Design of Thermal Conductivity Measurement Device for Molten Salts at Elevated Temperatures A Projec

Design of Thermal Conductivity Measurement Device for Molten Salts at Elevated Temperatures A Project Paper [Proposal] Presented to the Queensland University of Technology School of Chemistry, Physics and Mechanical Engineering In Partial Fulfilment of the Requirements for the Degree of Bachelor of Mechanical Engineering (EN40)

By Elaiza L. Luker SUPERVISORS Professor Ted Steinberg Ralf Raud

29th of April 2016

Abstract The use of Phase Change Materials (PCM) has been prevalent in latent heat storage applications because of its high energy storage density which plays a critical role in thermal energy storage (TES) applications (Wei Peng, Jie-min Zhou, Yuan Li, Ying Yang, & Mei-ru Guo, 2014).

However, in order to determine and verify the overall effectiveness of PCMs, its thermal conductivity needs to be known, and as the exploration of eutectic mixtures for TES applications is relatively recent, it is often not documented. The relevance of this research is contextualized and supported through evidence of previous work within the project overview and literature review.

This paper presents a design proposal of a thermal conductivity measurement device for use in measuring PCMs utilising the transient hot-wire method. A brief outline on the limitations and assumptions of the proposed design is also incorporated herein. Project management methods and challenges have been analysed and evaluated in order to ensure the quality and timely delivery of the project. Amongst rapid industrialization, the pursuit for cleaner energy to address challenges in sustainability is increasing more than ever; therefore, the design of this measurement device to contribute to the development of TES holds great significance in the harnessing the power of solar energy.

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Table of Contents Abstract ...................................................................................................................................... ii

Table of Contents ............................................................................................................. iii 1.0

Introduction ........................................................................................................... 4

1.1

Project Overview ...........................................................................................................4

1.2

Literature Review ..........................................................................................................5

1.2.1

Thermal Conductivity .................................................................................................5

1.2.2

Thermal Conductivity Measurement Method .............................................................6

1.2.3

Designs ........................................................................................................................7

1.2.4

Sample Material ..........................................................................................................8

1.3

2.0

Thesis Statement ............................................................................................................8

Approach/Methods ............................................................................................... 9

2.1

Design .............................................................................................................................9

2.1.1

Materials and Manufacturing ....................................................................................10

2.1.2

Limitations ................................................................................................................11

2.1.3

Assumptions ..............................................................................................................11

3.0

Project Management ........................................................................................... 12

4.0

Project Deliverables and Research Implications.............................................. 14

5.0

References ............................................................................................................ 15

Appendix 1: Design of Thermal Conductivity Measurement Device ......................... 16 Appendix 2: Project Gantt Chart .................................................................................. 17

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1.0 Introduction The thermo-physical properties of Phase Change Materials are very important in the design of efficient thermal energy storage for the continuous development of renewable energy sources.

The thermal conductivity of a material, is a particularly important for TES as they play a critical role in the energy storage and recovery process. However, due to the elevated temperatures and corrosive ambience at which the system will be operating under the measurement of the sample’s thermal conductivity proves to be challenging (A. Redkin, Yu. Zaikov, O. Tkacheva, & S. Kumkov, 2015). Key challenges encountered in measuring thermal conductivity includes: (a) internal effects of convection and radiation heat; (b) limitations in materials due to operating condition; (c) Producing a design that is sustainable and economically efficient (Nunes, Lourenco, Santos, & Castro, 2003). This paper aims to address these challenges; and establish and manufacture a viable design to measure thermal conductivity of PCMs under the stated conditions.

1.1

Project Overview The growth in demand for renewable energy sources, especially solar energy, requires

imperative developments in thermal energy storage (TES) capacity in order to maximize collection efficiency, as well as secure the availability and reliability of thermal energy at time intervals with little or no solar irradiation. The use of Phase Change Materials has been prevalent in latent heat storage applications due to its high energy storage density which plays a critical role in thermal energy storage applications (Wei Peng, Jie-min Zhou, Yuan Li, Ying Yang, & Mei-ru Guo, 2014).

