Marine energy - Wave, tidal, and other water current converters - Part 20: Design and analysis of an Ocean Thermal Energy Conversion (OTEC) plant - General guidance

IEC TS 62600-20:2019 establishes general principles for design assessment of OTEC plants. The goal is to describe the design and assessment requirements of OTEC plants used for stable power generation under various conditions. This electricity may be used for utility supply or production of other energy carriers. The intended audience is developers, engineers, bankers, venture capitalists, entrepreneurs, finance authorities and regulators.
This document is applicable to land-based (i.e. onshore), shelf-mounted (i.e. nearshore seabed mounted) and floating OTEC systems. For land-based systems the scope of this document ends at the main power export cable suitable for connection to the grid. For shelf-mounted and floating systems, the scope of this document normally ends at the main power export cable where it connects to the electrical grid.
This document is general and focuses on the OTEC specific or unique components of the power plant, particularly the marine aspects of the warm and cold water intake systems. Other established standards are referenced to address common components between the OTEC system and other types of power plants and floating, deep water oil and gas production vessels, such as FPSOs and FLNG systems. Relevant standards are listed within this document as appropriate.

General Information

Status
Published
Publication Date
17-Jun-2019
Current Stage
PPUB - Publication issued
Completion Date
18-Jun-2019
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IEC TS 62600-20:2019 - Marine energy - Wave, tidal, and other water current converters - Part 20: Design and analysis of an Ocean Thermal Energy Conversion (OTEC) plant - General guidance
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IEC TS 62600-20
Edition 1.0 2019-06
TECHNICAL
SPECIFICATION
colour
inside
Marine energy – Wave, tidal, and other water current converters –
Part 20: Design and analysis of an Ocean Thermal Energy Conversion (OTEC)
plant – General guidance
IEC TS 62600-20:2019-06(en)
---------------------- Page: 1 ----------------------
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---------------------- Page: 2 ----------------------
IEC TS 62600-20
Edition 1.0 2019-06
TECHNICAL
SPECIFICATION
colour
inside
Marine energy – Wave, tidal, and other water current converters –
Part 20: Design and analysis of an Ocean Thermal Energy Conversion (OTEC)
plant – General guidance
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 27.140 ISBN 978-2-8322-6915-2

Warning! Make sure that you obtained this publication from an authorized distributor.

® Registered trademark of the International Electrotechnical Commission
---------------------- Page: 3 ----------------------
– 2 – IEC TS 62600-20:2019 © IEC 2019
CONTENTS

FOREWORD ........................................................................................................................... 5

INTRODUCTION ..................................................................................................................... 7

1 Scope ............................................................................................................................ 11

2 Normative references .................................................................................................... 12

3 Terms and definitions .................................................................................................... 13

4 Abbreviated terms and acronyms ................................................................................... 15

5 Site specific and metocean design parameters .............................................................. 15

5.1 Environmental factors influencing design .............................................................. 15

5.1.1 General ......................................................................................................... 15

5.1.2 Seawater temperature ................................................................................... 16

5.1.3 Wind .............................................................................................................. 16

5.1.4 Waves ........................................................................................................... 16

5.1.5 Water depth and sea level variations ............................................................. 17

5.1.6 Currents ........................................................................................................ 17

5.1.7 Marine growth ................................................................................................ 17

5.1.8 Other meteorological and oceanographic information ..................................... 17

5.1.9 Water chemistry ............................................................................................. 17

5.1.10 Third party (collision, anchor impact, trawling, Unexploded Ordinance

(UXO) ............................................................................................................ 17

5.1.11 Soil/seabed conditions ................................................................................... 18

5.2 Biological impact ................................................................................................... 18

6 Floating OTEC – General information and guidance (closed cycle, deep water) ............. 18

6.1 Seawater considerations ....................................................................................... 18

6.2 Cold seawater system ........................................................................................... 19

6.2.1 Systems engineering considerations .............................................................. 19

6.2.2 Cold water pumping power considerations ..................................................... 20

6.2.3 CWP dynamic response ................................................................................. 20

6.2.4 Static Loads and bending moments ............................................................... 21

6.2.5 Suction collapse ............................................................................................ 21

6.2.6 Deflection by current and platform motions .................................................... 21

6.2.7 Analysis of loads and displacements ............................................................. 22

6.2.8 Recommendations for qualification of the Cold Water Pipe (CWP) ................. 22

6.2.9 Analysis approach ......................................................................................... 22

6.3 Warm seawater system ......................................................................................... 22

6.3.1 Warm water intake (screen) ........................................................................... 22

6.3.2 Warm water ducting and pumps ..................................................................... 23

6.3.3 Biofouling control ........................................................................................... 23

6.4 Seawater discharge arrangement and plume analysis ........................................... 23

6.4.1 Seawater discharge ducts .............................................................................. 23

6.4.2 Seawater pumps ............................................................................................ 23

7 Process system ............................................................................................................. 24

