Evaluating effective thermal conductivity of PCM-impregnated wood composite at phase change temperature

Journal of Energy Storage 138 (2025) 118788 Available online 6 October 2025 2352-152X/© 2025 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/bync/4.0/). Short communication Evaluating effective thermal conductivity of PCM-impregnated wood composite at phase change temperature Luisa F. Cabeza a,* , Saranprabhu Mani Kala a , Waqar Ahmed a , Meysam Nazari b , Nasko Terziev b , Sabrina Bischof c , Sebastian Gritsch c , Emiliano Borri a , Gabriel Zsembinszki a a GREiA Research Group, University of Lleida, Pere de Cabrera 3, 25001, Lleida, Spain b Department of Forest Biomaterials and Technology, Swedish University of Agricultural Sciences, Vallv¨ agen 9C, 756 51, Uppsala, Sweden c BOKU University, Institute of Environmental Biotechnology, Department of Agricultural Sciences, Konrad-Lorenz-Strasse 20, 3430, Tulln an der Donau, Austria ARTICLE INFO Keywords: Effective thermal conductivity Experimental study Phase change material Thermal energy storage Wood ABSTRACT This study examines the effective thermal conductivity of wood composite impregnated with phase change material (PCM) during the PCM phase change process. Incorporating PCMs into wood stabilizes its shape and boosts energy storage capacity, broadening its applications in buildings while reducing energy usage. Most previous studies on PCM focused solely on acquiring the thermal conductivity in their solid state, leaving a gap in research concerning their liquid state. In this study, a PCM-impregnated wood composite was developed with bio-based binders. The effective thermal conductivity was analysed across a temperature spectrum in which the impregnated PCM, ethyl palmitate, exists in three distinct phases: solid, liquid, and mushy (a mix of solid and liquid phases). The results indicate a notable variation in effective thermal conductivity during the phase change process. At 10 ◦C, the solid PCM has an effective thermal conductivity of 0.13 W/m⋅K, while at 40 ◦C, its liquid state maintains the same value. However, a significant 48 % increase in effective thermal conductivity was observed during the phase change temperature measured using a hot disk instrument. These findings are expected to provide valuable insights for both researchers and industrial sectors that use PCMs for TES. 1. Introduction Phase change material (PCM) impregnated wood composites recently gained significant attention as a sustainable passive thermal energy storage (TES) solution [1]. Their sustainable properties improve the thermal efficiency of buildings [2]. Researchers explored the integration of PCMs into materials like gypsum boards, concrete [3], and wood [4] to enhance their thermal mass and overall energy efficiency [5]. Additionally, researchers investigated natural substances like wood [6], plant fibres [7], and bio-based polymers [8] as supporting matrices for PCMs, aiming to create more environmentally friendly TES solutions [9]. While much research involving PCMs concentrated on assessing factors like melting temperatures and enthalpy [10,11], understanding the effective thermal conductivity of PCM-impregnated wood composites is also crucial for improving TES performance [12]. This information is essential for refining numerical models, aiding material selection, and optimizing these composites for real-world applications. During the phase change, the PCM contained in the wood composite will exist in two states: solid and liquid, but also as a mushy state. The thermal conductivity of the PCM in the mushy state will not resemble the thermal conductivity of either the solid phase or the liquid phase of the PCM. Furthermore, the heat transfer properties of the PCM differ significantly between solid and liquid states [13,14]. Research on the effective thermal conductivity of wood composites containing PCMs in their liquid state is limited. Additionally, there is a significant lack of studies investigating how these composites behave during the temperature range in which the PCM undergoes phase change. Gaining insights into the effective thermal conductivity changes in PCM-impregnated wood composites during these phase change processes is essential, as it affects thermal response and the extent of heat transfer. This study developed a PCM-impregnated wood composite using ethyl palmitate as the PCM and bio-binders. The uniqueness of this research lies in evaluating the effective thermal conductivity of the wood composite during these critical phase change states. These findings are expected to provide valuable insights for both researchers and industrial sectors that use PCMs for thermal energy storage. Furthermore, the study emphasizes * Corresponding author. E-mail address: [email protected] (L.F. Cabeza). Contents lists available at ScienceDirect Journal of Energy Storage journal homepage: www.elsevier.com/locate/est https://doi.org/10.1016/j.est.2025.118788 Received 29 April 2025; Received in revised form 4 August 2025; Accepted 4 August 2025 Journal of Energy Storage 138 (2025) 118788 2 the importance of understanding the thermal properties of PCMs throughout their phase change process. 