Phase Change Material Heat Sink Flight Experiment Results

47th International Conference on Environmental Systems ICES-2017-016 16-20 July 2017, Charleston, South Carolina

Phase Change Material Heat Sink Flight Experiment Results

Gregory J Quinn1

Hamilton Sundstrand Space Systems International, Inc., Windsor Locks, Connecticut, 06096

Thomas Ahlstrom.2, Hung Le

3, and Rubik Sheth

4

NASA Johnson Space Center, 2101 NASA Parkway, Houston, Texas, 77058

A flight experiment was conducted on the International Space Station (ISS) to prove out

operation of a microgravity compatible paraffin wax phase change material (PCM) heat

sink. A PCM heat sink can help to reduce the overall mass and volume of future exploration

spacecraft thermal control systems (TCS). Vehicles such as the Orion Multipurpose Crew

Vehicle can use PCM heat sinks to temporarily store thermal energy during mission phases

where the radiators are unavailable or too warm to reject the heat as it’s generated, or

sublimating water would require significant expendable mass. The experiment was

conducted specifically to prove out a heat sink design that incorporates a novel phase

management approach to prevent high pressures and structural deformation that often

occur with PCM heat sinks undergoing cyclic operation. The PCM heat sink test article was

incorporated into an ISS double EXPRESS rack, where it underwent performance testing

and acceptance testing at NASA Johnson Space Center. The experiment was delivered to

the ISS on the SpaceX 9 mission in the summer of 2016. It was successfully installed into the

ISS and run remotely for several months to exercise the PCM heat sink. Freeze and thaw

cycles were conducted with a range of coolant flow rates and heater powers to characterize

the performance of the technology with regard to heat storage and wax pressure

management. Heat storage performance met the objectives of the tests, but the novel phase

management approach had mixed results. Wax cavity pressures remained low in some tests,

but not others.

Nomenclature

ATTIC = Advanced Thermal Technology

Integration Center

EMC = Electromagnetic Compatibility

EMI = Electromagnetic Interference

EXPRESS = Expedite the Processing of Experiments

to the Space Station

g2/Hz = Power spectral density

Grms = Root mean square acceleration

GSE = Ground support equipment

Hz = Hertz

ISS = International Space Station

ISPR = International Standard Payload Rack

ITCS = Internal Thermal Control System

JSC = NASA Johnson Space Center

kg = Kilograms

kJ = Kilojoule

kPa = Kilopascals

lb = Pound

LTL = Low Temperature water Loop

MDP = Maximum Design Pressure

MSFC = Marshall Space Flight Center

MTL = Moderate Temperature water Loop

PCM = Phase Change Material

PGW = Propylene glycol and water

POIC = Payload Operations Integration

Center

PRCU = Payload rack checkout unit

psid = Pounds per square inch differential

psig = Pounds per square inch gauge

scc/s = Standard cubic centimeters per

second

TCS = Thermal Control System

TECHX = Thermal Electric Cooler Heat

Exchanger

TReK = Telescience Resource Kit

1 Staff Research Engineer, Space Systems, 1 Hamilton Road, M/S 1A-2-W66

2 EC7 / EVA Tools, Equipment, & Habitability Systems Branch, Crew and Thermal Systems Division.

3 EC6 / Thermal Systems Branch, Crew and Thermal Systems Division.

4 EC6 / Thermal Systems Branch, Crew and Thermal Systems Division.

International Conference on Environmental Systems

2

I. Introduction

HASE change material (PCM) heat sinks use the latent heat of fusion to store energy at a constant temperature.

They can undergo multiple freeze and thaw cycles, which makes them useful as spacecraft thermal capacitors. A

space vehicle thermal control system (TCS) would use a thermal capacitor to store excess energy on-board when

there are spikes in heat generation or periodic reductions in radiator capacity, then dissipate the energy at a later

time. Thus, a PCM heat sink can save a substantial amount of mass and volume because the balance of the system

can be designed to typical rather than worst-case conditions. This approach can reduce the size of radiators, or the

volume of expendables for a sublimator or evaporator1,2

.

Despite the system level mass savings that PCM heat sink technology offers to spacecraft, there are

shortcomings that resulted in significant risks for inclusion in vehicles that have pumped fluid thermal control

system. These shortcomings stem from density changes that occur during the phase change. For example, when a

sealed container of wax is solidified from one surface, the void space that occurs due to contraction is located at the

opposite end of the container from the cold surface. When the cold surface is then heated again by the coolant, the

wax begins to melt from that point, but is trapped by the rest of the solid wax. If the solid wax prevents the melting

wax from expanding, locally high pressures and structural failure occure.

