The simple principle of an energy without intermittence or fluctuation
In the intertropical zone, the existence throughout the year of ocean stratification makes it possible to reach, from a depth of 1,000 m, cold waters whose temperature difference with the surface water remains greater than or equal to 23°C. This delta is the minimum to be able to exploit the ocean thermal energy conversion and to operate a thermodynamic Rankine cycle:
- The open cycle consists, via exchangers, of vaporising cold water directly into the warm environment and turbining this steam, whose condensation can also produce desalinated water,
- The closed cycle uses an intermediate liquid, often an ammonia solution, in a double exchanger, which evaporates, turbines and condenses.
By activating a variable number of exchangers, power can be modulated on demand. This is basic energy, i.e. without intermittence or fluctuation. The thermodynamic cycle has a low efficiency and part of the electrical energy produced is consumed by pumping, which implies a large flow (3 m3/s) for a significant production (1 MW), but the resource is, in a first approach, almost inexhaustible.
A French historic and geographic interest
The principle of OTEC was laid down as early as 1881 by Arsène d’Arsonval. The first small-scale prototypes were then tested by Georges Claude from 1930 to 1950. Ifremer then designed an ambitious project for a power station and fish farm for Tahiti that the oil counter-shock of the 1980s postponed. The rest of the story is American, Japanese, Korean and Indian. Today, however, several French industrial groups are investing in this technology, especially as the French Overseas Territories offer a geographical opportunity. Indeed, many islands have a potential that is independent of the seasons, with a resource close by when bathymetry above 1,000 m is located close to the coast. Relatively rich and populated, they can justify the implementation of OTEC in their energy mix and act as ambassadors of this technology to neighbouring countries.
Well-identified technological locks with expensive solutions
Whether the OTEC plant is floating or onshore, its first cost and risk factor remains the large diameter cold water pipe to ensure the flow, limiting pressure drops and heat loss, with a design that is resistant to the extreme events of intertropical hurricanes. Next comes the question of biocolonisation and corrosion in exchangers with highly optimised surfaces and circuits. Numerous advances, methods and patents have been made and registered recently. The full-scale demonstration, which would result in a system of several tens of MW, is the real technical and economic challenge that will make this sector take off, perfectly adapted to complete a mix of renewable energies. Without subsidies, one must be able to accept today a LCOE of 400 €/MWh. Any cost reduction therefore remains the object of research.
R&D contributes to the progressive development of the OTEC
Since its creation, France Energies Marines has been working with some of its industrial and academic partners on the potential environmental impact of deep water discharge at the surface at the end of the cycle. Indeed, cold water, loaded with nutrients, could lead to a risk of eutrophication. Thesis work carried out from 2013 to 2016 (IMPALA project) showed that a good arrangement of these discharges under the lighted zone avoids primary growth and therefore cuts short a runaway of the ecosystem.
At the international level, there are few European actors on the subject of OTEC, France Energies Marines is active within a task of the Technological Collaboration Programme on Ocean Energy of the International Energy Agency (TCP/OES). This task is dedicated to the drafting of a white paper on OTEC which provides synthetic but essential information to inform decision-makers. Following a presentation of the technological characteristics, the physical potential and the financial paradigm specific to island environments in which investment in OTEC must be mobilised, the opportunity to develop joint activities by exploiting deep waters is presented. These joint activities relate to nutrients, rare earths or metals, unused frigories and desalination.
The Institute has recently initiated a collaborative project on multi-criteria optimisation for the supply of isolated grid (OPTILE project).
The resolution of cross-cutting issues applicable to OTEC
Other technologies are similar to OTEC in terms of ocean thermal exploitation processes, but also in terms of the joint problems of pumping large flows and large discharges. These include air conditioning using seawater and heat pumps drawing on the stabilised thermal reservoir of seawater. These technologies can be used in temperate zones. Several R&D activities carried out by the Institute can contribute to the resolution of cross-cutting issues, even if the primary purpose is no longer the production of electrical energy. This work focuses on site characterisation, environmental and societal integration, as well as system design and monitoring.
Photo credit: Naval Energies
Understanding and monitoring of ORE impacts on the benthic compartment via a measurement platform dedicated to passive acoustic
Impacts on micro-plankton of artificial upwelling inputs
Multi-criteria optimisation for offgrid marine renewable electrical production
Subsea power cables interactions with environment and associated surveys
Consideration of biofouling using quantification protocols useful for engineering
Monitoring of polyamide mooring lines
Collaborative initiative for monitoring the environmental effects of ocean energy development
Dynamic umbilicals for floating marine renewable energies technologies - Phase 2
Durable and flexible polyamide moorings for offshore renewable energies
Technology Collaborative Programme on Ocean Energy Systems
Accounting for biofouling through established protocols of quantification
Quantitative evaluation of metals released into the marine environment from the galvanic anodes of ORE structures.
Dynamic umbilicals for offshore renewable energies
Design and in-service monitoring of power cables and moorings
Ecosystem approach of the impact of offshore wind farms
Marine life monitoring
Training in the field of offshore renewable energies
Yann-Hervé De Roeck
Environmental and Societal Integration R&D Manager
Published on 02/11/2020