Recent Developments in Ocean Thermal

Energy

April 1980

NTIS order #PB80-201825
Library of Congress Catalog Card Number 80-600074

For sale by the Superintendent of Documents, U.S. Government Printing Office

Washington, D.C. 20402
PREFACE

This Technical Memorandum was prepared in response to a request from

the Chairman of the Subcommittee on Energy Development and Applications of

House Committee on Science and Technology. The Committee requested that the

Office of Technology Assessment provide an update of its study of Ocean

Thermal Energy Conversion (OTEC), which was published in May 1978.1

This Technical Memorandum reviews the status of OTEC technology

developments as of April 1980. It discusses major technical accomplishments

occurring after publication of the 1978 report and principal technical

uncertainties that remain; it also attempts to summarize numerous documents

and other available information in order to provide a concise update on the

status of OTEC technology.

The Memorandum

Program staff, with

recently retired as

University of Rhode

was developed over a two-month period by the OTA Oceans

the assistance of Dr. Herman Sheets, a consultant who

the head of the Department of Ocean Engineering at the

Island. In preparing the report recent DOE and industry

reports were reviewed and there were consultations with those directly

involved in recent technology development projects.

JOHN H. GIBBONS

Director
Staff for Ocean Thermal Energy

Updated Report

Eric Willis, Assistant Director

Science, Information and Transportation Division

Robert Niblock, Oceans Program Manager

Peter Johnson, Project Director

Prudence Adler

Kathleen Beil

Carolyn Gilmore

Consultants

Gary Baham

Denzil Pauli

Herman Sheets
TABLE OF CONTENTS

1. Introduction and Findings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

11. The OTEC Program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

111. The Ocean Resource . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Island and Gulf Coast Resources . . . . . . . . . . . . . . . . . . . . . . . . .

Plant Ship Resources . . . . . . . . . . . . . . . . . . . . . . .............o

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..O

I V . The Technology Base . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Significant Accomplishments . . . . . . . . . . . . . . . . . . . . . . . ● COOCC

Mini-OTEC . . . . . . . . . . . . . . . . . . . . . . .. .. .. . . .

Progress in Understanding Bio-Fouling . . . . . . . . . . . . . . . . . . .

Status of OTEC Component Development .. ..

Platforms .. .. .. .......... O..... . . . . . . . . . . . . . . . . . . .

Cold Water Pipe . . . . . . . . . . . . . . . . . . . . . . .. . . . . . 

Heat Exchanger . . . . . . . . . . . . . . . . . . . . . . .....0. . . . . . . .

Power System Components .. .. .. . . . . . . . . . . . . . . . . . . .

Electrical Power Transmission Cables . . . . . . . . . . . . . .

OTEC-1 ● *oee***e 9********- ● 8**e**m*0 ● ***9*9*** ● ***9

Pilot Plant Design for an OTEC Powered Ammonia Plant

Ammonia Conversion/Fuel Cells . . . . . . . . . . . . . . . . . . . . .

Sea Solar Power-Plant Designs . . . . . . . . . . . . . . . . . . . . .

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

1

3

9

9

12

12

13

16

16

16

17

17

18

19

2 0

22

2 4

26

2 6

29

3 0
TABLES & FIGURES

Table 1: OTEC Funding Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Table 2: Milestones and Funding of OTEC Testing Program . . . . 6

Figure 1: Thermal Resource Availability for Hawaiian Islands, Puerto

Rico, and the Gulf of Mexico . . . . . . . . . . . . . . . . . . . . . . 11

Figure 2: Schematic of Mini-OTEC . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Figure 3: OTEC-1 Floating Test Platform . . . . . . . . . . . . . . . . . . . . . 26

Figure 4: Baseline Pilot Plant Concept Design . . . . . . . . . . . . . . . 28
I. INTRODUCTION AND FINDINGS

OTEC is a proposed system for extracting useful energy from the solar

heat stored in vast surface waters of tropical and semi-tropical oceans.

OTEC systems aim to utilize the temperature difference between warm surface

and cold, deep ocean water to power turbines and produce electricity. Some

designs would use electricity for at-sea production of energy intensive

products such as ammonia. The federal Department of Energy is sponsoring a

major effort to develop the OTEC system as a future source of energy.

Much additional work has been done on OTEC since OTA’s original report

was issued. There have been a number of specific technical achievements and

the funding level for the program has grown from about $15 million annually

to a current annual spending pace of $40 million. Since the OTEC program

was established in the Energy Research and Development Administration five

years ago, the Federal government has invested over $100 million in it. In

addition, a number of important technical projects have been privately

funded.

The DOE is weighing data and awaiting results from additional tests

before making a key decision on whether to support the first large pilot

plant. Some critics of the DOE approach believe it is overly cautious and

that enough information is now available to justify an immediate decision to

build several pilot plants. Other critics feel that other energy

technologies are more meritorious~

MAJOR FINDINGS

The principal findings which can be summarized from this OTA analysis
- 2 -

of current OTEC technology are:

1. The technology base for OTEC has improved over the past two

years, and has consequently lowered the technical risk involved

in constructing a moderate-sized (10-40 megawatt) pilot plant.

It has not, however, developed to the point where the costs of

large commercial plants can be accurately estimated.

2 . The most significant technical accomplishments which have

occurred over the past two years are the small scale (10

kilowatt) demonstration of system feasibility of Mini-OTEC (a

barge-mounted test plant) and the improvements in several aspects

of heat exchanger performance through laboratory and sea tests.

