Design and Simulation of Axial Turbine for Ocean Thermal Energy Conversion ( OTEC )

 decreasing in fossil energy reserves about 3% every year and has not been matched by the discovery of new energy reserves. Therefore, it is necessary to increase the use of renewable Energy to meet energy needs. Renewable energy is energy derived from sustainable natural processes. Indonesia located in the tropical area, it has a lot of potential ocean energy. OTEC (Ocean Thermal Energy Conversion) is one of many renewable energy sources from the ocean. OTEC or Ocean Thermal Energy Conversion is one of the latest technologies that used the temperature difference between deep and shallow seawater. OTEC system generally used ammonia (NH3) as working fluid. Ammonia is used because it has a relatively low boiling point compared to water. OTEC system consists of evaporators, turbines, generators, condensers, and pumps. In this research, the authors focused on the design of lab-scale OTEC turbines. 2 stage turbine will be varied the tilt which is 0, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50 degree. The computational fluid dynamics (CFD) method is used in this research to simulate the OTEC Turbine. Based on the simulation results, the highest efficiency and net power is a 2 stage 40 degree turbine with 57.45% of efficiency and 287.25 kW of generated power. Keywords CFD, ocean thermal, OTEC, renewable energy, turbine,


I. INTRODUCTION 1
Energy has an important role in the achievement of social, economic and environmental goals for sustainable development, and national economic activities.Energy use in Indonesia is increasing rapidly in line with economic growth and population growth.Indonesia faces a decreasing in fossil energy reserves about 3% every year [1], and has not been matched by the discovery of new energy reserves.Therefore, it is necessary to increase the use of renewable Energy to meet energy needs.Renewable energy is energy derived from sustainable natural processes.
Studies and projects on renewable energy sources have been actively conducted around the world to solve the challenge of energy supply and concomitant environmental issues.OTEC (Ocean Thermal Energy Conversion) is one of many renewable energy sources from the ocean.
OTEC or Ocean Thermal Energy Conversion is one of the latest technologies that used the temperature difference between deep and shallow seawater that drive generators to produce electrical energy.In the tropical oceans between approximately 15° north and 15° south latitude, the heat absorbed from the sun warms the water in the mixed layer to a value near 28°C that is nearly constant day and night and from month to month.The annual average temperature of the mixed layer throughout the region varies from about 27°C to about 29°C.Beneath the mixed layer, the water becomes colder as depth increases until at 800 to 1000 m (2500 to 3300 ft), a temperature of 4.4°C [2].This temperature does not change dramatically throughout the year, with varying degrees due to weather and seasonal changes, and the 1 Irfan Syarif Arief, Department of Marine Engineering, Institut Teknologi Sepuluh Nopember, Surabaya 60111, Indonesia, Email : irfansya@ its.ac.id 2 Tony Bambang Musriyadi, Department of Marine Engineering, Institut Teknologi Sepuluh Nopember, Surabaya 60111, Indonesia, Email : tobac@its.ac.id 3 Desta Rifky Aldara, Department of Marine Engineering, Institut Teknologi Sepuluh Nopember, Surabaya 60111, Indonesia, Email : destarifkyaldara@gmail.com temperature difference between day and night turns only has an effect of about 1°C [3].
OTEC cycle generally used ammonia (NH3) as working fluid.Ammonia is used because it has a relatively low boiling point compared to water.OTEC system consists of evaporators, turbines, generators, condensers, and pumps.In this research, the authors focused on the design of lab-scale OTEC turbines to get the highest efficiency and net power.

II. METHOD
The turbine modelling used Auto blade.The blades angle designed based on the enthalpy difference between the inlet and outlet condition in turbine.Figure 1.shown the 2D drawing of turbine blades, and Table 1 shown the number of blades.After the modelling finished, it simulated by using FINE Turbo with the parameter as shown in Table 2. Simulation started to know the torque, mass flow and efficiency of turbine.

A. Preliminary
Indonesia has a lot of energy resources, both in the fossil resources, and renewable natural resources.In renewable natural resources, Indonesia has excellent potential, one of them is the ocean thermal sector.National Energy Council has mapped the potential of Ocean Thermal Energy Conversion.Theoretical resource: 4.676.689MW, Technical resource: 216.609MW, practical resource: 60.985 MW [1][10] [11].

