otec is not a dream

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Chapter 3

OTEC Is Not a Dream

The Viability of Land-Based Power Plants

Since its discovery in principle by d’Arsonval in 1881, followed by itsexperimental proof by Claude and Boucherat in 1926, OTEC has beenstudied for its practical application for around 100 years. Although somepeople in the past rejected it as impractical since it could only generaterelatively small amounts of electricity, all experts now consider that to beuntrue.

It is already possible to generate electricity economically with today’stechnology in tropical zones where the surface temperature does not fall inwinter. Considering the inventiveness of human beings we can be sure thatevaporators, condensers and intake pipes will all be improved. When thathappens, power generation in tropical zones will become yet more economical,and even generation in middle latitudes with a smaller thermal differencemay become possible. It would be even more economical if the sea-waterthat was pumped up could then be recycled for some other purpose, too.

OTEC systems can be constructed in two locations: on land and out atsea. Dr. Claude mainly studied the on-land type, which requires long intakepipes for pumping up sea-water, high construction costs and large amountsof energy to operate the pumps (Figure 23). Generation costs are high andthe net amount of power generated is low.

However, this type does have a long history of development and thisexperience can be put to good use. Moreover, once a plant is constructed,it has the merit that it can be maintained and repaired easily: in particular itwill not be affected by meteorological conditions such as typhoons. Thismeans that it would be economical if used over a long period. Then the on-land type is even more effective if its sea-water is utilized for other purposesafter generation. This is an important point and will be explained later.

How long would the intake pipes have to be if an OTEC power plant wasconstructed on land in Japan? In the case of the Abidjan Project, pipes fourkilometers long were used to pump up waters from a depth of 430 meters.However, to construct a plant in Toyama Bay in Japan, it would be enoughto have pipes no longer than two kilometers long to reach a depth of 400meters (Figure 24). For Tokunoshima Island in Kyushu, the figure would

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be three kilometers (Figure 24). The topography of the sea bottom inToyama Bay shows a gentle downward slope, while around TokunoshimaIsland it shows a gentle incline for the first two kilometers from the coast lineand a suddenly steep drop after that. The length of the pipes depends onbottom topography, and the ways in which pipes can be fixed on the seabottom differ according to the geological properties of the sea bottom: rock,sand, mud and so on.

Power Plants at Sea

It is also possible to construct power plants on the sea surface or in thesea. This way the water intake can be installed vertically from the plant, andpipes can be much shorter. Construction and operation costs are cheaper;

Figure 23. OTEC generation on land.

Figure 24. Possible locations of DOW intake for Tokunoshima Island and Toyama Bay.

DOW intake pipe

DOW intake pipeTokunoshimaIsland

Toyama Bay

Turbine

Evaporator

Warm sea-water

Cold sea-water

PumpCondenserWaterdischarge

Ikujihana

Ikuji

OTEC Is Not a Dream 33

damage to pipes is less since they are not fixed on the sea bottom; and coldwater gains little or no warmth in the process of pumping up. As a result,net power generation is higher. Moreover, no land is required for plantconstruction.

On the other hand, it would be necessary to build a large structure on thesea surface or in the sea, which would have to be firmly anchored down andprotected from the winds and waves. Cables would be required to deliverelectricity from the plant. Since the only practical experience we have isthrough Mini-OTEC and OTEC-1, much more research and experimentingwould have to be done to make this practicable.

Various research organizations and private enterprises have proposedvarious ideas for designs of OTEC power plants at sea. One of them is toinstall a generating system in a floating structure like a ship. As existingshipbuilding technology could be exploited, this would not be difficulttechnically. But since it would be floating on the sea surface it would workonly where the sea is calm: typhoons and other adverse climatic conditionswould cause damage and threaten safety.

In a system designed jointly by the Comprehensive Institute forElectronics and Technology and Ishikawajima-Harima Heavy Industries,the cold water intake is installed in the bows of a ship (Figure 25).

The TRW Company of the United States has designed a plant not in the

Figure 25. OTEC plant on board ship designed jointly by the Comprehensive Institute forElectronics and Technology and Ishikawajima-Harima Heavy Industries

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shape of a ship but in the shape of a cylinder. Warm surface sea-water istaken into the system through a filter in the middle part of the cylinder, andafter being evaporated by an evaporator it is expelled from the other side(Figure 26). Cold water is taken into the system through a pipe extendingfrom the central part, and after cooling the steam expelled from the turbineit is released from the bottom and sides of the system. As can be seen in thediagram, the pressure of the used sea-water being expelled helps the wholesystem to maintain its position and alignment. This is called “dynamicpositioning”: it was developed as a technique for sea floor oil explorationand drilling, and has already been used in actual practice.

