THERMAL POWER

Ocean thermal energy conversion — past progressand future prospects

D.E. Lennard, BSc(Eng), CEng

Indexing terms: Energy conversion and storage, Load and voltage regulation, Power systems and plant, Ocean thermal energy conversion, Naturalresources, Thermal power

Abstract: An overview is provided of the currentstatus of ocean thermal energy conversion,together with an historical perspective and theprogress anticipated in the next ten years. Theproblems yet to be resolved are briefly described,and the importance of providing risk assessmentsis stressed so that funding agencies may be per-suaded of the commercial prospects for this base-load generating system consisting predominantlyof existing technology. It is concluded that alimited market exists for island developingnations, with prospects for substantial growth asOTEC plant efficiency improves or as oil pricesincrease.

1 Introduction and history

The last 15 years have seen growing interest in, andcareful evaluation of, renewable sources of energy as theworld has become increasingly energy conscious. Thishas been, in part, due to substantial rises in the cost of oilduring that period which have brought the economics offossil-fuel energy and renewable energy much more intoline. While oil prices in the recent past have weakened,and most forecasts suggest this will continue for someyears, the overall long-term trend of prices for finite non-renewable resources is upward. The two key questionswhich remain are: just how competitive are the costs forrenewable energies and how reliable is their technology?Although ocean thermal energy conversion (OTEC) hasexisted in theory for just over 100 years, its technologicalstatus now owes very much to developments in offshoreactivities associated with the oil and gas industries sincethe late 1960s, more particularly as exploitation of thosefossil fuel resources has moved into deeper waters withharsher climates. There is also a substantial set ofopportunities for drawing on materials and technologydeveloped for other applications, in other industries, andthese are described later. There have also been extensiveprogrammes devoted specifically to OTEC in a numberof countries during recent years, and these also aredescribed.

Paper 5190A(Sl), first received 16th October 1985 and in revised form23rd June 1986The author is Managing Director of D.E. Lennard & Associates Ltd.and also of Ocean Thermal Energy Conversion Systems Ltd., 40 Brox-bourne Road, Orpington, Kent BR6 OAY, United Kingdom

OTEC is based on the extraction of energy from thetemperature difference existing between the warm surfacewaters of the oceans in extensive tropical and subtropicalareas, and the deep waters in those same areas whichhave flowed from the polar regions, predominantly theAntarctic. Fig. 1 indicates the general magnitude of thesetemperature differences around the globe. The energyextraction process is based on well established heatengine cycles with an overall efficiency of some 2\%,compared with a figure of typically 30% for a fossil-fuelpower station. To obtain the 2j% efficiency, a tem-perature difference of some 20°C is required, but clearlythe larger the temperature difference, the greater the effi-ciency of the plant. As the cold water is at depths ofaround 1000 m, and the flow of water required per mega-watt of power output is in the range of 4-8 m3/s, thescale of the plant (shown schematically in Fig. 2) will beapparent. Fig. 2 shows an OTEC plant at sea, t*ut theycan also be located on land as indicated in Fig. 3. Clearlyeach combination (open or closed power cycle, land-based or floating) will give rise to a particular set of prob-lems.

Practical realisation (engineering of the system) istherefore the key issue in considering the future ofOTEC, together, of course, with its economics.

The history of OTEC began when d'Arsonval, in 1881,first proposed the idea, using a heat engine working onthe Rankine cycle: in effect the common domesticrefrigerator but with the input and output reversed. Fiftyyears was to elapse before another Frenchman, Claude,attempted to turn the OTEC concept into practical usein Cuba. Both d'Arsonval's concept and Claude's realis-ation used an open-cycle version of the heat engine, andClaude attempted both land-based and floating versionsof the plant.

Some of the problems which he did not overcomeincluded sealing problems of the vacuum evaporator inthe open-cycle plant, and the cold-water pipe proved dif-ficult to lay, difficult to maintain in position, and the fric-tion in the pipe and heat losses through the pipe wallswere excessive. In short, the technology was just notavailable at that time to engineer a plant, in addition towhich the economics were not attractive.

France persevered, although on an intermittent basis,through the 1930s, 40s and 50s, culminating in plans for a7 MW land-based plant in the Ivory Coast, abandonedonly after the design was complete when a decision wastaken to build a hydroelectric plant nearby. After 80years then, OTEC remained an energy source attractivein principle, but not achieved in practice. As so often, thescientific attraction and the engineering reality were twodifferent things.

IEE PROCEEDINGS, Vol. 134, Pt. A, No. 5, MAY 1987 381

AON

2ON

2OS

40 S160W KOW 120W 100W 80W 60W 40W 20W 20E

AON

20N

wmmmm

20S -

AOSAOE 60E 80E 100E 120E KOE 160E 180 E 160W

Fig. 1 The ocean thermal resource; worldwide distribution

Annual average of temperature differences in °C between the ocean surface and a depth of 1000 m. (Source: Reference 1)• water depth less than 1000 m

2 The technology

2.1 Power circuitThe core of an OTEC plant is the thermodynamic workcycle. The ideal cycle suggested by Carnot has practicallimitations connected with control of the condensationprocess for the working fluid, and also the size of thecompressor. A modification, the Rankine Cycle, meetsthese objections, and while the ideal efficiency of the cycleis less than that for Carnot, the practical efficienciesachieved are only a little different — and the plant will besmaller. Fig. 4 gives an indication of a closed-cyclevariant, using ammonia as the working fluid.

