Proposal for Introduction of Ocean Thermal Energy Conversion (OTEC) to the Energy Sector

of Bangladesh Shifur Rahman Shakil1*, Md.Anamul Hoque2

1,2BRAC University, Bangladesh *E-mail: shifur.shakil@gmail.com

Abstract— Bangladesh is among the worst victims of the global energy crisis, but it is also in one of the most advantageous positions to make transition towards the renewable energy path. In order to solve the power crisis problem by introducing innovative renewable technologies, this paper proposes and analyses the factors behind OTEC technology establishment in Bangladesh. The main objective of this paper is to propose and present the need to set up this new cleaner and efficient source of renewable energy by analyzing its suitability on geographical position .The site of Bangladesh is naturally gifted area to establish Ocean Thermal Energy Conversion (OTEC) as Bangladesh lies just beneath the tropic of cancer and on the shore of the Bay of Bengal, the two vital elements: constant sunlight and large littoral areas needed for OTEC can easily be found in this region.

Keywords— Renewable energy, Ocean Thermal Energy, Infrastructural problem, environmentally sustainable, Clean energy, Green energy, OTEC sites

I. INTRODUCTION Indigenous energy sources are urgently needed in developing countries and OTEC is ideally suited to provide the required thousands of megawatts of electric power with the least environmental impact. Moreover, OTEC can be stated as one of the cleanest forms of renewable energy technology as there is zero emission of carbon dioxide in its procurement and conversion of the energy. A developing country like Bangladesh where power disruption became an acute problem due to the over dependency on fossil fuels , renewable energy especially OTEC can be a new alternative, the reason being there are many sites that are suitable for constructing OTEC plant in Bangladesh. Not only Bangladesh, OTEC has a future prospective all over the world in the field of renewable energy. The oceans cover a little more than 70 percent of earth surface. This makes oceans the world’s largest solar energy collector and energy storage system. On an average day, 60 million square kilometers of tropical seas absorb an amount of solar radiation equal in heat content to about 250 billion barrels of oil [1]. Proper designed of OTEC plants produce no greenhouse gases (GHG) or other pollutants. OTEC systems can produce fresh water besides electricity that can contribute

a significant role in island areas where fresh water is scarce [2].

II. OCEAN THERMAL ENERGY CONVERSION

Ocean Thermal Energy Conversion (OTEC) is an energy technology that converts solar radiation to electric power. The system uses the ocean’s natural thermal gradient to produce electricity. It consists of pumping cold ocean water to the surface and uses the temperature differences between surface water and the deep sea water. The warm surface water runs a thermal engine to generate electricity. It is anticipated that, on an annual basis, the amount of solar energy absorbed by the oceans is equivalent to at least 4000 times the amount presently consumed by human. Though the efficiency of OTEC is nearly about 3 percent; we need less than 1 percent of the renewable energy to satisfy all our desires towards energy [3].

III. PROCESS AND TECHNOLOGY STATUS (SUITABLE FOR BANGLADESH)

To proceed beyond experimental plants and towards commercialization in developed nations, a scaled version of a 100 MW plant must be designed and operated. The operational data is needed to earn the support required from the financial community and developers. Considering a 4-module system, a 1/5-scaled version of a 25 MW module is proposed as an appropriate size. The 5 MW pre-commercial plant is also directly applicable in some SIDS (small- island- developing -states)[4]. Warm water is collected from the surface of the tropical ocean by pumping and it is taken through a heat exchanger (evaporator) where the heat is used to vaporize a working fluid usually Propane or Ammonia. The vapor expands through a turbine which is connected to a generator. Cold water from the deep ocean is pumped up to the plant and taken through a condenser (another heat exchanger) where the Propane vapor now can return to the liquid state to be pumped back to the evaporator.

Proceedings of 2013 2nd International Conference on Advances in Electrical Engineering (ICAEE 2013)19-21 December, 2013, Dhaka, Bangladesh

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A small fraction of the power from turbine is used to pump the water through the system and to power other internal operations, while most of it is available as net power. There are three types of OTEC designs: Closed cycle, Open cycle and Hybrid cycle. Closed loop cycle OTEC is similar to open loop cycle OTEC, but uses a working fluid, like ammonia, that boils at a lower temperature than water. The working fluid creates steam and runs a turbo generator as shown in Figure 1. The steam is condensed with the cold water from lower depths, and the ammonia condenses to the working fluid

Figure 1: Closed cycle loop OTEC

The Open cycle OTEC uses warm surface water directly as working fluid. Placing the warm sea water in a low pressure container causes it to boil (flash evaporator).In some schemes the expanding steam drives a low pressure turbine attached to a generator. The steam is then condensed into liquid in a heat exchanger by exposing it to cold water pumped from down below. This method also produces pure desalinized fresh water as a byproduct, also the salt and impurities are left in the slow pressure container which is shown at Fig.2.

