THEAND M (also known by its English acronym OTEC for Ocean Thermal Energy Conversion) is a relatively unknown renewable energy but with considerable energy potential! ETM had already been discussed a few years ago on le forum energies. Brought back to the forefront by the legislative elections in Polynesia, in particular at the initiative of the Heiura – Les Verts party, it is interesting to take an interest in the new advances in this promising technology.

Brief reminder of the ETM principle

Already mentioned by Jules Verne in his novel "Twenty Thousand Leagues Under the Sea" in the 19th century, the production of thermal energy from the seas is made possible by the difference in temperature between the water on the surface and the water in the depths. It is then a question of pumping the water in these two places using different pipes. The difference in temperature between “hot” water coming from the surface and “cold” water coming from the depths must be at least 20°, which explains why this solution is only possible in certain hot regions of the globe!!

The plants can operate with 3 different types of cycles: closed cycle, open cycle with production of fresh water, or mixed cycle. The sea then provides thermal energy which makes it possible to operate an evaporator which in turn produces kinetic energy. This energy then allows a turbine to produce mechanical energy which is in turn transformed into electrical energy by an alternator. The following video helps to better understand this process. Be careful however, the NEMO project discussed there was finally put on hold in 2018. It consisted of the establishment by France of an ETM plant in Martinique, but the latter no longer seems to be in the pipeline.

The efficiency of the power stations remains quite low: it is only around 6% and part of the energy produced is reinvested to circulate the ammonia in the pipes. But seawater is an inexhaustible and free resource, which is the strength of this technology. On the other hand, unlike solar energy which can only be produced during the day, or wind energy which depends on climatic conditions, the production of energy by ETM power plants does not experience any interruptions. Eventually, ETM power plants should be able to operate directly at sea, taking the form of floating platforms anchored off the coast. But this optimized version does not seem to have been implemented yet. On the other hand, there are already ETM plants installed on the coasts.

Un hainan university certificate in China also offers a solution capable of combining energy production and fresh water production while solving the yield concerns mentioned above.

An example of an ETM plant: the Makai plant in Hawaii

This plant was launched in August 2015 by the American group Makai Ocean Engineering Inc. on the island of Hawaii in the Pacific Ocean. It operates using a closed ammonia cycle and has a capacity of 100 KW. It pumps hot water around 24° on the surface, and cold water at 4° in depth.

In order to protect the oceans, the pipes of the Makai plant are made of titanium sheets. Microfilters at their ends are there to prevent marine species from being drawn into the pipes when the water is pumped. The following video provides an overview of its facilities:

In addition to its power plant, the Makai group is also working on improving its offshore facilities. For example, they are developing and improving software to facilitate the laying of cables or pipes on the seabed. They also carry out research with the aim of improving the efficiency of the pipes used in marine installations (ETM, but also sea water air conditioning, etc.).

Although information about ETM projects filter little, other countries such as China, Japan, Korea, India, as well as many Pacific islands are currently interested in this technology. It is very likely that several other projects will see the light of day in the next decade.

In China, several universities have filed patents in the field of marine energy. One can, for example, cite a yantai university certificate which proposes the installation of an OTEC type system not on a platform, but on an ocean liner. The system then makes it possible to provide the energy necessary to move the liner forward. This idea of ​​using the energy produced on board a boat for its operation is also mentioned in a patent filed by Ahmed BYAH. Such technology, if proven effective, could revolutionize sea transport.

The technical constraints of ETM

In addition to the water temperature constraint already mentioned, the installation of an ETM plant also leads to other technical concerns. Installed on the coast, the plant will need relatively long pipes to pump the sea water necessary for its operation. It must of course be installed as close as possible to the sea. The floating version of the ETM plant must for its part be firmly anchored at sea so as not to drift. Transporting energy to the continent will also require technical prowess. Another possible option would be to use these floating power plants to supply an offshore activity, also installed on the floating platform.

The marine environment also requires taking into account the effects of corrosion on installations, which is particularly important in salt water. The proliferation of bacteria or living marine organisms such as algae, or shells on the pipes may also require regular maintenance. This proliferation is known as biofouling. In this area, research is underway to find ecological solutions to combat this process. The techniques currently used are for the most part harmful to the environment, but we can for example mention a proposal for anti-fouling paint based on silicone which would preserve the environment.

To learn more

In addition to energy production, ETM plants operating in open circuit or hybrid systems can help produce fresh water from sea water. On the other hand, cold water drawn from the depths of the oceans may also contribute to the air conditioning of certain facilities. It is this process of using deep waters (without the involvement of an ETM plant) that was used for the air conditioning of a public hospital in Tahiti, Polynesia.

This project was carried out with the participation of the French company Airaro. The following video briefly introduces this company:

Coupling electricity production, fresh water production and air conditioning using ETM technologies would make it possible to drastically reduce energy expenditure in the southern islands, while improving the living conditions of their inhabitants.

There are many possibilities for producing energy from the sea or the oceans. We could, for example, cite wave energy produced by waves, but also the concept of offshore wind turbines, also called offshore wind turbines. In France, an international event presenting renewable marine energies takes place every year, a sign of the interest shown in the subject. Appointed seanergy, it will take place in Le Havre in Normandy between June 15 and 17, 2022.

Follow them ETM news and technological advances on this discussion.