Solar PV-DG Hybrid Systems; Key challanges in integration
India has about 7,000 MW of diesel generating capacity in MW class, and has over 2,50,000 mobile telecom towers and about 5 million diesel based agricultural pump sets, which are a fit case for the solar PV DG hybrid opportunity which is not yet tapped. While the cost of off-grid solar PV has also reached to about 100 Rs/Wp, integrating solar PV with the captive DG sets can save significant amount of diesel and can become a cost effective solution for the country.
Considering the fact that solar PV electricity is costing half the cost of the DG electricity, many large consumers have already started adopting the solar PV DG hybrid systems. There are many companies which are ready to take up the task of diesel abatement through installation of solar pv systems in tandem with the existing DG sets under the RESCO model. The National Solar Mission has also targeted the off-grid solar power generation and capital subsidy is available in the smaller systems. Apart from the capital subsidy on the off-grid systems, there are additional benefits such as accelerated depreciation which makes it attractive to the companies opting for the solar PV DG hybrid generation.
While the solar PV-DG hybrid systems are at nascent stage of their development, there are few technical challenges which are faced by the developers in integrating the solar PV with the existing off-grid captive power generating DG sets. These challenges are as follows;
While the consumer load is variable in nature the integration of solar PV with the DG system need to handle the sudden spikes and drops in demand due to load variation. Apart from the demand variation, there is a variability in the solar PV component due to sudden clouds which leads to some advanced control to integrate the solar PV with the DG sets so that it can take care of this variability in the demand and supply. While working with the DG sets, solar PV is considered as a negative load and the variability in the solar PV generation and variability in the electricity demand leads to sudden change in the rotation speed of the DG set which in turn changes the frequency of the electricity being supplied to the loads. The controls on the DG sets needs to sense this change the frequency and accordingly change the fuel supply to match the electricity demand with the generation. The major challenge is to manage the ramp up and ramp down rates of the DG set. Maintaining the electricity frequency in a narrow band is also important because most of the electrical loads also operate in a narrow frequency band.
Apart from the frequency there is a requirement of controlling the voltage as most of the loads attached to the system also operate in a narrow voltage range. Voltages on the grid outside of these limits reduce efficiency and could cause significant damage. Diesel generator control systems are designed to monitor and control both frequency and voltage to ensure safe and efficient grid operations. In this manner diesel generators are able to provide power quantity, matching supply to demand, and power quality, maintaining frequency and voltage.
In order to handle the variable loads as well as the variable electricity supply through solar PV, it is important to have a spinning reserve. For this purpose one or more DG sets are kept in a standby mode with no power output. or more smaller Dg sets are kept running in a standby mode with no power output. Although the fuel to keep this generators running is “wasted” because no electricity is being produced it means the full production capacity of the generator is available as spinning reserves that can be deployed if demand suddenly increase or in the case of an accident that damages one of the generators providing power to the grid. In some cases there is a battery bank which act as a spinning reserve to handle the variability of the electricity supply from the solar PV.