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Dossier Océan et énergie - Énergie thermique des Mers

Ocean Thermal Energy Conversion: an opportunity for the Maritime Industry with early application to Islands.

GAUTHIER Michel. IOA Acting Chairman[1];

LENNARD Don. Ocean Thermal Energy Conversion System Ltd; 

Abstract: Recent study predicts a global growth of the shares for Renewable Energy to respond to the increasing World demand for primary energy. Among Renewables only rapid development of solar energy systems could permit to match the demand. Ocean Thermal Energy Conversion (OTEC) is one candidate. 

OTEC transforms the heat stored in the surface water of tropical oceans into mechanical work to produce useful energy. The main advantages of OTEC are :

and it is predicted early OTEC applications will be to Small Isolated Islands scattered in the tropical ocean regions. 

Very many sites are candidates for few tens MW OTEC/DOWA facilities. They represent the early market for OTEC, and priority island locations have already been identified. These potential early users are most often developing or poor countries which will not have the resources to pay at their real cost for the small plants adapted to their needs and the development of OTEC implies a long term strategy and international co-operation with government support from rich industrialised countries, and groupings such as the European Union with its Lomé and Cotonou Agreements. 

Introduction

Recently, attention of Solar Energy advocates was caught when reading a press release published in the March 2001 issue of “International Ocean Systems”. In this journal an article revealed that the NIOT, the Indian “National Institute for Ocean Technology” had ordered from a US company a large mooring line swivel with an acoustic release for the mooring of a 1MW OTEC barge. This announcement indicated that a significant step ahead was in course of development for OTEC technology. In this paper the authors will briefly review the basis of the OTEC process for the production of both electricity and fresh water, and revisit some past main attempts to design and promote the construction of OTEC plants adapted to the specific needs of small isolated island communities located in the ocean tropical belt, and make reference to other priority sites for such plants. 

Basic OTEC principles and history. 

The ocean surface is a huge captor of solar energy and most of this energy is stored under the form of thermal energy in the ocean surface layer. It is estimated that the annual amount of solar energy absorbed by the ocean is equivalent to several thousands times that consumed by humans. (i.e. 9 billion toe in 1990). The water in the surface layer of open ocean does not mix easily with deep water and the vertical temperature distribution shows a rather rapid change in temperature within the first few hundred metres below the surface. In the tropical region the water temperature of the sea surface upper layer ranges from 26 to 29 °C when that of the deep water at 1000 metres is quite uniformly ranging around 4 °C. This observation made by oceanographers in the mid - 1880s is at the origin of the Ocean Thermal Energy Conversion principle. It consists in using the ocean surface reservoir of warm water as the heat source of a machine and the deep water reservoir as its cold sink. The basic design of an OTEC power plant can be copied from the design of the steam engine using the Rankine cycle i.e. using hot source for heating and evaporating a « working fluid » in a boiler/evaporator, then expanding the working fluid vapour produced through a turbine before condensing the low-pressure vapour in a condenser using the cold sink to absorb and reject the remaining thermal energy outside the system. 

In fact this conversion of thermal energy into useful mechanical (and then electrical energy) is identical to that of our traditional power plants using fossil or nuclear fuels. Except that the temperatures of the hot source and the cold sink are quite different. For traditional power plants the temperature difference is greater than one hundred degrees, but it is within the range of 20 to 25° C for OTEC. Thermodynamics laws say that the useful energy that can be extracted from a given quantity of thermal energy is proportional to this difference in temperatures and this is translated for Ocean Thermal Energy, as for other solar energy, in saying OTEC is a “diffuse” energy. By principle such diffuse energy conversions plants are at no-cost-for-fuel but they necessitate large installations to produce significant power. Hence OTEC plants are relatively high capital cost facilities.

Because of the levels of warm and cold temperatures available in the ocean the choice of the working fluid is one determinant option for the development of the technology best adapted to OTEC. 

