Ocean Thermal Electric Conversion (OTEC) is making a comeback as our attention moves to protecting the ocean and harnessing its power. We examine the state of play for OTEC.
Ocean Thermal Electric Conversion (OTEC) is a clean energy technology that uses the temperature difference between warm surface water and cold deep water in the ocean to generate electricity. This process mimics nature’s own energy cycle and has huge potential for tropical island nations and coastal regions. This post explains how OTEC works, its benefits and challenges, and why it matters for a sustainable future.
1. How OTEC Works
The Basics
OTEC works by using the heat from warm surface seawater to vaporize a low-boiling-point fluid (like ammonia). This vapor spins a turbine to generate electricity. Then, cold deep-sea water condenses the vapor back into liquid, and the cycle repeats.
Three Main Types
1. Closed-cycle OTEC – uses a working fluid like ammonia in a closed loop.
2. Open-cycle OTEC – uses seawater directly as the working fluid, producing both electricity and fresh water.
3. Hybrid-cycle OTEC – combines both closed and open systems.
2. Why OTEC Matters for the Planet
Clean and Renewable
OTEC uses no fossil fuels and emits no greenhouse gases during operation. It taps into the largest solar energy collector on Earth – the ocean.
Base-load Energy
Unlike solar or wind, OTEC can produce constant power 24/7, which is especially valuable for remote islands and coastal communities.
3. Environmental and Social Benefits
Fresh Water Production
Open-cycle and hybrid OTEC systems can also produce desalinated water, helping solve water scarcity problems.
Aquaculture and Cooling
Cold water brought up from the deep sea can be used for fish farming, agriculture, or cooling buildings, making OTEC a multi-use system.
4. Technical and Economic Challenges
Efficiency Issues
OTEC plants have relatively low efficiency because the temperature difference between surface and deep water is small—typically around 20°C. This limits power output and requires large volumes of water.
Cost and Scale
Building OTEC plants, especially offshore ones, is currently expensive. Commercial-scale plants are still in the demonstration stage due to high infrastructure costs and technical complexity.
5. Real-World Examples
Hawaii, United States - The Natural Energy Laboratory of Hawaii Authority (NELHA) has tested small-scale OTEC systems for decades and is a global leader in this field.
Japan - Japan’s Saga University and companies like IHI Corporation have built pilot OTEC plants in Okinawa to support clean energy goals.
Martinique and Mauritius - These island nations have explored OTEC as a way to cut fossil fuel imports and stabilize electricity supply.
Global OTEC Ltd - Global OTEC Ltd is developing a commercial-scale floating OTEC platform called "Dominique" targeted for deployment in São Tomé and Príncipe. The project aims to provide stable, off-grid, renewable power for tropical island nations and is designed with replicable and modular technology to speed up adoption across the global south.
6. Community Perception
Positive Views
Supporters highlight OTEC’s ability to provide constant renewable energy and fresh water, especially to remote or climate-vulnerable regions. The technology is praised for having low carbon emissions and for promoting energy independence among small island states.
Negative Views
Critics allege that the technology remains expensive and unproven at commercial scales. This view ignored the successful implementations in section 5 with a technology readiness level of 8 (TRL 8). The economics of both projects have been enhanced by the. Environmental concerns include the potential for deep-sea ecological disruption from upwelling cold water and the long-term effects of thermal discharge.
7. Conclusion
Ocean Thermal Electric Conversion (OTEC) offers a promising pathway to generate clean, continuous energy using the ocean’s natural heat gradient. While still developing, its potential to deliver electricity, fresh water, and support food security makes it a key technology to watch—especially for island nations on the frontlines of climate change.
8. References
Ocean Thermal Energy Conversion (OTEC) – Technology Brief, IRENA, BIRI 9 https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2014/IRENA-OTEC-Technology-Brief.pdf
NELHA Hawaii OTEC Demonstration, Natural Energy Laboratory of Hawaii Authority, BIRI 8 https://nelha.hawaii.gov/category/main/energy/otec/
Renewable Ocean Energy, ESMAP World Bank, BIRI 9 https://www.esmap.org/sites/default/files/esmap-files/OTEC%20%20Update%20World%20Bank%20102013v2_Reduced.pdf
Ocean Energy Systems Annual Report 2022, International Energy Agency (IEA-OES), BIRI 9 https://www.ocean-energy-systems.org/publications/annual-reports/oes-annual-report-2022/
OTEC in Hawaii and beyond, BIRI 6 https://events.development.asia/learning-events/ocean-thermal-energy-conversion-otec-hawaii-and-beyond
OTEC in Kumajima, Japan, BIRI 6 https://events.development.asia/learning-events/ocean-thermal-energy-conversion-otec-viability-catalyst-transformative-island
Global OTEC Ltd Project Overview, Global OTEC Ltd, BIRI 5 https://www.globalotec.co.uk/dominique
OTEC Economics, Otec International, BIRI 5 https://tethys-engineering.pnnl.gov/sites/default/files/publications/Vega-Martin-2024.pdf
Opportunities and Challenges for OTEC in Small Islands, Springer Nature, BIRI 7 https://link.springer.com/referenceworkentry/10.1007/978-1-4419-0851-3_695
