowatt-hours of electricity are in a portable trailer behind the flag. This system has been part of several emergency training exercises in Colorado and has been exhibited on the National Mall in Washington, D.C.
PV systems with batteries for storage are excellent for supplying electricity when and where you need it. These systems are especially suitable in areas where utility power is unavailable or utility line extensions would be too expensive. The ability to store PV-generated electrical energy makes the PV system a reliable source of electric power both day and night, rain or shine. PV systems with battery storage are used all over the world to provide electricity for lights, sensors, recording equipment, switches, appliances, telephones, televisions, and even power tools!
Speaking about the "Solar Independence" PV system, John Thornton, engineer at the National Center for Photovoltaics, says, "The objective is to raise people's awareness about the value of these technologies. And the only way to do that is to show them the technology."
PV systems with batteries can be designed to power equipment that requires dc or ac electricity. People running conventional ac equipment will add a power conditioning device called an inverter between the batteries and the load. Although a small amount of energy is lost in converting dc electricity to ac, an inverter makes PV-generated electricity behave like utility power so it can operate everyday ac appliances, lights, and even computers.
We operate PV/battery systems by connecting the photovoltaic modules to a battery, and the battery, in turn, to the load. During the day, the PV modules charge the battery, and then the battery supplies power to the load as needed. A simple electrical device called a charge controller keeps the batteries charged properly and helps prolong their life by protecting them from overcharging or from being completely drained.
The U.S. Fish and Wildlife Service needed a cleaner, quieter power source for this facility on an island 30 miles west of San Francisco, California — home to biologists, volunteers, sea lions, and thousands of birds. Before a 9.1-kilowatt PV-hybrid system was installed, staff had to run loud, expensive diesel generators during the day, and they had no electricity after work. A quiet PV system from Applied Power Corporation now generates electricity around the clock. Its industrial battery bank stores up to 3 days' worth of energy, and a new, smaller generator serves as backup in case of heavy power usage or extended periods of bad weather.
Batteries make PV systems useful in more situations, but also require some maintenance. The batteries used in PV systems are similar to car batteries, but they're built somewhat differently to allow more of their stored energy to be used each day. (They're said to be "deep cycling," like the batteries used on golf carts.) Batteries designed for PV projects pose the same risks and demand the same caution in handling and storage as automotive batteries. We need to check the fluid in unsealed batteries periodically, and protect them from extremely cold weather.
The amount of electricity that can be used after sunset or on cloudy days is determined by the output of the PV modules and the nature of the battery bank. Including more modules and batteries increases system costs, so energy usage needs to be studied carefully to determine the best system size for the load. A well-designed PV-battery system balances cost and convenience with meeting the user's needs, and it can be expanded if those needs change. See an example of a PV-battery system used in a modern residence for more information.