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Sononucleation of inorganic phase change materials [Elektronische Ressource] / Eva Doris Günther

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Sononucleationof InorganicPhase Change MaterialsDissertationvonEva GüntherPhysik-DepartmentTECHNISCHE UNIVERSITÄT MÜNCHENLehrstuhl E19, Physik-DepartmentBayerisches Zentrum für Angewandte Energieforschung e. V.Sononucleation of inorganic phasechange materialsEva Doris GüntherVollständiger Abdruck der von der Fakultät für Physik der Technischen UniversitätMünchen zur Erlangung des akademischen Grades einesDoktors der Naturwissenschaften (Dr. rer. nat.)genehmigten Dissertation.Vorsitzender:Univ.-Prof. Dr. Martin ZachariasPrüfer der Dissertation:1. Univ.-Prof. Dr. Ulrich Stimming2. Univ.-Prof. Dr. Wolfgang Voigt, Technische Universität Bergakademie FreibergDie Dissertation wurde am 25.08.2009 bei der Technischen Universität Müncheneingereicht und durch die Fakultät für Physik am 20.01.2011 angenommen.Abstract – KurzfassungAbstractMany inorganic phase change materials, and particularly salt hydrates, show strongsubcooling, which has negative effects on their performance as heat or cold storagematerials. In this work, a suggested reduction of subcooling by ultrasonic treatment(“sononucleation”) was investigated. The nucleation temperature as function of pres-sure was experimentally quantified up to 800 MPa for three salt hydrates. Variousexperiments showed that sononucleation is an effective, robust and reliable techniquefor solidifying water, but ineffective for solidifying salt hydrates.

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Published 01 January 2011
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Sononucleation
of Inorganic
Phase Change Materials
Dissertation
von
Eva Günther
Physik-DepartmentTECHNISCHE UNIVERSITÄT MÜNCHEN
Lehrstuhl E19, Physik-Department
Bayerisches Zentrum für Angewandte Energieforschung e. V.
Sononucleation of inorganic phase
change materials
Eva Doris Günther
Vollständiger Abdruck der von der Fakultät für Physik der Technischen Universität
München zur Erlangung des akademischen Grades eines
Doktors der Naturwissenschaften (Dr. rer. nat.)
genehmigten Dissertation.
Vorsitzender:
Univ.-Prof. Dr. Martin Zacharias
Prüfer der Dissertation:
1. Univ.-Prof. Dr. Ulrich Stimming
2. Univ.-Prof. Dr. Wolfgang Voigt, Technische Universität Bergakademie Freiberg
Die Dissertation wurde am 25.08.2009 bei der Technischen Universität München
eingereicht und durch die Fakultät für Physik am 20.01.2011 angenommen.Abstract – Kurzfassung
Abstract
Many inorganic phase change materials, and particularly salt hydrates, show strong
subcooling, which has negative effects on their performance as heat or cold storage
materials. In this work, a suggested reduction of subcooling by ultrasonic treatment
(“sononucleation”) was investigated. The nucleation temperature as function of pres-
sure was experimentally quantified up to 800 MPa for three salt hydrates. Various
experiments showed that sononucleation is an effective, robust and reliable technique
for solidifying water, but ineffective for solidifying salt hydrates. Contrary to what is
proposed in literature, it is shown that peak pressures during cavitation in the ultra-
sonic field cannot be made responsible for sononucleation. Instead, sononucleation
could be explained by a surface mechanism, which is effective in pure substances or
solutions, but not in salt hydrates.
