Enhancing hydropower flexibility through tailrace hydrokinetic energy recovery: A Water-Energ-Ecosystem Nexus Perspective

Hydropower remains one of the most reliable and flexible renewable energy sources and continues to play a vital role in stabilising electricity systems with growing shares of wind and solar power. Yet, in practice, hydropower operation is increasingly shaped by non-power objectives such as environmental flow requirements, water supply security, flood management, and ecosystem protection. These competing demands, combined with climate-driven hydrological variability and evolving electricity market structures, limit the extent to which hydropower can respond to price signals and support renewable integration. Addressing these challenges calls for holistic, policy-relevant approaches that explicitly recognise the interdependencies between water, energy, and ecosystems.

This study explores how hybridising conventional reservoir-based hydropower with downstream hydrokinetic energy recovery can enhance operational flexibility without compromising water-resource or environmental constraints. A nonlinear optimisation framework is developed to co-ordinate hydropower generation, tailrace hydrokinetic extraction, and grid interaction under time-of-use electricity tariffs. The model explicitly represents reservoir dynamics, climatic drivers (inflow, precipitation, evaporation), and mandatory environmental flow releases, while capturing the site-specific relationship between hydropower discharge and tailrace flow velocity. A rolling-horizon formulation is adopted to reflect short-term operational planning and evolving hydrological conditions.

The approach is demonstrated using an existing hydropower plant in southern Poland, where limited hydrokinetic recovery (approximately 3% of main discharge) can be achieved without affecting upstream hydraulic performance or ecological flow regimes. Results show that coordinated operation improves reservoir stability, reduces reliance on peak-period grid imports, and lowers annual operational energy costs by 1.81% compared to conventional operation. Over the plant lifetime, the hybrid configuration yields a 3.95% reduction in total costs with a break-even period of approximately 3.7 years. Sensitivity analyses highlight electricity pricing and financial parameters as stronger drivers of system performance than hydrokinetic capital costs.

Overall, the study demonstrates that hybrid hydropower-hydrokinetic systems offer a practical and policy-compatible pathway to strengthen the water-energy-ecosystem nexus, enhance climate resilience, and unlock additional flexibility from existing hydropower infrastructure in low-carbon electricity transitions.