Lecture Notes ENGG4000 Lecture 15

ENGG4000 Renewable and Sustainable Energy OTEC-Lecture-15

Dr Hussein A Kazem Faculty of Engineering 1

Ocean Thermal Energy Conversion (OTEC)

Part-I

By Dr Hussein A Kazem



1 Introduction

The most plentiful renewable energy source in our plant by far is solar radiation: 170,000 TW

fall on Earth. Harvesting this energy is difficult because of its dilute and erratic nature. Large

collecting areas and large storage capacities are needed, two requirements satisfied by the

tropical oceans. Oceans cover 71% of Earth’s surface. In the tropics they absorb sunlight and top

layers heat up to some 25oC. Warm surface waters from the equatorial belt of pole ward melting

both the arctic and Antarctic ice. The resulting cold waters retune to the equator at great depth

completing a huge planetary Thermosyphon.

Harnessing solar power from ocean uses the basic technology known

as Ocean Thermal Energy (OTE), or sometimes Ocean Thermal

Energy Conversion (OTEC), which was invented in 1881 by a French

scientist years ahead of his time by the name of Jacque Arsene

D'Arsonval. Before describing the mechanics of the system, let's first

understand the natural resource and its potential.

The ocean covers most of the earth's surface. More than 300 times

what the world now consumes in electricity is available from the solar

energy that is constantly stored in the upper layers of the tropical

ocean. This takes place throughout the equatorial zone around the

world or about 20 degrees north and south of the equator - where most of the world's population

lives. This area is also where the greatest increase in demand for new power exists, because

population growth is greater in this region and where the standard of living has been rather low,

and now more people with more wealth are demanding more electricity.

Man-made solar collectors are still somehow expensive to build, require enormous amounts of

acreage and do not work at night when advanced societies require electricity around the clock.

Contrary to this is the ocean, the largest solar collector in the world. It is already there so to tap

its riches is most prudent. The modern day term to describe this process is OTE or OTEC.

To operate a sea solar power plant involves both a heat source and a heat sink. Therefore, the 80

degrees F surface water in the tropical oceans serves as the heat source and typically 3,000 feet

below the surface is the heat sink or the cold bottom water, which is 40 degrees F. This

temperature difference or delta T is sufficient to operate vapor turbines, which drive generators

and produces electricity and fresh water as a byproduct. This is the OTE concept.



But while it is true that the ocean's free seawater can supply an infinite amount of energy and

produce electricity for most of the world's population, the technical challenge is to design an

OTE plant that is economically efficient or at a reasonable capital cost.

2. Facts about OTE Renewable Energy:

Oceans are the largest solar collectors on earth

Oceans are already built and paid for

Man-made solar collectors only work when the sun shines

OTE's base load power operates 24 hours per day

Stored solar energy throughout the equatorial zone could provide 300 times the

world's consumption of electricity

Sea Solar Power Provides An Endless Source of Energy With No Polluted Air or Global Warming

Fig. 2 Ocean region applicable for OTE. Numbers on the map refer to temperature differences in degrees Celsius. The greater the difference, the better the resource.

3. OTEC and Sea Solar Power, 1880's – Present:

Although Ocean Thermal Energy has a long and interesting

history, it was J. Hilbert Anderson and his son, James H.

Anderson Jr., founders of Sea Solar Power Inc., were the first to

propose a way to harness this unusual resource economically.

Prior to beginning an exhaustive study of OTE in 1962, Hilbert

Anderson had extensive experience designing refrigeration and

heat power cycles. In 1965, father and son published “Large

Scale Sea Thermal Power,” the first of several papers explaining

how established practices in refrigeration and ocean engineering

SSP 4MW vapor turbine

inlet casting



could be innovatively applied. Some OTE Milestones:

1880’s -- OTE first proposed by French physicist Jacques d’Arsonval.

1920’s -- Georges Claude, one of d’Arsonval’s students, builds a test plant using the warm water effluent from a power plant in France.

1930’s -- Claude builds a 60kW open cycle land-based plant at the Bay of Mantanzas, Cuba. Claude later attempts a floating plant.

1940’s -- French energy company starts construction of land-based plant at Abidjan, Ivory Coast. Abandons project when hydroelectric dam goes into service nearby.

