The role of run-of-river hydropower dispatchability in power system flexibility

Abstract

To meet emission targets set by the European Commission for 2050, the European power sector needs to reduce its emissions by 95-100% compared to 1990 levels. These targets started a transition towards a low-carbon power system, in which intermittent renewable energy sources (iRES) play an increasingly important role. iRES are non-dispatchable and not fully predictable and as the penetration of these sources increases, so will the need for flexible generation by conventional thermal generators and dispatchable RES. An important source of flexibility is hydropower, because of its fast ramping abilities and its storage availabilities when reservoirs are present. The level of flexibility hydropower can provide depends on the type of plant. One of these types is run-of-river (RoR) hydropower, which is known to have limited storage capacity and is thus mainly dependent on the inflow from the river. However, it is unclear how much storage capacity European RoR plants have and how dispatchable they really are. Therefore this research investigates the dispatchability of European RoR hydropower capacity and evaluates its role in providing power system flexibility. The dispatchability of RoR plants is assessed based on the natural inflows they receive and the storage capacity the plants have. A database is constructed containing technical and hydrological details of 126 plants. Subsequently, the impact of RoR dispatchability on the European power system is analysed by modelling these plants with the detailed data in a low-carbon power system model. This detailed approach is compared to an aggregated modelling approach, where RoR plants are aggregated per geographical region. Additionally we study the impact of annual variation of water availability for RoR generation on the power system by modelling an average, a dry and a wet inflow scenario. Lastly, by means of interviews with RoR operators and experts we investigate if there are important aspects, other than water inflow and storage size, affecting the dispatchability of RoR plants. For 84 of the 126 plants in the database, data on storage size is found. Of the 84 plants, 28 plants can be classified as pure RoR plants (less than two hours of storage) and the remaining 56 plants can be classified as pondage RoR plants (more than two but less than 400 hours of storage). On average, natural inflow for RoR plants is highest during spring and early summer but there is a considerable variation in inflow patterns between regions due to the geographical diversity of river regimes. Modelling these aspects in a low-carbon power system shows that the flexibility of RoR plants is overestimated when using an aggregated approach. As a result of the limited flexibility of RoR plants, flexible generation has to be provided by more expensive flexible generators such as biothermal and GT capacity. In an aggregated approach, total generation costs tend to be underestimated by 4%. An analysis of the inter-annual variation of the water availability for RoR generation shows that the power system is mainly sensitive to a dry inflow scenario. In such a scenario RoR generation decreases with 28 TWh (-20%) compared to an average inflow scenario, resulting in increased generation costs as the system is more dependent on more expensive generators. A wet inflow scenario mainly results in water spill by the plants due to the limited storage size of the plants and limited turbine capacity. Finally, interview results show that besides storage capacity and inflows, there are a lot of other operational constraints for RoR hydro plants which are specific for each water course. Most of these factors concern controlled river flows and can therefore be included in the power system model by imposing a minimum generation level on the plants. This research shows that the role of RoR plants in providing power system flexibility is limited due to their limited storage size. Policy makers could use this knowledge when preparing the power system for higher iRES penetration towards 2050. Future research on hydropower flexibility can build upon this work by making use of the constructed database of European RoR plants.