# Ocean thermal gradient: Abundance

Terawatts in principle, kilowatts in practice

Source page: https://www.materialinsights.org/energy/e_ocean_thermal/abundance/

What this tab covers How much of this energy there is: the natural flow or stock, the share that could be harnessed, and how much is used today, compared with world primary energy use (592 EJ in 2024).

A very large flow in principle (several terawatts), but almost untapped: the largest operating plants are around 100 kW.
 A flow: renewed continuously

#### How much there is

| Measure | Value | Compared with world energy use | Note |
|---|---|---|---|
| **Maximum net OTEC power (3-D ocean model)** | about 30 TW^[2](https://ideas.repec.org/a/eee/renene/v50y2013icp532-540.html) | about 1.6 times world primary energy use (945 EJ vs 592 EJ) | Would require flows large enough to strengthen the global overturning circulation. |
| **Net power with little effect on ocean temperatures** | about 7 TW^[2](https://ideas.repec.org/a/eee/renene/v50y2013icp532-540.html) | about 37% of world primary energy use (220 EJ) | Considered the more practical upper bound. |
| **Net power limit, 1-D model, by discharge design** | 2.7-4.3 TW^[10](https://www.mdpi.com/2077-1312/6/1/25/htm) | about 14-23% of world primary energy use | Mixed discharge 2.7 TW; separate warm and cold discharges 4.3 TW. |
| **IRENA estimate of global OTEC potential** | 300 EJ per year^[1](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2014/Ocean_Thermal_Energy_V4_web.pdf) | about half of world primary energy use | IRENA calls this 90% of the global ocean energy potential. |
| **United States resource estimate (DOE assessment)** | 4,642 TWh per year^[3](https://www.nationalacademies.org/read/18278/chapter/7) | about 2.8% of world primary energy use (16.7 EJ) | Judged 'unrealistically high' by the National Academies review. |
| **Largest plant operating in Hawaii (grid-connected 2015)** | 0.1 MW^[5](https://www.power-eng.com/2015/08/31/makai-ocean-engineering-connects-world-s-largest-otec-plant-to-u-s-grid/) | negligible |  |
| **Kumejima demonstration plant, Okinawa, Japan (built 2013)** | about 0.1 MW^[6](https://eos.org/?p=175224) | negligible | Deep water is reused for cooling, prawn farming and agriculture. |

Estimates span an order of magnitude because the resource is self-limiting: pumping deep cold water up and discharging it warms the deep ocean and cools the surface, shrinking the very gap the plant relies on. One-dimensional models give a few terawatts depending on how effluent is discharged; a three-dimensional circulation model gives about 30 TW at most, but only about 7 TW without noticeably altering ocean temperatures.

World primary energy use was 592 EJ in 2024 (Energy Institute Statistical Review 2025), so even the cautious 7 TW figure is more than a third of it. Actual use is a rounding error: Makai's 100 kW plant in Hawaii (grid-connected in 2015) and a demonstration plant of about 100 kW on Kumejima, Japan, built in 2013.

#### 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. [Estimates of global Ocean Thermal Energy Conversion (OTEC) resources using an ocean general circulation model (Rajagopalan and Nihous, Renewable Energy 50)](https://ideas.repec.org/a/eee/renene/v50y2013icp532-540.html) Elsevier, 2013
3. [An Evaluation of the U.S. Department of Energy's Marine and Hydrokinetic Resource Assessments, chapter on ocean thermal energy](https://www.nationalacademies.org/read/18278/chapter/7) National Academies Press, 2013
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. [The Century-Old Renewable You've Never Heard Of (Kumejima OTEC plant)](https://eos.org/?p=175224) Eos (AGU), 2022
6. [A Preliminary Investigation of the Effect of OTEC Effluent Discharge Options on Global OTEC Resources (Nihous, JMSE 6)](https://www.mdpi.com/2077-1312/6/1/25/htm) MDPI, 2018

## Other sections

- [Summary](https://www.materialinsights.org/energy/e_ocean_thermal/index.md)
- [Overview](https://www.materialinsights.org/energy/e_ocean_thermal/overview/index.md)
- [Correlation](https://www.materialinsights.org/energy/e_ocean_thermal/correlation/index.md)
- [How energy resides](https://www.materialinsights.org/energy/e_ocean_thermal/resides/index.md)
- [Fluctuation in time](https://www.materialinsights.org/energy/e_ocean_thermal/time/index.md)
- [Across the Earth](https://www.materialinsights.org/energy/e_ocean_thermal/geography/index.md)
- [Impact on nature](https://www.materialinsights.org/energy/e_ocean_thermal/nature/index.md)
- [Relationships](https://www.materialinsights.org/energy/e_ocean_thermal/links/index.md)