# OTEC and salinity power: Efficiency

Why a 20 °C gap caps OTEC at about 7%

Source page: https://www.materialinsights.org/energy/e_otec/efficiency/

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 service

#### Efficiency by measure

| Measure | Range | Type | What is measured |
|---|---|---|---|
| **OTEC Carnot limit** | 7%^[1](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2014/Ocean_Thermal_Energy_V4_web.pdf) | 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](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2014/Ocean_Thermal_Energy_V4_web.pdf) | Commercial | Share of gross electrical output used to pump warm and cold seawater |
| **Early pilot (Japan/Nauru, 120 kW)** | 90%^[1](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2014/Ocean_Thermal_Energy_V4_web.pdf) | In service | Share of output consumed by pumping and plant operation |
| **Salinity mixing energy (theoretical)** | 1.4 to 15 MJ per m3^[2](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2014/Salinity_Energy_v4_WEB.pdf) | 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/m2^[2](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2014/Salinity_Energy_v4_WEB.pdf) | Commercial | Maximum net power per square metre of membrane |
| **PRO membranes, laboratory** | 4.4 to 16 W/m2^[2](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2014/Salinity_Energy_v4_WEB.pdf) | 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](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2014/Salinity_Energy_v4_WEB.pdf) | 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](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2014/Ocean_Thermal_Energy_V4_web.pdf) |
| **Seawater pumping** | 20-30% of gross output^[1](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2014/Ocean_Thermal_Energy_V4_web.pdf) |
| **Salinity pre-treatment and pumping** | large but not quantified^[2](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2014/Salinity_Energy_v4_WEB.pdf) |

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

1. [Ocean Thermal Energy Conversion: Technology Brief](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2014/Ocean_Thermal_Energy_V4_web.pdf) IRENA, 2014
2. [Salinity Gradient Energy: Technology Brief](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2014/Salinity_Energy_v4_WEB.pdf) IRENA, 2014

## Other sections

- [Summary](https://www.materialinsights.org/energy/e_otec/index.md)
- [Overview](https://www.materialinsights.org/energy/e_otec/overview/index.md)
- [Correlation](https://www.materialinsights.org/energy/e_otec/correlation/index.md)
- [Techniques](https://www.materialinsights.org/energy/e_otec/techniques/index.md)
- [Resources and labour](https://www.materialinsights.org/energy/e_otec/resources/index.md)
- [Impact on the biosphere](https://www.materialinsights.org/energy/e_otec/biosphere/index.md)
- [Relationships](https://www.materialinsights.org/energy/e_otec/links/index.md)