To compensate the poor quality of grid supply (low voltage, fluctuating frequency and frequent interruptions), high tariffs (much higher than actual cost of supply), unfair impositions (peak hour restrictions and unplanned load shedding) and unresponsive attitude of State Electricity Boards, many industries were forced to isolate themselves totally from the state grid and be on their own. For a reliable operation of the industry, they necessarily have to employ captive generation with a redundancy.
Captive or stand-alone PV systems are designed to operate independently from the electric utility grid, and are generally designed and sized to supply certain DC and/or AC electrical loads. Worldwide, stand alone solar installations are very popular while in India almost all captive power plants are of the grid-tie. It is often a good idea to start with small and very simple stand alone solar PV system first and then progress from there.
Direct-coupled system is the simplest type of stand-alone PV system, with the DC output of a PV module or array being directly connected to a DC load. Due to no electrical energy storage (batteries) in direct-coupled systems, the load only operates during sunlight hours, making these designs suitable for common applications such as ventilation fans, water pumps, and small circulation pumps for solar thermal water heating systems.
The “Small stand-alone” system is an excellent system for providing electricity economically.These systems are used primarily for RV power, lighting, cabins, back up and portable power systems. The size of the photovoltaic array (number of solar panels) and battery will depend upon individual power requirements. The solar panels charge the battery during daylight hours and the battery supplies power to the inverter as needed. The inverter changes the 12 volt batteries DC power into 230V volt AC power, which is the most useful type of current for most applications. The charge controller terminates the charging when the battery reaches full charge, to keep the batteries from “gassing-out”, which prolongs battery longevity.