Analysis and design of a solar heating ocean thermal energy conversion (sh otec)

25 2.3 Thermodynamic Analysis of Regenerative Solar-Heating Ocean Thermal Energy Conversion RSH-OTEC Cycle considering Heat Transfer Area .... -- - ii - Chapter 3: Analysis and Optimiza

Nguyen Van Hap

Analysis and Design of a Solar-Heating Ocean Thermal Energy Conversion (SH-OTEC)

Analysis and Design of a Solar-Heating Ocean Thermal Energy Conversion (SH-OTEC)

Supervisor: Prof Geun Sik Lee

A Dissertation

Submitted to the Graduate School of University of Ulsan

In partial Fulfillment of the Requirements

for the Degree of

Doctor of Philosophy

by

Nguyen Van Hap

School of Mechanical Engineering University of Ulsan, Republic of Korea

May, 2015

ACKNOWLEDGMENTS

First of all, I would like to express my deepest gratitude to my supervisor, Prof Geun

Sik Lee, who not only gives me encouragement, acclaim, and providing enthusiasm, as

well as direction but also supports me financial help during my study period in the

University of Ulsan I am extremely indebted to my supervisor for his ideas, suggestions,

comments and corrections to this thesis I would also like to thank Prof Mok In Lee for

his many helpful instructions about the deep knowledge of mathematics during the time

I carried out my dissertation

Thanks also go to the committee members for their helps, as well as comments and

suggestions I sincerely thank the University of Ulsan for the scholarship support and

opportunity to study in Korea I am greatly thankful to the professors and staffs at the

school of Mechanical and Automotive Engineering for providing the valuable expertise,

the helpful suggestions, the necessary environment for doing research and the other

kindly helps

My deepest gratitude, thank and love go to my family members for their love,

unfailing support, encouragement and advice during the time I am away from home I

also thank all members of the Entire Energy Harmony laboratory for their useful

supports I am also thankful to the Union of Vietnamese Students in University of Ulsan

and Korea, as well as the friends for spiritual helping and consulting about abroad living

experience

Ulsan, May, 2015

Nguyen Van Hap

- i - CONTENTS Abstract iv

List of figures vii

List of tables x

Nomenclature xi

Abbreviations xiv

Chapter 1: Introduction 1

1.1 An overview of Ocean Thermal Energy Conversion (OTEC) 1

1.2 Problem statement, necessary, objectives and thesis layout 4

Chapter 2: Investigation of effects of solar radiation, working fluid properties and thermodynamic cycles on OTEC 8

2.1 OTEC history and application 8

2.2 Effects of solar radiation on solar-heating OTEC (SH-OTEC) 10

2.2.1 Principle and mathematical model of SH-OTEC 11

2.2.2 Given data 16

2.2.3 Results and discussion 16

2.2.3.1 Effect of working fluids 16

2.2.3.2 Effect of solar radiation 20

2.2.4 Summaries 25

2.3 Thermodynamic Analysis of Regenerative Solar-Heating Ocean Thermal Energy Conversion (RSH-OTEC) Cycle considering Heat Transfer Area 25

2.3.1 System description of Regenerative Solar-Heating Ocean Thermal Energy Conversion (RSH-OTEC) Cycle 26

2.3.2 Mathematical model 28

2.3.2.1 Thermodynamic model 28

2.3.2.2 The models of Heat exchangers 31

2.3.3 Results and discussion 33

2.3.4 Summaries 41

- ii - Chapter 3: Analysis and Optimization of Heat Exchangers of Solar-Heating Ocean Thermal Energy Conversion (SH-OTEC) Using High-Performance Commercial Tubes 43

3.1 Introduction 43

3.2 Computational Modeling 46

3.2.1 Mathematical model of evaporator and condenser 47

3.2.2 Pressure drop 50

3.2.3 Objective functions 51

3.2.4 GA optimization 53

3.3 Results and discussion 56

3.3.1 Analysis of heat exchangers of SH-OTEC 58

3.3.2 Multi-objective optimization with GA 63

3.4 Summaries 64

Chapter 4: Analysis and Design of a Radial Turbine for Ocean Thermal Energy Conversion 66

4.1 Introduction 66

4.2 Modeling and analysis 68

4.2.1 Geometrical model 68

4.2.2 3-D Numerical analysis and Computational model 79

4.2.3 Boundary conditions 81

4.2.4 Grid independence 82

4.3 Results and discussion 82

4.4 Summaries 88

Chapter 5 : Structural analysis of the nozzle and rotor of radial turbine for SH-OTEC application 89