3 . Very little has been done recently to evaluate the potential

ocean thermal energy resources available for major OTEC

commercialization; the present DOE development strategy does not

adequately consider the future resource availability.

4 . The OTEC program within DOE has grown in size and scope over the

past two years and many competent technical groups have been

involved in recent OTEC work. However, it is not certain that

the project team could adequately respond to a major acceleration

effort which would entail pilot plant construction prior to FY

82.
- 3 -

11. The OTEC Program

Within the DOE Office of Solar Technology, OTEC is one of five major

systems development projects. 2 In 1978 when OTA published its OTEC

assessment the DOE Program was just beginning to reach its present level of

attention. The 1978 OTA report described the work leading up to this

major Department of Energy program, and analyzed the status of the

technology, the economic projections, and the government funding plans.

That report presented three options of future federal funding for

consideration ranging from a no funding approach to a “systems development

funding” approach which would entail annual costs of several hundreds of

millions of dollars. The present Department of Energy program appears to be

patterned after a middle-range approach which OTA described as an “R&D

funding” approach.

Since 1978 much additional research work has been done on OTEC through

this approach. A number of specific technical achievements have been

recorded and DOE program funding has increased to almost $40 million

annually; it is planned to remain at about that level through FY 81.

The past and future federal funding by major category is shown in a

budget breakdown in Table 1. It can be seen from the budget breakdown that

the DOE is switching emphasis from technology development to hardware

testing. In fact, the OTEC-1 test platform which is scheduled to go to sea

this June is currently using about one half of the program funds. 3

Table 2 shows the DOE OTEC-1 schedule and the schedule for an OTEC

Pilot Plant. DOE has assumed that enough information will be available by

the beginning of FY 82 to make a decision on whether to proceed with a 1O-4O

61-689 0 -80-2- .
- 4 -

MW pilot plant. Such a plant would require a major increase in funding for

construction, testing and evaluation.

DOE also has plans to fund studies of competing pilot power plant

designs. Recent construction cost estimates for a 40MW pilot plant range

between 150 and $300 million and the number of pilot plants which could or

should be built is a matter of considerable debate. The proposed

legislation which is now before Congress (S-1830 and HR 5796) suggests an

accelerated program which would include construction of one or more plants

to attain a goal of 100 megawatts by 1986 and 500 megawatts by 1989. The

Congressional Budget Office estimates that such an accelerated program would

cost $1 billion over the next five years. 4

The DOE program strategy is to concentrate on the US island market

potential for early OTEC pilot and demonstration plants but not to make a

decision on the first or subsequent pilot plants until more test results are

in. Bennett Miller of DOE, testifying before the House Science and

Technology Committee in February 1980, stated that if results from OTEC-1

and other tests are encouraging and a decision is made to build an OTEC

pilot plant, it is anticipated that detailed design and construction can be

started in FY 1982 and completed in FY 1985. This pilot plant is planned to

be of a scale sufficient to demonstrate the performance and reliability of

both the total system as well as the individual components and to provide

5,6

enough information for a decision to build a commercial size facility.

While the strategy of building initial systems for island markets is

considered logical by many, the pace of the program has been criticized by

several of the private companies and researchers who have been involved in

OTEC development over the past years. Some also claim that the plant ship
- 5 -

TABLE 1

Category

Management

Planning

Advanced Research -

Technology Develop -

Mostly

Testing (OTEC-1) -

Total

Cum. Total

OTEC Funding Summary

By Category

($ in Millions)

Fiscal Year

’75

0 . 1

1.6

1.2

2.9

2.9

’ 7 6

2 . 1

2 . 7

3 . 5

0 . 3

8 . 6

11. 5

’77

2 . 1

1 . 2

4 . 5

3 . 4

3 . 3

14. 5

26. 0

’ 7 8

2 . 5

1 . 6

5 . 0

10. 9

9 . 0

29. 0

55. 0

’79

3 . 7

3 . 0

1 . 0

11. 6

19. 2

38. 5

93. 5

Projected

’ 8 0

4 . 7

3 . 0

1 . 0

9 . 4

19. 5

37. 6

31. 1

’ 8 1

4 . 5

2 . 9

1 . 0

8 . 2

20. 0

36. 6

67. 7

Total

7 Years

19.7

16.0

17.2

43.8

71.0

167.7

Source: U.S. Department of Energy, Ocean Energy Systems

Program Summaries for FY 77, 78 and 79 and

Solar Energy Program Summary Document for FY 81.
- 6 -

TABLE 2

Milestones and funding of OTEC Testing Program -

LEGEN D A BEGIN MILE$ToMc

OCEAN SYSTEMS V END MILESTONE

ENGINEERING TEST & DEVELOPMENT o DECISION MILESTONE

A COMPLETED TASK

Source: U. S. Department of Energy, Solar Energy Program

Summary Document FY 1981.
- 7 -

option to develop an OTEC system which could be located in the tropical

oceans to produce ammonia or other energy intensive products has not been

adequately considered in the strategy.7

Whether the program could or should be accelerated and what the

appropriate level of government involvement might be are the subjects

addressed by pending legislation. The pending legislation includes two

types of bills both of which have been introduced in the House and Senate.

