Capacity optimization of existing reservoir hydropower expansion and its impact on power system flexibility

Expanding the capacity of an existing reservoir hydropower plant, with the primary aim of enhancing its flexibility, poses a complex optimization problem that need consider current operational constraints. This study's focus on hydropower capacity expansion to enhance flexibility contrasts with the traditional goal of primarily promoting electricity generation. To address this challenge, a novel Capacity Expansion Optimization Model (CEOM) is proposed. This model is designed to estimate the optimal expansion capacity of existing hydropower infrastructure, ultimately maximizing the flexibility that hydropower can provide to the power system. This study explores the true potential of existing reservoir hydropower flexibility. Based on an analysis of the principles and characteristics of hydropower flexibility, a mathematical expression is developed to more accurately represent the upward flexibility of hydropower. The CEOM integrates reservoir operation simulation and flexibility mathematical expression for application in the secondary design stage of existing hydropower. Two regimes of the CEOM are considered: one focusing solely on the optimal design for expanding the capacity of a single reservoir hydropower plant, and the other considering the joint expansion of cascade hydropower, taking into account hydraulic connections. The proposed CEOM is validated on two real cases: Goupitan hydropower with an annually regulated reservoir and Silin hydropower with a daily or weekly regulated reservoir along the Wujiang River in Guizhou, China. The models are solved using Particle Swarm Optimization (PSO). The results indicate that annually regulated hydropower provides an average of 0.80 units of upward flexibility per unit of capacity expansion, while daily or weekly regulated hydropower provides a corresponding upward flexibility of 0.59 units. Jointly expanding cascade hydropower offers a broader range of upward flexibility compared to individual hydropower expansion. Furthermore, impact analysis suggests that the optimal upward flexibility provided by annually regulated hydropower capacity expansion increases sharply with reservoir inflow due to larger reservoir storage. However, the optimal upward flexibility provided by daily or weekly regulated hydropower capacity expansion does not increase with inflow. This implies that, for daily or weekly regulated hydropower, the limiting factor of flexibility provided by capacity expansion comes from the size of the reservoir capacity. Additionally, this paper highlights the significant untapped potential of existing conventional hydropower and aims to provide guidance for scientific and engineering organizations on the optimal utilization of capacity expansion.