Phase Change Materials (PCMs) have enormous potential to contribute towards the successful development of renewable energy storage. PCMs store latent heat when they undergo a phase change between a solid and a liquid, and release heat when the liquid solidifies; this can have important applications in thermal energy storage (TES). Currently, one of the major hurdles preventing the expansion of solar renewable energy is a lack of storage, being able to store the energy harnessed from solar panels and using it later is essential to development. PCMs are being applied to this problem by making use of their ability to store thermal energy, which can be used for heating. Thermal conductivity plays an important role in this research as the higher the thermal conductivity, the smaller the temperature range required to charge and discharge the stored energy. Unfortunately, most of the PCMs possessing the qualities desired for good TES, such as chemical stability and high energy storage capacity, also have low thermal conductivities. Fang et al. (2013) increased the thermal conductivity of an eicosane based PCM through the addition of graphene nanoplatelets (GNPs), in an effort to create a new composite PCM that would fulfill the requirements needed for good TES, while also possessing a high thermal conductivity.
Figure 1. Illustration depicting how Phase Change Materials store and release heat as they move between solid and liquid phases1.
Fang et al. (2013) used the alkane eicosane with a purity higher than 99 wt% as their base, and added GNPs with a purity higher than 99.5 wt% to produce their mixture. They created eicosane-GNP solutions with weight loadings of 1, 2, 5, and 10% respectively, in addition to keeping pure eicosane aside for a reference. The Hot Disk TPS 2500 S thermal conductivity instrument was used to measure the thermal conductivities of the products (Figure 2). The TPS 2500S can perform both absolute isotropic and anisotropic thermal conductivity measurements in a single test, and tests between a range of 0.005 and 1000 W/mK. It is popular among researchers for its fast test times and easy sample preparation, has an accuracy better than 5%, and a reproducibility better than 1%. It performs measurements according to the ISO/DIS 22007-2.2 testing standard.
Figure 2. The Hot Disk TPS 2500S Thermal Conductivity Testing System.
Fang et al. (2013) tested the thermal conductivity of their PCM composites over a temperature range of 10-35°C that was controlled by a water bath. A differential scanning calorimeter was used to test the melting point and latent heat fusion of the composites. The Hot Disk TPS can use either a single sided or a double-sided sensor to performed measurements. Regardless of which sensor is used, excellent thermal contact must be achieved between the sample and the sensor. For the double-sided sensor, this is accomplished by sandwiching it between two identical sample pieces, and applying a small amount of pressure (Figure 3). For the single sided sensor, a single sample piece is placed on it, and a weight is placed on top to ensure proper contact (Figure 3).
Figure 3. Diagram of the sample set-up required to ensure good results using both the double-sided and the single sided sensor.
Data provided by the Hot Disk TPS 2500S indicated that the addition of GNPs to eicosane had a substantial, positive influence on the resulting thermal conductivity. The thermal conductivity of the sample containing 10 wt% GNP at 10°C increased by 400% relative to pure eicosane. The researchers noted that this was a much better result than those previously obtained by experiments attempting to raise the thermal conductivity of eicosane using a metallic oxide addition, such as CuO. Results from the differential scanning calorimeter indicated that the heat storage capacity of the composites did drop; as GNPs do not store heat, it is logical that the heat storage capacity will drop as eicosane is replaced in the solution. Despite this, Fang et al. (2013) concluded the drop in heat storage capacity did not outweigh the added benefit of an increased thermal conductivity.
Fang et al. (2013) concluded that the 10 wt% eicosane-GNP composite has excellent potential as a PCM. By comparing their results to those in the literature and discovering the superior results of GNPs over metal oxides, Fang et al. have inspired future work quantifying the exact type of GNPs that will produce the best PCM composite. The ability to create an effective thermal energy storage system through the use of PCMs will enable us to further development on solar energy as a replacement for fossil fuels, pointing us towards a greener future.
Note: For comprehensive results and an in-depth discussion, please follow the link in the references section to the scientific paper.