Improving concentrating solar power plant performance by increasing steam turbine flexibility at start-up

Among concentrating solar power technologies, solar tower power plants currently represent one of the most promising ones. Direct steam generation systems, in particular, eliminate the usage of heat transfer fluids allowing for the power block to be run at greater operating temperatures and therefore further increasing the thermal efficiency of the power cycle. On the other hand, the current state of the art of these systems does not comprise thermal energy storage. The lack of storage adds to the already existing variability of operating conditions that all concentrating power plants endure due to the fluctuating nature of the solar supply. One way of improving this situation is increasing the operating flexibility of power block components to better adapt to the varying levels of solar irradiance. In particular, it is desirable for the plant to achieve fast start-up times in order to be available to harness as much solar energy as possible. However, the start-up speed of the whole plant is limited by the thermal inertia of certain key components, one of which is the steam turbine. This paper studies the potential for power plant performance improvement through the increase of steam turbine flexibility at the time of start-up. This has been quantified by carrying out power plant techno-economic studies in connection with steam turbine thereto-mechanic behavior analysis. Different turbine flexibility investigations involving the use of retrofitting measures to keep the turbine warmer during offline periods or changing the operating map of the turbine have been tested through multi-objective optimization considering annual power performance and operating costs. Results show that reductions of up to 11% on the levelized cost of electricity are possible through the implementation of these measures.