Figure 2: Open cycle loop OTEC

A Hybrid cycle system combines the features of the closed and open cycle systems. Here the warm water enters a vacuum chamber and is flash evaporated, similar to the open cycle process. The steam vaporizes the low boiling point fluid like ammonia or propane within a closed cycle loop then the vaporized fluid runs the turbine. The steam condenses within the heat exchanger and produces desalinated water.

IV. ENVIRONMENTAL IMPACT The water being from the deeper region of the ocean, it contains nutrient and comprises of a different salinity level.

OTEC system involves of bringing up highly mineral enriched water from ocean depth. This mineral rich water, when dropped on the ocean surface helps in promoting the growth of photosynthetic phytoplankton. However as there is a question of maintaining the ocean mixed layer biota and maintenance of the natural surface temperature anomalies so the release of the water is conducted in a precise and controlled manner to avoid disrupting the natural balance. In case of any other off shore operations, there is a possibility of natural disruption of unintended fish or seabirds’ attraction; noise created from the plant might interfere with animal communication. Also, there is a fear of lubricants and anti-biofueling chemicals entering the ocean. Concerning the OTEC system, the release and redistribution of a huge amount of water periodically will cause some natural changes in geological stratification, ocean salinity, oxygen and nutrient levels near the site. It might affect the natural life habitat of some organisms. Some organisms base their behaviors on certain temperature or salinity gradients [5], while others may be affected by increased nutrient levels [6].

V. APPROACH FOR REDUCING COST OF OTEC PLANT

Researchers have been working on reducing the overall cost of OTEC systems. Keeping the cost and overall efficiency in mind, Rankine cycle, Open or Claude cycle, Mist-life cycle, Kalina cycle and Uhera cycle were few concepts which were introduced in OTEC[7]. Amongst them Rankine cycle is much more accepted among the engineering community. As for the working fluids, ammonia remains the fluid of choice for closed-cycle OTEC system along with propylene. Here we have confined ourselves to a closed system using Rankine cycle and ammonia. At OTEC conditions, there are considerable amount of experimental data available for heat exchangers using ammonia and seawater, both for boiling and condensation [08]. The reason for choosing this combination is for its practicality and the low risk factor with high yield from the seawater resources in the form of staging the Rankine cycle. Staging allows maximum potential extraction of heat and power from a set of given resources.

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Figure 3: Temperature-entropy (T-S) diagram for a Single and two-staged Carnot cycles in general (without scaling) Figure 3 illustrates the nature of staging. A temperature-entropy (T-S) diagram shows both a single-stage and a two-stage Carnot cycles. Advantages of staging are best illustrated with Carnot cycles, without any loss of applicability. Both cooling and heating lines for the warm and cold seawater are indicated in this figure. The single stage working fluid state points are indicated by points ABCD. The area within this rectangle represents the amount of power that can be generated from that cycle. The temperature approach at points A and C are dictated by the Minimal Internal Temperature Approach (MITA) on the evaporator and condenser, respectively. If the cycle is staged as two separate cycles, indicated by the rectangles, AFJK and GHCI, one can still maintain the same temperature approach in the two evaporators and condensers. However, the overall area covered by these two rectangles is substantially greater than the single-stage cycle. It is possible to increase the number of stages in order to increase power yield from a given set of resources while maintaining a given set of approach temperatures. However, cost tradeoffs will limit the number of stages that can economically be used.

VI.GEOGRAPHICAL ANALYSIS FOR SUITABLE OTEC

POWER PLANT SITES IN BANGLADESH:

The best locations for OTEC are in places where the temperature gradient of the ocean water is substantial. This is because the efficiency of the system depends on the temperature difference between the higher and lower portions of the ocean. Temperature difference between the ocean surface and the water at a depth of 1000 meter varies from less than 18 degree Celsius to more than 24 degree Celsius. OTEC can be sited in principle almost anywhere in the tropical ocean-generally between Tropic of Cancer and Tropic of Capricorn. A temperature difference between the surface and 1000 meter depth better than 22 degree Celsius is usually available here[9].

