TECHNICAL NOTE
Channel forma tion and visualization of melting and
crystallization behaviors in direct-conta ct latent heat
storage systems
Sven Kunkel
1
| Philipp Schütz
2
| Frederik Wunder
1
| Stefan Krimmel
2
|
Jörg Worlitschek
2
| Jens-Uwe Repke
3
| Matthias Rädle
1
1
Center for Mass Spectrometry and
Optical Spectroscopy, Hochschule
Mannheim — University of Applied
Sciences, Mannheim, Germany
2
Competence Centre for Thermal Energy
Storage, Hochschule Luzern — University
of Applied Sciences and Arts, Horw,
Switzerland
3
Dynamics and Operation of Technical
Systems, Technische Universität Berlin,
Berlin, Germany
Correspondence
Frederik Wunder, Center for Mass
Spectrometry and Optical Spectroscopy,
Hochschule Mannh eim — University of
Applied Sciences, Paul-Wittsack-Straße
10, Mannheim 68163, Germany.
Email: f.wunder@hs-mannheim. de
Summary
Thermal storage syste ms are an essential component for incr easing the share of
renewable ene rgies in reside ntial heatin g and for the valorization of waste heat. A
key challenge for the widespread applicat ion of thermal storage systems is their
limited power-to-capacity ratio. One potent ial solution for this challenge is repre-
sented by direct-contact lat ent heat st orage systems, i n which a phase change
material (PCM) is in direct contact with an immiscible heat transfer fluid (HTF).
To demonstrate the applicability of the d i rect-contact concept for domestic hot
water production, a PCM with a p hase change temperature of 59  C is chos en. To
enable cost-efficient implementation of t h e storage system, a eutectic mixture o f
two salt hydrates, magnesium chloride hexahydrate and magnesium nitrate hexa-
hydrate, is chosen as the PCM. One ke y asp ect for the direct-contact concept is
that, during discharge, the HTF channe l si nt h eP C Md on o tb e c o m ec l o g g e dd u r -
ing the solidification of the PCM. In this study, the formation and topology of the
channels in direct-contact system s under an optimized flow cond ition are investi-
gated via visual observation and X-ray computed tomogr aphy. The elucidation of
the channel structure provides information on the melting and crystallization
behaviors of the PCM, which are shown schematically.
KEYWORDS
computed tomography, direct contact latent heat storage, heat transfer surface, latent heat
storage, phase change material, thermal heat storage
1 | INTRODUCTION
Because of the increasing demand for energy, the finite
nature of fossil fuels, and concerns about environme ntal
impacts, it is nece ssary to increase the use of re newable
energy sources such as solar, biomass, and wind energies.
Because of their temporary nature, the effective use of
these energy sources depend s on the availabi lity of effi-
cient energy s torage systems.
1
Because of the natural var-
iability of sun and wind as heat and electricity sources,
an effective storage technology is required to compensate
for the time lag betwee n energy supply and demand.
2
Received: 24 September 2019 Revised: 10 January 2020 Accepted: 10 January 2020
DOI: 10.1002/er.5202
This is an open access article under the terms of the Creative Commons Attribution-NonCom mercial License, which perm its use, distribution and reproduction in any
medium, provided the original work is properly cite d and is not used for comme rcial purposes.
© 2020 The Authors. International Journal of Energy Resea rch published by John Wiley & Sons Ltd
Int J Energy Res. 2020 ;44:5017 – 5025. wileyonlinelibrary.com /journal/er 5017

In this investigation, we focus on thermal storage sys-
tems. There are three types of thermal energy storage sys-
tems: sensible heat storage, latent heat storage, and
thermochemical he at storage systems.
3
Despite their high
energy storage densities of the reactants of approximately
500 kWh/m
3
, thermochemical stora ge systems have a
major disadvantage in that the technology is complex
and requires large investments.
4,5
In comparison with
sensible heat storage systems, latent heat storage systems
have the following advantages
6
:
• Extraction and sup ply of thermal energy at an approxi-
mately constant temperature
• Approximately five times higher energy storage den-
sity ( Δ ϑ =1 0 K )
For a solid/liquid phase change, phase change mate-
rials (PCMs) can be subdivided into two main categories:
inorganic and organic s ubstances.
7
Inorganic substanc es
include salt hydrate s, salts, metals, and alloys, while
organic substances include paraffins, nonparaffins, and
polyalcohols. Organic nonp araffins include a variety of
substances such as fatty acids. In add ition, eutectic mix-
tures of inorganic and/or organic substances are also
used as PCMs.