A generating system installed on a turret built on the sea floor has also

Figure 26. Ocean-type OTEC plant designed by TRW. Cylindrical in shape, it is kept inposition by discharging water in various directions.

Separator Generator

Ammonia tank

Ventilator

Turbine

Condenser

Cold water pump

Cold water discharge

Warm water discharge

Warm water pump

Cold water intake

Ballast tankCold water discharge

EvaporatorWarm waterintake

Ventilator

Deck

Gate valve

Cold water


been proposed. This is also technology that has already been applied for oilexploitation. The system is partially submerged, and the intake pipes areinstalled vertically in the center of the turret. At present oil rigs sited inwaters at depths of 300 to 600 meters use this technology, so that it could beapplied for an OTEC plant constructed in shallow waters. This technologymight well be appropriate for OTEC plants in the Sea of Japan, to the westof the country.

A system which was almost wholly submerged would avoid the effectsof winds and strong sea currents at the sea surface. If such a submergedsystem could be constructed, it would be easier to keep the plant in position,and it would be safer even if the system was hit by a typhoon or rough seas.The drawback is that construction costs would be high and maintenance ofthe plant would be difficult.

Figure 27 shows a system designed by the Lockheed Co. in America,

Figure 27. Submersible OTEC plant designed by Lockheed

Outside diameter

Cold water intake

Outside diameter

Anchor chainAnchor

Total length

Main body for OTEC

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consisting of four generators weighing 10,000 tons each and a totalgenerating capacity of 160,000 kilowatts. Constructed with 260,000tons of concrete, it is about 180 meters tall, not including the intake pipe;only 18 meters of it rise above the sea surface, and at its widest part itis 74 meters in diameter. The intake pipe is also made of 35 tons ofconcrete; it is expandable and can be stretched to a maximum of 300meters. To prevent it from being displaced by winds and ocean currents,the whole system is moored on the sea bottom with steel chains thatalone weigh 2,600 tons.

The next Figure 28 shows a submerged system, designed in 1978 by aJapanese committee for research and development of OTEC, which isstreamlined in shape to lessen the effects of ocean currents.

The Cost of Electricity

At present, no country in the world is using OTEC for its everydayelectricity supply: its only application so far is for experimental purposes.Therefore, cost can only be estimated in theory. In Japan, a group of experts

Figure 28. Submersible OTEC plant designed in Japan


for the Sunshine Project have made such an estimate. Their calculationswere made on the basis of generating 100,000 kilowatts using four 25,000kilowatt generators with ammonia as the working fluid, and applied toseveral typical OTEC system designs.

The simulation provided for a system to be constructed on Tokuno-shima Island in Kagoshima Prefecture, or in Toyama Bay, assumingtemperatures of 26 to 28 degrees Celsius at the surface and 0.75 to 0.77degrees Celsius at the lower depths. Seventeen to 26 percent of the 100,000kilowatts of electricity generated would be used for operating the system,giving a net amount of 74,000 to 83,000 kilowatts. It was estimated that thecost of construction would be 44 to 56 billion yen.

As is shown in Figure 29, 40 percent of the total cost would be forconstruction of evaporators and condensers. The reasons for such a highcost are that this would be the first attempt at this kind of construction, andthat the expensive metal titanium would have to be used. The cost would bereduced once some parts could be mass-produced in factories and thetechnology could be adapted for using aluminum alloy. Then, it would beno more than about 20 percent of the total cost. The cost of turbines,generators and pumps would come next at 30 percent of the total cost. Then,about 17 percent would be required for buildings and anchorage equipmentfor keeping position, and 7 percent for intake pipes. Although this estimatedamount would vary according to which type of system was selected andwhere it was constructed, the results of estimates made in the US and Europealso show similar proportions. These figures seem to be more or less correct.

Figure 29. Estimated construction cost for an OTEC plant in Japan

Turbine, generatorpump

Subsidiary equipment

Working fluid

Holding structure

Cold water intake pipes

Evaporator

CondenserTotal constructioncost

About 50billion yen

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The resulting production cost of one kilowatt of electricity per hourwould be 12 to 13 yen, to which 0.66 yen should be added for the cost ofdelivering the electricity to land with a 20-kilometer-long power line. In theUS and Europe, a similar estimate was carried out independently for OTECpower plants with a net output of 100,000 to 400,000 kilowatts, and indicatedan even cheaper cost.