B. Ocean Thermal Energy Conversion
Ocean Thermal Energy Conversion (OTEC) used the temperature difference between deep and shallow seawater that drive generators to produce electrical energy.The cycles are based on a Rankine cycle of heat energy stored in the seawater into electrical energy [2].In OTEC operation, the working fluid is pumped back to the evaporator after condensation (conserved), as shown in Fig 2 .Ammonia is commonly used in this cycle because it has a relatively low boiling point with seawater as a fluid to evaporate and condense.

C. Rankine Cycle
The Rankine cycle closely describes the process by which steam-operated heat engines commonly found in thermal power plants.This cycle used two phases of working fluid, there are liquid and vapor. in a simple Rankine cycle consists of 4 main components namely condenser, pump, boiler, and turbine [4].The difference of OTEC power plant and thermal power plant is the boiler replaced by evaporator and the boiling point of OTEC cycle is lower so that water is not suitable for working fluid in OTEC system.Fig. 3. Shown the diagram of Rankine cycle.

D. Steam Turbine
A turbine is a rotary mechanical device that extracts energy from a fluid flow and converts it into useful work.The rotating part is the rotor, while the nonrotating part is the stator or turbine housing.Moving fluid acts on the blades so that they move and impart rotational energy to the rotor.The work produced by a turbine can be used for drive the load (i.e.electrical generator, pump, compressor, propeller, etc.).The working fluid may be water, steam, or gas [5].
Steam turbine is a driving force that converts steam potential energy into kinetic energy and then converted into mechanical energy in the form of turbine rotation.Turbine axle, directly or with reduction gear, connected with the mechanism to be driven.If the torque and RPM of turbine are known, then turbine power can be calculated using the equation In the steam turbine, vapor is expanded in the nozzle so, obtained the vapor velocity (c1) that will enter to the rotor on turbine.The rotor rotates with the velocity (u).It needs c1 and u ratio with a certain value so that the steam flow out of the nozzle works optimally.Thus, can be obtained inlet and outlet angle [6].Velocity triangle can be seen in Fig. 4.
Angle α1 and β1 shall be made in such a way, according to the vapor velocity.Value of α1 is free to determined, but should be as small as possible.The optimum value of α1 is between 14°-20° [7].From α1 can be found:

E. Heat Exchanger
Heat exchanger used to transfer heat between two or more fluids.The fluids separated by a solid wall to prevent mixing or they may be in direct contact.In OTEC system used heat exchanger that is evaporator and condenser.evaporator replace the boiler function in the steam power plant.An evaporator used to turn the liquid form of a chemical substance such as water into its gaseous-form/vapor.A condenser used to condense a substance from its gaseous to its liquid state, by cooling it.The design calculation for heat exchange means is essentially determining the heat transfer coefficient and heat transfer area (A) of the following equations

F. Pump
A pump is a device that moves fluids by mechanical action.Pumps operate by some mechanism, usually reciprocating or rotary, and consume energy to perform mechanical work for moving the fluid.The characteristics of the pump are determined by the volume of the pumped fluid (V), Head losses (H), Condition on each side of suction.One of the important factors in sizing a pump is total head requirements.a pump head is maximum height that the pump can achieve pumping against gravity [12].Total head is the sum of Head static, Head Pressure, Head Velocity, Head Loss.
1. Head Static Head static is the maximum height reached by the pipe after the pump.Head static pump is calculated from the pump inlet till the end of discharge.

Head Pressure
Head Pressure is the diference of pressure on the suction and discharge.

Head Velocity
Head Velocity is difference velocity of fluid between in suction and discharge of pump.

Head Loss
Head loss is energy loss per unit weight of the fluid in the drainage of fluid in the piping system.Head Losses including head major and head minor in suction and discharge.

 Head Major
Major losses are associated with frictional energy loss per length of pipe depends on the flow velocity, pipe length, pipe diameter, and a friction factor based on the roughness of the pipe, and whether the flow is laminar or turbulent.Head Major can be calculated with the equation: OTEC system consists of turbines, generators, evaporators, condensers, and pumps as shown on Fig. 5.In this chapter will explain how to design Ocean Thermal Energy Conversion (OTEC) turbine, simulation of turbine design, heat exchanger calculation, pump calculation, and net power calculation to be delivered to the consumer.

A. Design and Drawing of OTEC Turbine
In this sub-chapter will be explaining the design process using Autoblade software and simulation process using Fine Turbo.The first process is to determine the working fluid state at the inlet and outlet of OTEC's turbine, then, determining the angle of the turbine blades, and the last is the drawing of OTEC's turbines by using Autoblade software.