In collecting solar energy for storage, technologies and processes for Concentrating Solar Thermal Power (CSTP) is used. CSTP is a process that utilizes mirrors in concentrating solar thermal energy onto a small area within a heat exchanger in order to attain very high temperatures which is absorbed by a heat transfer fluid or PCMs in an intermediate cycle (Pitz-Paal, 2008).

PCMs are advantageous as TES due to its capacity to store and emit large quantities of energy during solid to liquid transformations as latent heat (LH), whilst only requiring small volumes of the heat transfer fluid. For LH storage and discharge, smaller temperature fluctuations are required to attain high storage density relative to sensible heat (SH) storage, which stores as a liquid or solid form rather than storage during a substance’s phase change (Pielichowska & Pielichowski, 2014).

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In recent studies, salt mixtures have shown compelling results in their application as as PCM for TES. However, due to its ongoing exploration the different mixtures’ accurate material properties, especially in its liquid state, are yet to be documented (Rosenzweig, 2012; Nelle, 2012).

For PCMs to be viable and efficient in its application, conditions of thermal, physical, kinetic and chemical properties must be satisfied. A thermal property which holds heavy significance within an effective LH PCM for TES is high thermal conductivity; therefore, a thermal conductivity measurement device must be designed in order to holistically assess the performance and applicability of PCMs at different phases (Pielichowska & Pielichowski, 2014). As the PCMs will be subject to various operating conditions including corrosive environments and elevated, ambient temperatures, it is important to consider these factors in the design of the thermal conductivity measurement device to ensure the reliability of results for the research.

1.2

Literature Review

1.2.1 Thermal Conductivity Thermal conductivity (k) is a material property which indicates its capacity to conduct heat energy through a medium (solid or liquid). It operates through the transfer of energy contact and interaction between particles with high energy to low energy (Çengel & Ghajar, 2015). The principles of thermal conductivity is governed by Fourier’s law of heat conduction, which is influenced by geometry, temperature difference and rate of heat transfer (refer to Equation 1-1).

!"#$% = −()

%* %+

Equation 1-1

Where !"#$% = ,-./ 12 ℎ/-. .,-452/, .ℎ,167ℎ 814968.:14 ( = .ℎ/,;-< 814968.:=:.> ?-,-;/./, 9@ = 8ℎ-47/ :4 ./;?/,-.6,/ 9A = .ℎ:8(4/55 12 ;-./,:-< Equation 1-1 indicates a negative value for heat transfer as heat always flows towards particles with lower energies (lower temperature). It can also be seen from the equation, thermal 5

conductivity is directly proportional to surface area (A) and temperature change ( B@ ) and indirectly proportional to thickness (BA). In TES applications, there is a requirement for higher thermal conductivity within PCMs in order to ensure that good heat conductivity in storing and releasing energy through LH. 1.2.2 Thermal Conductivity Measurement Method In measuring thermal conductivity, various methods can be employed including: steady state and transient measurement techniques. For measuring materials in its liquid state, the transient hot-wire (THW) method has become a more conventional means of thermal conductivity measurement due to its practicality and timeliness (J. P. Garnier, et al., 2008). THW involves the use of a thin and short platinum wire immersed evenly within an infinite sample. This methodology utilises the Joule’s effect whereby a constant electrical current is passed through the wire within the sample and the temperature rise with respect to the change in time is measured. The thin wire has a two-fold purpose in that it acts as a heat source for the system and also a temperature gauge (Nelle, 2012). Determining the change in temperature, BT, with changes in the magnitude of electrical currents is essential in assessing, calculating, and verifying the accuracy of, the sample’s thermal conductivity (as temperature changes is dependent to this) at different phases (Alvarado, Marín, Juárez, Calderón, & Ivanov, 2012).

