Chapter 13

Ocean Thermal Energy Conversion

Ocean thermal energy conversion (OTEC) is a method to produce electricity by using the temperature differences between warm ocean surface and cool deep ocean water to run a heat engine. If temperature difference is greater, then more energy will be produced. About 70% of the earth’s surface is covered by oceans, which are continuously heated by the sun. Extracting the solar energy stored in an ocean is carried out by exploiting the temperature difference between warm surface water and cold deep sea water.

Low-grade heat from renewable energy sources is considered to be a good candidate to generate electricity. Among those sources, OTEC and solar energy are typically utilized in converting low-grade heat into power generation and other applications.

OTEC systems use the ocean’s natural thermal gradient to drive a power-producing cycle. As long as the temperature difference between warm surface water and cold deep sea water is greater than about 20°C, an OTEC system can produce a significant amount of power. Suitable locations for OTEC systems in the world have been identified. It was found that natural ocean thermal gradients necessary for OTEC operation generally exist between latitudes 200°N and 200°S. OTEC can, therefore, be sited anywhere across about 60 million square kilometres of tropical oceans anywhere there is deep cold water lying under warm surface water. This generally means between the Tropic of Cancer and the Tropic of Capricorn. Surface water in these regions, warmed by the sun, generally stays at 25°C or above. Ocean water more than 1,000 metres below the surface is generally at about 5°C.

It would not be profitable to use an OTEC power plant in the Baltic Sea, because the average temperature is about 8°C–10°C.

13.1 INTRODUCTION

Low-temperature heat obtained from renewable energy resources, such as solar thermal, geothermal, ocean thermal, etc. is presently converted into electricity and utilized for direct heating applications. About 70% of earth’s surface is covered by ocean which is continuously heated by solar heat. Solar heat is stored as uneven distribution of heat between warm surface water and cold deep ocean water (called gradient) from where it is harnessed as ocean thermal energy.

OTEC sites that are located between the Tropic of Cancer and Tropic of Capricorn (23.5°N and 23.5°S of equator) found to be best locations. Ocean water with temperature gradient of 5°C and more is known as ocean thermal energy.

However, significant amount of electric power can be generated in the location where a temperature difference of 20°C and above exists between warm surface water and cold deep water.

In many regions, ocean surface water is generally maintained at 25°C or above and more than 1,000 metres below the surface is generally at about 5°C. Since average temperature in Baltic Sea is about 10°C, setting up of OTEC electrical power plant is not profitable.

Therefore, OTEC is an energy technology that converts solar radiation to electric power through heat of ocean water. These systems use ocean’s natural thermal gradient. As long as the temperature difference between the warm surface water and the cold deep water below 600 metres by about 20°C, an OTEC system can produce a significant amount of power. Thus, oceans are vast renewable resources with the potential to produce thousands of kW of electric power.

The cold deep sea water used in the OTEC system is also rich in nutrients, and it can be used to cultivate plant and marine organism near the shore or on land.

13.2 PRINCIPLE OF OCEAN THERMAL ENERGY CONVERSION

The basic principle of ocean thermal energy conversion (OTEC) is explained as follows:

The warm water from the ocean surface is collected and pumped through the heat exchanger to heat and vapourize a working fluid, and it develops pressure in a secondary cycle. Then, the vapourized working fluid expands through a heat engine (similar to a turbine) coupled to an electric generator that generates electrical power. Working fluid vapour coming out of heat engine is condensed back into liquid by a condenser. Cold deep ocean water is pumped through condenser where the vapour is cooled and returns to liquid state. The liquid (working fluid) is pumped again through heat exchanger and cycle repeats. It is known as closed-cycle OTEC.

If ocean surface water is high, enough propane or similar material is used as working fluid; otherwise, for low-temperature surface water, fluid such as ammonia with low boiling point is used.

