A Numerical Calculation Tool Design for the Performance Assessment of a Bench-Scale Thermochemical Heat Storage System

A Numerical Calculation Tool Design for the

Performance Assessment of a Bench-Scale

Thermochemical Heat Storage System

Diyue Wang

Master of Science Thesis

KTH School of Industrial Engineering and Management

Energy Technology TRITA-ITM-EX 2020:602

Master of Science Thesis EGI 2020:MJ240X TRITA-ITM-EX 2020:602

A Numerical Calculation Tool Design for the Performance Assessment of a Bench-scale Thermochemical Heat Storage

System

Diyue Wang

Approved Date Examiner

Viktoria Martin

Supervisor

Saman Nimali Gunasekara Course Code

MJ 240X

ABSTRACT

Thermochemical heat storage (TCS) is a technology to convert the heat and cold energy into chemical energy, via reversible chemical reactions, to be stored for heating and cooling applications. TCS technology is gaining interest for its very compact energy storage densities offering attractive thermal energy storage (TES) alternatives to decrease energy-related greenhouse gas(GHG) emissions and contribute to sustainable development.

This thesis is part of the umbrella project “Neutrons for Heat Storage (NHS)”, funded by Nordforsk. The objective of the NHS project at KTH is to design, construct and operate a bench-scale TCS system using strontium chloride (SrCl2)-ammonia (NH3) as the solid-gas reaction pair for low-temperature heat storage applications (40-80 ℃). This system has been thus numerically designed, adapted to practical considerations, and is now being built at Energy Technology, KTH.

Within this background, this thesis, as its primary objective, designs a calculation tool for evaluating the experimental performance of the above described bench-scale TCS system. A thorough explanation of the thesis methodology is presented here, including preparing the composites (SrCl2 impregnated into expanded natural graphite), the system’s risk analysis, and the critical focus is on the systems’ performance evaluation parameters, and the mathematical design of the calculation tool. A review of relevant literature is also conducted to identify the most pertinent performance evaluation parameters of this TCS system. For the consideration of user-friendliness, simplicity, and effectiveness, the calculation tool is designed using Ms. Excel. Here, energy efficiency, reaction advancement, reaction advancement rate, the real thermal energy density per mass, and actual thermal energy density per volume are chosen as the parameters to best-represent the system’s performance (i.e., Key Performance Indicators (KPIs)), calculated based on mass balance and energy balance expressions, primarily. Using this calculation tool, concerning this experimental bench-scale system, the user can visualize the obtained experimental data, calculate the defined KPIs of the system, and seek the potential to improve the current system.

A group of test data is assumed (consulting the reaction equilibrium curve, thus ensuring that they fall within realistic experimental conditions) to check the calculation tool's accuracy and function. For the lack of experimental data, the results of the test data are not ideal. However, thanks to these assumed test data, it is proven that the calculation tool functions correctly. The calculation process can be finished in several minutes, saving a lot of time otherwise required for the data analysis after the experiments. It also functions as the test model to analyze the experimental data.

In conclusion, this project designed and presents a functioning calculation tool to evaluate the experimental performance of a bench-scale experimental TCS system (being built and commissioned at KTH) for the reaction between SrCl2 and NH3. Some suggestions related to future improvements are proposed as well. For instance, the calculation tool is not automatic enough because it involves manual operation at specific points. Therefore, one of the future tasks is to add the ability to identify the reaction pressure vs temperature curve against the equilibrium conditions and defining whether the process is absorption or desorption automatically. Besides, currently, much electrical equipment is employed in the system, which decreases the system's sustainability, whereas, in future work, the layout of the system can be improved. The system's exergy performance is not analyzed in the thesis report, which can be chosen as another future task.

SAMMANFATTNING

Termokemisk energilagring (TCS) är en teknik som omvandlar värme och kyla till kemisk energi via reversibla kemiska reaktioner, som ska lagras för uppvärmning och kylning. Intresset för TCS-teknik ökar idag för sin mycket kompakta energilagringstäthet som erbjuder ett attraktivt alternativ för termiskenergilagring (TES) för att minska energirelaterade växthusgasutsläpp (GHG) och bidra till hållbar utveckling.

Denna avhandling är en del av paraplyprojektet ”neutroner för värmelagring (NHS)”, finansierat av Nordforsk. Målet med NHS-projektet vid KTH är att designa, konstruera och driva ett TCS-system i bänksala med strontiumklorid (SrCl2) och ammoniak (NH3) som reaktionspar för fast-gas reaktion för värmelagringsapplikationer vid låg temperatur (40-80℃). Detta system har därmed numeriskt utformats och anpassats till praktiska användningsområden och byggs nu på institutionen för Energiteknik på KTH. Med denna bakgrund utformar detta projekt som sitt primära mål, ett beräkningsverktyg för att utvärdera den experimentella prestandan för det ovan beskrivna TCS-systemet i bänkskala. En grundlig förklaring av metoden presenteras här, inklusive förberedning av kompositerna (SrCl2 impregnerat i en expanderad naturlig grafit) samt systemets riskanalys. Kritiskt fokus ligger på systemens prestandaparametrar och den matematiska utformningen av beräkningsverktyget. En genomgång av relevant litteratur genomfördes också för att identifiera de mest relevanta parametrarna. Med hänsyn till användarvänlighet, enkelhet och effektivitet, är beräkningsverktyget utformat med hjälp av Excel. Här väljs energieffektivitet, reaktionsprogression, förändring av reaktionsprogression, den verkliga termiska energidensiteten per massa och praktisk termisk energitäthet per volym som parametrar för att bäst representera systemets prestanda (dvs Key Performance Indicators (KPIs)). Dessa KPIs beräknades främst baserat på massbalans och energibalansuttryck. Med hjälp av detta beräkningsverktyg för detta TCS-systemet på bänkskala kan användaren visualisera den erhållna experimentella datan, beräkna de definierade KPI:erna för systemet och hitta potentialen att förbättra det nuvarande systemet.

En mängd testdata antogs (genom att hänvisa till reaktionens jämviktskurva, vilket säkerställer att de faller inom realistiska experimentförhållanden) för att kontrollera beräkningsverktygets noggrannhet och funktion. På grund av avsaknaden av experimentella data är resultaten av testdata inte optimala. Men med hjälp av den antagna testdatan är det bevisat att beräkningsverktyget fungerar korrekt. Beräkningen kan avslutas efter några minuter, vilket sparar mycket tid som annars krävs för dataanalysen efter experimenten. Det fungerar också som en testmodell för att analysera experimentdata. Sammanfattningsvis utformade och presenterade detta projekt ett fungerande beräkningsverktyg för att utvärdera experimentella prestanda för ett experimentellt TCS-system på bänkskala (byggs och tas i drift vid KTH) för reaktionen mellan SrCl2 och NH3. Några förslag relaterade till framtida förbättringar föreslås också. Beräkningsverktyget är till exempel inte helt automatiserat eftersom det behöver manuell inmatning vid specifika punkter. Därför är en av de framtida uppgifterna att lägga till förmågan att identifiera reaktionstrycket mot temperaturkurvan i förhållande till jämvikten och att definiera om processen är absorption eller desorption automatiskt. För närvarande används mycket elektrisk utrustning i systemet vilket minskar systemets hållbarhet, medan systemet i framtiden kan förbättras. Systemets exergiprestanda analyseras inte i rapporten, vilket kan lämnas för framtida arbete.

FOREWORD

Upon completing the master’s thesis, I am grateful to the ones who encourage and support me in the last half-year. Firstly, profound gratitude should go to my supervisor Dr. Saman Nimali Gunasekara, who is always positive, hardworking, organized, and patient. She guides me with great patience throughout the entire thesis process and shows me the right attitude working as a researcher. Significantly impacted by the Covid-19 pandemic, the thesis's topic has been changed from operating and analyzing the experiments into designing the numerical tool for calculation. Besides, I was in a severe panic in April because of the Covid-19 pandemic. I would like to thank my parents, who encourage me to keep positive even during the worst pandemic period. You are always there for me. Also, I appreciate Dr. Anastasiia Karabanova, who helps me fit in a lab assistant's role and build a cautious attitude towards the lab diary. I would also like to thank my friend Sixiang Lyu and Martin Tholander, who accompany me during the pandemic.

Moreover, I am grateful to Viktoria, who offers her guidance during the meetings as my examiner. As my opponent, Sabarish provides a lot of critical and valuable feedback. Thank you all so much.