The first type (S 1830 and HR 5796) requires the Department of Energy to

prepare a comprehensive plan leading to OTEC commercialization demonstration

goals. The second type (HR 6154 and S 2492) provides a licensing system for

OTEC and financial incentives for commercial and demonstration facilities.

The technical risks involved in a decision to accelerate the program are

described in section IV of this paper. It appears, however, that whether a

pilot plant is built next year or the year after will be determined more by

policy direction and management capability of the Department of Energy than

by technical considerations. 8

The OTA review of past DOE funding and development work indicates that

the number of focused technical accomplishments is modest compared to the

money spent. However, the work has helped to build an institutional

capacity consisting of a number of experienced technical groups. The DOE

staff is located in several field offices as well as in Washington

headquarters. Several systems contractors also provide management support

to each of the DOE offices. The groups now involved in OTEC work are large

and diversified, include many competent technicians and some major private

companies with relevant experience. They are also spread over many

locations which makes it difficult to coordinate the diverse pieces of work.
-8 -

In their multiyear program plan, DOE has projected that, even with an

accelerated effort, large budget additions would not be needed before FY 82.

Given the present DOE management system, it does not appear likely that

OTEC pilot plant construction could be initiated much earlier than the FY 82

date now planned.
- 9 -

111. The Ocean Resource

OTEC plants make use of the differential temperature between the

surface and deep ocean water masses. Thus, potential energy resources are

greatest in tropical regions of the ocean where surface temperatures remain

warm (about 80oF) throughout the year, and where cold water is available at

reasonable depths.

OTA’s 1978 report noted that no one had undertaken total assessment of

the ocean’s thermal resources and their relationship to the amount and kind

of energy needed in specific locations. It did not appear that a

commercialization strategy for OTEC could be developed without, having more

detailed information and analysis of the potential thermal energy resource.

Since that report, it does not appear that DOE has completed even a

preliminary assessment of this kind which could be used in their own

9

planning and commercialization of OTEC power systems.

It has been estimated that over 20 million square miles of suitable

ocean area exists worldwide for OTEC sites 1O. The DOE estimates the upper

extractable limit of this renewable resource as 200 quads (1015 Btu) per

year. (One quad per year of electrical output is roughly equivalent to

11,000 megawatts operating 100% of the time) The magnitude of the thermal

resource available to the United States for exploitation has been estimated

by DOE to be tens of quads per year. These resource numbers have not been

documented by DOE; it has not identified the sites for each estimate; and it

has not stated the assumptions used in making these estimates.11

Island and Gulf Coast Resources

Resource development strategy has also not been defined in existing DOE
- 10 -

planning documents. The DOE “island strategy” for electrical power

generation has targeted the U.S. island market as ideal for OTEC

development. OTA recommended in the 1978 report that it would be beneficial

to emphasize such specific development of island sites in the Caribbean and

Pacific Oceans.

DOE has projected incremental baseload electricity needs of U.S.

islands and the Gulf Coast. OTEC seems well-suited to the U.S. island

market because the cost of alternative incremental generation capacity is so

high for islands. The DOE strategy for commercialization also depends

heavily on penetration into the much larger continental U.S. electricity

demand in the Sunbelt region adjacent to the Gulf of Mexico. In this

market, OTEC plants must compete with baseload fossil and nuclear generating

plants.

Even though DOE future projections appear to rely heavily on developing

OTEC sites in the Gulf of Mexico, this has not been based on a detailed

analysis of the total resource that would be available to the U.S. Some

researchers have developed dynamic models of the ocean temperature in the

Gulf but these have yet to be verified. OTA made a very rough estimate of

total potential OTEC electrical production from the Gulf of Mexico in its

1978 report and concluded that about 15,000 megawatts may be available for

U.S. markets. While this estimate has also not been verified, it does

indicate that there may be some limits on resources in this particular

12, 13 region.

Another question to be addressed in analyzing the Gulf of Mexico

resource is that of U.S. jurisdiction. Most of the resource now identified

is outside of 200 miles from U.S. shores and inside 200 miles from Cuban and
- 11 -

Figure 1

THERMAL

HAWAIIAN ISLANDS

GULF OF MEXICO

RESOURCES

PUERTO RICO

GA

BROWNSVILLE

36°F .0 MONTHS

36°F -12 MONTHS

DEPTH TO RESOURCE 3960’

NOTE: MONTHLY 1 o SQUAR E

DATA USED IN ANALYSIS

THERMAL RESOURCE AVAILABILITY FOR THE HAWAIIAN ISLANDS, PUERTO RICO, AND THE GULF OF MEXICO

Source : U.S. Department of Energy, Draft Multiyear Program Plan, Ocean

Systems, Oct., 1979.
- 12 -

Mexican coastlines. This is now considered international waters but it is

by no means easily accessible to large U.S. electrical markets.

Plant Ship Resources

The OTEC plant ship concept uses the electricity generated from the

thermal temperature differences to produce an energy intensive product such

as ammonia. This concept avoids the requirement of the electric cable from

the OTEC plant site to the user. Instead, a product will be shipped by

barge or by pipeline to the end user.

To gain maximum OTEC efficiency, there is an incentive for the plant

ship to be located where maximum temperature differences exist between the

surface and deep ocean water. These locations may be in the tropical

Atlantic or Pacific or even in the large

Hawaiian Island chain, all of these both

ocean regions near the long

regions having very large resource

potential. Thus, the plant ship may be located in international waters, a

considerable distance from the product markets of the continental United

States.

