Menu déroulant

Dossier Océan et Énergie - Énergie Thermique des Mers

OTEC : A neglected marine energy renewable 

Le Club des argonautes - 2005, November

 

The ocean receives annually from the sun an amount of energy equivalent to more than 1000 times that of the world's primary energy demand. This energy is stored in the form of Heat - in the water surface layers of the oceans. It is then redistributed between ocean and atmosphere. It causes winds, sea waves, clouds, rains - and warms up the polar regions. It determines the Earth climates.


The idea to convert the Ocean Thermal Energy into electricity (OTEC) was born more than one hundred years ago.

Demonstrating the process was feasible at sea was made in the years 1930s by the French engineer Georges Claude already questioning about decline of fuel resources, coal at that time. 

After  World War 2, exploitation of oil, first on land and then offshore, meant postponing an answer to the recurring question on the future of energy supply for the Industrial world. 

However, The 1973 oil crisis showed evidence of the supply vulnerability and revived OTEC Research & Development (R&D) activities in France, USA and Japan. The decrease of pressure on oil market in 1986 led France government to re-examine the case for OTEC, then abandon OTEC R&D and leave the leadership to the USA and Japan.
 

OTEC: how does it work?

When heating a liquid up to its boiling temperature, it is transformed into vapour which can be used to power a turbo-alternateur, before going through a condenser where it cools down and transforms back to liquid. With OTEC, the “Heat” necessary to transform the liquid into vapour is drawn from the water surface layer in the ocean warmest regions where temperature can reach 25 to 28°C and the "Coldness" necessary to the vapour to condensae is supplied by the deep water, pumped at depth under the thermocline, where the temperature is close from 0° C. 
Hence OTEC is similar to the process used in our modern power plants, whichever the fuel - fossil or nuclear. Only the operating conditions are different. The small temperature difference available for OTEC - about 20°C between the hot source and the cold source - renders the process poorly efficient. However, the energy that may be extracted, i.e. the net amount available for the end user (or final energy), may be as high as five times the power used by the plant, according to considered difference of temperature .

 

For the past 20 years these two countries maintained some momentum in the research for technical solutions and economical options to render OTEC more and more attractive.

They have optimised the characteristics of components: heat exchangers and turbines, conforted the reliability of the marine components - especially for the construction and the deployement of Cold Water Pipes, and developed the concept of "multi-products” OTEC plants up to some tens of MW. 

This "multi-products" concept aims to optimizing other usages of Deep Ocean Water (DOW) : for desalinated water and aquaculture products, and for other products matching the demand from small isolated communities located close from the resource. Also they have studied extrapolation to large size plants up to several hundreds MW for the offshore production of liquid synthetic fuels (hydrogen, ammoniac and methanol) to be transported by tankers and satisfying the demand of industrialized countries located in region far from the resource.

At last, data acquired during the two past decades by running experimental plants enabled better evaluation of negative and positive environmental impacts caused by the still cold and nutrient rich Deep Ocean Water effluent. 

During the same period of time developed the idea that clean and renewable energy will become more and more necessary for lessening the vulnerability of traditional fuels supply caused by political embargo or resource depletion, and also for mitigating as much as possible the severe and durable negative effects their usage causes to our environment . 

To these reasons one should add that of the change - in course - of the balance of the energy demands between rich and poor countries. From the beginning of the Industrial era the richest have been both the most important consumers and the greatest polluters. Tomorrow the poorest will come first because their demographic growth and their increasing demand for improving their life standard. Well, note these countries from the “South” are also those where the OTEC resource is the most easily accessible. 

 

Future Energy Prospects : estimating the demand for OTEC.