1960’s -- Andersons’ first papers: MIT Bachelor’s Thesis, 1963, Power Magazine, 1964, and American Society of Mechanical Engineers technical paper and Mechanical Engineering Magazine in 1965.

1972 -- Hilbert Anderson addresses National Science Foundation’s Solar Energy Panel.

1973-- US Energy Research and Development Administration (ERDA) launches OTE program, eventually spending over $200 million on conceptual, environmental, and feasibility studies.

Early 1980’s --ERDA confirms that OTE is technologically and commercially viable.

ERDA’s successor, the Department of Energy, invites private industry to take over.

Mid-1980’s --Mini-OTE floating plant using a refrigerant working fluid operates in Hawaii.

Late 1980’s --Tokyo Electric builds a small land-based demonstration plant using a refrigerant working fluid on Nauru (www.otecnews.org/articles/nauru/html).

1990’s -- By the early 1990’s it becomes clear from the experience of pilot plants and heat exchanger tests in ocean conditions that OTE heat exchangers can be made from inexpensive aluminum alloys and don’t require expensive cupronickel or titanium. It also becomes clear that bio-fouling of the heat exchanger surfaces can be controlled with small amounts of chlorination.

2000’s ---India’s Institute for Energy Studies has been working on construction of a small floating plant (www.niot.res.in).

Price of oil bottoms at $10/barrel in1998 and then, in 2006 and 2007, surges to between

$60 and $80/b. For tropical island nations and others who generate electricity from oil,

OTE becomes especially attractive.



4. What are we Waiting for?

Ocean Thermal Energy is clearly well within the capability and ingenuity. Scientist believes its

implementation will be comparable to the dawn of “big oil”, our fuel of choice for the last 100

years. As OTE technology further matures, costs will continue to drop. Fossil fuel costs,

however, will continue rising, perhaps exponentially.

Fig. 4

What's Taking So Long?

Some reasons why OTE has been neglected include:

Energy independence not considered an urgent national policy goal.

Global warming not taken seriously.

An acceptable and non-threatening price of oil until 2005.

It’s a new power source and a new technology

5. How does it Work?

Approximately 70% of the earth is covered by

water. In equatorial regions oceans cover nearly

90% of this surface segment of the earth. These

oceans collect and store solar energy. Because

water arranges itself by temperature and density,

the warmest water, being the lightest, is on the

surface. Cold heavier water lies deep below.

A stationary floating plant skims off a small

percentage of the surface layer to use as the heat source. For the heat sink, the plant has a large

diameter submerged pipe to pump up the heavier frigid water below.



A small amount of heat is extracted from the warm water and a lesser amount is put into the cold

water. The net difference in energy flow is turned into electricity and fresh water and/or fuels

and other useful products. Electricity is transmitted to shore through an underwater cable.

A sea solar power electric plant can be described as having 10 major elements. They are (1)

boiler; (2) condenser; (3)vapor turbine generator; (4) working fluid; (5) working fluid pump; (6)

warm water pump; (7) cold water pump; (8) cold water pipe; (9) electric cable to shore; (10)

integrated floating structure.

The warm surface ocean water is pumped to the boiler, which transfers heat to the working fluid,

turning it into a high-pressure vapor. The turbine generator spins as the vapor rushes through it to

reach the low-pressure condenser, which is cooled by the nearly freezing water brought up from

the ocean depths. After condensing, the working fluid is sent back to the boiler to be reused and

to repeat the cycle.

Pumps are needed to bring the cold water up from the deep and through the condenser. Other

pumps move the warm water through the boiler. The power for moving these very large water

flows, plus the power to move the working fluid from the condenser to the boiler, consumes

about 20% of the total power generated (120MW Gross for 100MW Net).

Fig. 6



6. The Benefit of Sea Solar Power:

Benefits and Bi-Products No fuel burned - All the problems of air and water

pollution, strip mining and waste disposal are

eliminated. No smoke, fumes, oil spills or thermal

pollution. No greenhouse gases.

Road to Energy independence - OTEC stands as the

best option for countries near the Ocean and other

nations to gain energy independence. At the moment, a

handful of petroleum-rich countries have a choke hold

on the rest of the world. Countries can become independent by using their own nearby ocean

waters to supply a large part of their electricity, fresh water, fuel, and food.

Fishing - Cold water, drawn from the depths, is nutrient-rich and can significantly increase

fishing yields.