Summary

The Department of Energy has projected that OTEC will be able to serve

major U.S. markets through the use of Gulf of Mexico sited electrical

generating systems and plant ships in the tropical oceans. They have

projected 50,000 megawatts for the Gulf of Mexico and Plant ships by the

year 2010. It is not stated, however, what portion of this number would be

electrical generation or plant ships. Since the types of systems are very

different and since there is a huge difference in possible resources between

Gulf of Mexico and the tropical oceans, a strategy for resource evaluation

is urgently needed. More attention should also be given to careful analysis

of all feasible ocean thermal resources that will have an effect on OTEC

development.
- 13 -

I v . The Technology Base

The technology base for OTEC has improved over the past two years.

Substantial and significant work has been accomplished.

The OTA report in 1978 detailed the history and background of the

development of the OTEC concept. As stated in 1978, no technological or

scientific breakthroughs are needed for OTEC to become a commercial reality.

However, there are still formidable engineering development challenges in

getting from the present state of development to many, large economically

competitive commercially operating systems.

Two basic uses have been proposed: baseload electrical generation and

the power supply for manufacture of an energy-intensive product such as

ammonia. These have been the most thoroughly examined of the potential OTEC

uses. There are conceptual designs of systems for both applications which

have changed only slightly since 1978*

Regardless of the design and end use, each OTEC would require an ocean

platform, a heat exchanger and a cold water pipe. If the system were to

provide electricity to a busbar, it would require underwater transmission

lines and a mooring system. An OTEC used to produce a product such as

ammonia would probably have a propulsion system enabling it to move from

site to site, thus capitalizing on areas where the greatest differences of

temperature exist between water at the surface and at the cold water pipe

inlet. A large commercial system would be of about 400 megawatt capacity.

The present program is directed primarily at developing the technology which

could be incorporated into future possible commercial systems.
- 14 -

Within a logical technology development process, the construction and

operation of a pilot plant would be very desirable to fully test a total

system design under seagoing conditions. Only after a pilot plant test

program is well underway can any accurate estimates of long term commercial

economic and technical feasibility be established. Unfortunately for such

systems as OTEC, even a pilot plant program is likely to be very costly.

OTEC technology has been developed to the stage where a moderately

sized (10-40 MW) pilot plant can probably be designed and constructed. The

most significant technical risks are in the areas of cold water pipes, heat

exchangers and electrical transmission cables. These three areas probably

need component tests and evaluations prior to building a complete system for

a pilot plant. OTEC-1 will be a floating test platform intended to evaluate

heat exchangers of one megawatt* size. Other component tests are also

planned.

OTEC development work has shown substantial progress during the last

two years. A significant event was the operation of Mini-OTEC during the

summer of 1979 in Hawaii, showing that a small OTEC system can generate net

electrical output. The progress in fouling countermeasures and the

development of heat exchangers with overall heat transfer coefficients of

about 1000 are also note-worthy.** As a result, there is now more

confidence in the prediction of OTEC technical performance.

* Plant electrical output.

** Large heat exchangers in standard electrical power plants usually

operate with heat transfer coefficients of less than 400.
.

- 15 -

Figure 2

Schematic of Mini-OTEC

A Small Scale System Concept Test Bed

Which Operated off Hawaii During Aug.-Oct. , 1979

,CONTROL STATION

/ /

AMMONIA POWER PLANT

COLD WATER TRANSFER HOSE

/

EVAPORATOR HEAT EXCHANGER AND CONDENSER

HEAT EXCHANGER

SUPPORT BARGE

/

COLD WATER PIPE POSITIONING

BUOY

, /

COLD WATER PIPE

Source: Lockheed Missiles and Space Co.
- 16 -

The following sections describe the status of the major technological

developments which make up a complete OTEC system.

Significant Accomplishments

Significant accomplishments in the OTEC program have been the operation

of Mini-OTEC and technical progress in understanding fouling and keeping

heat exchanger surfaces clean.

Mini-OTEC

The Mini-OTEC program was a joint venture costing about $2.5 million

between the State of Hawaii, Dillingham Corporation, and the Lockheed

Missile and Space Company with the U.S. Navy furnishing the barge for the

program. Mini-OTEC’s purpose was to generate net power according to the

temperature difference between the warm and cold water resource. This was

considered by many to be and the proof of principle for an OTEC system. The

cold water pipe was fabricated of 24-inch diameter polyethylene in a length

of 2150 feet. It started operation in August 1979, and ran for a period of

about 2 months. During this time, the Mini-OTEC plant operated for 2 weeks

continuously and met its design goal of 50 to 55 kilowatt gross electrical

power with 10 to 12 kilowatt net electrical power output. The 40 kilowatt

difference was used to power pumps and other auxiliaries. Heat exchangers

were fabricated of titanium and were of the plate type design. Although

this was essentially a private venture, it benefitted from a combination of

technology developed under government sponsorship and off-the-shelf

14 commercial hardware.

Progress in Understanding Bio-Fouling

Another accomplishment in the OTEC program during the last three years
- 17 -

was progress in the field of micro-fouling (slime formation on heat

exchanger elements). The OTEC program has recognized the importance of

bio-fouling, corrosion, and the selection of materials for establishing the

feasibility of the entire OTEC system. It was recognized early that the

selection of materials would significantly affect cost and performance of

the entire OTEC system.

