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Enhancing Thermal Properties of PCM Using Nano Materials

Final Report Summary - ENTAS (Enhancing Thermal Properties of PCM Using Nano Materials)

Energy storage technologies have long been a subject of great interest from both academia and industry and are crucial for achieving the European climate energy objectives as defined in the European Union’s (EU) “20-20-20” targets and in the European Commission’s (EC) Energy Roadmap 2050. The importance of energy storage technologies for a progressively decarbonised European energy system is also clearly stated in DG ENER’s Working Paper “The future role and challenges of Energy Storage Energy”.

The focus of this project is on Latent Heat Storage (LHS) which is currently a key international priority and is connected with a phase transformation of the storage materials (PCMs), typically changing from solid phase to liquid and vice versa. Stored energy is equivalent to the heat (enthalpy) for melting and solidification.

The aim of the Project, performed at the Faculty of Engineering & Environment, has been to develop the cost effective novel Phase Change Material (PCM) composites with enhanced and long lasting thermo-physical properties for application in compact and efficient Latent Heat Thermal Storage Systems. Novel PCM composites being investigated in this study include a PCM matrix with carbon additives to enhance their thermo-physical performance.

One of the main objectives has been to perform systematic micro-structural and morphological analysis of the PCM composites with carbon additives (1-6 wt.%) to establish correlation between the above and thermo-physical properties of the PCM composites.

Study is concentrated on enhancing paraffin wax type PCM which is ideally suited for accumulation of solar thermal energy and utilisation of this heat for domestic hot water production and space heating. Expandable graphite was investigated to improve its thermo-physical performance.

A number of PCM composites have been analysed. These include Paraffin wax and Lauric acid mixed with two types of expanded graphite. Paraffin wax has a melting point of 53-57°C (ASTM D 87, Sigma-Aldrich) and Lauric acid has 98 % purity (Sigma-Aldrich). Expandable graphite
FireCarb TEG-315 (A-type) and FireCarb TEG-160 (B-type) were used as a filler, respectively. These two types of expandable graphite (EG) were supplied by LKAB Minerals Ltd (Derby, UK).

Raw expandable graphite was firstly treated in a microwave chamber and then mechanically stirred with molten pure paraffin wax or Lauric acid maintained at the 70-80 deg. C temperature. Expanded graphite was further dispersed in molten PCM by placing the mixture container in an ultrasonic bath for 60 minutes. The prepared PCM composites contained 2, 4 and 6 wt.% of EG. Samples of PCM composites were prepared in the form of cylinders with the diameter of 13 mm. Distribution of expanded graphite in the mixture was investigated using the optical microscope InfiniteFocusSL (Alicona) and environmental scanning electron microscope (ESEM) FEI/Philips XL30 and MERA3/TESCAN, see Fig. 1.

KD2 Pro thermal properties analyser (Decagon, USA) was used to measure thermal properties of phase change materials (Fig. 2). The measurement technique is based on using the transient linear heat source method and the analyser measures the thermal conductivity, thermal diffusivity and volume specific heat of solids and thermal conductivity of liquids with the error not exceeding ±10%.

Differential Scanning Calorimetry analysis of PCM composites to determine the latent heat was performed in argon gas environment using a Setaram EVO131 DSC. Heating and cooling rates were kept constant at 5°C/min. The mass of samples was varied from 15 mg to 25 mg in the experiments.

Finally, a FTIR spectra analysis of paraffin wax/expanded graphite compositions was conducted deploying a Perkin Elmer Spectrum 2 FTIR spectrometer using its micro-ATR unit, see Fig. 3. The images obtained using scanning electronic microscopes were analysed to quantify distribution of expanded graphite in the composite using the four colour scheme: black-dark grey-light grey-white with black and white colours representing pure PCM and expanded graphite, respectively. Statistical analysis has been performed for the obtained images to determine the independent effects of the type of EG deployed and its concentration.

Analysis of results shows that overall expanded graphite B is better distributed in the composites for the deployed mixing process and homogeneity in distribution improves with the increase for graphite concentration from 2 to 6 wt. % (Fig. 4).

Measurements of the thermal conductivity demonstrate that at the 6 wt. %. concentration of the expanded graphite the thermal conductivity is 0.977 W/m°C and 1.263 W/m°C for paraffin wax PCM composite with A- and B-type EG, respectively, which is the enhancement by factor of 3.79 and 4.9 respectively.

Results of DSC measurements demonstrate that the solid-solid transition takes place between 29°C and 44°C with melting process occurring between 49°C and 61°C in pure paraffin wax whilst the same processes in the composite with EG B takes place between 28°C and 43°C and 48°C and 60°C, respectively. The total heat storage effect due to phase transition and change for paraffin wax is 144 J/g whilst for compositions with A-type EG it is about 136 J/g. In the case of composites with EG B it was observed that there is only a single thermal effect corresponding to the solid-liquid phase change, compared to composites with EG A. This melting process takes place between 45°C and
52°C. Heat of fusion of such compositions varies between 140-154 J/g. In composites with EG B the temperature range in which the solid-liquid phase change takes place became significantly narrow in comparison with compositions with A-type expanded graphite in which phase change processes cover the range between 28°C and 61°C. This narrowing of the temperature range for phase transition provides a considerable advantage in operation of thermal storage systems.

The results of Fourier transmittance infrared radiation analysis show that in the composite with EG B there is some kind of molecular interaction between paraffin wax and EG B, which is the reason for the above observed behaviour during the phase transition process, see Fig. 5. According to supplier of both types of expandable graphite, the technology of manufacturing is the same and these EGs
differ from each other only in the initial sizes of particles.

We believe that such intermolecular interaction between paraffin wax and expanded graphite was not reported previously and the nature of this intermolecular interaction requires further thorough study.

The more detailed description of the above obtained results can be found at

The discovered positive effect makes it possible to develop Solar Latent Heat Energy Storage systems which are more compact due to increased latent heat of PCM composite used and a shorter charging/discharging period due to the narrowed temperature range for melting/solidification process.

Results obtained will be used to develop such novel effective Solar Thermal Energy Storage systems as a part of Project 723596—Innova MicroSolar—H2020-EE-2016-2017/H2020-EE-2016-RIA-IA (2016-2020).