# Ocean thermal gradient: Across the Earth

Why tropical islands with steep slopes are ideal

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

What this tab covers How the resource varies with latitude, climate, terrain and geology: the richest and poorest regions and the leading countries.

Confined to the tropics, where surface water stays at 25-30 °C over deep water near 4 °C; best near steep coasts and islands where deep water lies close to shore.

The resource is strongest in a broad band around the equator and fades towards the subtropics, where winter surface cooling closes the gap. Steep island slopes matter because they bring 1,000 m water depth within a short pipe of the shore.

#### Rich and poor regions

| Region | Resource | Level |
|---|---|---|
| **Tropical Pacific islands (e.g. Micronesia, Samoa)** | Micronesia 1,134 and Samoa 1,331 TWh/yr (DOE estimate), but demand there is small^[3](https://www.nationalacademies.org/read/18278/chapter/7) | high |
| **Hawaii** | 143 TWh/yr^[3](https://www.nationalacademies.org/read/18278/chapter/7) | high |
| **Puerto Rico and US Virgin Islands** | 39 TWh/yr^[3](https://www.nationalacademies.org/read/18278/chapter/7) | medium |
| **Caribbean and Pacific islands; African coasts** | many islands have surface water at 25-30 °C; many African countries have viable resource within 25 km of the coast^[1](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2014/Ocean_Thermal_Energy_V4_web.pdf) | high |
| **Gulf of Mexico** | 13% of the continental US resource; not viable in winter^[3](https://www.nationalacademies.org/read/18278/chapter/7) | low |

IRENA counts at least 98 nations and territories with an OTEC resource. The economics favour small tropical islands that import diesel for power, need fresh water and have deep water close offshore, which is why the pilot plants are in Hawaii, Okinawa, Nauru and Korea, and why projects have been planned for Martinique and southern China.

Within the United States, the National Academies review found that territories in the Pacific dominate the estimated resource, while the continental share (394 TWh/yr, mostly the Florida Straits and East Coast) is under 9% of the total.

#### 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. [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

## 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)
- [Abundance](https://www.materialinsights.org/energy/e_ocean_thermal/abundance/index.md)
- [Fluctuation in time](https://www.materialinsights.org/energy/e_ocean_thermal/time/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)