The constantly increasing emissions of greenhouse gases as well as numerous other gases toxic for human organisms necessitate urgent measures in the direction of energy saving and reducing fossil fuel consumption. Significant contributions to achieving both objectives can be expected by improving the energy performance of buildings to secure lower energy consumption for heating and cooling, and by efficiently utilizing the power generated by means of the thermal storage of untapped heat. In addition, the more efficient utilization of renewable energy sources (RES) shall result in an increase in the share of RES in gross final energy consumption and, consequently, in reduced CO2 emissions.
Among diverse technologies, phase change materials (PCMs) offer considerable advantages, rendering them ideal for thermal energy storage as well as for preserving constant temperatures for longer time spans. PCMs capitalize on the latent heat of fusion and, therefore, store more thermal energy than sensible heat materials. Moreover, during the phase change, PCMs maintain an almost stable temperature, thus contributing to the reduction of the required heat and cooling loads of the buildings [1
]. Despite the aforementioned advantages, PCMs require longer time spans to change from the solid to the liquid state and vice versa, due to their low thermal conductivity coefficient. The complexity of the equations describing the phenomenon of the heat flow through the PCM (as a result of the phase change process and the random movement of the fusion-solidification front) does not lend itself to the formation of analytic relations for the calculation of the temperature distribution in the PCM and the heat flow. Consequently, for the purposes of the mathematical analysis and the description of the phenomenon, both the numerical solution and mathematical simulation of the equations are required as well as the experimental calculation of the relevant quantities. Several computational methods have been developed for the accurate simulation of the heat flow through the PCM, describing, with adequate accuracy, the zone where both the liquid and solid phases co-exist (mushy zone). It is typically considered that the material in the mushy zone is solid, partly porous and transforming as the melted material increases. One method of simulation for PCM behavior utilized by several researchers [2
] is the enthalpy-porosity method, according to which the enthalpy of the PCM also depends on the ratio of the melted PCM. Another methodology used to solve the heat flow through PCMs by means of finite elements is the lattice Boltzmann method. Both the stability and the efficiency of the method in solving the particular heat transfer problem have been studied by several researchers [6
]. In addition to the numerical methods used for the solution of the differential equations describing the phenomenon of the heat transfer to and from the PCM, ready-to-use software has also been developed for the simulation of each PCM application. EnergyPlus [11
] and TRNSYS [12
] are two commercial software packages widely used for modeling the phase change of the PCM. Further to the numerical solution of the problem arising from each application of PCMs, the calculation of the thermal properties of PCMs (the latent heat of fusion-solidification, specific heat capacity, temperature range of the phase change, and thermal conductivity coefficient) is also imperative as they change during the cooling and heating processes in the material. By utilizing Differential Scanning Calorimetry (DSC), researchers Li et al. [15
] calculated the thermal properties of several commercially available PCMs for different values of temperature variation in the chamber. Based on the measurements, the conclusion was that both the temperature at which phase change takes place and the enthalpy variation of the material depend on the rate at which the temperature change occurs. Therefore, further research is required on whether the values of the PCM’s thermal properties match the respective values in each application. Similar were the conclusions reached by researchers M. Iten et al. [16
], who studied the effect of specific heat capacity on the phase change process. The results reveal the dependence of the specific heat capacity of the PCM on the rate of temperature change, which may lead to incorrect calculations and thus conclusions if not considered during simulation. Due to the difficulties in the experimental calculation of the thermal properties of PCMs, semi-empirical correlations have been developed to associate the PCM thermal properties with the temperature of the material. G. Ferrer et al. [17
] developed empirical correlations to calculate the viscosity and specific heat capacity of fatty acids as well as the viscosity of paraffins [18
The present paper includes the two-dimensional study of organic PCMs such as paraffin by building a model in Matlab (R2016b, MathWorks) code and by utilizing commercial software, such as Comsol and Fluent (Multiphysics 5.2). The analysis concerns the melting speed and front as well as the heat storage. Moreover, the analysis is repeated with nanoparticles being used as thermal conductivity enhancers so as to accelerate the phase change process.