8
A large number of organic and inorganic
substances have melting points in a te chnically relevant
range and substantial enthalpies of fusion. Nevertheless,
besides having a suitable melting point, the majority of
PCMs does not meet the criteria for a suitable storage
medium
9
as their enthalpy of fusion is too low or they
are corrosive or simply too expensive. A recent overview
of suitable PCMs is given in Zalba et al.
10
In this study,
we focus on salt hydrates. Similarly, to paraffins and fatty
acids, they have melting temperatures between 0  C and
100  C. Fatty acids were excluded because they are about
three times more expensi ve than paraffins.
8
Compared
with paraffins, salt hydrates have several advantages
11
:
• High volumetric latent heat stora ge capacity (approxi-
mately 200-600 kJ/L
3
)
• Low cost and widesprea d availability
• Sharp phase change (paraf fins are mixtures of several
hydrocarbons and thus have a melting temperatur e
range rather than a sharp melting point as in the case
of salt hydrates
8
)
• High thermal conductivity (paraffin of approximately
0.2 W/m
2
 K
12
and salt hydrates of approximately
0.5 W/m
2
 K
13
)
• Nonflammable
However, salt hydrates also have some disadvantages,
which make their use as a latent heat storage mediums
more difficult
10
:
• Undercooling: The liquid PCM cools to below the
melting point of the PCM during energy removal with-
out solidifying.
• Phase separation can occur in PCMs that consist, for
example, of several components. Dur ing a melting and
crystallization cycle, it is possible that phases partially
separate. The individual phases have different proper-
ties, which also differ from the desired properties of
the PCM.
• Corrosion: Many inor ganic substances, as well as salt
hydrates, are corrosive toward s metals. This must be
considered during the construc tion of the stor-
age tank.
• Water sorption due to the hygroscopy behavior in
many salt hydrates leads to changes of the stoichio-
metric water content reducing the phase change
enthalpy.
To improve the performance of latent heat storage
systems with salt hydrates as PCMs, nucleating agents
and thickeners help to prevent both undercooling and
phase separation.
1
In 1957, Etherington suggested
another solution for the problems of undercooling and
phase separation.
14
In this approach, the storage mate-
rial, the salt hydrate (disod ium phosphate dode-
cahydrate), and the heat transfer fluid (HTF) (oil) are in
direct contact, and the media are not miscible. The HTF
is injected into the lower part of the storage tank and
rises through the liquid PCM in the form of droplets, as
the density of the HTF is lowe r than that of the PCM .
Because of the resulting agitation, undercooling and
phase separation of the PCM are reduced.
FIGURE 1 Schematic illustration of a direct-contact latent
heat storage system
15
5018 KUNKEL ET AL .

In this study, a direct-contact latent heat storage sys-
tem employing a eutectic mixture of two salt hydrates as
the PCM, namely, magnesium chloride hexahy drate and
magnesium nitrate hexahydrate, and a mineral oil as the
HTF is investigated. Figure 1 illustrates a direct- contact
latent heat storage tank.
In Figure 1, the PCM is in a completely melted state;
that is, the storage system is loaded. If oil at a tempera-
ture below the melting temperature of the PCM is
injected into the storage tank, the PCM cools down and
begins to crystallize in the vicinity of the oil droplets.
During this crystallization process, a channel structure is
formed in the PCM. If the PCM is completely crystallized,
a channel structure remains within the PCM. In this
state, the storage tank is discharged. To charge the system
again, oil at a temperature above the melting temperature
of the PCM is pumped through the channel structure to
liquify the PCM.
Besides direct-contact systems, indi rect-contact latent
heat storage systems have also been described in the liter-
ature.
16-18
In these systems, the PCM and the HTF are
physically separated by a wall. For instance, a piping s ys-
tem for the HTF separates it from the PCM . Compared
with indirect heat transfer, direct heat transfer offers the
following advantages
3,19,20
:
• Higher charging and discharging rates due to
improved heat transfer between the PCM and the HTF
• Higher storage density, since there is no piping system
in the storage tank
• Extended period of high transfer rate during charge
because solidified PCM with low therm al conductivity
around the piping system does not screen the heat
transfer
Further studies on direct-contact latent heat storage
systems are described in the literature.