5.1 Introduction 89

5.2 Methodology 91

5.2.1 Mathematical model 91

5.2.2 Computational modeling 93

- iii - 5.2.3 Meshing and grid convergence 96

5.2.4 Boundary conditions 101

5.3 Results and discussion 101

5.3.1 Structure of nozzle 101

5.3.2 Structure of rotor 106

5.4 Summaries 113

Chapter 6: Conclusions and Future Works 115

References 119

List of publications 126

Appendix 128

- iv -

Analysis and Design of a Solar-Heating Ocean

Thermal Energy Conversion (SH-OTEC)

By Nguyen Van Hap School of Mechanical Engineering University of Ulsan (UOU)

ABSTRACT

The most critical problems in the world are global warming, which leads to climate

changes, energy security, pollution and depletion of non-renewable fossil resources

OTEC (Ocean Thermal Energy Conversion) systems operate as heat engines They

receive ocean thermal energy to transform this energy into electrical OTEC is

considered as a technology solution for providing a clean and reliable energy solution

Hence, in this thesis, we perform design of an OTEC system using solar heating to

generate electrical To construct a demonstration plant before commercialization of

OTEC, the study proposes and analyses the performance of system in order to select a

suitable thermodynamic cycle and the best working fluid for OTEC system The

components of OTEC system such as condenser, evaporator and turbine are modeled

and simulated in order to obtain their sizes OTEC design is carried out as follows

Firstly, we investigate effects of solar radiation as well as type of working fluid on

thermal efficiencies of OTEC The solar radiation considered as the secondary heat

source is used to increase the warm seawater temperature in order to improve thermal

efficiency The collectors in type of flat plate are modeled for solar collector of

solar-heating OTEC The results show that each of working fluid has still advantages and

- v -

disadvantages None of any substance is a perfect working fluid In summary, R152A

followed by R600, R600A and R134A are the most suitable working fluids for OTEC

Besides, the required area of collector for increasing the collector outlet temperature by

200C for 1kW work output after the solar collector array varies from 49.91m2 to

96.84m2 in Ulsan and from 42.27m2 to 82.24m2 in Nha Trang depending on the months

in year As a fixed solar-collector area of 90m2, the monthly average of annual gain of

thermal efficiency achieves about 6.23% in Ulsan and 8.2% in Nha Trang These

efficiencies are approximately 4 and 3 times, respectively, higher than the performance

of the typical OTEC without solar heating SH-OTEC located in Nha Trang, Vietnam

leads to the monthly average efficiency of 31.6% higher than that of the one located in

Ulsan, Korea

For selection of SH-OTEC thermodynamic cycle, the regenerative cycle is a

potential thermodynamic cycle for OTEC Thermodynamic efficiencies of the

regenerative cycle for SH-OTEC are higher than those of the basic cycle Adopting the

regenerative cycle made it possible to reduce the heat transfer irreversibility associated

with evaporator and condenser The total heat transfer area in the regenerative cycle is

reduced approximately 10.5% to 29.3% for almost working fluids

Secondly, we concentrate on optimal design of the heat exchangers of SH-OTEC

with working fluid of R134a using high-performance commercial tubes One of the

most challenges for commercialization of OTEC is to reduce the sizes of heat

exchangers Optimization of heat exchangers including evaporator and condenser of

OTEC is considered as one of important tools to reduce the electricity price which

generated by OTEC The Genetic Algorithm (GA) with two objective functions

including capital cost i.e heat transfer area and operating cost is used in order to

achieve a trade-off between these two objective functions simultaneously As the results,

the surface heat transfer area and pressure drop are strongly dependent on the number of

tubes and number of tube passes as well The heat transfer surface area of the condenser

accounts for very high percentage of total heat transfer area of SH-OTEC in comparison

- vi -

with the evaporator Pareto fronts of evaporator and condenser obtained from

multi-objective GA provide designers or investors with a wide range of optimal solutions that

they can select projects suitable for their financial resources

Finally, the preliminary design of a radial inflow turbine with high efficiency using

the working fluid of R134a for OTEC application is conducted in order to obtain the

geometrical dimensions Based on these results, three-dimensional turbine model is built

for CFD analysis Flow characteristics inside the turbine including the volute and the

nozzle are investigated by using the CFD software of ANSYS CFX Within a pertinent

number of nozzle guide vane ranging from 10 to 15, the turbine efficiency is higher than

