HIGH THEMAL ENERGY STORAGE DENSITY MOLTEN SALTS FOR PARABOLIC

TROUGH SOLAR POWER GENERATION

by

TAO WANG

RAMANA G. REDDY, COMMITTEE CHAIR

NITIN CHOPRA

YANG-KI HONG

A THESIS

Submitted in partial fulfillment of the requirements

for the degree of Master of Science

in the Department of Metallurgical and Materials Engineering

in the Graduate School of

The University of Alabama

TUSCALOOSA, ALABAMA

2011

Copyright Tao Wang 2011

ALL RIGHTS RESERVED

ii

ABSTRACT

New alkali nitrate-nitrite systems were developed by using thermodynamic modeling and

the eutectic points were predicted based on the change of Gibbs energy of fusion. Those systems

with melting point lower than 130oC were selected for further analysis. The new compounds

were synthesized and the melting point and heat capacity were determined using Differential

Scanning Calorimetry (DSC). The experimentally determined melting points agree well with the

predicted results of modeling. It was found that the lithium nitrate amount and heating rate have

significant effects on the melting point value and the endothermic peaks. Heat capacity data as a

function of temperature are fit to polynomial equation and thermodynamic properties like

enthalpies, entropies and Gibbs energies of the systems as function of temperature are

subsequently induced. The densities for the selected systems were experimentally determined

and found in a very close range due to the similar composition. In liquid state, the density values

decrease linearly as temperature increases with small slope. Moreover, addition of lithium nitrate

generally decreases the density. On the basis of density, heat capacity and the melting point,

thermal energy storage was calculated. Among all the new molten salt systems, LiNO3-NaNO3-

KNO3-Mg(NO3)2-MgKN quinary system presents the largest thermal energy storage density as

well as the gravimetric density values. Compared to the KNO3-NaNO3 binary solar salt, all the

new molten salts present larger thermal energy storage as well as the gravimetric storage density

values, which indicate the better thermal energy storage capacity for solar power generation

systems.

iii

DEDICATION

This thesis is dedicated to everyone who helped me and guided me through the trials and

tribulations of creating this manuscript. In particular, my family and close friends who stood by

me throughout the time taken to complete this masterpiece.

iv

ACKNOWLEDGEMENTS

I am pleased to express my gratitude and appreciation to my advisor, Professor Ramana

G. Reddy, for his patience and guidance during my graduate study and the entire research work. I

am greatly benefited from his experience, knowledge and enthusiasm for scientific research.

I would like to express my sincere thanks to Dr. Nitin Chopra and Dr. Yang-Ki Hong for

serving on my committee. Their valuable suggestions and comments are very insightful for my

research work.

I would like to thank all the research colleagues of Dr. Reddy‟s research group, special

thanks to Dr. Divakar Mantha for his valuable suggestions and comments. I would like to extend

my gratitude to U.S Department of Energy for the financial support.

Finally, I would like to thank my parents and my fiancée, whose invaluable

understanding and loving support helped me through the difficult times.

v

TABLE OF CONTENTS

ABSTRACT ii

DEDICATION iii

ACKNOWLEDGEMENTS iv

LIST OF TABLES viii

LIST OF FIGURES x

CHAPTER 1. INTRODUCTION 1

CHAPTER 2. LITERATURE REVIEW 11

2.1 Melting point 11

2.2 Density 15

2.3 Heat capacity 18

CHAPTER 3. OBJECTIVES 22

CHAPTER 4. THERMODYNAMIC MODELING OF SALT SYSTEMS 24

4.1 Thermodynamic modeling 24

4.2 Calculations 27

vi

CHAPTER 5. EXPERIMENTAL PROCEDURE 30

5.1 Melting point determination of molten salt mixtures 30

5.1.1 Materials 30

5.1.2 Apparatus and Procedure 30

5.2 Heat Capacity determination of molten salt mixtures 32

5.3 Density determination of molten salt mixtures 33

CHAPTER 6. RESULT AND DISCUSSION 34

6.1 Melting point determination 34

6.1.1 DSC equipment calibration 34

6.1.2 Results 35

6.1.3. Discussion 41

6.2 Heat capacity determination 51

6.2.1 Heat capacity calibration 51

6.2.2 Results 52

6.2.3 Thermodynamic properties 55

6.2.4 Discussion of Gibbs energy change for molten salts 84

6.3 Density determination 86

vii

6.3.1 Density calibration 86

6.3.2 Results and discussions 82

6.4 Thermal energy storage density of molten salts 90

CHAPTER 7. CONCLUSION 94

REFERENCES 96

APPENDIX 104

APPENDIX A 104

APPENDIX B 109

APPENDIX C 114

APPENDIX D 118

APPENDIX E 123

APPENDIX F 128

APPENDIX G 133

APPENDIX H 138

APPENDIX I 143

viii

LIST OF TABLES

2.1. Melting point of various nitrate salt systems 12

2.2. Melting point of various carbonate salt systems 13

2.3 Melting point of various fluoride/chloride salt systems 14

2.4 Melting point of various hydroixde salt systems 15

2.5 Density coefficients A and B of nitrate salts 16

2.6 Density coefficients A and B of carbonate salts 17

2.7 Density coefficients A and B of chloride/fluoride salts 17

2.8 Density coefficients A and B of molten salt mixture with hydroxide salts 18

2.9 Heat capacity of alkali nitrate salt at 500oC 19

2.10 Heat capacity of alkali carbonate salt at 500oC 19

2.11 Heat capacity of fluoride/chloride salt at 500oC 20

2.12 Heat capacity of hydroxide salt at 500oC 21

4.1 Calculated composition and melting point for multi-component systems 29

6.1 Calibration data of melting points with different samples 35

6.2 DSC results of melting point, transition point and change of enthalpy 41

ix

6.3 Fusion and solid phase transition temperature for individual salts 42

6.4. Melting points of candidate systems as function of temperatures 51

6.5 Calibration data of heat capacities with different samples 52

6.6 Heat capacity of selected new TES molten salt mixtures 54

6.7 Change of Gibbs energy values at 623.15K for molten salt systems 85

6.8 Calibration of density measurements with different pure nitrate salts 86

6.9 Coefficient of A and B for density determination of salt #1 to salt # 9 87

6.10 Extrapolated value of density and heat capacity at 500oC of salt #1 to salt #9 91

6.11 Energy density of salt #1 to salt #9 compare to solar salt 92

6.12 Gravimetric storage densities for solar salt and new molten salts 93

x

LIST OF FIGURES

1.1 Theoretical and engineering energy conversion efficiency as function of temperature 6

1.2 Gravimetric storage density for different energy storage systems

as function of temperature 8

5.1 Photography of set-up for DSC equipment 31

6.1 Melting point calibration with indium sample 34

6.2 Melting point calibration with KNO3 sample 35

6.3 DSC endothermic peaks of LiNO3-NaNO3-KNO3 salt. 36

6.4 DSC endothermic peaks of NaNO3-NaNO2-KNO3 salt. 37

6.5 DSC endothermic peaks of LiNO3-NaNO3-KNO3-MgK salt. 37

6.6 DSC endothermic peaks of LiNO3-NaNO3-KNO3-NaNO2 salt. 38

6.7 DSC endothermic peaks of LiNO3-NaNO3-NaNO2-KNO3-KNO2 salt. 38

6.8 DSC endothermic peaks of LiNO3-NaNO3-KNO3-KNO2 salt. 39

6.9 DSC endothermic peaks of LiNO3-KNO3-NaNO2-KNO2 salt. 39

6.10 DSC endothermic peaks of LiNO3-KNO3-NaNO2-Mg(NO3)2 salt. 40

6.11 DSC endothermic peaks of LiNO3-NaNO3-KNO3-Mg(NO3)2-MgKN Salt. 40

6.12 DSC plot of 69.8wt% KNO3 -30.2wt% NaNO2 binary system 43

xi

6.13 DSC plot of 27.0wt% NaNO3-73.0wt% KNO3 binary system 45

6.14 DSC plot of 45.8wt%LiNO3-54.2wt%KNO3 binary system 45

6.15 DSC plot of 46.0wt% NaNO3-54.0wt% KNO3 binary system 46

6.16(a) DSC endothermic peaks of LiNO3-NaNO3-KNO3-KNO2 salt

for 20oC/min heating rate. 47

6.16(b) DSC endothermic peaks of LiNO3-NaNO3-KNO3-KNO2 salt

for 5oC/min heating rate. 48

6.17(a) DSC endothermic peaks of LiNO3-KNO3-NaNO2-KNO2 salt

for 5oC/min heating rate. 49

6.17(b) DSC endothermic peaks of LiNO3-KNO3-NaNO2-KNO2 salt

for 20oC/min heating rate. 49

6.18 Heat capacity data plot of LiNO3-NaNO3-KNO3 ternary system

as function of temperature 53

6.19 Heat capacity of LiNO3-NaNO3-KNO3 in liquid state from 403.15-623.15K 54

6.20 Change of Gibbs energy as function of temperature for molten salt systems 85

6.21 The densities of the salt #1 to salt #5 as function of temperature 87

6.22 The densities of the salt #6 to salt #9 as function of temperature 89

6.23 Densities of the salt #1, salt #2 as function of temperature compared to

the equimolar NaNO3-KNO3 binary system and pure KNO3. 90

6.24 Gravimetric storage density comparison of different energy storage

xii

systems as function of temperature 93

1

CHAPTER 1

INTRODUCTION

Renewable energy sources such as wind, solar, water power, geothermal and biomass are

playing more and more significant role in our energy supply. Because the cheap cost and infinite

amount of energy storage inside the resource, solar energy is emphasized since 20th

century and

viewed as promising alternative method to satisfy the large energy consumption every day in the

world, reduce the emission of carbon and strengthen the economy.