In an open-cycle OETC, warm ocean surface water is pumped into a low-pressure boiler to boil and produce steam. Then, the steam is used in steam turbine to drive an electrical generator for producing electrical power. The cold deep sea water is used in condenser to condense steam.

Some fractions of electrical power generated by OTEC plants are used for operating and controlling equipments involved in power plants, and high electrical power is used for feeding to several other energy consumers.

13.3 OCEAN THERMAL ENERGY CONVERSION PLANTS

There are two different kinds of OTEC power plants, namely land-based power plant and floating power plant.

13.3.1 Land-based Power Plant

The land-based power plant will consist of a building as shown in Figure 13.1.

Figure 13.1 Land-based OTEC power plant

It is constructed on shore and accommodates all parts of OTEC plants. It requires laying down long pipes from plant site on shore to two extreme points of necessary temperature gradient. One pipe is used to collect warm ocean surface water through screened enclosure near the shore. Another long pipe lay down on the slope deep into the ocean to collect cold water.

A third pipe is used as outlet to discharge used water again in ocean via marine culture ponds deep down the ocean. Cost of pipe installation and maintenance is very expensive, and land-based plant is also very expensive. Since large electricity is used to pump water through long pipes, the net electricity reduces considerably.

Land-based OTEC plant has the advantage of savings on electrical transmission line and connectivity to electrical power grid.

13.3.2 Floating Power Plant

Floating power plant is built on a ship platform exactly where required temperature gradient sufficient for OTEC plant is available. The working principle of ocean thermal energy conversion (OTEC) is same as that of land-based power plant. Undoubtedly, the cost savings exist on piping system, but long transmission line is required to transmit electrical power from plant to sea shore.

Owing to high installation cost of long underwater power cables and its inefficiency and many other associated problems, floating OTEC plants are considered for the production of fuels, such as hydrogen, on the platform itself by the electrolysis of water.

Figure 13.2a Submersible OTEC plant designed by lockheed

Figure 13.2b Floating OTEC power plant

Cold water pipe is the largest single item in the land-based plant design, as the slopes are seldom larger than 15° or more. If 1,000-metres-long vertical pipe with 10 to 15 m diameter used in floating plant, the length of land-based plant considering slope will be about three times.

13.4 BASIC RANKINE CYCLE AND ITS WORKING

The basic Rankine cycle shown in Figure 13.3 consists of the following:

1. An evaporator
2. A turbine expander
   

   Figure 13.3 OTEC Rankine cycle

3. A condenser
4. A pump
5. A working fluid

In open-cycle OTEC, warm sea water is used as working fluid, whereas in closed-cycle type, low-boiling point ammonia or propane is used.

Warm ocean surface water flows into the evaporator which is the high-temperature heat source. A fluid pump is utilized to force the fluid in a heat evaporator where liquid fluid vapourizes. Then, the vapour of boiling fluid enters the turbine expander coupled with an electrical generator to generate electrical power. The vapour released from the turbine enters into condenser where it condenses. The cold deep sea water is pumped through the condenser for heat rejection from vapour fluid and condenses it as liquid fluid. The liquid fluid is again pumped through evaporator and cycle repeats.

As temperature difference between high- and low-temperature ends is large enough, the cycle will continue to operate and generate power.

13.4.1 Selection of Working Fluids

The steam Rankine cycle and organic Rankine cycle are the two main types used in OTEC systems, and the choice of working fluids plays an important role in design and performance of OTEC. Water is the only working fluid for steam Rankine cycle, but a large number of working fluid is available for organic Rankine cycle. The working fluid has the following properties:

1. Chemical stability and compatibility: Certain organic fluids are more prone to decompose when subjected to high pressure and temperature which results in material corrosion of different parts of plants, explosion etc. Thus, working fluid should be chemically stable and compatible with materials and structures of OTEC plants.
2. Heat transfer coefficient: Low-thermal resistance of working fluids improves heat transfer.
3. Flash point: A working fluid with a high flash point should be used in order to reduce flammability.
4. Specific heat: A working fluid with a low specific heat should be used to reduce load on the condenser.
5. Latent heat: A working fluid with a high latent heat should be used in order to raise the efficiency of heat recovery.
6. Safety: Working fluid should be non-corrosive, non-toxic, and non-inflammable having maximum allowable concentration and explosion limit for safe and efficient operation of OTEC plants.
7. Environmental acceptability: Low-toxicity working fluid minimizes water contamination. The environmental risk of OTEC plant is low.
8. Cost and availability: The ease of availability and low cost of working fluid is also important.

