CEN/CLC/TR 17603-31-10:2021

SLOVENSKI STANDARD
01-oktober-2021
Vesoljska tehnika - Priročnik o toplotni zasnovi - 10. del: Kondenzatorji s faznimi
prehodi
Space engineering - Thermal design handbook - Part 10: Phase - Change Capacitor
Raumfahrttechnik - Handbuch für thermisches Design - Teil 10: Kondensatoren mit
Phasenübergängen
Ingénierie spatiale - Manuel de conception thermique - Partie 10: Réservoirs de
matériaux à changement de phase
Ta slovenski standard je istoveten z: CEN/CLC/TR 17603-31-10:2021
ICS:
31.060.99 Drugi kondenzatorji Other capacitors
49.140 Vesoljski sistemi in operacije Space systems and
operations
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

TECHNICAL REPORT
CEN/CLC/TR 17603-31-
RAPPORT TECHNIQUE
TECHNISCHER BERICHT
August 2021
ICS 49.140
English version
Space engineering - Thermal design handbook - Part 10:
Phase - Change Capacitor
Ingénierie spatiale - Manuel de conception thermique - Raumfahrttechnik - Handbuch für thermisches Design -
Partie 10 : Réservoirs de matériaux à changement de Teil 10: Kondensatoren mit Phasenübergängen
phase
This Technical Report was approved by CEN on 21 June 2021. It has been drawn up by the Technical Committee CEN/CLC/JTC 5.

CEN and CENELEC members are the national standards bodies and national electrotechnical committees of Austria, Belgium,
Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy,
Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Republic of North Macedonia, Romania, Serbia,
Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and United Kingdom.

CEN-CENELEC Management Centre:
Rue de la Science 23, B-1040 Brussels
© 2021 CEN/CENELEC All rights of exploitation in any form and by any means Ref. No. CEN/CLC/TR 17603-31-10:2021 E
reserved worldwide for CEN national Members and for
CENELEC Members.
Table of contents
European Foreword . 8
1 Scope . 9
2 References . 10
3 Terms, definitions and symbols . 11
3.1 Terms and definitions . 11
3.2 Abbreviated terms. 11
3.3 Symbols . 12
4 Introduction . 14
5 PC working materials . 15
5.1 General . 15
5.1.1 Supercooling . 15
5.1.2 Nucleation . 19
5.1.3 The effect of gravity on melting and freezing of the pcm . 20
5.1.4 Bubble formation . 21
5.2 Possible candidates . 21
5.3 Selected candidates . 28
6 PCM technology . 52
6.1 Containers . 52
6.2 Fillers . 52
6.3 Containers and fillers . 53
6.3.1 Materials and corrosion . 53
6.3.2 Exixting containers and fillers . 56
7 PCM performances . 60
7.1 Analytical predictions . 60
7.1.1 Introduction . 60
7.1.2 Heat transfer relations . 61
8 Existing systems . 67
8.1 Introduction . 67
8.2 Dornier system . 68
8.3 Ike . 81
8.4 B&k engineering . 101
8.5 Aerojet electrosystems . 106
8.6 Trans temp . 116
Bibliography . 126

Figures
Figure 5-1: Temperature, T, vs. time, t, curves for heating and cooling of several PCMs.
From DORNIER SYSTEM (1971) [9]. . 18
Figure 5-2: Temperature, T, vs. time, t, curves for heating and cooling of several PCMs.
From DORNIER SYSTEM (1971) [9]. . 19
Figure 5-3: Density, ρ, vs. temperature T, for several PCMs. From DORNIER System
(1971) [9]. . 47
Figure 5-4: Specific heat, c, vs. temperature T, for several PCMs. From DORNIER
System (1971) [9]. . 48
Figure 5-5: Thermal conductivity, k, vs. temperature T, for several PCMs. From
DORNIER System (1971) [9]. . 49
Figure 5-6: Vapor pressure, p , vs. temperature T, for several PCMs. From DORNIER
v
System (1971) [9]. . 50
Figure 5-7: Viscosity, µ, vs. temperature T, for several PCMs. From DORNIER System
(1971) [9]. . 51
Figure 5-8: Isothermal compressibility, χ, vs. temperature T, for several PCMs. From
DORNIER System (1971) [9]. . 51
Figure 6-1: Container with machined wall profile and welded top and bottom.
