3.6.2 Autonomous photovoltaic systems
STAND-ALONE PV SYSTEMS INFORMATION stand-alone photovoltaic systems are usually utility power inverters. They generally include solar charging units, photovoltaic storage batteries and controls or regulators as shown in figure 3.15. STAND-ALONE PV SYSTEMS INFORMATION installed on the ground or on the roof will require a mounting structure, and if AC power is desired, an inverter is also required. In many autonomous photovoltaic systems batteries are used for energy storage, as they can represent up to 40% of the total cost of the stand-alone PV system throughout its lifetime [ 33 ].
Figure 3.15 . Diagram of autonomous photovoltaic system with battery storage power DC and AC loads [8].
AUTONOMOUS PHOTOVOLTAIC SYSTEMS INFORMATION: These batteries cause losses in the PV system due to limited availability of time and energy to recharge the battery in addition to inadequate battery maintenance. Therefore, a charge controller is used to control the system and prevent overcharging and overcharging the battery. Overcharging reduces battery life and may cause gassing, while overcharging can lead to sulphurization and stratification, which results in reduced battery efficiency and lifetime [ 34–37 ].
Batteries are often used in stand-alone PV systems to store energy produced by the PV generator during the day and supply electrical loads as needed (during night or cloudy weather). Additionally, batteries in monitoring systems are required to operate at MPP in order to supply electrical loads with constant voltages. Almost, most of the batteries used in stand-alone PV systems are deep cycle lead-acid batteries [ 38 ]. These batteries have thicker lead plates that make them tolerate deep discharges. The thicker the lead plates, the longer the battery life. The heavier the battery for a given group size, the thicker the plates and the better the battery will tolerate deep discharges [ 39 ].
All deep-rolled batteries are rated at ampere-hour capacity (AH), the amount of usable energy that can be stored at rated voltage [40]. A good charge rate is about 10% of the total battery capacity per hour. This will reduce electrolyte losses and damage to the plates [38]. A photovoltaic system may need to be large enough to store enough energy in batteries to meet the demand for power during many days of cloudy weather, known as "autonomy days". The Institute of Electrical and Electronic Engineering (IEEE) (IEEE Std 1013-1990) [41], for the selection, charging and testing of autonomous photovoltaic systems (IEEE Std 1361-2003) [42]] and for their installation and maintenance ( IEEE Std 937-2007) [43].
Nickel-cadmium batteries are also used for stand-alone PV systems, but are often expensive and “may have voltage compatibility issues with some inverters and chargers” [ 44 ]. However, their main advantage is that they are not affected by temperature like other types of batteries, so they are mostly recommended for industrial or commercial applications in cold locations. IEEE has also developed some guidelines for installation and maintenance (IEEE Std 1145-1999) [ 45 ].
AUTONOMOUS PHOTOVOLTAIC SYSTEMS INFORMATION: To extend battery life and make the system work efficiently, a charge controller is required to regulate the flow of electricity from the PV modules to the battery and load. The controller keeps the battery fully charged without overcharging it. Many controllers have the ability to detect excess electricity drawn from the batteries to the load and stop the flow until sufficient charge is restored to the batteries. The latter can significantly extend battery life. However, controllers in a stand-alone PV system are more complex devices that depend on the state of charge of the battery, which in turn depends on many factors and is difficult to measure. The controller must be sized to handle the maximum current generated. Certain features should be considered before selecting a controller, such as adjustable set points such as high and low voltage cut-offs, temperature compensation, low voltage warning and reverse current protection. In addition, the controller should ensure that no current flows from the battery to the array at night.