A new PCM heat sink was built and incorporated into an ISS flight experiment to prove out thermal performance

and a novel phase management technique that eliminates hydraulically locked wax3. The experiment simulates the

pumped coolant loop of a crewed spacecraft. Three test programs were carried out on the PCM heat sink and the

integrated experiment. The first test program was conducted on the PCM heat sink on its own using laboratory

equipment to simulate a thermal control system. This allowed the unit to be tested in a wide range or orientations

with respect to gravity to evaluate the effectiveness of the phase management features. These tests also imparted the

highest thermal loads on the unit, because the laboratory equipment was not limited by the power available on the

ISS. The second test program was carried out after the PCM heat sink was integrated into the EXpedite the

PRocessing of Experiments to the Space Station (EXPRESS) rack, but before it was launched. These tests helped to

certify the experiment for launch and integration into the ISS, and also created a baseline set of performance data on

the ground to compare with microgravity results. The final test program was carried out on the ISS. Control of the

experiment was handled remotely on the ground, where the test operator had access to real-time telemetry. This

paper details the results of the three PCM flight experiment test programs.

II. PCM Heat Sink Test Article

The PCM heat sink test article is a type of vacuum brazed aluminum heat exchanger. The first fluid is

pentadecane paraffin wax, which is contained in eight sealed layers of the heat exchanger. Pentadecane has a

melting point of 10 °C and a solid-solid phase change at -2 °C, with a total latent heat capacity of 205 kJ/kg. The

second fluid is a 50/50 mixture of propylene glycol and water (PGW). PGW is the coolant used for the internal TCS

on the Orion crew vehicle. It remains liquid below -8 °C, which allows it to chill the pentadecane below the solid-

solid phase change temperature. Table 1 shows the size and capacity of the heat sink. Figure 1 shows the PCM heat

sink before it was filled with wax. Further details of the design, its sizing, and its manufacture can be found in

reference 3.

Table 1. PCM Heat Sizing

Envelope: L x W x H (m) 0.363 x 0.345 x 0.193

Coolant 50/50 Propylene glycol and water (PGW)

Nominal Coolant Flow Rate 0.0118 kg/s

Wax Melt Point 10°C (50.0 °F)

Heat Storage Rate 200 to 979 Watts

PCM Mass 3.9 kg (8.7 lb)

Latent Heat Capacity 636 kJ at 10 °C (50 °F) plus 170 kJ at -2.25 °C (28 °F)

Max. Coolant Outlet Temperature at end of melt 11.2°C (52.2 °F)

Maximum Coolant Pressure 827 kPa (120 psig)

Allowable Coolant Pressure Drop 34.5 kPa at 0.66 L/min and -10°C

(5.0 psid at 93.6 lb/hr and 14 °F)

Leakage < 1 x 10-5

scc/s Helium at maximum pressure

Melt Cycle Life 17,390 freeze/thaw cycles

Assembled Mass 15.7 kg (34.6 lb)

P


3

III. Laboratory Setup and Procedures

Performance testing of the heat sink in the

laboratory had several objectives, including

demonstration of heat storage and regeneration

capabilities at both the ISS experiment design

conditions and the higher power conditions

scaled from the Orion vehicle. The higher

power test points could not be done after the

heat sink was integrated into the EXPRESS

rack due to electrical power limits for the

experiment. PGW pressure drop was tested in

the laboratory setup as well. The heat sink was

also tested in four different orientations with

respect to gravity to test the effectiveness of the

phase management features. Varying the

orientation of the heat sink was not practical

once it was integrated into the EXPRESS rack.

A. Test Setup

The PCM heat sink schematic for

laboratory testing is shown in Figure 2 below.

The setup was configured to allow the PCM heat sink to be tested with a constant heat input via the coolant heater

and to be re-frozen using the constant temperature bath. Pressure within the wax cavities depends largely on the

gravity field, so testing was conducted with the heat sink in four different orientations, shown in Figure 3.

The heat sink was instrumented with four pressure transducers that sensed the pressure within the wax cavities

located next to the two PGW flow layers. Several surface thermcouples were also fixed to the heat sink to help

determine where the wax melt front was located over time, and what kinds of thermal gradients were set up in the

item. Figures 4 and 5 show the locations of the surface mounted thermocouples and pressure transducers.

N2

NC

NO

V4

V5

NONC

C.T. Bath

V1

V3

Pump

T4

Variac

FM

T1T2

T5-T11

In-Line Heater

PCM

PDP

Tank

Ball

valves

OT

Controller

V2

V6

V7

N2

TC

P1-P4

To

DAC

S100 psig

T3

T12-

T15 Figure 2. Laboratory PCM Test Schematic.

Figure 1. Brazed and Welded PCM Heat Sink.


4

B. Laboratory Procedures

Testing in the laboratory was done according to the procedures listed below. Due to limitations of the laboratory

chiller setup, the minimum temperature it could reach was 0 °C. Therefore, the laboratory tests did not use the solid-

solid phase transition of pentadecane.