Compared with other power plants, OTEC plants are expensive toconstruct, but the fuel costs nothing. Other types of power plants have to paythe cost of fuel, which varies in general according to current prices. Figure30 shows a projection of electricity and fuel costs for various types ofgeneration in the year 2000 compared with 1977.

The comparison is based on construction costs of $900 for nuclearpower plants and $720 for thermal power plants per kilowatt of output, andthree different models of OTEC: $1500 (in the case of the cheapestconstruction cost), $2000 (medium) and $2500 (the most expensive) (Figure30). Each OTEC estimate includes the cost of delivery to land over adistance of 40 kilometers.

Even supposing that the cost of coal remained the same, by 2000 the costof OTEC-produced electricity would be lower than thermally producedpower, as long as OTEC power plant construction costs could be reduced.Similarly, if the price of uranium, the fuel for nuclear power plants, rose byonly 0.8 percent, OTEC-generated electricity would be cheaper. If coal wentup by 1.4 percent or more per year, OTEC-generated electricity wouldbecome more economical by the year 2000 even with a power plant constructedin the medium cost range. In actual fact, the price of coal rises by more than

Figure 30. Changes in cost of electricity for different fuels up to 2000

Uni

t el

ectr

icit

y pr

ice

% increase in fuel cost per year

Coal

Atomic power

OTEC (high)

OTEC (medium)

OTEC (low)


8 percent per year, which leaves no doubt that OTEC-generated electricitywill soon beat it for economy. Similarly, if the price of uranium rises bymore than 2.7 percent per year, OTEC-generated electricity from a medium-cost plant will be competitive. What is more, if the costs for environmentalconservation and waste disposal are added to the production costs of coal, oiland nuclear power plants, a much higher figure is reached. At present, notenough thought is given to those factors.

Reasons for the Stable Thermal Difference in the Oceans

Because the cold water in the ocean depths will never run out, OTEC canexploit unlimited energy from the sea. How does this natural mechanismwork?

Sunlight strikes the sea. It warms up the surface and water near thesurface, so that the water temperature rises in shallow waters. Since heat isalways transmitted from warm areas to cold, the water temperature at the seabottom should gradually rise. However, the thermal conductivity of sea-water, if it takes place only by molecular diffusion, is very low: it would takeone to seven years for heat to be transmitted to a depth of one meter.

Of course, sea-water does transmit heat when it is mixed up, in the sameway as the temperature of water in a bathtub becomes uniform when we stirit up. This is called turbulence. But vertical movements such as turbulencein sea-water are far rarer than horizontal movements such as currents, so thatheat transmission to deeper parts of the sea takes a very long time.

When convection and turbulence occur in sea-water, heat transfersrelatively quickly from the top to bottom. Convection occurs in winter and/or at night, when the upper layer is cooled and replaced by relatively warmerwater from below; turbulence is mostly caused by waves and currents. Bothphenomena happen only in shallow depths.

For this reason, sea-water from the surface to 100 meters deep has arelatively high temperature that differs little throughout those 100 meters(Figure 31). Then the temperature falls rapidly from this point down todepths of 1,000 meters: beyond there, it remains steady at 4 to 6 degreesCelsius. This means that a huge cold water mass remains on the oceanbottom with a thin high-temperature layer of 100 meters floating on it. Inthe tropics, this thin layer may be 20 to 30 degrees Celsius warmer than thecold mass below. Further north or south, the temperature difference variesaccording to the season: in summer it is high, and in winter, low.

Although vertical heat transfer is slow, most heat should be transmitteddownward, given time, so that the temperature of deep water in the tropicsought to rise gradually. But despite the passage of millions of years, it hasnot risen. There must be some mechanism that cancels out the naturalthermal conduct and keeps the deep water constantly cold.

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One theory to explain this is a huge system of convection that involvesall the oceans of the earth. Sea-water becomes heavier when it is cooleddown in the high latitudes. This heavy water gradually sinks and flows tothe deep seabeds of the world, including the tropics. There it will begradually warmed up by heat from the surface, and will rise to the surfaceitself. The whole process is shown in Figure 32.

As the figure shows, the places where heavy water is supposed to beformed are in high salinity areas: off Greenland in the Atlantic Ocean and inthe Weddell Sea in the Antarctic Ocean. Comparing the northern Atlanticand the northern Pacific, salinity is a little higher in the northern Atlantic.This is generally explained by the heavier rainfall in the northern Pacific,which dilutes the sea-water. When the north Atlantic and the north Pacificare cooled at the same time, the sea-water in the north Atlantic becomes

Figure 31. Changes in ocean water temperature with depth. Down to about 200 meters it variesgreatly with season and geographical location.