A first option was imagined in 1881 (Arsène d’Arsonval 1851-1940). It was suggested to use any fluid having an appropriate vapour pressure at a temperature close to that of warm seawater. After cooling in a surface condenser the liquid working fluid was re-introduced by pumping in the evaporator, and recycled in the circuit. This concept is known under the name of closed-cycle OTEC.

Among the many fluids initially suggested as potential candidates ammonia -NH3- was effectively tested at sea for the first time in 1978 on board of the Mini-OTEC-1 closed-cycle floating facility offshore Hawaii. Other fluids like propane, butane and freon are also main potential candidate working fluids for closed-cycle OTEC. Freon was first tested in the Japanese onshore 100 kW close-cycle OTEC plant experiment at Nauru in 1981.

Main technical difficulties with closed-cycle OTEC is the design of large surface heat exchangers with low rate of corrosion by sea water that otherwise might leak and mix with the working fluid, and low fouling growth that could drastically reduce thermal exchanges and jeopardise plant efficiency. Industry claims available technology would enable the construction of modular closed-cycle OTEC plants with capacity of hundreds of MW.

Another option was proposed and tested at sea in Cuba in the 1930s (G.Claude 1971-1956). In this second concept the working fluid was the warm seawater itself which forms vapour when boiling in an evaporator maintained at the appropriate low pressure. After driving the turbine the vapour was directed to the condenser stage. To maintain the appropriate pressure in the system requires specific gas exhaust equipment to remove air gas introduced in the system by the degassing of seawater and leakage through the structure. Vapour was condensed either through a direct contact condenser by mixing with cold sea water or directed through a surface condenser where desalinated water can be recuperated as a by-product of the thermal process. In both cases the condensed vapour was not reintroduced in the working fluid circuit and Claude’s concept is known as the open-cycle OTEC.

In 1992, almost 60 years after Claude’s first experiment, a 210 kW open-cycle OTEC plant was built onshore Hawaii USA and successfully tested during one year by the Pacific International Center for High Technology Research (PICHTR) at the Natural Energy Laboratory of Hawaii (NELH). Main technical difficulties for open-cycle OTEC come from the low vapour pressure of the working fluid that imposes very large size turbines. Industry claims that available technology would enable the construction of modular open-cycle OTEC plants with capacity of tens of MW.

OTEC for small Islands. 

The solar resource is abundant and widely distributed in the tropical and sub-tropical regions and ocean thermal energy is available 24 hours a day all year round and hence presents the main advantage to be available for decentralized base-load production.

OTEC technology is simple. Its thermo-machinery implies low temperature and low pressure components and its sea water pumping subsystem is off-the shelf for small plants with power capacity of a few Megawatts. 

OTEC operation is thought to have benign environmental impacts when the extraction of energy is kept below 0.2 MW/km² . But environmental issues are not fully investigated yet and this value is calculated for open sea operation and should be adapted to local condition for coastal installation. 

With the possibility for OTEC also to produce desalinated (potable) water as a by-product where it is needed, the OTEC development has been thought from its early beginning to be beneficial first to inhabitants of small islands scattered in the oceans’ tropical belt. Additionally, the cold deep water is free of pathogens but rich in nutrients and can therefore be used for other purposes – the so-called Deep Ocean Water Applications (DOWA) – for example pharmaceuticals, the growing of fish at greatly enhanced rates, and even agriculture. Some of these may be useful to island states in adding to the variety and total quantity of their GDP.

