Applications

Phase-Change Material (PCM)

Measuring the Thermal Conductivity of PCMs

Phase change materials (PCMs) are widely used in energy-efficiency initiatives for Thermal Energy Storage (TES). 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). This phenomenon may be used to moderate daytime temperature variations in a housing environment, to help mitigate the urban heat island effect, to ensure thermal comfort in bedding, cool electronics and for a variety of other applications. 

Figure 1. Energy vs. Temperature plot showing the isothermal heat storage effect of PCMs

The ideal PCM is stable, chemically inert, and non-flammable. As well, it also has a high latent heat of phase change, remains solid through the phase change, and has 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 – thus a highly competitive field of research has emerged to optimize the properties of these materials, through the addition of flame retardants, thermal conductivity enhancers, and shape stabilizing components, with the thermal conductivity being a key performance metric of these materials.

C-Therm TCi Thermal Conductivity and Effusivity Analyzer

Figure 2. C-Therm TCi Thermal Conductivity Analyzer

The C-Therm TCi Thermal Conductivity Analyzer provides the optimal solution for measuring Phase-Change Materials as it is the only commercial instrument that offers the versatility to test the thermal conductivity of solids, liquids, powders, pastes, and textiles.

Case Highlight #1:

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).

Data obtained by Li et al on the thermal conductivity of their mixed SSPCMs

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.

Measuring Thermal Effusivity

The C-Therm TCi Thermal Conductivity Analyzer additionally measures thermal effusivity (thermal inertia).

Case Highlight #2:

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 hydroscopics (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: 

(kρCp)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.

  

Figure 1. Thermal inertia (effusivity) of gypsum board with an embedded paraffin-based PCM. (Source:  http://dx.doi.org/10.1080/01694243.2016.1215011 )  

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 TCi 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 TCi 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.

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