Figure 4: Suitable location for OTEC plant

From the figure 4, it can be seen that for Bay of Bengal the temperature difference between surface and sub surface (1000m) sea water ranges from 20 degree Celsius to 22 degree Celsius. So, OTEC technology is expected to be feasible in the Bay of Bengal which helps to generate electricity. Here we

mainly focus on Cox’s Bazaar where OTEC plant can be constructed. The annual average temperature in Cox's Bazaar remains at about a maximum of 34.8 °C and a minimum of 16.1 °C. As Bangladesh lies beneath the tropic of cancer, the temperature difference between the ocean surface and the water at a depth of 1000 meter in the Bay of Bengal is usually between 20°C to 22 °C. The worst thing is that there is yearly cyclone and land slide from hilly areas which is quite obvious as most of these cyclone origin from the Indian Ocean. Considering the OTEC site selection criteria as mentioned in multiple papers by L.A. Vega, who is one of the key researchers and an internationally figure on this subject matter , factors like geographical location, surface and undersea temperature, land area, population, electric energy sources, electricity demand ,current availability and future electricity demand have been considered here[10]. The political climate supportive of large infrastructural development in Cox’s Bazaar is suitable enough (better if under strict observation) which can be considered the major prerequisite for constructing OTEC plants. Moreover, support of foreign investment from the standpoints of taxation, permitting, and emigration (or working visas) for foreigners in Bangladesh is quite satisfactory and it can be improved if proper government arrangements are made. According to the Table I, there are quite few large constructional infrastructure apartments and hotels which are gradually increasing day by day as Cox’s bazaar is becoming famous internationally for tourism. The below table illustrate the characteristics of Cox’s bazaar:-

Table I: Characterestics of Cox’s bazaar

Though the density of population per sq.km is 705 in Cox’s bazaar, there is insufficient supplies from main power-grid and diesel powered generators as a result there are willing buyers for electricity that would be produced by OTEC. Buyers mainly include hotels, resorts and restaurants authority. Currently, there is no local power station available which is discussed in Table II. Significant amount of hotels along with some under construction including international chains like Radisson also apartments and condos are rapidly developing furthermore Cox’s Bazaar being the largest beach in the world (interesting site) attracts many foreign tourists who demand smooth electricity facilities including air-conditioning and

OTEC Site Suitability Cox’s Bazaar Beach

Districts

Cox’s Bazaar

Area Covers

2491.86 sq.km (District)

Population

51,918

Famous For

Major Tourist Spot World’s Longest 125 km long Sea Beach

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chilled water in tourist filled hotels and resorts . The present source of water in Cox’s bazaar comes from the sea-water distillation facility and the deep of ground. There are large amount of local fishing along with commercial fish farm, also one of the biggest market of dry fishes. Recently there is a rise of organic products so fresh water can help develop organic farm which will boost the tourism sector. All the following things gives a suggestion that Cox’s bazaar can be appropriated as one of OTEC sites in Bangladesh. The following table gives a full description about the possible potential buyers of electricity that produce from this possible OTEC sites, the electric energy sources, demand of electricity of this place and finally the amount of available electricity.

Table II: Study on Cox’s bazaar for OTEC sites

The Plants in Cox’s bazaar can be land based, shelf based, or floating based. Land based power plant located at the coast has several advantages over that of the deep water plant. These do not need sophisticated moorings, lengthy power cables, or more elaborate maintenance requirements in Open Ocean. Here the system can be protected better from storm and heavy seas. In addition, land based plants can exploit better other opportunities such as the production of fresh water .On the other hand one major disadvantage is the exposure of the coast to turbulent wave actions. Also the deep sea cold water in this case has to be brought from a long distance away, and the discharge has to be carried a long distance too before releasing at the proper path. To avoid the above disadvantages, OTEC plants are often mounted to the continental shelf at the depths of up to 1000 meters. It is something similar to off shore oil rings, and faces the similar open ocean conditions. The platform requires extensive pilling to maintain a stable base. Power delivery needs long under water cable. Floating off shore plants solve some of the above problems. However, moorings and power cables with floating plants are more susceptible to damage at depths more than 1000 meter. VII. Capital Cost: 50 MW OTEC sites in Cox’s bazaar A 50 MW OTEC plant can be easily introduced as many private companies are willing to do this in Cox’s bazaar. This study will help many companies to illustrate the cost to set up

