April 14, 2014 – Seawater is proving to be one way to combat climate change by reducing fossil fuel dependency for some ocean island nations. Taking a page from land-based geothermal power which uses the coolness below ground in heat exchange systems, islands are using the thermal energy gradient in a column of seawater to generate electricity.
The technology is known as Ocean Thermal Energy Conversion or OTEC. A French defense contractor, DCNS Group, is the latest to deploy it in the island of Reunion in the Indian Ocean. It plans a second project in Martinique which is expected to come online in 2015.
Lockheed Martin, the American defense contractor, has been working with OTEC for over a decade. When I previously wrote about this technology I described a pilot project Lockheed was building in Southern China. The company currently plans to have an OTEC power plant operating offshore in Hawaii.
Why offshore? Because OTEC projects need to be on or near the water. The Lockheed and DCNS technology above the water looks very much like a marine production oil platform (see picture below).
Because the ocean is a great energy storage medium, in fact, the largest on the planet, we can take advantage of the temperature gradient that occurs in a column of water and use it to our advantage. Surface water can be as warm as 30 degrees Celsius (86 Fahrenheit) off islands like Reunion and Martinique. At depths of 1,000 meters (3,300 feet), however, that water remains a constant 4-5 degrees Celsius (39-41 Fahrenheit). Go deeper and you approach freezing temperatures or below freezing. The difference in temperature between surface and deep water is what makes OTEC work.
OTEC technology is built using a membrane that serves as a heat exchanger. The warm surface water is exposed to a liquid with a low boiling point. DCNS uses ammonia. When it gasifies the ammonia drives a turbine which is attached to a gemerator. The second part of the OTEC technology involves drawing the cold from below to act as a coolant. This condenses the ammonia back to a liquid state where the process can then be repeated.
Key to OTEC’s successful deployment is the finding of island and coastal locations that currently experience high energy costs because fossil fuels need to be imported for energy generation. The second key is locations where there is a sufficiently high temperature gradient in nearby ocean and seas. Japan, China, the Bahamas, Curacao, South Korea and Hawaii are current locations where OTEC is under development or being considered. For countries looking to lower their carbon footprint with all the right keys in place, OTEC may prove to be a strong renewable energy play.