The wind energy was used as a clean energy to generate electricity back to late 19th

century.

However, this renewable energy source was not emphasized due to the cheap price of fossil fuel.

The re-emergence happened in mid 1950s when the amount of traditional energy source was

found apparently decrease. The development of wind energy usage continued and in 1990 the

first mega-watt wind turbine was launched, which was viewed as a symbol of shift to large scale

wind energy utilization [1-2]. The modern application of wind energy mainly relies on wind

turbine. On the basis of aerodynamic, wind turbine generates certain net positive torque on

rotating shaft and then converts the mechanical power to electrical power. As an electrical power

generator, wind turbine is connected to some electrical network to transport the electricity to

battery charging utilities, residential power systems and large scale energy consuming systems.

In general, most of wind turbines are small scale and can only generate 10KW electrical power.

Only few of the wind turbine systems operate with capacity as large as 5MW. Although the

usage of wind energy can reduce the emission of carbon oxide, the noise pollution and high cost

2

limit its large scale application. Since the wind is not transportable, the electrical energy can only

be generated where the wind blows, which also decrease the flexibility of wind energy.

Water power is another term of alternative power supply and it was used for irrigation,

operating machines like watermill even before the development of electrical power. The modern

application of water power is to generate electricity by using the gravitational force of falling or

flowing water. These days, there are various ways for the water power application. The most

traditional method is to store the water in dam and generate electricity by converting the

potential energy; pump storage is a different way to utilize water power and can change its

output depending on the energy demand by moving water between reservoirs at different

elevations. In the low energy demand period, excess energy is used to lift water to the higher

level, while in the peak period of energy demand, the water is released back to the lower

elevation through water turbine. Water power can also be converted by taking advantage of

naturally raise and fall of tide to satisfy the demand of electrical energy consumption [3].

Although the usage of water power can reduce the emission of carbon dioxide and cost, it will

destroy the ecosystem because of the large land required for construction. There will be methane

emission from the reservoir; the potential failure hazard of dam is also a fatal issue and flow

shortage caused by drought may also create serious problem. As result of that, water power

technique is not a long-term alternative choice.

Geothermal energy is the energy form generated inside the earth. At the very beginning of

the planet formation, a large amount of thermal energy was stored from the radioactive decay of

minerals, volcanic activity and solar energy absorption. Because of the temperature difference

3

between the core and the surface of planet, the thermal energy stored inside the earth is driven to

the outer surface in the form of heat. This form of renewable energy source can be applied to

generate electrical power and heat for industrial, space and agricultural applications.

Theoretically, the deposited amount of geothermal energy is adequate to supply the energy

consumption in the world. However, most of the geothermal energy is stored deeply near the

core of the earth, the deep drilling and exploration of geothermal energy is very expensive and

limits the large-scale use of this renewable energy source [4].

Biomass is a renewable energy source used to generate heat or electricity with living or

recently living organism such as wood, waste, (hydrogen) gas and alcohol fuels. The biomass

energy can be converted to electrical energy by thermal method such as combustion, pyrolysis,

and gasification. Several specific chemical processes may also be able to convert the biomass

energy to other forms. The main problem arise from application of biomass is air pollution which

contains carbon monoxide, NOx (nitrogen oxides), VOCs (volatile organic compounds),

particulates and other pollutants. And the level of air pollution, to some extent, is even above that

of traditional fuel resource. Some other possible issue like transportation and sink of carbon also

limit the wide usage of this type of alternative energy [5].

Among all the renewable energy sources, solar energy is the most suitable alternative

energy for our future life. It is clean, cheap, abundant, without any noise, air pollution, no

transportation issue and easy to be obtained anywhere in the earth. Inside the core of the Sun,

hydrogen fuses into helium with a rate of 7×1011

kg/s and generates very powerful nucleation

power. This type of nucleation explosion creates ultra high temperature in the core of the Sun,

4

which reaches approximately 16 million K degrees. Although the Sun is not perfectly black body,

it still radiates abundant power with the energy density as 1.6×107 watts/m

2 [6-7]. Because of the

enough amount of hydrogen underneath the surface of the Sun, the radiation given arise of from

the nucleation explosion can continue at least 5 million years with the same rate and strength.

The energy reaching the earth is vastly reduced mainly caused by the absorption and spreading

of the radiation. It is easily to understand that there are numerous amorphous objects all around

the entire universe which can absorb certain portion of the radiation for the Sun. Moreover, the

light generated from the spherical object such as the Sun fills all the available space between the

origin to the destination. Even though the energy will not be lost in the travelling process, due to

the long distance between the Sun to the earth, the surface area of the sphere which is formed

with the Sun as center and the distance as radius is much larger than that of the earth. As the

result of that, only 1340W/m2 finally reaches the upmost surface of the earth. Even though the

final amount of the received solar energy is very small compared to that is initially radiated from

the Sun, the average daily solar radiation falling on one area in the continental United States is

equivalent in total energy content to 11 barrels of oil. In summary, the solar energy itself is

relatively unlimited, useful, clean and almost unexploited energy and definitely can behave as

the promising mean for the future energy supply [8].

There are several different methods to take advantage of the solar energy and all the

methods can be distinguished into three group: solar parabolic trough, solar tower and solar dish.

Parabolic trough is constructed by silver coated parabolic mirror and there is a Dewar tube going

through the length of the mirror and set on the focal point, all the radiation is concentrated on the

tube and transfer by heat transfer fluid to the thermal energy storage unit. Solar tower are used to

5

capture solar energy with thousands of mirrors and focus the concentrated sunlight to the top of

the tower which is located in the middle of the heliostats. The thermal energy storage medium

within the tower was heated to high temperature and transferred to thermal energy storage tank

and eventually sent to steam pump. The solar dish is built with a large, reflective parabolic dish

which concentrates all the received sunlight to one spot. There is normally a receiver located on

the focal point and transform the solar energy to other forms of useful energy. The working

upper limit temperature of solar parabolic trough system is the lowest among these three systems,

normally its maximum working temperature is within the range from 400-500oC; the solar tower

has higher maximum working temperature which ranges from 500-1000oC; the solar dish has the

highest working upper limit temperature which reaches 700-1200oC [9].

The energy conversion efficiency is the most concerned parameter in the solar energy

storage application and the theoretical and real engineering efficiency are given in fig 1.1 as

function of temperature. The theoretical conversion efficiency can be up to 80% while in real

application, the value is always less than 70% regardless of collectors. The actual efficiency

increases with temperature in the whole working temperature. As a result of that, the thermal

energy storage materials in solar parabolic trough, for instance, should be able to work stably at

the upper limit temperature of this type of collection system which is 500oC to ensure the highest

efficiency [9, 10].

6

Fig 1.1 Theoretical and engineering energy conversion efficiency as function of

temperature

Solar energy can be stored in three major forms: sensible heat, latent heat and

thermochemical heat. Sensible heat storage was utilized based on the heat capacity and the

change as function of temperature of storage materials in the charging and discharging process

which correspond to the absorbing and withdrawing energy processes, respectively. The sensible

heat stored from the melting point to the maximum working temperature can be expressed by

equation 1 [9].

[1]

Where m is the mass of storage material, Tmp and TH are melting point temperature and high

temperature in the same phase, respectively, Cp(T) is the heat capacity at different temperature.

Because the sensible heat storage materials remain in a single phase in the working temperature

range, the charging and discharging processes are completely reversible for unlimited cycles.

7

Latent heat storage is operated by absorbing and withdrawing energy in the charging and

discharging processes accompanied with fusion of materials [9]. The latent heat collected

throughout the working temperature range can be expressed by equation 2 as following:

[2]

Where T is temperature in solid state, Tmp is melting point temperature of storage material, TH is

the high temperature in liquid state and is enthalpy of fusion.

