Phase change materials (PCMs) are increasingly used in energy-efficiency initiatives for Thermal Energy Storage (TES) and for passive heating or cooling. PCMs are interesting because they store the thermal energy of a phase change, called the “latent” heat of phase change, on heating, and then when cooled, they can give off the stored heat by reversing the phase change (Figure 1).
Figure 1: Phase Change Cycle of Common PCMs
PCMs are applied in an increasing number of applications including moderating daytime temperature variations in housing environments, optimizing thermal comfort in bedding, passive cooling of electronics and thermal management of batteries.
The ideal PCM is stable, chemically inert, non-flammable and has a high thermal conductivity to maximize the efficiency of the heat-transfer during the phase change. Unfortunately, many materials which would otherwise make very attractive PCMs, such as paraffin and natural oils (e.g., coconut oils) are flammable, lose structural integrity with their phase change and suffer from low thermal conductivity. A highly competitive field of research has emerged to optimize the properties of these material through the addition of additives in effort to increase the effective thermal conductivity. The thermal conductivity is a critical attribute of the overall PCM’s effectiveness.
MTPS Sensor with Liquid Cell
C-Therm’s MTPS method is the ideal tool for characterizing the thermal conductivity of PCMs owing to its ability to test both the solid and liquid states of the material continuously.
The TCi is housed in our Golden facility and does most of the heavy lifting for our LHS® industrial product development and production QC for standard thermal conductivity measurements. The machine has been working fine.”
Fatty Acid Ester and Paraffin Based Mixed Solid-State PCMs
In a paper published in Applied Sciences in 2016, Lee et al developed fatty acid ester and paraffin based mixed solid-state phase change materials (SSPCMs). Coconut oil and n-hexadecane were the base PCMs, and exfoliated graphite nanoplatelets (xGNP) were used the characterize the thermal conductivity. They employed the C-Therm TCi Thermal Conductivity Analyzer to characterize the thermal performance of their composite PCMs. Data obtained using the MTPS sensor (Figure 3) showed that the composition of the final mixed SSPCM had a substantial effect on the thermal conductivity (with more coconut oil being related to a higher thermal conductivity) but also that the mixed SSPCM had a much higher thermal conductivity than either pure coconut oil (0.321 W/mK) or pure n-hexadecane (0.154 W/mK).
Figure 3. Data obtained by Li et al on the thermal conductivity of their mixed SSPCMs.
The researchers noted that there seemed to be a trade-off between economics and heat capacity, with fatty acid esters being more economical but having a lower heat capacity than paraffin based materials. Additionally they noted that the thermal conductivity of the mixed SSPCMs were 284% higher than the thermal conductivity of the pure PCMs, suggesting that the mixed SSPCMs would be suitable to application in buildings.
Using Thermal Effusivity to Investigate the Thermal Performance of PCMs and PCM Composites
Phase change materials (PCMs) are substances with a high latent heat (typically of fusion) which may be used to store a large degree of heat energy by melting and crystallizing at a certain temperature. PCMs can be organic, inorganic, eutectics, and hygroscopic (where the phase change is not a change of fusion but rather of absorption and desorption of water vapor). A key performance metric of a PCM is its ability to exchange heat with its surroundings – a metric which is often referred to as “thermal inertia” or, more commonly, “thermal effusivity.” A higher thermal effusivity allows a material to be thermally activated in a more rapid manner – and therefore more thermal load can be stored during a dynamic thermal process. In short: PCMs with higher thermal effusivity can absorb or release more thermal energy, faster.
Thermal effusivity is governed by the following equation:
e = (k∙ρ∙Cp)1/2
Where e is the thermal effusivity, ρ is density, Cp is the mass specific heat capacity at constant pressure, and k is the thermal conductivity. Thermal effusivity may be expressed equivalently in units of Ws1/2/m2K or J/s1/2m2K.
How thermal effusivity describes the ability of a material to exchange heat with its surroundings is a large part of why not only k is important to PCM performance, but also the volumetric heat capacity, or ρCp. Given density information as a function of temperature, thermal conductivity information as a function of temperature, and a DSC curve which provides specific heat data, it is possible to calculate the thermal effusivity of a material as a function of temperature, which Korean scientists recently did in a paper published in the Journal of Adhesion Science and Technology. Their results are seen above in Figure 1.
However, this approach may be difficult, as it is often hard to obtain accurate density and thermal conductivity data during a phase transition – which can introduce error to the process and is time-consuming as it requires collection of thermal expansion and thermal conductivity data as well as DSC data. Researchers are increasingly benefitting from the ability to directly measure the thermal effusivity instead of calculating it – and thus reduce the error introduced by assumptions of constant density or thermal conductivity.
The C-Therm TCi Thermal Conductivity Analyzer is primarily known for its ability to measure the thermal conductivity of materials – however, it also directly measures the thermal effusivity of materials. It is compliant with existing standards for the measurement of thermal effusivity via the Modified Transient Plane Source method (ASTM Standard D7984).
A sample of paraffin wax, a commonly used base for many organic phase-change materials, was obtained and its thermal effusivity was monitored as a function of temperature on cooling through the phase change. The resulting data is plotted below in Figure 2:
Figure 2. Measured thermal effusivity of paraffin as a function of temperature.
At the highest point, the measured thermal effusivity is 1.49 x 103 Ws1/2/m2K. Aside from the phase transition highlighted in pink, the data plotted also exhibits a peak near 40°C and another near 22°C – it is fairly common for paraffins to exhibit crystal-crystal transitions in the latter, and the former is explainable by the melt of a minor component with a shorter chain length than the bulk of the wax. The performance of paraffin in this metric is best illustrated by comparison to the results obtained by the Korean group, whose shape-stabilized PCM exhibited a thermal effusivity at its peak of 103.19 x 103 Ws1/2/m2K – nearly two full orders of magnitude larger. As expected from the well-known thermal performance issues of pure paraffin in phase-change applications, this suggests that paraffin has difficulty exchanging heat with its surroundings, which limits its utility as a PCM.
For more information on how the C-Therm Trident can be applied to testing of phase-change materials, our technical library has a collection of papers on PCMs, and we also frequently highlight PCM applications on our blog or in our webinars. If you would like to learn more about how the Trident could be used to solve your PCM thermal conductivity testing problem, contact us today. A C-Therm representative will be happy to discuss your application and to recommend a solution.