As a result of the work done during the last two years, considerably

more knowledge is available regarding micro-fouling and its accumulation for

the design of heat exchangers for long-range operation and high performance.

Information is available on a variety of cleaning methods for both the

mechanical and chemical systems. In addition, ultrasonic cleaning methods

are being investigated. Consequently, it appears that for some designs

micro-fouling can be overcome by appropriate countermeasures. 15

Status of OTEC Component Development

Platforms

Of the various OTEC components, the platform represents relatively few

technological problems. The platform for a 100-400 MW commercial OTEC power

plant is approximately the same size as very large oil drilling platforms.

The building material can be reinforced concrete or steel; such platforms

have been built in a number of industrial countries. Thus, the size and

design of the platform does not represent new concepts or technology.

However, long-life and survivability represent factors which require

additional attention. The station-keeping and mooring will require specific

data and designs for a site. A system for mooring large commercial plants

in deep water is beyond the state-of-the-art and engineering development may
- 18 -

be required. Numerous engineering studies of platforms and moorings have

been completed over the past few years; however, except for Mini-OTEC and

OTEC-1 testbeds, none have been constructed.16

Cold Water Pipe

The fabrication, deployment, and connection of the cold water pipe to

the platform will require a substantial engineering effort. For the

Mini-OTEC plant, the 24 inch polyethylene cold water pipe performed

satisfactorily. It can also be expected that for OTEC pilot plants, in the

10 to 40 megawatt range, the cold water pipe may not be a insurmountable

problem. However, for large plants (400 MW) where the cold water pipe can

be approximately 100 feet in diameter and up to 3000 feet long, it will be

considerably more difficult to design and build a pipe which can be

subjected to movements in all three axial directions and in rotation about

several axes. A substantial amount of work is presently being undertaken

for cold water pipe design and analysis. Several configurations and

materials have been proposed as feasible candidates. However, long lifetime

requirements and survivability are presenting uncertainties for the large

pipes. Dynamic loadings on the pipe due to wave action and stresses due to

platform motions are recognized as problems affecting pipe design. At this

time, rigid materials such as reinforced concrete and steel are being

analyzed together with more compliant materials such as a variety of

plastics with reinforcements and possibly nylon-reinforced rubber. It is

also quite possible that the design of the cold water pipe will be

location-dependent, similar to the cooling water discharge pipes from the

condensers of existing central station power plants. Considerable physical

oceanographic data will be required to optimize the location of the OTEC
- 19 -

plant to determine the best cold water pipe design. once a successful

design for the cold water pipe has been established, there will be a need

for production engineering and the establishment of manufacturing facilities

for the cold water pipe. Pipe materials will be an important consideration

and their selection may be affected by size and volume. Advanced handling

procedures will be needed for the large cold water pipes. It will be

important to undertake ocean testing of the cold water pipe. 17, 18

Heat Exchanger

The heat exchanger for the closed cycle OTEC plant represents the most

important component because of its size, weight, and cost. A variety of

designs have been proposed and tested. Some of these units are of the shell

and tube type with the sea water inside the tubes and the evaporating

ammonia on the shell side. Various types of heat transfer enhancement

techniques, such as flutes to promote local turbulence, have been analyzed

and tested on both the water and ammonia sides of tubular heat exchangers.

Plate heat exchangers have also been tested with ammonia side enhancement.

As a result of the many analytical and experimental data which have been

accumulated during the last two years, substantial progress has been made.

An OTEC heat exchanger with a total heat transfer coefficient of about 1000

can be expected in a modern heat exchanger design. This type of design

would have no system to enhance heat transfer on the water side so that it

can be easily cleaned for fouling purposes. This total heat transfer

coefficient is about two to three times the value attainable two or three

years ago. If the heat exchanger has no enhancement on the waterside there

is no increase in pumping power. It can be expected that the same high heat

transfer coefficient will be achieved with the plate and fin type heat

exchanger, The experimental confirmation of the high heat transfer
- 20 -

coefficient performance is a significant advancement in heat exchanger

technology. If a chemical fouling countermeasure is used, then additional

enhancement on the waterside can be used, possibly further increasing the

overall heat transfer coefficient.

At this time, titanium promises the greatest reliability for an OTEC

heat exchanger. Stainless steel and aluminum offer opportunities for less

expensive heat exchanger materials and also result in lower fabrication

cost. However, additional studies and experimentation are needed for these

materials to guarantee the same reliability and long life as titanium when

19, 20 subjected to the anti-fouling countermeasures.

Other working fluids besides ammonia have been suggested. These

include various combinations of hydrocarbons and freons. For these fluids

copper-nickel could be used in the heat exchanger and thus the problems of

fouling and corrosion would be substantially reduced. Some basic work on

additional fluids may be justified so that large potential changes in

performance are not overlooked. Plastic heat exchangers have been

suggested. Such units may offer the potential of lower cost. However it may

take several years for these new materials to meet all the tests for

endurance. overall, considerable progress has been made in heat exchanger

design and performance. Its technical performance can now be estimated with

greater confidence than before. Long-range development of a lower cost heat

exchanger material will be desirable. Additional tests to optimize cleaning

methods will be needed to minimize the cost of cleaning while meeting

performance and environmental requirements.
- 21 -

Power System Components

The power turbine is a component which has received only limited

attention. This appears justified in view of such major problems as heat

exchanger design and fouling. However, the power turbine with ammonia as a

working fluid in the sizes contemplated for a commercial plant has never

been built. Since the heat of evaporation for ammonia and the enthalpy drop

through the turbine are considerably less than those of existing steam power

turbines, there may be a need to study turbine stability as well as turbine

control. In addition, it may be desirable to study the effects of ammonia

leaks on the entire power plant system.

