Numerical Investigation On Latent Thermal Energy Storage System With Spiral Coil Tube And Composite PCM

Latent thermal energy storage(LTES)technology applied in recovery of industrial waste heat,solar thermal applications,and energy conversation of buildings,etc.can solve the mismatches of thermal energy in supply and demand,space and time,and thus increasing the efficiency of thermal energy utilization.While,the heat transfer process in LTES is unstable and non-liner,which makes incapable of the traditional design techniques of heat exchangers.And in the present LTES systems,the structures of heat exchangers are complicated or un-optimized,and thermal conductivities of the used phase change materials(PCMs)are low.These drawbacks greatly limit the applications of LTES technology.In this work,a rapid charging tube-in-tank LTES system is developed by applying a compact spiral coil tube and paraffin/expanded graphite composite PCM with a high thermal conductivity simultaneously.To investigate the factors in performance of the LTES system,numerical simulation validated by experiment firstly is applied,in which the realizable k-epsilon model is chosen to model turbulence and the equivalent specific heat capacity is selected to model phase change process.Compared to experiment,the relative error of the time for PCM finishing phase change process is in 20%.And temperature increasing curves of PCM,temperature differences of inlet and outlet of water are similar and close in simulation and experiment,indicating that the numerical model is adequate enough to simulate the performance of a similar LTES system.Detailed analysis of factors on performance of the LTES system with single spiral coil tube has been done in this work.And the results are as follows: Increasing Re or ?T will give rise to a shorter thermal energy storage time,but Re and ?T being too high simultaneously is not advisable.The optimal RH/RT for a shortest latent thermal energy storage time and a best temperature distribution in 3D model of the PCM tank is confirmed between 0.53 and 0.64,and the value is also valid for a larger LTES system.Decreasing the pitch of spiral coil tube when the radius is constant can shorten the thermal energy storage time,but the effect is remarkable when the pitch is little.And increasing the packing density of PCM can also shorten the energy storage time,because of the increasing thermal conductivity.To achieve the specific value the RH/RT,a quasi-steady state method together with numerical validation is applied.The value for the LTES system with single spiral coil tube is finally found to be 0.61,and for LTES system with double spiral coil tubes,the values are found to 0.33 and 0.78 for the inner and outer tube,respectively.For the LTES system with double coil tube,when the ratio of heat transfer area of the inner and outer coil tube is 1:2,the inlet of water is more appropriate through the outer coil tube.And when the ratio is 1:3,water inlet through the inner coil tube is better.Finally,performance comparison of the LTES systems with single and double spiral coil tubes is made,and the results indicate that performance of the LTES system with double coil tube is better than that with single coil tube,and the time for 90 wt.% PCM finishing phase change process in the tank with double coil tube can be decreased by 25.3% compared to that with single coil tube,and for 95 wt.% PCM,the time can be decreased by 13.3%.More importantly,the performance enhancement potential for the LTES system with double coil tube is higher than that with single coil tube.