Ocean Thermal Energy Conversion: The Promise of a Clean Future

A. Hossain1, A. Azhim1,2, A. B. Jaafar2,3, M. N. Musa2 , S. A. Zaki1,2 & D. Noor Fazreen1,4

1 Malaysia Japan International Institute of Technology, Universiti Teknologi Malaysia (UTM) 2Ocean Thermal Energy Center, UTM

3Perdana School of Science, Technology and Innovation Policy, UTM 4Faculty of Sciences and Biotechnology, Universiti Selangor, UNISEL

Abstracts Due to the world’s heavy dependence on fossil fuels for electricity, pollution and global warming is on the rise. However, numerous countries are still relying on diesel generators as their main source of energy. There is a lack of practical alternative energy source that can meet the global energy demand without posing any threat to the natural environment. Ocean Thermal Energy Conversion (OTEC) is a concept that has the potential to address this growing issue. It is basically a mechanism that exploits the temperature difference between warm surface seawater and cold deep ocean water, to produce electricity. Although OTEC has a low energy density, the thermal energy in the ocean is vastly abundant. OTEC development has been dormant for a long time since it was first proposed in 1881. However, it has now regained recognition worldwide as a realistic solution to our world energy issue. Instead of having a great potential for power generation, it also carries the ability to produce high value products from the large volume of Deep Sea Water (DSW) that can be released as by- products of OTEC plant operation. These products can hold a very profitable place in industries such as pharmaceutical, food, cosmetics and mineral water production. Further research on ocean current and DSW properties can someday lead to the commercial used of OTEC. This paper is a review of the basic concept, present status and future prospects of OTEC around the world.

Keywords — Ocean Thermal Energy Conversion; Renewable

Energy; Clean Energy; Deep Sea Water

I. INTRODUCTION

Considering the growing world population and environmental problems, it is obvious that in this 21st century the conventional resources of energy such as oil, coal and uranium become unreliable. The obvious alternative energy sources such as wind, solar and geothermal power are considerable solutions to this problem. However in comparison to all these alternatives, ocean thermal energy is highly abundant, very stable and easily applicable in many industrial fields. Ocean Thermal Energy Conversion (OTEC) is a marine renewable energy technology util izing the temperature difference between deep cold ocean water and warm ocean surface water and generates electricity. Fig. 1 ill ustrates the global primary sources of energy in perspectives [1]. It is clear from Fig. 1, OTEC alone can meet the world energy demand, as observed from the world energy used in the year 2010.

According to L.A. Vega (2003), the amount of solar energy absorbed by the oceans in a year is equivalent to at least 4000 times the amount currently consumed on earth. For an OTEC efficiency of 3%, in converting ocean thermal energy to electricity, we would need less than 1% of this renewable energy to satisfy the world demand [2].

Fig. 1. Global Primary Source of Energy in Perspectives

Furthermore, OTEC makes it possible not only to produce

electricity but allows the release of massive amount of Deep Sea Water (DSW) which is rich in minerals and is highly applicable in several industries including pharmaceuticals, aquaculture (mariculture), cosmetics and mineral water production.

II . HOW OTEC WORKS

Sunlight can be absorbed by the surface ocean water and can only penetrate up to 100 meters water depth. The sunlight cannot reach the deep sea water level. Fig. 2 illustrates the temperature profile along the cross section in the Pacific [4]. The water in the lower half of all oceans is uniformly cold and OTEC plants take advantage of this feature. Typically, a depth of 600 to 1000 meters is used to generate electricity by an OTEC plant.

The principle of a basic OTEC plant is shown in Fig. 3. The main components are evaporator, condenser, turbine, power generator and pump. These components are connected via pipes that contain working fluids, typically ammonia [3]. The liquid working fluid is sent to the evaporator with a pump which is heated by the hot surface water of 25 to 30°C, and evaporated to vapour. The vapour then turns the turbine and activates the power generator, thereby generating electricity. The used vapour leaving the turbine is then condensed to li quid by the cold deep seawater of 4 to 10°C inside the condenser, and then recycled back into the evaporator. The process is thus repeated in order to maintain continuous electricity production. This is basically how a typical closed-cycle OTEC system works.

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[Source: Sakaguchi A., et. al., 2012]

Fig. 2. Temperature profile of ocean water

[Source: Ikegami, et. al., 2010]

Fig.3. A basic OTEC system

In an open-cycle OTEC system, the warm seawater is used as the working fluid. The warm seawater is “flash” evaporated in a vacuum chamber and steam is produced. The steam expands through a low-pressure turbine that is coupled to a generator to produce electricity. The steam leaving the turbine is then condensed by cold deep seawater through a cold water pipe. If a surface condenser is used in the system, the condensed steam remains separated from the cold seawater and provides a supply of desalinated water. A schematic diagram [5] of the cycle is shown in Fig. 4.