Pumps may require special attention because they will deliver large

amounts of salt water against a relatively low head. It has been proposed

to have two or more of these pumps operating in parallel. Such high

specific speed pumps may be difficult to operate satisfactorily in parallel

unless they are provided with a special control system. The pumps and their

power requirements critically affect a total power demand to start the OTEC

plant. Considerable attention must be given to the starting requirements of

the OTEC plant and the associated power supply.

A number of alternative power cycles have been investigated. They

include the open cycle, hybrid cycle, the foam cycle, and the mist cycle.

The open cycle has some merits for small units and its ability to supply

fresh water and should be pursued. However, substantial support of the

other cycles appears no longer justified because after considerable length

of study their technical and economic success is very much in doubt. Some

other innovative power cycles may be pursued as long-range research projects

until their technical feasibility and potential economic benefits are
- 22 -

credibly evaluated. The problem of such long-range R&D) is the lack of a

central evaluation authority. There could be benefits to investigating

cycles which require smaller amounts of cold water per megawatt of

21, 22, 23 electricity or systems which will reduce fouling.

Electrical Power Transmission Cables

Prior studies by the Office of Technology Assessment on the OTEC

Program pointed out the state of art of underwater electrical power

transmission citing examples of technology used in Norwegian

commercial developments have not extended that technology to

required for OTEC; nor is it expected that a commercial need

such extended technological development separate from OTEC.

waters. These

what is

will arise for

The analysis of potential power cable failure modes have been

undertaken by the Simplex Wire and Cable Company as well as Pirelli Cable

Systems, Inc. both of which have considerable experience in the design and

fabrication of underseas cables. Several designs have been prepared for

overcoming the severe problems associated with the riser cables and two

prototype cables incorporating different insulation techniques are being

manufactured for ultimate application to the 10-40 MW pilot plant. Three

cables of about 6 inches diameter each will be required. The prototypes

have to undergo extensive testing and if failures occur, will have to be

redesigned and recycled through testing. The whole process of design,

testing, redesign, retesting, preparation of manufacturing specifications,

manufacturing engineering, and further laboratory and field testing of the

cables to assure long life will take a minimum of 3 to 4 years. Plant

modifications for full cable length manufacturing and the associated detail

manufacturing engineering can then proceed followed by the actual
- 23 -

fabrication for a 100-400 MW OTEC. Manufacturing samples of the early cable

runs may well have to undergo further tests.

Thus a major development effort will be required to provide highly

reliable underwater power transmission cables to connect 1OO-4OO MW offshore

OTEC power plants to onshore consumers. Technologically , this cable must be

considered as two distinct parts: the ocean floor cable that runs from

shore to the OTEC site and the riser cable that connects the OTEC plant to

the ocean floor cable. The ocean floor portion will require fewer

technological advances as compared to the riser cable. Deeper depth

operating capability than present experience (1000 m to 1500 m as compared

to 550 m) will probably be achieved for the ocean floor cable without major

difficulty.

The technological advances required however for developing a long life

riser cable are considered to be significant. The riser cable will be

subject to continual accelerations induced by the platform motion in

response to the sea as well as its own response to ocean conditions. These

accelerations, pressures differentials, and specific weight and other

physical differences of the various elements of the riser can result in

early failure of the insulation. The development of reliable splicing

techniques for connecting the riser to the ocean floor cable and for repairs

will also require development and extensive life testing. A further

complexity will be introduced for transmission lines that are over 50 miles

long (most Gulf of Mexico sites fall within this category). These

transmission lines will probably have to be designed for very high voltage

DC rather than AC to minimize power losses. This will affect the selection

of insulations as well as the internal cable construction. In view of the
.

- 24 -

foregoing, it will be necessary that the cable design take into account

system aspects such as expected sea conditions, Platform movement and cable

laying techniques and capability, as well as the techniques of attachment of

the riser cable to the platform. Concepts of integration of the riser cable

with the platform cold water pipe will have to be weighed against

repairability , maintenance requirements, and technological trade-offs.

If several OTEC plants are to be installed within the next decade there

may not be enough cable manufacturing facilities in the United States or

possibly in the world to provide enough cables for the OTEC programs. New

cable manufacturing facilities in a coastal area may be needed together with

the requirements for cable laying ships or barges. With modern engineering

methods it may be possible to substantially reduce the cost of cable

manufacturing and cable laying. 24, 25, 26, 27

OTEC-1

The OTEC-1 floating test facility will begin at-sea operations in June

of this year. Converted from a Navy type T-2 tanker by Global Marine

Development, Inc., it is a test facility designed to evaluate heat exchanger

components. OTEC-1 includes a cold water pipe, pumps, and an ammonia

evaporation/condensation loop. The cold water pipe consists of a bundle of

three 48 inch diameter polyethylene pipes each 2100 feet long with a steel

cable running through to a weight on the bottom.

