OTEC and salinity power: Efficiency
Why a 20 °C gap caps OTEC at about 7%
What this tab covers How much of the energy available is actually captured or extracted, in per cent: theoretical limits, laboratory records, typical new equipment and in-service averages, each with exactly what is being measured.OTEC works across a temperature difference of only about 20 °C, so even a perfect heat engine could turn at most about 7% of the heat into work, and pumping the huge water flows eats 20-30% of gross output. Salinity power is limited instead by how much power each square metre of membrane delivers.
Chart: Efficiency ranges
Theoretical limit Commercial In serviceEfficiency by measure
| Measure | Range | Type | What is measured |
|---|---|---|---|
| OTEC Carnot limit | 7%1 | Theoretical limit | Heat taken from warm surface water to work, for about 25 °C surface and 4 °C deep water |
| OTEC pumping (parasitic) load | 20 to 30%1 | Commercial | Share of gross electrical output used to pump warm and cold seawater |
| Early pilot (Japan/Nauru, 120 kW) | 90%1 | In service | Share of output consumed by pumping and plant operation |
| Salinity mixing energy (theoretical) | 1.4 to 15 MJ per m32 | Theoretical limit | Free energy released mixing 1 m3 river water with 1 m3 seawater (1.4) or with saturated brine (about 15) |
| PRO membranes, commercial | 2.7 W/m22 | Commercial | Maximum net power per square metre of membrane |
| PRO membranes, laboratory | 4.4 to 16 W/m22 | Record | Hollow-fibre tests 4.4-16 W/m2; best net PRO lab result 14.4 W/m2 |
| Statkraft Tofte PRO pilot, output against rating | 50%2 | In service | Actual output (about 5 kW) against rated 10 kW |
Where the energy is lost
| Step | Loss |
|---|---|
| Second-law (Carnot) limit of a small temperature difference | at least 93% of heat rejected (calculated)1 |
| Seawater pumping | 20-30% of gross output1 |
| Salinity pre-treatment and pumping | large but not quantified2 |
OTEC's low efficiency matters less than it seems because the heat is free and available day and night; what it drives is size and cost, since a plant must move enormous volumes of water for each kilowatt.
Sources
- Ocean Thermal Energy Conversion: Technology Brief IRENA, 2014
- Salinity Gradient Energy: Technology Brief IRENA, 2014