Honeycomb filler with heat conduction fins. All the dimensions are in mm.
From DORNIER SYSTEM (1972) [10]. . 57
Figure 6-2: Fully machined container with welded top. Honeycomb filler. All the
dimensions are in mm. From DORNIER SYSTEM (1972) [10]. . 58
Figure 6-3: Machined wall container profile with top and bottom adhesive bonded.
Alternative filler types are honeycomb or honeycomb plus fins. All the
dimensions are in mm. From DORNIER SYSTEM (1972) [10]. . 59
Figure 7-1: Sketch of the PCM package showing the solid-liquid interface. . 61
Figure 7-2: PCM mass, M , filler mass, M , package thickness, L, temperature
PCM F
excursion, ∆T, and total conductivity, k , as functions of the ratio of filler
T
area to total area, A /A . Calculated by the compiler. . 65
F T
Figure 7-3: PCM mass, M , filler mass, M , package thickness, L, temperature
PCM F
excursion, ∆T, and total conductivity, k , as functions of the ratio of filler
T
area to total area, A /A . Calculated by the compiler. . 66
F T
Figure 8-1: PCM capacitor for eclipse temperature control developed by Dornier
System. . 71
Figure 8-2: 30 W.h PCM capacitors developed by Dornier System. a) Complete PCM
capacitor. b) Container and honeycomb filler with cells normal to the heat
input/output face. c) Container, honeycomb filler with cells parallel to the
heat input/output face, and cover sheets. . 71
Figure 8-3: PCM mounting panels developed by Dornier Syatem. b) Shows the
arrangement used for thermal control of four different heat sources. . 72
Figure 8-4: Thermal control system formed by, from right to left, a: PCM capacitor, b:
axial heat pipe and, c: flat plate heat pipe. This system was developed by
Dornier System for the GfW-Heat Pipe Experiment. October 1974. . 76
Figure 8-5: PCM capacitor shown in the above figure. 76
Figure 8-6: Temperature, T, as selected points in the complete system vs. time, t,
during heat up. a) Ground tests. Symmetry axis in horizontal position. Q =
28 W. b) Ground tests. Symmetry axis in vertical position. Q = 28 W. The
high temperatures which appear at start-up are due to pool boiling in the
evaporator of the axial heat pipe. c) Flight experiment under microgravity
conditions. Q not given. notice time scale. . 77
Figure 8-7: PCM capacitor developed by Dornier System for temperature control of two
rate gyros onboard the Sounding Rocket ESRO "S-93". . 79
Figure 8-8: Test model of the above PCM capacitor. In the figure are shown, from right
to left, the two rate gyros, the filler and the container. 79
Figure 8-9: Temperature, T, at the surface of the rate gyros, vs. time, t . Ambient
temperature, T = 273 K. Ambient temperature, T = 273 K.
R R
Ambient temperature changing between 273 K and 333 K. This
curve shows the history of the ambient temperature used as input for the
last curve above. References: DORNIER SYSTEM (1972) [10], Striimatter
(1972) [22]. . 80
Figure 8-10: Location of the thermocouples in the input/output face. The thermocouples
placed on the opposite face do not appear in the figure since they are
projected in the same positions as those in the input/output face. All the
dimensions are in mm. . 83
Figure 8-11: Prototype PCM capacitor developed by IKE. All the dimensions are in mm.
a: Box. b: Honeycomb half layer. c: Perforations in compartment walls. d:
pinch tube. e: Extension of the pinch tube. . 85
Figure 8-12: Time, t, for nominal heat storage and temperature, T of the heat transfer
face vs. heat input rate, Q . Time for nominal heat storage. Measured
average wall temperature at time t . Measured temperature at the center
of the heat transfer face at time t. . 86
Figure 8-13: Time, t , for complete melting and temperature, T, of the heat transfer
max
face vs. heat input rate, Q. Time for complete melting: measured.
calculated by model A. Calculated by models B or C. Average wall
temperature at time t : measured. calculated by model A.
