Compressed Expanded Natural 

Graphite (CENG) Processing for PCM 

Composites

Alex Bulk, Wale Odukomaiya, Ethan Simmons, and Jason Woods

The National Renewable Energy Laboratory

IMECE Paper 

2020-57439
doi: 10.1016/2013-04-031

http://large.stanford.edu/courses/2010/ph240/barile2/ IMECE Paper #2020-57432 2

Solar Energy Storage

Stanford University, 2010

Tung-Chai, Chi-Sun, 2013

Building Envelope Energy Storage

TES Batteries

Applications for Thermal Energy Storage
doi: 10.1016/2017-12-027 IMECE Paper #2020-57432 3

Compressed Expanded Natural Graphite (CENG)

Graphite foams

Benefits

• High thermal conductivity

• Low cost

• High porosity

• Low pore size (micro and 

nanometer-scale)

• High pore density

• Ability to be molded into many 

geometries

Compressed Expanded 

Natural Graphite

1. Graphite flakes are intercalated 

with acid

2. Thermal shock causes rapid 

expansion

3. Expanded graphite “worms” are 

compressed to form a CENG 

matrix

Mallow et al, 2018
IMECE Paper #2020-57432 4

CENG Processing:

Porosity: Well Characterized

Not Investigated:

1. Thermal Shock Temperature

2. Thermal Shock Exposure Time

*We will soak in Tetradecane (C14H30)
IMECE Paper #2020-57432 5

CENG Performance Parameters

Objective: Evaluate Effect of CENG Processing Parameters (Thermal 

Shock Temperature and Exposure) on CENG Performance Parameters

• Energy Density: Energy Stored per 

unit Volume – Improved by 

maximizing the % Volume of PCM

• Power Density: Rate of Energy 

Transfer per unit Volume –

Improved by maximizing the

Thermal Conductivity

Example Ragone Plot:

doi: 10.1021/5b01813

McCloskey, 2015
IMECE Paper #2020-57432 6

Saturation Experimental Procedure

Exposure 

Time (min)

Thermal shock temperatures (°C)

300 400 500 700

5 X X X X

30 X X X X

60 X X X X

120 X X X X

δδ =

mmCCCCCCCC

VVCCCCCCCC

φφ = 11 − δδ

ρρCCCC

VVff =

mmPP PP

ρρPP PP � VVCCCCCCCC

VVff,PP PP =

VVff

φφ

CENG Bulk Density:

CENG Porosity:

PCM Volume 

Fraction:

% PCM Pore 

Saturation:

1. Soak CENG in PCM

2. Remove at Time Interval

3. Dry and Weigh

4. Restart timer and soak to next interval: 

repeat

*Manufacturer recommends min shock temp @ 290 C
IMECE Paper #2020-57432 7

Thermal Conductivity Experimental Procedure

Porosity (%) Thermal shock temperatures (°C)

300 500 700

65% X X X

75% X X X

83% X X X

90% X X X

1. Shock mass of CENG required to achieve desired volume 

and porosity

2. Compress CENG puck to required thickness based on 

instrument resistance range

3. Use guarded heat flow meter to measure conductivity 
IMECE Paper #2020-57432 8

Thermal Shock Effect on PCM Saturation

95% Porous CENG -Increased shock 

temperature and exposure 

yield: 

• Greater rate of 

saturation

• Greater max saturation

-Shock exposure improves 

saturation, but has 

diminishing returns

*Cause of low shock 

temperature plateau starting at 

30 s – 1 min is unclear
IMECE Paper #2020-57432 9

Thermal Shock Effect on PCM Saturation

65% Porous CENG -Increased shock 

temperature and exposure 

yield: 

• Greater rate of 

saturation

• Greater max saturation

-Shock exposure improves 

saturation, but has 

diminishing returns

*Lower Saturation Rates at 

Lower Porosity
IMECE Paper #2020-57432 10

Thermal Shock Effect on 

Matrix Morphology

SEM Imaging of Expanded 

Graphite: 

Greater Shock Temperature and 

Exposure Time Yield:

• More pores per unit length

• Partial oxidation at 700 C 

associated with larger pores

• More accessible pores (pore walls 

broken)
IMECE Paper #2020-57432 11

Effect of Thermal Shock and Porosity on Thermal 

Conductivity

Thermal Conductivity vs. Porosity Measured at 20 °C:

• No significant effect of shock 

temperature

• Similar local maximum 

conductivity around 80- 83% 

porosity seen by Bonnisal et al.

• Higher pore density above 83% 

yields lowered conduction 

pathways

• Anisotropic layering under high 

compression at porosities below 

*Different Measurement Methods Between Studies 83% increases resistance

dois: 10.1016/S0017-9310(00)00309-4; 10.1016/S0008-

6223(01)00032-X; 10.1115/IPACK2017-74244
IMECE Paper #2020-57432 12

Summary

• Low thermal conductivity PCMs are impregnated in conductive CENG 

matrices to improve power density

• Past studies had only evaluated effect of CENG porosity and not thermal 

shock temperature or exposure time

• Greater shock temperature yields improved rate of PCM saturation and total 

saturation, as well as greater pore density

• Greater shock exposure improved saturation rates up to a day below 500 C

• Shock conditions did not affect conductivity but effect of porosity was 

consistent with literature
Thank you! Questions?

Alex Bulk, Buildings and Thermal Sciences Center, 

The National Renewable Energy Laboratory, 

abulk@nrel.gov, +1-303-384-6358

IMECE Paper 

2020-57432

NREL/PR-5500-78503