Ocean thermal gradient: Fluctuation in time
One of the steadiest renewable flows there is
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.One of the steadiest renewable flows: the temperature gap changes slowly with seasons and ocean cycles, so plants can run as baseload.
Variation by time scale
| Time scale | What happens | How large |
|---|---|---|
| Seconds to hours | Essentially steady; internal tides near Hawaii move isotherms up and down by 50-100 m.3 | 5-10% output swing over a tidal cycle |
| Seasonal | Surface water is warmer in late summer; the deep water hardly changes.3 | about 20% variation in annual output in Hawaii (1,000 m pipe) |
| Year-to-year | El Niño and La Niña shift surface temperatures in the central tropical Pacific.3 | up to 50% in that region |
| Year-to-year (Pacific islands) | Equatorial and southern islands show the highest inter-annual variability, linked to ENSO.4 | No single swing figure; modelled 2022 output of a 1 MW plant ranged from 7 to 8 GWh across 18 island sites |
How predictable it is
High: sea temperature profiles are well observed and change slowly, so output can be forecast weeks to months ahead; ENSO forecasts give some warning of year-to-year swings.
Capacity factors and utilisation
| Plant or measure | Value |
|---|---|
| OTEC plants (IRENA estimate) | 90-95%1 |
| Modelled 1 MW plants at 18 Pacific island sites, 2022 (7-8 GWh, calculated) | about 80-91%4 |
Unlike sun and wind, the ocean thermal resource has no night and almost no weather. The warm surface layer is a vast heat store and the deep water is fed by a circulation that turns over on a time scale of centuries, so the temperature gap moves only with the seasons and with ocean cycles such as ENSO.
That steadiness is OTEC's main selling point: IRENA puts plant capacity factors at about 90-95%, comparable with nuclear plants. The main uncertainties are year-to-year swings linked to El Niño in the central Pacific and the long-term question of whether large deployments would themselves erode the temperature gap.
Sources
- Ocean Thermal Energy Conversion: Technology Brief IRENA, 2014
- An Evaluation of the U.S. Department of Energy's Marine and Hydrokinetic Resource Assessments, chapter on ocean thermal energy National Academies Press, 2013
- Spatial and Temporal Variability of Ocean Thermal Energy Resource of the Pacific Islands (Posterari et al., Energies 17) MDPI (via PNNL Tethys), 2024