max
Calculated by models B or C. Measured temperature at the center of the
heat transfer face at t . . 86
max
Figure 8-14: Location of the thermocouples in the heat input/output face (f) and within
the box (b). The thermocouples placed on the opposite face do not appear
in the figure since they are projected on the same positions as those in the
input/output face. All the dimensions are in mm. . 89
Figure 8-15: PCM capacitors with several fillers developed by IKE. All the dimensions
are in mm. a: Model 1. b: Model 2. c: Model 3. d: Model 4. . 91
Figure 8-16: Measured temperature, T, at several points of the PCM capacitor vs. time
t. Model 2. Heat up with a heat transfer rate Q = 30,6 W. Points are placed
as follows (Figure 8-14): Upper left corner of the heat input/output
face. Center of the insulated face. Center of the box,
immersed in the PCM. Time for complete melting t , is shown by means
max
of a vertical trace intersecting the curves. 92
Figure 8-17: Time for complete melting, tmax, vs. heat input rate Q . Model 1.
Measured. Model 2. Measured. Calculated by using model A.
Model 3. Measured. Calculated by using model A.
Calculated by using model B. Model 4. Measured. . 92
Figure 8-18: Largest measured temperature, T, of the heat input/output face vs. heat
input rate, Q .Model 1. Measured. Model 2. Measured.
Calculated by using model A. Model 3. Measured. Calculated by
using model A. Calculated by using model B. Model 4. Measured. . 93
Figure 8-19: Location of the thermocouples in the heat input/output face. The
thermocouples placed on the opposite face do not appear in the figure
since they are projected on the same positions as those in the input/output
face. Thermocouples are numbered for later reference. All the dimensions
are in mm. . 96
Figure 8-20: PCM capacitor developed by IKE for ESA (ESTEC). All the dimensions
are in mm. a: Box. b: Honeycomb calls. c: Perforations in compartment
walls. d: Pinch tube. . 98
Figure 8-21: Measured temperature, T at several points in either of the large faces of
the container vs. time, t. Heat up with a heat transfer rate Q = 86,4 W.
Points 1 to 5 are placed in the heat input/output face as indicated in Figure
8-19. Circled points are in the same positions at the insulated face. Time for
complete melting, t , is shown by means of a vertical trace intersecting
max
the curves. . 99
Figure 8-22: Time for complete melting, t vs. heat input rate, Q . Measured.
max
Calculated by using the 26 nodes model. Overall thermal conductances in
−1 −1
the range 1,4 W.K to 5,6 W.K . . 99
Figure 8-23: Average temperature, T of either of the large faces vs. heat input rate, Q. t
= t . Heat input/output face. Measured. Calculated by the 26
max
− 1
nodes model. Overall thermal conductance 5,6 W.K . Calculated
−1
as above. Overall thermal conductance 6,7 W.K . Insulated face.
Measured. Calculated as above. Overall thermal conductance 5,6
−1 −1
W.K and 6,7 W.K . . 100
Figure 8-24: Set-up used for component tests. . 103
Figure 8-25: PCM capacitor developed by B & K Engineering for NASA. All the
dimensions are in mm. . 104
Figure 8-26: Schematic of the PCM capacitor in the TIROS-N cryogenic heat pipe
experiment package (HEPP). From Ollendorf (1976) [20]. . 104
Figure 8-27: Average temperature, T of the container vs. time, t , during heat up for two
different heat transfer rates. Q = 25 W. Q = 45 W. Component tests
data. . 105
Figure 8-28: Average temperature, T, of the container vs. time, t, during cool down.
Data from either component or system tests. Component tests, Q = 6,1
W. Freezing interval ∆t≅ 4,5 h. System tests, Q = 5,2 W. Freezing
interval ∆t≅ 5 h. Time for complete melting, tmax, is shown by means of a
vertical trace intersecting the curves. . 105
Figure 8-29: Set up used for the “Beaker” tests. . 108
Figure 8-30: Set up used for the "Canteen" tests. Strain gage.