Table of Contents

ABSTRACT ... 3 SAMMANFATTNING ... 4 FOREWORD... 5 Table of Parameters ... 7 List of Figures ... 8 List of Tables ... 9 1 Introduction ... 10 1.1 Background ... 12

1.2 Aim and Objectives ... 14

2 System Description ... 15

3 Literature Review ... 18

4 Methodology ... 21

4.1 Composites Preparation ... 21

4.2 Composites Loading into the Reactors ... 22

4.3 Risk Analysis ... 24

4.4 Performance Evaluation Parameters（KPIs） ... 25

4.5 Experimental TCS System’s Detailed Measurements Scheme ... 26

4.6 Mathematical Design ... 28

5 Results and Discussion ... 35

5.1 Description of the Calculation Tool ... 35

5.2 Discussion ... 42

5.3 Sustainability Analysis ... 45

5.3.1 Economic Analysis ... 45

5.3.2 Environmental Analysis ... 45

5.3.3 Social Analysis ... 46

6 Conclusion and Future Work ... 47

6.1 Conclusion ... 47

6.2 Future Work ... 48

6.2.1 Reaction conditions vs equilibrium conditions comparison ability ... 48

6.2.2 Improvements of the system layout ... 48

6.2.3 Exergy analysis ... 49

7 References ... 50

Table of Parameters

Symbol Identification

Unit

T The temperature measured by the thermal sensor ℃ (or K)

Tamb The temperature of the ambiance ℃ (or K)

CpReactor _{The specific heat capacity of the reactor J/(kg·K)}

Cpcomp _{The specific heat capacity of the composites J/(kg·K)}

Cpspacer _{The specific heat capacity of the spacers J/(kg·K)} Cpfins _{The specific heat capacity of the fins in the reactor J/(kg·K)} Cptubes _{The specific heat capacity of the tubes in the reactor J/(kg·K)} Cpnuts&bolts _{The specific heat capacity of the nuts and bolts on the reactor J/(kg·K)} CpTherm_180 _{The specific heat capacity of the therm-180 fluid in the thermal bath J/(kg·K)}

mcomp The mass of the compositions contained in the reactor (monoamine) kg m`comp The mass of the compositions contained in the reactor (octaammine) kg

mReactor The mass of the reactor kg

mspacer The mass of the spacers kg

mfins The mass of the fins in the reactor kg

mtubes The mass of the tubes in the reactor kg

mNuts The mass of the nuts on the reactor kg

mNH3 The mass of the ammonia mol/s

𝑉𝑉̇𝑇𝑇ℎ𝑒𝑒𝑒𝑒𝑒𝑒180 The volume flow rate of the Therm 180 fluid mol/s

MNH3 The molecular weight of ammonia g/mol

F1 The flow rate of ammonia during the absorption process mol/s F2 The flow rate of ammonia during the desorption process mol/s 𝜌𝜌𝑇𝑇ℎ𝑒𝑒𝑒𝑒𝑒𝑒180 The density of the Therm-180 fluid kg/m3

x(t) The reaction advancement \

dx(t)/dt The reaction advancement rate \

∆𝐻𝐻 The enthalpy of the reaction kJ/mol

∆𝑆𝑆 The entropy of the reaction J/(mol·K)

tabs The time of the absorption process s

tdes The time of the desorption process s

Pi The power of the electrical equipment W

p The power output of the system W

Ptheoretical The theoretical power of the system W

Qs,Reactor The sensible heat of the reactor kJ

Qs,comp The sensible heat of the composites kJ

Qs,spacer The sensible heat of the spacers kJ

Qs,finns The sensible heat of the fins kJ

Qs,tubes The sensible heat of the tubes kJ

Qs,Nuts The sensible heat of the nuts kJ

Q`s,Reactor The sensible heat of the reactor kJ

Q`s,comp The sensible heat of the composites kJ

Q`s,spacer The sensible heat of the spacers kJ

Q`s,finns The sensible heat of the fins kJ

Q`s,tubes The sensible heat of the tubes kJ

Q`s,Nuts The sensible heat of the nuts kJ

W The input work of the specific electrical equipment kJ

𝜂𝜂𝑠𝑠 The energy efficiency of the system %

𝛾𝛾𝑡𝑡𝑡𝑡,𝑒𝑒 The thermochemical thermal energy per mass kJ/kg

List of Figures

FIGURE 1CLASSIFICATION OF THERMAL ENERGY STORAGE (TES) ... 11

FIGURE 2.CLASSIFICATION OF THERMOCHEMICAL ENERGY STORAGE (TCS) ... 11

FIGURE 3.THE SYSTEM LAYOUT WITH PIPING AND INSTRUMENTATIONS ... 12

FIGURE 4.THE SYSTEM LAYOUT WITH PIPING AND INSTRUMENTATIONS ... 15

FIGURE 5.NH3 PHASE CURVE(BASED ON DATA FROM [1]) AND SRCL2∙NH3-SRCL2∙8NH3 EQUILIBRIUM PRESSURE-TEMPERATURE PLOT(BASED ON THE VAN’S HOFF EQUATION[7],[13]) ... 16

FIGURE 6.THE FINS, SPACERS, AND HEAT EXCHANGER TUBES IN THE REACTOR. ... 22

FIGURE 7.THE LAYOUT OF THE SYSTEM ONLY CONSISTS OF REACTOR A ... 26

FIGURE 8THE LAYOUT OF THE SYSTEM ONLY CONSISTS OF REACTOR B ... 27

FIGURE 9.THE MASS FLOW IN AND OUT OF THE SYSTEM ... 28

FIGURE 10.THE ENERGY BALANCE DURING ABSORPTION AND DESORPTION PROCESS ... 29

FIGURE 11.THE TEN WORKSHEETS OF THE CALCULATION TOOL... 35

FIGURE 12.THE “INTRODUCTION” WORKSHEET ... 36

FIGURE 13.THE “IDENTIFICATION” WORKSHEET... 36

FIGURE 14.THE LABEL OF THE CATEGORIES ... 36

FIGURE 15.THE “SENSOR DATA” WORKSHEET ... 37

FIGURE 16.TIME VSMEAN TEMPERATURE ... 37

FIGURE 17.TIME VSPRESSURE ... 37

FIGURE 18.CORRELATION BETWEEN MEAN TEMPERATURE AND PRESSURE IN THE REACTOR ... 37

FIGURE 19.TIME VSFLOW RATE (ABSORPTION) ... 38

FIGURE 20.TIME VSFLOW RATE (DESORPTION) ... 38

FIGURE 21.THE “SENSOR DISPLAY” WORKSHEET ... 38

FIGURE 22.THE WORKSHEET “THEORETICAL HEAT” ... 38

FIGURE 23.THE “COMPONENT DATA” WORKSHEET ... 39

FIGURE 24.THE “DISPLAY” WORKSHEET ...40

FIGURE 25.THE “PHASE CURVE” WORKSHEET ... 41

FIGURE 26.THE “REFERENCES” WORKSHEET ... 41

FIGURE 27.TIME VSMEAN TEMPERATURE (TEST) ... 42

FIGURE 28.TIME VSPRESSURE (TEST) ... 42

FIGURE 29.CORRELATION BETWEEN MEAN TEMPERATURE AND PRESSURE IN THE REACTOR (TEST) ... 42

FIGURE 30.TIME VSFLOW RATE(ABSORPTION)_TEST ... 43

FIGURE 31.TIME VSFLOW RATE(DESORPTION)_TEST ... 43

FIGURE 32.THE COMPARISON BETWEEN T-P REACTION CURVE AND THE EQUILIBRIUM CURVE... 43

FIGURE 34.DIAGRAM OF THE SUSTAINABLE ANALYSIS ... 45

FIGURE 33.THE DIAGRAM OF RE-USING THE WASTE HEAT ... 48

List of Tables

TABLE 1.THE KPIS OF THE SYSTEM ... 25

TABLE 2.THE PHYSICAL PROPERTIES OF THE COMPONENTS RELATED TO THE REACTOR ... 31

TABLE 3.THE MACRO USED IN THE CALCULATION TOOL ... 39

TABLE 4.THE OPERATION TIME OF THE ELECTRICAL EQUIPMENT AND ABSORPTION PROCESS_TEST ... 43

TABLE 5.THE KPIS OF THE SYSTEM_TEST ... 44

TABLE 6.THE RISK VALUE (ON THE PERSON) DURING REACTION MEDIA COMPOSITES PREPARATION ... 62

TABLE 7THE RISK VALUE (ON THE EQUIPMENT) DURING REACTION MEDIA COMPOSITES PREPARATION ... 62

TABLE 8THE RISK VALUE (ON THE PERSON) DURING BENCH-SCALE TCS RIG COMMISSIONING AND OPERATION ... 62

1 Introduction

In this chapter, the identification of thermal energy storage (TES) and different kinds of TES technologies are first introduced as valuable methods to decrease conventional energy's environmental pressure. The basis of this master's thesis project regarding the umbrella project (Neutrons for Heat Storage (NHS) ([1], [2]) is presented. Subsequently, the aim & objectives of this project are proposed. Energy is the foundation of the society of human beings. From the wood burnt by the homo sapiens to modern people's fossil fuels from the first industrial revolution, energy is involved in people's history. However, because of the first industrial revolution, people have relied too much on fossil fuels, leading to a wide range of environmental issues, such as global pollution and El Nino [3]. Therefore, more and more people and organizations have realized that some renewable energy sources must replace fossil-based energy. However, renewable energy usually has low energy quality and density. Hence, instead of searching for new types of energy, inspired by batteries' function, some researchers are working hard to store energy by transferring the heat or electricity into other kinds of energy that are easier to be stored. Thermochemical energy storage(TCS), belonging to the thermal energy storage (TES), is one of the main methods to realize the purpose [3].

Thermal energy storage, commonly called heat and cold storage, is storing heat or cold energy. Hence, the processes must be reversible. The thermal energy can be divided into sensible heat and latent heat energy. Considering the primary process for thermal energy storage technology is to store and re-use the heat stored, the TCS technology can be divided into sensible heat storage technology and latent heat storage technology if there are only physical processes [4]. Sensible heat storage technology consists of three kinds of working phases: gas, liquid, and solid. Simultaneously, the latent heat storage technology can be divided into the solid-liquid type, solid-solid type, and liquid-gas type[5]. When the processes involve chemical reactions, the reactant can be divided into solid, gas, and liquid. The classification of thermal energy storage technology (TES) can be found in Figure 1.

