Table 2 Summary of potential drivers of long-term hydropower generation and capacity factor decline
From: Hydropower capacity factors trending down in the United States
Driver | Description | Mechanism for reducing annual CF |
|---|---|---|
External environmental phenomena | ||
Decline in annual water availability | Decline in inflow to reservoirs caused by climate change or upstream catchment process change | Reduced water available for release to penstock; reduced reservoir head levels |
Shift in the seasonal flow signal | Change in the timing of water inflows to the reservoir, with a stronger flood season | Stronger flood leads to increased spills and thus a smaller proportion of water used for power generation |
Enhanced reservoir surface evaporation | Warming temperatures promote an increased loss of water from storage | Reduced water availability for generation |
Sedimentation | Accumulation of sediments in hydropower reservoirs | Loss of water storage capacity, leading to greater likelihood of spill periods |
Infrastructural change | ||
Wear-and-tear | Deterioration of power-generating equipment or dam infrastructure | More frequent outages and maintenance (planned or unplanned) and lower efficiency |
Capacity additions | Nameplate capacity increases with new unit installations | Planned capacity additions may be associated with an expected decrease in capacity factor if water is constrained |
Dam operational change relating to evolving river and reservoir needs | ||
Change in reservoir storage rule curves | Lowering target water levels as flood season approaches | Lower reservoir levels lead to reduced hydraulic head and thus less output (MWh) per unit turbined water (m3) |
Fish passage regulations | Requirements of non-turbined water release to promote fish passage | Decreasing proportion of water passing through turbines for generation |
General non-power operational changes | Changes to release, such as for navigable waters, water quality control (temperature, dissolved oxygen), and water supply for downstream municipal, industrial, or agricultural needs. | Several possible mechanisms, including increased non-powered spill, and loss of water for generation due to diversions directly from the reservoir |
Dam operational change relating to evolving power grid needs | ||
Emerging need for ramping capabilities | Hydropower plants increasingly used for their flexibility in balancing variable wind and solar generation | Reduced turbine efficiency of generation with ramping behavior; lower overall powered release |
Emergence of other must-take technologies | Increased deployment of wind and solar technologies with zero marginal production cost | Reservoir releases are non-powered if grid demands are satisfied by renewables |