Thermochemical heat storage is based on the heat capacity and its change as function of

temperature accompanied with chemical reaction. The thermochemical heat collected throughout

the working temperature range can be expressed by equation 3.

[3]

Where TL is the low temperature before the reaction, TR is the reaction temperature and

is the enthalpy of chemical reaction. Because of the destruction of the chemical bonds

in the reaction process, the charging and discharging process cannot be completely reversible,

which reduces the stability and recyclability of storage operation [10].

Sensible energy storage method is chosen to ensure the efficient usage of solar energy for

parabolic trough system of which the maximum working temperature ranges from 400-500oC.

Different from thermochemical heat storage, the sensible heat storage can achieve completely

reversible working condition under unlimited cycles. Also, fig 1.2 illustrates that the sensible

heat storage materials mainly work in the working temperature range for parabolic trough system,

8

and the gravimetric energy storage densities of sensible heat is higher than that of latent heat

materials [9 -11].

Fig 1.2 Gravimetric storage density for different energy storage systems as function of

temperature

Various materials are chosen to serve as thermal energy storage fluid for sensible heat

such as water, thermal oil, ionic liquid and molten salt [12]. The properties of different heat

transfer fluid determine the performance of solar energy heating system. In these days, the

efficiency and cost of output of electrical power mainly relies on the parabolic trough solar

power plant and the thermal storage fluid [12]. A large investment cost is needed to dispatch

100MW to 200MW energy by consuming the energy transfer fluids. Given by this situation, the

development of new thermal storage fluid with higher thermal energy storage density is

paramount to lower the expense for generating energy and a lot of effect has been put on design

of new systems [13-16].

9

Water is commonly used as heat transfer and thermal energy storage fluid in industry

because of its low cost, high heat capacity and high thermal conductivity. However, the

limitation for using this medium is also obvious that the temperature range within which the

liquid state can be assured is too small. It is well know that, water can only serve as thermal

energy storage liquid above the freezing point 0oC and below the boiling temperature 100

oC. In

practical experiment, the actual temperature range is even less than 100oC because of the large

amount of weight loss near the boiling temperature due to the high vapor pressure. Water is

possible to work above 100oC only if high pressure is also applied to avoid the phase

transformation, but the cost will be highly increased. Accordingly, water is only suitable to work

in low temperature below 100oC.

Thermal oils are also being used in the parabolic trough solar power plant and have very

low melting point as low as 12oC [17, 18]. However, the application of the oil for the thermal

energy storage liquid is limited by some disadvantages from the physic-chemical properties. The

upper limit for this oil is only 300oC and above that the liquid state cannot be maintained.

Moreover, the low thermal decomposition temperature, low density and low heat capacity result

in limited thermal energy storage capacity. Since the working temperature range is so narrow, the

rankie cycle efficiency is reduced when using the synthetic oil and the cost for generating power

is considered to be very expensive [19, 20].

Ionic liquid is another medium served as thermal energy storage fluid. The liquid

temperature range of ionic liquid is large, which is one of the main advantages of this type of

material. The high heat capacity and density ensure the efficiency of thermal energy storage of

10

ionic liquid. What‟s more, the excellent chemical stability and little vapor pressure increase the

lifetime [21-24]. However, as a result of the very serve corrosion problem to the liquid container

and the high cost, the usage of ionic liquid is still limited.

Considering various relative physic-chemical properties of thermal energy storage system,

molten salts have been proposed as a suitable group for a wide temperature range application.

They are being emphasized in the solar energy applications because of their low melting point

and high upper limit which can increase the stable working range. The high heat capacity

increases the thermal energy storage density of the heat storage system; excellent thermal

stability and negligible vapor pressure ensure the steadiness of cyclic repeating in the lifetime

[25]; low viscosity strengths the mobility and efficiency of the thermal storage liquid; low

utilization cost reduce the investment and protect the economy. The liquidus temperature range

of the molten salt varies from 150-600oC, combination of various salts can bring the melting

down and further increase the working temperature range. Due to these properties, molten salts

can be excellent thermal energy storage fluid in the solar power generation system.

11

CHAPTER 2

LITERATURE REVIEW

Several physical and thermodynamic properties of thermal energy storage fluid play

significant role in determining the efficiency and performance of solar energy storage systems. In

order to evaluate the feasibility of systems, the physic-chemical properties of several molten salts

should be reviewed. The three determining parameter which directly affect the thermal energy

storage capacity in systems are melting point, heat capacity and density.

There are large amount of melting point data available in the literature for various molten

salt system in previous literatures while those with melting point less than 120oC is very limited.

All the previous study on molten salt system revealed that five group of molten salts are

emphasized and commonly used: alkai or alkaline nitrates, carbonates, sulphates, chloride and

hydroxides. Although most of the systems have the same group of cation, the melting point

varies a lot from one to anther due to the different effect of anions.

2.1 Melting point

The melting points of individual and multi-component nitrate/nitrite systems are listed in

Table 2.1[26-31]. Among those systems, solar salt (NaNO3/KNO3: 60/40) is the thermal energy

storage medium which is currently being used with the freezing point of 221oC [27]. Although

the melting point for this system is highest in all the candidate mixtures in this group, the lowest

12

combined compound cost makes it widely used in solar energy storage field. Another ternary

system HITEC which contains NaNO3, KNO3 and NaNO2 has freezing point of 141 oC [28].

This combination brings the melting point down but the lack of combination of optimum features

limits its further application [29]. Some mixtures such as LiNO3-Ca(NO3)2-KNO3 are not often

utilized because they increase the compound cost at the same time of lowering the melting point

to around 120oC [30], moreover , the decreased melting point is still high compared to the

organic oil. There are also several systems have the melting points less than 100oC or even 60

oC,

they are not used in the parabolic trough solar power plant due to the decomposition of some

components during high temperature [31].

Table 2.1. Melting point of various nitrate salt systems

Compound Melting Point (ºC)

LiNO3 253

NaNO3 307

KNO3 334

Ca(NO3)2 561

Sr(NO3)2 570

Ba(NO3)2 590

NaNO3-NaNO2 221

NaNO3-NaNO2-KNO3 141

NaNO3-KNO3-CaNO3 133

LiNO3-KNO3-NaNO3 120

KNO3-CaNO3-LiNO3 117

LiNO3-KNO3-NHNO3 92

KNO3-NHNO3-AgNO3 52

The melting points of individual and multi-component carbonate systems are listed in

Table 2.2 [26, 32, 33]. Different from the nitrate salts, the melting points for both the individual

13

and multi-component carbonate systems are on the higher side. The lowest melting point is

achieved with lithium, sodium and potassium carbonate ternary system whose melting point is

still 277oC higher than that of the nitrate ternary system with the same cations [32]. Besides,

because of the thermal decomposition issue, the choice of component involved in the multi-

component carbonate systems is limited. Some salt like CaCO3 doesn‟t have stable form at high

temperature and the lack of multi-component system reduces the chance of the synthesis of low

melting point salt mixtures. Even though this group of salt is not thermally stable and the

working temperature range is relatively small, it is still viewed as possible candidate working at

high temperature due to its low price.

Table 2.2 Melting point of various carbonate salt systems

Compound Melting Point (ºC)

Li2CO3 732

Na2CO3 858

K2CO3 900

MgCO3 990

Na2CO3-K2CO3 710

Li2CO3-Na2CO3 496

Li2CO3-K2CO3 488

Li2CO3-K2CO3-Na2CO3 397

Alkali and Alkaline fluoride/chloride salts are also selected as one possible choice as the

thermal energy storage fluid and the melting point examined from previous literatures are given

in table 2.3 [34-38]. A lot of study has been done for this group of salt and the melting points

were found in the same range as the carbonate group. And for the pure salt, metal chloride salts

have lower melting point than metal fluoride ones.

14

Table 2.3 Melting point of various fluoride/chloride salt systems

Compound Melting Point (ºC)

LiF 849

NaF 996

KF 858

LiCl 610

NaCl 801

KCl 771

LiF-KF 493

LiF-NaF 652

LiCl-KF 487

LiF-NaF-KF 454

LiF-NaF-KF-MgF2 449

LiF-KF-BaF2 320

LiF-KF-CsF-RbF 256

Several studies were also conducted to determine the melting point of molten hydroxide

salts and the results are shown in Table.2.4. The data of pure salts and multi-component mixtures

merely in this group were not much determined in the literatures. Generally, they are mixed with

other groups of anion and form some low melting point salt mixtures [39-42]. On the basis of the

previous literature data, alkali hydroxide salts and their mixture with salts in other groups have

relatively lower melting point compared with pure carbonate and fluoride/ chloride group salt

mixtures. Most of the melting points given in Table 4 are lower than 300oC; sodium potassium

hydroxide binary mixture even reaches the melting point below 200oC. Accordingly, relatively

large temperature range can be obtained by using hydroxide salt mixtures or adding them as

additive.

