Proceedings of the 55th International Convention of Society of Wood Science and Technology

August 27-31, 2012 - Beijing, CHINA

Paper SP-3 1 of 5

A Novel Energy Saving Wood Product

with Phase Change Materials

Fang Chen 1, Amanda Kessel

2, Michael Wolcott

3*

1 PhD student, Composite materials and engineering center (CMEC), Washington

State University, Pullman, WA, 99163, USA. 2 Summer REU student in CMEC, Washington State University (2011).

3 Professor and Research Director, Composite materials and engineering center

(CMEC), Washington State University, Pullman, WA, 99163, USA.

* Corresponding author

wolcott@wsu.edu

Abstract

Phase change materials (PCM) are utilized for thermal energy storage. Structure and properties

of PCM have been studied extensively in the past because they are highly correlated to materials’

energy saving ability and application. However, with the quickly growing requirement of PCM

application in buildings, to reduce our reliance on air conditioning and heating through solar

energy storage, the problem of PCM leakage must be solved first. This study is focusing on

verification of the feasibility of manufacturing a novel energy saving wood product with PCM,

using wood as a low cost porous structure material for PCM instead of the porous materials that

are widely studied. Scanning electron microscope (SEM) imaging was carried on to investigate

the possibility of paraffin to get into lumen of wood chips. The ability of wood flour to absorb

paraffin is calculated. PCM and wood flours were compounded utilizing a parallel co-rotating

twin screw extruder. Melting and crystallization behavior of the blends were investigated by

differential scanning calorimeter (DSC). Based on the hypothesis that wood encapsulation and

polymer sealing can help to restrict leakage of PCM, a novel energy saving wood product with

PCM by using wood lumen as containers of PCM is possible to be made.

Keywords: Phase change materials, wood product, energy saving, composites, paraffin

Proceedings of the 55th International Convention of Society of Wood Science and Technology

August 27-31, 2012 - Beijing, CHINA

Paper SP-3 2 of 5

INTRODUCTION

Thermal energy can be stored in several ways, sensible heat, thermochemical, and latent heat.

Phase change materials possess high energy storage density and isothermal operating

characteristics making them very efficient materials for utilizing latent heat in application [1-3].

Many researchers are focusing on organic PCM, inorganic PCM, and eutectics[4] for passive

solar energy storage, all very promising areas for energy saving in buildings. Phase change

materials can be implemented in the walls of buildings. When absorbing energy instead of the

interior of buildings, the phase of PCM can be changed from solid to liquid. When the ambient

temperature drops, PCM begins to solidify, releasing stored thermal energy to the building and

stabilizing the interior temperature. The temperature of PCM will be maintained closer to the

desired temperature during each phase transition period for a long period of time until the phase

change is complete. In this way, PCM can be used to increase human comfort by decreasing

swings in internal air temperature[5]. Energy conservation is achieved through this reversible

and repeatable phase change.

The primary phase change of interest in the proposed work is the solid-liquid change in PCM.

Other types of phase change store and release latent heat energy during transition, but they are

impractical for most energy storage applications. In solid-liquid PCM, exudation occurs after the

PCM absorbs enough energy to complete the phase change to liquid, undermining its application

value. Controlling this leakage is the current major obstacle to the use of PCM in the thermal

envelopes of buildings. Encapsulation is widely applied to contain PCM to prevent leakage. But

this method comes with some issues, such as low flame retardancy and thermal conductivity with

the core-shell structure, low chemical and thermal stability, and high cost[6, 7]. To use porous

structure materials, such as expanded perlite (EP), diatom earth (DE), expanded graphite[8-10],

concrete[11], porous metallic foams[12] and so on, instead of conventional encapsulation

method is a promising way to capture PCM and protect it from leakage when melting.

For a mature cell in wood in which there is no protoplast, the open portion of the cell is known as

the lumen. Most wood cells in wood have two domains which are the cell wall and the cell

lumen[13]. The cell lumens are connected by pits by which cell walls are modified to allow

communication between the cells in the living plant[13]. The objective of this study is to verify

the feasibility of manufacturing a novel energy saving wood product with phase change

materials, using wood as a low cost porous structure material for PCM instead of other porous

materials which have already been widely studied.

MATERIALS AND METHODS

Materials. The materials used are paraffin (Technical Octadecane) from Roper Thermals,

Ponderosa pine flour from American wood fibers, and maple chips.

SEM. The maple chips are soaked in melt paraffin for 10 min followed by SEM imaging.

Proceedings of the 55th International Convention of Society of Wood Science and Technology

August 27-31, 2012 - Beijing, CHINA

Paper SP-3 3 of 5

Extrusion. Paraffin and WF (40, 60, 80 mesh) mixtures in 60/40, 70/30, and 80/20 (wt%) ratios

were prepared, respectively, and blended in a co-rotating twin screw extruder (Leistritz ZSE-

18HP) at 50oC.

DSC. The melting temperature and crystallization temperature of paraffin in composites were

determined using a deferential scanning calorimeter (DSC) in the range from 0 to 75oC. The

scanning rate was 10oC/min.

RESULTS AND DISCUSSION

Wood is a natural structural material with a honeycomb-like microstructure. Fig.1 shows a SEM

image of paraffin infiltration into maple lumen. The lumen size may influence paraffin

infiltration. If sealing a layer of plastic on the external surface of wood, we can achieve a good

encapsulation of PCM in wood composites.

Figure 1. Paraffin infiltration into maple lumen

The result above shows that it is possible to encapsulate PCM into wood. In addition, how much

PCM could be encapsulated? To estimate how much PCM could be penetrated into wood, the

density values in table 1 are used.

Species Density (g/cc)

Ponderosa Pine 0.35

Paraffin 0.65

Wood cell wall 1.5

Table 1 Density

Wood porosity (Ф) is calculated as follows.

77.0/1

/11

f

cellwall

Proceedings of the 55th International Convention of Society of Wood Science and Technology

August 27-31, 2012 - Beijing, CHINA

Paper SP-3 4 of 5

Assuming that paraffin fills wood lumen completely, we calculate the maximum paraffin weight

fraction in the mixture as follows.

59.0)1(

cellwallp

p

pW

In the equations above, ρ is density, f represents fiber, and p represents paraffin.

DSC results in figure 2 show that after blending with wood flour, paraffin maintains its phase

change and its energy saving ability is not removed. Besides, based on our study on miscibility

of polyethylene (PE)/Paraffin blends, after blending with PE, paraffin functions well, too.

Further, with the help of plastic, good encapsulation of PCM is expected to be achieved. Thus, it

is possible to make a novel energy saving wood product with phase change material (paraffin)

and polyethylene

Temperature (oC)

10 20 30 40

He

at

flo

w (

mW

)

-60

-40

-20

0

20

40

60

80

100

paraffin, heating

20wf, heating

30wf, heating

40wf, heating

20wf, cooling

30wf, cooling

40wf, cooling

paraffin, cooling

Figure 2. Influence of wood flour content on melting and crystallization of paraffin

CONCLUSIONS

It is possible to make a novel wood product with PCM by using wood lumen as containers of

PCM for efficient energy saving. The encapsulation of paraffin with wood flour and plastic

combined together needs to be studied. Composites formulation and wood lumen size need to be

considered as well.

References

Proceedings of the 55th International Convention of Society of Wood Science and Technology

August 27-31, 2012 - Beijing, CHINA

Paper SP-3 5 of 5

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Acknowledgments

I’m grateful to my professor Dr. Michael Wolcott’s advice, encouragement and patience. Useful

discussions with Dr. Bo Liu are greatly appreciated.