Fresh water - A potentially immense by-product of the power cycle, for drinking and

agriculture.

Abundant resource - 40 million square miles of Tropical Ocean suitable for OTEC close to

major populated countries. Ideal for southeast of US, southern tip of Baja California, Hawaii,

Caribbean, Central and South America, Africa, Middle East, India, Southeast Asia, China and

Taiwan, Philippines, Indonesia, Japan, and Australia.

Hurricane Resistance - Sea Solar Power plants are 95% submerged below wind and wave

action. Modern floating drill rigs have a higher profile, stay on station, and survive severe

storms.

Ocean Location - No effect on real estate market, no legal battles over plant sitting. No land

structures marring scenic vistas.

A Host of By-Products - A Sea Solar Power plant can also produce other energy intensive

products including: Hydrogen as fuel • Methanol as liquid fuel • Ammonia for fertilizer •

Industrial gases.

Many useful by-products can be obtained from the warm and cold water resource using a little or

all of the power produced in the plant. (See the diagram below.) Fish will benefit from the

nutrient rich cold water discharged from the plant beneath the surface layers. This will be like



fertilizing a field. Fresh water for human consumption and agriculture is an important by-product

and can be barged to shore. Hydrogen for fuel cells or direct fuel burning and/or methanol can be

produced for transportation fuels.

Fig. 7

Table 1 Potential Products at sample prices from 100MW Floating Plants.

Product(1) Output/yr Unit

Price Output/yr Metric Units

Unit Price

Metric Revenue/yr(1)

Electricity 788,400,000 kwhrs $--- 788,400,000 kwhrs $--- $---

Oxygen 219,000 tons $94.00 198,672 M.Tons $104 $20,586,000

Nitrogen 139,000 tons $82.00 126,098 M.Tons $90 $11,398,000

Ammonia 106,000 tons $170.00 96,161 M.Tons $187 $18,020,000

Hydrogen 17,000 tons $1,900.00 15,422 M.Tons $2,094 $32,300,000

Methanol 26,700,000 gals $1.90 101,059,500 liters $0.50 $50,730,000

Fresh water(2) 21,000,000,000 gals $0.003 79,485,000,000 liters $0.0008 $63,000,000

Fish 32,000,000 lbs. $1.00 14,514,880 kgs $2.20 $32,000,000

(1) Electricity only from first plants. Later plants, according to their design, will have a mix of

products.

(2) Fresh water plant only; much higher quantity



7. Why Alternative Energy Resources?

Energy Conservation and Increased Energy Efficiency:

Our modern global community is confronted with a multitude of challenging environmental

problems, and overwhelming consumption of fossil fuel is threatening man and earth's very

survival.

It is important to understand that present environmental conditions are the result of long-term

abuse and the process of solving problems to return to a sound and healthy environment is going

to be difficult. We are currently on a collision course, finding ourselves in a situation where too

many people are demanding too many goods and services. As the world's population expands, so

does industrial production, creating a higher standard of living for more of our citizens who will

then consume even more fossil fuel. The result is escalating damage to the global environment as

our precious finite natural resources are depleted.

As individuals we often find that too much of a good thing turns into a bad habit. The same could

be said about our thirst for, and our addiction to, fossil fuel. But, let's not be too harsh on the oil

producers and those visionaries who designed machines and gadgets that have enhanced our

comfort and standard of living. It has only been in recent years that scientists have been able to

identify environmental penalties associated with progress. Furthermore, heated debates continue

on the extent to which our current situation is man-made or natural. But the recent scientific

findings have created a wedge between the progressive industrialist seeking more goods at lower

cost, and those who demand a clean and safe environment.

Scientific evidence tends to side with the environmentalists-the facts are on their side and

strongly support the critical need to change the way we live. The wisest response is to select a

new course of action that allows economic expansion now--without harm to the environment in

the future. And beyond fossil fuels, new alarms are being sounded about the depletion of basic

resources such as the world's supply of fresh water -critical to support life and improve standards

of living --and the supply of fish--one of the global population's major sources of protein. Some

marine species face extinction because of over fishing while major underground water tables are

in steady decline around the world.