Temperature sensor. . 109
Figure 8-31: PCM capacitor developed by Aerojet ElectroSystems Company. The outer
diameter is given in mm. . 109
Figure 8-32: "Canteen" simulation of the S day. a) Heat transfer rate, Q vs. time, t. b)
PCM temperature, T, vs. time t. Data in the insert table estimated by the
compiler through area integration and the value of h in Tables 8-9 and 8-
f
10. . 110
Figure 8-33: Maximum diurnal temperature, T of the radiator vs. orbital time, t.
Predicted with no-phase change. Measured. Phase-change attenuated
the warming trend of the radiator for eleven months (performance
extension). . 110
Figure 8-34: Location of the thermocouples and strain gages in the test unit.
Thermocouples 12, 17 and 14 are placed on the base; 6, 7 and 8 on the
upper face; 9 and 10 on the lateral faces; 11 on the rim, and 26 on the
mounting hub interface. Strain gages are placed on his upper face. 114
Figure 8-35: PCM capacitor developed by Aerojet. All the dimensions are in mm. . 114
Figure 8-36: Average temperature, T, of the container vs. time, t, either during heat up
or during cool down. a) During heat up with a nominal heat transfer rate Q =
2,5 W. b) During cool down with the same nominal heat transfer rate. With
honeycomb filler. Mounting hub down. Measured. Calculated.
Cooling coils down. Measured. Calculated. Without honeycomb
filler. Cooling coils up. Measured. Calculated with the original
model. Calculated with the modified model. Cooling coils down.
Measured. Times for 90% and complete melting (freezing) are shown in the
figure by means of vertical traces intersecting the calculated curves.
Replotted by the compiler, after Bledjian, Burden & Hanna (1979) [6], by
shifting the time scale in order to unify the initial temperatures. . 115
Figure 8-37: Several TRANS TEMP Containers developed by Royal Industries for
transportation of temperature- sensitive products. a: 205 System. b: 301
System. c: 310 System. 1: Outer insulation. 2: PCM container. . 125
Figure 8-38: Measured ambient and inner temperatures, T vs. time, t, for several
TRANS TEMP Containers holding blood samples. a: 205 System. b: 301
System. c: 310 System. Ambient temperature. Inner
temperature. . 125

Tables
Table 5-1: Supercooling Tests . 17
Table 5-2: PARAFFINS . 22
a
Table 5-3: NON-PARAFFIN ORGANICS . 23
Table 5-4: SALT HYDRATES . 24
Table 5-5: METALLIC . 26
Table 5-6: FUSED SALT EUTECTICS . 27
Table 5-7: MISCELLANEOUS . 27
Table 5-8: SOLID-SOLID . 28
Table 5-9: PARAFFINS . 29
Table 5-10: PARAFFINS . 30
Table 5-11: PARAFFINS . 32
Table 5-12: NON-PARAFFIN ORGANICS . 34
Table 5-13: NON-PARAFFIN ORGANICS . 35
Table 5-14: NON-PARAFFIN ORGANICS . 37
Table 5-15: NON-PARAFFIN ORGANICS . 39
Table 5-16: NON-PARAFFIN ORGANICS . 40
Table 5-17: SALT HYDRATES . 42
Table 5-18: METALLIC AND MISCELLANEOUS . 45
Table 6-1: Physical Properties of Several Container and Filler Materials . 54
Table 6-2: Compatibility of PCM with Several Container and Filler Materials . 55

European Foreword
This document (CEN/CLC/TR 17603-31-10:2021) has been prepared by Technical Committee
CEN/CLC/JTC 5 “Space”, the secretariat of which is held by DIN.
It is highlighted that this technical report does not contain any requirement but only collection of data
or descriptions and guidelines about how to organize and perform the work in support of EN 16603-
31.
This Technical report (TR 17603-31-10:2021) originates from ECSS-E-HB-31-01 Part 10A.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN [and/or CENELEC] shall not be held responsible for identifying any or all such
patent rights.
This document has been prepared under a mandate given to CEN by the European Commission and
the European Free Trade Association.
This document has been developed to cover specifically space systems and has therefore precedence
over any TR covering the same scope but with a wider domain of applicability (e.g.: aerospace).