7.1 Working fluid selection .......................................................................................... 24

7.2 Heat exchanger (HX) selection .............................................................................. 25

7.3 Materials compatibility .......................................................................................... 25

7.4 Process system risks and hazards ........................................................................ 25

8 Platform type ................................................................................................................. 25

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IEC TS 62600-20:2019 © IEC 2019 – 3 –

8.1 General ................................................................................................................. 25

8.2 Mooring/Station keeping ....................................................................................... 26

8.2.1 Grazing OTEC plants (no power export cable required) ................................. 26

8.2.2 Non-grazing OTEC plants .............................................................................. 26

9 Power export ................................................................................................................. 27

9.1 General ................................................................................................................. 27

9.2 Design considerations ........................................................................................... 27

9.3 Platform based equipment .................................................................................... 27

9.4 Transmission cable ............................................................................................... 27

9.5 Land based equipment .......................................................................................... 28

10 Energy storage and transfer system .............................................................................. 28

10.1 General ................................................................................................................. 28

10.2 Hydrogen .............................................................................................................. 28

10.3 Ammonia .............................................................................................................. 28

10.4 Methanol ............................................................................................................... 28

10.5 Battery storage ..................................................................................................... 28

11 Land and shelf-based OTEC .......................................................................................... 29

11.1 General information and guidance......................................................................... 29

11.2 CWP design for land and shelf-based OTEC plants ............................................... 29

12 Risk based approach for the design and operations of OTEC plants .............................. 30

12.1 Risk assessment ................................................................................................... 30

12.2 Risk based design................................................................................................. 30

12.2.1 Risk assessment process .............................................................................. 30

12.2.2 Prototype testing ........................................................................................... 31

12.3 Risk based operational guidelines ......................................................................... 31

12.3.1 Floating plant ................................................................................................. 31

12.3.2 Operating plant .............................................................................................. 31

12.3.3 Product export risks/hazards ......................................................................... 31

13 Transportation and installation (T&I) .............................................................................. 32

14 Commissioning and handover ........................................................................................ 32

15 Operations, inspection and maintenance ....................................................................... 33

15.1 General ................................................................................................................. 33

15.2 Operations ............................................................................................................ 33

15.3 Inspection and maintenance ................................................................................. 34

15.4 Hazards and safety ............................................................................................... 35

15.4.1 Hazards ......................................................................................................... 35

15.4.2 Safety ............................................................................................................ 35

16 Decommissioning .......................................................................................................... 36

Annex A (informative) OTEC potential and its history ........................................................... 38

A.1 OTEC potential ..................................................................................................... 38

A.2 Installation sites .................................................................................................... 39

A.3 Previous OTEC projects ........................................................................................ 39

A.4 Open cycle OTEC ................................................................................................. 40

Bibliography .......................................................................................................................... 41

Figure 1 – Tropical ocean temperature-depth profile ............................................................... 7

Figure 2 – Working principle of closed cycle ocean thermal energy conversion [2] .................. 8

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– 4 – IEC TS 62600-20:2019 © IEC 2019

Figure 3 – Major power cycle components of a closed cycle OTEC plant ................................ 9

Figure 4 – Open cycle OTEC system .................................................................................... 10

Figure 5 – Example of a typical process for developing and testing an OTEC system

(land-based and floating) ................................................................................................... 12

Figure 6 – Seawater differential temperature with 95 % confidence intervals......................... 14

Figure 7 – Example of OTEC power definitions ..................................................................... 14

Figure 8 – Seawater flow considerations for floating OTEC ................................................... 19

Figure 9 – Major components of a closed cycle OTEC plant working fluid process

system .................................................................................................................................. 24

Figure 10 – ISO 19900 offshore standards relevant to OTEC platform design ....................... 26

Figure 11 – Simple risk evaluation matrix .............................................................................. 30

Table 1 – Indicative design consideration in selecting Cold Water Pipe parameters .............. 19

Table A.1 – Notable OTEC systems – Past and present ........................................................ 39

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IEC TS 62600-20:2019 © IEC 2019 – 5 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
MARINE ENERGY – WAVE, TIDAL, AND OTHER
WATER CURRENT CONVERTERS –
Part 20: Design and analysis of an Ocean Thermal Energy Conversion
(OTEC) plant – General guidance
FOREWORD

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Technical Specification are subject to review within three years of publication to decide

whether they can be transformed into International Standards.