2. Material and methods 2.1. Material This study developed a composite of impregnated wood particles with ethyl palmitate (as PCM), polymerised lignosulfonates (as binders) [15,16], plasticisers, and non-impregnated wood particles. The blending and moulding process was performed as described in the literature [17]. Table 1 shows the composition of materials used to produce the impregnated wood composite. 2.2. Thermal conductivity measurement The effective thermal conductivity of the PCM-impregnated wood composite was measured using a hot disk instrument, TPS 2200. The measurements were performed at various temperatures to understand how the effective thermal conductivity of wood composite changes with temperature. These temperatures were chosen to ensure that the PCM impregnated in the wood composite exists in three distinct phases: solid phase, liquid phase, and mushy (a mix of solid and liquid phases). The sensor was positioned between two samples to evaluate their effective thermal conductivity. The hot disk sensor functions as both a temperature sensor and a heat pulse source, allowing it to evaluate the thermal conductivity as detailed in Eq. (1) [18]. To ensure accurate measurements, the surface of the samples in contact with the sensor was polished to eliminate air gaps that could result in lower thermal conductivity readings. ΔTave( τ ) = P0 a⋅k⋅ π 3 / 2 ⋅D( τ )(1) where “P0” represents the overall power output from the sensor, “a” is the sensor radius, “k” is the effective thermal conductivity of the sample, “D( τ )” is the dimensionless time-dependent function, and “ΔTave( τ )” is the time-dependent temperature increase in the sensor. The measurement was performed using the red cable 8563 sensor. To determine the precise value of the effective thermal conductivity, the heating power and measurement duration for the samples were optimized according to the manufacturer procedure [18]. The optimal heating power and duration are 50 mW and 320 s for the impregnated wood composite, which varies depending on the measurement temperature. Fig. 1 illustrates the methodology used to measure the effective thermal conductivity of the wood composites. 2.3. Differential scanning calorimeter The heat flow versus time curve for the PCM (ethyl palmitate) was measured using a differential scanning calorimeter (DSC). Before conducting DSC measurements, the samples were pre-dried in a vacuum desiccator to remove moisture. For the heat flow measurements, two crucibles were employed: (i) one contained a small amount of the sample and (ii) an empty crucible (reference). Approximately 5.4 mg of dried sample was loaded in a 40 μ L aluminium crucible. The sample crucible and the reference (empty) crucible were hermetically sealed and positioned in the furnace chamber of the DSC. The measurements were conducted under an inert atmosphere over a temperature range of 5 to 35 ◦C, with a heating rate of 1 ◦C/min, and from 35 ◦C back to 5 ◦C at the cooling rate of 1 ◦C/min. Three continuous heating and cooling cycles were performed, but only the second and third cycles were analysed. 2.4. Uncertainty estimation There exist two categories of uncertainties related to the measurement of the thermophysical properties of the sample: Type-A and Type-B uncertainty. Type-A uncertainty is the average of three repeated measurements, while Type-B uncertainty pertains to the uncertainty associated with the measuring instrument. In the context of thermal conductivity measurements, the Type-B uncertainty attributed to the thermal conductivity equipment is approximately 0.05 %. Meanwhile, the Type-B uncertainty associated with the DSC equipment measures about ±0.1 ◦C and ±3 J/g for the enthalpy calculations. 3. Results and discussion 3.1. Heat flow vs temperature curve The heat flow curves of the PCM, ethyl palmitate, are illustrated in Fig. 2. The melting of the PCM occurs within the temperature range of 20.0 ◦C to 25.0 ◦C, while solidification happens in two ranges: approximately 19.5 ◦C to 18.0 ◦C and around 15.5 ◦C to 13.5 ◦C. The phase change enthalpy during the heating and cooling cycle is 182.4 J/g and 170.7 J/g, respectively. Wood particles impregnated with this PCM, exhibiting the specified latent heat, offer a promising option for building and energy storage applications. Table 1 Composition of materials present in the PCM-impregnated wood composite. Sample Wood Density of composite (g/cm 3 ) PCM Composition of PCM-impregnated wood composite Non-impregnated wood particles (wt %) Impregnated PCM wood particle (wt%) Binder (wt%) PCM impregnated wood Pine 0.510 Ethyl palmitate 30 40 30 Fig. 1. Thermal conductivity measurement via Hot disk TPS 2200 instrument. L.F. Cabeza et al. Journal of Energy Storage 138 (2025) 118788 3 3.2. Effective thermal conductivity The effective thermal conductivity variation with temperature for the impregnated wood composite is shown in Fig. 3. The effective thermal conductivity of PCM-impregnated wood composite remains constant at 0.13 W/m⋅K at both extreme temperatures (10 ◦C and 40 ◦C), regardless of the PCM state (solid phase or liquid phase). This value is