Figure 1 Classification of thermal energy storage (TES)

1.1 Background

The thesis project is based on the neutrons for the heat storage (NHS) project funded by Nordforsk, which is a Nordic collaboration between the Institute for Energy Technology (IFE), Amminex Emission Technology (AET), Technical University of Denmark (DTU), and KTH [6]. The NHS project's primary purpose is to design and develop a thermochemical heat storage system for low-temperature heat storage (40-80) ℃. At DTU and IFE, collaborating with AET, a 3D-COMSOL simulation for the reactor cell, which was decided by DTU, has been developed and validated based on the experimental data. Thanks to Neutron Radiography (NR) imaging technology, the reaction progression's experimental characterization is that the reactor cell has been reached. For further study, the preliminary design of a TCS system based on NH3-SrCl2 has been proposed by KTH, where the TCS reactor has been designed and calculated using CFD modeling in COMSOL Multiphysics [6].

The materials used in this project are Strontium Chloride and ammonia. Strontium Chloride (SrCl2) is a colorless cubic crystal with a molecular mass of 158.52 g/mol [7]. SrCl2 belongs to the low-toxic chemicals category. It has slightly irritating properties, e.g., if it comes in contact with eyes, skins, and inhalation when using it. SrCl2 is hygroscopic and thus is easily soluble in water and deliquescent in the air [7]. Ammonia (NH3) is a colorless gas with a pungent irritating smell. NH3 is easily soluble in water and ethanol. NH3 can burn the skin, eyes, and mucous membranes if in high-enough concentrations. Inhalation of concentrated NH3 can cause lung swelling and even death in extreme cases [8]. Therefore, a risk analysis is required to be performed in this project.

The packed-bed storage reactor designed is filled with layers of reaction media composite, which is expanded natural graphite (ENG) in which SrCl2 is impregnated. The ENG improves the composite’s heat and mass transfer performance and decreases the swelling that could happen during the reaction[9]. The main advantage of packed-bed storage is the high degree of stratification. When the temperature inside the reactor is uniformly distributed, the storage system is fully charged [4]. The salt (SrCl2) incorporated with ENG has been compared numerically with computational fluid dynamics (CFD) modeling with salt-only packed-bed in the reactors. According to these simulation results, for a reaction time of 15 hours, the salt-ENG composites' reaction advancement reaches over 0.85, whereas for the salt-alone reactor is only around 0.55 [10]. It would be interesting to compare the simulation results with the experimental results to validate and refine the numerical models. That is, therefore, one main objective of the operation of this bench-scale experimental TCS system.

The numerical design of this bench-scale TCS system, considered in this project, was previously designed on ASPEN PLUS ([6], [11]), the simulation software. This numerical design is based on the reversible reaction between ammonia and strontium chloride and comprises the component layout shown in Figure 3.

The process can be operated both in absorption and desorption directions, in each respective half-cycle of the system, to meet heating (heat release) and cooling (heat storage) purposes. The absorption cycle is utilized for heating (i.e., heat release) requirements, and desorption is for cooling (i.e., heat storage) requirements. Two identical reactors are arranged in this storage system, with an individual capacity of 0.5 kWh, to simulate the absorber and desorber. Apart from the two reactors, the system involves a liquid ammonia tank (c.f. Figure 3) that functions as the buffer and reactant (absorbate) provider. The absorption process begins from the ammonia tank (kept at, e.g., 10 bars at 25 ℃, maintaining NH3 at liquid state). From this NH3 storage tank, on the way to the absorber (79 ℃, 8 bar), as can be seen in Figure 3 the ammonia is heated by the heater (18-25 ℃) after passing the expansion valve (8-10 bar), as the expansion tends to cool the NH3. Whereas, the desorption process involves, by order, the desorber (82 ℃, 8 bars), Cooler1 (25-35 ℃, 8 bar), a compressor, Cooler2 (0-25 ℃, 10 bar), and the liquid ammonia storage tank (10 bars at 25 ℃), as Figure 3 shows. According to this previous numerical study, the thermal energy storage system's efficiency would be 67% for the absorption process and 61 % for the desorption process ([6], [11]).

In this TCS operation, the reaction can be divided into two steps. Firstly, after evaporating the liquid ammonia into gas, pure strontium chloride would react with ammonia (i.e., absorb) to produce monoamine (𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆2∙ 𝑁𝑁𝐻𝐻3). Then, monoamine would further react with ammonia to produce octaammine

(𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆2∙ 8(𝑁𝑁𝐻𝐻3)). The enthalpy of the reaction between monoamine and octaammine is 41.432 kJ/mol.

For the last ammine, the enthalpy is 48.1 kJ/mol [7], where the salt undergoes an enormous volume change and requires the most considerable energy change. Therefore, the designed system [6] excludes the last ammine conversion to enable a more straightforward practical operation (avoiding extreme temperature and pressure conditions and too large volume change), restricting the TCS reversible reaction to be only between monoammine and octaammine. Hence, the reaction is assumed to be restricted to Eqn. 3 and Eqn. 4, representing the exothermic process (i.e., absorption) and endothermic process (desorption), respectively [6].

𝑁𝑁𝐻𝐻3(𝑙𝑙)+ ∆𝐻𝐻𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒⇔ 𝑁𝑁𝐻𝐻3(𝑔𝑔) Eqn. 1

𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆2(𝑠𝑠)+ 𝑁𝑁𝐻𝐻3(𝑔𝑔)⇔ 𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆2∙ (𝑁𝑁𝐻𝐻3) + ∆𝐻𝐻 Eqn. 2

𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆2∙ 𝑁𝑁𝐻𝐻3(𝑠𝑠)+ 7𝑁𝑁𝐻𝐻3(𝑔𝑔)⇔ 𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆2∙ (𝑁𝑁𝐻𝐻3)8+ ∆𝐻𝐻 Eqn. 3

𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆2∙ (𝑁𝑁𝐻𝐻3)8+ ∆𝐻𝐻 ⇔ 𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆2∙ 𝑁𝑁𝐻𝐻3(𝑠𝑠)+ 7𝑁𝑁𝐻𝐻3(𝑔𝑔) Eqn. 4

1.2 Aim and Objectives

The main aim of this thesis project is to design a digital tool to calculate the required variables for evaluating the Key Performance Indicators (KPIs) of the TCS system. These KPIs are parameters that can be used to evaluate the effectiveness of achieving critical objectives by the system [12]. Utilizing TCS technology, this project's primary reaction is the reaction between ammonia (NH3) and strontium chloride (SrCl2), which is shown in Eqn. 3. The reaction could happen in either direction, which depends on the reaction conditions. Therefore, it enables the absorption and desorption process so that the system can function both for heating and cooling purposes.

The system is operated by the reaction between ammonia and strontium chloride to store and release heat to be applied for low temperature (40-80 ℃) applications. The primary focus of the thesis project is on digital tool design. As there is no experimental data available yet, certain experimental data will be assumed, be within the reaction conditions suitable for this system (consulting the system’s equilibrium curve) to test the calculation tool's function. The expectation here is that, after experiments, the user can input the real experimental data and receive the results immediately by employing this calculation tool. Therefore, the mathematical model should be as realistic and reliable as possible so that the calculation tool's results can be valid.

Therefore, the objectives of the thesis project are shown as following:

 _{Perform a literature review to critically evaluate the current status of the performance evaluation} of metal halide-ammonia TCS systems and thereby select the most suitable performance evaluations criteria for this particular system (including the KPIs)

 Perform the risk analysis for this experimental system operating under high pressure and temperature

 _{Design a digital tool to be used to evaluate the performance of the system using the experimental} measurements from real system operation, with the capability to provide all the identified KPIs and other critical performance indicators as tool outcomes automatically

 Critically and comparatively analyze and discuss the outcomes of the digital tool to ensure the inputs have been calculated in an accurate way to deliver the chosen outputs

 _{Based on the obtained results synthesis, propose suggestions to improve the performance of the} system for the bench-scale rig operation

2 System Description

After some improvements and adaptations (mainly concerning practical aspects, see section 1.1), the final designed system layout with piping and instrumentations is shown in Figure 4 [6]. There are fifty-seven thermal sensors planned to be used in the TCS system, gathering the temperature information along the absorption and desorption process. Thirty-eight of them are located in the reactors to measure the temperature inside and outside the reactors. Also, there are five pressure indicators in the system monitoring the pressure of the two reactors, compressed NH3 after the compressor, NH3 tank, and the expanded NH3 after the expansion valve, respectively, during the two processes. Moreover, there are two flow meters to measure ammonia's flow rate during the absorption and desorption. These parameters would be used for the incorporated KPIs calculation and evaluation of the TCS system.

Figure 4. The system layout with piping and instrumentations

The absorption half-cycle begins from the NH3 storage tank, passing through the expansion valve, the heater, and ends at the reactor. While the desorption half-cycle starts from the reactor, passing through cooler 1, compressor, cooler 2, and ends at the NH3 storage tank. The whole system (i.e, each reactor, respectively) begins at the initial step of the absorption process. After all the 𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆2∙ 𝑁𝑁𝐻𝐻3 is reacted into

𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆2∙ 8𝑁𝑁𝐻𝐻3, the absorption process is over. The fully-absorbed reactor operation is then switched into

the desorption process under the required pressure and temperature conditions. The two 3-way valves (controlled manually) enable the alternating connection of the two reactors into the relevant cycle paths to function as absorber and desorber during absorption and desorption.