13.5 CLOSED CYCLE, OPEN CYCLE, AND HYBRID CYCLE

There are three types of OTEC cycle designs, namely open cycle, closed cycle, and hybrid cycle.

1. In an open cycle, warm sea water is pumped into a flash evaporator as working fluid where it boils at low pressure and converts into steam. This steam expands through low-pressure turbine which drives an electrical generator and generates electricity. The steam released from turbine condensed in a condenser by deep sea cold water as non-saline water. When non-condensable gases are separated and exhausted, the non-saline water is either pumped in marine culture ponds for freshwater applications or finally discharged in sea surface water.
2. In closed cycle, organic fluid flows in a separate closed-cycle loop called organic Rankine cycle. Warm sea surface water pumped through another pipe vapourizes working fluid in heat exchangers to drive turbine generator, The fluid vapour condenses into liquid form by deep sea water pumped in condenser by a separate pumping system, The process of pumping liquid fluid in an evaporator cycle is repeated.
3. A hybrid cycle is a combination of both closed and open cycle.

13.5.1 Open-cycle OTEC

An open-cycle OTEC uses the warm ocean surface water as working fluid. It is a non-toxic and environment friendly fluid. The major components of this system are shown in Figure 13.4. It consists of evaporator, low-pressure turbine coupled with electrical generator, condenser, marine culture ponds, non-condensable gas exhaust, and pumps. Evaporator used in an open-cycle system is a flash evaporator in which warm sea water instantly boils or flash in the chamber that has reduced pressure than atmosphere or vacuum. It results in reduced vapourization pressure of warm sea water. A large turbine is required to accommodate large volumetric flow rates of low-pressure steam, which is needed to generate electrical power, and is used with other plant components in a similar manner. During vapourization process in an evaporator, oxygen, nitrogen, and carbon dioxide dissolved in sea water are separated and are non-condensable. They are exhausted by non-condensable gas exhaust system. Condenser is used to condense vapour or steam released from steam turbine is condensed by cold deep sea water and returned back to sea. If a surface condenser is used, condensed steam (desalinated water) remains separated from cold sea water and is pumped into marine culture ponds. To avoid leakage of air in atmosphere and to prevent abnormal operation of plants, perfect sealing of all components and piping systems is essential. The working principles of open-cycle OTEC plants are explained as follows with the help of Figure 13.4.

Figure 13.4 Open-cycle OTEC

1. The warm ocean surface water is pumped into flash evaporator where it is partially flashed into steam at a very low pressure. The remaining warm sea water is discharged into the sea.
2. The low-pressure vapour (steam) expands in turbine to drive a coupled electrical generator to produce electricity. A portion of electricity generated is consumed in plants to run pumps and for other work, and the remaining large amount of electricity is stored as net electrical power.
3. The steam with many gases (such as oxygen, nitrogen, and carbon dioxide) released from the turbine separated from sea water in an evaporator is pumped into condenser. The steam is cooled in a condenser by cold deep sea water.
4. The condensed non-saline water is discharged either directly in deep sea cold water or through the marine culture pond.
5. The non-condensable gases are compressed to pressure and exhausted simultaneously.
6. The warm ocean surface water is continuously pumped into evaporator and cycle repeats.