Kurzfassung
Viele anorganische Phasenwechselmaterialien, insbesondere Salyhydrate, zeigen deut-
liche Unterkühlung. Unterkühlung verzögert die Kristallisation und behindert den
Einsatz der Materialien als Energiespeichermedien. In dieser Arbeit wurde die Aus-
lösung der Kristallisation mit Hilfe von Ultraschall (Sononukleation) untersucht. Die
Nukleationsdruckkurve T (p) der Salzhydrate wurde experimentell im Bereich bisn
800 MPa quantifiziert. Ultraschall wurde im Experiment als effektive und robuste
Nukleationsmethode für Wasser bestätigt, aber als wirkungslos für die Salzhydrate
befunden. Es wurde gezeigt, dass die durch Kavitation im Ultraschallfeld erzeugte
Druckspitzen die Sononukleation im Wasser nicht auslösen können. Ein Oberflächen-
mechanismus kann aber die experimentellen Beobachtungen in den verschiedenen
Stoffsystemen zufriedenstellend erklären.
iContents
Contents
1. Background and motivation 1
1.1. Thermal energy storage (TES) . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1.1. Relevance of TES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1.2. Sensible and latent heat storage . . . . . . . . . . . . . . . . . . . . 3
1.2. Phase change materials (PCM) . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2.1. Organic PCM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2.2. Inorganic PCM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.3. Subcooling and nucleation in latent heat storages . . . . . . . . . . . . . . 7
1.3.1. Effect of subcooling on storage performance . . . . . . . . . . . . . 7
1.3.2. Reduction of subcooling . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.3.3. Aim of this work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2. Theory and context 11
2.1. Theory of phase change and nucleation . . . . . . . . . . . . . . . . . . . . 11
2.1.1. Stable phases – equilibrium thermodynamics . . . . . . . . . . . . 11
2.1.2. Subcooling – stabilizing the mother phase . . . . . . . . . . . . . . 16
2.1.3. Nucleation 1 – general concepts . . . . . . . . . . . . . . . . . . . . 21
2.1.4. Nucleation 2 – more types of nucleation . . . . . . . . . . . . . . . 26
2.1.5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
2.2. Context . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
2.2.1. Melting and nucleation temperatures . . . . . . . . . . . . . . . . . 30
2.2.2. Nucleation mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . 32
2.2.3. Physical conditions in insonicated liquids . . . . . . . . . . . . . . 38
2.2.4. Investigations of sononucleation . . . . . . . . . . . . . . . . . . . . 43
2.2.5. Summary of the scientific context of this work . . . . . . . . . . . . 48
3. Experimental work 49
3.1. General considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
3.1.1. Sample substances . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
3.1.2. Containers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
3.1.3. Time scales in nucleation experiments . . . . . . . . . . . . . . . . 59
3.2. Determination of the nucleation temperature at normal pressure . . . . 62
3.2.1. Setup and procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
3.2.2. Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
iiiContents
3.3. Determination of melting and nucleation temperatures at high pressures 69
3.3.1. Setup and procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
3.3.2. Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
3.4. Observation of the samples under ultrasonic treatment . . . . . . . . . . 77
3.4.1. Setup and procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
3.4.2. Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
3.5. Determination of the speed of solidification . . . . . . . . . . . . . . . . . 87
3.5.1. Setup and procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
3.5.2. Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
4. Discussion 93
4.1. General limitations of theory and experiment . . . . . . . . . . . . . . . . 93
4.1.1. Small clusters and the early stage of nucleation . . . . . . . . . . . 93
4.1.2. Application of nucleation theory to inorganic PCM . . . . . . . . . 98
4.1.3. Interpretation of nucleation experiments . . . . . . . . . . . . . . . 100
4.2. Discussion of new experimental data from this work . . . . . . . . . . . . 107
4.2.1. General issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
4.2.2. Nucleation by static pressure . . . . . . . . . . . . . . . . . . . . . . 113
4.2.3. Nucleation under ultrasonic irradiation . . . . . . . . . . . . . . . . 118
4.3. New insights in sononucleation . . . . . . . . . . . . . . . . . . . . . . . . 123
4.3.1. Statistics in static nucleation and sononucleation . . . . . . . . . . 123
4.3.2. Theory of nucleation by a direct pressure mechanism . . . . . . . 126