For plants up to 10 MW size the heat exchangers arejust within the state-of-the-art, using shell-and-tubedesigns with titanium tubes, but it is generally acceptedthat plate-fin heat exchangers will offer higher efficiencies

in this application, although, at present, control andremoval of biofouling will present potentially greaterproblems which still require confident solutions. Somepossible heat-exchanger arrangements are shown inFig. 5. Plate-fin heat exchangers offer an opportunity fortechnology transfer, as the superplastic forming capabil-ities of titanium and some types of aluminium, and thediffusion bonding capabilities of the former, appear tohave ready application to heat exchangers for OTECplants, and the aerospace developments of these materialsare providing a rapidly expanding database for their use.

The turbine too, while requiring some development forgaseous ammonia operation, is essentially state-of-the-art, and the generator will be a standard unit with theusual specification for offshore use, or use in a generalmarine environment.

However, the overall state of knowledge of this power

382 IEE PROCEEDINGS, Vol. 134, Pt. A, No. 5, MAY 1987

circuit cannot be considered as suitable for productionplants without further development testing, and this iswhere the need for a demonstration plant first becomesclear. As referred to elsewhere, the funding of such a

= ^ _ warm water^ ^ ^ (22-28'C)

cold water(4-7'C)

Fig. 2 General layout of a future 10 MW floating OTEC plant

plant (i.e. before the production technology is established)presents a difficulty, and to alleviate this it has been pro-posed, in some schemes, that the power circuit should bedivided into two or more entirely separate units. Thebuilding initially of only one unit will substantiallyreduce the absolute capital cost; one of the principalproblems at present of getting the first plant built. At alater stage the subsequent unit(s) can be added, takingadvantage of lessons learnt where relevant. The disadvan-tages of this are that each unit will be removed from theoptimum size for peak efficiency and electrical switchingarrangements will need to be added — increasing the costand the overall plant complexity. The redundancythereby built in, however, will alleviate the problems ofdown-time and interruption of power supply, andimprove the attractiveness of the plant to fundingagencies. On balance, it is believed that, for the demons-tration plant certainly and for early production plantspossibly, the lower efficiency and added complexity willbe justified.

2.2 Hull/structureFor land-based OTEC plants there is nothing uniqueabout the structure to house the plant, and costings willbe as accurate as for any other large 'power hall'. Forfloating variants, certainly for power plants up to about50 MW, the floating structure (probably in concreterather than steel) is within the scale and nature of existing

oil-production platforms in the North Sea. Some furtherdevelopments are necessary, but these will be much lessthan those which resulted in the Norwegian, Netherlandsand British structures now operating, and much lessambitious than the developments necessary for some ofthe structures now being designed, for example for theNorwegian Troll field.

Overall therefore, whether the plant is to be land-based or floating (or even a shelf-mounted plant asfavoured by some, where the structure is mounted on pilefoundations a few hundred metres offshore) the design,the construction scheme and cost can be calculatedwithin close tolerances.

2.3 Cold-water pipe (CWP) and collection of warmwater

This is where the design of land-based and floatingOTEC plants diverges considerably.

In the case of the latter, it will be located and floatingin the middle of the warm resource, and measurement ofsurface currents will enable a specific location to bechosen which will ensure adequate replenishment of thewarm resource as it is utilised in the power cycle. In thisrespect the floating plant has an advantage over the land-based version, where proximity of the coastline and theshelving seabed restricts to some extent access to thewarm resource and its adequate replenishment, almostcertainly requiring some form of short pipe or collector:as already noted the quantities required of both warmand cold water are very large because of the inherentinefficiency of a process working on such a small tem-perature difference. In addition, the exhaust warm andcold waters must be discharged with rather more care inthe land-based variant, if they are not to degrade theinput warm resource, and, for this reason, an exhaustpipe will need to be constructed, adding to cost and tofrictional losses in the system. It is for this reason that themodified land-based (or shelf-based) design is sometimespreferred.

Turning to the cold resource, the design is quite differ-ent for land-based and floating plants — and each has itsprotagonists. In both cases the collecting pipe must reachto the necessary depth for the cold water, which for com-parison purposes is usually taken as 1000 m. For thefloating plant the pipe will hang vertically (see Fig. 2) andmust be stabilised. The concept of a pipe some 8 m diam-eter (for 10 MW) and 1000 m long, with an ability to berepaired, is clearly an interesting design exercise. TheCWP for the land-based plant, on the other hand, will beup to 3000 m long in order to reach the cold resourceand due to potentially higher frictional losses associatedwith the greater length, the diameter will need to belarger than for the floating variant to reduce the flowvelocity and hold the frictional losses at the same value.Stabilisation of the pipe will be by stays or by saddlesgrouted or otherwise fixed to the seabed at intervals. Aswith the floating plant, replacement of damaged partsmust be feasible.