80% and the highest efficiency is shown at the 15 guide-vane nozzle

The components, especially the turbine of OTEC system are required long operating

life If the turbine blades and guide vanes are not able to withstand the stress subjected

on them, the blades and vanes will be fractured and the failure will takes place The

objective of this section is to determine the stress distribution, stress concentration and

deformation in 90 degree radial-flow turbine blades and nozzle guide vanes of OTEC

turbine by using FEM method The safety thicknesses of the blades and the nozzle guide

vanes will be estimated with respect to operating conditions From the 3D static

structural analyses, the guide-vane nozzle with trailing edge of 0.5mm presents stronger

and safer than shaping trailing-edge guide-vane nozzle For the turbine rotor, maximum

total deformation is located at outlet side close to tip of blades whereas the maximum

stress appears at the blade area near hub and close to the outlet side of the rotor Failure

occurs in case of 0.5mm blade thickness because the lowest safety factor is less than one

(only 0.758) The lowest safety factors from the other cases show that they are always

higher than one

Keywords: Ocean thermal energy conversion, Renewable energy, Solar heating,

Heat-exchanger optimization, Working fluid, Radial-inflow ORC turbine

- vii - LIST OF FIGURES Fig 1.1 Total net generation of electricity in the world during the last 30 years 2

Fig 1.2 Available temperature difference in the oceans (0-1000m) in degree Celsius 3 Fig 1.3 Diagram of open-cycle OTEC 4

Fig 1.4 Diagram of close-cycle OTEC 5

Fig 2.1 Schematic of SH-OTEC 11

Fig 2.2 T-s diagram of SH-OTEC 12

Fig 2.3 The calculation procedure for the present OTEC design using the software of TRNSYS 15

Fig 2.4 Volume flow rate of turbine inlet versus temperature difference Th between Twcol and TE with different working fluids 18

Fig 2.5 ORC thermal efficiency versus temperature difference Th between Twcol and TE with different working fluids 18

Fig 2.6 Pressure ratio versus ORC thermal efficiency among working fluids 19

Fig 2.7 The weather parameters 21

Fig 2.8 Hourly variations of SH-OTEC efficiency 23

Fig 2.9 Hourly variation of SH-OTEC efficiency for the average days of the month 23 Fig 2.10 Monthly average variation of SH-OTEC efficiency 24

Fig 2.11 Schematic Diagram of a regenerative SH-OTEC cycle 27

Fig 2.12 Temperature and entropy diagram of the regenerative SH-OTEC 27

Fig 2.13 First law efficiency as a function of pressure ratio for different working fluid 33

Fig 2.14 Thermal efficiencies of the cycles corresponding to various working fluids 35 Fig 2.15 Total heat transfer area of the cycles with respect to various working fluids 35 Fig 2.16 Variation of  w.r.t TIT 37

Fig 2.17 Irreversibility in the components of two cycles for 22 working fluids 39

- viii - Fig 2.18 Variation of total irreversibility w.r.t TIT of regenerative cycle for different working fluids 40

Fig 3.1 Configuration of SH-OTEC shell and tube evaporator 46

Fig 3.2 Configuration of SH-OTEC shell and tube condenser 48

Fig 3.3 Enhanced surface tubes 48

Fig 3.4 Reynolds number vs Number of tubes and tube passes 58

Fig 3.5 Heat transfer area vs Number of tubes and tube passes 60

Fig 3.6 Pressure drop vs Number of tubes and tube passes 61

Fig 3.7 Pareto front of evaporator 62

Fig 3.8 Pareto front of condenser 62

Fig 4.1 The layout of a 90o IFR turbine (a) and the triangle velocity diagrams (b) 70

Fig 4.2 Diagram of the volute and its cross-sections 72

Fig 4.3 Flow chart of radial-flow turbine calculation 75

Fig 4.5 The components of radial-inflow turbine 77

Fig 4.6 Complete assembled geometry of radial-inflow turbine 77

Fig 4.7 The rotor mesh 78

Fig 4.8 Mesh elements of turbine rotor at span 50 view 79

Fig 4.9 The velocity streamline passing through whole turbine domain 83

Fig 4.10 The Mach number contour 80

Fig 4.11 The pressure contour 85

Fig 12 The turbine efficiency and power with respect to number of guide vanes 86

Fig.13 Temperature distribution in the volute and rotor 87

Fig.5.1 Model of nozzle with 0.5mm trailing-edge guide vanes 94

Fig 5.2 Model of nozzle with sharp trailing-edge guide vanes 94

Fig 5.3 The models of rotor 95

Fig 5.4 Finite element mesh for the nozzle with shape trailing-edge guide vanes 96

- ix -

Fig 5.5 Finite element mesh for the nozzle with 0.5mm trailing-edge guide vanes 97

Fig 5.6 Computational mesh of the 90 degree inflow-turbine rotor 98

Fig 5.7 Grid independent investigation of the nozzles 100

Fig 5.8 Grid independent investigation of the rotor with 0.5mm blade thickness 100

