Nemo prepares to plunge into ocean thermal energy conversion

The expert: Jean Ballandras. Company Secretary, Akuo Energy. A graduate of Sciences Po (the University of Paris Institute of Political Research) and an alumnus of the ENA (National School of Administration) in Strasbourg, Ballandras was also technical advisor to the French president’s office from 2003 to 2005. From 2005 to 2011, he was Secretary-General for Regional Affairs in the French overseas department of Réunion, where he was responsible for the GERRI project (the official environmental strategy for Réunion).

What is ocean thermal energy conversion?
Jean Ballandras says: Ocean Thermal Energy Conversion, or OTEC, means using the difference in temperature between warm surface water and colder water (roughly 5°C) 1,000 metres below sea level. It’s an established technical principle – heat exchange – which has been studied for many years [see box]. In this case it relies on ammonia, a gas that condenses into a liquid below 20°C. Warm water, drawn from beneath the surface, gasifies ammonia in a heat exchanger. The gas expands and powers a turbine, coupled with an alternator that generates electricity. It returns to liquid in a condenser, where it’s cooled by the deep water brought up from the depths, which is generally around 7 or 8°C.
Further reading: Islands to switch to renewable energy

The Nemo project is a floating power plant which will be operational in 2018. It is being built by DCNS (formerly the French Shipbuilding Directorate) and Akuo Energy, a French renewable energy producer. Located off the east coast of Martinique, 7 kilometres from Bellefontaine, it will eventually produce 16 megawatts – enough to supply electricity to some 35,000 homes. © DCNS
How much power will nemo generate?
The Nemo floating installation in Martinique will consist of four modules and will generate 16 megawatts. We are also working on the (5.6 megawatt) Nautilus project, also in Martinique, an onshore plant which will be self-contained. Nemo’s 16 megawatts is the power produced, but some of that will be used to operate the pumps and the rest of the plant. So the net power available from Nemo will be 10.5 to 11 megawatts.
Ocean thermal energy conversion: an idea embraced by Jules VerneThe name Nemo is an acronym (New Energy for Martinique and Overseas) but it’s also a reference to Captain Nemo who commands the submarine in Jules Verne’s novel Nautilus. Through this character, in 1870, Jules Verne explains how: “by establishing a circuit between two wires immersed to different depths, [he would be able] to obtain electricity through the diverging temperatures they experience…” Jacques Arsène d’Arsonval, a doctor and physicist, explicitly described the principle in 1881 and engineer Georges Claude tried to use OTEC in Cuba and Brazil in the 1930s. But each time, the ocean got the better of the long underwater pipelines.Tests were conducted in the 1950s in Abidjan (in Côte d’Ivoire) and Guadeloupe then began in the 1970s in Japan and the United States, and even in Tahiti in the 1980s. Recently, between 2011 and 2014, DCNS developed a prototype for an OTEC plant on Réunion, at the University Institute of Technology in Saint-Pierre, on the south of the island. This validated the concept.

What are the limitations of otec?
Firstly, this energy can only be produced in the tropics. There needs to be a difference in temperature of around 20°C, which requires the surface water to be at least 25°C. In addition, the 1,000-metre depths must be easily accessible, so not too far from the coast. In concrete terms, OTEC is well-suited to volcanic islands with steeply sloping sea beds. This excludes coasts with a continental shelf. In Guyana, for example, you have to go 60 kilometres off the coast to reach a depth of 1,000 metres.

The Nautilus plant being built in Martinique, also in Bellefontaine, is more modest. However, it will be situated onshore and operational by 2017. It won’t just produce electricity (for 15,000 homes) – it will also supply cold water for cooling buildings, for aquaculture and for an EDF oil-fired plant.
What are the advantages?There are many. It’s a type of energy that’s easy to access, with simple installations. The principle is tried and tested and, although the predicted output is low (7%), it’s a free resource. And unlike most other renewable energies, it can be produced continuously – 24 hours a day, 365 days a year. It may dip slightly with the seasons, but these are not very extreme in the tropics. From a financial point of view, an investment has to be made upfront, but after that, as with a dam, operating costs are low. This renewable energy has a very low impact on the environment, and the installations have very little visual impact. Ammonia is used, and this substance should not be released into the environment, but steps have been taken to ensure the risk of this is minimal. Lastly, an OTEC plant can also be used directly, particularly to provide air conditioning, using the cold water retrieved from the ocean depths. Nautilus will do this.
Further reading: El Hierro: an island in the windIf the technology has been known for so long, why isn’t otec more widespread?Technically speaking, sufficiently robust pipelines have had to be developed (measuring several metres in diameter). The offshore ocean environment is very challenging. Economically speaking, the price of oil being so low for so long has provided little incentive to explore and use OTEC.Apart from nemo and nautilus, do any other projects exist?Yes: in the Philippines and Indonesia. And OTEC is soon to be deployed in the Caribbean and the Pacific. It’s a technology that’s well suited to island environments And this is just the beginning!To go further
Islands to switch to renewable energy
Tunnels under the sea à la Jules Verne
El Hierro: an island in the wind
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