15

Table 2.4 Melting point of various hydroixde salt systems

Compound Melting Point (ºC)

LiOH-LiF 427

NaOH-KOH 170

LiOH-NaOH 213

NaOH-NaNO2 232

NaOH-NaNO3 237

NaOH-NaCl-NaNO3 242

NaOH-NaCl-Na2CO3 282

2.2 Density

For the solar energy storage system, density for the thermal energy storage fluid is also

essential parameter. The density is needed for the size calculation as function of temperature and

assessing for the thermal stability of thermoclines. Besides, density as function of temperature is

used to evaluate the volume change in the process of freezing which contributes to potential

stress.

Alkali/alkaline nitrate salts were studied very much about their density as function of

temperature. All the results indicate that the density was decreased linearly as temperature

increases and any specific density value in the molten state can be expressed by equation as

equation 4:

= A-BT [4]

Where (g/cm3) is the density of salt, A (g/cm3) is the initial density value at 0oC and B

(mg/cm3·°C) is the density change slope as function of temperature. The coefficients are shown in

16

Table 2.5 for the nitrate group molten salts. Among these systems, pure sodium nitrate has the

largest value which reveals the high initial density value at low temperature [43]. Conversely,

lithium nitrate has the lowest value while it presents the smallest decrease trend as temperature

increases [44]. The densities and A, B values of multi-component nitrate salts were included in

the range of those two salts discussed above.

Table 2.5 Density coefficients A and B of nitrate salts

Compound A(g/cm3) B×103(g/cm3·°C)

LiNO3 1.922 0.556

NaNO3 2.334 0.767

KNO3 2.127 0.760

NaNO3-KNO3 2.134 0.773

KNO3-CaNO3-LiNO3 2.172 0.735

LiNO3-KNO3-NaNO3 2.083 0.715

Several experiments were also conducted to measure the density as function of temperature

for the individual and multi-component carbonate salt systems. The density of the carbonate salt

also follow the same trend as that of nitrate salt and the temperature dependence of density

followed the linear equation as discussed above. It is observed that all the carbonate salts have

higher initial density coefficient A than the nitrate salt. The largest value is reached to 2.511 and

even the lowest value in this group is greater than the maximum A of nitrate group [45-49].

What‟s more, the regression slope coefficient B of carbonate salt is lower compared to that of the

nitrate salt group. Accordingly, the salts in this group present larger density in the molten state

and the density coefficient A and B are given in Table 2.6.

17

Table 2.6 Density coefficients A and B of carbonate salts

Compound A(g/cm3) B×103(g/cm3·°C)

Li2CO3 2.303 0.532

Na2CO3 2.350 0.448

Na2CO3-K2CO3 2.473 0.483

Li2CO3-K2CO3 2.511 0.599

Li2CO3-Na2CO3 2.456 0.519

Li2CO3-Na2CO3-K2CO3 2.364 0.544

Density of metal fluoride and chloride molten salt were also examined and present similar

regression trend as temperature increases. The linear temperature dependence is also expressed

by the same equation. On the basis of previous literature data, the pure chloride salt shows lower

density than the fluoride salt with the same cation in the molten state [49]. What‟s more, the

sodium halide salt has the largest density value while the lithium halide salt has the lowest value,

which is very similar to the nitrate group salt. The density determination coefficient A and B are

given in Table.2.7.

Table 2.7 Density coefficients A and B of chloride/fluoride salts

Compound A(g/cm3) B×103(g/cm3·°C)

LiCl 1.766 0.432

NaCl 1.991 0.543

KCl 1.976 0.583

LiF 2.226 0.490

NaF 2.581 0.636

KF 2.469 0.651

LiF-NaF 2.520 0.818

LiCl-NaF-KCl 2.436 0.742

LiF-NaF-MgF 2.240 0.701

18

The density measurement of hydroxide was not conducted as much as those three anion

groups discussed above. Only few data of density is available for the alkali hydroxide salt when

added into other salt systems and temperature dependence also follows the linear regression

trend [32, 41]. The density determination coefficient A and B are given in Table 2.8.

Table. 2.8 Density coefficients A and B of molten salt mixture with hydroxide salts

Compound A(g/cm3) B×103(g/cm3·°C)

LiCl-LiOH 1.6 0.443

LiF-LiOH 1.65 0.471

2.3 Heat capacity

In the heating process, the temperature of solar energy storage molten salt is increase by

absorbing energy from the solar radiation. Conversely, the same amount of heat is released and

applied to heating system in the process of cooling. Heat capacity is the amount of heat required

to increase the temperature of certain material by 1 oC and can be viewed as the directly relevant

parameter to the energy storage ability. To some extent, the large heat capacity assures the

efficiency of the application of solar energy storage materials

The heat capacity of alkali/alkaline nitrate salt was investigated for both individual and

multi-component system in the previous literature. To simplify the comparison, only the heat

capacity value at 500oC is shown in all the following tables. In the liquid state, the heat capacity

increases with temperature following linear equation and the increasing slope is as small as 10-5

to 10-4

[50, 51]. Among those alkali nitrate salt systems, lithium nitrate has the largest heat

19

capacity at 500oC while potassium nitrate presents the lowest value at that temperature. In table

2.9, the heat capacity results at the selected temperature in literature are given.

Table 2.9 Heat capacity of alkali nitrate salt at 500oC

Compound Heat capacity(J/g·K)

LiNO3 2.175

NaNO3 1.686

KNO3 1.400

NaNO3-KNO3 1.533

LiNO3-KNO3 1.642

LiNO3-KNO3-NaNO3 1.681

For the carbonate salt systems, in the molten state, the heat capacity is almost constant and

almost independent with temperature [32, 33]. Same as the nitrate group salts, the heat capacity

for pure carbonate salt decreases as the atomic number of the alkali element increases, which

means the value for lithium carbonate is the largest and that of potassium carbonate is the

smallest. Generally, the heat capacity value for carbonate in molten state is larger than that in

solid state. However, the sodium-potassium carbonate binary system is an exception, for which

the heat capacity in solid state is larger than liquid state. In table 2.10, the heat capacity results of

carbonate salts at the selected temperature in literature are given.

Table 2.10 Heat capacity of alkali carbonate salt at 500oC

Compound Heat Capacity(J/g·K)

Li2CO3 2.50

Na2CO3 1.78

K2CO3 1.51

Na2CO3-K2CO3 1.57

20

Li2CO3-K2CO3 1.60

Li2CO3-Na2CO3 2.09

Li2CO3-K2CO3-Na2CO3 1.63

The heat capacity of fluoride/chloride salt was measured in several literature and found

that for the pure salt, the lithium halide has the biggest heat capacity data in the molten state

while the potassium halides shows the lowest heat capacity value. Similar to the carbonate group,

the heat capacity value of fluoride/chloride salt varies little with temperature in the liquid state

[32, 33]. The values at 500oC for the alkali/alkaline halides are shown in Table 2.11.

Table 2.11 Heat capacity of fluoride/chloride salt at 500oC

Compound Heat Capacity (J/g·K)

LiCl 1.48

NaCl 1.15

KCl 0.90

LiF-KF 1.63

NaCl-MgCl2 1.00

LiF-NaF-KF 1.55

KCl-MgCl2-CaCl2 0.92

The heat capacity of pure and multi-component hydroxide salt systems is limited in the

previous literature and the values in the liquid state follow linear equation which is observed for

all the molten salt discussed above [52]. The values at 500oC for the alkali/alkaline halides are

shown in Table 2.12.

21

Table 2.12 Heat capacity of hydroxide salt at 500oC

Compound Heat Capacity(J/g·K)

NaOH 1.88

LiOH-NaOH 2.21

NaOH-KOH 1.82

In summary, on the basis of comparison of various physic-chemical properties, molten

nitrate slats have relatively low melting point, excellent working temperature range, reasonable

density and high heat capacity. As the result of that, molten nitrate salt is suitable to be applied as

the thermal energy storage fluid in the solar energy storage system.

22

CHAPTER 3

OBJECTIVES

Based on review of previous literature data, it is found that there are several disadvantages

such as the high melting point, relatively low density value or poor heat capacity in liquid state

which limit the application of molten salt in certain groups in solar thermal energy storage

system. Conversely, alkali/alkaline nitrate salt is considered as the suitable choice and proposed

as the thermal energy storage liquid for high temperature.

Currently, the used thermal energy storage liquid is NaNO3 (60mol%)-KNO3 (40mol%)

binary system (solar salt) which has the melting point at 221oC [30]. Although the melting point

for this salt mixture is not the lowest, it is still emphasized because of its low investment cost.

