OCEAN THERMAL ENERGY CONVERSION-THE WORLD by sparkunder14

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									Chinese-German Joint Symposium on Hydraulic and Ocean Engineering, August 24-30, 2008, Darmstadt

OCEAN THERMAL ENERGY CONVERSION-THE MOST IMPORTANT RENEWABLE ENERGY IN THE WORLD
Nai-Kuang Liang
Chinese Ocean & Undersea Technology Association, Taipei I. INTRODUCTION Due to the overuse of fossil fuel, global warming becomes a world wide issue. We, human-beings, have sensed this serious problem and Kyoto Protocol, which constrains the CO2 output for all nations, soon became in effect. This means that the consumption of crude oil and coal should be controlled. Then the renewable energy will be paid more attention and be developed and becomes a part of our energy consumption. The ocean occupies about 70% of earth’s area. The renewable ocean energy is abundant and includes current, tide, wave and thermal. The ocean renewable energy has three types, i.e. kinetic, potential and thermal, in which the kinetic energy is the smallest, the potential energy in the second place and the thermal energy is the biggest. For a 1 m/s current speed, the seawater has only 5 cm velocity head. But one gram water releases 1 cal. heat as it decreases1°C. This heat can raise itself 420 meters height. Everyone has the experience that hiking is much exhausted than walking on the ground. And an air conditioner consumes much more electricity than a fan. The ocean current belongs to kinetic energy, the tide potential energy, the wave has both kinetic and potential energy and the ocean thermal energy conversion (OTEC) of course belongs to thermal energy. In the following, a brief description on the electricity generation principle, the technical status, shortcomings & merits and the potential of the above-mentioned ocean energy candidates are introduced. II. OCEAN ENERGY
B. Tidal power Tidal power is like a hydraulic power station. There is already a commercial tidal power plant in France. There are still many potential sites for tidal power in the world. C. Wave power There are many wave power patents but still no commercial wave power plant in the world. One of the best conceptual design is to use air as the energy transfer and accumulation medium. The main shortcoming is that the wave is too unsteady. Especially for the place where typhoons or hurricanes take place, while the wave is sometimes too large so that the wave energy absorber is very much likely to be destroyed.

Figure 1. wave power navigation buoy

A. Current power The ocean current power generation principle is similar to the wind power. It just uses the flowing seawater to drive a turbine to generate electricity. The shortcomings are the low energy density, the high construction and maintenance cost as water depth is large. Comparing with wave the merits are that the ocean current is relatively stable and the electricity generation technology is simple. Because the electricity should be transmitted to shore, the suitable sites are limited. Hence, the ocean current power has little potential.

A quasi-commercial wave power navigation buoy is shown in Fig. 1. As the buoy heaves in the sea, the air in a chamber is sucked or pressed through a wind turbine and electricity is generated.
D. Ocean Thermal Energy Conversion (OTEC) In 1881, a French scientist D’Arsonval proposed a concept as shown in Fig.2. A working fluid, e.g. ammonia, is put into a closed loop. The warm surface seawater flows through an evaporator to make the ammonia liquid evaporate by a heat exchanger. The ammonia steam is then condensed into liquid by the cold deep seawater through a condenser. A steam turbine is put in the loop between the evaporator and the condenser and the electricity is generated. This is called “Closed Cycle” OTEC.

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Chinese-German Joint Symposium on Hydraulic and Ocean Engineering, August 24-30, 2008, Darmstadt

Figure 2. Schematic diagram for closed cycle OTEC

In 1926, a French mechanical engineer, named Claude, envisaged that the closed cycle needed many heat exchangers and evaporators that had the bio-fouling problem. This will make the heat transfer efficiency decrease significantly. He proposed that the surface warm seawater flows into a partial vacuum flashing chamber, then a partial seawater will boil into vapor. Then the vapor will be condensed into water using cold deep seawater, ether by direct contact or through a heat exchanger. The latter will produce fresh water (Fig. 3). A steam turbine is located behind the flashing evaporator, the electricity is then generated. This process is called “Open Cycle” OTEC.

Figure 3. Schematic diagram for open cycle OTEC

After the first energy crisis, i.e. during 70s of the 20th century, U.S., Japan and French have endeavored much effort in the OTEC research. Many fruitful results were produced, especially in the power system, which is nearly commercialized. 1 MW generator has been tested at sea for a short period by an American Organization, i.e. OTEC-1 Project, which was the last big OTEC project sponsored by U.S. Government. 100KW land-based plant has been tested for two years by a Japanese company. The bio-fouling issue for the heat exchanger was also solved. For the Closed Cycle system, the ammonia steam pressure difference is quite large, i.e. about 9 atmospheric pressure. Therefore, the turbine size is not large. But for the Open Cycle system, the steam pressure difference is quite low. Therefore the turbine size should be very large.