During the next 20 years, experts foresee a need for 1500 gigawatts of additional power simply

to meet new demand. This equates to 15,000 power plants that are 100 MW's each and 59

million barrels of oil consumed each day. The World Bank estimates that the developing



countries alone will need to spend $100 billion each year for the next 30 years installing new

power plants most of which will be in the equatorial zone. These are astronomical figures that

could mean enormous quantities of fossil fuel and 2.2 billion tons of carbon dioxide released to

the atmosphere per year. Hence, an urgent needs to switch to alternative energy.

What is the answer? Do we build more efficient fossil fuel power plants? Do we retrofit existing

plants with energy recovery systems and incorporate greater use of co-generation? Do we design

more efficient household appliances and automobiles and should we not be concerned about why

do we not see a greater use of renewable?

Of course, we recognize the value of confronting all of these questions. Energy conservation and

increased energy efficiency should be aggressively implemented whenever economically

feasible. And yes, renewable energy should be encouraged, but traditionally renewable have not

been cost effective (the exception has been hydroelectric dams, but now there is concern about

building new dams because of the distorted use of rich agricultural land, the misuse of water and

the blockage of fish passages).

Renewable have not been cost competitive with conventional power generation because of the

availability factor. For example, wind only generates power when the wind blows. Solar

collectors only produce electricity when the sun shines and wave energy is not constant. So it is

the capacity factor that is so important and why renewable are not very economical or popular.

Fossil fuel plants have a high capacity factor.

What is needed is a technology that uses solar energy to generate electricity, produces fresh

water in the process, and operates 24 hours per day. We believe that by using advanced

technology a more efficient means of harnessing solar energy can be created, allowing the global

community to enjoy both its demand for progress and its respect for the environment. Such a

concept is moving from vision to reality.

Although there are many alternative sources of energy that show great promise, this site will

focus on ocean thermal energy (OTE) also called ocean thermal energy conversion (OTEC).

OTE takes advantage of the temperature difference between the solar heated surface water and

the deep cold bottom water, using the warm surface water as the heat source and the cold bottom

water as the heat sink. Ideal operating conditions are plentiful throughout the equatorial zone.

This is an economically efficient means to convert the solar energy in the upper layers of the



tropical oceans into low cost electricity, large volumes of desalinated water, and a variety of

other valuable by-products.

8. A Quick Look at Some Current Methods of Power Generation:

Oil and Natural Gas: Both in relatively short supply. New wells and extraction methods make costs rise for new supplies.

Coal: Less costly than oil but polluting and costly for waste cleanup. Mercury emissions enter the food chain and affect the health of unborn children. (Above three sources release carbon dioxide to the atmosphere, a major cause of global warming.)

Nuclear Power: Radioactive waste creates an environmental hazard for thousands of years. Serious malfunction of a nuclear plant or a terrorist attack could cause a disastrous meltdown. Hydro/Dams: Environmentally friendly but most available sites already utilized.

Wind Power: Environmentally friendly but resource is intermittent, requiring reliable backup power plants.

Solar Power: Environmentally friendly but needs large areas of unshaded land and only makes power when sun shines. Needs large storage systems for night loads and/or backup power plants on cloudy days and nights.

9. Ocean Thermal Energy Converters:

The power involved is enormous. For example, the Gulf Stream has a flow rate of 2.2 × 1012 m3

day-1 of water, some 20 K warmer than the abyssal layers. A heat engine that uses this much

water and that employs as a heat sink the cool ocean bottom would be handling a heat flow of

VTc , where T is the temperature difference, c is the heat capacity of water (about 4 MJ m-3

K-1) and V is the flow rate. This amount to 1.8×1020 J day-1 or 2100 TW. The whole world uses

energy at the rate of only ≈ 8 TW. These order of magnitude calculations are excessively

optimistic in the sense that only a minuscule fraction of this available energy can be practically

harnessed. Nevertheless, ocean thermal energy holds some promise as an auxiliary source of

energy for use by humankind.

Figure 8 shows a typical temperature profile of a tropical ocean. For the first 50 m or so near the

surface, turbulence maintains the temperature uniform at some 25 C. It then falls rapidly

reaching 4 or 5oC in deep places. Actual profiles vary from place to place and also with the

seasons.



Fig. 8 Typical ocean temperature profile in the tropics

It is easier to find warm surface water than sufficiently cool abyssal waters which are not readily

available in continental shelf regions. This limits the possible sittings of ocean thermal energy

converters.

