Solar electrical energy: Topology of usage
Solar electricity can be used in a variety of ways. Batteries and chargers are used in many of these methods as their system components along with solar panels. The most common types of usage of solar electricity are stand-alone solar system with or without backup and grid-tied system.
Stand-alone solar system with backup
The stand-alone system is an excellent system for providing electricity from solar power economically. This type of system consists of a number of solar modules, batteries, chargers and an inverter as the main system components. Solar panels charge the battery during daytime and the battery supplies the power to the inverter as needed. The inverter converts the DC voltage to AC 120V/220V as required to suit the appliances. In some applications, DC is used directly without an inverter, in which case normal household appliances and equipment cannot be used, and special DC operated equipment is required. The charger’s role is to keep the battery charged and prevent it from overcharging. Some chargers are equipped with MPPT capability which can charge the battery with better utilization of solar power. The battery capacity, panel wattage, battery depth of discharge (DOD), inverter capacity and charger size are the key factors of design in a standalone solar system. These parameters are designed on the basis of daily energy usage, peak power requirements, autonomy of operation required and insolation of the area.
Figure 3: Stand-alone solar electric system with backup
Stand-alone solar electricity systems are found in RV power, portable power systems and cabin lighting, etc. Most of the underdeveloped countries where electricity is not available in rural areas use these systems for home lighting. Such a typical home lighting unit consists of a solar panel up to 150W, 12V lead acid battery, and a charge-cum load controller and required number of LED or CFL lights. Here direct DC is used for the lights. The controller keeps tracks of the charging as well as discharging of the battery.
Stand-alone solar system without backup
Another combination of a stand-alone system is without backup battery. In this case, the charger and the battery are absent in the system. The system consists of only a solar module and an inverter. As the sun shines, the inverter starts working; but as sun sets or sun is covered by clouds, the inverter stops working and the power is down.
Figure 4: A typical stand-alone solar system without backup
A typical application of this kind of system is in remote and rural areas for underground water pumping and irrigation. The inverter is designed to operate pump motors. As long as sunlight is available, the pumps are running and water is flowing.
Here, total water throughput in a day and insolation are key parameters in the system design. Based on these parameters, the panel size, inverter size and pump specified are determined.