The 1999 OCDE report : " Energy, the Next Fifty years " established several scenarios for the evolution of the World primary energy demand, starting from 1990 data with population of 5,26 billions and a consumption of primary energy of 8,98 Gtoe, namely 1,7 toe per inhabitant.
In the most pessimistic business-as-usual scenario, the power consumption of primary energy was predicted at 24 Gtoe in 2050, for a population of 10 billions. 
In the " ecological " (Green) scenario, in agreement with the Kyoto protocole, the figure for primary energy raises to 14 Gtoe for a population of 10 billions. This " Green " scenario was also the cheapest with a capital investment of 24 billion US dollars over fifty years. It was based on the assumption of increasing fourfold the renewable energy production: from a 1,6 Gtoe (in 1990) to 5 Gtoe (in 2050), with an intermediary step of 2,3 Gtoe in 2020. 
Supplying 5 to 10% of these needs thanks to OTEC could be the aim for an European Union R&D program. 

 

See :

Ocean Thermal Energy Conversion (OTEC)and &Deep Ocean Water Applications(DOWA).market opportunities for European industry.

 

Doing nothing when facing this unavoidable prospect is running the risk to be confronted with both :

Then, like the sailors who anticipate the threat of a troublesome horizon the GEONAUTES must modify their route before it becomes too late. 

 

Possible routes are numerous: 

  1. energy saving, 

  2. cleaner production and 

  3. development of clean renewable energy resources. 

OTEC because it is, stable, abundant and renewable offers production potential accessible to all countries and commensurable to their needs. 

 

The OTEC resource

The tropical ocean surface, where the water temperature difference between surface and 1000 metres depth is greater than 22°C, is estimated to be 60 millions square kilometres. This zone is coloured in red on the picture. The conversion into electricity of this thermal resource (of solar origin) would probably sustain a yealy production of 100.000 TWh. 

Note: the mean monthly value of the temperature difference between surface and 1000 meters depth is more than 22°C in the red zone, between 20°C and 22°C in the yellow zone, and between 18°C and 20°C in the pink one. 

 

American and Japanese OTEC promoters have imagined OTEC development could be accomplished in three phases, over a time period of several decades.

The first phase will be that for the development of " multi-products facilities " responding to the present needs of the poor countries from the “ South ” region having direct access to the resource. Development would be undertaken in partnership with, and the financial and technical support of the international community of industrialized countries. The development cost being covered partly thanks the contribution allocated by countries from the “ North ” for the development of those from the “ South ” , and partly from the income generated by taxes linked to international agreements aiming towards decrease GHg content of the atmosphere.
During the second phase, the experience acquired during the exploitation of first phase plants will enable the construction of OTEC facilities whose capacity matching better and better the demand of future energy markets to supply electricity to coastal towns in tropical zones. 
Then, during its ultimate phase OTEC development will be oriented towards offshore production of synthetic fuels to be exported anywhere in the whole world (The Hydrogen Economy of 2050: OTEC Driven? de Makai Ocean Engineering, Inc.).

In parallel with technical progress and cost reduction resulting from OTEC development as imagined above, the experience gained from operating facilities of greater and greater size over longer and longer time period will enable : 

  1. a better understanding of the effects of OTEC exploitation on the environment,

  2. to approach estimation of the exploitation limits, and 

  3. to optimize the most valuable operating procedures to return water effluents back to the ocean.

After pioneering R&D into the OTEC during the first half of the past century, after having devoted heavy investments from 1982 to 1985 to study a 5 MW pilot plant for Tahitian, France abandoned the project in 1986 for the sole reason the production cost would not compete with that of traditional energy in Tahiti ! This was rather an evidence for the production case by a pilot plant exploiting a new resource.

This abandon is regrettable as is also regrettable the absence of will in France and Europe for taking follow-up actions and organize a technical watch on the continuation of programmes and progresses accomplished in foreign countries for OTEC development. This absence of follow-up is risky since it could jeopardize our industrial competitivity in case OTEC opens new markets opportunity and it is all the more surprising than France and Europe have many overseas Territories with direct access to OTEC, a resource other countries still consider full of promise.