Hydropower representation in energy system models: Aggregation, operational constraints and flexibility

Hydropower plants provide crucial flexibility in energy systems: their ability to change their electricity output quickly enables them to balance the varying production of weather-dependent sources, such as wind and solar. This capability makes them invaluable in systems with increasing shares of variable renewable energy sources. Large-scale energy system models are important tools for analysing future scenarios, identifying potential problems, and testing solutions. Due to model scale and data availability, hydropower is often highly simplified. However, this can overestimate hydropower’s flexibility for balancing, distorting the assessment of future energy systems.

This dissertation analysed hydropower representation in an energy system optimisation model at the country- and continent-level. For the Finnish energy system, two case studies were implemented: Oulujoki and Kemijoki. Both rivers represent heavily regulated catchments with many hydropower plants, where an upstream plant’s outflow impacts a downstream plant’s inflow. Implementing such cascading systems so that one unit represents the whole cascade hides critical dynamics. The study highlights mismatches in system costs and emissions, as well as misrepresentations of reservoir volumes, spillages and ramping capabilities.

Furthermore, the application of artificial operational constraints was analysed in two different contexts: (a) simulating environmental flow constraints in cascade hydropower systems and (b) regulating aggregated hydropower in large-scale systems without the need to add plant-level detail. Environmental flow constraints are measures to prevent negative impacts of hydropower operation on riverine ecosystems. Their addition reduces harmful operational practices, such as fast output changes, but lowers revenues for hydropower operators. When combined with batteries or hydrogen storage, economic losses can be reduced. In addition, applying historically observed operating ranges to highly simplified hydropower setups in a continent-level energy system model decreases overestimated ramping rates and capacities. This directly impacts modelled cross-border transmission, system costs, utilisation of electricity storage and emissions.

Different types of energy system models and hydropower representations have established uses, but known biases need to be addressed to ensure more reliable estimates of future scenarios and necessary solutions.