13.5.2 Closed-cycle OTEC

The schematic of closed-cycle OTEC is shown in Figure 13.5. It has different arrangement when compared to open-cycle OTEC. Organic fluid with low boiling point is used as working fluid. Ammonia liquid is the most widely used working fluid. Working fluid flows in a closed loop and perfectly sealed piping system. Working fluid circulates around the loop continuously. Warm ocean surface water flows through completely separate piping system and discharges in upper surface of ocean. Warm surface sea water and working fluid piping are placed very closely to each other in a heat exchanger to transfer warm sea water heat into working fluid. The cold deep sea water piping system is in contact with working fluid piping system in a condenser where working fluid condenses to its liquid state. Other components of both open- and closed-cycle OTECs are similar. Working principles of closed-cycle OTEC are as follows:

1. Working fluid is pumped through heat exchangers in a closed loop cycle which is perfectly leakage proof.
2. Warm sea surface water is pumped through separate pipe in heat exchanger in close contact with fluid closed loop cycle
3. Warm sea water transfer its heat energy to working fluid in heat exchanger and working fluid vapourizes.
4. The fluid vapour makes the turbine to rotate and drive an electrical generator to produce electricity.
5. Fluid vapour leaving the turbine is cooled and condensed as liquid fluid and is pumped again to repeat cycle.
6. Cold deep sea water is pumped through a separate pipe in condenser for providing efficient cooling of working fluid.

Figure 13.5 Closed-cycle OTEC plant

13.5.3 OTEC Hybrid Cycle

As shown in Figure 13.6, a hybrid cycle combines the features of both closed-cycle and open-cycle systems. Warm sea water is pumped into a vacuum chamber where it is used to flash and produces steam. Working fluid in another closed cycle loop is evaporated and vapourized by steam in vacuum chamber. The fluid vapour rotates the turbine and drive an electric generator to produce electricity.

Figure 13.6 OTEC hybrid cycle

13.6 CARNOT CYCLE

The Carnot cycle is the most efficient thermodynamical cycle by exploiting the warm sea surface water and cold deep sea water.

Figure 13.7 Carnot efficiency P–V diagram

Let W be the work done by the system (energy exiting the system as work), Q_H be the heat put into the system (heat energy entering the system),

T_C be the absolute temperature of the sea surface and

T_H be the absolute temperature of the deep sea water hot reservoir.

Carnot efficiency (η) is given by the following equation:

(13.1)

13.7 APPLICATION OF OTEC IN ADDITION TO PRODUCE ELECTRICITY

OTEC schematic diagram and applications are shown in Figure 13.8. Ocean thermal converting plants provide several products for use by mankind. These are explained as follows:

Figure 13.8 OTEC plant and applications

1. Electricity: Electrical energy is the primary product of OTEC plants. Laying down long transmission and distribution cables up to the sea shore for domestic and industrial applications is not practical from economic view point. OTEC plants are, therefore, considered for other products and applications.
2. Hydrogen production: Electricity produced from OTEC plants is used for separating water in hydrogen and oxygen by the method of electrolysis of water. Hydrogen is considered as the second best usable form of energy after electricity. Use of deep sea cold water and OTEC electricity for hydrogen production signifies the important applications of OTEC plants.
3. Ammonia and methanol production: OTEC electricity can be used to obtain by-products, such as ammonia and methanol, that can be transported either by tankers or through pipe lines to on shore applications
4. Desalinated water: Desalinated water is produced in an open-cycle and hybrid-type OTEC plants through surface condenser. It is freshwater and widely used as water resource for drinking, agriculture, and industry.
5. Aquaculture: Nutrient-rich cold deep sea water provides sufficient environment for fish farming which may create a profitable business activities.
6. Chilled soil agriculture: Chilled soil agriculture is another application of OTEC plants. Cold deep sea water flowing through underground pipes chills the surrounding soil. The temperature difference is maintained between plant roots in the cool soil and plant leaves in the warm air, and thus, the tree and plants grows. The amount of food that can be produced in this way is very large, larger in market value than the electric power produced by the plant.
7. Air conditioning: Because the temperature is only a few degrees, cold water can be used as a fluid in air condition systems.