1. Start coolant pump and chiller

2. Direct coolant through chiller and PCM to freeze the wax

– Check that all TC’s drop below 2 °C (35 °F)

3. Turn on heater to desired power

4. Bypass the chiller

5. End test when unit is fully melted or meets any of the conditions below

– All TC’s above 18 °C (65 °F) or

– Any pressure transducer > 7 psig or

– Any TC > 50 °C (122 °F)

Figure 5. Location of pressure transducers and

thermocouples on the heat sink for laboratory

testing

Figure 4. Location of thermocouples on the

top of the heat sink (layer 1)

g

Hot Coolant in

Vertical, PGW ports

on top

Vertical, PGW ports

on Bottom

Hot Coolant in

On Edge

Hot Coolant in

Figure 3. Heat sink orientations in laboaratory testing

“Flow up” “Flow down” “On Edge”

“Flat”


5

IV. Double Locker and EXPRESS Rack Test Setups

Performance testing of the PCM heat sink in the Double Locker on the ground and within the EXPRESS Rack

on orbit have the same objectives as those of the laboratory tests, including demonstration of heat storage and

regeneration capabilities at both the ISS experiment design conditions and at moderately higher power conditions.

Both ground and on orbit testing provide performance data for the PCM Heat Sink in a progressively higher fidelity

environment.

A. EXPRESS Rack

The EXPRESS Rack is a standardized payload rack system that houses and supports experiments aboard the ISS.

The philosophy behind the EXPRESS Rack is to enable fast and efficient integration of multiple payloads by

standardizing hardware interfaces, providing commonly required resources and streamlining the certification

approach. EXPRESS Racks are mounted in an International Standard Payload Rack (ISPR) and remain on orbit

while payloads are exchanged in and out as required. Each rack is configurable into “lockers” of varying sizes.

Payloads housed in the EXPRESS Rack utilize resources provided by the EXPRESS Rack including electrical

power, low temperature cooling water, moderate temperature cooling water, vacuum, cabin air, and numerous

communications infrastructure. Payloads may be controlled by the ISS crew, remotely from the ground by the

Payload Rack Officer at Marshall Space Flight Center (MSFC), or by the Payload Developer via TREK system

connected to MSFC. This experiment was controlled by the Payload Developer via TREK.

B. Double Locker Ground Setup and Testing

For Double Locker ground testing, the PCM Heat Sink is hard mounted on a tray with four Titanium mounting

features, designed to minimize heat conduction to and from the Heat Sink. The removable tray is mounted on a

slide mechanism on the top portion of the double locker. Thermocouples and pressure transducers on the PCM Heat

Sink are connected to the data acquisition and controller built into the payload via a data cable jumper. The PCM

Heat Sink together with the double locker comprises the Phase Change HX Payload (or simply payload). The Phase

Change HX payload includes three distinct and fluidically isolated loops: Propylene Glycol Water (PGW) loop, Low

Temperature water Loop (LTL), and Moderate Temperature water Loop (MTL). Cool Internal Thermal Control

System (ITCS) water from the MTL flows into the payload through the MTL supply hose to cool the payload’s

avionics via a four pass pressed in stainless steel tube cold plate and exits the payload through the MTL return hose.

Cold ITCS from the LTL flows into the payload through the LTL supply hose through a custom made stainless steel

cold plate and exits the payload through the LTL return hose. During the freezing cycle the LTL fluid provides the

heat sink needed to reject energy from the Thermal Electric Cooler Heat Exchanger (TECHX) which provides lift

for the PGW loop. The PGW loop provides the necessary cooling to freeze the wax within the PCM Heat Sink.

Figure 6. Discreet fluid loops within Double Locker


6

The pressure systems (PGW Loop, LTL and MTL) and the electrical system are mounted within the double

locker and are thermally insulated. Fluid, power, and data connections are made via appropriate connectors

mounted on the front panel of the double locker.

For ground double locker tests the Phase Change HX Payload is not installed in an EXPRESS Rack. Power,

LTL and MTL are provided by Ground Support Equipment (GSE) power supplies and chiller carts via GSE

jumpers. Telemetry is routed to a GSE computer via an Ethernet cable. All ground thermal tests were performed

with closed cell white foam insulation shrouding the payload and additional open cell beige insulation overlay, with

the exception of the front panel, to simulate the expected low convective heat transfer on-orbit condition.

In addition to thermal tests, ground testing consisted of a suite of environmental verification and acceptance

testing as required by the ISS Program for all payloads. These tests include: leak and proof pressure testing,

Acoustics, Vibration, Electromagnetic Interference (EMI) and Electromagnetic Compatibility (EMC), Power

Quality, and Payload rack checkout unit (PRCU) tests.

For leak and proof pressure testing, each component was verified at its Maximum Design Pressure (MDP) for

leak, proof and burst pressures. Once assembled, the payload was verified for leak at 1.0X MDP (95 psia for

the PGW Loop and 121 psia for the MTL/LTL). This was done via three successive procedures. A vacuum leak

check with sprayed helium on the PGW, MTL, and LTL loops was followed by a Helium pressure decay on the

MTL and LTL systems, and finally a pressure decay test on the PGW loop. These procedures were completed

on December 2, 2015 in the Advanced Thermal Technology Integration Center (ATTIC) at JSC.