Fig 5.9 Total deformation distribution of the nozzle with shape trailing-edge guide

Fig 5.13 Variation of maximum total deformation and von Mises stress in the nozzle

with shape trailing-edge guide vanes 105

Fig 5.14 Variation of maximum total deformation and von Mises stress in the nozzle

with 0.5mm trailing-edge guide vanes 105

Fig 5.15 Distribution of total deformation of rotor 107

Fig 5.16 Distribution of von Mises stress of rotor 109

Fig 5.17 Maximum total deformation and von Mises stress among the rotor models 109

Fig 5.18 Variation of maximum total deformation on operating conditions in the rotor

with 0.8mm blade thickness of OTEC turbine 110

Fig 5.19 Variation of maximum von Mises stress on operating conditions in the rotor

with 0.8mm blade thickness of OTEC turbine 111

Fig 5.20 Safety factor distribution of the OTEC rotor 112

- x - LIST OF TABLES Table 2.1 Initial data of the SH-ORC 16

Table 2.2 The parameters of system with different working fluids 17

Table 2.3 The required area of collector for Ulsan and Nha Trang 20

Table 2.4Input parameters for SH-OTEC 30

Table 2.5 Input parameters of heat exchangers 30

Table 2.6 The optimum values of fp 33

Table 3.1 Operating conditions of SH-OTEC 56

Table 3.2 Geometrical parameters of tubes 57

Table 3.3 The input parameters for multi-objective GA 63

Table 3.4 Some optimal points of evaporator 64

Table 3.5 Some optimal points of condenser 64

Table 4.1 The input variables and calculated geometry parameters of radial turbine 68 Table 5.1 The material properties of rotor and nozzle 98

- xi -

NOMENCLATURE

A : Area (m2)

A C : Heat transfer area of condenser (m2)

A E : Heat transfer area of evaporator (m2)

A t : Total heat transfer area of heat exchangers (m2)

B :inlet width of rotor (m)

C : Cost ($), absolute velocity (m/s)

H : Annual operating time (h/year)

h : Enthalpy (kJ/kg), heat transfer coefficient (W/m 2 K)

i : Interest rate (%)

I : Irreversibility (kW)

k : Thermal conductivity (W/m.K)

L : Length (m)

m : Mass flow rate (kg/s)

N : Number of fins, tube rows, angular velocity

n : Number of tubes, tube passes

Nu : Nusselt number

ny : Number of years

p : Pass

P : Pumping work (W) pressure (Pa)

P c : Pressure and critical pressure

Pr : Prandlt number

P T : Tube pitch (m)

Q : Heat transfer rate (W)

R : Root, radius

- xii -

Re : Reynolds number

R f0 : Fouling resistance of working fluid side

R fi : Fouling resistance of seawater side

 : Ratio of specific heat

 : Relative flow angle

I.H.E : Internal heat exchanger

IFR : Inflow-Radial Turbine

LMTD : Log mean temperature different

ORC : Organic Rankine Cycle

OTEC : Ocean Thermal Energy Conversion RMS : Root Mean Square

SH-OTEC : Solar-heating ocean thermal conversion

TIT : Turbine inlet temperature

- 1 -

Chapter 1:

Introduction

1.1 An overview of Ocean Thermal Energy Conversion (OTEC)

Total energy consumption in the world over the last three decades has increasing

sharply because of the population growth and development of economic Global

electricity generation has risen about 2.6 times within the last 30 years, as shown in Fig

1.1 (U.S EIA, 2014) As illustrated in this figure, approximately 67% of the world

electricity has been produced by using fossil fuels, whereas percentage of renewable

energy resources used has increased in recent years but it only takes up about 20% In

the same period, total CO2 emissions from the consumption of energy have gone up to

75% In near future, the world energy consumption will grow by 56% between 2010 and

2040 Renewable energy power is the world's fastest-growing energy source, increasing

2.5% per year However, fossil fuels continue to supply nearly 80% of world energy use

through 2040 Moreover, it shows that there is a constant growth rate for CO2 emissions

The worldwide energy-related carbon dioxide emissions are projected to rise to 45

billion metric tons (about 46%) in 2040 (U.S EIA, 2014) In company with the increase

of worldwide energy demand, fuel price is also rising because of the scarcity of natural

fossil fuel resources

OTEC utilizes the ocean thermal gradient between the warm surface water and the

cold deep sea water to drive an Organic Rankine Cycle (ORC), which transforms heat

into mechanical work that is used for the electricity production Therefore, OTEC is a

heat engine, which does not consume any fossil fuels to generate electricity The

amount solar energy absorbed by the oceans is equivalent to at least 4000 times the

amount presently consumed by humans (Vega, 2003) The resource for OTEC has better