14,15,20-37
In these
studies, only a few simplified descriptions of the melting
and crystallization behaviors are given.
34,36
In particular,
the determination of heat transfer surfaces has not been
described.
To support a deeper understanding of direct heat
transfer and the design of the storage system, this study
provides a detailed explanation of the melting and crys-
tallization behaviors of the PCM. The investigation is
based on volumetric images from the H TF channels
recorded by X-ray computed tomography (CT). The heat
transfer surface is then subsequently determined
directly from the images for one model storage tank.
For the design of the storage tanks, however, it is of
interest to understand how the heat transfer surface is
distributed in the PCM. This allows conclusi ons to be
drawn about which areas melt quickly and which areas
melt slowly. To facilitate the X-ray analysis, a model
storage tank was built, which has only one oil inlet at
the bottom. The internal surface area is an important
property for any heat exchanger, therefore this studies goal
is to advance the design and s cale up of direct-contact
PCM heat storage systems.
2 | MATERIAL AND METHODS
2.1 | Phase change material
The PCM used was a stable eutectic mixture of the two
salt hydrates: magnesium chloride hexahydrate and mag-
nesium nitrate hexahydrate. The PCM has a melting
point of 59  C and is well suited for storing heat from
renewable energy sources or for storin g low-temperature
waste heat. The mineral oil Fragolt herm Q-7, which is
optimized for cooling and heating in process engineering
plants, was employed as an HTF. Table 1 shows the prop-
erties of the PCM and the mineral oil.
The support material of the storage tank is also
required for the investigation of the channel structure by
means of X-ray CT. Therefore, polycarbonate was
selected for this pu rpose.
2.2 | Experimental setup and
measurement method
The experimental setup shown in Figure 2 was used for
the investigation of the channel geometry. The set up con-
sists of an oil receiver tank (A), an electrical gear pump
TABLE 1 Material properties of the PCM and mineral oil as
the heat transfer fluid (HTF)
PCM Oil
Melting point,  C5 9
38
Less than
− 40
Heat of fusion, kJ/kg 132.2
38
Not relevant
Heat capacity solid, kJ/kg  K 1.86 (40  C)
38
Not relevant
Heat capacity liquid, kJ/kg  K 2.42 (70  C)
38
2.257 (70  C)
40
Heat conductivity solid,
W/m  K
0.678 (53  C)
38
Not relevant
Heat conductivity liquid,
W/m  K
0.510 (65  C)
38
0.123 (70  C)
40
Density liquid, kg/m
3
1550
38
797 (70  C)
40
Density solid, kg/m
3
1630
38
Not relevant
Viscosity, mPa  s 35 (62.5  C)
39
4.51 (50  C)
40
Abbreviation: PCM, phase change material.
KUNKEL ET AL . 5019

(B) delivering a pressure drop-independent mass flow,
two heat exchangers (C and D), and the storage tank
(E) containing the PCM . The storage tank is made of
transparent polycarbonate and has a height of 350 mm
and an internal diameter of 40 mm. The transparent
polycarbonate mak es it possible to obs erve the melting
and crystallization behaviors. At the bottom of the stor-
age tank, there is one orifice with a diameter of 6 mm,
which allowed the input of oil into the storage tank. Two
different channel configurations were investigated: an
artificially created cylindrical chan nel and a channel
under real flow conditions. The artificial cylindrical
channel had a diameter of 6 mm and was created usin g a
cylindrical inset. This channel geometry serves as a vali-
dation method for the measurement and evaluation
method, as the shell surface can be calculated based on
the known geometry.
To measure the pressu re and oil temperature in the
pipeline, a pressure transmitter and eight thermocouples
(type K) were integrated. To create the artificial cylin dri-
cal channel, a steel pipe with an outer diameter of 6 mm
was inserted into the oil inlet opening in the bottom of
the storage tank containing the liqui d PCM. Subse-
quently, oil was intr oduced at a temperature below the
melting point of the PCM, such that the PCM began to
crystallize on the wall of the steel pipe. Onc e the entirety
of the PCM had crystallized, it was possible to remove
the pipe, and a chan nel with an inner diameter of 6 mm
remained inside the PCM. To create a channel under real
flow conditions, complete melting of the PCM is
required. The PCM was overheated to a temperatur e of
approximately 68  C, and oil at a temperature of 49  C was
then injected into the storage tank. The oil mass flow was
set to 6.50  10
− 4
kg/s. The PCM then cooled down to its
melting point, and crystallizat ion began. In this case, a
branched channel structure is formed within the PCM .