In 1994, Avery & Wu wrote a book entitled “Renewable energy from the ocean a guide to OTEC”, which collected all the OTEC research information at that time. They estimated that about 3 M3/S flow rate of warm and cold seawater can generate 1 MW net power. Under the assumption that the sea surface temperature remains unchanged, the tropical and subtropical ocean of water depth greater than 1000 meters can generate over 10 minion MW power. The Taipower capacity is only 0.3% of the amount. In 1980, the U.S. Government was going to sponsor U.S. private companies to develop a commercialized OTEC. However, the Regan’s administration terminated the project. There are two types of OTEC power plant. One is the land-based or shelf-mounted OTEC which are suitable to the eastern Taiwan. The latter has much more sites than the former. A schematic diagram of the shelf-mounted OTEC plant is shown in Fig. 4. It is estimated that the eastern Taiwan has 4 5 GW potential. The technology issue is only the underwater cold water pipe. The pipe diameter must be greater than 7 meters and the pipe length is greater than 4 km. Another type is the floating plant which may be moored or freely drifting. The electricity is then used to electrolyze water to produce hydrogen and oxygen. To synthesize hydrogen, oxygen and carbon, methanol is produced, which can be used as fuel. Fig. 5 is a design of a 160 MW grazing OTEC plantship profile, which can produce 1750 MT methanol per day. The plantship length is 175 meters. Ammonia is another alternative fuel production. Because ammonia is NH3, there will be no CO2 as we use ammonia as fuel. Both of them can be used in the fuel cell. In the book of Avery & Wu (1994), a financial estimate of producing methanol by OTEC employed the Adjusted Present Value (APV) as a profit index. As APV is positive, then the project can win profit. Assuming the plantship number increases, the APV is shown in Fig. 6. At that time the crude oil price is 20 US dollar per barrel. In Fig. 6, the first plant investment amount $1280 million in 1990 value is the maximum value, $985M the nominal value, $800M the minimum, $495M is the cost estimate for the construction in Japan and $330M the cost estimate for constructing the plant in Korea. ITC is the investment tax credit. From Fig. 6, as the methanol sale price is greater than $0.43 per gallon, the APV becomes positive for $330M cost estimate.

Figure 4. Schematic diagram for shelf-mounted OTEC

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Chinese-German Joint Symposium on Hydraulic and Ocean Engineering, August 24-30, 2008, Darmstadt

Figure 5. Profile of 160MW OTEC methanol plantship (Avery & Wu,1994)

Figure 7. Global ocean temperature difference (Avery & Wu, 1994)

Figure 6. Profit diagram for 200-MWe OTEC methanol fuel production (Avery & Wu, 1994)

Except cold water pipe, all other OTEC technologies are available. Only some of them should be modified to meet the marine requirements. Comparing with the shelf-mounted OTEC, the cold water pipe of OTEC plantship is much easier. And the exploitable ocean area is also much more. Hence, the grazing OTEC plantship has more potential. The warmest sea area in the world is called “warm pool”, of which most of the area doesn’t belong to any country. As shown in Fig. 7, the monthly mean seawater temperature difference for the warm pool is greater than 24°C. According to the thermodynamics, the energy transform efficiency for the 24°C temperature difference is 13% higher than that of 21°C temperature difference. Although the warm pool is the typhoon generation source area, the typhoon is still young and moves away to the north. Hence, the typhoon in the warm pool is not a very big problem. Especially for area where the latitude is less than 5°, there is almost on typhoon.

Although the construction material cost now is much higher than that in 1994, this only influences the initial cost. However, the fossil fuel price raises much, which benefits OTEC much more. In the long run, the crude oil price will never drop and the CO2 release decreasing pressure becomes stronger. In 1980, the Carter’s administration U.S. has concluded that the OTEC can be commercialized. Now, it is the time to develop OTEC in order to solve Human’s energy and environment problem. III. DISCUSSION AND CONCLUSION As mentioned above, OTEC is the most promising ocean energy. However, only after commercialization the renewable energy development is then succeeded. Why is the OTEC still not commercialized? The reasons are: 1. The crude oil price was well controlled in the last 20 years before 2007. 2. The best place of natural conditions for OTEC is in the Equator zone of the Southeast Asia, where the industrial level is not high enough. 3. A 2-3MW model plant, which is not economical, is necessary to prove the technology which can be commercialized. This task should be sponsored by a non-profit organization. The most suitable area for OTEC is the tropical ocean, where the seawater temperature is the highest and there are no typhoons, such as Bali, Indonesian (Fig. 8). An international cooperation may execute the OTEC pilot plant experiment at a site of favorable marine environment. In the future, commercialized OTEC plants can be also set up in the public open ocean of the equator zone.

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Chinese-German Joint Symposium on Hydraulic and Ocean Engineering, August 24-30, 2008, Darmstadt

Figure 8. Water depth of a part of Indonesia

The OTEC-related businesses include ocean data monitoring, ocean engineering, maritime transportation, ship building, heat exchanger manufacture, light concrete, electricity generation, electrolyze technology, methanol and ammonia synthesis, international ocean law, marine environment protection, marine fishery, etc. Because the deep seawater is nutrient rich, the OTEC waste water may benefit the natural fish production. However, the most challenging technology is the Cold Water Pipe (CWP), which is a structural and ocean engineering topic, i.e. our field! If a moored platform is used, the generated power can be transmitted to shore. In the future, other than grazing OTEC plantship, a moored artificial island can be considered, on which open cycle can be installed due to a larger space than a plantship. Moreover, for a larger scale OTEC the construction, management and maintenance costs will be greatly dropped down. A successful renewable energy should be commercialized. The influencing factors are the fossil fuel price, technology level, natural conditions, environment pressure and the plant scale. Examining the factors, only the technology level need to be upgraded and integrated. Almost all the technologies are already existed. We only need to modify them for applying in the marine environment. Sooner or later OTEC will finally become commercialized. We may imagine that our energy ships sail or artificial islands are moored in the world ocean, extracting ocean thermal energy or transporting clean fuel to different countries. How beautiful and peaceful this picture is! The ocean is just our big solar energy collector.

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