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2nd International Conference on Ocean Energy (ICOE 2008), 15th – 17th October 2008, Brest, France

Impacts Study of OTEC Seawater Effluent Discharge

2nd International Conference on Ocean Energy (ICOE 2008), 15th – 17th October 2008, Brest, France

Jacques Merle 1, Gerard Nihous2 , BrunoVoituriez3, Raymond Zaharia3, Michel Gauthier3

1 Club des Argonautes, France, ,

2 Hawaii Natural Energy Institute, University of Hawaii, Honolulu, U.S.A, 3 Club des Argonautes, France.

2nd International Conference on Ocean Energy (ICOE 2008), 15th – 17th October 2008, Brest, France

Abstract

Ocean Thermal Energy Conversion (OTEC) seems to be a renewable inexhaustible source of energy if we consider that 2/3 of the solar radiative energy heating the earth is stocked in the ocean. OTEC operations rely on the existence of sustained vertical temperature differences through the water column, and therefore on elevated surface temperatures generally found in inter-tropical areas. The Ocean Thermal Energy resource can be partially converted into mechanical and electrical power by utilizing the existing thermal stratification between warm surface water and cold deep water. Number of technological constraints, associated with the low process efficiency, lead to consider that OTEC could product a theoretical energy power of 10 Tera Watts (TW = 1012 watts) which represent about two time the energy consumption of humanity predicted for the year 2040 (Nihous 2005 and 2006). But this estimate does not take into account the degradation of the oceanic thermal gradient structure and the biological perturbations induced by the impact of the massive sea water intakes and effluent discharge, the later having a temperature and chemical composition different from the ambient values depending of the depth of the discharge. This important and new limitation, known as the “impacts of artificial upwelling”, lead to an OTEC maximal power energy production of 3 TW (Nihous 2006). This theoretical estimate should be evaluated more precisely in consideration of the local constraints and of a number of parameters like the scale of the OTEC operation (overall power generation capacity), the spatial distribution of power plants and the effluent discharge strategy. In the latter case, multiple choices are available and environmental responses will vary according to the depth at which effluents are released, mixed or not. It is therefore critical, using various modeling tools, to carefully evaluate impacts from OTEC seawater intakes and effluent discharge under various scenarios in order to simultaneously optimize OTEC power production and minimize its potential disruption of the ocean environment. A positive impact of these “artificial upwelling” on the ocean biological productivity should also be studied coupling biological models with ocean-atmosphere physical models.

1. INTRODUCTION

Ocean Thermal Energy Conversion (OTEC) has recently received renewed attention as the search for renewable, clean energies capable of replacing costlier fossil fuels has intensified. The most accessible reserves of oil, coal and natural gas have actually started to decline. Stored as heat in the surface layer of tropical oceans, solar energy can be partially converted into mechanical and electrical power by utilizing the existing thermal stratification between warm surface water and cold deep water. The conversion process, conceived by the end of the 19th Century and tested in the 1930s, uses warm surface water and cold deep water to respectively feed an evaporator and a condenser on either side of a turbomachine operating on a so-called Rankine cycle.

The ocean is a renewable almost inexhaustible source of energy. More than 2/3 of the solar radiative energy heating the earth is stocked in the upper layers of the ocean. But there are important differences by latitudes in the heat budget at the surface and the temperature vertical gradient structure. Then we can only expect an average of 20°C temperature difference, that lead to a low 3% thermodynamic process efficiency, between the warm upper waters and the deeper cold waters, in some regions of the tropical ocean.

Another crucial limitation of the exploitation of this energy is the sustainability of the resource. But, may be more important, is the impact of this exploitation on the tropical ocean thermal structure and the general ocean circulation, particularly the meridional thermohaline circulation which is possibly implied in climate oscillations mostly in the Atlantic.

Artificial upwelling of deep nutrient-rich water in oligotrophic areas should also be investigated as a biological impact of OTEC. Several recent studies have tentatively tried to estimate these sustainable resources and there limitations and impacts, using simple models (Nihous 2005, 2006, 2007).

