Salinity gradient: Fluctuation in time
Following the river's seasons, floods and droughts
Following the river's seasons, floods and droughts What this tab covers How availability changes over seconds, days, seasons, years and longer, how predictable it is, and what that means for the plants that use it.Follows river discharge, so it varies with the seasons and with droughts and floods, but changes slowly and predictably compared with sun or wind.
Variation by time scale
| Time scale | What happens | How large |
|---|---|---|
| Seasonal | Global extractable potential tracks river flow, which is dominated by Northern Hemisphere rivers; highest from May to September.3 | Monthly variability resolved for 1,078 rivers; no single global swing figure published |
| Within a site (environmental limits) | Only part of a river's flow can be diverted without harming the estuary, so output is capped in low-flow months.2 | Practical potential about 4% of theoretical (625 of 15,102 TWh/yr) |
| Hybrid plants | Plants using concentrated brine (for example from desalination) decouple output from river flow and raise energy per cubic metre, up to about tenfold with saturated brine.1 | up to 15 MJ vs 1.4 MJ per m3 mixed |
How predictable it is
High on daily to monthly scales, because river discharge is gauged and changes gradually; lower year to year, when droughts reduce fresh-water flow.
Capacity factors and utilisation
| Plant or measure | Value |
|---|---|
| Statkraft Tofte prototype: actual vs technical output | about 50% (about 5 kW of 10 kW)1 |
A salinity plant has the same timetable as the river that feeds it: snowmelt and monsoon seasons raise output, dry seasons and droughts cut it. Because large rivers integrate rain over huge basins, flow changes over days and weeks rather than minutes, which makes output easier to forecast than wind or solar.
No commercial plant has run long enough to publish a capacity factor. The Norwegian prototype delivered about half its technical rating before Statkraft ended its osmotic power investment at the end of 2013.
Sources
- Salinity Gradient Energy: Technology Brief IRENA, 2014
- Practical global salinity gradient energy potential (Alvarez-Silva, Osorio and Winter, Renewable and Sustainable Energy Reviews 60) Elsevier, 2016
- Extended assessment of the globally extractable salinity gradient energy from river mouths (Alvarez-Silva, Roldan-Carvajal and Arevalo, Sustainable Energy Technologies and Assessments 91) Elsevier (via PNNL Tethys), 2026