Scope
Solid-liquid phase-change materials (PCM) are a favoured approach to spacecraft passive thermal
control for incident orbital heat fluxes or when there are wide fluctuations in onboard equipment.
The PCM thermal control system consists of a container which is filled with a substance capable of
undergoing a phase-change. When there is an the increase in surface temperature of spacecraft the
PCM absorbs the excess heat by melting. If there is a temperature decrease, then the PCM can provide
heat by solidifying.
Many types of PCM systems are used in spacecrafts for different types of thermal transfer control.
Characteristics and performance of phase control materials are described in this Part. Existing PCM
systems are also described.
The Thermal design handbook is published in 16 Parts
TR 17603-31-01 Thermal design handbook – Part 1: View factors
TR 17603-31-02 Thermal design handbook – Part 2: Holes, Grooves and Cavities
TR 17603-31-03 Thermal design handbook – Part 3: Spacecraft Surface Temperature
TR 17603-31-04 Thermal design handbook – Part 4: Conductive Heat Transfer
TR 17603-31-05 Thermal design handbook – Part 5: Structural Materials: Metallic and
Composite
TR 17603-31-06 Thermal design handbook – Part 6: Thermal Control Surfaces
TR 17603-31-07 Thermal design handbook – Part 7: Insulations
TR 17603-31-08 Thermal design handbook – Part 8: Heat Pipes
TR 17603-31-09 Thermal design handbook – Part 9: Radiators
TR 17603-31-10 Thermal design handbook – Part 10: Phase – Change Capacitors
TR 17603-31-11 Thermal design handbook – Part 11: Electrical Heating
TR 17603-31-12 Thermal design handbook – Part 12: Louvers
TR 17603-31-13 Thermal design handbook – Part 13: Fluid Loops
TR 17603-31-14 Thermal design handbook – Part 14: Cryogenic Cooling
TR 17603-31-15 Thermal design handbook – Part 15: Existing Satellites
TR 17603-31-16 Thermal design handbook – Part 16: Thermal Protection System

References
EN Reference Reference in text Title
EN 16601-00-01 ECSS-S-ST-00-01 ECSS System - Glossary of terms
TR 17603-30-06 ECSS-E-HB-31-01 Part 6 Thermal design handbook – Part 6: Thermal
Control Surfaces
TR 17603-30-11 ECSS-E-HB-31-01 Part 11 Thermal design handbook – Part 11: Electrical
Heating
All other references made to publications in this Part are listed, alphabetically, in the Bibliography.
Terms, definitions and symbols
3.1 Terms and definitions
For the purpose of this Standard, the terms and definitions given in ECSS-S-ST-00-01 apply.
3.2 Abbreviated terms
The following abbreviated terms are defined and used within this Standard.
air traffic control (aerosat)
ATC
Brennan & Kroliczek
B & K
Gessellschaft für Weltraumforschung
GfW
heat pipe experiment package
HEPP
HLS
international heat pipe experiment
IHPE
institut für kernenenergetik (university of Stuttgart)
IKE
long duration exposure facility
LDEF
methyl-ethyl ketone
MEK
multilayer insulation
MLI
phase-change material
PCM
systems improved numerical differencing analyzer
SINDA
stainless steel
SS
second surface mirror
SSM
stoichiometric day, see clause 8.5
S day
tungsten-inert gas
TIG
television and infra-red observation satellite
TIROS
tag open cup
TOC
tag closed cup
TCC
transporter heat pipe
TPHP
Other Symbols, mainly used to define the geometry of the configuration, are introduced when
required.