We present in this paper an overview of the results of these researches and propose some direction of investigation using more complexes models: Oceanic General Circulation Models (OGCM) coupled or not with Atmospheric General Circulation Models (AGCM). The objective is not to restrict the development of OTEC but rather to show its profound link to the oceanic environment, like the other marine energies resources, all issued of the sun, and to advise on the large gap between the theoretical potential of these energies and their practical amount of exploitation.

2nd International Conference on Ocean Energy (ICOE 2008), 15th – 17th October 2008, Brest, France

2. THE HEAT RESOURCE OF THE OCEANS

2.1 The heat budget

2nd International Conference on Ocean Energy (ICOE 2008), 15th – 17th October 2008, Brest, France

Nearly 160 W/m2 enters the ocean from the sun radiation representing more than 2/3 of the total solar flux received by the earth. If integrated over the whole sea surface area, an amount of 52 Peta watts (10 watts) is stored in the ocean. On reverse, the atmosphere is almost transparent to this incident solar energy flux. Most of the energy which is necessary to the atmosphere to entertain its dynamic, is provided by the ocean through the air-sea interactions. The two dominant factors which drive the heat budget at the surface are the net incident solar radiation, positive for the ocean, and the latent heat transfer to the atmosphere associated to the evaporation. The net solar radiative flux depends mostly on the latitude, since the latent heat loss by evaporation occurs mainly at the western hedge of the ocean basins where warm currents (like the Gulf Stream in the Atlantic and the Kuro Shivo in the Pacific) reach regions of cold dry air in middle latitudes (20 ° - 50 °).

2nd International Conference on Ocean Energy (ICOE 2008), 15th – 17th October 2008, Brest, France

Then the repartition of the net heat exchange across the ocean surface presents a very large spatial variability (figure 1). We observe a positive heat input in the tropics of an average of 75 Watts/m2 and an enormous loss of heat by evaporation in regions where the warm western currents can release a flux of 150 W/m2. Then globaly, the tropics are traping heat since the middle and high latitudes are losing heat and feeding the atmosphere. These large latitudinal differences of the ocean heat budget are at the origine of the ocean meridional overturning circulation made of warm surface waters flowing poleward, particularly northward (Gulf Stream and Kuro Schivo as exemples of nortward warm currents), compensated by cold deep waters returning to the south and upwelling in the intertropical regions (Figure 2).

2nd International Conference on Ocean Energy (ICOE 2008), 15th – 17th October 2008, Brest, France

2.2 The tropical thermal structure

The intertropical thermal structure is the result of this heat budget repartition. Near the equator, the heat gain in the eastern part of the tropical basins, mostly dû to the relatively cold surface waters minimazing the evaporation, warms a surface mixed layer which accumulates and deepens in the west under the forcing of the trade-winds stress and the associated equatorial currents. Thus “warm pools” of waters over 28°C and with a depth of almost one hundred meters (An average of 150 meters in the Pacific and 100 meters in the Atlantic) concentrate a large part of the energy stored by the ocean (Figure 3). The Pacific “warm pool” linked to the eastern upper warm waters of the Indian ocean represents the main energy reservoir of the planet, feeding the atmosphere.

On a more global scale, under the mixed layer and the thermocline, the cold waters are upwelling between latitude 30°N and 30° S and on a depth between about 1.000 and 300 meters (Figure 2). Thus in the tropics, the thermocline is sharp and we can observe a very large temperature difference ∆T, between the warm surface mixed layer and the cold deep waters. ∆T should be at least equal or over 20 °C for a minimum efficiency of the heat engine [1, 2]. And the depth of the pumping of cold waters should also be minimized and restricted to a maximum average of 1.000 meters. It appears that such a 20°C difference between the surface and deep temperatures could be found in some place of the tropics on depths of only 400 to 500 meters.

Moreover two doming zones appears at around 10 ° of latitude, symmetrical to the equator, where we can find temperature of less than 9 ° C at only 400 meters depth (Figure 4).