The ammonia-SrCl2 thermochemical storage system's equilibrium conditions (and the resulting equilibrium pressure versus temperature curve) can be obtained by employing Van't Hoff equation [13], as shown in Eqn. 5. 𝑃𝑃𝑒𝑒𝑒𝑒= 𝑃𝑃0∙ 𝑒𝑒𝑥𝑥𝑝𝑝 �−∆𝐻𝐻_𝑅𝑅 𝑅𝑅 0𝑇𝑇 + ∆𝑆𝑆 𝑅𝑅0� Eqn. 5

Where: 𝑃𝑃𝑒𝑒𝑒𝑒 is the equilibrium pressure (Pa)

𝑃𝑃0 is the reference pressure (equals to 1 Pa)

∆𝐻𝐻𝑅𝑅 is the enthalpy of the reaction (J/mol)

∆𝑆𝑆 is the entropy of the reaction (J/(mol∙K))

𝑅𝑅0 is the ideal gas constant (equals to 8.314 J/(mol∙K))

𝑇𝑇𝑒𝑒𝑒𝑒 is the equilibrium temperature (K)

Hence, the desorption and absorption zones can be chosen as in Figure 5 [6]

Figure 5. NH3 phase curve(based on data from [1]) and SrCl2∙NH3-SrCl2∙8NH3 equilibrium pressure-temperature plot(based on the Van’s Hoff equation[7], [13])

Similarly, during the desorption process, the thermal bath functions as the external heater as the reaction are endothermic. With the heat offered by the thermal bath, the reaction would keep going until 𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆2∙

8𝑁𝑁𝐻𝐻3 is fully converted into 𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆2∙ 𝑁𝑁𝐻𝐻3. Unlike the absorption process, the NH3 storage tank should

3 Literature Review

As mentioned in Section 1.2, to reach higher thermal density and performance efficiency, various efforts have been invested in storing and reusing heat and cold energy. During the last five years, both short-term TCS and long-short-term TCS have experienced considerable developments. Unlike the other TES systems (i.e., sensible TES and latent TES), the evaluation of TCS systems requires consideration of heat transfer and mass transfer. A TCS system's performance usually depends on its working pairs, chemical reaction, physical processes, the thermodynamic cycle, and the heat exchangers (HEXs) in the system [9].

The reactants for the absorption TCS system are usually MeX-water or MeX-NH3. MeX means the metal compound, such as MnCl2. For example, according to the related research, the working pairs include CaCl2-NH3 ([14]), SrCl2-NH3 ([15], [16]), CaO-H2O ([17]–[19]) , MnCl2-NH3 ([9], [20]), MnCl2-CaCl2-NH3 ([21]), MnCl2-SrCl2-NH3 ([22]).

Ammonia (NH3) is considered as one of the best working fluids in the absorption TCS systems as ammonia has no pressure limitations caused by the hydration process as in the water-based systems [23]. Even though the metal chloride-ammonia sorption systems tend to have comparatively high storage densities, they have certain disadvantages. One of the main issues is that these chemically reactive salts have low thermal conductivities, which leads to a long time taken for a cycle completion [23]. Also, swelling and agglomeration might happen during the reaction process, which leads to higher heat losses and insufficient reaction completion [23]. In Critoph et al., 2004 [23], the system using granular beds of pure MnCl2 only has a thermal conductivity of around 0.1 W/(m·K). Whereas the one using ENG- MnCl2 as the composites has a thermal conductivity of 10-40 W/(m·K). To reduce or overcome the impact of these disadvantages, forming composites of these salts with a material with better thermal conductivity has become one of the best solutions in the related research. The MnCl2 is embedded into the thermal conductivity enhancement material, usually expanded graphite, to form the composites. Then, the composites are used as the reactant. Thanks to the composites, the swelling and agglomeration phenomena also decrease distinctly [23].

Li et al., 2013 [14] analyzed a thermochemical energy storage system using a CaCl2-NH3 sorption reaction. The system consists of a low-temperature evaporator, a low-temperature Solid/Gas (S/G) reactor, a high-temperature evaporator, and a high-temperature S/G reactor. This study mainly focuses on the energy aspects, evaluating the system concerning the useful heat released/stored, the TES density, the system's power, and its energy efficiency. The experimental results show that the CaCl2-NH3 TCS system can store thermal energy during summer days, which can be used in winter days. In another study, Li et al., 2015[15] evaluated a SrCl2-NH3 TCS system both in a short-term and long-term TES operation. The system contains an NH3 storage tank, an evaporator, an S/G reactor, and a condenser. In this paper, the short-term mode is the operating mode of storing thermal energy on sunny days and using the stored thermal energy on cloudy days. Simultaneously, the long-term mode is the mode to store solar energy in summer and use it in winter. The energy storage density is identified as a parameter to evaluate the TCS system's performance, which is the energy stored by the composites divided by the composites' mass. Apart from the energy storage density, energy efficiency, and power during heat production and cold production are studied individually in this paper.

kinetic and potential energy changes, neglecting the heat losses, one single cycle involving both absorption and desorption process, and the S/G reactors turning in steady-state devices immediately [16]. Also, the reactor's mass and the evaporator are assumed to be five times that of the reactants. It offers approaches to calculate the mass balance, energy balance, and the exergy balance of different components because of its function as a closed system. When calculating the mass balance, the decreased mass was considered as equal to the increased mass. For example, during the desorption process, the decreased mass of octaammine is equal to the sum of the increased mass of monoamine and ammonia. When calculating the heat balance of different components, the heat input was considered as equal to the heat output. Similarly, the exergy balance of different components was also reached. However, when calculating the exergy balance, the standard chemical exergy of SrCl2 and NH3 was not stated in the article [16]. Besides, as the system operates under a cycle involving both absorption and desorption process, the system's overall entropy change is assumed to be 0 [16]. Hence, the mass balance and energy balance proposed are comprehensive and inspiring, but the exergy balanced approach can be improved.

Yan, Wang, and Li, 2018 [9] and Mofidi et al., 2017 [20] studied TCS systems using MnCl2-NH3 as the working pair. Yan, Wang, and Li, 2018 [9] are more interested in the operating temperature, corresponding heat production per volume, and heat storage efficiency. In contrast, Mofidi et al., 2017 [20] focus on building a numerical model for calculating the heat transfer coefficient, respectively, during the absorption and desorption processes. Similarly, Yan et al., 2019 [22] designed a TCS system using MnCl2-SrCl2-NH3 as the reactants. Their emphasis is on the system's energy performance, in-terms of such as the released/stored heat per mass of reactant, energy efficiency, reaction advancement, and energy efficiency. Also, Jiang et al., 2016 [21] evaluated a TCS system using MnCl2-CaCl2-NH3, concentrating on its energetic aspects, including the released and stored heat, thermal density, and energy efficiency.

According to these discussed literature, the TCS system evaluations mainly focus on the system's mass balance and energy balance. According to the mass balance, the stored/released heat can be calculated according to the related reaction shown by Eqn. 6, serving for the energy calculation. When calculating the thermal energy, the heat can be divided into two main parts, which are sensible heat and reaction heat. The reaction heat calculation is based on Eqn. 6 as well, corresponding to the amount of the reactants in the system, as shown by Eqn. 7. When calculating the sensible heat, the literature mentioned above calculates the heat by time series, as explained by Eqn. 8. A common attribute of these discussed literature is that the heat losses are neglected during the calculation. Also, the standard chemical exergy value of the reactants was not mentioned in any of the analyzed literature.

�𝑀𝑀𝑒𝑒𝑀𝑀 + 𝐴𝐴_{𝑀𝑀𝑒𝑒𝑀𝑀 ∙ 𝐴𝐴 + ∆𝐻𝐻}𝐴𝐴𝑏𝑏𝑠𝑠𝑏𝑏𝑒𝑒𝑒𝑒𝑡𝑡𝑏𝑏𝑏𝑏𝑏𝑏�⎯⎯⎯⎯⎯⎯⎯� 𝑀𝑀𝑒𝑒𝑀𝑀 ∙ 𝐴𝐴 + ∆𝐻𝐻 𝐷𝐷𝑒𝑒𝑠𝑠𝑏𝑏𝑒𝑒𝑒𝑒𝑡𝑡𝑏𝑏𝑏𝑏𝑏𝑏 �⎯⎯⎯⎯⎯⎯⎯� 𝑀𝑀𝑒𝑒𝑀𝑀 + 𝐴𝐴 Eqn. 6 𝑄𝑄𝑒𝑒𝑒𝑒𝑒𝑒𝑡𝑡𝑡𝑡𝑏𝑏𝑏𝑏𝑏𝑏= � 𝑚𝑚̇ ∙ ∆𝐻𝐻 ∙ 𝑑𝑑𝑑𝑑 Eqn. 7 𝑄𝑄𝑠𝑠𝑒𝑒𝑏𝑏𝑠𝑠𝑏𝑏𝑏𝑏𝑙𝑙𝑒𝑒 = � 𝑆𝑆𝑒𝑒∙ 𝑚𝑚 ∙ 𝑑𝑑𝑇𝑇 𝑇𝑇𝑖𝑖+1 𝑇𝑇𝑖𝑖 Eqn. 8

𝜂𝜂 = 𝑄𝑄𝑏𝑏𝑜𝑜𝑡𝑡

4 Methodology

This chapter introduces the methodology applied in this project. Because of the coronavirus issue happening during the thesis project, the thesis topic has been changed from a purely experimental project to designing a calculation tool to evaluate the experimental rig's performance. However, to better design the digital tool, some essential experimental preparations were still required, such as preparing the composites (SrCl2+ENG). ENG stands for Expanded Natural Graphite. Besides the contributions to preparing the composites, a risk analysis of the experimental rig was also conducted in this project. Therefore, these contributions are also explained briefly in this chapter, besides the numerical calculation tool's main contribution. After searching, comparing, and analyzing the literature related to the TCS based on SrCl2 and ammonia's reaction, the best-suited parameters to evaluate the system performance are proposed. Moreover, the mathematical design applied in the calculation tool development is explained for transparency, clarity, and any eventual upgrades required by the actual experimental rig operation.

