Dossier Océan et énergie - Énergie Thermique des Mers

Sommaire IOA News Letters


By Tim Downs

This article was to have been a simple description work I carried out for the Hawaii Natural Energy Institute during 1988-90. However, I would like to begin with a personal view of why I believe work in OTEC is important.

At the root of all our future science and engineering into natural resources there should lie a motivation to understand them first and then use them sensibly .

Growing up in Britain, one is very aware of its industrial heritage. The poet Wordsworth wrote glowingly about the pioneers of industrial science and technology:

..... those to whom the harmonious doors
Of Science have unbarred celestial stores,
To whom a burning energy has given
That other eye which darts thro 'Earth and Heaven.

'An Evening Walk' (1788-9)

Later, he became disillusioned about the spirit of his 'Inventive Age' with it's 'new and unforeseen creation'. Sensitive to the monumental changes he observed, he wrote of 'unremitting fires' and the permanent curtain of smoke they disgorged.

Fundamental to the environmental problems of resource exploitation is the way that human beings view their place in the natural world. The relationship between human beings and Nature has logically been governed by utility. Reaching an extreme state, this motivation is characterized by swift and ignorant consumption that is inherently fallacious in its benefit.

It is true that human beings can change neither the laws of cultural evolution nor those of organic evolution. However , we can understand these laws, act with recognition of them, and thereby influence their outcome for the better. This approach of 'enlightened self-interest' is exemplified in the work on OTEC systems. I was struck from this outset by the implications of this form of energy production-its local and, more significantly, its global potential.

I suspect that the scientists and engineers who believe in OTEC, share my hope that we are entering a new era-one that reflects our precarious relationship with natural resources, and uses it as motivation for assuming the true responsibility of knowledge:

an active conscience. By coming to terms with Life as a continuum, we can compromise with Nature to ensure its survival and therefore our own.

'No philosopher's or poet's fancy, no myth of a primitive people has ever exaggerated the importance, the usefulness, and above all, the mar-vellous beneficence of the Ocean for the community of living things.
--- L J Henderson, ' The Fitness of the Environment'

Much has been written about the prospect of using the nutrient rich cold effluent from OTEC plants to support mariculture. My brief was to consider the conceptual design of a structure that could be deployed in offshore or nearshore locations, and would allow the cold water outflow to be retained in the photic zone long enough to stimulate phyto-plankton growth. The scale of the floating OTEC facility was viewed to be very large, of 100-1000 MW capactity.

Among the aspects considered were:

Form and Mode of Operation;
Hydrodynamic Support-Buoyancy;
Hydrodynamic Loading and Response;
Mooring and Free-grazing Options;
Materials-strength and longevity;
Constructions Details-novel structural elements;
Deployment Logistics; and
Model and prototype requirements.

The scale of the mariculture structure was also very large, having a characteristic length of 500m-1000m so as to use a substantial but measured proportion of the could OTEC outflow. It was considered to be made of highly flexible material. Residence time requirements are such that use of the outflow must be partial, since a Very Large Scale OTEC (VLS OTEC) facility would handle extremely large fluid flows.

The commercial aspects of offshore OTEC-associated mariculture are clear. However, there are important problems that must be recognised:

Biofouling of OTEC plants due to mariculture waste;
OTEC downtime impact on bio-mass; and
Interference between mariculture structure and OTEC plant-differential environmental loading and response.

The last aspect is a fundamental one that leads to the scenario of a no-physical-link approach to the problem (as has been suggested for the CWP/Support Structure interaction problem). Structure loading on a floating mariculture structure comes from static pressures due to the containment of heavier-than-ambient fluid, and from waves and currents that impose dynamic pressures. Unusual problems arise when trying to estimate hydrodynamic loading and response for a very flexible body. For wave loads on rigid vertical cylinders, for example, we may apply different methods depending on the relative size of body and wave:

D/L >1
conditions approximate pure
D/L >0.2
diffraction increasingly important
D/L <0.2
Morison equation usable
(where D= characteristic length; L= wavelength)

The concept of highly-flexible-body strain (and resultant stress) is hard to focus upon. Resistance to deformation and load transmission intuitively seem very small and, indeed, the scale of the structure means that this may well be advantageous. Structural planes will tend to readily deform with water particle motion. Lack of stiffness means that estimation of response is not possible in the traditional way. After all, the term 'natural frequency' has no meaning for such a body and non1inearities are intrinsic.

Deep water mooring is as much of a problem for any mariculture structure as it is for the parent OTEC facility. [The author is presently working on a floating form for deep water OTEC support structures that may overcome some of the stationkeeping problems of an OTEC facility.] Free grazing OTEC plants and any free ranging associate structure have serious implications and some means must be found for economic control within acceptable CWP/Support Structure acceleration ranges.

Biofouling of a mariculture structure is a serious problem, mainly because it causes an increase in current drag and can cause the deterioration of structural materials. Indeed, the rapid formation of marine growth will cause frictional drag to increase and turbulent layer flow to dominate over a wider range of Reynolds numbers.

Several innovative structural elements were used with a view to maintaining the form of the structure, including some drawn from aerospace applications that offer low weight/high strength advantages.

The work also included an investigation of the mechanism for natural cold eddies and upwellings, and the feasibility of imitating them using OTEC outfolws.

I offer my thanks to those in Hawaii who supported me in this work, especially Pat Takahashi, Ludwig Seid1 and Karl Bathen.