Experimental Study On The Thermocline Storage Tank With Phase Change Material

Out of the demand of stable heat-supply,the electric power load regulation capability is significantly weakened by combined heat and power(CHP)despite of its energy saving benefit,which aggravates the accommodation problem of renewable energy.The application of thermal energy storage(TES)has been proved effective to improve this situation.Combined the advantages of conventional water TES tank and latent heat storage using phase change material(PCM),the energy storage density of water thermocline storage tank can be substantially enhanced when the PCM is employed.Encapsulated paraffin wax with metallic shell is selected to build the PCM packed bed,which is arranged within the water tank.Having appropriate phase change temperature,it is designed to charge heat by heat exchanger with the steam turbine extraction and provide hot water to the district heating network.Aiming at investigating the feasibility of such water thermocline storage tank with encapsulated paraffin wax packed bed(PCM-water tank),experimental results with detailed analysis are presented in this work.At first,the investigation of basic characteristics of charging/discharging are presented.To discuss the feasibility of this TES technology,a comparison with conventional water tank is shown under various working conditions with different flow rates and working temperature ranges.Aiming at the stratification performance and efficiency of both TES systems with and without PCM packed bed,factors including dimensionless temperature,thermocline thickness,charging/discharging power and energy efficiency are analyzed.The results indicate that the performance of PCM-water tank is not as good as the water tank in stratification,but the advantage of encapsulated paraffin wax packed bed is obvious especially when operating at small flow rate,releasing much more thermal energy with qualified hot water.During the discharging process,the outlet water temperature stays close to the phase change temperature for much longer time.In practical application,to take advantage of the latent heat storage,it is suggested to employ the PCM whose phase change temperature is properly higher than the required heat-supply temperature.Following the basic characteristics analysis,investigation on the working patterns with complex working conditions based on the actual operational demands are shown.It is found that when the charging is periodic,both shorter non-charging duration and larger charging flow rate may result in stabler hot outlet water temperature for heat-supply,but the influence of the former appears to be more significant.As for the study on the performance of cyclic operation,convergence behavior is observed as the cycle number increases during the evolutions of temperature distribution,charging/discharging duration and efficiency.What's more,the speed of such convergence and the final status when the convergence is completed are relevant to the working flow rate,working temperature range and the completeness of charging/discharging.