4.1 Composites Preparation

Before filling the reactor with composites, the composites need to be prepared. The composites mentioned in this project are ENG impregnated with SrCl2. For this, several steps are required to be taken.

Firstly, a piece of ENG with the size of 29𝑐𝑐𝑚𝑚 × 19𝑐𝑐𝑚𝑚 × 2𝑐𝑐𝑚𝑚 is cut out from the larger SIGRATHERM®L20/1500 board [24]. The surface of the cut ENG should be as smooth as possible. This cut ENG block’s real dimensions were then measured. The required mass of SrCl2 powder (here 95% pure SrCl2 is used) was also measured, which was calculated to account for 1.6g of 33.3 wt% SrCl2 solutions of 1 cm3 _{of ENG. Here, a 10 % margin has been applied to this exact requirement of the mass} of SrCl2, ensuring the maximum amount of impregnation. The salt is kept in one or more beakers and dried along with the cut ENG block at 150 ℃ under vacuum conditions 24 hours. This drying was performed to remove any eventually absorbed moisture both in the ENG and this SrCl2, mainly as SrCl2 is a very hygroscopic material. When determining the required amount of SrCl2, it is assumed that all the SrCl2 available has been converted into hexahydrate of SrCl2 due to ambient moisture. After being dried for 24 hours, the mass of the dried ENG and SrCl2 are measured. Afterward, the ENG is soaked by submerging entirely in 99% pure ethanol for 5 hours.

Meanwhile, an aqueous solution of SrCl2 is prepared, where the required amount of water is calculated based on the mass of SrCl2. It is calculated based on the data from earlier composites preparation experiments performed by the project researchers at DTU to obtain an optimal impregnation. Therein, 1.6 g of 33.3% w/w SrCl2 aqueous solution is required to soak 1 cm3 _{of ENG. Directly after soaking in} ethanol is finished, the ENG block is then soaked, kept completely submerged in the SrCl2 solution for 72 hours. As ENG has a lower density than water or ethanol, it tends to float when inserted in these liquids, and thus two beakers containing water were kept on top of the ENG block during both of the soaking processes. Then, the SrCl2 soaked ENG block, i.e., the composite, is dried at 90 ℃ for 24 hours in an oven, followed by vacuum drying at 150 ℃ for another 24 hours. At certain instances, several composites blocks were prepared simultaneously.

4.2 Composites Loading into the Reactors

The reactor can be divided into several components, including the layers, the spacers, the heat exchanger tubes, the reactor’s body, the reactor’s lids, and nuts and bolts. The fins are in function of holding the composites, the holes on which are the ammonia paths. As the composites are soft, the spacers help to keep the distance between different layers so that the composites would not be crushed. The heat exchange tubes are in charge of transferring thermal energy. The nuts and bolts connect and seal the reactor’s lids and body. The composites are cut into 4 cm wide layers and packed to fit into the reactor's inner cylindrical space. Then, the composites layer height was decided as 4 cm. In total, there are eight fins and seven composites-packed layers in one reactor.

Figure 6. The fins, spacers, and heat exchanger tubes in the reactor.

The useful volume for filling the composites in one reactor 𝑉𝑉𝑜𝑜𝑠𝑠𝑒𝑒𝑢𝑢𝑜𝑜𝑙𝑙 can be calculated according to the

formulae below: 𝑉𝑉𝑜𝑜𝑠𝑠𝑒𝑒𝑢𝑢𝑜𝑜𝑙𝑙= (𝑉𝑉𝑙𝑙𝑒𝑒𝑙𝑙𝑒𝑒𝑒𝑒𝑠𝑠− 𝑉𝑉𝑠𝑠𝑒𝑒𝑒𝑒𝑡𝑡𝑒𝑒𝑒𝑒𝑠𝑠− 𝑉𝑉𝐻𝐻𝐻𝐻𝐻𝐻 𝑡𝑡𝑜𝑜𝑏𝑏𝑒𝑒𝑠𝑠) × 7 Eqn. 10 𝑉𝑉𝑙𝑙𝑒𝑒𝑙𝑙𝑒𝑒𝑒𝑒𝑠𝑠=𝜋𝜋 ∙ 𝐷𝐷 2_{∙ ℎ} 𝑙𝑙𝑒𝑒𝑙𝑙𝑒𝑒𝑒𝑒 4 × 7 = 5372.37 𝑐𝑐𝑚𝑚3 Eqn. 11 𝑉𝑉𝑠𝑠𝑒𝑒𝑒𝑒𝑡𝑡𝑒𝑒𝑒𝑒𝑠𝑠=𝜋𝜋 ∙ ℎ𝑠𝑠𝑒𝑒𝑒𝑒𝑡𝑡𝑒𝑒𝑒𝑒∙ (𝐷𝐷 2_{− 𝑑𝑑}2₎ 4 × 7 = 103.78 𝑐𝑐𝑚𝑚3 Eqn. 12 𝑉𝑉𝐻𝐻𝐻𝐻𝐻𝐻 𝑡𝑡𝑜𝑜𝑏𝑏𝑒𝑒𝑠𝑠=𝜋𝜋 ∙ 𝑑𝑑𝑡𝑡𝑜𝑜𝑏𝑏𝑒𝑒𝑠𝑠 2_{∙ ℎ} 𝑡𝑡𝑜𝑜𝑏𝑏𝑒𝑒𝑠𝑠 4 × 5 Eqn. 13

Where:𝑉𝑉𝑙𝑙𝑒𝑒𝑙𝑙𝑒𝑒𝑒𝑒𝑠𝑠 is the volume between the fins (cm3)

𝑉𝑉𝑠𝑠𝑒𝑒𝑒𝑒𝑡𝑡𝑒𝑒𝑒𝑒𝑠𝑠 is the volume of the spacers between the layers and the inner wall of the reactor (cm3)

𝑉𝑉𝐻𝐻𝐻𝐻𝐻𝐻 𝑡𝑡𝑜𝑜𝑏𝑏𝑒𝑒𝑠𝑠 is the volume of the heat exchanger tubes running vertically between the layers (cm3)

ℎ𝑙𝑙𝑒𝑒𝑙𝑙𝑒𝑒𝑒𝑒 is the height of each layer, which is 40 mm

ℎ𝑠𝑠𝑒𝑒𝑒𝑒𝑡𝑡𝑒𝑒𝑒𝑒 is the height of each spacer, which is 40 mm

ℎ𝑡𝑡𝑜𝑜𝑏𝑏𝑒𝑒𝑠𝑠 is the total height of the heat exchanger tubes in the assigned zone for composites, which

is 430 mm

𝑑𝑑 is the diameter up to the inside surface of the spacers, which is 156.3 mm 𝑑𝑑𝑡𝑡𝑜𝑜𝑏𝑏𝑒𝑒𝑠𝑠 is the diameter up to the outside surface of the tubes, which is 10 mm

As a result, the maximum volume available for the composites in one reactor is 5099.73 cm3_.

Therefore, the maximum amount of mass of the salt (SrCl2) in one reactor can be calculated, as shown below:

𝑚𝑚𝑠𝑠𝑒𝑒𝑙𝑙𝑡𝑡 = 0.31 𝑔𝑔/𝑐𝑐𝑚𝑚3× 𝑉𝑉𝑜𝑜𝑠𝑠𝑒𝑒𝑢𝑢𝑜𝑜𝑙𝑙 𝑐𝑐𝑚𝑚3= 1588.73 𝑔𝑔 Eqn. 14

As the molar mass of SrCl2: 𝑀𝑀𝑆𝑆𝑒𝑒𝑆𝑆𝑙𝑙2 is 158.52 g/mol, the number of moles of SrCl2 in the reactor

𝑛𝑛𝑆𝑆𝑒𝑒𝑆𝑆𝑙𝑙2can be calculated, as shown below:

𝑛𝑛𝑆𝑆𝑒𝑒𝑆𝑆𝑙𝑙2 =

𝑚𝑚𝑠𝑠𝑒𝑒𝑙𝑙𝑡𝑡

𝑀𝑀𝑆𝑆𝑒𝑒𝑆𝑆𝑙𝑙2

= 10.06 𝑚𝑚𝑚𝑚𝑆𝑆 Eqn. 15

Thus, the maximum number of moles of monoammine is 10.03 mol in one reactor.

According to the reaction shown in Eqn. 3, the moles of ammonia required for a complete absorption reaction (to convert all SrCl2⋅NH3 to SrCl2⋅8NH3) in one reactor is:

𝑛𝑛𝑁𝑁𝐻𝐻3= 10.06 ∗ 7 = 70.41 𝑚𝑚𝑚𝑚𝑆𝑆

The reaction enthalpy of the system for monoammine to octaammine conversion, ∆𝐻𝐻 = 41.43 𝑘𝑘𝑘𝑘/ 𝑚𝑚𝑚𝑚𝑆𝑆𝑁𝑁𝐻𝐻3. Hence, the maximum possible stored/released heat of one reactor is 2917.3 kJ (0.81 kWh), and

4.3 Risk Analysis

Risk analysis is the systematic analysis of a system, process, and similar, using available and reliable information to identify the hazards and evaluate the risk to people, equipment, and the environment. A risk analysis can be divided into three main steps: identifying the hazards, the analysis of the consequences, and the measures to reduce or avoid the consequences [25]. The hazards can be considered, the hidden danger to operators, equipment, and the environment. Therefore, the hidden risks must be identified first. Then, the reasons for the hidden dangers should be analyzed. The consequences of the hidden dangers should be identified and mentioned. Finally, the measures to reduce or avoid the impact of the consequences should be proposed. The risk analysis is enormously important, as it can significantly increase the operators' safety and the project's financial condition. Accordingly, this current project's risk analysis starts by first explaining the system, identifying hazards, analyzing the risks, and reducing or avoidance, as described in the following.

