There are mainly four types of solar power stations.
- Stand Alone or Off Grid type Solar Power Plant
- Grid Tie type Solar Power Plant
- Grid Tie with Power Backup or Grid Interactive type Solar Power Plant
- Grid Fallback type Solar Power Plant.
Let us discuss a brief introduction of each type of solar power plant.
Stand Alone or Off Grid Solar Power Station
This is most commonly used photo-voltaic installation used to provide localized electricity in absence of conventional source of electric power at certain location. As the name prefers this system does not keep any direct or indirect connection with any grid type network.
In standalone system the solar modules produce electric energy which is utilized to charge a storage battery and this battery delivers electricity to the connected load. Standalone systems are normally small system with less than 1 kilo watt generation capacity.
Grid Tie Solar Power Station
In some countries facility is available of selling power to the local or national grid. This is gaining popularity in Europe and the United States. This system facilitates both electric utility companies as well as the consumers. Here consumers can generate electricity by their own plant and can sell the surplus to the electricity utility company through grid connected to their plant. As the consumers sell the power they can earn money as return of their investment for installation of captive power plant on the other hand electric utility companies can reduce their capital investment on their own plant for power generation. In a grid-tie solar system, consumers consume electricity produced by solar captive power plant during sunny day time and also export surplus energy to grid but at night while solar plant does not produce energy, they import electric energy from grid for consumption. The main disadvantage of this system is that if there is a power cut in the grid, the solar modules should be disconnected from the grid. This system is not always very profitable especially where overall maximum demand of the system does not occur at the peak sunny period of the day. In hot climate where the power demand for air conditioning machines becomes maximum during peak sunny period of the day, this grid tie solar power generation system works most efficiently.
Grid tie solar systems are of two types one with single macro central inverter and other with multiple micro inverters. In the former type of solar system, the solar panels as well as grid supply are connected to a common central inverter called grid tie inverter as shown below. The inverter here converts the DC of the solar panel to grid level AC and then feeds to the grid as well as the consumer’s distribution panel depending upon the instant demand of the systems. Here grid-tie inverter also monitors the power being supplied from the grid. If it finds any power cut in the grid, it actuates switching system of the solar system to disconnect it from the grid to ensure no solar electricity can be fed back to the grid during power cut. There is on energy meter connected in the main grid supply line to record the energy export to the grid and energy import from the grid.
As we already told there is another type of grid-tie system where multiple micro-inverters are used. Here one micro inverter is connected for each individual solar module. The basic block diagram of this system is very similar to previous one except the micro inverters are connected together to produce desired high AC voltage. In previous case the low direct voltage of solar panels is first converted to alternating voltage then it is transformed to high alternating voltage by transformation action in the inverter itself but in this case the individual alternating output voltage of micro inverters are added together to produce high alternating voltage.
Grid Tie with Power Backup Solar Power Generation
It is also called grid interactive system. This is a combination of a grid-tie solar power generation unit and storage battery bank. As we said, the main drawback of grid tie system is that when there is any power cut in the grid the solar module is disconnected from the system. For avoiding discontinuity of supply during power cut period one battery bank of sufficient capacity can be connected with the system as power backup.
Grid Fallback Solar Power Generation
Grid fallback is most reliable and stable system mainly used for electrifying smaller households. Here solar modules charge a battery bank which in turn supplies distribution boards through an inverter. When the batteries are discharged to a pre-specified level, the system automatically switches back to the grid power supply. The solar modules then recharge the batteries and after the batteries are being charged up to a pre-specified level again the system switches back to solar power. We do not sell electricity back to the electricity utility companies through this system. All the power that we produce is utilized for ourselves only.
Although we do not have any direct earning benefit from this system but the system has its own big advantages. This system is most popular where there is no facility of selling power to the grid.
Grid fallback system has all advantages of grid interactive system except power selling, but it adds benefit of using own power whenever it is required irrespective of position and condition of sun in the sky.