There are other significant differences between CWPdesign for land-based and floating plants, and, at present,the greater number of development programmes (seelater) favour the land-based concept. The author,however, is firmly of the view that, taking the overallengineering view, the floating variant is to be generallypreferred.

IEE PROCEEDINGS, Vol. 134, Pt. A, No. 5, MAY 1987 383

2.4 MooringsThe only moorings required for the land-based plant arethe location devices for the CWP and, possibly, theexhaust and the warm-water collector. In the case of the

floating plant, the whole structure (perhaps 45 000 t for a10 MW unit) has to be located in up to 2000 m waterdepth. While 'fixed' location is not necessary as for an oildrilling platform, the flexible location necessary still

discharge(another option)

cold water in,from 1000m

Fig. 3 Schematic diagram of land-based OTEC plants: onshore and shelf-mounteda Land-based, b tower-based

requires extension beyond the present state-of-the-art. Itis the general view that, while tension leg moorings nowin use in the North Sea Hutton Field could have applica-tion for OTEC use, catenary nodes are presently mostsuitable for development in this application. Fig. 6 indi-cates three of many possible options under consideration

warm water in

evaporator

warm water out

gaseous ammonia

generator

cold water out gaseousammonia

cold water in

Fig. 4 Schematic diagram of closed-cycle OTEC circuit

at present.

2.5 Electrical transmissionHere again, the land-based plant is to be preferred,because power takeoff is identical to that for any otherland-based power-generating system. The floating planthowever has to transmit its power from the plant downto the seabed, along the seabed, and ashore. Early(demonstration) floating OTEC plants will be locatedclose to shore (preferably no more than 2 or 3 km) andtransmission will then be with alternating current, withlosses of about 0.05% of input power per kilometre.When the technology of floating OTEC plants is wellestablished, plants located well offshore may be con-sidered, and beyond 30 km DC transmission will be pre-ferred with its much lower losses, at about 0.01% ofinput power per kilometre, but with the additional inver-sion and conversion losses to be added to this. A furtherbenefit of conversion to direct current for offshore trans-mission is that reconversion to alternating current can bematched to user requirements. Also, the AC generatingsets offshore can be optimised for frequency, whereas forstraight AC transmission offshore the generating fre-

384 IEE PROCEEDINGS, Vol. 134, Pt. A, No. 5, MAY 1987

IEE PROCEEDINGS, Vol . 134, PC. A, No . 5, M A Y 1987

quency must match the user frequency (unless additionalconversion equipment is incorporated, with its ownlosses), which, in general, will result in larger electricalgenerating sets than would otherwise be necessary.

clear opportunities for extending the OTEC state of theart in substantial steps, and much of the developmentprogrammes mentioned later is devoted to work on theseopportunities.

Fig. 6 Three options for layout of individual tethers in a multipoint moora Simple anchor or piled fixing, with catenary moorb Catenary moor with clump weight and anchor or pilec Catenary moor with intermediate standoff buoy fixed to moor. Further length of mooring (chain) then runs from buoy to anchor or pile

These views on transmission systems are not univer-sally held, and the combination of expense for the controlcircuits needed for DC transmission and the theoreticalcapability of transmitting 10 MW of power at 33 kV, fordistances up to 50 km using low-capacitance cables, areargued as favouring the AC system. A further pointargued in favour of alternating current is that there is noneed to optimise the energy conversion efficiency of theturbine/generator sets offshore, and directly connectedsynchronous generation is simpler and cost-effective.

Apart from these aspects, the design of multimegawatttransmission lines for the seabed and the riser presentproblems of ambient pressure, and weight of cable, notyet fully solved. For this reason, both redundancy andseparation of phases is likely to be the preferred initialsolution, once again increasing the switching arrange-ments necessary. It is clear therefore that, consideringpower transmission in isolation, the land-based plant hasa specific advantage over the floating plant.

2.6 Technology transferThe opportunity for application of aerospace materialdevelopments to heat exchangers for OTEC plants hasalready been mentioned. Three further examples are forthe material of the CWP, of the moorings, and for thestrength elements of the power transmission cables,where aramid fibres (either in simple form or as part ofcomposites) have much to offer. With an initial applica-tion in the aerospace field, they too appear to provide

This Section has briefly examined the overall techno-logical status of OTEC. However, engineering success isdependent not only on technology but on economicattractiveness and financial feasibility. In the followingSections recent progress and prospects for the rest of thecentury are reviewed.

3 The last ten years — progress

3.1 Economic and financial aspectsAfter the French programme for the Ivory Coast was ter-minated virtually nothing happened to OTEC for over 10years, at which time the USA and Japan began to take aninterest, followed shortly afterwards by a European pro-gramme. Much had changed in the intervening period,including the considerable increase in knowledge andpractice of the industry which was exploiting offshore oiland gas, but more than anything the sudden and largeincrease in oil prices in the early 1970s. At a stroke, theOPEC countries had changed the basis of economic com-parison for power generation between fossil fuels andrenewable energies. Whereas the situation before hadbeen one of economic disinterest however good the tech-nology, the situation then was reversed — with the onuson proof of technical viability, because the economicswere 'comparable' with oil-fired power generation.

