# OTEC and salinity power: Techniques

Ammonia turbines, vacuum boilers and ion-sorting membrane stacks

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

What this tab covers The methods used to capture this energy: how each works, and how they compare on efficiency, cost, life-cycle emissions, maturity and scale.

All five methods are at research or small pilot stage, with well under 1 MW installed.

#### Comparison

| Method | Efficiency | Cost | Life-cycle emissions | Maturity | Scale |
|---|---|---|---|---|---|
| **OTEC, closed cycle**^[1](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2014/Ocean_Thermal_Energy_V4_web.pdf)^[4](https://www.power-eng.com/2015/08/31/makai-ocean-engineering-connects-world-s-largest-otec-plant-to-u-s-grid/)^[5](https://www.jstage.jst.go.jp/article/gre/2/0/2_43/_article/-char/en)^[6](https://eos.org/?p=175224)^[12](https://english.ryukyushimpo.jp/2013/06/14/10571/) | Below the 7% Carnot limit, minus 20-30% for pumping | USD 16,400-35,400/kW at 1-10 MW, about 8,430/kW at 50 MW (USD 2010); LCOE about USD 0.07-0.19/kWh for large floating plants | 71.5 g CO2e/kWh predicted (hybrid design, single study) | pilot | Makai, Hawaii, 100 kW (2015); Kumejima, Japan, 50 kW at start-up (2013), later described as about 100 kW |
| **OTEC, open cycle**^[1](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2014/Ocean_Thermal_Energy_V4_web.pdf)^[3](https://tethys-engineering.pnnl.gov/sites/default/files/publications/IRENA_Innovation_Outlook_Ocean_Energy_2020.pdf) | Lower than closed cycle | 50 MW about USD 10,751/kW (USD 2010) | No study found | pilot | Hawaii 1 MW plant ran 1993-1998; a 50 MW design would make 120,000 m3 of fresh water a day |
| **OTEC, hybrid**^[1](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2014/Ocean_Thermal_Energy_V4_web.pdf)^[3](https://tethys-engineering.pnnl.gov/sites/default/files/publications/IRENA_Innovation_Outlook_Ocean_Energy_2020.pdf)^[5](https://www.jstage.jst.go.jp/article/gre/2/0/2_43/_article/-char/en) | Similar to closed cycle | No separate figure | 71.5 g CO2e/kWh predicted (1 MW, Malaysia) | research | Design output about 2.28 million litres of water a day per MW |
| **Pressure-retarded osmosis (PRO)**^[2](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2014/Salinity_Energy_v4_WEB.pdf)^[7](https://sj.jst.go.jp/news/202508/n0825-01n.html) | 2.7 W/m2 commercial membranes | USD 0.15-0.30/kWh projected for 2020; membranes (EUR 10-30/m2) are 50-80% of capital cost | No study found | pilot | Statkraft 10 kW pilot closed 2013-2014; Fukuoka, Japan, plant opened 2025, about 880,000 kWh a year expected |
| **Reverse electrodialysis (RED)**^[2](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2014/Salinity_Energy_v4_WEB.pdf)^[8](https://tethys.pnnl.gov/publications/life-cycle-assessment-salinity-gradient-energy-recovery-using-reverse-electrodialysis) | Lower power density than PRO | USD 0.11-0.20/kWh projected for 2020; 50 kW pilot about EUR 7.33 million | Comparable to other renewables in a 2021 LCA (no single figure) | pilot | REDstack 50 kW pilot, Afsluitdijk, Netherlands (2013) |

IRENA's 2020 database counted only 0.23 MW of OTEC and 0.05 MW of salinity capacity worldwide. OTEC costs fall steeply with size, so it needs a first plant of tens of megawatts. Salinity power is held back by membrane cost and fouling.

#### How each method works
**OTEC, closed cycle**

Warm surface water boils a working fluid such as ammonia; the vapour drives a turbine and is condensed by cold water pumped from 800-1,000 m depth.^[1](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2014/Ocean_Thermal_Energy_V4_web.pdf)^[4](https://www.power-eng.com/2015/08/31/makai-ocean-engineering-connects-world-s-largest-otec-plant-to-u-s-grid/)^[5](https://www.jstage.jst.go.jp/article/gre/2/0/2_43/_article/-char/en)^[6](https://eos.org/?p=175224)^[12](https://english.ryukyushimpo.jp/2013/06/14/10571/)
**OTEC, open cycle**

Warm seawater flash-boils in a vacuum chamber and the steam drives a turbine; condensing it with cold seawater yields fresh water.^[1](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2014/Ocean_Thermal_Energy_V4_web.pdf)^[3](https://tethys-engineering.pnnl.gov/sites/default/files/publications/IRENA_Innovation_Outlook_Ocean_Energy_2020.pdf)
**OTEC, hybrid**

A closed cycle generates power, then the warm water is flash-evaporated for fresh water.^[1](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2014/Ocean_Thermal_Energy_V4_web.pdf)^[3](https://tethys-engineering.pnnl.gov/sites/default/files/publications/IRENA_Innovation_Outlook_Ocean_Energy_2020.pdf)^[5](https://www.jstage.jst.go.jp/article/gre/2/0/2_43/_article/-char/en)
**Pressure-retarded osmosis (PRO)**

Fresh water seeps through a membrane into pressurised seawater, adding volume on the high-pressure side; part of that flow spins a turbine.^[2](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2014/Salinity_Energy_v4_WEB.pdf)^[7](https://sj.jst.go.jp/news/202508/n0825-01n.html)
**Reverse electrodialysis (RED)**

Fresh and salt water flow through stacks of membranes passing only positive or only negative ions, creating a voltage directly.^[2](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2014/Salinity_Energy_v4_WEB.pdf)^[8](https://tethys.pnnl.gov/publications/life-cycle-assessment-salinity-gradient-energy-recovery-using-reverse-electrodialysis)

#### 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
3. [Innovation Outlook: Ocean Energy Technologies](https://tethys-engineering.pnnl.gov/sites/default/files/publications/IRENA_Innovation_Outlook_Ocean_Energy_2020.pdf) IRENA (copy hosted by PNNL Tethys), 2020
4. [Makai Ocean Engineering connects world's largest OTEC plant to U.S. grid](https://www.power-eng.com/2015/08/31/makai-ocean-engineering-connects-world-s-largest-otec-plant-to-u-s-grid/) Power Engineering, 2015
5. [Prediction of emission intensity for 1 MW onshore H-OTEC in Malaysia (Phua et al., Grand Renewable Energy proceedings)](https://www.jstage.jst.go.jp/article/gre/2/0/2_43/_article/-char/en) J-STAGE, 2022
6. [The Century-Old Renewable You've Never Heard Of (Kumejima OTEC plant)](https://eos.org/?p=175224) Eos (AGU), 2022
7. [Japan's first osmotic power plant begins operations in Fukuoka](https://sj.jst.go.jp/news/202508/n0825-01n.html) Japan Science and Technology Agency (Science Japan), 2025
8. [Life cycle assessment of salinity gradient energy recovery using reverse electrodialysis (Mueller et al., Journal of Industrial Ecology 25)](https://tethys.pnnl.gov/publications/life-cycle-assessment-salinity-gradient-energy-recovery-using-reverse-electrodialysis) Wiley (via PNNL Tethys), 2021
9. [Ocean thermal power generation to start on Kume-jima](https://english.ryukyushimpo.jp/2013/06/14/10571/) Ryukyu Shimpo, 2013

## 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)
- [Efficiency](https://www.materialinsights.org/energy/e_otec/efficiency/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)