- 2 -

distribution at tropical oceans Performance of OTEC is directly dependent on the

available temperature difference between surface and deep ocean waters As can be seen

in Fig 1.2, the contour of temperature difference in the oceans (0-1000m) is revealed

The tropical regions have great potential to exploit OTEC than the rest Meanwhile, the

area with the annual temperature difference exceeds 18°C takes up 136 million km2 or

37% of the total ocean surface (Nihous, 2007)

Based on an integration of cold seawater, OTEC systems are classified two main

types as closed-cycle OTEC and open-cycle OTEC Besides, the other type called a

hybrid cycle is also used A hybrid cycle combines the features of the closed- and

open-cycle systems

Open-cycle OTEC:

The diagram of an open-cycle OTEC is presented in Fig 1.3 Open-cycle OTEC uses

Fig 1.1 Total net generation of electricity in the world during the last 30

years (U.S EIA, 2014)

16000 18000 20000 22000 24000 26000 28000 30000

32000

Total electricity in the world Total fossil fuel electriccity Total nuclear electricity Total renewable electricity Total CO2 emissions

- 3 -

warm sea water directly to make electricity and water considered as the working fluid

The components of the OTEC system such as the evaporator, turbine, and condenser

operate at vacuum pressure A fraction of the warm seawater in form of vapor is

generated by flash evaporation at low pressure in evaporator The low pressure

expanding steam flows through a turbine to produce power The liquid water, which

condensed in condenser, must be compressed to pressures required to discharge it from

the OTEC system

Close-cycle OTEC:

Closed-cycle OTEC uses organic Rankine cycle with organic working fluid at a low

boiling point to generate the mechanical power in order to create electricity The

principle diagram of a closed-cycle OTEC is shown in Fig 1.4 The warm seawater

supplies the evaporator with heat source and the heat released from condenser is

discharged into the cold sea water In comparison with open-cycle OTEC, close-cycle

OTEC is easily integrated with other secondary heat sources as solar, waste heat from

industrial sources for improvement of OTEC efficiency and reduction of component

sizes of OTEC system

Fig 1.2 Available temperature difference in the oceans (0-1000m) in

degree Celsius (www.nasa.gov)

- 4 -

1.2 Problem statement, necessary, objectives and thesis layout

The world is facing the crises due to human activities such as unsolved problems of

global warming and climate change These problems might lead to energy crisis, global

environmental disaster So, using renewable energy for human energy demand also

contributes to economic and environmental benefits Developing an OTEC plant is the

urgent need for solving challenges of the serious environmental and the energy crisis at

this time In this study, we analyze and design a solar-heating OTEC (SH-OTEC) for

power generation Moreover, OTEC can be considered as a baseload power plant

because OTEC electricity is able to be supplied all day and night (24/7) and whole year

The other OTEC benefit is to have aptitude co-generating the secondary products such

as desalination, air-conditioning

Fig 1.3 Diagram of open-cycle OTEC

~Turbine

Steamgenerator

OutVacuum pumpGenerator

- 5 -

The objectives of this research are listed as follows:

- Analyze and investigate effects of solar radiation on thermal efficiencies of OTEC

The solar radiation considered as the secondary heat source is used to increase the warm

seawater temperature in order to improve thermal efficiency of OTEC system Local

solar radiation data including two different regions such as temperate region (Ulsan,

Koean) and tropical zone (Nha Trang, Vietnam) are chosen for simulations

- Conduct extensive analyses and comparisons among different working fluids with

respect to selection criteria Based on these criteria as the thermodynamic and heat

transfer properties, pressure ratio, environmental impacts, volume flow rate and safety,

the best working fluid will be selected for SH-OTEC plant

- Different forms of thermodynamic cycles had applied for organic Rankine cycle

which is a Rankine cycle using an organic working fluid In this study, two kinds of