The case for cost 'comparability' is crucial to theacceptability of OTEC. On the basis of capital cost alone,OTEC (and all other renewables) show up badly against

386 IEE PROCEEDINGS, Vol. 134, Pt. A, No. 5, MAY 1987

oil-fired power generation. Oil-fired plant would typicallycost a few hundred pound sterling per installed kilowatt,while OTEC would a few thousand pound sterling perkilowatt. On the basis of capital cost therefore the resultis 'no contest', oil will win every time by a very largemargin. However, fuel oil is now much more expensive,while OTEC fuel is still free. Maintenance costs for welldeveloped oil-fired plant are low, whereas the main-tenance costs of low-efficiency OTEC plant will be rela-tively high. High capital cost of OTEC means high totalinterest charges to be serviced in relation to the interestcharges for the oil-fired plant. It is clear that a conven-tional comparison of capital costs only is not valid andone effective way to incorporate the actual situation,taking note of the points just mentioned, is shown in Fig.7. The origin of this comparison lies with the European

6 8 10time in operation, years

12

Fig. 7 Cost comparison for 100 MW OTEC and oil-fired generatingplants

Assumptions:oil price = S40/barreloil price increase = inflationinterest rate = 10%plant availability = 5000 h/year

oilOTEC

(i) Total annual payments for oil and OTEC plants arranged to be the sameinitially(ii) At this point, absence of fuel costs for OTEC has resulted in nearly all thehigher capital costs for OTEC being repaid. OTEC repayments now drop rapidly.Oil-fired costs continue to increase, as fuel costs increase. Need to purchase fuelreduced early capital repayments, which also continue to at least the 15th year(iii) All OTEC capital costs repaida Oil: capital repayments and fuel costsb OTEC: capital repaymentsc OTEC: maintenance and operational costsd Oil: maintenance and operational costs

group of companies which formed Eurocean; the actualvalues incorporated in the Figure are the latest onesavailable from UK proposals.

The basis of the Figure is that payments for a 100 MWplant whether OTEC or oil-fired are arranged to beequal. While the payments for OTEC are capital, oper-ational and maintenance, those for oil are capital, oper-ational, maintenance and fuel. Despite the high capitalcost of OTEC, after less than 10 years its capital isrepayed and further costs are operational and main-tenance only. For oil, however, so much of the paymentshave gone on oil fuel that costs continue to rise asshown: typically to 15 years or even beyond. Even whenthe capital cost of the oil-fired plant has been defrayed,the payments for oil fuel continue. Just how the price ofoil will change in the future is open to considerabledoubt, but the basic presentation is one which substantialand important parts of the financial community findacceptable. Clearly with oil presently at a lower pricethan shown, the period for pay back of OTEC capital

will be longer (although lower oil prices will result inlower construction costs for the OTEC plant, it should benoted). Once the balance point has been reached (duringwhich generating costs will, by definition, be the same forOTEC and oil-fired power), it is seen that OTEC gener-ating costs drop to about 30% of their former value, atwhich time costs are as low as Norwegian hydropower.However, that low price is a bonus to be looked for inthe future. At present, there is a need to corroborate atdemonstration or full size the validity of Fig. 7.

A more conventional presentation of costing (in thiscase for a 10 MW demonstration plant), and which alsoincludes a derived generating cost, is shown in Table 1.

Table 1: Cost of generating electricity from OTEC 10 MWdemonstration plant

Cost items

Capital cost (1) £35.34 x 1 o6

30% grant (2) £10.60 * IP 6

70% of capital, to be repaid (2) £24.74 * 106

Annuity factor over 10 yrs (3, 4) : 5.8892Annual repayment of capital

and interest £4.20 * 106

Inspection and maintenance (5), j £2 4 7 * 1 0 6

Operational (6) and Insurance (7)J __Fixed annual charge £6.67 * 1 0 6

Fixed annual charge rate £667/kW

Unit generating cost for 8.46 p/kWhelectricity (8)

or (9) 12.68 cents/kWh

Assumptions:(1) Uninflated capital cost (1985 values); excluding research,

development and test, which are separately funded(2) For the demonstration plant only, 30% of the capital cost is

obtained as a direct grant(3) For the demonstration plant, 70% of capital, plus interest, to be

repaid in equal annual instalments over only 10 years; zeroresidual value is also assumed even though design life is 25years

(4) Interest rate: 11 %/annum(5) Annual cost, 2% of total capital cost(6) Annual cost, including crew costs, 3% of total capital cost(7) Annual cost, 2% of total capital cost(8) Utilization at 90% on the basis of the multiple power pods

fitted, i.e. 7884 hr/yr(9) $1.5 = £1

Merchant Bank prepared figures for this same demons-tration plant show a real rate of return on total capitalemployed of 10.3% (nominal 15.8%).