A hybrid-cycle OTEC system combines the features of both the closed-cycle and open-cycle systems. In this system, warm seawater enters a vacuum chamber where it is evaporated into steam, which is similar to the open-cycle evaporation process. The steam vaporizes the working fluid of a closed-cycle loop on the other side of an ammonia vaporizer. The vaporized fluid then drives a turbine to generate electricity. The steam condenses within the heat exchanger and provides desalinated water.

[Source: Ghosh, et. al., 2011]

Fig. 4. A schematic diagram of an open-cycle OTEC system

III . HISTORY AND PRESENT STATUS OF OTEC

The concept of OTEC for electricity generation was invented by a French physicist Mr. Jaques D‟Arsoval in 1881. Studies on OTEC application has been conducted in100 years. In the past, the application of OTEC was said to be impractical as it only can generates small amount of electricity but the improvement in the technologies, all experts now considered that to be untrue [6]. Table 1 summarizes the development of OTEC over the years in different parts of the world.

A. The Study in Saga University, Japan

OTEC study in Japan began in 1974 with the Sunshine Project by the Japanese government [7]. The primary aim of this project was to study and establish an OTEC system by investigating methods to increase the efficiency of OTEC plant. In 1977, Saga University successfully constructed 1 kW OTEC plant. In 1980, a 50 kW offshore OTEC plant was established where many experiments were performed. The following year, Tokyo Electric Co. successfully experimented with an OTEC system in the Republic of Nauru, producing up to 120 kW of electricity [7].

In 1981, a new method for utilising the temperature differences in the ocean to produce power was proposed and known as Kalina cycle.[15]. Up until then, the primary focus of study had been on the well- known Rankine cycle. The Kalina cycle was able to use a mixture of ammonia and water to operate, which gave it an advantage over the Rankine cycle that requires a pure substance (such as ammonia) [8]. In 1982, Kyushu Electric Co. also of Japan succeeded in constructing a 50 kW OTEC plant. This plant was based on a closed loop cycle that util izes the waste heat from a diesel generator. Later in 1994, Uehara and his colleague developed a new system using shell- and-plate type evaporator and condenser. This was later known to be the Uehara cycle and experiments which this cycle has shown high efficiency [9].

B. Experimental Study in India

In 1993, National Institute of Ocean Technology (NIOT) was formed by the Department of Ocean Development (DOD). In early 1977, a 1 MW gross OTEC plant was established by NIOT in Chennai, which was the first ever MW range plant established anywhere in the world. NIOT has been exploring the participation of national and international expertise for a joint research and development since then. This OTEC plant was mainly used for desalination project for fresh water.

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Year Activity 1881 D'Arsonval (France) conceived of OTEC 1926 Claude (France) began research for commercial use 1933 Claude built power generating ship (1200KW) 1964 Anderson's proposal for a power generation in the sea 1970 OTEC research results examined by the board of

investigation into new power generation methods (Japan) 1973 Saga University, Japan, commenced research study on

OTEC technology-power generation 1974 OTEC research commenced as part of Sunshine project

plan (Japan) 1974 ERDA project (USA) commenced 1974 First international OTEC conference (USA) 1977 Saga University succeeded with 1kw of power 1979 Mini-OTEC (USA) succeeded with 50KW of power

1980 Saga University performed experiments on the sea, off Shimane in Japan Sea

1981 Tokyo Electric Power Co., succeeded with 120KW of power on Nauru

1982 Kyushu Electric Power Co. succeeded with 50KW of power at Tokunoshima, off Kagoshima, Japan

1985 Saga University completed 75KW of power plant 1988 Inauguration of Organization of OTEC Study (Japan)

1989

Agency of Science and Technology (Japan) began study of utili zation of Deep Sea Water (DSW) off Toyama in Japan Sea

1990 IOA (international OTEC Association) was organized by Taiwan, USA and Japan)

1993 210KW open cycle system completed in Hawaii 1994 Saga University constructed a new cycle plant

1995 Saga University started on testing new 4.5KW cycle plant (Kalina cycle, Uehara cycle)

1997

Signing of collaboration memorandum with National Institute of Ocean Technology (NIOT), India, on OTEC study

2003 Saga University Completed 30KW multipurpose OTEC Plant in Imari, Saga, Japan

2005 OPOTEC (Organization for the Promotion of the Ocean Thermal Energy Conversion) established in Saga, Japan

2013

A 1.25 MW OTEC power plant was built in Japan‟s Kumejima Island, which supplies 10% of the island‟s total electricity consumption

TABLE I. OTEC THROUGH THE YEARS

[Source: adapted from Noda, et. al., 2002]

C. Other Regions

Other than Japan and India, new projects in the USA, Taiwan, South Pacific Islands and Cuba are currently in advance. In the USA, after the experimental tests of 200 kW open cycle system, together with low cost of crude oil, the study of OTEC has been inactive for nearly 15 years. However in 2008, Lockheed Martin established a 10,000 kW OTEC in Hawaii.