3.3 Symbols
cross-sectional area, [m ]
A
modulus of elasticity, [Pa]
E
maximum energy stored in the PCM device, [J]
Emax
thickness of the PCM device, one-dimensional model,
L
[m]
mass, [kg]
M
heat transfer rate, [W]
Q
temperature, [K]
T
melting (or freezing) temperature, [K]
TM
temperature of the components being controlled, [K]
T0
reference temperature, [K]
TR
excursion temperature, [K], ∆T = T0−TM
∆T
− −
1 1
specific heat, [J.kg .K ]
c
−
heat of fusion, [J.kg ]
hf
−
heat of transition, [J.kg ]
ht
− −
1 1
thermal conductivity, [W.m .K ]
k
vapor pressure, [Pa]
pv
heat flux to the PCM device, one-dimensional model,
q0
−
[W.m ]
heat flux from the PCM device to the heat sink, one-
qR
−
dimensional model, [W.m ]
interface position, measured from x = 0, one-
s(t)
dimensional model, [m]
time, [d], [h], [min], [s]
t
time for complete melting, [h]
tmax
time for melting 90% of the volume of the PCM, [h]
t90
geometrical coordinate, one-dimensional model, [m]
x
−
2 1
thermal diffusivity, [m .s ], α = k/ρc
α
thermal expansion coefficient, volumetric (unless
β
−
otherwise stated), [K ]
µ dynamic viscosity, [Pa.s]
−
ρ density, [kg.m ]
−
surface tension, [N.m ]
σ
ultimate tensile strength, [pa]
σult
−
χ isothermal compressibility, [Pa ]

Subscripts
Container
C
Filler
F
Phase-Change Material
PCM
Total
T
Liquid
l
Solid
s
Introduction
Solid-liquid phase-change materials (PCM) present an attractive approach to spacecraft passive
thermal control when the incident orbital heat fluxes or the onboard equipment heat dissipation
fluctuate widely.
Basically the PCM thermal control system consists of a container which is filled with a substance
capable of undergoing a phase-change. When the temperature of the spacecraft surface increases,
either because external radiation or inner heat dissipation, the PCM will absorb the excess heat
through melting, and will restore it through solidification when the temperature decreases again.
Because of the obvious electrical analogy this thermal control system is also called PCM capacitor.
To control the temperature of a cyclically operating equipment, the PCM cell is normally sandwiched
between the equipment and the heat sink.
When the PCM system aims at absorbing the abnormal heat dissipation peaks of an equipment which
somehow the excess heat to a sink, the cell is placed in contact with the equipment, without interfering
in the normal heat patch between equipment and heat sink.
For achieving the thermal control of solitary equipment, the PCM capacitor may be used as the sole
heat sink, provided that the heat transferred during the heating period does not exceed that required
to completely melt the material.
PC working materials
5.1 General
The ideal PCM would have the following characteristics:
1. Melting point within the allowed temperature range of the thermally controlled
component. In general between 260 K and 315 K.
2. High heat of fusion. This property defines the available energy storage and may be
important either on a mass basis or on a volume basis.
3. Reversible solid-to-liquid transition. The chemical composition of the solid and liquid
phases should be the same.
4. High thermal conductivity. This property is necessary to reduce thermal gradients.
Unfortunately most PCM are very poor thermal conductors, so that fillers are used to
increase the conductivity of the system.
5. High specific heat and high density.
6. Long term reliability during repeated cycling.
7. Low volume change during phase transition.
8. Low vapor pressure.
9. Non-toxic.
10. Non-corrosive. Compatible with structural materials.
11. No tendency to supercooling.
12. Availability and reasonable cost.
Relevant physical characteristics of phase-changing system are discussed, from a general point of
view, in the following pages.
5.1.1 Supercooling
Supercooling is the process of cooling a liquid below the solid-liquid equilibrium temperature without
formation of the solid phase. Supercooling when only one phase is present is called one-phase
supercooling. Supercooling in the presence of both solid and liquid, or two-phase supercooling,
depends upon the particular material and the environment surrounding it. The best way to reduce
supercooling is to ensure that the original crystalline material has not been completely molten In such
a case the seeds which are present in the melt tend to nucleate the solid phase when heat is removed.
Nucleating catalysts are available for many materials.
Several PCMs have been tested under repeated heating and freezing cycles by DONIER SYSTEM. The
main purpose of these tests has been the detection of eventual supercooling phenomena. Among the
tested materials, the normal paraffins did not show any supercooling tendency. This result is also
confirmed by the available literature. However, according to investigations carried out by Bentilla,
Sterrett & Karre (1966) [5], contamination of the paraffins with water leads to supercooling of the
molten substance down to 283 K. Care should therefore be taken to prevent this contamination.