an OTEC plants as both CC-OTEC & OC-OTEC can be manufactured in Cox’s bazaar. CC-OTEC system needed typical double-hulled vessels and could be constructed in various shipyards all over the world. The estimate for the CC-OTEC plant is $451M and for the OC-OTEC plant $551M [11] The OC-OTEC system needs a vessel that is about three times wider (beam direction) than the standard tanker and container ships. The OC-OTEC plant would be shorter at 176 m but beamer at 90 m resulting in a displacement of 247,400 tons [11]. If we draw a graph (fig-5) by plotting the data nominal plant size MW-net Vs CC, installed capital cost ,we see that OTEC capital cost ($/kW) is a strong function of plant size (MW). For convenience and future reference a least-squares curve fit is provided: CC ($/kW) = 53,000 * MW ^(-0.42)

Figure 5: Capital Cost Estimated for OTEC Plants.

OTEC power will be cost effective if the unit cost of power is comparable with other power plants such as wave, hydro and diesel. However, it is important that all capital costs and ongoing maintenance/service costs are included so that the individual technologies are compared on a level playing field. Table III illustrates a comparison of the cost for constructing around 1 MW plants using different technologies [12]. Table III: Comparison of Unit Cost of OTEC with Conventional Energy Sources in the Pacific Region

Plant Capacity

Plant life (yr)

Annual output(GW)

Cost (US$/kW)

Wave 1.5 40 9 0.062-0.072 Hydro 1.2 40 5 0.113 Diesel 0.9 20 5 0.126 OTEC 1.256 30 8.8 0.149

VIII. EFFICIENCY IN OTEC PLANT SITES IN COX’S BAZAAR:

We can use equation 1 (Carnot’s equation) to get an idea about the efficiency of OTEC [13]:

…………. (1)

OTEC site suitability Cox’s Bazaar Beach Potential Buyers

Sub-Marine Cable Station

54 Shrimp Farms Fish export processing zone

Major International Hotels and Motels (Radisson, Best-Western)

Electric Energy Sources

From Chittagong District

Demand

45 Mega Watt

Available

9 Mega Watt

QT

TTW′−=

377

Here, W = Work obtained (energy);

Surface water temperature (°K); he deep water temperature (°K); Thermal Value

Typical temperatures in the Cox’s bazaar sites:

% Q being the order of 0.5 for practical considerations in the conversation, the efficiency comes to slightly more than 3%.This may seem like a low efficiency, but this does not make OTEC less competitive than other renewable energy sources. For example, it needs to handle no more than a hydroelectric plant of the same capacity and the resources available in the case of OTEC which is vast, 300 times more than mankind’s current total power usage [14].

IX.CONCLUSION With the demand for electricity in Bangladesh increasing every day, potential energy sources need to be sought out in order to alleviate the ensuing energy crisis. As Bangladesh is also a developing country, it has to be borne in mind that these energy sources have to be inexpensive, preferably local and must have probation for quick setup for power generation. Although solar, wind and hydropower offer interesting alternatives they require huge funding, which would strain the economy and take a longer time to recover the costs. Though the initial cost of constructing OTEC sites is huge, but it gives profit within 5 years besides it gives fresh water along with seafood which is very indispensable for the people of coastal area. This paper looks into the prospect of OTEC in Bangladesh and where in the country, there is suitable infrastructure building condition for its implementation. OTEC power systems satisfy the criteria for suitable energy systems in Bangladesh, and if the suggestions given in this paper are implemented, it would be a giant leap forward in solving the present energy crisis of the nation.

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[6] G. W. Boehlert and B. C. Mundy,“Vertical onshore and offshore distributional patterns of tuna larvae in relation to physical habitat features.” MEPS 107:1-13,1994

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[10] L.A.Vega, “Ocean Thermal Energy Conversion(OTEC)”,1999

[11] L. A. Vega, “Economics of Ocean Thermal Energy Conversion (OTEC): An Update”. National Marine Renewable Energy Center,University of Hawai'i.

[12] R.Mario, “OCEAN THERMAL ENERGY CONVERSION ANDTHE PACIFIC ISLANDS”, Faculty of Science and Engineering, Saga University, Japan. [13] M. R. Islam, Nature Science and Sustainable Technology, Nova Publishers, 2008 . [14] J.Baird, “OTEC and Energy Innovation: The Willie Sutton Approach”.

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