IV. FUTURE PROSPECTS A. Site Selection Criteria for OTEC Plant

The most important physical criterion for OTEC site selection is the accessibil ity of deep cold seawater [10]. For an OTEC plant to generate a significant amount of power, the temperature difference between the surface and deep ocean water must be at least 20°C. Therefore a suitable site for OTEC plant operation must have an ocean depth of at least 700 meters or more. Fig. 5 is a graphical representation of the distribution of the temperature difference between surface seawater and the ocean floor around the globe [11]. The darker areas within the ocean represent a bigger temperature difference than others.

[Source: Vega, 2010]

Fig. 5. OTEC Potential in the Tropics and Subtropics

B. Potential for OTEC Energy

It is important to calculate how much energy can be exploited through OTEC, but at present no firm estimate can be given. In 2010, Vega listed 99 main lands and islands which have OTEC potential within 200 nautical miles from the coast. Among these locations, 38 are within the Americas, 23 within Africa and 38 within the Indian/Pacific Ocean.

C. Multi -Industrial application of DSW

Aside from supplying electricity, OTEC is also capable of extracting very large volumes of DSW for its operation. DSW is referred to ocean water from a depth of 200 meters or below sea level and accounts for 95% of all seawater. It has cold temperature, is abundant in minerals and is pathogen free and stable. DSW circulated the world in duration of 2000 years, and the upwelling of DSW occurs regularly in the oceans and seas throughout the world [12]. It has a lucrative market for many industries and below is an explanation of some of its applications.

1) Air conditioning After the utilization of DSW in the OTEC plant, the

temperature of the water is still low and cold. Therefore it can be used as chilli ng source for air conditioning or in nearby greenhouses. Such air conditioning system provides a better energy saver properties compared to the ordinary electrical refrigeration methods.

2) Mineral Water Production The mineral concentration in DSW is high and is known to

possess many medicinal properties. Recent research has also shown anti- obesity and anti-diabetic effects of DSW in mice [13]. Therefore it is possible to produce high quality mineral water as a by-product of the OTEC plant.

3) Aquaculture Due to its nutritional value, DSW can be used effectively for

aquaculture to increase the growth rate of the culture and decrease the disease outbreak.

4) Lithium Extraction One very common method of industrial lithium production is the

extraction of lit hium-chloride from seawater. Since DSW is much purer and cleaner than surface seawater, it can be economically more suitable for lithium extraction by reducing cleaning intervals.

5) Food, Cosmetics and Pharmaceuticals The nutritional properties of DSW also make it a valuable source

for the food, cosmetics and pharmaceutical industries. In Japan, DSW is used in the production of „Sake‟, „Tofu‟, etc. Some cosmetic products based on DSW has also reached the Japanese market and gained tremendous public favour [14].

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V. CHALLENGES

The main challenge of developing a commercial-scale land- based OTEC plant is the capital cost. Studies show that OTEC plants smaller than 50 MW cannot compete economically with other present energy alternatives [15]. Although the initial construction cost of OTEC plant can be a big challenge, the profit generated from the additional products that OTEC plants can produce, can more than make up for this cost. The expected output of the main by-products of OTEC is shown in Table II [14].

TABLE II . EXPECTED OUTPUT OF BY-PRODUCTS

Gross Power Output (MW) 1 10 Net Power Output (MW) 0.7 7.5 Net Electricity (MWh/year) 4,900 52,500 Up-well ing DSW (t/h) 4,700 43,300 Fresh Water (t/h) 1,100 10,000 Hydrogen (Nm3/h) 2,000 22,000 Lithium Chloride (kg/day) 30 260 Mineral Water (bottle/day) 16,000 150,000

VI. CONCLUSION

OTEC not only has the potential to satisfy the global demand for energy but can one day become a power solution to three other greatest global issues of clean energy, fresh water as well as food. It holds the promise of fuelling our future.

ACKNOWLEDGMENT

This research is funded partly by UTM Research University Grant (GUP), QJ130000.2424.00G62

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[14] Kobayashi, H. Jitsuhara, S and Uehara, H. The present status and features of OTEC and recent aspects of thermal energy conversion technologies (2004) p. 1-8

[15] Kalina, A., Generation of electricity by means of a working fluid, and regeneration of working fluid. US Patent no. 4,346,561. Aug 31st, (1982)

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