10. OTEC Configurations:

Two basic configurations have been proposed for OTECs:

1. Those using hydraulic turbines, and

2. Those using vapor turbines.

The first uses the temperature difference between the surface and bottom waters to create a

hydraulic head that drives a conventional water turbine. The advantages of this proposal include

the absence of heat exchangers.

Consider a hemispherical canister as depicted in the left hand side of Figure 9. A long pipe

admits cold water, while a short one admits warm water. The canister is evacuated so that, in the

ideal case, only low-pressure water vapor occupies the volume above the liquid surface. In

practice, gases dissolved in the ocean would also share this volume and must be removed.

This configuration was proposed by Beck (1978).

At a temperature of 15oC, the pressure inside the canister is about 15 kPa (0.017 atmospheres).

At this pressure, warm water at 25oC will boil and the resulting vapor will condense on the parts

of the dome refrigerated by the cold water. The condensate runs off into the ocean, establishing a

continuous flow of warm water into the canister. The incoming warm water drives a turbine from



which useful power can be extracted. The equivalent hydraulic head is small and turbines of

large dimensions would be required.

Fig. 9 Hydraulic OTECs

To increase the hydraulic head, Zener and Fetkovich (1975) proposed the arrangement of Figure

9 (right). The warm surface water admitted to the partially evacuated dome starts boiling. The

resulting vapor condenses on a funnel-like surface that seals one of the two concentric cylinders

in the center of the dome. This cylinder receives cold water pumped from the ocean depths,

which chills the steam-condensing surface. The collected condensed water subsequently flows

into the central pipe creating a head that drives the turbine. The efficiency of the device is

substantially enhanced by the foaming that aids in raising the liquid.

OTECs developed in the 1980s were of the vapor turbine type. They can use open cycles

(Figures 10A and B), close cycles (Figure 10C) or hybrid cycles (Figure 10D). The open cycle

avoids heat exchangers (or, if fresh water is desired, it requires only a single heat exchanger).

However, the low pressure of the steam generated demands very large diameter turbines. This

difficulty is overcome by using a close (or a hybrid) cycle with ammonia as a working fluid.

Most work has been done on the close-cycle configuration, which is regarded as more

economical. However, the costs of the two versions may turn out to be of comparable.



Fig. 10 OTEC configurations include the open-cycle type without distilled water production (A), the open-cycle type with distilled water recovery (B), the close-cycle (C), and the hybrid-cycle (D).

10.1 Closed cycle:

In the closed-cycle systems, a low boiling-point fluid (e.g., ammonia, CFCs HCFCs or low-MW

hydrocarbons) is used as a working fluid in the heat engine (power cycle). A working fluid

receives heat from the water at the ocean’s surface (the temperature can be as high as 20–25o C)

and then boils to become vapour. The resulting vapour is then used to drive a turbine, which, in

turn, leads to the production of electricity. When the vapour has passed through the turbine, it

becomes saturated liquid (or the mixture of saturated liquid and vapour). This saturated liquid (or

the liquid vapour mixture) is condensed to liquid (normally saturated liquid), which is then

pumped back to the heat exchanger to restart the cycle. Most of the OTEC systems used

worldwide are closed-cycle.



Fig. 11

10.2 Open Cycle:

Open-cycle systems utilize the warm water at the ocean’s surface to produce electricity. When

this warm water is contained in a low-P container, it boils, and the resulting vapour is used to

drive a turbine. To enhance the efficiency of the open cycle OTEC power plants, this system is

incorporated with a solar thermal system, with the main purpose of increasing the sea-water’s

temperature. Sea-water is passed through a solar panel, to pick up heat, before entering the low-P

chamber. Open-cycle OTEC systems can produce electricity with the output capacity of ~40–50

kW.

Fig. 12

10.3 Hybrid:

A hybrid system combines the features of both closed and open-cycle systems. The system is

essentially a closed-loop system with a low-boiling-temperature fluid, but the seawater, which is



used to vaporize the working fluid, is passed through a vacuum or low-P chamber before

entering the heat exchanger (to provide to boil the working fluid) – the main objective is to

increase the heat transfer efficiency between the seawater vapour and the working fluid.

Fig. 13

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Lecture Notes ENGG4000 Lecture 15