Fig 1.4 Diagram of close-cycle OTEC

~

Warm sea water

Evaporator

Warm-sea-water pump

Cold sea water

Condenser

Turbine

Generator

Working fluid pump

water pump

Cold-sea -

- 6 -

thermodynamic cycles such as the basic cycle and the regenerative cycle are adopted to

estimate a suitable one for SH-OTEC due to these cycles are much less complex and

need less maintenance than the others

- Analyze exergy to estimate irreversibility of the SH-OTEC components as turbine,

heat exchangers and pump

- The capital cost of an OTEC plant is proportional to the size of the plant The main

duty for the design of an OTEC plant is minimum cost So, this work optimizes the

condenser and evaporator of SH-OTEC by using the multi-objective optimization

method with Genetic Algorithm (GA) Two kinds of objective functions of both heat

exchangers are considered: capital cost stands for heat transfer area and operating cost

stands for pressure drop Two design variables for optimal problem selected are tubes

and passes, respectively

- Estimate sizes of ORC turbine as the rotor, nozzle and volute for OTEC system by

performing preliminary design

- Investigate the flow characteristics of working fluid in the turbine using CFD

method in ANSYS CFX

- Carry out the structural analysis of solid part of rotor and nozzle with FEM method

in order to evaluate the failure of the models

The structure of dissertation are organized as follows:

Chapter 2 starts with a literature review of OTEC technology at the current

state-of-the-art In the second part of this chapter presents the effects of solar radiation

and some working fluids on efficiency of OTEC system In the last part, the analytical

results of thermodynamic cycles, heat transfer properties among working fluids and

irreversibility of the components and system are illustrated and discussed

Chapter 3 introduces the non-gradient optimization method using GA for

optimal design of heat exchangers In this chapter, multi-objective GA is proposed for

- 7 -

optimal design of condenser and evaporator of OTEC Simultaneous optimization of

both cost functions of capital cost and operating cost is presented Besides, effects of the

geometry of heat exchangers on cost functions are also revealed

Chapter 4 describes the process of preliminary design of ORC turbine with high

performance applying for OTEC application The dimensions of turbine obtained from

this step are reported Then the CAD model of the turbine built for CFD simulation in

order to estimate the flow characteristics in the turbine Depending of turbine efficiency

on variation of number of guide-vanes is presented in this chapter

Chapter 5 focuses on the structural analyses of solid parts of the turbine

including the rotor and the nozzle Using FEM method in ANSYS, von Mises stress and

deformation of solid parts of the turbine are shown in this section Besides, safety

design is discussed through report of the failure of turbine material among the models

Chapter 6 is the last chapter and overall conclusions are drawn regarding the

works illustrated in this dissertation It also shows the recommendations from the study

and future works

-8-

Chapter 2:

Investigation of effects of solar radiation,

working fluid properties and

thermodynamic cycles on OTEC

2.1 OTEC history and application

Since Jacques Arsene D'Arsenoval, a French physicist, proposed the OTEC concept in

1881 and George Laude first demonstrated in Matanzas, Cuba in 1930, produced 22kW

of electricity There are many efforts to construct OTEC plants from the original idea of D'Arsenoval The first Mini-OTEC plant used a closed-cycle OTEC was built in Hawaii, USA in 1979 which generated more than 50kW of gross power with the net output of 18kW Another demonstration OTEC plant of 100kW gross power built and tested in Nauru Island in 1981 by Japanese researchers Besides, India tested 1MW floating OTEC pilot plant near Tamil Nadu in 2002 Saga University together with various Japanese industries installed and was testing a new OTEC plant at Kume Island in 2013 The largest OTEC facility of 16MW gross 100MW net offshore plant is planning to design and construct by DCNS group partnered with Akuo Energy in 2014 (Wikipedia.com) There has been increasing interest in developing OTEC technology both theoretical and

empirical over past few years There are about 50 countries to examine implementation

of OTEC plant as a sustainable source of energy and fresh water, including India, Korea,

Palau, U.S.A, Philippine and Papua New Guinea (Ahmadi et al., 2013; Meegahapola et

al., 2007) A closed cycle demonstration OTEC plant with R134a as a working fluid was

-9-

designed and constructed by Faizal and Ahmed (2013) They found that the thermal

efficiency of system and work done by turbine increased with increasing the operating

temperature difference The maximum efficiency of the system was achieved about 1.5%

Tahara et al (1995) evaluated the energy balance and CO2 reduction potential of OTEC

They found that the CO2 emission decreased by about 140,000 t-C/year for a 100MW

system of OTEC The CO2 emission with the coal fired power plant is 53 times higher

than that with the OTEC system Straatman and Van Sart (2008) described the concept

of a unique OTEC system combined with an offshore solar pond which is called hybrid system (OTEC-OSP) Yeh et al (2005) studied the effects of the temperature and flow rate of cold seawater on the net efficiency of an OTEC system They also considered the

other parameters including the dimensions of pipes and the seawater flow rate A certain

flow rate of cooling seawater showed the available maximum work output The higher

the warm seawater flow rate is, the larger the work output is Sun et al (2012) carried out

the theoretical optimization to maximize the net power output of ORC in OTEC with

working fluids as R717 and R134a Ammonia stands for a good working fluid from net

power output point of view Yang & Yeh (2014) performed theoretical study to determine

optimal operating parameters for maximal objective parameters in an OTEC system for