However, there are some drawbacks for this binary nitrate mixture. The main disadvantage is

the high melting point. In evenings or in winter, the molten salt can easily freeze and block the

pipeline. As a result of that, some auxiliary cost should be added to overcome this problem and

the total investment will be increased.

Development and synthesis of newer molten salt mixtures with freezing point lower than

those currently used for thermal energy storage applications is necessary for higher efficiency of

utilization of solar energy and getting rid of any unnecessary cost. The approach to develop

lower freezing point molten salt mixtures is by the prediction of new eutectic mixtures and also

23

by the development of new nitrate compounds. Besides these two most well known systems,

several other mixtures were also studied. Preliminary evaluation of several new molten salt flux

systems based on requirements for thermal energy storage systems, mainly including freezing

point, density, heat capacity, viscosity, and thermal energy storage density. The promising

candidate low melting point molten salt system should satisfy the requirements that eutectic

melting temperatures are lower than 220oC and the thermal energy storage densities are higher

than binary solar salt. It is known that the melting point can be lowered by the addition of one or

more ABNO3 nitrate compounds where A and B are cations. Consequently, several multi-

component systems which have more constituent salts than solar salt were came up with and

studied with little fundamental data on the physic-chemical properties at the required operating

conditions available at present.

In this thesis, the new systems with simulated eutectic compositions were tested for their

experimental melting points, heat capacities using the Differential Scanning Calorimetry (DSC)

technique which is considered to be the accurate instrument for thermodynamic data analysis

[53-59]. Some significant thermodynamic properties such as heat capacity, enthalpy, and entropy

and Gibbs energy were calculated in the thesis to evaluate the energy change of the system in the

phase change process and the potential of being applied in the parabolic trough solar power plant.

The energy density was obtained by using the experimental measured density and heat capacity

of the mixtures in molten state. Finally, 9 down-selected systems were present and discussed.

24

CHAPTER 4

THERMODYNAMIC MODELING OF SALT SYSTEMS

4.1 Thermodynamic modeling

To lower the melting point of solar energy storage system, multi-component system is

applicable. Thermodynamic model was introduced to predict the eutectic temperature of salt

systems based on the Gibbs energies of fusion of individual salt and that of mixing of constituent

binary systems. At the eutectic temperature, the Gibbs energies in the liquid state and solid state

of salt are equal. In thermodynamics, Gibbs energy of fusion can be expressed by the equation

given as follows:

G = H-TS [5]

Where H is the change of enthalpy of fusion and S is the change of entropy of fusion. Equally,

the entropy change of fusion can be expressed by differentiating G and the equation is given:

[6]

It is known that the change in entropy can be expressed in terms of change in heat capacity in the

melting process as:

[7]

25

If the change of heat capacity is assume to be independent of temperature, the integral of

from Tm to T can be shown as:

[8]

where Sm is the entropy of fusion at the melting point which is equal to . Accordingly,

Eq.8 can be rewritten as:

[9]

Substituting Eq. 9 in Eq. 6 and integrating the equation from Tm to T we get,

[10]

Eq. 10 illustrates that by using the change of heat capacity, melting point and enthalpy of fusion,

the Gibbs energy change at any temperature can be obtained.

The standard Gibbs energy of fusion of a salt „1‟ can be expressed in terms of the activity

of the salt as:

[11]

where is the molar excess Gibbs energy and X1 is the molefraction of the salt „1‟. Gibbs

energy of fusion at any give temperature T is expressed by Eq 7 in terms of its molefraction and

partial molar excess Gibbs energy.

26

Take LiNO3-NaNO3-KNO3 as an example in which the integral molar excess Gibbs energy is

composed of the summation of the Gibbs energies of three constituent binary system and one

ternary. The expression of the integral excess Gibbs energy is given by Eq.12.

[12]

Gibbs energies of the three constituent binary systems, LiNO3-NaNO3, LiNO3-KNO3, and

NaNO3-KNO3 of the LiNO3-NaNO3-KNO3 ternary system are taken from the literature [48, 49].

The Gibbs energies of mixing or the integral excess Gibbs energies of the three constituent

binary systems of the LiNO3-NaNO3-KNO3 ternary system are given below:

LiNO3-NaNO3 Binary System

J/mol [13]

LiNO3-KNO3 Binary System

J/mol [14]

NaNO3-KNO3 Binary System

J/mol [15]

When assume the intergral excess Gibbs energy of to be zero, the excess Gibbs energy in

the ternary system can be expressed by the summation of three constituent binary systems:

[16]

27

Generally, the partial molar excess Gibbs energies are reduced from the integral molar

excess Gibbs energy and can be expressed by the generalized equation for certain “m”

component salt as:

[17]

In the ternary system, the i value equals to 1,2 and 3, and the partial molar excess Gibbs energy

of mixing for each component can be expressed as follows:

[18]

[19]

[20]

Based on Eq. 7 and the partial molar excess Gibbs energy of individual component, the

Gibbs energy in the fusion can be expressed as Eq.21- 23.

[21]

[22]

[23]

4.2 Calculations

The fusion of the ternary salt system is defined by solutions of Eq. 21-Eq. 23. Newton-

Raphson method can be used to solve these three non-linear equations by linearizing the non-

linear equations using the Taylor series and truncating the series to first order derivatives.

28

Consider the three non-linear functions F, G, and H in three variables, x, y, and z. The three

equations that are solved for the three variables are written as:

F(x, y, z) = 0;

G(x, y, z) = 0;

H(x, y, z) = 0; [24]

The partial derivatives of the function F with respect to x, y and z are given as:

;

;

; [25]

Similarly, the partials derivatives can be expressed for the other two functions G and H.

Newton-Raphson iterative method of solving the three equations in three variables

essentially deals with the solution of the incremental vector in the matrix equation given below.

[26]

For the initial values of x, y, and z, (say xi, yi, and zi) the right hand side vector contains the

values of the functions at the initial values (xi, yi, and zi). The 3×3 matrix on the left hand side

contains the partial derivatives of the functions with respect to the three variables at the initial

values. Solutions of the matrix equation (Eq. 26) result in the increments of the variables x, y,

and z. The variables for the next iteration will then be xi + x, yi + y, and zi + z. The

process of solving the matrix equation (Eq. 26) is continued until the increments in the variables

x, y, and z is less than a very small quantity. The iteration process is then said to be

29

converged and the values of the variables at convergence of the solution are the roots of the

system of the three fusion equations.

The composition of LiNO3, NaNO3 and KNO3 and the eutectic temperature is solved by

using the Newton-Raphson iterative method. Different from the data in previous literature, the

eutectic temperature for the ternary is 116oC. Besides, the composition for each component is

also different from those published in literatures. The new molten ternary system is composed of

25.92 wt% LiNO3, 20.01 wt% NaNO3, and 54.07 wt% KNO3. The similar method is also applied

to other multi-component systems to determine the composition and eutectic temperature. The

predicted melting points for new solar energy storage system are given Table.4.1.

Table 4.1 Calculated composition and melting point of multi-component molten salts systems

System Composition (wt%) Calc. Tmp

LiNO3 NaNO3 KNO3 NaNO2 KNO2 Mg(NO3)2 MgKN (°C)

Salt #1 25.9 20 54.1 - - - - 116

Salt #2 - 16.1 54.7 29.2 - - - 123.8

Salt #3 17.5 14.2 50.5 17.8 - - - 98.6

Salt #4 11.5 10.4 27.4 - - - 50.7 98.6

Salt #5 17.2 13.9 47.6 17.2 4.1 - - 95.7

Salt #6 9 42.3 33.6 - 15.1 - - 100.0

Salt #7 19.3 - 54.6 23.7 2.4 - - 108.1

Salt #8 19.3 - 55.9 23.9 - 0.9 - 100.8

Salt #9 15.4 17.2 32.4 - - 8.3 26.7 103.6

30

CHAPTER 5

EXPERIMENTAL PROCEDURE

5.1 Melting point determination of molten salt mixtures

5.1.1 Materials

Ternary, quaternary and quinary nirate and nitrite mixtures were tested in the thesis. Most

components in the mixtures don‟t require any pre-preparation and can be used as received. The

only exception is new developed MgKN which was composed of 66.67 mol% KNO3 and 33.33

mol% Mg(NO3)2. This unique compound is synthesized from magnesium nitrate hexahydrate

(98%, Alfa Aesar) and potassium nitrate (ACS, 99.0% min, Alfa Aesar) and added into the

mixture as one single component. As received magnesium nitrate hexahydrate is dehydrated

before synthesizing MgKN compound. Weighted amount of magnesium nitrate taken in a

stainless steel crucible and placed on a hot plate in an argon atmosphere. Temperature of the salt

is measured with a thermocouple immersed in the salt. The temperature was held at 523.15 K for

2 hours. The salt solidifies to a white mass. The temperature of the salt is then raised to 573.15 K

slowly to remove any traces of moisture and to ensure complete dehydration. The complete

removal of water is ascertained by weight loss.