IEC TS 62600-20, which is a Technical Specification, has been prepared by IEC technical

committee 114: Marine energy - Wave, tidal and other water current converters.
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– 6 – IEC TS 62600-20:2019 © IEC 2019
The text of this Technical Specification is based on the following documents:
Draft TS Report on voting
114/286/DTS 114/299A/RVDTS

Full information on the voting for the approval of this Technical Specification can be found in

the report on voting indicated in the above table.

A list of all parts in the IEC 62600 series, published under the general title Marine energy -

Wave, tidal and other water current converters, can be found on the IEC website.

This document has been drafted in accordance with the ISO/IEC Directives, Part 2.

The committee has decided that the contents of this document will remain unchanged until the

stability date indicated on the IEC website under "http://webstore.iec.ch" in the data related to

the specific document. At this date, the document will be
• transformed into an International standard,
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.

IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates that it

contains colours which are considered to be useful for the correct understanding of its

contents. Users should therefore print this document using a colour printer.
A bilingual version of this publication may be issued at a later date.
---------------------- Page: 8 ----------------------
IEC TS 62600-20:2019 © IEC 2019 – 7 –
INTRODUCTION

Seventy percent of the Earth’s surface is ocean. Most solar energy striking the ocean is

absorbed within the upper 100 m and is retained as thermal energy. Expanding slightly as it

warms the surface seawater layer is reheated by additional sunlight resulting in temperatures

often exceeding 25 °C in tropical latitudes. Deep seawater is much cooler, typically,

about 4-5 °C at depths varying from 800 m to 1 000 m, as shown in Figure 1. This deep cold

water is replenished from the polar regions by the thermohaline ocean circulation. From the

temperature difference that exists between these upper and deep layers of the ocean,

significant quantities of energy can be sustainably extracted by a process called Ocean

Thermal Energy Conversion, OTEC.
Figure 1 – Tropical ocean temperature-depth profile

The temperature difference between the ocean layers in the tropics changes very little during

daily or even yearly cycles and shows a moderate and predictable seasonal variation. This

steadiness creates an attractive characteristic in that OTEC can generate non-intermittent

(sometimes referred to as ‘base-load’) power. Due to the relative simplicity of the process,

OTEC is expected to have a very high capacity factor compared to most other forms or

renewable energy. Capacity Factor is the ratio of actual electrical energy output over a given

period of time, relative to the maximum possible electrical energy output over the same

amount of time. The maximum possible energy output of a given installation assumes its

continuous operation at full nameplate capacity over the relevant period of time. OTEC power

output reliability and predictability is appealing when compared to the intermittency and hence

low capacity factor of most renewable energy sources.
a) Working principle

OTEC converts a sustainable, low-grade heat source, ocean thermal energy, into electricity by

applying a thermodynamic cycle. The theoretical maximum thermal conversion efficiency is

determined by the Carnot cycle, where absolute ocean temperatures are applied in Kelvin. An

example of the Carnot efficiency with a hot source of 27 °C and a cold source of 4 °C is:

η_Carnot= 1-T_cold/T_hot = 1-(4+273,15)/(27+273,15) =7,66 %

This efficiency assumes that the conversion is done by an ideal, reversible heat engine. In

practice, the actual heat transfer is irreversible due to temperature differences in the heat

exchangers and other factors. These heat transfer losses and the actual performance of the

---------------------- Page: 9 ----------------------
– 8 – IEC TS 62600-20:2019 © IEC 2019

turbine and generator shall be accounted for when calculating the actual efficiency. The non-

ideal, actual efficiency would thus be in the range of 3 % to 4 %.