For further background, a cost breakdown of com-ponents is indicated in Table 2, and contingency cost

Table 2: Percentage component costs for 10 MW OTECplant

Site specific data 1.7Heat exchangers 20.2Cold-water pipe 6.1Moorings 4.9Electrical transmission (sea bed and riser) 8.3Pumps, turbines, generators and control 12.9Hull, including warm-water circuit 18.0Installation and maintenance 4.1Start-up and test 8.0Miscellaneous 1.9Unknowns 13.9

variations are shown in Table 3.None of these address the question of risk, and this

too must be 'acceptable' if an OTEC project is to befinanceable. 'Acceptable' in this context means that the

IEE PROCEEDINGS, Vol. 134, Pt. A, No. 5, MAY 1987 387

Table 3: Contingency cost estimates for items of floatingOTEC plants

Percentage of target costs

Capital costs

Heat exchangers

Cold water pipe

Warm water circuit

Moorings

Sea bed power cableand junctions

Riser power cableand junctions

Pumps, turbines,generators

Hull

Operational costs

Operational labour

Maintenance labour

Replacement equipment

60 80 100 120 140 160

rate of return on the investment must match the level ofrisk involved. The programme of work over the lastdecade has shown good progress in reducing the areas ofdoubt within OTEC technology and achieving levels ofrisk which match the rate of return on investment in away which is becoming 'acceptable' to the financial com-munity.

3.2 Other OTEC applicationsNone of this progress would have been possible in theabsence of OPEC oil price increases and, as a result ofthis new-found economic attractiveness, OTEC applica-tions have been examined for many activities beyond thebasic one of power production envisaged by d'Arsonvaland Claude. Adequate financial returns on an OTECinvestment can only be achieved if there is a market forthe output. Power requirements in the tropical or sub-tropical areas of the ocean, where the temperature differ-ence is adequate, certainly do exist, but the potential forgenerating power from OTEC is much greater than thatmarket alone can justify — for some period ahead.Byproducts from OTEC power have therefore beenexamined, two of which are a direct use of the OTECcycle itself, the remaining ones making onsite use of thepower produced rather than requiring it to be exportedas electricity.

The first two are aquaculture and the production ofpotable water. The cold deep water for the OTEC plantis nutrient rich, and phytoplankton can convert the nutri-ents to organic materials on which shell fish can feed andgrow. Potable water is an immediate product of open-cycle OTEC where the evaporated warm sea water iscondensed, while in the case of closed-cycle OTEC low-temperature distillation/multistage flash techniques areappropriate. Aquaculture and potable water production,which fit in well with the large quantities of warm andcold water moved by an OTEC plant, can be incorpo-rated separately or together, in the limiting case using upall the external power to drive pumps and otherequipment for the desalination and aquaculture activities.

The remaining options make use of OTEC electricalpower in situ to realise energy intensive products. Hydro-gen would be produced by electrolysing water, and theOTEC power then stored and transported as liquid

hydrogen to a market. Alternatively, hydrogen may beproduced as an intermediate product, being used, in turn,to form ammonia. At present, use of ammonia fertilisersis determined in part by production capacity fromnatural gas and the use of these fertilisers in the thirdworld (much of it in the tropical and subtropical zoneswhere OTEC processes are available) could be a majorcontribution to world food production. An alternative toenergy transportation by means of liquid hydrogen is toenhance the quality of the power derived from an OTECplant in heat terms, which is then stored in molten saltsand transported to shore for reconversion and use pre-dominantly in industrial processes.

Also, the production of some metals from their ores(aluminium from bauxite is a good example) requiresconsiderable power input, and the location of an OTECplant onshore by the bauxite deposits or transportationof the deposit to an offshore plant with the refined metalbeing shipped back on the return journey, are options forOTEC power consumption. Finally, the economy of amanganese nodule mining operation (given an acceptedinternational legal regime for these operations) wouldcertainly benefit from onsite processing, and, once again,the richest sites for manganese nodules are in areas of theocean particularly suitable for OTEC activities.

While the economic viability of these processes varies(and some are certainly not economical at the presenttime) they illustrate the range of possible uses for OTECpower, a range which gives flexibility to the ultimate usefor which OTEC power is used, and which gives rise to adiversified market.

It is this variety of markets, coupled with the techno-logical developments of the offshore oil and gas activitiesalready referred to, which has encouraged a number ofcountries to become involved in OTEC developmentprogrammes during the last decade.

3.3 OTEC development programmesThe countries now involved in OTEC work are France,India, the Netherlands, Japan, Sweden, the UK and theUSA. In addition, there was the joint European pro-gramme of Eurocean which involved French, Italian,Netherlander and Swedish firms.

Very briefly, and in chronological order, progressbegan as follows:

3.3.1 United States of America: In terms of expenditure,this has been by far the largest OTEC programme under-taken. It began effectively in 1972, growing year by yearand culminating in mini OTEC (50 kW gross, 10-15 kWnet power) deployed off Hawaii in 1979 and OTEC-1(1 MW) also off Hawaii in 1980/81 — in both cases thetests extending over a period of months. These were bothfloating plants, and overall efficiencies in excess of 2.5%were achieved for temperature differences of 21°C.