A study of opportunities for OTEC in island locations around the world, undertaken in an unpublished report for the UK’s Department of Trade and Industry, included a number of relevant factors in addition to the fundamental one of the thermal resource. As a result it was possible to derive a priority listing. The factors included the present electrical generating system on each island considered, and whether it was based on oil, coal or hydro. Nuclear power was not an issue since none of the small islands would ever have a total power demand to justify a nuclear installation, and this remains the case today. The pricing of those fuels was important, and anomalies were critically examined. For example, in one Pacific island where the introduction of a hydropower system was expected to reduce generating costs, they did in fact rise. In the case of another island which had a hydropower system installed, both the dam and the lake were discovered to be in a rain shadow area causing the power supply to be unreliable. For a Carribean island, the initial survey marked it at a lower level of priority because the temperature difference in the location was some 22-23°C. However, when the preferred site for an OTEC plant was decided upon, the temperature difference there was found to be >24°C. Since for every 1°C increase in temperature difference, the efficiency of an OTEC plant increases by some 10%, the priority for that island increased. Factors in addition to temperature difference which were covered in this prioritising process included the distance from shore to the site (from zero to 100km), the scale of current velocities in the area (from less than 0.2m/s to greater than 0.6m/s) and the frequency of storms in the area and their severity. Clearly, the thermal resource was the key starting point though, and this was assessed for values of temperature difference from 19°C to >23°C, and for the depth at which this difference occurred from as little as 700m to a more usual 1000m. Each factor had a weighting applied to it in relation to the others, and within each values were assigned. For example, the factor with the highest weighting was the temperature difference, and the values for that varied from 8 where the ΔT was 23°C down to 1 where it was 19°C. Finally, the demand for power, and its anticipated rate of growth were assessed, and some islands were removed from consideration because the demand was less than would economically justify an OTEC plant.

Taking all these factors into account the island locations which came out as the best opportunities for OTEC were: Papua New Guinea; Fiji Island; St. Lucia; Jamaica; Bahamas; Trinidad and Tobago; Cayman; Guam; and the Pacific Island Trust Territories. To take just two of those a little further, the preferred site in Fiji Island is on the second island - Vanua Levu – where the thermal resource was less than 1km from shore in an inlet rather like a Norwegian fjiord; and for St. Lucia the high ΔT was within 3km from shore on the west coast near Soufriere, due to the volcano which had exploded in times past, leaving Petit and Gros Piton on the shore as evidence of its existence. So encouraging was this site that a detailed survey of temperatures, depths and currents was undertaken for the whole of Soufriere Bay as far south as Gros Piton Point and out to 10 km. It is detail such as this which is necessary if proper prioritization is to be done. 

A further example to demonstrate the interest of small islands for OTEC is the design of an onshore 3.5 MWe open-cycle OTEC electricity plant for the Guadeloupe Island in the 1950s. The Cold water pipe was 4.2 km long and 1.95 m in diameter. The design of the surface condenser allowed fresh water production of 5000 m3 per day. The global investment cost was 5075 million F (1959) i.e. about 70 million Euro present value [1]. The economic study included different hypotheses on CWP life time ( i.e. 10, 15 or 30 years) and interest rates ( zero, 4 and 8%). Three sale prices were considered for fresh water, starting at 0.57 Euro/m3, to 1.16 and 1.74 Euro/m3. The highest price (120 F/m3 ,1958 or 1.74 Euro/m3) was that of water obtained from traditional desalination plants at the end of the 1950s and the lowest (0.57 Euro/m3 ) was an estimated limit for the long term. Detailed results are given Table 1.

 

Table 1
Summary of Guadeloupe OTEC economic study

 

Loan rate

0%

4%

8%

Reference cost of fuel/electricity 

in Euro/kWh

0,09

0,10

0,11

Cold Water Pipe Life Time

Price of Fresh Water in Euro/m3

30 years

0,71

1,54

2,62

15 years

1,30

2,01

2,96

10 years

1,60

2,31

3,29

 

The Guadeloupe OTEC study showed that assuming a loan rate equal to 8%, a cold water pipe life time of 30 years and a reference price of 0.11 Euro/kWh for electricity, the sale price of fresh water as a by-product had to be equal or greater than 2.62 Euro/m3 to render the OTEC plant competitive. With a lower loan rate of 4%, commercial viability was obtained with prices of 0.1 Euro/kWh and 1.54 Euro/m3. But the studies concluded OTEC was not economical because there was no market for desalinated water and Guadeloupe OTEC projects were abandoned in 1959, respectively, for economic reasons.