Theory of the hot-wire method A thin wire consists of an initial electrical resistance, CD , at standard room temperature.

However, upon the application of an electrical current, I, onto the wire through the aforementioned Joule’s effect, the electrical energy is transformed into heat energy. This consequently provides a heat flow onto the eutectic mixture and the induced temperature change, @(.), triggers a variation in the wire’s electrical resistance, R. With a known constant value of I and a variation in R(t), the measured energy V at the ends of the wire will evidently change with time (Equation 1-2) (Alvarado, Marín, Juárez, Calderón, & Ivanov, 2012).

G . = C . H Equation 1-2

Where C(.) = CD (1 + KB@ . )

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B@ = @ . − @D σ = temperature coefficient of the metallic wire resistivity In measuring the voltage changes and voltage changes over time for different values of I, CD can be estimated by applying very low currents and measuring the amount of voltage drop. In knowing this relationship, further post processing can be conducted to determine the gradient of voltage drop and relate it to thermal conductivity (Alvarado, Marín, Juárez, Calderón, & Ivanov, 2012).

1.2.3 Designs Nelle (2012), proposed a potential design for a thermal conductivity measurement device utilizing the transient hot-wire method. Figure 1-1 shows a diagram of the device consisting of an L-shaped bracket fixed onto the cover to hold the wire in an appropriate place within the crucible.

As seen in the drawing, the bracket is kept at a fair distance from the metallic wire and this was to ensure that it the bracket did not interfere with the wire’s measurement requirements of an ‘infinite’ sample and hence ensuring data accuracy.

Figure 1-1: Thermal Conductivity Measurement Device (Nelle, 2012)

In this design the following factors were also considered: (a) high temperature corrosion, (b) ease of maintenance and use, (c) appropriate insulation, (d) appropriate placement of the wire within the sample for measurement (Nelle, 2012). A main advantage in adopting this design lies

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in its simplicity for maintenance and ease of positioning and immersion of the thin wire appropriately within the crucible and sample.

1.2.4 Sample Material A thermal conductivity measurement device needs to be designed in order to measure the following eutectic mixtures: Sodium Nitrate (NaNO3); Sodium Chloride (NaCl) and Sodium Sulfate (Na2SO4); Sodium Chloride (NaCl) and Sodium Carbonate (Na2CO3); Sodium Carbonate (Na2CO3), Lithium Carbonate (Li2CO3) and Potassium Carbonate (K2CO3).

1.3

Thesis Statement The aim of this paper is to design, build, verify and test a transient hot wire for

measurement of thermal conductivity for eutectic mixtures as phase change materials for use as in thermal energy storage applications.

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2.0 Approach/Methods In conducting a literature review, it was determined that utilising a transient hot-wire method is the most practical means of measuring thermal conductivity for the scope of this project and its operating conditions. Herein, the proposed method for the design of the measurement device will be discussed and the required equipment and steps that will need to be taken in order to pursue its completion. The limitations and assumptions in various design will also be evaluated and considered to account for possible errors and allow for modifications and improvements as the design undergoes further refinements and developments. Various designs were evaluated and assessed in order to determine the most viable solution. The process of establishing this proposed design will be further detailed within the progress report as a sequel to this proposal.

2.1

Design In developing upon Nelle’s (2012) thermal conductivity measurement device, the same

design principles will be employed herein with practical materials and manufacturing modifications to better suit the specialized application of this project.

Figure 2-1 shows the proposed design method for a thermal conductivity measurement device. As depicted on the illustrations below, the thin wire is seen to go through the crucible cover (b) and out through the side sealed with alumina tubing. The crucibles currently do not have dimensions; however, relevant engineering drawings are incorporated in Appendix 1.