Acoustics evaluations were performed in the Acoustics Noise Control Lab anechoic chamber at JSC on January

7, 2016. Measured noise levels for continuous pump operation were compared to NC-32 and NC-34 full octave

band limits specified in SSP 52000 Rev N and SSP 57000 Rev R respectively. Intermittent levels for period

cycling of the three-way valve were also measured and compared to levels allowed in SSP 52000 and SSP

57000.

Figure 8. Insulated Double Locker for

ground testing

Figure 7. Connections on Front Panel of Payload Double

Locker.

PGW Loop

Connectors (4)

Electrical

Data Cable

Captive

Fasteners

Phase Change

HX Loop Manual

Valve

(Connected to ISS LTL

Water Loop)

MTL

Inlet/Outlet

Connections

Circuit

Breaker

LED Status

Indicator

LTL Inlet/Outlet Connections


7

Figure 9. Acoustics evaluation set up in anechoic chamber

Protoflight Random Vibration Acceptance test was conducted in the Static and Dynamic Test Facility, General

Vibration Lab at JSC on January 15, 2016. The payload was configured in launch configuration with vibration

dampening foam, packed inside an M1 CTB and strapped to the vibration table. This setup simulates the soft

stowed configuration in the launch vehicle. Random Vibration tests were performed for 60 seconds duration

with frequency spectrum from 20 – 2000 Hertz, with varying power spectral densities (g2/Hz). The composite

vibration load was 3.27 Grms, on all three axes, which corresponds to loads expected from the commercial

cargo vehicles4.

Figure 10. Random Vibration Acceptance Test setup

EMI/EMC tests were conducted in the EMI Test Facility at JSC between January 25 and February 2,

2016. The payload was tested in three operating modes: standby, freezing and thawing. EMI/EMC

tests conducted includes:

Radiated Emissions RE02

Conducted Emissions CE01, CE03, CE07

AC and DC Magnetic Fields

Radiated Susceptibility RS02, RS03

Conducted Susceptibility CS01, CS02, CS06

Right Surface

Microphone

Top Surface

Microphone

Front

Surface

Microphone

Back

Surface

Microphone


8

Power Quality test was conducted in the Electrical Power Systems Test Facility on February 9 - 10, 2016.

Test results of the Phase Change Heat Exchanger Power Quality Test were compared to ISS User

Electric Power Specifications and Standards Document SSP 52000-IDD: Express Rack Payloads IDD

and data used to verify payload’s compatibility with the ISS Electrical Power System.

PRCU Tests were conducted at JSC while remotely connected to the PRCU test rack at Marshall Space

Flight Center between March 4-10, 2016 and comprised of three tests: End-to-End data interface with

EXPRESS Rack, Commanding and Telemetry interface with EXPRESS Rack, and data monitoring for

fire detection. Additionally a packet collision test was completed on March 16, 2016.

C. EXPRESS Rack On-Orbit Test Setup

For on-orbit EXPRESS Rack testing, the aforementioned Phase Change HX Payload (Double Locker

configuration) is installed inside EXPRESS Rack 8 (ER8) in the US Lab on the International Space Station.

Jumpers connect the payload to either the upper or lower connector panel on ER8 and to the LTL connectors on

the Z-Panel below the EXPRESS Rack. The PCM Heat Sink is located in the top half of the double locker,

behind its own front panel and is connected to the PGW loop via PGW fluid jumpers with Quick Disconnect

(QDs) external to the payload. This design allows on-orbit removal and replacement of the PCM Heat Sink.

Figure 11. Double Locker Payload installed inside EXPRESS Rack

Tests are remotely initiated by using a Labview shell that sends commands to the EXRESS Rack via the

Telescience Resource Kit (TReK). TReK is a suite of software applications and libraries used to monitor and

control payloads in space. TReK provides access to remote services provided by the ISS Payload Operations

Integration Center (POIC). TReK provides support to receive, process, record and display payload telemetry.

TReK also provides command capabilities such as creating, modifying, storing, and uplinking commands.

Command capabilities also include monitoring, recording, and tracking command activities.

V. Thermal Performance Results

Thermal performance of the PCM heat sink was tested in three configurations; in the laboratory prior to

integration with the EXPRESS Rack; during checkout testing at NASA Johnson Space Center while in the Double

Locker configuration; on orbit on the ISS. The thermal response of the heat sink is not dependent upon gravity, but

is affected by the surrounding thermal environment, PGW flow rate, PGW heater power, and heat sink starting

temperature. Overall results show that the PCM heat sink exceeded its thermal storage requirements, but that the

inlet and outlet header configuration was likely creating a thermal short-circuit. The thermal short-circuit made the

actual test results deviate from the simplified 2-D model predictions.