Another example is the design of an OTEC plant for Tahiti [2]. The project plan was to build an OTEC 5 MWe pilot plant to deliver 10 to 15 % of the electricity consumed on the island and to be distributed on the existing distribution grid. The power goal was to be obtained for the maximum seasonal temperature difference at the Tahiti site, which was recorded as 24° C when warm water reaches 28 °C and cold water is pumped at about 4°C from 1024 metres depth. Final optimisation of the ratio of Cold Water Pipe length (and so of the temperature of CW and of the CWP cost) over the annual electricity production led to the final pumping depth at 700 m, with a 2.3 m diameter and 2 km long pipe, made of fibre- glass and syntactic foam materials. 

The Energy Subsystem was composed of two identical and autonomous modules composed of a large vacuum vessel divided into three compartments, which contain the evaporator at the centre and two condensers at the sides. A turbo-generator unit was placed above the modules. It was composed of two turbine wheels at the ends of the alternator shaft. The gross power of each turbo generator unit was 3.3 MW generated by a 40 kg. s1 steam flow directed to the turbine wheels with an outside diameter of 4.5 m equipped with turbine blades 1.1 metres long developed for the low pressure turbine stage of nuclear plants. 

The power balance of the energy modules shows that about 10 % of the gross power was consumed for venting, 10 % for pumping cold water and 4 % for pumping warm water. The electric net power was expected to be about 75 % of gross power. 

The cost estimate for the construction of the 5 MW plant was 500 Million French Francs (1985) i.e.: about 19 Euro/W elect , 1996. The production cost was 1.5 FF/kWh (1985) or ~0.3 Euro/kWh (1996); this price was about twice the cost of Diesel electricity on the island. The Open Cycle design offered the possibility of a fresh water by-product of 7000 cubic metres per day but this was of no economical interest for the Tahiti market.

For India [3] the total OTEC potential in Indian EEZ water is estimated to be 180 GW, with a power density of 0.2 MW/km² [4] but the priority seems to be to develop small OTEC plants to supply electricity to archipelagos of islands such as the Lackshedweep, Andaman & Nicobar Islands. In India the interest for OTEC was first raised early in the 1980s with the preliminary design of a 1 MW closed-cycle floating plant using ammonia as working fluid. The size of 1 MW is acknowledged as the minimum size to confirm results obtained today from theoretical studies and small size experiments, and to demonstrate the overall capacity for OTEC to supply commercial electricity. The construction of such a plant by India, with the assistance of the Japanese University of Saga, might be a stepping stone in the learning phase of the development of the OTEC technology necessary to scale up OTEC to tens and hundreds of MW.

Conclusion 

Very many sites – some of which have been mentioned - are candidates for few tens of MW OTEC/DOWA facilities. They represent the early market for OTEC, and priority island locations have already been identified. These potential early users are most often developing or poor countries which will not have the resources to pay at their real cost for the small plants adapted to their needs and the development of OTEC implies a long term strategy and international co-operation with government support from better off industrialised countries, or groups such as the EU.

 


 

REFERENCES

[1] Gauthier, M. “OTEC economics and electricity costs : a little of OTEC history” . IOA Vol 11 N°4, Winter 2000 Retour

[2] Institut Français de Recherche pour l’Exploitation de la Mer. Publication Interne, 1985 . Retour

[3] Ravindram,M. “The Indian 1 MW Floating OTEC plant –An Overview”. IOA Newsletter Vol11, N°2,Summer 2000 Retour

[4] Avery W. & Wu C. “Renewable Energy from the Ocean A guide to OTEC” . 1 Oxford University Press 1994. Retour