3. IMPACTS OF OTEC

3.1. The basic questions

The impact of OTEC on the ocean environment will essentially consist of massive seawater intakes and effluent discharge, the latter having a temperature and composition a priori different from ambient values. The magnitude of this impact will mostly depend on the scale of OTEC operations (overall power generation capacity),

on the spatial distribution of power plants and on the effluent discharge strategy. In the latter case, multiple choices are available and environmental responses will vary according to the depth at which effluents (from evaporator and condenser) are released, mixed or not. It is therefore critical to carefully evaluate impacts from OTEC seawater intakes and effluent discharge under various scenarios in order to simultaneously optimize OTEC power production and minimize its potential disruption of the ocean environment. Such studies concern the global impact of future intensive OTEC exploitation. They are different from those necessary to

evaluate the local impact of any OTEC plant construction. Optimization itself could be based on different metrics according to possibly different objectives: e.g. to maximize power production, or to promote biological production from the artificial upwellings generated by the discharge of nutrient-rich OTEC effluents into the photic layer.

3.2. Importance of artificial Upwellings.

Natural upwellings are produced by wind stress over the ocean under certain conditions that favor a divergence of surface waters (proximity of a coastline or of the Equator). This locally induces the upward motion of deeper, colder nutrient-rich waters toward the surface.
Upwellings play an important role in the global energy balance of the ocean and of the Earth. In tropical regions, they allow an accumulation of heat that is later transferred to higher latitudes by major currents (Gulf Stream in the Atlantic and Kuro Shiyo in the Pacific). They also strongly mediate interactions between the ocean and the atmosphere by inducing meteo-oceanic oscillations that affect climate across the entire tropical belt, such as the phenomenon known for centuries as El Niño (ENSO).

Wherever they occur, upwellings boost biological productivity as well. The high nutrient concentrations generally found in deeper waters are advected upward into the photic layer where photosynthesis is promoted. This increase in primary productivity benefits the entire food chain. In certain cases (cf. below), artificial upwellings related to OTEC operations could contribute to a local increase in primary production. While seemingly positive, the consequences of such biological effects should be assessed from a long-term perspective involving multiple trophic levels.

In view of the foregoing, potential impacts of artificial anthropogenic perturbations of the thermal and chemical structures of the ocean upon ocean dynamics itself, atmospheric dynamics and marine biological processes must be evaluated carefully in the context of an ultimate and massive deployment of OTEC plants within tropical regions. Thresholds could exist beyond which such perturbations could permanently alter oceanic circulation and, possibly, atmospheric circulation as well. This could substantially constrain the acceptable scale of OTEC-related perturbations upon the environment. Such thresholds should be assessed as precisely as possible.

A first analysis of the sustainability of the heat resource with a simple one dimensional model has been used to estimate the value of the theoretical amount of energy that could be extracted […..].

4. ORDER-OF-MAGNITUDE OF SUSTAINABLE OTEC RESOURCE.

(Gerard Nihous…)

5. CONCLUDING REMARKS.

The order of magnitude of a theoretical sustainable exploitation of OTEC resource is close to 3 TW which represents almost an electrical power capacity as large as that needed by mankind in 2040 (5 TW). But many environmental issues arise from a prospective deployment of OTEC technologies, the more so as effluent discharge options may vary with different targeted outputs.

For a given design configuration, several questions should be addressed :

How will ocean dynamics adapt to a perturbation of the oceanic thermal structure?

What effects perturbations of the ocean surface temperature and the onset of an artificial heat sink may have on the atmosphere and its dynamics?

How can marine biota respond to nutrient enrichment and different temperatures?

These questions give rise to research priorities which can be articulated along three axes, with a definite dependence on the selected effluent discharge option:

- Study of the perturbations of the thermal structure of the ocean and of the dynamic response of the ocean as a result of OTEC operations.

- Study of the coupling of thermal and dynamic oceanic perturbations with atmospheric behavior.