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 have forced many industries 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.
There are three distinct configurations one can choose from when setting up solar based captive power plan, which are standalone, grid tied (grid-interactive only (no battery backup) and grid-interactive with battery backup) and hybrid systems. The most prominent technology option in India is the grid tied system, and one of emerging hybrid systems is the solar PV-diesel hybrid. Each of the above mentioned categories has a different connotation, and would need to be treated accordingly, for effectively bringing the surplus capacity into the utilities’ network.
Stand-alone PV systems are designed to operate independent of 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.
A grid-tied solar power system (also referred to as grid-intertied, or on-grid or utility-interactive (UI)) produces solar electricity that is fed directly into the utility grid, hence the term grid-tied, as the system is tied, literally, to the grid. In US and Europe, in a grid tied system any surplus power generated is sold back to the power utilities. When the sun is not shining one can buy power back from the utilities. This is done using net metering; a method of metering the energy consumed and produced using a renewable energy generator.
And finally, systems with more than one source of power are called hybrid systems. It is often desirable to design asystem with additional source of power. The most common type of hybrid system contains a gas or diesel powered engine generator. Another hybrid approach is a PV/Wind system. Adding a wind turbine to a PV system provides complementary power generation.
Selecting the best system design for a particular application is important. Often, a combination of techniques can dramatically reduce costs and improve the reliability of a system, or provide the design flexibility to cover a wide range of applications.