Direct‐Use and Ba ery Charging Solar Energy Systems
Solar energy captured by photovoltaic (PV) panels can be used directly or stored using ba eries in order to meet various needs. Whether to use the energy directly or rely on a bank of ba eries depends on your needs, and will affect the system installa on, maintenance and opera on costs. Direct‐use solar energy systems are simpler and less costly than ba ery‐storage systems, and require minimal maintenance. Pumping water for homes, livestock watering and irriga on are examples of direct‐use, off‐grid solar energy systems. Since water can be needed during cloudy periods when the PV system cannot power the pump, water storage tanks and the pumping system are designed to pump enough water in a single day to fill the storage tank with enough water for three or four cloudy days. Larger storage tanks are less costly than banks of ba eries, and they also require li le maintenance. If your livestock requires 500 gallons of water per day, a direct‐use solar powered pumping sta on should be capable of pumping 2,000 gallons of water in a single sunny day to fill a tank that can water the ca le through three cloudy days. A float switch will stop the pump when the tanks are full, preven ng overflows and muddy areas around the tank. Allowing the pump to shut off also extends the pump life. Off‐grid systems that incorporate banks of ba eries are far more complex and more costly than direct‐
use systems. It is cri cal that all the system components be sized properly or it will not func on well, if at all. The bank of ba eries must be sized to provide all the energy needs during three or four cloudy days with li le if any solar energy available to recharge the ba eries. When selec ng ba eries, select ba eries that can deliver the needed energy without being discharged more than 50 percent of their capacity. Completely discharging lead‐acid ba eries will greatly reduce the number of charge‐
discharge cycles. As with direct‐use systems, the PV system should have sufficient capacity to fully charge the ba eries in a single day. Never connect PV panels directly to the ba eries. Photovoltaic panels can generate voltages high enough overcharge and damage the ba eries. Always use a charge controller that is suitable for the voltages and currents of your system. The purposes of the charge controller are:  Regulate charge voltage and current to prevent overcharging  Prevent ba ery current discharging back through the PV modules  Gives an indica on of ba ery charge level  Prevents over‐discharging ba eries by disconnec ng the loads at a specified voltage  Some may also track energy usage But, the ba eries should not be discharge more than 50 percent of their capacity, so the usable energy storage will be: 1,200 wa •hours X 0.50 = 600 wa •hours Now that we know how much usable energy storage is available from each ba ery, we need to determine how many ba eries are needed: 3,600 wa •hours ÷ 600 wa ‐hours= 6 ba eries When several ba eries are required, you must create strings of ba eries in series to deliver the required voltage, and strings in parallel to deliver the required current. You should not have more than three strings of ba eries in parallel, as it can be difficult to charge the strings equally. Select larger ba eries in order to minimize the number required. Determining energy storage capacity of ba eries can be confusing. Specifica ons of some ba eries are listed as the number of hours at a certain current, or a certain number of amp‐hours. Others are listed by their wa ‐hours or kilowa ‐hours, which makes ba ery selec on easier. If your water pump requires 200 wa s and is expected to run up to 6 hours per day, then its daily energy requirement will be: 200 wa s X 6 hours = 1,200 wa •hours per day If the ba eries must power the pump for 3 cloudy days, then the total energy storage requirement is 1,200 w•hr per day X 3 days = 3,600 w•hr If you plan to use 12‐volt deep‐cycle marine ba eries rated for 5 hours at 20 amps current, then their total energy storage capacity is: 5 hours X 20 amps = 100 amp•hours 100 amp•hours x 12 volts = 1,200 wa •hours In our example, a 12‐volt pump would require 6 marine ba eries wired in parallel. But, a 24‐volt pump would use three parallel strings of ba eries with two ba eries in series per string. Backup generator
All off‐grid systems are likely to experience mes when the cloudy periods are longer than the system is designed for. A backup generator and ba ery charger can be used to recharge ba eries. Some pump controllers have generator inputs that allow the pump to run using generator power. The University of Tennessee Extension offers its programs to all eligible persons regardless of race, color, na onal origin, sex, age or disability and is an Equal Opportunity Employer.