To investigate the channel structure, the entire storage
tank was removed from the experi mental setup. Subse-
quently, the channel structure was investi gated by means
of X-ray CT. The XT H 225 ST CT system from Nikon
was used for this pu rpose. The X-ray radiation was gener-
ated with a rotating anode tube with a maximu m acceler-
ation voltage of 225 kV. For the measurement of the
storage system, an acceleration voltage of 120 kV and a
tube current of 550 μ A were employed. The storage sys-
tem was measured in two overlapping segments (top and
bottom), with a field of view of 2000 × 2000 pixels
recorded by the X-ray detector . The cross-sectional
images were subsequently reconstructed by the Nikon
reconstruction software, based on the Feldkamp-Davis-
Kress algorithm.
41
The dimension of the storage tank resulted in a reso-
lution of 93 μ m. After the measurement started, the sam-
ple stage rotated 360  , and images were taken
continuously during the measurements. After approxi-
mately 38 minutes, the measur ement was finished.
The images of the two consecutive measurements
were then combined manually by matching the two
image stacks according to position and prominent
features.
2.3 | Evaluation method
The volumetric images were analyzed by the Volume
Graphics Studio Max 3.1.2 software from Volume
FIGURE 2 Schematic of the
experimental setup
37
: A, oil receiver
tank; B, pump; C, cooling; D, heating; E,
storage tank
5020 KUNKEL ET AL .

Graphics (Heidelberg, Ger many). The channel visualiza-
tion and analysis of the heat tran sfer surface were per-
formed with the inclusion module. The selection
threshold for the analysis process wa s based on the inte-
grated automatic threshold procedure. The program
myVGL from Volume Graphi cs was subsequently
employed to measure and display individual chann el-
specific conditions, such as the local diameter of the
channel.
3 | RESULTS AND DISCUSSION
3.1 | Channel formation during
solidification
Figure 3 shows the crystallizat ion behavior
schematically.
At the be gi nnin g of the so li dif ic at io n behav io r, th e
PCM is com pl et el y melt ed , and the oil ris es in the P CM in
the f or m of dro pl et s (Fig u re 3 A) . Th e oil en te rs t he st or ag e
tan k in t he fo r m of a je t, wh ic h is fo ll ow ed by de cay of th e
jet . Af te r th e de ca y, si ngl e o il dr opl et s ar e f orme d. By
int ro du ci ng oil at a te mper at ur e belo w the ph as e cha nge
temp er at ur e of the PC M, the PC M begi ns to c ool do wn .
Su bseq ue nt ly , the PCM st ar ts to so li di fy, wher eb y so me
soli d PC M pa rt ic les fo rm in th e li qu id PC M. In ad di ti on , a
la ye r of soli d PC M is forme d at t he PC M/ oil ph as e bo un d-
ary at th e uppe r pa rt of the st ora ge t ank. Th is la ye r
ap pear s as a kind o f fo am. Th e re ma in in g por ti on of the
soli d PC M is kept in su sp en si on by the oi l flow . So me soli d
PCM pa rt ic le s sink to the bo tt o m of the sto ra ge tan k
bec au se th e so li d pa rt ic le s hav e a h ig he r de nsi t y tha n th at
of the li qu id PCM. In t he lowe r pa rt of the st ora ge ta nk,
the oil je t for ms a sing le cyli nd r ic al chan ne l. Ov er time , an
inc re as in g amoun t of soli d PCM par ti cles ar e for me d,
whic h ar e kept in susp en si on (F ig ure 3B ). Be ca use of th e
fo rmat io n of soli d PCM part ic le s, the oil fl o w is hind er ed ;
this fl o w resi st an ce in cr ea ses wi th hei gh t in the st or ag e
tan k. Ne ve rt he le ss, in the mi dd le of the sto ra ge t an k, th e
je t deca y ca n st il l form a st r ongl y br an che d ch an nel st r uc-
tur e. Howe ve r , in th e to p pa rt , th e hi ndr an ce is so st ro ng
tha t on ly a main cha nne l is fo rmed . Fi nal ly , the PCM
so lidi fi es c om pl etel y (F igu re 3C ).
In order to determine the channel structure, the solid-
ified storage tank was transferred to an X-ray CT system
where a stack of volumetric images were obtained and
recombined. Figure 4 shows the channel structures of the
artificial cylindrical channel (Figure 4A) and of one chan-
nel under real flow conditions (Figure 4B) within the
PCM, obtained by means of the CT system.
