OTEC and salinity power: Techniques

Ammonia turbines, vacuum boilers and ion-sorting membrane stacks

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 cycle¹⁴⁵⁶¹²	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 cycle¹³	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, hybrid¹³⁵	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)²⁷	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)²⁸	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.¹⁴⁵⁶¹²

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.¹³

OTEC, hybrid

A closed cycle generates power, then the warm water is flash-evaporated for fresh water.¹³⁵

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.²⁷

Reverse electrodialysis (RED)

Fresh and salt water flow through stacks of membranes passing only positive or only negative ions, creating a voltage directly.²⁸

Sources

Ocean Thermal Energy Conversion: Technology Brief IRENA, 2014
Salinity Gradient Energy: Technology Brief IRENA, 2014
Innovation Outlook: Ocean Energy Technologies IRENA (copy hosted by PNNL Tethys), 2020
Makai Ocean Engineering connects world's largest OTEC plant to U.S. grid Power Engineering, 2015
Prediction of emission intensity for 1 MW onshore H-OTEC in Malaysia (Phua et al., Grand Renewable Energy proceedings) J-STAGE, 2022
The Century-Old Renewable You've Never Heard Of (Kumejima OTEC plant) Eos (AGU), 2022
Japan's first osmotic power plant begins operations in Fukuoka Japan Science and Technology Agency (Science Japan), 2025
Life cycle assessment of salinity gradient energy recovery using reverse electrodialysis (Mueller et al., Journal of Industrial Ecology 25) Wiley (via PNNL Tethys), 2021
Ocean thermal power generation to start on Kume-jima Ryukyu Shimpo, 2013