Equally important as these practical examples of float-ing OTEC systems was the US Government's OceanThermal Energy Conversion Act of 1980, passed by theCarter Administration, which, in addition to providingfor a simplified federal licensing system for OTEC install-ations in US waters, also made both commercial anddemonstration OTEC facilities eligible for Federal LoanGuarantees, and allowed US owners of OTEC facilitiesto use the capital construction fund tax regime previouslyavailable only to vessel owners under the MerchantMarine Act. These are excellent examples of a govern-ment creating a suitable fiscal and legal framework toencourage development of a new technology which can

388 IEE PROCEEDINGS, Vol. 134, Pt. A, No. 5, MAY 1987

benefit the nation. The same 1980 Act also establishedgoals for manufacture of OTEC power ranging from100 MW in 1986 through to 500 MW by 1989 and noless than 10000 MW by 1999.

Following the change of Administration in 1981, theprogramme was modified and concentrated on develop-ments geared towards a 40 MW land-based OTECdemonstration plant, and the two approved schemeswere again located in Hawaii. Completion by the twogroups of Phase 1 of this project was followed, in 1983,by award of a contract to one group for Phase 2, toinclude complete conceptual design of the plant. Early in1985, tenders were invited for construction of a 50 MWshelf-mounted plant in Hawaii, as the next stage of thedevelopment.

In addition the US programme, which included someforeign industrial participation, has also prepared OTECplans for locations which include Guam, the NorthernMarianas, Puerto Rico and the (US) Virgin Islands.

Perhaps the most significant feature of these OTECproposals is their location. All are islands, reflecting thehigh cost of oil when transported to island situations. Asnoted in a paper by the PREST Group of the Universityof Manchester, many islands are dependent solely on oilfor their electrical power generating capacity, and thisfactor, combined with the transportation costs of the oil,makes island sites prime targets for OTEC plants.

3.3.2 Japan: Following private industrial initiatives inthe early 1970s, the ubiquitous MITI (Ministry of Inter-national Trade and Industry) became involved in theJapanese OTEC programme through the SunshineProject in 1974. It was a less diversified programme thanthat of the USA and the Japanese have worked towardsestablishment of a viable land-based plant, with thefurther intention to have a 10 MW floating plant oper-ational during 1991.

After closed-cycle tests of a 1 kW system in 1979, oneJapanese group established a 50 kW unit on Tokuno-shima Island in 1982. Another group had reached agree-ment with the government of the central Pacific island ofNauru; the location has an excellent temperature differ-ence (24°C) and a substantial demand for power formineral ore refining activities. The plant for Nauru wasoperational during 1981/82 with a design gross poweroutput of 100 kW, and a net output of 15 kW. It is highlysignificant that the maximum power output under testwas 120 kW (gross) and 31.5 kW (net). In other words, a100% improvement in net power developed over andabove the design value — an indication of commendableconservatism of the Japanese engineers when estimatingperformance of this plant.

In addition to the 10 MW floating plant referred tohere, the Japanese are believed to be advancing plans fora 20 M W land-based plant on the island of Nauru, basedon the experiences of their 1981/82 experiments.

3.3.3 Eurocean: This grouping of 26 European com-panies was formed in 1970, and, in 1975, initiated areview of all forms of ocean energy. A year later a sub-group of 9 companies from France, Italy, the Netherlandsand Sweden began work on OTEC and produced theirpreferred options at the end of 1977. Although its role asa catalyst was by then fulfilled, Eurocean, and itsmembers, took proposals further forward in terms ofplans for a floating plant, and also (with a Belgianmember involved), for a combined OTEC/desalination/aquaculture scheme of 1 MW size, these stages being

reached in 1979 and 1980. From this point on, asplanned, the members took their commercial OTECplans forward on essentially national bases. The co-operative progress was made very economically, withcosts divided among the participating members.

3.3.4 France: After a gap of nearly 20 years, the Frenchgovernment (through CNExO — the Centre Nationalpour Exploitation des Oceans, reformed in 1984 as a partof Ifremer — Institut francais de recherche pourl'exploitation de la mer) revived its interest in OTEC, inthe range of 1-5 MW, particularly because of the sub-stantial number of dependencies and overseas areas ofthe nation in OTEC-attractive locations, many of whichwere islands dependent on oil for their power generatingcapacity. One such location was Tahiti, where the currentFrench programme is aimed. Both open- and closed-cycleland-based systems have been examined, initially by twogroups. In 1982, these two groups were fused into one,which is now proceeding with designs for a 5 MW land-based plant in Tahiti, and the decision on closed or opencycle will be taken this year, based on experiments beingconducted at present. These cover heat-exchangersystems for the closed cycle, evaporators and condensersfor the open cycle, together with material evaluations forthe cold-water pipe and detailed site surveys in Tahiti.Once the cycle has been selected, design of the plant isplanned for completion by the end of 1986, with con-struction starting then, and operation of the prototype in1990.

3.3.5 The Netherlands: Apart from their own pro-gramme, aimed initially at the Netherlands Antilles, and,more recently Bali, the Netherlands provided one of theforeign companies contracted to work for the US pro-gramme; particularly for their skills in the constructionof concrete structures for a floating OTEC plant.

