 Solution

Hydropower can convert a renewable water flux efficiently, has low operating fuel requirements, and reservoir schemes can provide rapid dispatch, inertia and storage. Long plant lifetimes can make capital expenditure worthwhile, while pumped storage shifts energy rather than producing a new primary resource.

Limitations include finite favorable sites, seasonal drought, sedimentation, habitat fragmentation, blocked fish migration, displaced populations and major civil-engineering/failure risks. Reservoir evaporation and methane emissions can matter, especially in warm flooded landscapes, so life-cycle emissions are not uniformly zero. Run-of-river plants avoid some reservoir impacts but offer less controllable output. A project assessment must compare useful annual energy and firm capacity, ecological and social costs, and the counterfactual electricity system.