5 working fluids Ahmadi et al (2013) and Kazim (2005) proposed a hydrogen production

cycle using electricity by OTEC Uehara et al (1996) and Ikegamia et al (2006) used an

integrated hybrid cycle and an open OTEC cycle for desalination

In spite of a large potential of power production by OTEC, the low thermal efficiency

of OTEC system due to the small temperature difference between the surface water and the cold seawater prevents OTEC from becoming a commercial large-scale technology (Rajagopalan and Nihous, 2013) The net efficiency of a practical OTEC system is approximately 1% to 2% (Berger and Berger, 1986) To improve thermal efficiency of OTEC, the secondary heat source as solar energy or waste heat was also used to increase

the warm seawater temperature Yamada et al (2009) carried out a performance

-10-

simulation of a solar boosted ocean thermal energy conversion (SOTEC) in Japan The

solar collector was used to heat the temperature of warm seawater in order to increase

overall thermal efficiency of SOTEC Tong et al (2008) proposed a solar energy reheated

power cycle The solar collector introduced into OTEC system, which will greatly

improve the temperature difference and the cycle performance as well To be applied in

industry, the net power output of the system should be at least 50kW Kim et al (2009)

used the condenser effluent from a nuclear power plant to improve the thermal efficiency

of OTEC They built the computer simulation programs for some thermodynamic cycles

with various working fluids The results showed that the system efficiency increased

approximately 2% for using condenser effluent from a nuclear power plant and the

effective temperature difference between the surface and deep seawater for electricity

generation is higher than 15oC Nguyen and Lee (2013) evaluated heating for OTEC by

using solar energy (SH-OTEC) in South Korea Most of the above studies were

concentrated on improving the efficiency of OTEC

From an engineering point of view, OTEC is considered as a heat engine without

fuel consumption Energy resource supplying for operating OTEC is free and unlimited

Besides, thermal efficiency of OTEC system is not as high as the performance of

traditional power plant Therefore, requirements for the present OTEC technology are to

improve the overall cost effectiveness including increase of thermal efficiency of OTEC

system as well as it components and economical design of heat exchangers, turbine, pipe

and so on

2.2 Effects of solar radiation on solar-heating OTEC (SH-OTEC)

In order to improve the performance of system, the secondary heat sources are

added by means of solar radiation, waste heat from factories and so on Besides, the

selection of a suitable working fluid for system is also a way to enhance the net efficiency

Most of the above mentioned studies have focused on discussing the organic Rankine

-11-

cycle They fixed the parameters of the added heat In this section, we carry out the

simulations to determine the effects of weather conditions on the efficiency of a

solar-heating ocean thermal conversion system by using TRNSYS software ver.16 and EES

software ver.6.8 Besides, one more research is needed to select a most suitable working

fluid for SH-OTEC

2.2.1 Principle and mathematical model of SH-OTEC

Fig 2.1 Schematic of SH-OTEC

~

Warm sea water

Evaporator

water pump Solar collector

Warm-sea-Cold sea water

Condenser

Turbine

Generator

Working fluid pump

water pump

5 6

8 7

-12-

Fig 2.2 T-s diagram of SH-OTEC

The layout of the basic components of a SH-OTEC system is sketched in Fig 2.1 In

this figure, the SH-OTEC system consists of a turbine, an evaporator, a condenser, and a

collector The liquid working fluid in the evaporator is heated and evaporated to be vapor

state by the warm seawater from the collector system The vapor flows into the turbine

and its energy is converted into work The low pressure flow exiting from the turbine is

condensed to be liquid state in the condenser The waste heat of condensing process is

received by the cold seawater The pump supplies the working fluid to the evaporator and

makes it possible to perform a thermodynamic cycle of Rankine cycle

The above described thermodynamic processes and the corresponding

temperature-entropy (T-s) diagram are shown in Fig 2.2 Here, TE, Tc and Twcol are the evaporating,

condensing temperature and the seawater temperature leaving the solar collector,

respectively Twi, Two and Tci, Tco are the inlet temperature and the outlet temperature of

the warm seawater and the inlet temperature and the outlet temperature of the cold

seawater, respectively QE and QC are the heat rate at the evaporator and the condenser,

78

65

1-2: Expansion2-3: Condensation3-4: Pumping

4-1: Preheating and Vaporization5-6: Cold seawater heating7-8: Warm seawater cooling