13.8 ADVANTAGES, DISADVANTAGES AND BENEFITS OF OTEC

13.8.1 Advantages

1. Ocean thermal energy is a renewable, clean natural resource available in abundance.
2. It is pollution-free and has no greenhouse effects.
3. It is a good source of freshwater and portable water.

13.8.2 Disadvantages

1. High cost: Electricity generated by OTEC plants is more expensive than electricity produced by chemical and nuclear fuels.
2. Complexity: OTEC plants must be located where a difference of about 20°C occurs year round. Ocean depths must be available fairly close to shore-based facilities for economic operation. Floating plant ships could provide more flexibility.
3. Acceptability: For the large-scale production of electricity and other products, OTEC plants are poorly acceptable due to their high costs.
4. Ecosystem damage: It is obvious by setting OTEC plants.
5. Lower efficiency: A higher temperature difference between ocean surface warm water and cold deep ocean water is required for highly efficient operation of plant.

13.8.3 Benefits as a Measure of the Value of OTEC

Economic and other benefits are the value of OTEC plants. These include the following:

1. It is a clean, renewable natural resource available in plenty.
2. It has no environmental problems and greenhouse effects.
3. It is a source of base load electricity and fuels such as hydrogen, methanol, and ammonia.
4. It provides freshwater for drinking, agriculture, and industry.
5. It encourages chilled agriculture and aquaculture.
6. Self-sufficiency, no environmental effects, and improved sanitation and nutrition are the added benefits for island.

SUMMARY

• Basic thinking behind ocean thermal energy conversion (OTEC) was first suggested in 1881 by a French physicist Jacques d’Arsonval which involves extracting useful energy from the solar heat stored in the oceans.
• Ocean thermal energy conversion is a potential source of renewable energy that creates no emissions.
• It uses the ocean’s temperature difference between warm surface water and cold deep sea water to generate both electricity and potable water. (See more at: http://www.otecorporation.com/ocean-thermal-energy.)
• OTEC is an energy technology that converts solar radiation to electric power. These systems use the ocean’s natural thermal gradient. As long as the temperature difference between warm surface water and cold deep sea water below 600 meters by about 20°C, an OTEC system can produce a significant amount of power.
• The cold sea water used in the OTEC process is also rich in nutrients, and it can be used to cultivate both marine organisms and plant life near the shore or on land.
• Sea water air conditioning (SWAC) is a clean method of air conditioning buildings, using cold deep sea water in place of polluting standard refrigerants.
• The main advantages of OTEC are that the method is fuel-free, has a low environmental impact, can supply pure water for both drinking and agriculture, can supply refrigeration and cooling, and can provide a coastal community with reliable energy. The disadvantages include high capital cost, potential for hostile ocean environment during construction and use, and an overall lack of familiarity with OTEC technology.
• There have been several analyses of the feasibility of full-scale implementation of OTEC. While some of these investigations are contradictory to each other, research with actual mini OTEC plants is proving that OTEC systems will one day become a feasible, efficient, and renewable source of energy.
• It is very expensive.

REVIEW QUESTIONS

1. Write a short note on ‘ocean thermal energy’.
2. Explain the difference between fixed dome-type and floating dome-type biogas plant.
3. What is the basic principle of OTEC?
4. What are the main types of OTEC power plants? Describe their working principle in brief.
5. Describe the ‘closed-cycle’ OTEC systems. Write its advantages over ‘open-cycle systems’.
6. Explain Carnot efficiency for an OTEC plant with the help of a thermodynamic cycle on a T–S plane.
7. What is the limitation of open-cycle OTEC systems?
8. State the merits and demerits of OTEC plants.
9. State the expression for energy and power in ocean waves.
10. Explain how the ocean temperature differences can be used to generate electrical power.
11. Discuss in detail OTEC systems based on (i) open cycle (ii) closed cycle.