The corresponding system in this thesis project is the TCS experimental rig that is being constructed. That system- and its preparation-incorporated hazards can be divided into two categories: the hazards during the preparation of the composites and the hazards during the rig's experimental operation. Hence, the risk analysis is also presented here in two categories: composites preparation and experimental rig operation.

The reactor is an essential component of the whole system, where the reaction between ammonia and SrCl2

happens. There are two identical reactors, and each of them functions as the absorber and desorber alternatingly, under different process conditions. By impregnating the SrCl2 into ENG boards (c.f. section

4.1), reaction media composites are prepared to improve the reaction efficiency and the reaction advancement rate, as well as to minimize the solid reactant swelling phenomenon. Each thermostat bath is connected to each reactor, functioning as the external cooler during the absorption process and as the external heater during the desorption process. The heat exchange inside each reactor is enhanced by using perforated fins made of Aluminum (Al), through which five Al tubes carrying the heat transfer fluid (a silicon oil) pass. The reaction media is sandwiched between these fins and tubes, with seven layers (4 cm height each) is packed between eight fins in each reactor. Therefore, there are hazards related to the chemical components (i.e., SrCl2 and

NH3), heat, pressure, fluid leakage, and heavy components.

Also, multiple valves control the operation and the stopping of the system, with an expansion valve that functions as the central controller of the system's absorption half-cycle. There are two 3-way valves in the system, which decide the direction of the mass flow and control each reactor's operating process, allowing process alteration. Hence, there are hazards concerning the leakage of NH3 through these valves (e.g., system

limit conditions are exceeded).

Besides, around 57 temperature sensors are utilized to monitor and gather temperature data at different locations for operational control and further analysis. The ammonia flow would be monitored by two separate flow meters in the system, placed just before the absorption reactor and right after the desorption reactor. Meanwhile, the pressures inside the ammonia tank, the two reactors, and that after the compressor are measured, using pressure transducers, for control and analysis. As a result, there are hazards related to irregular/improper monitoring and management of the system regarding malfunctions in the thermal sensors, flow meters, and pressure transducers.

There is an electrical heater within the absorption process before the reactor fully ensures the ammonia is in the gas phase before entering the absorption reactor. There are two plate heat exchangers and one compressor on the desorption process to increase the ammonia pressure and decrease the ammonia flow after the desorption reaction. The temperature sensor before the ammonia tank and the pressure tap after the compressor help to monitor the condition before the ammonia tank to ensure that the ammonia flowing into the ammonia tank would maintain in the liquid phase. In this part of the process, therefore, there are hazards related to electrical equipment.

4.4 Performance Evaluation Parameters（KPIs）

In this work, the primary reactants during the process are monoamine, octaammine, and ammonia. The system utilizes ENG impregnated with SrCl2 to increase the composite's overall thermal conductivity for better heat transfer and faster chemical reaction and decrease the volume change impact.

In this section, the selection of the most relevant and essential parameters to evaluate the TCS system performance is explained, done mainly based on a literature review. The evaluation parameters need to be identified first, followed by the identification of the KPIs. The mathematical formulae to calculate the parameters would then be derived and verified to ensure the accuracy of the calculation tool. The KPI is the value of evaluating the effectiveness of achieving critical objectives by the system [2], summarized in Table 1 below.

Based on the literature review discussed in Chapter.3, several parameters are chosen as the performance evaluation parameters, including the amount of released/stored heat (kJ), the energy efficiency of the reactor (%), the total time for absorption/desorption (s), the power of the absorber/desorber (W), the thermal density in each reactor per mass of solid reactant (kJ/kg), the thermal density per volume (kJ/L), and the reaction advancement. These parameters can reflect the system's performance, consequently devoting to the KPIs of the system.

Among these, the Storage Capacity (KPI 1) reflects the heat released/stored by each reactor in the TCS system. Energy Efficiency is considered the second KPI, which shows the rate between the released/stored heat and the reactor's total energy input. KPI 3, the Minimum Cycle Length, mirrors the minimum time required to complete a single cycle. The cycle means the time for absorption or desorption, depending on which process is considered in the system analysis. Nevertheless, this KPI can stand for absorption or desorption time when the specific process is under operation.

The next KPI is Power, which is the ratio between the released/stored heat and the time it takes, which indicates the heat releasing/storage ability of the TCS system per unit time. KPI 5, which is Thermal Density per Mass, shows the ratio between the released/stored heat and the mass of the composites after the reaction, reflecting the real performance of the TCS system during the experimental operation. Similarly, KPI 6, which is Thermal Density per Volume, shows the ratio between the released/stored heat and the composites' useful volume, indicating the system performance during the experimental operation. The last KPI is chosen as the Maximum Reaction Advancement, which presents the reactant conversion percentage. Even though exergy analysis is mentioned in some literature, the methodology to reach the results could not be fulfilled because the standard chemical exergy of monoamine and octaammine is unknown. Consequently, the exergy analysis is not included in this report.

Table 1. The KPIs of the system

KPI 1 KPI 2 KPI 3 KPI 4 KPI 5 KPI 6 KPI 7

Storage

4.5 Experimental TCS System’s Detailed Measurements Scheme

There are two reactors involved in the TCS system, as is mentioned in Chapter.2, which can function individually. These are labeled as reactor A and B, respectively. Among the whole process, all the thermal sensors, pressure indicators, and flow meters have been labeled individually, as depicted in Figure 7 and Figure 8, corresponding to the system’s connection to reactor A and B, respectively. During the absorption process, T24 and P3 show the temperature and pressure of the ammonia tank individually. Then, T25 and P4 indicate the temperature and pressure after the expansion valve, respectively. After the electrical heater heats the ammonia flow, the ammonia's temperature and flow rate are measured using T26 and F1 sensors.

The thermal sensors and pressure indicator directly related to Reactor A are labeled as TA1-TA23 and P1, shown in Figure 7 below. In contrast, the ones directly associated with Reactor B are labeled as TB1-TB23 and P5, shown in Figure 8. From TA/B 1 to TA/B 18, the thermal sensors measure the composites' temperature at different levels of layers in the reactors. In contrast, TA/B 19 measures the temperature outside the reactor because the temperature inside and outside the reactor are different. TA/B 22 shows the temperature before entering the reactor, while TA/B 22 measures the heat transfer fluid (HTF)'s temperature when it is exiting the reactor. TA/B 20 measures the temperature of the HTF exiting the reactor (to be sent to the thermal-bath), and TA/B 21 indicates the temperature of the HTF entering the reactor (coming from the thermostat bath). The pressure indicator P1 measures Reactor A's pressure, while P5 measures the pressure inside Reactor B.

The thermal sensors, pressure taps, and flow meter assigned within the desorption process have been labeled. T27 and F2 measure the ammonia flow temperature and flow rate after the 3-way valve of the desorption path. Cooler 1 is set between the compressor and the 3-way valve to decrease the ammonia temperature coming from the desorber. The inlet and outlet temperatures of the secondary fluid (i.e., district cooling water) side of cooler 1 are marked as T31 and T32. T28 is in charge of measuring the NH3 temperature after cooler1. As the pressure and temperature of the ammonia flow would change significantly after the compressor, P2 and T29 measure the pressure and temperature after the compressor. Like cooler1, cooler 2 is used to decrease the ammonia flow temperature, pressurized after the compressor, to ensure the ammonia gas is turned into liquid completely. T30 indicates the temperature of NH3 before the ammonia tank, while T33 and T34 show cooler 2's secondary fluid (i.e., district cooling water) inlet and outlet temperature, respectively.

4.6 Mathematical Design

When designing the calculation tool, the mathematical methodology is vital to ensure the tool's accuracy. In this section, the mass balance and energy balance analysis of the system is explained.

The mass balance means the summary of the mass flow in and out of the system. Generally, the system's initial mass equals the sum of the system's final mass and the mass flowing out of the system [9]. The system boundary of the absorber/desorber is the outside surface of the absorber/desorber, as shown in Figure 9. The formulation during the absorption and desorption process is shown as Eqn. 16 and Eqn. 17.