9

A. Laboratory Thermal Results

Thermal performance results are summarized in Table 2. The PCM heat sink met the melt performance

specifications at the design flow rate and powers of 0.82 L/min / 979 W (Tests 9A-9C) and 0.82 L/min / 400 W

(Test 7) by storing over 710 kJ of energy before the coolant outlet temperature exceeded 11.2 °C (52 °F). For

comparison, the latent heat of the pentadecane was 636 kJ at the primary phase change temperature of 10 °C (50 °F).

In each of these tests, the heat sink was conditioned to 2 °C (35 °F) at the start of the test. Stored energy was

calculated based on the heat removed from the PGW, beginning when the coolant inlet temperature reached 10 °C

(50 °F) and ending when the coolant outlet temperature reached 11.2 °C (52 °F).

Table 2. Heat storage during melt, with coolant outlet less than 11.2 °C (52 °F)

Test

Flow

Rate

(L/min)

Flow

Rate

(lb/hr)

Power

(W)

Stored Energy (kJ)

Lab Results

Integrated

Ground

Results ISS Results

1 1.06 150 200 692 666 703

2 1.06 150 300 720 Not tested 738

3 1.06 150 400 747 Not tested 763

4 1.06 150 450 Not tested Not tested Not Tested

5 0.82 116 200 690 Not tested 729

6 0.82 116 300 712 717 753

7 0.82 116 400 826 Not tested Tests were

ongoing as

of the

writing of

this paper

8 0.82 116 450 813 Not tested

9A 0.82 116 979 785 Not tested

9B 0.82 116 979 858

9C 0.82 116 979 851

10 0.35 50 200 571 Not tested

11A 0.35 50 790 592 Not tested

11B 0.35 50 790 569

11C 0.35 50 790 643

11D 0.35 50 790 613

11E 0.35 50 790 611

11F 0.35 50 790 611

The amount of heat stored while maintaining temperature control was assessed against coolant flow rate. Test

data showed a very slight trend of increased heat storage with increased flow rate for the lower power runs (200 W

and 300 W). That trend reversed when heater power was increase to 400 Watts. The 2-D model predicted that

increases in flow rate should result in less heat storage at all heater powers. Figure 12 shows the test data plotted

with the model predictions. At 400 Watts, the figure shows that the model and test data trends match, with

increasing coolant flow reducing the amount of thermal energy stored before the outlet temperature rises. This is

explained by the fact that higher flow rates and higher velocities bring the warm coolant downstream and out of the

heat sink before the wax can draw as much of the heat out. Lower flow rates should give the fluid a greater residence

time inside the flow passages where it can give up heat. The fluid moves slow enough in the fins to be laminar, and

thus have a constant Nusselt number within the flow rates that were tested. It is possible that thermal “short

circuiting” was taking place between the inlet and outlet headers of the heat sink. Short circuiting would occur

between the headers due to conduction through the aluminum closure bars. This would explain why lower power

tests, which were run for a longer period of time, stored less energy than higher power tests. Thermal energy would

be conducted from the inlet to the outlet for the entire melt period, and bring the outlet temperature higher than if the


10

headers were on opposite ends of the heat sink. It would also explain the overall lower heat storage results

compared with the model, which can be seen in all of the tests.

Heat storage was also assessed as a function of power added to the coolant. Both the test results and the model

predictions showed that higher power tests resulted in more energy storage. This was expected because higher power

tests created hotter coolant inlet temperatures and a greater build-up of stored sensible heat. Put another way, the

hotter coolant warms up the inlet of the heat sink to higher temperatures. Figure 13 shows the trend of increased

total heat storage with increased coolant power. It also shows that the model predictions for heat storage are higher

than the test data.

The model was also run using the recorded inlet temperature from test 9A. Figure 14 shows how the measured

coolant outlet temperature rises faster than predicted. Again, this could be due to the thermal short circuiting

Figure 13. Stored energy as a function of heater power during laboratory testing

Figure 12. Stored energy as a function of coolant flow rate during laboratory testing


11

between the coolant headers. Figure 15 shows that the total stored energy within the PCM heat sink is well-predicted

by the 2D model. This is expected because there are few other thermal masses in the test setup, and losses to the

room are minimal.

B. EXPRESS Rack Results

Thermal results from the ground double locker tests and the ISS tests are consistent with the laboratory tests, as

shown in Table 2. The data show the same trends of increased heat storage with increased heater power. Of note, is

that the ISS tests had about 5% more heat storage than the JSC ground tests. This can be attributed to differences in

insulation and losses to the surroundings.