5.1.1.1 Experimental investigations
An experimental investigation has been performed by DORNIER SYSTEM with the aim of finding out
the variables having any influence on the supercooling behavior of the PCM; namely: total number of
cycles, cooling rate, and type of container.
In these experiments, the temperature of the test chamber was alternatively increased and decreased
at constant steps within a range of 240 to 320 K. The results are summarized in Table 5-1, and Figure
5-1 and Figure 5-2.
The temperature history of both the PCM inside the containers and the honey comb packages was
measured by means of copper-constant thermocouples, and continuously recorded. The test points
were located in the center of the container.
5.1.1.2 Results
Water and paraffins did not show any tendency to supercooling during freezing, not even at the
− −
2 1
maximum realizable cooling rate of 2x10 K.s .
Acetic acid showed supercooling of different orders of magnitude. It could not be determined whether
or not supercooling is a function of cooling rate. Supercooling was also independent, at least not
noticeably dependent, on the number of cells constituting the container. If however, Al-chips were
added to facilitate heterogeneous nucleation, supercooling was reduced during the first cycles,
although it augmented when the test time increased.
Supercooling of disodium hydroxide heptahydrate amounted to 3-3,5 K regardless of cooling rate and
number of temperature cycles.
Table 5-1: Supercooling Tests
PCM Container Number of FIRST CYCLE Maximum Comments
Cycles Supercooling
Plateau Temperature [K] Supercooling
DODECANE Aluminium Cell. 146 265 NO NO
Honeycomb adhesive-
bonded to end plate.
TETRADECANE Aluminium Cell. 50 280 NO NO Slightly ascending
Honeycomb adhesive- temperature.
bonded to end plate.
HEXADECANE Test tube without 50 293 NO NO
honeycomb.
Aluminium Cell. 70 300 ∆T =9,5 K ∆T = 10 K
Honeycomb adhesive-
bonded to end plate.
ACETIC ACID Neck flash with splinters. 70 301 NO ∆T = 6,5 K Normal supercooling:
∆T= 5 K
Aluminium Cell. 23 299 ∆T = 1,5 K ∆T = 5 K
Honeycomb inserted (not
adhered).
DISODIUM Neck flash with splinters. 36 286,5 ∆T = 3,5 K ∆T = 3,5 K No plateau
HYDROXIDE temperature during
22 287 ∆T = 2,5 K ∆T = 3 K
HEPTAHYDRATE heating-up.
DISTILLED WATER Aluminium Cell. 85 274 NO NO
Honeycomb adhesive-
bonded to end plate.
NOTE From DORNIER SYSTEM (1971) [9].
Figure 5-1: Temperature, T, vs. time, t, curves for heating and cooling of several
PCMs. From DORNIER SYSTEM (1971) [9].
Figure 5-2: Temperature, T, vs. time, t, curves for heating and cooling of several
PCMs. From DORNIER SYSTEM (1971) [9].
5.1.2 Nucleation
Nucleation is the formation of the first crystals capable of spontaneous growth into large crystals in an
unstable liquid phase. These first crystals are called nuclei.
Homogeneous nucleation occurs when the nuclei may be generated spontaneously from the liquid
itself at the onset of freezing. The rates of formation and dissociation do not depend on the presence or
absence of surfaces, such as container walls or foreign particles.
Heterogeneous nucleation occurs when the nuclei are formed on solid particles already in the system
or at the container walls. Supercooling (see clause 5.1.1) can be considerably reduced or even
eliminated when heterogeneous nucleation is present.
5.1.3 The effect of gravity on melting and freezing of the pcm
Gravitational forces between molecules are comparatively small relative to intermolecular or
interatomic forces; for example, the ratio between gravitational and intermolecular forces between two
−
molecules of carbon dioxide is of the order of 10 .
Since phase-change is controlled by intermolecular forces, the rate of melting or freezing should be the
same under microgravity as it is under normal gravity conditions, provided that thermal and solutal
fields are the same.
However, phase change happens to be indirectly influenced by gravity through the following effects:
1. Convection in the liquid phase which propagates nuclei and enhances heat transfer, and
2. thermal contact conductance between the heated (or cooled) wall and the PCM.
