Ocean thermal gradient: Impact on nature
No fuel burned, but huge volumes of seawater moved
What this tab covers What taking energy from this source does to land, water, air and climate, wildlife and ecosystems, with life-cycle emissions and land use where measured.No fuel and no combustion, but large volumes of pumped seawater disturb marine life and change local water temperature and nutrients; effects of a large fleet on ocean circulation are uncertain.
Life-cycle emissions 71.5 g CO2e/kWh7 Single life-cycle study for a 1 MW hybrid OTEC plant in Malaysia (prediction); about 85% below a 3 kW pilot. No harmonised range exists.Effects on nature
| Area | Effect | Size | How it is reduced |
|---|---|---|---|
| Marine life (intakes) | Organisms are trapped against intake screens (impingement) or drawn through the plant (entrainment); both can be lethal.8 | Not quantified at commercial scale | Screens on both warm and cold water intakes |
| Water quality (discharge) | Effluent is cooler, denser and richer in nitrogen and phosphorus than surface water, which can trigger algal blooms.1 | Discharge at 70 m or deeper leaves the top 40 m unaffected | Deep discharge; separate warm and cold outfalls |
| Chemicals | The warm-water side needs a biocide such as chlorine to keep heat exchangers clean.8 | Biocide expected below US Clean Water Act limits | Dose control |
| Ocean and climate | Very large deployment would cool the tropical surface and could strengthen the thermohaline circulation.2 | Little effect up to about 7 TW; marked effects near 30 TW | Limit total deployment |
| Noise | Pumps and turbines may disturb marine mammals.8 | Not quantified | Not specified; careful siting |
OTEC burns nothing, so it emits no combustion gases in operation; its life-cycle emissions come mainly from building pipes, heat exchangers and platforms, and the only published estimate for a megawatt plant (71.5 g CO2e/kWh) is a model result rather than a measurement.
The real environmental question is water. A 10 MW plant moves about 100,000 m3 per hour (IRENA), and the effects of discharging that much cold, nutrient-rich water into nutrient-poor tropical seas have not been observed at scale. Deep, separate discharge limits the effect on the surface layer, and the same deep water can be reused for cooling, aquaculture and agriculture, as on Kumejima.
Sources
- Ocean Thermal Energy Conversion: Technology Brief IRENA, 2014
- Estimates of global Ocean Thermal Energy Conversion (OTEC) resources using an ocean general circulation model (Rajagopalan and Nihous, Renewable Energy 50) Elsevier, 2013
- Prediction of emission intensity for 1 MW onshore H-OTEC in Malaysia (Phua et al., Grand Renewable Energy proceedings) J-STAGE, 2022
- Ocean Thermal Energy Conversion (OTEC) Environmental Impacts (fact sheet based on NOAA material) PNNL Tethys