Figure 4 shows a visualization of the channels
extracted from the X-ray CT images. Figure 4A displays
the storage system with the artificial cylindrical channel.
The optical analysis revealed that the ave rage diameter
was 6.20 mm (±0.08 mm) and the shell surface was
93.65 cm
2
. With a height of 319 mm, the surface area of
the shell should therefore be 62.13 cm
2
(±6.55 cm
2
). The
discrepancy with the measured shell surface can be
explained by the surface roughness of the chann el wall.
Figure 4B presents the visualization of the structure of
the channel, which was created under real flow condi-
tions. Beginning at the oil inlet, a nearly cylindrical
section of the channel with a height of approximately
90 mm and a diameter of approximately 2.4 mm
(±0.2 mm) can be observed. The heat transfer surface
was approximately 6.8 cm
2
. This section is genera ted by
the oil jet resulting from the oil mass flow. The jet then
decays, which causes a branch to form within the PCM.
In the next section (beginning at a height of about
110 mm), the structure is heavily branched. It is in this
section that the largest heat transfer area is provided.
This section is succeeded by another (between 260 and
320 mm), in which the individual branc hed channels rec-
onnect. The numb er of branches is thereby reduced. In
contrast to previously reported findings,
36
an uneven
channel structure was formed. This structure fundamen-
tally influences the melting of the PCM . In this example,
the shell surface of the channel was 168.1 cm
2
, which
also corresponds to the heat transfe r surface.
FIGURE 3 Schematic illustration
of the crystallization behavior of the
phase change material (PCM) in three
different phases: A, initial situation,
where PCM is completely melted; B,
intermediate situation, where solid and
liquid phases of the PCM coexist; C,
storage tank in the discharged state,
where PCM is solid
KUNKEL ET AL . 5021

FIGURE 4 Visualization of the
structures formed under real flow
conditions within the phase change
material (PCM), obtained by means of
the X-ray computed tomography
(CT) system of the following: A, the
artificial cylindrical channel; B, a
channel
FIGURE 5 Schematic
illustration of the melting behavior of
the phase change material (PCM)
divided into six different phases. A,
Initially, the PCM is completely
solid. B, The channel structure
expands via melting of the PCM and
formation of a liquid PCM layer at
the channel outlet. C, The channel
structure expands further, and the
liquid PCM layer at the channel
outlet grows as well. D, The
threshold diameter is reached, at
which point the liquid PCM flows
back into the channel structure. E, In
the melting process, the middle
section melts first. F, The storage
tank is then in a charged state,
wherein the PCM is liquid
5022 KUNKEL ET AL .

3.2 | Melting behavior
Figure 5 schematically shows the melting behavior of
the PCM.
At the beginn ing of the char ging proces s, the PCM is
complete ly solidified, and a channel s t ructure is present
wi t h in t h e P C M. T h e oi l t h e n f lo w s t hr o u gh t h e pr ev al ent
channel structure, and the channels are comp letely filled
with oil (Figure 5A/ Figure 3C). By injecting oil with a tem-
perature above the melting temperature of the PCM into
the system, the PCM heats up and b egins to me lt . Convec-
tive heat transfer oc curs, and the channel structure
expands. The oil flow partially rem oves liquid P CM from
the canal. This res ults in the formation of a liquid PCM
la ye r at th e ch an nel ou tl et (Fi gu re 5B). T he cha nn el str uc -
ture continues to expand, and the laye r of liquid PCM also
continue s to grow (Figure 5C). If the channel st ructure
reaches a threshold diamete r , the layer of liquid PCM flows
back into the ch annel stru cture. The cha nne ls a re then
filled with liquid PCM. In consequence, an oil jet forms,
which is followed by decay of the jet and the formation of
single oil droplets. The liquid PCM is further heated by the
oil, while the liq u id PCM m elts the solid PCM via conduc-
tive heat transfer (Figure 5D). Further areas of the PCM are
melted by this additional s upply of heat. From optical
recordings of th e charging pro cess, it can be s een that the
mi dd le sec t io n of t he st or a ge sy s t em f ir st me l t s co mpl et e ly ,
followed by the lower section. The PCM/ oil phase bound-
ary is within the channel structure (Figure 5E) . Eventually,
the top section is melted until the entire PCM is in the liq-
uid s t ate (F igure 5F). Figure 6 sh ows a photog raph of the
intermediate state illustrated in Figure 5E.