-13-

respectively The Rankine cycle is assumed a saturated cycle The thermal duties of each

component of SH-OTEC can be expressed as follows

The power produced by the turbine is

 1  t G

f

t m h h η η

where t, Gand m f are the turbine and generator efficiencies and the mass flow rate

of working fluid, respectively

The heat flow rate of the condenser, Q defined as C

E cw

Δt c

fp η

h h m

cwp

cw cw

η

H.g m

where W fp, W are the required powers of pump for the working fluid and the cold cw

seawater, respectively; g is the acceleration due to gravity and H is total pressure difference; fp, cwp are pump efficiency of the working fluid and cold seawater, respectively Tong et al (2008) suggested that the required power of the warm seawater

I A

Q η

a i L R n R sc

I

T T U F I

T T U F τα F

(2.8)

where

F R Overall collector heat removal factor

U L Collector overall heat loss coefficient

U L/T Thermal loss coefficient dependency on T

I T Solar radiation

() Transmittance-absorptance product

T i Fluid inlet temperature

T a Ambient air temperature

𝑄̇ Useful heat transfer rate of the collector

C

A Aperture area of the collector

The Rankine cycle efficiency Rand the net cycle efficiency net are represented as follows:

-15-

E

fp t

R

Q

w w

E

cw fp

t net

Q

w w

Input: The type of solar collector, "Type 1b"

The solar collector area The mass flow rate of warm sea water The latitude and solar radiation of Ulsan

Calculate Twcol

Assume Qcol =QECalculate R from Eq.2.9 and net from Eq.2.10

under the calculated values of wt, wfp, wcw, and

mf from the given working fluids (Table 2.2)

END START

-16-

2.2.2 Given data

Table 2.1 Initial data of the SH-ORC

Turbine generator power (W t) kW 1 Evaporating temperature (T E)

Condensing temperature (T ) C

Cold seawater temperature (T ) wi

Warm seawater temperature (T ) ci

The operating conditions of the Organic Rankine Cycle (ORC) are shown in Table 2.1

Firstly, to estimate the effects of working fluids on SH-OTEC, the system is assumed that

the evaporating temperature is heated by the warm seawater from collector array and

reaches 350C The thermodynamic properties of fluids and the performance of the ORC

are simulated by using the simulation tool EES (Engineering Equation Solver) (Klein,

2013) Secondly, the procedure, as shown in Fig 2.3 is used to determine the effects of

weather conditions on the efficiency of a SH-OTEC system The flat plate solar collector which called ‘Type 1b’ in TRNSYS software also used for heating the warm seawater before flowing into the evaporator

2.2.3 Results and discussion

2.2.3.1 Effect of working fluids

The characteristic of 13 working fluids are investigated for a 1kW power system The

results are revealed in Table 2.2 As can be seen in Table 2.2, the saturated pressure in

the evaporator is observed as an important operating criterion The working fluids of

isopentane, ethanol and water have the low pressure The minimum pressure value is only

5.63kPa with water In contrast to water, R125 has the highest pressure followed by R22

and NH3 The group of good evaporating pressure is R600, R600A, R152A, R134A and

isobutene

Fig 2.4 shows the dependence of volume flow rate of the turbine inlet on T h with different working fluids R125, NH3, R22 and R152A have small volume flow rate which

-18-

are suitable for economic criterion From Table 2.2, among 13 working fluids, the group

of high volume flow rate such as water, ethanol and isopentane is the worst choice for

SH-OTEC

Fig 2.4 Volume flow rate of turbine inlet versus temperature difference Th between

Twcol and TE with different working fluids

Fig 2.5 ORC thermal efficiency versus temperature difference Th between Twcol and

TE with different working fluids

-19-

Fig 2.6 Pressure ratio versus ORC thermal efficiency among working fluids

Fig 2.5 illustrates the effect of the variation of temperature difference Th on ORC thermal efficiency The organic Rankine cycle efficiency increases with increasing solar

collector outlet temperature when the condensing temperature TC is kept constant The

working fluids with high thermal efficiency increase are water followed by ethanol and

NH3, respectively On the other hand, R290 has the smallest increase among all working

fluids

Fig 2.6 shows the effects of pressure ratio on the ORC performance among 13 working

fluids As shown in this figure, the group of high efficiency such as water, ethanol and

ammonia also have high pressure ratio, the corresponding values are 4.58, 4.38 and 2.2

respectively Ammonia is the best working fluid in this case However, it is not selected

because of its high evaporating pressure and odorous and toxic properties R125 is the

worst working fluid with efficiency of 4.8% while pressure ratio is about 2 The potential

group of working fluid for OTEC is R600, isopentane, R152A and R134A respectively