Although less has been written about the Netherlandsprogress than about most of the others, it has movedforward in a sound, steady way from 1980 onward. Not-withstanding the American work, land-based plants havebeen central to their own programme, with the closedcycle taking preference. The Indonesian (Bali) scheme isfor a 0.25 MW size plant and plans for this began in1982.

3.3.6 Sweden: As with the Netherlands, Sweden tooprovided one of the foreign-based contractors for the USOTEC programme, this time in the heat-exchanger field,and again like the Netherlands (and to some extentFrance) developed its own programme out of that initi-ated with Eurocean. From 1980, work developed onclosed-cycle land-based OTEC prototypes, and in thespring of 1983 a project scheme for a 1 MW plant inJamaica was initiated. This is now being evaluated, priorto a decision being taken on design of a larger size plant,once again land-based.

It will again be noted that the location (as too for theNetherlands) is an island site with oil generated elec-tricity.

3.3.7 United Kingdom: Following extensive preliminarywork, the British OTEC programme came formally intobeing at the beginning of 1981 with the establishment ofa company devoted exclusively to its development, and in1982 the UK Department of Trade and Industry (DTI)offered its involvement in the programme. Working withother relevant companies, and with the financial sector,

IEE PROCEEDINGS, Vol. 134, Pt. A, No. 5, MAY 1987 389

work has concentrated on development of a floating10 MW closed-cycle demonstration plant. During 1983,two of the university groups in the Science and Engineer-ing Research Council's (SERC) Marine TechnologyProgramme also started programmes of related workdealing with the dynamic response of the system, aspectsof design for the heat exchangers and features of eco-nomic and risk assessment of OTEC systems, and thiswas developed for a further two years to 1987 under jointSERC and DTI funding; the programme is directly rele-vant to, and planned in co-ordination with, the industrialCompany's work noted above.

The preferred conceptual design has been selected,although work continues on potentially encouragingalternatives, and three sites (all islands) are being evalu-ated in the Caribbean and the Pacific. Present plans callfor completion of the final design, construction andinstallation in time for initial production of power in1991.

3.3.8 India: Plans in India centre on small land-basedOTEC plants for island locations. There are a number ofsites (including the Andaman and Nicobar Islands, andthe Lakshadweeps) which satisfy the principal economiccriterion absolutely; the price of oil delivered to theislands is extremely high. The Indian programme inte-grates interests from technological institutes, oceano-graphic centres and government, and, starting in 1981,has now progressed to the stage of establishing designrequirements. No firm date for construction and install-ation of the pilot facility has been set at the time ofwriting.

3.4 The '1985' situationJust 12 years after the OPEC price rises which breathedlife into programmes of work on 'high-cost' renewableenergies, 7 national OTEC programmes have togethermade substantial progress. For a number of locations,particularly islands, the economics now look attractive,both in terms of generating cost and (for financiers) interms of a return on investment commensurate with risk.What are the next steps?

4 The next 10 years — prospects

The key to further progress in ocean thermal energy isthe construction of representative scale plants. Fully eco-nomic sizes will be of the order of 5-8 MW for land-based plants and 25-40 MW for floating plants. Later, bythe turn of the century, OTEC plants of size comparableto established generating systems (100-400 MW) shouldbe feasible. However, it is the gap between the relativelysmall-scale tests so far carried out and the fully economicplants which must be bridged in the next decade. Tomake the gaps suitable for engineering extrapolation thismeans construction of the 1 MW land-based plants and10 MW floating plants now planned, referred to earlier inthe paper. However, at prices of thousands of pound ster-ling per kilowatt also mentioned earlier, the sums ofmoney are very large: £40M for a 10 MW demonstrationplant for example. While the world is well attuned toresearch-and-development costs it is still trying to cometo terms with demonstration projects, purely because ofthe scale of funds which must be provided for a singleunit. Only nuclear power attracts spending of these andgreater sums on a regular basis for test purposes, andwhatever arguments may be advanced for that spending,they do not apply to OTEC. Rather, it is that OTEC

390

holds out the prospect of competitively priced power gen-eration in a major geographical sector of the world whichis, on the whole, energy-deficient and short of funds topurchase more oil to increase that energy base. Estimatesshow that, despite the high initial costs, OTEC plants willsave, in displaced oil purchases, 100% more than thosecosts during their operational life, which must be goodnews for the developing countries.

As much of an OTEC plant (land-based or floating) iswell established technology, it has an advantage oversome other of the renewable energies. But significant fea-tures of an OTEC plant must still be shown to work inpractice at a representative scale: these features weretouched on earlier in Section 2. Market surveys showthat there is a need for both floating and land-basedvariants, indicating that about half of the present loca-tions for pure OTEC plants are suitable only for thefloating version.

The availability of new materials and powerful analyti-cal techniques is considerably easing the remaining prob-lems, and high confidence levels are now placed in theirsuccessful solution. There is substantial scope for tech-nology transfer from other industries, as indicated above,and this is already happening. This paper does not allowspace to go into detailed technical points; suffice it to saythat the programmes previously described offer a suffi-cient level of redundancy to encourage the belief that suc-cessful solutions will be found well within the decade.