Figure 15. Total heat stored in the PCM heat sink is well-predicted by the model

Figure 14. Test data shows the coolant outlet temperature increasing earlier than the model predicts


12

Heat storage shown in Table 2 is the amount of energy stored in the heat sink starting from when the coolant inlet

temperature reached the melting temperature of 10 °C (50 °F), and ending when the coolant outlet temperature

reached 11.2 °C (52 °F). Unlike the laboratory tests, the integrated ground tests and the ISS tests started with the

heat sink at -8 °C (18 °F) in order to test the PCM heat sink’s response with the second phase transition that occurs

at -2.2 °C (28 °F). Figure 16 shows the coolant inlet and outlet temperatures from one of the JSC ground tests. The

first and second phase transitions are marked in the figure. The difference in latent heat of the two transitions results

in a significantly shorter plateau period during the first transition. After the first phase transition is complete, the

whole heat sink begins to warm up again. By the time the coolant inlet reaches 10 °C, it appears that the effect of the

first phase transition has dissipated. This would explain why the laboratory tests, which started at 2 °C (35 °F) had

similar heat storage results to the EXPRESS rack tests.

In Test Point 1 conducted on the ground with the double locker, the total stored energy in the PCM heat sink was

also calculated from the time the coolant inlet reached the -2.2 °C phase transition until the outlet reached 11.2 °C.

During that time, it stored 1107 kJ of thermal energy, or 440 kJ more than when looking at just the primary phase

transition (667 kJ seen in Table 1). Considering that the latent heat of the second phase transition is just 170 kJ, the

heat sink stored 270 kJ in sensible heat as it warmed up from the isothermal temperature of -8 °C to the temperature

distribution present when the coolant inlet reached 10 °C (the red vertical line in Figure 16).

VI. PCM Cavity Pressure Results

Phase change material heat sinks for spacecraft must be designed to accommodate the large volume change that

occurs at the melt temperature of the material. Pentadecane wax expands upon melting and can create thousands of

pounds of pressure if it is hydraulically locked. A new phase management design was used for the ISS experiment to

prevent the wax from becoming hydraulically locked during the melt phase of the heat sink. The design was based

on a laboratory prototype that underwent over 100 freeze and thaw cycles in all orientations with respect to gravity

without developing significant pressure within the sealed wax cavities. However, changes were made to the design

to allow for tighter packaging, and greater strength to withstand launch loads. Pressure results of ground based

laboratory tests and the ISS microgravity tests are discussed below.

Figure 16. Temperature response of double locker configuration showing the first and second phase

transitions


13

A. Laboratory Pressure Results

The laboratory tests of the PCM heat sink were conducted before the unit was integrated into the EXPRESS

rack. Table 3 shows the results of those wax cavity pressure tests, which varied coolant flow rate, coolant heater

power, and orientation with respect to gravity. These results show that there were combinations of heat sink

orientation, flow rate and coolant power that resulted in melt-induced pressure spikes in the heat sink. The results

fall into three categories. The first type of result had a small pressure rise during melting and is indicated by the

white rows of data. The second type of result had a fairly steep rise in wax cavity pressure during the melt phase, but

not at a rate that would force an immediate shut down (orange rows). The third type of result was a rapid pressure

spike that occurred during the melt phase, forcing a shut down (red rows).

Table 3. Wax cavity pressure responses

Melt Orientation

Flow Rate

(L/min)

Power

(Watts) Pressure Response During Melt

Flow Down 1.06 300 Gradual, low rise







Flow Down 0.35 400 Sharp Spike, Test Stopped



Flow Down 0.35 790 Steep rise, Test allowed to finish



Flow Up 0.35 790 No spikes



On Edge 1.06 790 No spikes

On Edge 0.82 200 Steep rise, Test allowed to finish


On Edge 0.82 450 Sharp Spike, Test Stopped

On Edge 93 790 Steep rise, Test allowed to finish





Flat 1.06 200 Gradual, low rise

Flat 0.82 400 Steep rise, Test allowed to finish

Flat 0.35 300 Sharp Spike, Test Stopped

Flat 0.35 400 Sharp Spike, Test Stopped




14

The most benign orientations with respect to gravity proved to be the “Flat” and “Flow Up” orientations, as

referenced in Figure 3. The “On Edge” orientation proved problematic with all but the lowest power and highest

flow rate conditions. The “Flow Down” orientation was expected to be free from pressure spikes in all cases, but

several tests at the lowest flow rate of 0.35 L/min resulted in relatively high pressures, some of which forced shut-

downs. Figure 17 shows wax cavity pressures measured during similar melt cycles with the PCM heat sink in three

different orientations. The “Flat” orientation produced no melt-induced pressure spike, but the “Flow Down” and

“On Edge” orientations caused significant pressures within the wax cavity. The “On Edge” test was shut down due

to the steep increase in wax cavity pressure. The “Flow Down” test showed how the the pressure within the wax

cavity gradually decreased during the melt phase after a quick initial rise.

