Proceedings of the 55th International Convention of Society of Wood Science and Technology
August 27-31, 2012 - Beijing, CHINA
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
[email protected]
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
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Proceedings of the 55th International Convention of Society of Wood Science and Technology
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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.
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Proceedings of the 55th International Convention of Society of Wood Science and Technology
August 27-31, 2012 - Beijing, CHINA
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 ZSE18HP) 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
Ponderosa Pine
Paraffin
Wood cell wall
Density (g/cc)
0.35
0.65
1.5
Table 1 Density
Wood porosity (Ф) is calculated as follows.
  1
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1 /  cellwall
 0.77
1/  f
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Proceedings of the 55th International Convention of Society of Wood Science and Technology
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Assuming that paraffin fills wood lumen completely, we calculate the maximum paraffin weight
fraction in the mixture as follows.
Wp 
 p
 0.59
 p  cellwall (1   )
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
100
paraffin, heating
20wf, heating
30wf, heating
40wf, heating
20wf, cooling
30wf, cooling
40wf, cooling
paraffin, cooling
80
Heat flow (mW)
60
40
20
0
-20
-40
-60
10
20
30
40
o
Temperature ( C)
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
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August 27-31, 2012 - Beijing, CHINA
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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.
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