Crucible Cover

Thin Wire

Alumina Crucible Alumina Tube (a)

(b)

Figure 2-1: Solidworks Drawing of Proposed Design (normal (a) and with hidden lines visible (b)

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In order to clamp the thin wires on the outside, it will first be rolled onto an alumina tubing and clamped appropriately to prevent breakages. The wires will then be attached to a power supply to input current and a voltmeter to measure the voltage drop which is used to determine the wire’s resistance necessary in determining the PCM’s thermal conductivity through post processing of information.

2.1.1 Materials and Manufacturing The proposed design aims to achieve simplicity through minimalizing the amount of raw material and manufacturing processes needed. This is because alumina, as a result of its rarity in common use, is both difficult and expensive to manufacture in small quantities. Therefore, the use of alumina tube inserted through a discreet hole on the side of the crucible will enable ensure that the thin wire remain straight within the vessel. Keeping this in mind, with the presence of holes within sections of the crucible which are exposed to PCMs, relevant sealants must be applied in order to keep the substances contained.

In choosing the correct sealant for application, it must be noted that it has to be effective under the same high temperature and corrosive conditions that the storage vessel and PCMs operate under. Therefore, research was conducted in alumina based sealants as the main vessel, made out of this same material, is known to be effective. The material properties of sealants were analysed and it was determined that the adhesives: 503-Ceramabnd, 503-VFG-C-Cermacoat, 503Thinner from Aremco are the most compelling materials for use due to its low porousity, easy application and its specifications indicating the desired operating conditions (Aremco Products Inc, 2015).

These sealants; however, will need to be tested prior to manufacturing and assembly and if they prove to be effective in their purpose design modifications will be applied to further simplify the current design proposal. If this is the case, the alumina tube will and the wire will be inserted and sealed vertically at the base of the crucible in order to minimise errors in the positioning and orientation of the thin wire. Thermal conductivity is ideally measured through keeping the wire perfectly vertical as it acts as a heat source and thermal sensor.

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2.1.2 Limitations There are limitations associated to this design in terms of its reliability on the sealants working perfectly. Furthermore, due to the nature of the design, the delicate wire must be carefully calibrated upon each use, compromising time efficiency. Another significant limitation is that this system cannot be used on any crucible without first drilling a hole onto it. As a result, this method is destructive to the individual crucibles and hence may be costly if continually using different vessels to measure.

2.1.3 Assumptions Some key assumptions that are made in this design is that the distance between the wire and the crucible’s wall is adequate to provide an ‘infinite’ sample to measure thermal conductivity.

The design also assumes the ideal functionality of the sealants. Furthermore, in measuring the thermal conductivity accurately, it is assumed that the wire is perfectly vertical; however, this may not be the case and hence some errors may be induced during this process. Lastly, convection and radiation heat will also be contributing factors to the increased heat detected by the wire and hence some errors in measurements will be induced from this.

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3.0 Project Management In order to manage the successful completion of this project, key objectives were detailed and comprehensive literature review was established in order to highlight the focus of this project and reinforce design with relevant theory and established work. A project Gantt chart with a breakdown of the work to be completed during the first phase of the project is also incorporated herein in order to keep track of progress and address all the necessary elements in the design’s completion (refer to Appendix 2).

A break-down of the preliminary design phase, detailed in Figure 3-1, defines the necessary processes required to attain a final design with appropriate quality control and testing in order to incrementally evaluate the design progress at each stage. Regular moderation of the designs will allow for flexibility in modifications, whereas testing only after the manufacture and assembly of the final design is high risk in terms of cost and time efficiency, as well as quality.

Initial Design Phase and Materials Selection • Perform design calculations according to relevant literature • Research and source appropriate materials to satisfy constraints and requirements

Design Finalisation and Simulation • Finalise Design Calculations and Materials to be used • Simulate the design through Solidworks to generate an engineering model

Material Sourcing and Prototyping • Obtain the relevant materials and equipment and conduct comprehensive tests to verify its performance.