Drawing OTEC Turbine
The angle of the turbine blades is designed so as to produce optimum work on the turbine.Value of α1 is free to determined, but should be as small as possible.The optimum value of α1 is between 14°-20° [7].In this research, the value of α1 = 15 °.Following is the result of blade's angle calculation by using equation 8 to 13: 15° 31.78°24.1° 21.1° Drawing process is done by using Autoblade, the 3D model of OTEC turbine can bee seen on Fig. 6.After modelling process, next is processing by using FINE Turbo.

13
B. Performance of OTEC Turbine After simulating process.The performance results of each turbine model will be compared to obtain the best OTEC turbine for laboratory scale.Simulation using Numeca Fine Turbo obtained numerical data, such as mass flow balance (inlet and outlet), efficiency, and torque.The result of simulation can be seen on Table 3.Based on Table 3. Can be seen that the highest efficiency and OTEC turbine power is 2 stage 40 Degree with 610.6571 kW and 57.45% of efficiency, and the lowest is 2 stages 0 degree turbine with 99.59 kW and 41.17% of efficiency.From 2 stage 0 degree until 2 stage 40 degree the power of OTEC turbine increased and going down on 2 stage 45 degree OTEC turbine.

Calculation of Volume Flow Rates
In order to an Ocean Thermal Energy Conversion (OTEC) system to work, it needs ammonia as a working fluid, warm seawater used to change the ammonia phase from liquid to vapor, and cold seawater used to change the vapor phase to liquid.The volume flow rates can be calculated by using equation 15.Table 4 shown the result of volume flow rates calculation.

Calculation of Pump
Ocean Thermal Energy Conversion (OTEC) system use about 3 pumps are used to pump the working fluid ie ammonia, warm seawater, and cold seawater.In this subchapter will be explained about the calculation of the pump, including the calculation of head and power.
 Working Fluid Pump Fig. 7 is a pipeline of the working fluid.The pump's total head should be provided for the planned amount of seawater.Total head can be calculated by using equation 17 to 19, and the power gained by the fluid can be calculated by equation20.Table 5 is the result of working fluid pump calculation.
 Evaporator Pump Fig. 8 is a pipeline of the evaporator.The pump's total head should be provided for the planned amount of seawater.Total head can be calculated by using equation 17 to 19, and the power gained by the fluid can be calculated by equation 20.

Calculation of Nett Power
Nett power is the power that distributed to the consumer.Generator is needed to generate power that will be distributed to the consumer.In this research, generator efficiency is assumed to be 85%.Table 8. shown the power generated by generator.
After knowing the power generated by the generator, the pump calculation needs to be done to know the nett power to be distributed to the consumer.Fig. 10.Can be seen that a single-stage OTEC turbine produces nett power of 31.09kW.For the 2 stage OTEC turbine, the lowest power is a straight turbine model with a nett power of 31.62 kW, while the highest is generated by a 40 degree turbine with a nett power of 287.73 kW.For 3 stage OTEC turbines producing the highest power among all models, the resulting nett power is 351.37 kW. 2. Adding the number of stages can improve the efficiency and power generated by the turbine.The highest efficiency and net power is a 3 stage 40 degree turbine with 351.37 kW generated power, and 65.02% efficiency.Lowest is single stage turbine with nett power 31.09kW, and efficiency 45.85%.

Figure. 3 .
Figure. 3. Diagram of Rankine Cycle kW) T = Torque (Nm) RPM = Revolution per minutes : absolute velocity of steam inlet and outlet from the nozzle International Journal of Marine Engineering Innovation and Research, Vol.3(1), Des.2018.008-017 (pISSN: 2541-5972, eISSN: 2548-1479) 11 w1 and w2 : relative velocity of steam inlet and outlet from the rotating blades u : circumference velocity of the rotating blade α1 and α2 : angle of the nozzle β1 and β2 : angle of the rotating blades Ψ : speed coefficient transfer rate (Watt) U = Overall heat transfer coefficient (W/m 2 .o C) LMTD = Logarithmic mean temperature difference ( o C) F = LMTD correction factor friction factor (unitless) L = the pipe length (m) d = the hydraulic diameter of the pipe D (m) g = the gravitational constant (m/s 2 ) v = the mean flow velocity V (m/s)  Head Minor Minor loss is a pressure loss in components like valves, bends, tees and similar.Head minor can be calculated with the equation: rate (m 3 /s) g = the gravitational constant (m/s 2 ) Htotal = Total head (m) III.RESULT AND DISCUSSION

TABLE 3
Table 6 is the result of evaporator pump calculation.

Table 9 .
shown nett power to be delivered to the consumers.