Flat: When the heat sink is lying flat, the ullage space caused by contraction during freezing of the wax is

distributed at the top of each PCM layer. Melting wax can easily expand into that space along the top parting sheet

or end sheet. The phase management feature had little effect on how the wax expands into the ullage space, and

prior testing of prototypes without those features showed no pressure problems with the flat orientation. Testing of

this heat sink was mostly consistent with those prior observations. The exception was the test conducted with 0.35

L/min flow and 300 Watts into the coolant, which had a pressure spike ten minutes into the run, and was shut down.

Flow Up: The orientation where the heat sink is stood on its end, with the inlet and outlet headers on the bottom

was one that showed no pressure spikes. The pressure transducers were located at the top of the heat sink, where it

was expected that the ullage space would be located. When the melt front reached the pressure transducers it is

likely that it also reached the ullage space so that the transducers never detected a pressure rise.

Flow Down: Most prior PCM heat sinks experienced the highest pressure spikes in this orientation, which has

the coolant flowing upwards in the primary coolant passages. The phase management features used in the previous

laboratory prototype eliminated those pressure spikes. Testing of the flight experiment showed that the changes

made to the pressure management features were a detriment to their ability to fully eliminate the melt-induced

pressure spikes. In particular, low flow rates and high power resulted in rapid pressure spikes that became more

severe with repeated tests. Tests conducted with a higher flow rate showed some pressure rise and some trend

toward increased pressure rises with repeated tests, but none that shut down the test before the unit had finished

melting. This indicated that the phase management features were working in those conditions.

Figure 17. Wax cavity pressure measurements showing different results for different orientations with

respect to gravity


15

On Edge: The flight experiment was tested lying on its edge, with the expectation that there would be no melt-

induced pressure spikes. However, the pressure transducers connected to the lower edge of the PCM layers showed

pressure spikes in several tests. The pressure was higher when the unit was tested with lower coolant flow rates.

Tests at the higher flow rates of 0.82-1.06 L/min, were allowed to run to completion, and showed a trend of higher

melt pressure with higher coolant power. These results also indicate that the changes made to the phase management

features from the earlier prototype were not beneficial.

B. ISS Pressure Results

PCM heat sink pressure results from the ISS testing in mircrogravity have been assessed for test points 1-3 and

5-7 shown in Table 4 below. The plan started with coolant flow rates and powers that were shown to produce the

lowest wax cavity pressures on the ground. Progressively higher powers and lower flow rates were tested to

challenge the PCM heat sink’s ability to maintain low wax melt pressures. This test program set out to answer

several questions, including:

How does the wax pressure respond in microgravity compared with different orientations on the ground?

What coolant powers and flow rates might cause melt-induced pressure spikes?

Do repeated freeze and thaw cycles conducted at the same conditions result in different pressure responses?

Table 4. Pressure response of the wax cavities in microgravity

On-Orbit

Test Point

Flowrate

(L/min)

Power

(Watt)

Pressure Response During

Melt

Pressure Response Cycle to Cycle

1 1.06 200 Small spike during 1st melt

transition. Small gradual

increase during 2nd

melt

transition.

Similar sized increases from cycle to cycle

with absolute increase caused by

increasing amount of dissolved gas coming

out of solution



increase during 2nd

melt

transition.




out of solution



decrease during 2nd

melt

transition.




out of solution; noticeable during

temperature increase between the two melt

transitions.


transition. Larger spike during

first half of the 2nd

melt

transition, followed by large

decrease during the second half

of the 2nd

melt transition.




out of solution; noticeable during

temperature increase between the two melt

transitions.




melt

transition, followed by small


of the 2nd

melt transition.

Test system communications issues did not

allow for completion of more than two

cycles. Rerun required.




melt

transition, followed by small


of the 2nd

melt transition.


during the 1st melt transition but with

larger increase at the start of the 2nd

melt

transition.

9 0.35 200 Not yet completed

10 0.35 300 Not yet completed


16

Testing began with Test Point 1 in the table, with coolant flowing at 1.06 L/min and 200 Watts in the heater.

Pressure and temperature results are shown in Figure 18 below. They revealed that the pressure during melting

stayed low, but that the pressure after the melt was complete continued to rise. Each cycle resulted in higher

pressures within the cavities when the experiment reached the same automatic cycle temperature (line 1 in the

figure). In addition, the wax pressure during melt cycles 1, 3, 4 and 6 reached a peak after the unit appeared

completely melted, then decreased again even while the heat sink’s temperature continued to slowly approach 30 °C

(86 °F) (circle 2 in the figure). These data also show that the heat sink’s wax pressures slowly decreased to below

atmospheric pressure during a long shutoff period between cycles 5 and 6, while the temperatures remained constant

(line 3 in the figure). These pressure responses had not been observed during the laboratory tests. However, they can

be attributed to air coming out of solution from the wax, then slowly re-desolving in between cycles. Testing of an

analogous setup on the ground repeated the phenomena. While the wax loaded into the PCM heat sink had

undergone a de-gasing process, the ground tests showed that a longer and more intensive degassing protocol was

necessary to drive most of the air out of solution. The remainder of the ISS tests were assessed knowing that

varying amounts of air were present in the ullage spaces withing the wax cavities Rest periods between cycles were

also added to allow the air to redesolve, which helped to prevent the cavities from reaching their pressure shutoff

point.

