Thermal Balance of a Water Thermal Accumulator Based on Phase Change Materials

The arrangement of a water thermal accumulator (WTA) containing phase change materials (PCM) is presented and analyzed. The hot or cool water is used as a working body. The accumulator contains two concentric cylindrical tubes. The inner tube is used for hot or cool water flowing, while the volume between the inner and outer tubes is filled with PCM. The thermal energy in the accumulator is stored as a result of flowing the hot water through the inner tube due to the phase transition in PCM. This accumulated energy can be extracted from PCM as a result of flowing the cool water through the inner tube. For the enhancement of the thermal conduction coefficient, the PCM is doped with the nanocarbon particles having a thermal conductivity coefficient exceeding that of PCM by 4-5 orders of magnitude. The thermal balance of the accumulator is calculated on the basis of the solution of the time-dependent heat conduction equation by taking into account the heat absorbed and released as a result of the phase transition as well as the convection thermal exchange in the melted PCM. The calculation results determine the interconnection between the thermal conductivity of PCM and the characteristic time of thermal exchange between PCM and the working body. The calculations indicate that the characteristic thermal exchange time decreases as the thermal conduction coefficient enhances, so that the dependence becomes close to saturation at the thermal conductivity coefficient of about 5 W/m K. Such a coefficient can be reached by doping the paraffin-based PCM with a reduced graphene oxide at a content of about 2% (weight).