4.3.3. Other mechanisms that could explain sononucleation . . . . . . . 132
4.4. Conclusions and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
4.4.1. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
4.4.2. Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
5. Summary 139
6. Acknowledgements 141
A. Indexes A-1
A.1. Publications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A-1
A.2. Nomenclature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A-2
A.3. Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A-5
B. Appendix B-1
B.1. Are quantum effects relevant? . . . . . . . . . . . . . . . . . . . . . . . . . . B-1
B.2. Pressure dependency of the critical radius . . . . . . . . . . . . . . . . . . B-3
B.3. Analysis of high pressure melting and nucleation curves . . . . . . . . . . B-5
B.4. Effect of variable compressibility on adiabatic heating . . . . . . . . . . . B-8
B.5. Effect of evaporative cooling during cavitation . . . . . . . . . . . . . . . . B-12
B.6. Details of the observations of solidifying salt hydrates . . . . . . . . . . . B-14
iv1. Background and motivation
In this chapter, the background and motivation of this work is presented. First, thermal
energy storage as an important technology in the energy supply and demand system is
introduced. The advantage of latent heat storage compared to sensible heat storage
is explained next. Then, materials used as latent heat storage media are introduced
with their specific advantages and limitations. In particular, the problem of subcooling
and nucleation in inorganic phase change materials is presented, and the need for new
solutions is demonstrated.
1.1. Thermal energy storage (TES)
Energy storage is a main approach to manage mismatch between power demand and
supply. The storage of energy in form of heat or cold is called thermal energy storage.
Although other forms of energy storage such as electro-chemical (e.g. in batteries),
electrical (e.g. in capacitors) or potential (e.g. in reservoirs for hydropower plants)
are more widely-used and well-known today, thermal energy storage is an important
technology and a key concept in energy efficient systems.
1.1.1. Relevance of TES
From one perspective, energy storage is a very interesting technology for conventional
power generation. An illustration is given in figure 1.1a. To provide a varying power
supply in an energy infrastructure without storages, either the loads of power plants
are varied, or whole power plants are switched on and off. In general, power plants
running in partial load have lower efficiencies than when running in full load. For
plants running on fossil fuel, a lower efficiency is equivalent to a higher emission of the
greenhouse gas CO per generated kWh. Energy from power plants which are idle most2
of the time and running only during peak hours is very expensive due to high relative
investment costs. In countries like Japan, off-peak power is cheaper by up to a factor of
five when compared to peak power [1]. If peaks in the energy demand, like for example
the large cooling load during hot hours of the day, are satisfied from a storage, the time
profile of power generation can be smoothed and the peak in power generation can be
significantly reduced.
From another perspective, energy storage is used to adapt a given energy supply to a
different demand curve, as sketched in figure 1.1b. A good example is solar thermal
power, where the primary power supply is highly fluctuating. With the help of storage
11. Background and motivation
surplus demand
from storage surplus supply
to storagesurplus supply surplus demand
to storage from storage
supply demand
demand supply
time time
(a) supply management (b) demand management
Figure1.1.: Storage technology is used to manage mismatch between demand and supply.
technology, the supply can be adapted to a more continuous demand.
With the help of thermal energy storage and peak-shifting, energy efficiency in
conventional technologies can be greatly improved, and renewable energy sources can
be used [2]. While peak shifting is a general storage topic, a more specifically thermal
energy storage topic is the use of waste heat. Heat is a by-product of all irreversible
energy conversions. In German energy production, the losses sum up to about 45% of
the energy delivered to the consumers, as shown in figure 1.2.




(a) (b)
Figure1.2.: Amount of primary energy (a) and forms of secondary energy as delivered to the
consumer (b). Graphs based on data for Germany [3].
While the main amount of energy is transported via the electric grid or directly
delivered as fuel and gas as shown in figure 1.2b, the final use of energy is often thermal.
The demand for heat covers about two thirds of the total energy demand in Germany,
as shown in figure 1.3b.
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