5.1.2 Apparatus and Procedure

31

Differential scanning calorimetry (DSC) analysis was performed using Perkin Elmer

Diamond DSC instrument and the setup is shown in fig. 5.1. Heat flow and temperature can be

recorded in the instrument with an accuracy of 0.0001 mW and 0.01 K respectively. The

measurements were made under purified nitrogen atmosphere with a flow rate of 20cc/min and at

a heating rate of 5 K/min.

Fig 5.1 Photography of set-up for DSC equipment

After dehydration if necessary, each component was weighed to an accuracy of 0.1mg with

the electrical balance and mixed thoroughly in a stainless steel crucible. Later, the mixture is

heated up to certain temperature at which the entire salt melts. At this temperature the salt

mixture was held for about 30 minutes. The salt mixture is allowed to air cool to ambient

temperature. This procedure is repeated 3 to 4 times to get the well-mixed compound. Standard

aluminum pan with lid used for DSC measurements are weighed before the experiment. Small

amount of the synthesized compound is placed carefully in the aluminum pan and closed with

the lid. The lid is crimped by a sample press and the pan is weighed. The weight of the sample is

32

determined by removing the weight of the pan and lid. For the determination of melting point

and heat capacity (20-25) mg of the sample was used.

Perkin-Elmer Diamond Differential Scanning Calorimeter (DSC) is used to measure the

melting point and heat capacity of compound. The crimped ample pan was immediately put

inside the sample chamber of DSC after preparation and held at 523.15 K for 10 hours to remove

the trace amount of moisture possibly caught in the process of loading sample and also to ensure

a homogeneous mixture. In the experimental procedure, a temperature range from 298.15 K to

523.15 K was set with a heating rate of 5 K min

1 followed by a cooling cycle at the same rate.

This cycle is repeated for at least 6 times to ensure good mixture of the sample and

reproducibility of the results.

5.2 Heat Capacity determination of molten salt mixtures

To start Cp measurement, the same procedure as that of melting point determination is

followed with an addition of „iso-scan-iso‟ steps to the program after 5-cycle temperature scan.

Starting from 298.15 K, the temperature was held for 5 minutes before and after each scan step.

Small temperature scan range is chosen to avoid thermal resistance between device and testing

sample except when the temperature is approaching the melting temperature. The upper limit for

the Cp measurement was set to 623.15 K in our experiments. Since the change in the molar heat

capacity of the salt in the liquid state is very small, the Cp data in the liquid state can be easily fit

to an equation and extrapolated to higher temperatures. To get the value of molar heat capacity

of the sample, heat flow curve for the baseline of the empty sample pan also needs to be obtained

immediately following the identical “iso-scan-iso” steps which were used for the actual sample

33

run. The difference of heat flow between the actual crimpled sample and the empty sample pan is

the absolute heat absorbed by the test sample.

5.3 Density determination of molten salt mixtures

Density measurement was carried out with standard densitometer which has fixed volume.

Initial weight of the densitometer is measured and noted. Salt composition, of which the density

is measured, is placed in a beaker on a hot place. The densitometer is also placed on the same hot

plate. The temperature is set to a fixed value above the melting point of the salt and is measured

by a thermocouple. After the salt is melted and when the temperature shows stable reading, the

molten salt is poured in to the densitometer up to the set mark on the sensitometer bottle. The

weight of the densitometer with the molten salt is measured. The weight difference between this

weight and the weight of empty densitometer gives the weight of the molten salt at the fixed set

temperature. By knowing the fixed volume in the densitometer, the density of the salt at that

temperature can be calculated. This procedure is repeated at least three times to accurately

determine the density of the salt.

34

CHAPTER 6

RESULT AND DISCUSSION

6.1 Melting point determination

6.1.1 DSC equipment calibration

Before the actual melting point measurement, pure indium, zinc metal and several

individual salts were used to calibrate the DSC equipment. For metals, only one sharp peak was

observed for each and the heat flow curve for indium metal is shown in fig 6.1. However, larger

and boarder peaks are found for salts, just like the condition illustrated in fig 6.2 for pure

potassium nitrate. Based on the results shown in Table 6.1, the experimental data for melting

points and enthalpies of fusion have excellent agreement with the literature values [60-63]. The

variation of point is within 0.7% and the variation of change of enthalpy is less than 3%.

Figure 6.1 Melting point calibration with indium sample

35

Figure 6.2 Melting point calibration with KNO3 sample

Table 6.1 Calibration data of melting points with different samples

Sample

Lit.

Tmp

Expt.

Tmp

Lit.

Ttrans

Expt.

Ttrans

Lit.

ΔHfusion

Expt.

ΔHfusion

Lit.

ΔHtrans

Expt.

ΔHtrans

°C °C °C °C J/g J/g J/g J/g

Indium 156.6 156.3 - - 28.6 27.8 - -

Zinc 419.5 418.8 - - 108.6 106.8 - -

LiNO3 256.7 255.0 - - 361.7 363.3 - -

NaNO3 310.0 308.1 277.0 275.3 177.7 175.6 14.7 15.2

KNO3 337.0 337.2 133.0 133.2 99.3 100.5 53.8 52.9

6.1.2 Results

Differential scanning calorimetry (DSC) was used to determine the melting point and any

solid state phase transitions of the salt mixture. A low scanning rate was chosen to record the

heat flow curve as function of temperature in order to improve the sensitivity of detection [64]. It

helps to pick up any small endothermic peaks and also avoids the thermal resistance between the

36

internal furnace and sample. Nine systems were chosen to test and the eutectic composition is

already listed in Table 4.1.

All the selected systems are composed of alkaline nitrate and nitrite and most of them have

three basic components which are lithium, sodium, potassium nitrate or nitrite. All the quaternary

and quinary systems were developed on the basis of the LiNO3-NaNO3-KNO3 baseline ternary.

Figure 6.3-6.11 shows the DSC plot of all the salt systems. DSC plots for each system were

collected for at least five runs (each run with fresh salt preparation) to ensure the reproducibility.

All the onset temperatures, peak temperatures, predicted temperatures, enthalpy of fusion for

melting peaks and the solid phase transformation temperatures are given in Table.6.2.

Figure 6.3 DSC endothermic peaks of LiNO3-NaNO3-KNO3 salt.

37

Figure 6.4 DSC endothermic peaks of NaNO3-NaNO2-KNO3 salt.

Figure 6.5 DSC endothermic peaks of LiNO3-NaNO3-KNO3-MgK salt.

38

Figure 6.6 DSC endothermic peaks of LiNO3-NaNO3-KNO3-NaNO2 salt.

Figure 6.7 DSC endothermic peaks of LiNO3-NaNO3-NaNO2-KNO3-KNO2 salt.

39

Figure 6.8 DSC endothermic peaks of LiNO3-NaNO3-KNO3-KNO2 salt.

Figure 6.9 DSC endothermic peaks of LiNO3-KNO3-NaNO2-KNO2 salt.

40

Figure 6.10 DSC endothermic peaks of LiNO3-KNO3-NaNO2-Mg(NO3)2 salt.

Figure 6.11 DSC endothermic peaks of LiNO3-NaNO3-KNO3-Mg(NO3)2-MgKN salt

Table 6.2 illustrates that the predicted melting point is close to the experimental

determined value and most deviation is within 10% except for system #9. The great agreement

41

between experimental and calculated data verifies the accuracy and feasibility of the

thermodynamic modeling.