within the anticipated thermal conductivity range for wood composites [19]. Moreover, the measured effective thermal conductivity is roughly 9 % higher than the value (0.118 W/m⋅K) reported by P´ asztory et al. [19], considering that the material tested by P´ asztory et al. is a non-PCM-impregnated wood with a density comparable to the sample developed in this study. This enhancement may result from the incorporation of PCM, which boosts the thermal conductivity of the impregnated relative to the non-impregnated composites. This theory closely aligns with the study conducted by Nazari et al. [20] on impregnating bio-PCMs into pine and beech wood particles, which demonstrated a 33 % and 11 % improvement in thermal conductivity. The PCM-impregnated wood composite shows a significant increase (48 %) in effective thermal conductivity, particularly between 20 ◦C and 25 ◦C, closely aligned with the phase change temperature of the PCM ethyl palmitate. Chen et al. [21] noted a similar trend in the thermal conductivity of the PCM during measurements within the phase change temperature range. During the phase change process, the PCM takes up a significant amount of heat while keeping the material temperature nearly constant. This small temperature change has a significant effect on the increase in the effective thermal conductivity. The hot disk machine evaluates thermal conductivity by monitoring the average temperature changes of a sensor located between the samples according to Eq. (1). As the PCM changes phases, the heat pulse from the sensor is absorbed by the sample, which inhibits any temperature rise. Consequently, the adjacent sensor registers only a small average temperature change. This results in a significant increase in the effective thermal conductivity during the phase change process, in accordance with the hot disk method. However, further investigation is required to understand the mechanism behind the notable rise in the effective thermal conductivity of the PCM-impregnated wood composite. 4. Conclusions This study developed a PCM-impregnated wood composite by impregnating wood particles with ethyl palmitate. The heat flow curves of the PCM, ethyl palmitate and the effective thermal conductivity of PCM-impregnated wood composite were measured at different temperatures. The effective thermal conductivity of the impregnated wood composite was found to be 0.13 W/m⋅K at 10 ◦C and 40 ◦C, where the PCM exists in solid and liquid phases, respectively. However, the enhancement in the effective thermal conductivity was especially significant (48 % increment) during the temperature range in which the impregnated PCM undergoes a phase change. Studies detailing the thermal conductivity properties of PCMs during phase change temperatures and their significant increases were not previously reported in the literature. Understanding this parameter is crucial, as the key aspect of PCMs is their latent heat absorption and release during phase changes. This study will be beneficial for researchers and industries focusing on the application of PCMs in TES systems. CRediT authorship contribution statement Luisa F. Cabeza: Writing – review & editing, Supervision, Resources, Project administration, Methodology, Funding acquisition, Formal analysis, Data curation, Conceptualization. Saranprabhu Mani Kala: Writing – original draft, Visualization, Methodology, Investigation, Formal analysis, Data curation. Waqar Ahmed: Writing – review & editing, Investigation. Meysam Nazari: Writing – review & editing, Formal analysis. Nasko Terziev: Writing – review & editing, Supervision, Resources, Project administration, Funding acquisition, Formal analysis. Sabrina Bischof: Writing – review & editing, Supervision, Resources, Funding acquisition, Formal analysis. Sebastian Gritsch: Writing – review & editing, Formal analysis. Emiliano Borri: Writing – review & editing, Validation, Supervision, Methodology, Investigation, Formal analysis. Gabriel Zsembinszki: Writing – review & editing, Methodology, Investigation, Formal analysis. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This project was co-funded by the European Union's Horizon Europe Research and Innovation Programme under grant agreement 101135629 (BIOBUILD). Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or REA. Neither the European Union nor the granting authority can be held responsible for them. This paper is part of the RYC2023-044196-I, funded by MCIU/AEI/10.13039/501100011033 and FSE+. This work was partially funded by the Ministerio de Ciencia e Innovaci´ on - Agencia Estatal de Investigaci´ on (AEI) (PID2021Fig. 2. Heat flow vs temperature curve of the PCM ethyl palmitate. Fig. 3. Variations in thermal conductivity of impregnated bio-composite with respect to temperature. L.F. Cabeza et al. Journal of Energy Storage 138 (2025) 118788 4 123511OB-C31 - MCIN/AEI/10.13039/501100011033/FEDER, UE). This work was partially funded by Ministerio de Ciencia e Innovaci´ on - Agencia Estatal de Investigaci´ on (AEI) (RED2024-153629-T). This work is partially supported by ICREA under the ICREA Academia programme. 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