Manufacture and Assembly • Determine and apply appropriate methods and technologies to manufacture the finalised design • Assemble the system for prototyping and testing

Prototyping,Test and Evaluate • Test the design, evaluate and apply relevant modifications in order to control result quality and accuracy Figure 3-1: Engineering Design Process for the design and manufacture of the thermal conductivity measurement device.

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Weekly meetings with the supervisor are conducted in order to continually receive feedback and allow for modification as the project progresses. Challenges that will need to be overcome within this project is effective time management due to the magnitude of work required.

This will be combatted through the employment of a rigid project outline and active engagement in research through performing regular laboratory work or related research. Another challenge that may arise are delays in progress due to waiting time between phases and external factors, in order to account for this the project plan has been constructed with some flexibility to account for unforeseen circumstances and ensure the quality of project delivery.

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4.0 Project Deliverables and Research Implications The main project deliverable of manufacturing an effective and practical thermal conductivity measurement device will be applied through conducting tests and measurements on the eutectic mixtures detailed in section 1.2.4 for further work. In designing a thermal conductivity measurement device for TES applications, this will enable further research in Phase Change Materials and the identification of its thermos-physical properties. This is important as the thermal conductivity of substances is imperative in the energy storage and recovery. This development will greatly contribute to the progress of TES and consequently renewable energy sources which will eventually reach its peak in demand as the society makes its transition to cleaner energy to maintain sustainability.

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5.0 References Çengel, Y. A., & Ghajar, A. J. (2015). Heat and Mass Transfer: Fundamentals and Applications (5th Edition ed.). New York: McGraw-Hill Education.

A. Redkin, Yu. Zaikov, O. Tkacheva, & S. Kumkov. (2015, November 13). Molar thermal conductivity of molten salts. International Journal of Ionics.

Alvarado, S., Marín, E., Juárez, A. G., Calderón, A., & Ivanov, R. (2012). A hot-wire method based thermal conductivity measurement apparatus for teaching purposes. European Journal of Physics, 33, 897-906.

Aremco Products Inc. (2015). Product Catalogue.

J. P. Garnier, J. P. Maye, J. Saillard, Thévenot, G., Kadjo, A., & S. Martemianov. (2008, April). A New Transient Hot-Wire Instrument for Measuring the Thermal Conductivity of Electrically Conducting and Highly Corrosive Liquids using Small Samples.

International Journal of Thermophysics, 29(2), 468-482 J. P. Garnier , J. P. Maye, J. Saillard, G. Thévenot, A. Kadjo, S. Martemianov.

Nelle, S. (2012). A Device for Measuring Thermal Conductivity of Molten Salt Nitrates at Elevated Temperatures for Use in Solar ermal Power Applications. Leigh University.

Nunes, V. M., Lourenco, M. J., Santos, F. J., & Castro, a. C. (2003). Importance of Accurate Data on Viscosity and Thermal Conductivity in Molten Salts Applications. Journal of Chemical & Engineering Data, 48, 446 - 450.

Pielichowska, K., & Pielichowski, K. (2014, March 21). Phase Change Materials for Thermal Energy Storage. Progress in Materials Science, 67-123.

Pitz-Paal, R. (2008). Concentrating Solar Power. In T. M. Letcher, Future Energy: Improved, Sustainable and Clean Options for our Planet (pp. 171-174). Elsevier.

Rosenzweig, B. (2012). A Calibration Procedure for Measuring the Thermal Conductivity of Molten Salts at Elevated Temperatures. Leigh University.

Wei Peng, Jie-min Zhou, Yuan Li, Ying Yang, & Mei-ru Guo. (2014, February). A dynamic technique for the measurement of thermal conductivity of molten salt based on cylindrical melting model. Journal of Thermal Analysis and Calorimetry, 115(2), 17671777.

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Appendix 1: Design of Thermal Conductivity Measurement Device

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Appendix 2: Project Gantt Chart