Table 6.2 DSC results of melting point, transition point and predicted melting point

System Tmp Ttrans ΔHfusion

Calculated, °C Onset, °C Peak, °C Peak, °C J/g

Salt #1 116.0 99.4 119.1 104.3 60.0

Salt #2 123.8 115.0 124.0 NA 9.7

Salt #3 98.6 94.0 99.9 NA 24.4

Salt #4 98.6 94.0 101.0 NA 6.0

Salt #5 95.7 91.0 95.0 NA 6.2

Salt #6 100.0 93.0 96.0 NA 8.6

Salt #7 108.1 99.2 100.3 79.3 6.0

Salt #8 100.8 101.0 101.9 85.3 5.9

Salt #9 103.6 83.4 89.2 NA 9.3

6.1.3. Discussion

It is observed that the first curve is different from last ones shown in the DSC plots and

this phenomenon is common for all the melting point measurement with DSC technique. This

happened because in the first cycle, the moisture caught by salt mixture, especially the lithium

nitrate, was removed in the process of heating. Moreover, the partially solidified sample in the

sample loading process can be re-homogenized in the first heating cycle [65-67]. In figure 6.3,

6.9 and 6.10, more than one endothermic peak was found. The first smaller endothermic peak

refers to solid state phase transition of the salt mixture. The second larger endothermic peak

refers to the melting of the salt. Normally, the onset temperature of transition is taken as the

experimental transition point for any metallic sample. However, in case of molten salts mixtures,

since the thermal conductivity is low [68-74], the complete transition is ensured only at the peak

42

transition temperature. The thermal gradient which exists due to the low thermal conductivity of

the salt results in internal heat flow which enhances the mixing in the salt. Thus, the transition

temperature is defined as the peak temperature of phase transition. For salt No.1, the small

endothermic peak happened before and was connected to the main peak which occurred at

390.27K. The first endothermic peaks for salt No. 7 and 8 occurred at almost the same

temperature because of the similar composition for these two compounds. Since the small

amount of magnesium nitrate and potassium nitrite contained in these two compounds, the small

endothermic peak can hardly be related to these two components. Obviously, the rest three major

components must have something to do with the first peaks happened before the melting peaks

for both cases. Each component among the major three ones were tested to find out any possible

solid phase transition peaks of them and the results shown in Table. 6.3, which reveals that

lithium nitrate doesn't have any phase transition peak in solid state while potassium nitrate and

sodium nitrite both own the solid phase transformation peaks before their melting peaks.

Table 6.3 Fusion and solid phase transition temperature for individual salts

System Tmp, °C Ttrans, °C ΔHfusion, J/g ΔHtrans, J/g

LiNO3 255.0 - 363.3 -

KNO3 337.2 133.2 100.5 52.9

NaNO2 431.1 41.70 111.9 8.80

The further investigation was carried out by running the KNO3-NaNO2 (55.0 wt% and 23.8 wt %)

binary compound with the very similar weight percentage as that in salt No. 7 (54.6 wt% and

23.7 wt%) and salt No. 8 (55.9wt% and 23.9wt%). By converting the weight percentage of the

studied binary system into 100% scale, the weight fraction for sodium nitrate and potassium

nitrate can be rewritten as 69.8wt% and 30.2wt%. The DSC plot for this binary system was

43

shown in fig 6.12. Although the solid transition and melting temperature were brought down by

adding lithium nitrate, the shape of the plots in fig. 6.9 and 6.10 are identical to that shown in fig.

6.12. The enthalpy of solid state transformation of the binary salt was also converted to that in

both quaternary systems by using the weight fraction occupied by the binary system and the

comparable change of converted enthalpy between the binary system and two quaternary systems

indicates the relevance of the solid transition peaks in salt #7 and #8 to the combined effect of

potassium nitrate and sodium nitrite.

Figure 6.12 DSC plot of 69.8wt% KNO3- 30.2wt% NaNO2 binary system

The similar analysis was applied to No.1 salt to find out the reason for the presence of a

small peak adherent to the main melting peak before the melting point. Sodium nitrate and

potassium nitrate binary system was synthesized based on the weight fraction of these two

constituent salts in No. 1 salt. DSC plot for the sodium nitrate-potassium nitrate binary system in

Fig. 6.13 with the converted composition which is essentially same as that in the No.1 ternary

system shows smooth heat flow curve before the melting peak, which means the solid transition

peak in ternary is not simply relative to the binary system. Assumption was made that the solid

44

phase transformation peak in the ternary salt is resulted from a multiple effect, i.e. the

combination of one of the eutectic binary system involved in the ternary salt mixture and the

other binary system which is composed of the rest components. The statement is verified that the

small peak in salt #1 is mainly caused by the solid phase transformation peak in lithium nitrate-

potassium nitrate eutectic binary system given the similar shape of the DSC plots in Fig. 6.14.

Since in salt No.1 there is excess amount of sodium nitrate to form the lithium nitrate-sodium

nitrate binary system, the rest sodium nitrate can interact with potassium nitrate and form new

sodium-potassium nitrate system which is shown in fig.6.15. Besides, a solid phase

transformation peak is observed in fig.6.15 which has a very small area and won‟t change the

shape of phase transformation peak in fig.6.14 when these two binary systems are combined and

form salt #1. The enthalpies of solid state transformation in two binary salts were also converted

to that in salt #1 by using the weight fractions occupied by both binary systems. The difference

of the change of converted enthalpies between the lithium-potassium nitrate eutectic binary and

ternary system is filled by the binary mixture which is composed of the rest components: sodium

nitrate-potassium nitrate. The comparable converted values of enthalpy change between salt #1

and its two constituent binary systems further verify the assumption that the solid phase

transformation happened in salt #1 is mainly due to the combined effect of LiNO3-KNO3 eutectic

binary system and NaNO3-KNO3 binary system.

45

Figure.6.13 DSC plot of 27.0wt% NaNO3-73.0wt% KNO3 binary system

Figure.6.14 DSC plot of 45.8wt%LiNO3-54.2wt%KNO3 binary system

46

Figure.6.15 DSC plot of 46.0wt% NaNO3-54.0wt% KNO3 binary system

Unlike those discussed mixtures above, salt No.2, Salt No.4, Salt No.5 and Salt No.6 have

only one relatively board melting peak and the heat flow curve before and after are very stable.

Similarly, there is no solid transformation peaks observed in salt No.3, salt No.7 and salt No.8.

However, the heat flow after the melting peak in these cases are not stable and the main

endothermic peak is followed by a small hump which is considered to be the recrystallization

process once the compound entered into the liquid state. When the process is finished, the heat

flow curve returns to steady state.

Heating rate is a significant parameter when collect the heat flow curves by using DSC

technique. Fig 6.16(a) and Fig 6.16(b) illustrate the difference of melting point for salt No.6 due

to the change of heating rate. If the heating rate is 20oC/min, the peak temperature and onset

temperature for the melting peak is 96.69oC and 92.21

oC, respectively. Once the heating rate is

decreased to 5oC/min, these two temperatures will also be lowered to 96.14

oC and 91.90

oC. The

difference is resulted from the diverse amount of thermal resistance between the testing sample

and the furnace inside the DSC instrument [75]. Under higher heating rate, the decisive thermal

47

resistance is raised due to the low thermal conductivity medium between the furnace and the

actual sample. The insensitivity of gas heat conduction medium in DSC results each unit of

temperature increase on one side cannot have an immediate response on the other side of the gas.

Consequently, the sample holder which is connected the furnace has a higher temperature than

that inside the sample. In this condition, the value of temperature profile collected as the sample

holder temperature is larger than the actual temperature. The deviation will be much smaller

when the heating rate is reduced. In the case, the thermal resistance will be decreased because of

the lower temperature gradient of the gas medium in the heating process. As a result of that, the

collected temperature from the sensor attached to the sample holder will be very close to the

actual temperature inside the testing sample.

Figure 6.16(a) DSC endothermic peaks of LiNO3-NaNO3-KNO3-KNO2 salt for 20oC/min

heating rate.

48

Figure 6.16(b) DSC endothermic peaks of LiNO3-NaNO3-KNO3-KNO2 salt for 5oC/min

heating rate.

Besides the difference of temperature while using higher and lower heat rate, the solution

of DSC will also be affected by different heating rate. Fig. 6.17(a) shows the DSC plot for salt

No. 7 using the heating rate as 5oC/min and the DSC plot in Fig. 6.17(b) is collected under the

heating rate as 20oC/min. It can be observed that in the lower heating rate, two small separated

peaks can be viewed as two parts of the solid phase transformation process, while in Fig. 6.17(b)

two small peaks before the melting peak merge and present as a board hump. The qualification

of resolution can be executed by the term named resolution factor RMKE which is calculated as

the ratio of the peak heat flow value of the separated peaks to that of the concave point between

two peaks. The equation for determining RMIKE is given in Eq. 27 [76, 77].

RMIKE =hpeak/hmin [27]

49

Figure 6.17(a). DSC endothermic peaks of LiNO3-KNO3-NaNO2-KNO2 salt for 5oC/min

heating rate.

Figure 6.17(b). DSC endothermic peaks of LiNO3-KNO3-NaNO2-KNO2 salt for 20oC/min

heating rate.

In the case of lower heating rate, the RMKE is determined to be 1.5 and the value for higher

heating rate is not available because the concave point of heat flow doesn‟t exist from

Fig.6.17(b). Since the higher RMKE value indicates better resolution, it can be stated that the

50

lower heating rate also results in greater sensitivity of the equipment to pick up any small

endothermic peaks.