The first heat exchangers to be tested on OTEC-1 are one megawatt,

conventional titanium shell-and-tube designs for evaporation and for

condensing furnished by TRW under contract to DOE. Each is about 50 feet

long by 10 feet in diameter and contains 6,000 tubes. Following 8-9 months
- 25 -

allocated to tests on this design, it is planned that the ship will be

returned to port, the heat exchangers removed, and up to four smaller 0.2

megawatt units of advanced design installed for tests.28

The actual expenditures for the design and conversion of OTEC 1 are now

projected to be about $7 million more than the original budgeted amount of

$33 million. This was caused by a number of factors including difficulties

encountered after the mothballed tanker was carefully inspected to see which

systems needed replacement. Such an overun is not unusual in large

engineering development projects and it might be expected that future

overuns could occur when the much more difficult OTEC technology development

and testing work is undertaken.

To what extent the results of OTEC-1 tests are necessary for the design

and construction of a pilot plant is a matter of considerable debate. At

present, it is not possible to determine because DOE has not defined which

pilot plant concept or strategy they wish to pursue. If a concept for a

pilot plant had been selected, a logical program of component testing aboard

a test platform could be developed. For some pilot plant concepts OTEC-1

may have limited usefulness. For others, component testing could include

heat exchangers, cleaning methods, parts of cold water pipes and electrical

riser cables. It now appears that only one type of heat exchanger, which

may or may not be suitable for a pilot plant, will be tested on OTEC-1 prior

to FY 82.

It is too early to report any accomplishments from the OTEC-1 test

platform but it appears that most of the hardware has been built on schedule

and by 1981 some initial at-sea heat exchanger test results should be

available.
- 26 -

Figure 3

HELlCOPTER DECK

--4 EVAPORATOR

\

COLD-WATER PIPE CONDENSER

\ ~

iii WARM-WATER

INTAKE

OTEC-1 FLOATING TEST PLATFORM being designed and built water pipe and ammonia evaporation, condensation, and recirGlobal Marine Development, Inc. and TRW is a converted culation. TRW designed the heat exchanger under a separate

Navy tanker. It will be used to evaluate different (OTEC compo- contract with the Department of Energy. The ship will be annents and operation of the heat exchange loop, including cold chored off Ke-ahole Point near the island of Hawaii.

Source: Quest, New Technology at TRW Defense and Space Systems Group,

Autumn, 1979.
- 27 -

A conceptual integrated design of a 40 megawatt pilot plant has been

completed under contract to DOE by the Applied Physics Laboratory of Johns

Hopkins University using the APL plant ship concept. This design calls for

a concrete platform approximately 450 feet long of almost 100,000 tons

displacement. It’s four modules would generate 10 megawatts each. A

lightweight concrete cold water pipe 30 feet in diameter and 3,000 feet long

with four cold water pumps is proposed. The design can be either a moored

plant with an electrical cable to shore or a grazing plant and include an

ammonia conversion plant. The Johns Hopkins Applied Physics Lab is now

completing a report on this design which includes a complete system concept

design. Heat transfer tests of some laboratory conducted ultra-sonic

cleaning tests of the heat exchanger indicates promising results. A section

of the cold water pipe is being fabricated for test. APL has estimated the

cost of a 40 MW pilot plant with no profit or contingencies to be $140 -

$160 million. 29

Ammonia Conversion/Fuel Cells

The prospect for developing an OTEC system for operating in the

tropical coceans has led to design studies of ammonia conversion systems for

these plants and of fuel cells which may be powered by ammonia produced by

these plants.

APL’s first OTEC Plant Ship concept used the solid polymer electrode

(SPE) electrolysis system to produce hydrogen from seawater. The cells use

electricity to manufacture hydrogen at voltages of 1.6 to 1.9 volts (direct

3 0

current).
Figure 4

Baseline pilot >

Proposed b y

by Johns H ~1 ‘

t ‘~ant Concept Design

‘p ‘s ApPlied physics ‘ab 40 ~E lectrfcal m- 125 STPD &

60,000 Ton ~

‘ncrete nU~l Ollia 30 ‘t” ‘i-ter ‘ight concrete cold ‘ater ‘iP’=

‘oUrce: ‘Ohns %Jkins @Plied phYsic5 Lab.
- 29 -

Currently, General Electric has a 50 KW unit for generating electricity

from hydrogen operating in the laboratory. The electrodes on these cells

are made of platinum and blends of other noble metals. They have in some

instances operated over 20,000 hours with negligible deterioration. GE iS

working on ways to reduce the noble metal content of the cell electrodes. 31

Avery has published a paper including cost estimates for an

ammonia-OTEC fuel cell cycle using SPE cells. The paper claims a 50-55%

efficiency of the SPE fuel cell cycle using present systems going up to 60

to 65% achievable at low current densities with further R&D effort. While

such systems may be possible in the future, considerable R&D effort will be

needed.32

Sea Solar Power-Plant Designs

A somewhat different approach to OTEC plant design has been suggested

by Sea Solar Power, Inc. of York, Pennsylvania. They have concentrated R&D

attention on a system using a halocarbon instead of ammonia as a working

fluid and incorporating their patented high performance heat exchanger.

They have built and tested a small working model and have prepared a

conceptual design for a 100 megawatt plant. Much of their work has been

funded internally. Some of their concepts deserve development attention in

future OTEC designs because breakthroughs in heat exchangers could be the

most significant factor for future economic viability.33
- 30 -

References

10 U.S. Congress, Office of Technology Assessment, Renewable Ocean Energy

, Sources - Ocean Thermal Energy Conversion, May 1978.