Figure 6 confirms that the PCM in the m iddle
section of the storage tank had already melted while the
PCM in the top a nd bottom sect ions was still solid. From a
qualitative point of view, this s tate provides the largest heat
transfer surface. Although the heat transfer surface is
r e d u c e di nc o m p a r i s o nw i t ht h e m i d d l es e c t i o n ,t h el o w e r
pa rt o f th e PCM me lt s sub se qu ently. This may be explained
by the high heat flux in this area due to the highest temper-
ature gradient b etween the oil and PCM. Finally, the PCM
in the top section melts as the rest of the PCM is already
liquid. The individual channels have merged by this sta ge,
to form a main chann el. The he at transfer surface is small.
Previous studies have described and discussed,
20,31
among other thin gs, the melting and crystallization
behaviors of PCMs. This publication supports our fin ding
that the middle section melts first. These results suggest
that an efficient storage system should have a low height
in order to reduce the area in the top section of the stor-
age tank wherein PCM is solid (Figure 6). As a conclu-
sion, it can be said that direct-c ontact latent heat storage
tanks should be built wider rath er than taller. It is not
expected that the residence time of the oil within the
PCM would be insufficient to achieve efficient heat trans-
fer in shorter systems. A direct-contact latent heat storage
system employing erythr itol (density
42
: 1480 kg/m
3
) and
an oil as the HTF has been described previously.
36
In this
study, it was shown that efficient heat transfer can be
achieved even with PCM heights as low as 0.2 m.
For initial designs, a total PCM height of approxi-
mately 0.30 m is therefore recommended, even for larger
storage tanks.
4 | CONCLUSIONS
In this study, the melting and solidification behaviors of
a PCM within a direct-contact latent heat storage system
were experimentally investigat ed. The considered system
consisted of a vertical cylindrical storage tank conta ining
a eutectic mixture of two salt hydrates as the PCM and
FIGURE 6 Image of the state illustrated in Figure 5E
KUNKEL ET AL . 5023

a mineral oil as the HTF. By combining visua l observa-
tions with a volumetric cross-sectional analysi s by X-ray
CT, the melting and solidification behaviors were eluci-
dated, and design principles for such storage systems
were visualized, derived, and proposed. The conclusions
are summarized as follows.
Visual and X-ray CT inspections confirmed a hierar-
chical melting behavior. The middle section of the PCM
melted first, followed by the bottom section , and finally
the top section. During the solidification behavior, a
channel structure providing an increased heat transfer
surface is formed in the middle section. The total surface
area was 168.1 cm
2
in the investigat ed geometry.
The X-ray CT images confirme d that the channel
structure was divided into three sections: an unbranche d
section in the lower part of the PCM , succeeded by a
strongly branched section in the middle, and a less
branched section at the upper part. With the help of these
images, the melting and crystallization behaviors were
illustrated in detail.
For the design of direct-co ntact latent heat storage
systems, it is recomm ended to build storage systems
wider rather than taller, as the channel structure in the
upper part of the PCM is less branc hed and the heat
transfer surface is thereb y reduced. This can lead to del-
ayed melting and reduce d storage performance.
The elucidation of the m elting and crystallization
dynamics was an important undertaking to deepen the
understanding o f direct heat trans fer and will assist greatly
in t he d esi gn o f dir ec t-c ont ac t lat en t h ea t st o ra ge sy st ems .
ACKNOWLEDGE MENTS
The authors acknowled ge Prof Dr Christoph Zollikofer
and Dr Jody Weissmann for proving the possibility to
perform the computed tomography experiment s at the
facilities of University of Zurich.
NOMENCLATU RE
d
c
diameter of the PCM channel (mm)
d
s
diameter of the storage tank (mm)
ABBREVIATIONS
CT computed tomograp hy
HTF heat transfer fluid
P pressure gauge
PCM phase chan ge material
T thermocouple
ORCID
Frederik Wunder https://orcid.org/0000-00 03-3990-
8297
Stefan Krimmel https://orcid.org/0 000-0002-5462-2548
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How to cite this article: Kunkel S, Schütz P,
Wunder F, et al. Channel format ion and
visualization of melting and crystallization
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https://doi.org/10.1002/er.520 2
KUNKEL ET AL . 5025

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