Figure 9. The mass flow in and out of the system

𝑚𝑚𝑆𝑆𝑒𝑒𝑆𝑆𝑙𝑙2∙𝑁𝑁𝐻𝐻3+ 𝑚𝑚𝑁𝑁𝐻𝐻3 = (1 − 𝑥𝑥(𝑑𝑑)𝑒𝑒𝑒𝑒𝑚𝑚)𝑚𝑚𝑆𝑆𝑒𝑒𝑆𝑆𝑙𝑙2∙𝑁𝑁𝐻𝐻3+ 𝑥𝑥(𝑑𝑑)𝑒𝑒𝑒𝑒𝑚𝑚𝑚𝑚𝑆𝑆𝑒𝑒𝑆𝑆𝑙𝑙2∙8𝑁𝑁𝐻𝐻3+ (1 − 𝑥𝑥(𝑑𝑑))𝑚𝑚𝑁𝑁𝐻𝐻3 Eqn. 16 𝑚𝑚𝑆𝑆𝑒𝑒𝑆𝑆𝑙𝑙2∙8𝑁𝑁𝐻𝐻3− 𝑚𝑚𝑁𝑁𝐻𝐻3 = (1 − 𝑥𝑥(𝑑𝑑)𝑒𝑒𝑒𝑒𝑚𝑚)𝑚𝑚𝑆𝑆𝑒𝑒𝑆𝑆𝑙𝑙2∙8𝑁𝑁𝐻𝐻3+ 𝑥𝑥(𝑑𝑑)𝑒𝑒𝑒𝑒𝑚𝑚𝑚𝑚𝑆𝑆𝑒𝑒𝑆𝑆𝑙𝑙2∙𝑁𝑁𝐻𝐻3+ (1 − 𝑥𝑥(𝑑𝑑))𝑚𝑚𝑁𝑁𝐻𝐻3 Eqn. 17

Where: 𝑚𝑚𝑆𝑆𝑒𝑒𝑆𝑆𝑙𝑙2∙𝑁𝑁𝐻𝐻3 is the maximum mass of the monoammine contained in the composites

𝑚𝑚𝑆𝑆𝑒𝑒𝑆𝑆𝑙𝑙2∙8𝑁𝑁𝐻𝐻3 is the maximum mass of the octaammine contained in the composites

𝑚𝑚𝑁𝑁𝐻𝐻3 is the mass of ammonia measured by the flowmeter

𝑥𝑥(𝑑𝑑) is the reaction advancement by time series

Reaction advancement x(t) shows the conversion of the reactants. During the absorption process, x(t) is defined as in Eqn. 18. The range of x(t) is from 0 to 1. When x(t) = 0 , the composites only contain SrCl2·NH3. While, x(t) = 1 means the composites only consist of SrCl2·8NH3.

𝑥𝑥(𝑑𝑑) = 𝑚𝑚𝑆𝑆𝑒𝑒𝑆𝑆𝑙𝑙2∙𝑁𝑁𝐻𝐻3

(𝑚𝑚𝑆𝑆𝑒𝑒𝑆𝑆𝑙𝑙2∙𝑁𝑁𝐻𝐻3)𝑒𝑒𝑒𝑒𝑚𝑚

Eqn. 18

𝑥𝑥(𝑑𝑑) = 𝑚𝑚𝑆𝑆𝑒𝑒𝑆𝑆𝑙𝑙2∙8𝑁𝑁𝐻𝐻3

(𝑚𝑚𝑆𝑆𝑒𝑒𝑆𝑆𝑙𝑙2∙8𝑁𝑁𝐻𝐻3)𝑒𝑒𝑒𝑒𝑚𝑚

Eqn. 19

The mass of ammonia flowing into the reactor (absorption process) can be calculated by Eqn. 20, while the one flowing out of the reactor (desorption process) is calculated by Eqn. 21. F1 is the flowrate value measured by the flowmeter before the absorber, while F2 is the flowrate value measured by the flowmeter after the desorber.

𝑚𝑚𝑁𝑁𝐻𝐻3 = 𝜌𝜌𝑁𝑁𝐻𝐻3∙ 𝐹𝐹1∙ 𝑑𝑑𝑑𝑑 Eqn. 20

𝑚𝑚𝑁𝑁𝐻𝐻3 = 𝜌𝜌𝑁𝑁𝐻𝐻3∙ 𝐹𝐹2∙ 𝑑𝑑𝑑𝑑 Eqn. 21

However, the mass of monoamine and octaammine, which indeed reacted with ammonia at a given time cannot be directly measured experimentally. Consequently, reaction advancement x(t) is unknown. Nevertheless, the value of reaction advancement can be obtained after performing the energy balance calculations.

During the absorption process, the energy balance corresponds to the heat released by the heat source (the salt), i.e., the reaction heat, equal to the heat absorbed by other components and materials related to the absorber [16]. The simplified system sketch and system boundary used in energy balance is shown in Figure 10. The only heat source during the desorption process is from the HTF coming from the thermal-bath. During the absorption process, the heat balance expression is shown as Eqn. 22. The thermostat bath would absorb extra heat to ensure the progress of the reaction. Corresponding to the desorption process is demonstrated as Eqn. 23, where the thermostat bath would provide heat for the reaction.

𝑄𝑄𝑠𝑠,𝑒𝑒𝑒𝑒𝑒𝑒𝑡𝑡𝑡𝑡𝑏𝑏𝑒𝑒+ 𝑄𝑄𝑠𝑠,𝑡𝑡𝑏𝑏𝑒𝑒𝑒𝑒+ 𝑄𝑄𝑠𝑠,𝑡𝑡𝑜𝑜𝑏𝑏𝑒𝑒𝑠𝑠+ 𝑄𝑄𝑠𝑠,𝑠𝑠𝑒𝑒𝑒𝑒𝑡𝑡𝑒𝑒𝑒𝑒𝑠𝑠+ 𝑄𝑄𝑠𝑠,𝑢𝑢𝑏𝑏𝑏𝑏𝑠𝑠+ 𝑄𝑄𝑠𝑠,𝑏𝑏𝑜𝑜𝑡𝑡𝑠𝑠&𝑏𝑏𝑏𝑏𝑙𝑙𝑡𝑡𝑠𝑠+ 𝑄𝑄𝑇𝑇ℎ𝑒𝑒𝑒𝑒−𝐵𝐵𝑒𝑒𝑡𝑡ℎ+ 𝑄𝑄𝑠𝑠,𝑁𝑁𝐻𝐻3= 𝑄𝑄𝑒𝑒,𝑠𝑠𝑒𝑒𝑙𝑙𝑡𝑡 Eqn. 22

𝑄𝑄𝑠𝑠,𝑒𝑒𝑒𝑒𝑒𝑒𝑡𝑡𝑡𝑡𝑏𝑏𝑒𝑒+ 𝑄𝑄𝑠𝑠,𝑡𝑡𝑏𝑏𝑒𝑒𝑒𝑒+ 𝑄𝑄𝑠𝑠,𝑡𝑡𝑜𝑜𝑏𝑏𝑒𝑒𝑠𝑠+ 𝑄𝑄𝑠𝑠,𝑠𝑠𝑒𝑒𝑒𝑒𝑡𝑡𝑒𝑒𝑒𝑒𝑠𝑠+ 𝑄𝑄𝑠𝑠,𝑢𝑢𝑏𝑏𝑏𝑏𝑠𝑠+ 𝑄𝑄𝑠𝑠,𝑏𝑏𝑜𝑜𝑡𝑡𝑠𝑠&𝑏𝑏𝑏𝑏𝑙𝑙𝑡𝑡𝑠𝑠+ 𝑄𝑄𝑒𝑒,𝑠𝑠𝑒𝑒𝑙𝑙𝑡𝑡+ 𝑄𝑄𝑠𝑠,𝑁𝑁𝐻𝐻3= 𝑄𝑄𝑇𝑇ℎ𝑒𝑒𝑒𝑒−𝐵𝐵𝑒𝑒𝑡𝑡ℎ Eqn. 23

𝑄𝑄𝑠𝑠,𝑡𝑡𝑏𝑏𝑒𝑒𝑒𝑒 is the sensible heat absorbed by the composites in the reactor (kJ)

𝑄𝑄𝑠𝑠,𝑡𝑡𝑜𝑜𝑏𝑏𝑒𝑒𝑠𝑠 is the sensible heat absorbed by the heat exchanger tubes in the reactor (kJ)

𝑄𝑄𝑠𝑠,𝑠𝑠𝑒𝑒𝑒𝑒𝑡𝑡𝑒𝑒𝑒𝑒𝑠𝑠 is the sensible heat absorbed by the spacers in the reactor (kJ)

𝑄𝑄𝑠𝑠,𝑢𝑢𝑏𝑏𝑏𝑏𝑠𝑠 is the sensible heat absorbed by the fins in the reactor (kJ)

𝑄𝑄𝑠𝑠,𝑏𝑏𝑜𝑜𝑡𝑡𝑠𝑠&𝑏𝑏𝑏𝑏𝑙𝑙𝑡𝑡𝑠𝑠 is the sensible heat absorbed by the nuts and bolts on the reactor (kJ)

𝑄𝑄𝑇𝑇ℎ𝑒𝑒𝑒𝑒−𝐵𝐵𝑒𝑒𝑡𝑡ℎ is the sensible heat absorbed by the HTF from the thermostat bath (kJ)

𝑄𝑄𝑠𝑠,𝑁𝑁𝐻𝐻3 is the sensible heat absorbed by the remaining ammonia in the reactor (kJ)

𝑄𝑄𝑒𝑒,𝑠𝑠𝑒𝑒𝑙𝑙𝑡𝑡 is the reaction heat produced by the reaction between the salt and NH3 (kJ)