2. U.S. Department of Energy, Solar Energy Program Summary Document FY

1981, January 1980.

3. U.S. Department of Energy, Ocean Energy Systems, Fiscal Year 1979,

Program Summary.

4. U.S. Congress, Ocean Thermal Energy Conversion, Research Development

and Demonstration Act, Senate Report No. 96-501, December 14, 1979.

5 . Statement of Bennett Miller before the House Science and Technology

Committee Subcommittee on Energy Development and Applications, February

13, 1980.

6. U.S. Department of Energy, Briefing on OTEC Program Status by W.

Richards, February 14, 1980.

7 . Johns Hopkins, Applied Physics Laboratory, Briefing on status of 40 MW

pilot plant design studies, February 25, 1980.

8 . U.S. Department of Energy, Draft Multiyear Program Plan, Ocean

Systems, October 1979.

9 . Letter; Lloyd F. Lewis, Division of Planning and Technology Transfer,

DOE, April 5, 1979, and Memo, Lloyd Lewis to OTA, February 15, 1980.

10. Letter from W.H. Avery, Applied Physics Laboratory, Johns Hopkins

University, to Russell W. Peterson, Office of Technology Assessment,

August 25, 1978.

110 Multiyear Program Plan, Executive Abstract for Ocean Systems,

Department of Energy, Division of Central Solar Technology, Ocean

Systems Branch, October 1979.

12. Renewable Ocean Energy Sources, Part 1 Working Papers - Energy

Conversion, The Baham Corporation, Prepared for the Office of

Technology Assessment, May 1978.

13. Renewable Ocean Energy Sources, Part 1 Ocean Thermal Energy Conversion,

The Office of Technology Assessment, May 1978.

14. Technical briefing to OTA by F.E. Naef of Lockheed Corp. , Washington,

D.C., March 10, 1980.

150 Phone conversation between Dr. Herman Sheets, Analysis and Technology,

Inc. , and Dr. Norman Sather and Dr. Joseph Draley, Argonne National

Laboratory, March 1980.
- 31 -

1 6 .

1 7 .

1 8 .

1 9 .

2 0 .

2 1 .

2 2 .

2 3 .

2 4 .

2 5 .

2 6 .

2 7 .

2 8 .

2 9 .

3 0 .

3 1 .

U.S. Department of Energy, Draft Report of the Working Groups, 6th OTEC

Conference, (Washington, D.C.: June 19-22, 1979), unpublished.

Presentation to the Ocean Energy Systems Council by ETEC, “Equipment

Experiment Instrumentation and Data Reduction on OTEC 1“, La Jolla,

California November 13-14, 1979.

TRW Systems and Energy, Cold Water Pipe Preliminary Design Study, Final

Report, (Redondo Beach, California: TRW, November 20, 1979).

Phone conversation between Dr. Herman Sheets, Analysis and Technology,

Inc. and Dr. Norman Sather, Argonne National Laboratory, March 1980.

Phone conversation between D. Pauli, OTA and Ralph Mitchell, Harvard

University, April 11, 1980.

University of Oklahoma, School of Chemical Engineering, Use of Mixtures

as Working Fluids in Ocean Thermal Energy Conversion Cycles--Phase II.,

(Norman, Oklahoma: University of Oklahoma, February 1978).

U.S. Department of Energy, Draft Report of the Working Groups, 6th OTEC

Conference, (Washington, D.C.: June 19-22, 1979), unpublished.

Oak Ridge National Laboratory Report of the Proceedings of the OTEC

Alternative Cycles Contractors Information Exchange Meeting, December

1978.

Bamford, Thomas B. et al, Riser Segment Design of Underwater Electric

Power Transmission Cable System, Simplex Wire and Cable Company,

(Portsmouth, New Hamphire: 1978).

Rumbaugh, Jeffery H. et al. , “Thermal Energy Conversion: Tapping the

Sea Depth”, Spectrum, Vol. 16, No. 8 (August 1979).

Phone conversation between D. Pauli, OTA, and Robert E. Perry,

Electrical Power Research Institute, April 10, 1980.

Phone conversation between D. Paul, OTA, and E. Ken Roberts, Simplex

Wire and Cable Company, April 11, 1980.

Douglass, R.H. , OTEC: Solar Energy from the Sea, in Quest-New

Technology at TRW, Autumn 1979.

Johns Hopkins Applied Physics Lab, OTEC Briefing at Columbia Maryland,

February 25, 1980.

Avery, W.H. , R.W. Blevins, G.L. Dugger, and E.J. Francis, “Executive

Summary, Maritime and Construction Aspects of Ocean Thermal Energy

Conversion (OTEC) Plant Ships,” Applied Physics Laboratory, SR 76-1A,

April 1976.

Telecon: G. Baham, OTA to L. Nuttal, General Electric Company, March

14, 1980.
- 32 -

32. Avery , W.H. , “The OTEC Contribution to Energy Needs of all Regions of

the U.S. ,“ undated.

33. Spencer R. Liverant and James H. Anderson Jr., Sea Solar Power Inc.,

OTEC briefing to OTA, Washington, D.C., March 11, 1980.

U.S. GOVERNMENT PRINTING OFFICE : 1980 0 - 61-689