The OTEC process can be configured with different cycles: open, closed and hybrid. The

choice of which system will be optimum will normally be based on site characteristics, such as

local power and fresh water demand.
b) Closed cycle

Closed-cycle OTEC systems are based upon the Rankine thermodynamic cycle and use a

refrigerant-type process working fluid, contained within a closed piping system. Liquid

working fluid is pumped into an evaporator heat exchanger where heat from the warm

seawater causes the working fluid to vaporise. This vapour is piped to a turbine where its

enthalpic energy drives a turbine-generator. The turbine’s vapour exhausts to a condenser

heat exchanger, where it condenses to a liquid by the cooling effect of the cold seawater.

The liquid working fluid then drains to the working fluid pump, completing the cycle. Major

components and flows of a Closed Cycle OTEC plant are illustrated in Figure 1 and Figure 2.

Design considerations associated with these components will be discussed in Clause 5.

Within the evaporator, the warm seawater transfers its heat to the boiling working fluid,

becoming less warm. Similarly, heat from the condensing vapour causes the cold deep

seawater passing through the condenser to become less cold. The heat flow from warm water

is 3 % to 6 % larger than the heat flow into the cold water. This difference is the energy

usefully extracted by the turbine or lost due to friction.

The working fluid will have fluid properties that vary with the specific type used, such as R717

(anhydrous ammonia), R32, R134a or others. The evaporation and condensation properties

and heat exchanger design performance should normally be selected to attain optimum

efficiency for a particular working fluid. Within the process system, the highest pressure

occurs at the working fluid pump outlet, the lowest pressure occurs in the condenser and the

most significant pressure drop will take place within the turbine.
Figure 2 – Working principle of closed cycle ocean thermal energy conversion [2]
____________
Numbers in square brackets refer to the Bibliography.
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IEC TS 62600-20:2019 © IEC 2019 – 9 –
Figure 3 – Major power cycle components of a closed cycle OTEC plant
c) Open cycle

Open-cycle OTEC uses a vacuum process to exploit the different boiling pressures of warm

and cold seawater. The working fluid is used only once and is continually replenished, hence

the term “open” cycle. The process is as follows: Warm seawater enters a large evaporation

chamber at approximately 96 % vacuum, where a small fraction of the seawater vaporizes to

low pressure steam and the remaining seawater supplies the needed heat of vaporization.

The cooled warm seawater is pumped from the evaporator. The low-pressure steam passes

through a mist separator, drives a low pressure turbine and exhausts into the condensing

chamber, which is maintained at approximately 98 % vacuum. The steam condenses directly

onto cold seawater droplets within the condenser chamber and the slightly diluted cool

seawater mixture is pumped from the condenser. Continuously-running vacuum compressors

maintain the chamber vacuum by removing dissolved air and other trace gases that enter with

the seawater flows.

Alternately, a large condensing surface heat exchanger can segregate the steam from the

cold seawater, yielding quantities of fresh water suitable for drinking water or irrigation. Thus

open cycle OTEC can be configured to produce both electricity and fresh water.

Both closed cycle and open cycle OTEC use the Rankine thermodynamic cycle. The primary

difference is that open cycle systems use large vacuum chambers and a very high-volume low

pressure steam turbine, whereas closed cycle uses heat exchangers, a smaller turbine and a

working fluid pump. A schematic diagram of the open cycle OTEC system is given in Figure 4

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– 10 – IEC TS 62600-20:2019 © IEC 2019
Figure 4 – Open cycle OTEC system
d) Hybrid cycle

A hybrid cycle combines features of both the closed-cycle and open-cycle systems to yield

both electricity and desalinated water. Heat exchangers, vacuum chambers and other

components may be arranged in numerous stages to extract additional thermal value from the

“used” warm and cold seawater flows.
---------------------- Page: 12 ----------------------
IEC TS 62600-20:2019 © IEC 2019 – 11 –
MARINE ENERGY – WAVE, TIDAL, AND OTHER
WATER CURRENT CONVERTERS –
Part 20: Design and analysis of an Ocean Thermal Energy Conversion
(OTEC) plant – General guidance
1 Scope

This part of IEC 62600 establishes general principles for design assessment of OTEC plants.

The goal is to describe the design and assessment requirements of OTEC plants used for

stable power generation under various conditions. This electricity may be used for utility

supply or production of other energy carriers. The intended audience is developers,

engineers, bankers, venture capitalists, entrepreneurs, finance authorities and regulators.

This doc
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