1. Convection can be induced by volume forces and/or by surface forces.
When the density gradient (due to thermal, solutal, or other effects) is not aligned with
the body force (gravitational) vector, flow immediately results no matter how small the
gradient. On the other hand, when the density gradient is parallel to but opposed to the
body force, the fluid remains in a state of unstable equilibrium until a critical density
gradient (more precisely, a critical Grashof number) is exceeded. The Grashof number
gives the ratio of buoyancy to viscous forces.
Convection from the interfaces is produced by surface tractions due to surface tension
gradients (which again can be due to thermal, solutal, or other effects). If the temperature
gradient is parallel to the undisturbed interface, flow immediately results no matter how
small the gradient. When the temperature gradient is normal to the undisturbed
interface, convection appears provided that a critical Marangoni number is exceeded.
This Marangoni number is defined as the ratio of surface tension gradient forces to
viscous forces.
A priori information on which type of convection prevails under given circumstances can
be obtained from an order of magnitude analysis (Napolitano (1981) [19]). Nevertheless,
convection is presumably negligible for most PCM capacitors because of the low
temperature gradients (associated to the high thermal conductances usually required)
and of the reduced characteristics lengths of the enclosures when a metallic filler is used (
see clause 6.2).
Convection due to shrinkage forces associated to phase change is normally small when
solid and liquid densities are not too different.
2. Gravity presses the denser phase against the lower wall of the container.
At the onset of the cool down the liquid remains in contact with the lower wall, be it
cooled or heated, while the upper part of the cell is empty because of the void (ullage)
which is usually provided for safe operation at high temperatures when using rigid
containers (see clause 6.1). Thence, the temperature of the cooled wall is expected to be
lower when the device is cooled from above (poor thermal conductance), that when
cooled from below.
As soon as the cool down progresses, the solid, which is assumed to be denser than the
liquid, makes contact with the lower wall enhancing the heat transfer to it, because the
solid phase has normally a higher thermal conductivity than the liquid.
Reliable cooling data of PCM cells are not easily found in the literature since cooling is
difficult to control precisely. Data reported in Figure 8-36b, Clause 8.5, do not support the
above prediction, rather the average wall temperature is higher (not smaller) when the
solid is pressed by gravity against the cooled wall. Notice, however that according to
Table 5-9, clause 5.3, liquid 1-Heptene is more conductive than the solid.
During heat up, the insulation is similar. Since the frozen PCM is usually denser than the
liquid, the solid falls to the bottom of the cell. When heating up from below, the solid
PCM remains close to the heated face, with a thin liquid layer amid them, whereas when
heated from above a void space or, at most, a thick gap filled with the liquid appears
between the heated face and the solid PCM. Thence, the heated wall temperature is
expected to be higher when the device is heated from above than when it is heated from
below. This effect has been observed experimentally, see for instance, in clause 8.3 the
tests corresponding to Figure 8-14. Melting time is barely affected by the orientation of
the gravity vector.
Metallic fillers (see clause 6.2) have the two-fold effect of increasing the thermal contact
conductance between container and PCM, and of impeding liquid motion. Migration of
the liquid by capillary pumping toward selected parts of the container can be achieved by
varying the cross sectional area of the filler cells as in the PCM device shown in clause
8.4.
5.1.4 Bubble formation
Bubbles can affect PCM operation in several ways: the thermal conductivity will be altered; bubbles in
the liquid phase will cause stirring actions; small bubbles in the solid phase can take up some of the
volume shrinkage, thereby avoiding the formation of large cavities.
There are several types of bubbles likely to occur during PCM performance under microgravity: PCM
vapor bubbles, cavities or voids from volume shrinkage, and gas bubbles.
The most persistent bubbles seem to be those formed by dissolved gases. During solidification,
dissolved gases can be rejected just as any other solute at the solid-liquid interface. During the reverse
process of melting, bubbles previously overgrown in the solid can be liberated. In an one-g field,
bubbles would be more likely to float to the top and coalesce. Under microgravity, bubbles would be
trapped in the frozen solid.
In order to reduce the amount of dissolved gases it is suggested to boil the PCM in liquid form under
reduced p
...