Solar thermal hybrid applications have the potential to be an important, low-cost option for adding green power to the existing coal based generation fleet, allowing utilities to meet renewable energy targets, reduce plant emissions and lower fuel costs. The hybrid system will inject solar-generated steam / water into the TG cycle of the fossil plants, resulting in a lower reliance on fossil fuels. This approach solves the problem of fluctuations in output associated with solar power and hence maintains the reliability of supply to the grid. Solar energy can be used to either increase power or reduce fuel consumption. Further, Incremental cost increases in steam cycle equipment are smaller than that for standalone solar facility.
Integration of solar thermal plant with 210 MWe coal fired unit of NTPC Dadri (Unit #4) is based on the aforesaid concept. In this hybrid plant, controlled quantity of feed water is tapped from inlet of HP Heater-6, passed through Solar Heat exchanger and again looped back at the exit of HP Heater-6. This shall reduce the extra steam extraction from HP turbine and will help to reduce coal consumption or produce extra units of electrical energy by the plant.
TECHNOLOGY BRIEF
Linear Fresnel mirrors has been installed in solar field which generates pressurized hot water at 80 bar & 280°C. This hot water passes through Solar heat exchanger where it dumps its heat. Solar heater is placed parallel to existing HP Heater #6 (figure 1). A fraction of feed water is made to pass through the solar heater and gets heated up. The control logics ensure that the exit temperature of the feed water at solar heater matches with the feed water exit temperature of HP Heater #6 within close limits of ±2°C. This temperature control is achieved by adjusting the feed water flow through the solar heater. In this process certain amount of steam which was being extracted to heat the entire feed water in HP Heater gets saved and goes on to do additional work in turbine.
