CFD investigation of level fluctuations in steam accumulators as thermal storage: A direct steam generation application

Concentrating solar power plants with direct steam generation have the possibility to store the generated energy in thermal form very easily. This is achieved by using two-phase accumulators in which the heat is stored directly in tanks filled with water and steam in a saturated state. The Llo solar power plant, owned by SUNCNIM in France, has nine such tanks connected in parallel. During the unloading of the storage, steam is withdrawn from the accumulators and the operators observe oscillations in the liquid level between the tanks. To ensure that the turbine is never supplied with liquid water, the maximum filling of the tanks is limited to a certain safety level, resulting in a reduction of the storage capacity of the accumulators. The objective of our study is to understand the origin of these oscillations in order to optimize the filling of the accumulators and thus the performance of the power plant. The exploitation of the power plant's operating data allowed us to identify several points that can cause level instability. To study and understand this phenomenon, a computational fluid dynamics modelling was carried out. Several parametric studies were performed in order to select the boundary conditions and the models to be applied to the system. Finally, our study shows that the liquid levels vary mainly due to the pressure imbalance between the tanks resulting from the pressure drops of the turbine feed network. A first solution, simple to implement in the real system, is proposed to mitigate these fluctuations.