Preparation Of Graphite Foam/eutectic Salt Composite Phase Change Material And Numerical Modeling Of The Energy Storage Unit

Thermal generation system is the main pattern of solar energy is solar thermal applicatons; however, the environment puts a large impact on it. Hence an effective high temperature energy storage system needs to be explored in order to improve the utilization efficiency and the sustainability of thermal generation. On the basis of research background, this paper has prepared a new graphite foam/eutectic salt composite phase change material which is suitable to be used in a solar thermal power plant by making use of graphite foam with high thermal conductivity. The stability of prepared material was tested in the laboratory before analyzing the phase change process of energy storage unit using numerical simulation method. Taking phase change time and average heat flux as assessment criteria, factors that have influence on the properties of energy storage unit were investigated. Following are the main conclusions:(1) After 20 cycles, 50 cycles and 100 cycles of heat test, the loss rate of eutectic salt in our prepared composite material were 0.33%, 2.99% and 4.60%, respectively, while the structure has almost no change. The result illustrate that this composite phase change material is so sustainable that it have a long life cycle and a high application value.(2) The simulation of phase change process indicates that the high thermal conductivity of graphite foam reduce phase change time and increase the heat transmission during the process. It is shown that the local thermal non-equilibrium model of graphite foam have little impact on sensible heat temperature field while large influence on phase change process temperature field. The local thermal non-equilibrium model could be able to reflect the phase change process of composite material objectively and explain the energy transmission during this process theoretically. The addition of graphite foam decreases the energy storage capacity to some degree, but greatly shortens the phase change time according to the analysis of energy change in eutectic salt over time. The average heat flux during phase change process was presented here to analyze the benefits of combining graphite foam with eutectic salt on the basis of heat change efficiency.(3) Taking phase change time and total average heat flux as assessment criteria, the factors investigated in the numerical simulation were inlet temperature, inlet velocity of hot fluid, tube length and tube pitch. The phase change time reduced as the inlet temperature increased. The fitting equation of phase change time and total average heat flux under the effects of the mentioned factors were obtained respectively.(4)The phase change became longer as the porosity of graphite foam increased, however, the average heat flux increased to a maximum value before dropping. The porosity of graphite foam corresponding to the maximal average heat flux is 0.711 when the phase change unit showed the best properties of energy storage. The fitting equations were 2q ? ?23034.7? ?33044.8? ?2304.6 and 2q ?24027.3? ?34089.4? ?2326.6 in the process of fusion and freezing respectively.