Hanna Ek Fälth, Physical Resource Theory

Hydropower is the largest carbon-neutral electricity source worldwide and a key flexibility resource in electricity systems, capable of adjusting its output across time scales from seconds to years. Yet its operational capabilities are constrained by, among other factors, the physics of how water flows through networks of dams and reservoirs and the technical characteristics of turbines. Moreover, hydropower operations affect river ecosystems, creating a tension between decarbonization and the protection of aquatic biodiversity. Understanding what hydropower can realistically deliver requires both improved modeling and analysis of these limits and their consequences.

This thesis aims to advance both the modeling approaches and substantive understanding of hydropower in electricity systems. To situate this work, the thesis first examines how another flexibility strategy, potential transmission expansion, affects the cost of low-carbon electricity systems. It then turns to hydropower, addressing three research questions: How does hydropower modeling detail affect the realism of modeled operations, and does the choice of representation affect electricity system model results such as costs and electricity prices? How do physical, technical and regulatory constraints affect hydropower's operational capabilities? And what are the electricity system implications of regulatory constraints on hydropower? While the modeling approaches and insights are general, the case study applications focus on Swedish hydropower within the Nordic and Northern European electricity system.

The thesis builds on four appended papers as well as additional analyses. New models are developed that represent individual hydropower plants at high technical detail, both standalone and embedded within an electricity system dispatch model. Using these models, the representation of hydropower in energy system models is scrutinized by comparing formulations ranging from fully aggregated national representations to individual turbines, showing that the choice of modeling approach is consequential for the results obtained. Hydropower's ability to sustain high output during prolonged periods of high demand is quantified under real physical, technical and regulatory constraints. The effects of environmental regulations on Swedish hydropower, including minimum flow requirements and restrictions on rapid flow changes, are assessed both in terms of hydropower operations and electricity system costs. The findings are discussed in light of ongoing regulatory processes for environmental adaptation of hydropower in Sweden.