Water temperature

Mixed layer Seasonal thermocline

Permanent thermocline

Hypolimnion

Dep

th (

m)


heavier, because of its high salinity, and sinks.The water that sinks in the north Atlantic flows slowly to the Antarctic,

where it merges with cold water that has sunk from the Weddell Sea, and thenflows on through the Indian Ocean to the Pacific Ocean. It takes water thatsinks near Greenland about 2,000 years to reach the Antarctic Sea beforemoving on to the Pacific Ocean and the Indian Ocean. Then it rises by onecentimeter per day, taking a further 2,000 years to come to the surface.

This cold water originating from Greenland and the Antarctic is believedto compensate for any warmth transmitted from the surface to the depths and,as a result, the deep water temperature has always remained constant. It issaid that the total amount of water that sinks in those two areas is about 40megatons (one megaton is equal to one million tons) per second, a hugeamount which is equivalent to the mass of water carried by the Kuroshiocurrent along the Japanese Islands. Naturally, since water could notaccumulate in the Indian and Pacific Oceans, the surface water displaced bythe deep water flowing into those oceans flows back to the Atlantic Oceanat the surface, to be cooled again and sink once more.

The Potential for OTEC Energy

It is important to calculate how much energy can be exploited throughOTEC, but at present no firm estimate can be given. For this purpose, itwould be necessary to know everything about sea-water temperatures andsea currents all over the world. Although much is known about seatemperatures, ocean currents are still not fully understood, and muchoceanographic research remains to be done. However, in order to put OTEC

Figure 32. Water circulation in the oceans. Water sinks off Greenland and in the Weddell Sea,and wells up in the Indian and the Pacific Oceans.

Greenland

Weddell Sea

Shallow

Deep

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to practical use, it is necessary to identify the places where OTEC could beexploited, and roughly how much energy might be potentially availablethere.

The solar energy that irradiates the earth surface is said to be 83.6trillion kilowatts. Since two-thirds of the earth surface is covered by sea,55.1 trillion kilowatts per second of solar energy strike the sea surface. IfOTEC could use just two percent of this energy, one trillion kilowatts ofelectricity could be produced. This figure is probably a maximum: in reality,there are many places in the sea where the temperature difference of sea-water is negligible, which would considerably reduce the actual figure.

The American Dr. Zener estimated the amount of energy that could beobtained if sea-water between latitudes 20 degrees north and 20 degreessouth was used. He estimated that 60 billion kilowatts of electricity couldbe produced with variations in sea surface temperature within a range of onedegree Celsius. This figure is considered to be the most realistic at present.

For Japan, considering that OTEC is feasible with a temperaturedifference of 20 degrees Celsius or more between the sea surface and a depthof 1,000 meters, only southern Kyushu fulfills the necessary conditions(Figure 33). Most of the main island of Honshu has temperature differencesof less than 20 degrees Celsius, and the surface temperature falls even furtherin winter.

There have been other, more modest estimations. One example is the

Figure 33. Temperature differences between the surface and 500 meters


model made by Drs. Wick and Schmidt, which proposes a 12-degree Celsiustemperature difference between the top 100 meters at the surface and a depthof 400 meters, and 1,000 years to exchange completely the upper and lowerwaters. Their estimate indicates a potential 50 billion kilowatts, of which 20billion kilowatts might be generated by OTEC.

Another modest figure was obtained based on using 10 percent of the 40megatons per second and assuming no significant temperature difference.By applying the currently available OTEC technology, that would enable 10billion kilowatts to be generated.

A Japanese committee for OTEC estimated the amount of energypotentially usable for Japan. Figure 34 shows the situation of ocean energywithin the area where Japan could utilize it exclusively: the ExclusiveEconomic Zone (EEZ), within 200 nautical miles (370 kilometers) from thecoast. The total amount was estimated at 30 billion kilowatts. This figurecorresponds to about 8.6 billion tons of oil per year, which is about 20 timesas much as the total energy consumed in Japan in 1980. Supposing only onepercent of this energy was used, it would be possible to reduce oil consumptionby about one billion tons.

It may be necessary to study the oceans and the climate surroundingJapan in greater detail for a more precise estimate, but what is certain is thata great deal of energy could be extracted from the seas both to the east andto the west of Japan. What is more, it could be used for ever.

deep. No figures for coastal areas less than 500 meters deep.

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Figure 34. Heat energy available from water 600 meters deep in the Exclusive Economic Zoneof Japan (dotted line). Figures show 100 billions of kilowatt/hours per year in an area of 1degree latitude by 1 degree longitude.

otec is not a dream

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