Figure 19 shows pressure traces from one of the wax cavities during two cycles at Test Point 1 and two cycles at

Test Point 6. The results show that there is only a small pressure response during the first, solid-solid phase

transition. They also show that the wax pressure rises quickly during the initial melt phase. In Test 1, with 1.06

L/min flow rate and 200 Watts, the initial pressure jump during melting is low. Pressure continues to rise after the

initial jump due to thermal expansion of the wax and of the air trapped in the cavity. In Test 6, with 0.82 L/min flow

rate and 300 Watts, the initial jump in pressure is steeper and higher. For the first cycle under these conditions, the

pressure reached 13 psia before gradually dropping back down to 10 psia as wax continued to melt and more of the

Figure 18. Data from eight freeze/thaw cycles at 1.06 L/min coolant flow and 200 W heater power, a)

pressure measurements and b) temperature measurements.


17

void pockets became exposed to the melt front. Pressure rose again due to thermal expansion of the liquid wax and

air pockets.

The second cycle of test point 6 resulted in a shut down due to the pressure measured during the melt phase. This is

similar to what was seen in Figure 17 for the “On Edge” test. Two phenomena could be responsible for the cycle-to-

cycle difference in pressure response seen at the beginning of the melt phase. The first issue is the dissolved gas

coming out of solution during each cycle. The second issue might be migration of wax to different locations within

the cavities, as it’s drawn to the coldest surfaces due to surface tension effects. It was expected that the wax would

reset itself to fill the entire heat sink once it was 100% melted though, since the void space is only about 2% of the

total volume while the heat sink is at the cycle transition temperature.

Overall, the pressure response of the PCM heat sink in microgravity most closely resembles the response seen on

the ground in the “Flat” orientation. There are cycle to cycle differences in the response, with a trend towards higher

pressures until a “rest” period occurs. Much of the trend can be explained by the presense of air in the cavity that

evolved from the loaded wax. The results don’t indicate whether the melt-induced pressure spikes, like the one that

occurred during the second cycle of Test Point 6, are caused by dissolved gas, or some amount of hydraulically

locked wax not addressed by the phase management features of the heat sink.

VII. Conclusion

The PCM heat sink flight experiment was successfully launched, installed, and operated on the ISS. It met its

thermal performance goals, providing sizing data for flight programs. The phase management features of the

experiment were meant to eliminate pressure spikes from expansion of melting wax. However, the ground test data

and ISS microgravity data show that the features, as implemented for the experiment, are only partially effective.

Higher flow rates and lower coolant temperatures during the melt phase result in lower pressures within the wax

cavities, both on the ground and on the ISS. Higher power and lower flow rate conditions tended to result in melt-

induced pressure spikes that forced some test points to shut down. ISS testing also revealed that a significant amount

of dissolved air is present in the wax. The dissolved gas affected the pressure response from cycle to cycle and could

be affecting the pressure response that occurs at the beginning of each melt cycle.

Lessons learned from the ISS experiment are being applied to the next iteration of vehicle level PCM heat sinks.

These lessons include the need for a more thorough degass protocol, and for improvements to the phase

management features.

Figure 19. Pressure response of one wax cavity for two cycles of test points 1 and 6.


18

Acknowledgments

Funding for this work was provided by the NASA Space Technology Mission Directorate. The authors want to

thank the ISS program for their guidance and help in making the experiment a success, Space X for launching the

experiment to the International Space Station, and the crew of the ISS for installing the experiment into the orbiting

laboratory.

References 1 Quinn, G., Hodgson, E., and Stephan, R., “Phase Change Material Trade Study: a Comparison between Wax and Water for

Manned Spacecraft,” 41st International Conference on Environmental Systems, AIAA 2011-5229, Portland, OR, July 2011.

2 Ungar, E., Navarro, M., Hansen, S., and Sheth, R., “Water Phase Change Heat Exchanger System Level Analysis for Low

Lunar Orbit,” 46tht International Conference on Environmental Systems, ICES-2016-359, Vienna, Austria, July 2016.

3 Quinn, G., Stieber, J., Sheth, R., and Ahlstrom, T., “Phase Change Material Heat Sink for an International Space Station

Flight Experiment,” 45th International Conference on Environmental Systems, ICES-2015-167, Wellevue, WA, July 2015.

4 SSP 50835D, “ISS Pressurized Volume Hardware Common Interface Rquirements Document,” Revision D, Type 4, NASA

Johnson Space Center, April 2013.

Phase Change Material Heat Sink Flight Experiment Results

Documents

Transcript of Phase Change Material Heat Sink Flight Experiment Results