How then are the demonstration plants to be funded?The United Nations Conference on New and RenewableSources of Energy in 1981 highlighted OTEC as a front-runner among renewable energies, and the UnitedNations has published a Guide to OTEC for DevelopingCountries in furtherance of this. Both Brandt Reports laygreat stress on the need for energy in, and the sharing oftechnology with, developing nations — to the benefit ofdeveloped and developing nations alike. Stable energyprices will be a major influence on steadying the econo-mies of the developing nations. These are factors whichwill influence, presumably favourably, international insti-tutions such as the World Bank, the European Com-munity and others. Funds secured in support ofdemonstration plants will provide the means to reducethe dependency of some less fortunate nations on con-tinued aid to purchase oil, and help to break the viciouscircle of poor nations becoming poorer. Once thedemonstration plants have operated, and theirundoubted teething problems been resolved (it is inevit-able that there will be some problems because we aretalking of practical engineering developments not theory,and the quantification of these problems is the object ofthe risk analyses), then conventional funding of full-scaleplants can be undertaken.

In deciding on the nature of these full-scale plants, therole for OTEC, in addition to straightforward power gen-eration (as described in other OTEC applications), willneed to be carefully reassessed against market studies,which it is suggested should be undertaken at intervalsnot greater than two years. There is, in purely practicaleconomic terms, substantial logic in moving forward on,for example, a combination plant of OTEC, desalinationand aquaculture — to provide power, water and aproduct to sell for a number of island communities. Towhat extent, however, does the added complexity of thecombination plant, over and above that of a simpleOTEC plant, affect the reliability of operation; can theratio of power-out/water-out/aquaculture products bevaried sufficiently in a single plant to cater for varying

IEE PROCEEDINGS, Vol. 134, Pt. A, No. 5, MAY 1987

demands for each product — which, in turn, may dependon factors remote from the country with the combinationplant, for example the demand for oysters thousands ofkilometres away?

These doubts suggest that, at this stage, all practicalactivity should be geared towards full validation andacceptability of the demonstration-scale simple OTECplant, floating and land-based. Dilution of effort fromthis objective, for any reason, is likely to delay unneces-sarily all forms of OTEC and OTEC-combination plants.When the validation of the simple plant has been carriedout, to the satisfaction of the technical, financial and,inevitably, the political community, then, and only then,should practical progress be advanced on alternativeOTEC applications.

the size and cost of OTEC plants for a given poweroutput, for example the plate-fin heat exchangers, men-tioned earlier, will have a knock-on benefit as theirimproved performance results in a decrease in their sizeand mass, a consequent reduction in mass of the hull andits dimensions, a further consequent reduction inmooring loads and the size of moors, and so on.

Both developing and developed nations together canbenefit from OTEC, initially as user and manufacturer,respectively, and later through technology transfer, as apartnership.

The key step forward now is construction ofdemonstration-scale OTEC plants, and, within Europe,the topic is very appropriate to the marine technologyaspects of the current Eureka initiative.

5 Conclusions

OTEC is a base-load renewable energy system — anadvantage over other renewables having need of bufferstorage to achieve this, with commensurate added costs.OTEC is environmentally benign, for the number andsize of plants which can be built in the next 20 years, theeffect on the overall environment will be negligible, par-ticularly so for the floating plants. During that period,work can be carried out to determine the effect, if any, offarms of OTEC plants in close groupings, which could bein position during the period from 2005-2015.

The consensus of results from OTEC programmes todate is that OTEC power is competitive with oil-generated power, now, for a limited number of islandlocations, and this position will improve if the price of oilbegins to rise again in absolute terms. It will alsoimprove substantially as technological advances decrease

6 Bibliography

1 COHEN, R.: 'Energy from the ocean', in 'Marine technology in the1990s'. Royal Society Conference Proceedings, London, 1982, pp.143-175

2 LENNARD, D.E.: 'Ocean thermal energy conversion — a base loadrenewable energy system', in 'Energy: money, materials and engineer-ing'). I.Chem.E. Conf. Proc, 1982, London, UK, T6/15-T6/25

3 DUNBAR, L.E.: 'Market potential for OTEC in developing coun-tries'. Proceedings, 8th Ocean Energy Conference, Washington, USA,1981, pp. 947-956

4 BRANDT, W.: 'North-South: A programme for survival' (PanBooks, London, 1980)

5 BRANDT, W.: 'Common crisis North-South: cooperation for worldrecovery' (Pan Books, London, 1983)

6 ITO, F., and SEYA, Y.: 'Present situation and future outlook forOTEC power generation', Energy Exploration and Exploitation, 1983,2, pp. 99-111

7 'A guide to OTEC for developing countries' (United Nations, NewYork, 1984)

8 'World energy balance 2000-2020' (World Energy Conference Head-quarters, London, 1984)

IEE PROCEEDINGS, Vol. 134, Pt. A, No. 5, MAY 1987 391