Besides the down-selected 9 compounds, some more salt mixtures were also tested. Most

of them were not selected to the final candidate for the thermal energy storage application

because of their higher melting point. Table 6.4 gives some of the trial systems measured with

DSC technique. It is illustrated that the melting points of mixtures with lower or even no content

of lithium nitrate turn out to be higher than those with sufficient amount of lithium nitrate. For

most of the mixtures with melting point lower than 120oC, the amount of lithium nitrate should

be larger than 8.1wt%. Also, all of the systems in table 6.4 with lithium nitrate less than 1.5wt%

have melting point higher than 140oC. Based on the observation above, it is concluded that the

lithium nitrate can be used as an additive to bring the melting point down for thermal energy

storage systems.

51

Table 6.4 Melting points of candidate systems as function of temperatures

System Composition (wt%)

Onset

Temp

Peak

Temp

(oC) (

oC)

LiNO3 – NaNO3 – KNO2 10.7 45.9 43.4 89.0 91.0

LiNO3 - KNO3 - NaNO2 19.6 56.4 24.1 102.4 104.6

LiNO3 - NaNO3 - KNO3 - KNO2 9.0 42.3 33.7 15.1 93.0 96.0

LiNO3 - NaNO3 - NaNO2 - KNO2 8.1 45.4 6.5 40.1 90.0 91.0

LiNO3 - KNO3 - NaNO2 - KNO2 19.3 54.6 23.7 2.4 99.2 100.3

LiNO3 - KNO3 - NaNO2 – Mg(NO3)2 19.3 55.9 23.8 0.9 101.0 102.0

LiNO3 - NaNO3 - KNO3 - Mg(NO3)2

- MgK 15.4 17.2 32.4 8.3 26.7 83.4 89.2

LiNO3 - NaNO3 - KNO2 – Ca(NO3)2 1.4 39.0 33.3 26.3 125.0 147.0

NaNO3 - KNO3 - NaNO2 - KNO2 42.5 16.3 7.1 34.1 140.7 144.7

NaNO3 - KNO3 - KNO2 – Mg(NO3)2 43.2 14.6 38.0 4.2 138.6 142.1

NaNO3 - NaNO2 - KNO2 - Ca(NO3)2 45.1 9.2 41.0 4.8 115.0 139.0

LiNO3 - NaNO3 - NaNO2 - KNO2 -

Ca(NO3)2 1.5 39.3 3.7 32.3 23.2 138.0 148.0

LiNO3 - NaNO3 - KNO2 - Ca(NO3)2 -

Mg(NO3)2 1.4 37.9 31.3 27.5 2.0 133.9 153.4

6.2 Heat capacity determination

6.2.1 Heat capacity calibration

DSC was also calibrated for the heat capacity measurement. Lithium nitrate, sodium

nitrate and potassium nitrate were examined for the heat capacities from room temperature to

upper limit temperature for the instrument. In liquid state, the heat capacity values for each salt

can be fit to straight line with trace amount of increasing trend. Since the temperature range from

the onset temperature of liquid state to the upper limit of DSC is relatively small, the heat

capacity values for pure individual salts can be viewed as constants. The comparison between the

theoretical and experimental heat capacity data is given in Table 6.5. Except lithium nitrate, the

52

experimental heat capacities data for the rest two systems are almost same as the literature. Even

for lithium nitrate which demonstrates the biggest difference from the literature data, the 2.8%

vibration is still within a reasonable range

Table 6.5 Calibration data of heat capacities with different samples

Sample Lit. Cp Expt. Cp

J/g.K J/g.K

LiNO3 2.18 2.12

NaNO3 1.69 1.67

KNO3 1.40 1.39

6.2.2 Results

The materials used in the heat capacity measurements are the same as those in the melting

point experiments. Molar heat capacities of the all compound were measured by the DSC

equipment from room temperature to 623.15 K. The heat flow is recorded as a function of

temperature in “iso-scan-iso” steps at intervals of 20 K. The „iso stage‟ refers to isothermal

holding at a particular temperature, „scan stage‟ refers to the heat flow recording at a heating rate

of 5 K min1

up to a an increment of 25 K, followed by another isothermal holding stage. This is

a standard procedure followed in the measurement of heat capacity of materials using the DSC

equipment [63, 64]. This procedure of heat capacity measurement has two advantages; (i) any

heat fluctuations during the recording are avoided by the isothermal steps and (ii) any phase

transition can be highlighted by the choice of temperature range. The absolute heat flow to the

sample is determined by subtracting the heat flow collected by running a baseline curve with an

empty pan. Because the heat capacity measurement in the heating process corresponds to

53

collecting the value of required heat flow at each temperature, all the heat capacity plots have the

same shape with that of heat flow in the melting point measurements. Take the heat capacity plot

of LiNO3-NaNO3-KNO3 ternary system as an instance which is shown in fig 6.18, the heat

capacity curve also has two different peaks. The first little peaks corresponds to one occurs at

390.27K which was observed in fig 6.3, the second large and sharp peak happened right after the

small one is prevalent to the endothermic peak with the peak temperature as 390.27 K. Similarly,

after the phase transformation, the heat capacity in liquid state becomes very stable and increase

with temperature linearly with little slope.

Fig 6.18 Heat capacity data plot of LiNO3-NaNO3-KNO3 ternary system as function of

temperature

The heat capacity change as function of temperature for salt No.1 was illustrated in fig

6.19. Based on the trend of heat capacity in the liquid state, any value for the system in the liquid

can be extrapolated. The expressions for heat capacity in liquid state for the new molten salt

systems were discussed and given in the next section.Table.6.6 shows the specific heat capacity

54

of the all the selective compounds measured at 623.15 K and extrapolated at 773.15K. Besides,

the molar heat capacities at 773.15K are given in Table 6.6 of all the salts.

Fig 6.19 Heat capacity of LiNO3-NaNO3-KNO3 in liquid state from 403.15-623.15K

Table 6.6 Heat capacity of selected new TES molten salt mixtures

System Expt. (623.15K) Extrapolated(773.15K) Extrapolated(773.15K)

Cp, J/g.K Cp, J/g.K Molar Cp, J/mol.K

Salt #1 1.53 1.70 152.1

Salt #2 1.43 1.68 151.5

Salt #3 1.48 1.55 218.3

Salt #4 1.53 1.66 141.1

Salt #5 1.53 1.70 144.0

Salt #6 1.51 1.63 143.5

Salt #7 1.56 1.67 144.3

Salt #8 1.55 1.68 141.0

Salt #9 1.61 1.70 193.7

55

6.2.3 Thermodynamic properties

The standard thermodynamic properties such as entropy, enthalpy, and Gibbs energy for

salt mixtures are determined from the experimental data of melting point and heat capacity in the

temperature range of the present study and expression for determining these properties are given

in equation 28-30. In thermodynamics, all these three properties are related to heat capacity and

its variances with temperature. In the studied temperature range (298.15K-623.15K), they can be

described as expression includes heat capacity:

[28]

[29]

[30]

Where Tt is the solid transformation temperature, Tmp is the melting point, ΔHt is enthalpy of

solid phase transformation and ΔHfusion is enthalpy of fusion. The standard thermodynamic

properties, entropy, enthalpy and Gibbs energies as function of temperature for each compound

are expressed in the following section.

6.2.3.1 LiNO3-NaNO3-KNO3 (Salt #1)

The heat capacity data can be divided into two sections for LiNO3-NaNO3-KNO3

compound; (i) solid state 1 (323.15-384.15) K (ii) liquid state (403.15-623.15) K. Accordingly,

56

the heat capacity data are fit to two separate polynomial equations corresponding to the three

phases of the compound.

6.2.3.1.1 Heat capacity of solid state 1: (298.15-384.15) K

The heat capacity data for LiNO3-NaNO3-KNO3 compound in the solid state 1 in the

temperature range of 298.15 to 384.15 K is fit to a second order polynomial equation. Eqn. (31)

gives the polynomial equation along with the least square fit parameter (R2) in the temperature

range for the solid state 1 of the compound.

[31]

( ) K

R2 = 0.982

6.2.3.1.2 Heat capacity of liquid state: (403.15-623.15) K

The heat capacity data for LiNO3-NaNO3-KNO3 compound in the liquid state in the

temperature range of 403.15 to 623.15 K is fit to a linear equation. Eqn. (32) gives the linear

equation along with the least square fit parameter (R2) in the temperature range for the liquid

state of the compound.

J/K.mol [32]

57

R2 = 0.947

Heat capacity data of the LiNO3-NaNO3-KNO3 compound in the solid state follows a

second order polynomial curve whereas the heat capacity is linear in the liquid state.

6.2.3.1.3 Thermodynamic properties of solid state 1(298.15-384.15) K:

J/K.mol [33]

J/mol [34]

[35]

J/mol

6.2.3.1.4 Thermodynamic properties of liquid state 2(403.15-623.15) K:

[36]

58

J/K·mol

J/mol [37]

[38]