When calculating the sensible heat, the heat is decided by the initial temperature, final temperature, mass, and specific heat capacity [3]. Accordingly, the specific formulae to calculate the sensible heat are presented below: 𝑄𝑄𝑠𝑠,𝑒𝑒𝑒𝑒𝑒𝑒𝑡𝑡𝑡𝑡𝑏𝑏𝑒𝑒= � 𝑚𝑚𝑅𝑅𝑒𝑒𝑒𝑒𝑡𝑡𝑡𝑡𝑏𝑏𝑒𝑒∙ 𝑆𝑆𝑒𝑒𝑅𝑅𝑒𝑒𝑒𝑒𝑡𝑡𝑡𝑡𝑏𝑏𝑒𝑒∙ 𝑑𝑑𝑇𝑇 𝑇𝑇`𝑖𝑖+1 𝑇𝑇`𝑖𝑖 Eqn. 24 𝑄𝑄𝑠𝑠,𝑡𝑡𝑏𝑏𝑒𝑒𝑒𝑒= � 𝑚𝑚𝑡𝑡𝑏𝑏𝑒𝑒𝑒𝑒∙ 𝑆𝑆𝑒𝑒𝑡𝑡𝑏𝑏𝑒𝑒𝑒𝑒∙ 𝑑𝑑𝑇𝑇 𝑇𝑇𝑖𝑖+1 𝑇𝑇𝑖𝑖 Eqn. 25 𝑄𝑄𝑠𝑠,𝑠𝑠𝑒𝑒𝑒𝑒𝑡𝑡𝑒𝑒𝑒𝑒𝑠𝑠= � 𝑚𝑚𝑠𝑠𝑒𝑒𝑒𝑒𝑡𝑡𝑒𝑒𝑒𝑒𝑠𝑠∙ 𝑆𝑆𝑒𝑒𝑠𝑠𝑒𝑒𝑒𝑒𝑡𝑡𝑒𝑒𝑒𝑒𝑠𝑠∙ 𝑑𝑑𝑇𝑇 𝑇𝑇𝑖𝑖+1 𝑇𝑇𝑖𝑖 Eqn. 26 𝑄𝑄𝑠𝑠,𝑢𝑢𝑏𝑏𝑏𝑏𝑠𝑠= � 𝑚𝑚𝑢𝑢𝑏𝑏𝑏𝑏𝑠𝑠∙ 𝑆𝑆𝑒𝑒𝑢𝑢𝑏𝑏𝑏𝑏𝑠𝑠∙ 𝑑𝑑𝑇𝑇 𝑇𝑇𝑖𝑖+1 𝑇𝑇𝑖𝑖 Eqn. 27 𝑄𝑄𝑠𝑠,𝑡𝑡𝑜𝑜𝑏𝑏𝑒𝑒𝑠𝑠 = � 𝑚𝑚𝑡𝑡𝑜𝑜𝑏𝑏𝑒𝑒𝑠𝑠∙ 𝑆𝑆𝑒𝑒𝑡𝑡𝑜𝑜𝑏𝑏𝑒𝑒𝑠𝑠∙ 𝑑𝑑𝑇𝑇 𝑇𝑇𝑖𝑖+1 𝑇𝑇𝑖𝑖 Eqn. 28 𝑄𝑄𝑠𝑠,𝑏𝑏𝑜𝑜𝑡𝑡𝑠𝑠&𝑏𝑏𝑏𝑏𝑙𝑙𝑡𝑡𝑠𝑠 = � 𝑚𝑚𝑏𝑏𝑜𝑜𝑡𝑡𝑠𝑠 𝑒𝑒𝑏𝑏𝑎𝑎 𝑏𝑏𝑏𝑏𝑙𝑙𝑡𝑡𝑠𝑠∙ 𝑆𝑆𝑒𝑒𝑏𝑏𝑜𝑜𝑡𝑡𝑠𝑠&𝑏𝑏𝑏𝑏𝑙𝑙𝑡𝑡𝑠𝑠∙ 𝑑𝑑𝑇𝑇 𝑇𝑇`𝑖𝑖+1 𝑇𝑇`𝑖𝑖 Eqn. 29 𝑄𝑄𝑠𝑠,𝑁𝑁𝐻𝐻3= � 𝑚𝑚𝑁𝑁𝐻𝐻3∙ 𝑆𝑆𝑒𝑒 𝑁𝑁𝐻𝐻3_{∙ 𝑑𝑑𝑇𝑇} 𝑇𝑇𝑖𝑖+1 𝑇𝑇𝑖𝑖 Eqn. 30 𝑄𝑄𝑒𝑒,𝑠𝑠𝑒𝑒𝑙𝑙𝑡𝑡 = 𝑥𝑥(𝑑𝑑) ∙ 𝑄𝑄𝑒𝑒,𝑠𝑠𝑒𝑒𝑙𝑙𝑡𝑡𝑒𝑒𝑒𝑒𝑚𝑚 Eqn. 31 𝑄𝑄𝑇𝑇ℎ𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑙𝑙−𝐵𝐵𝑒𝑒𝑡𝑡ℎ= � 𝜌𝜌𝑇𝑇ℎ𝑒𝑒𝑒𝑒𝑒𝑒180∙ 𝑉𝑉𝑇𝑇ℎ𝑒𝑒𝑒𝑒𝑒𝑒180∙ 𝑆𝑆𝑒𝑒𝑇𝑇ℎ𝑒𝑒𝑒𝑒𝑒𝑒∙ 𝑑𝑑𝑇𝑇 𝑇𝑇21 𝑇𝑇20 Eqn. 32

Where: 𝑇𝑇𝑏𝑏+1 is the temperature in the reactor when time is i+1 (℃)

𝑇𝑇𝑏𝑏 is the temperature in the reactor when time is i (℃)

𝑇𝑇`𝑏𝑏+1 is the average temperature between the inside and outside of the reactor when

𝑇𝑇`𝑏𝑏 is the average temperature between the inside and outside of the reactor when time

is i (℃)

𝑚𝑚 is the mass of the specific components (kg), where the subscripts Reactor, comp, spacers, fins, tubes, nuts and bolts, NH3 mean respectively reactor, composites, spacers, fins, heat exchanger tubes, nuts and bolts, ammonia.

𝑆𝑆𝑒𝑒is the specific heat capacity of the specific components (𝑘𝑘𝑘𝑘/(𝑘𝑘𝑔𝑔 ∙ 𝐾𝐾))

𝑄𝑄𝑒𝑒,𝑠𝑠𝑒𝑒𝑙𝑙𝑡𝑡𝑒𝑒𝑒𝑒𝑚𝑚 is the maximum reaction heat released/stored by the salt, which is 2906.51

kJ(according to Section 4.2)

𝑇𝑇20 is the temperature before entering the thermal-bath (K)

𝑇𝑇21 is the temperature after the thermal-bath (K)

𝑆𝑆𝑒𝑒𝑇𝑇ℎ𝑒𝑒𝑒𝑒𝑒𝑒 is the specific heat capacity of the therm-180 fluid (𝑘𝑘𝑘𝑘/(𝑘𝑘𝑔𝑔 ∙ 𝐾𝐾))

𝜌𝜌𝑇𝑇ℎ𝑒𝑒𝑒𝑒𝑒𝑒180 is the density of the therm-180 fluid (kg/m3)

𝑉𝑉𝑇𝑇ℎ𝑒𝑒𝑒𝑒𝑒𝑒180 is the volume flow rate of the therm-180 (L/s)

The therm-180 fluid is chosen as the HTF of the system, which is a silicon-based thermal fluid from Lauda [26]. There are two reactors made of SS316L involved in the TCS system, which have the same physical properties. There are eight perforated fins made of an aluminum alloy EN AW 1050A in the reactor, which function as extended heat transfer surfaces. The perforations allow an easier passage for the ammonia. Also, there are seven spacers, made of SS316L, used inside each reactor to protect the soft composite layers from being crushed due to their weight and the fins' weight. Also, there are five heat exchanger tubes in the reactor, made of aluminum alloy EN AW 6060. Moreover, sixteen pairs of nuts and bolts are used on each reactor to hold the reactor lids in place, which are made of SS316L. The mass of the reactor 𝑚𝑚𝑅𝑅𝑒𝑒𝑒𝑒𝑡𝑡𝑡𝑡𝑏𝑏𝑒𝑒, the mass of the spacers in the reactor 𝑚𝑚𝑠𝑠𝑒𝑒𝑒𝑒𝑡𝑡𝑒𝑒𝑒𝑒𝑠𝑠, the mass of the fins

in the reactor 𝑚𝑚𝑢𝑢𝑏𝑏𝑏𝑏𝑠𝑠, the mass of the heat exchanger tubes 𝑚𝑚𝑡𝑡𝑜𝑜𝑏𝑏𝑒𝑒𝑠𝑠And the mass of the nuts and bolts on

the reactor 𝑚𝑚𝑏𝑏𝑜𝑜𝑡𝑡𝑠𝑠&𝑏𝑏𝑏𝑏𝑙𝑙𝑡𝑡𝑠𝑠 are assumed to be constant during the absorption and desorption process. The

mass of these components has been measured in the lab. As the specific heat capacity of SS316L, Al EN AW 6060, and Al EN AW 1050A would not change too much between 10 and 100℃[16], [27], [28], it is assumed that the specific heat capacity of these materials would remain constant during the absorption and desorption processes. The used values of the physical properties and materials of these components are shown in Table 2. The reactor's temperature is measured by eighteen thermal sensors (T1 – T18), the mean value of which is used to calculate the sensible heat of the spacers, the fins, the HEX tubes, and composites. The reactor's outside surface temperature is measured by the thermal sensor labeled as T19, which calculates the reactor's sensible heat and nuts and bolts. The density and specific heat capacity of the HTF therm-180 can be found on the datasheet from LAUDA [26], where the density of therm-180 is found by 694.2-1.05*T (kg/m3_{). The specific heat capacity of therm-180 is} 0.0015*T+1.2103 (kJ/(kg·K)), as functions of temperature (K).

Table 2. The physical properties of the components related to the reactor

Component Mass Material Specific heat

Capacity

One reactor 54.45 kg SS316L 0.5 kJ/(kg·K) [16]

Spacers in one reactor 0.6 kg SS316L 0.5 kJ/(kg·K) [16]

Fins in one reactor 2.06 kg Al EN AW 1050A 0.9 kJ/(kg·K) [28] Tubes in one reactor 2.53 kg Al EN AW 6060 0.9 kJ/(kg·K) [27] Nuts and Bolts on one