Study On Heat Transfer Performance And Optimization Design Of Shell And Tube Latent Heat Thermal Energy Storage System

Latent thermal heat storage is a new technology that utilize the latent heat of phase change material(PCM)when the phase change happend to realize the heat storage and transformation.It has been widely focused becusse the advantages of large heat storage density,approximate isothermal heat storage process.Latent thermal heat storage technology can effectively collect low grade energy,and solve the mismatch between energy supply and employ in the aspect of time and strength,which makes it is valuable in the field of building energy efficiency,such as solar energy heat utilization,building envelope,heating and air conditioning system.Latent thermal storage technology is one of the most important technical ways to improve the efficiency of energy utilization and realize the sustainable development of building.Low heat release/absorb rate is a problem of latent thermal energy storage system faced in the scope of buiding efficiency,which cause a contradiction between energy storage capacity and energy requirement.The most effective way to solve this problem is to enhanbe thermal conductivity of PCM and optimize the heat transfer performance of device.In order to improve the energy storage performance of latent thermal energy storage system,the optimal method on heat transfer performance of latent thermal enery storage device and its geometric parameters should be considerd once the PCM is determined.In order to solve above problems,research of heat transfer performance strengthen and geometry optimization design of horizontal shell and tube laten thermal energy storage system were carried out in this paper.Numerical simulation and experimental study were adopted to investigate the thermal performance of adding fins in inner tube and eccentric geometry,the effect of heat transfer enhancement and its mechanism in the melting and solidification process were revealed.During melting process,the natural convection plays a very important role in melting rate.Adding fins can increase the heat exchange area but obstruct the intensity of natural convection,these two factors affect the PCM melting performance,so the number of fin have an optimal value which is related to the temperature of thermal boundary.In this paper,averaged wall heat flux as an indicators was put forward to evaluate the optimal number of fins.Thermal conductivity was the dominant heat transfer way in the process of solidification,so the solidification rate increased monotonously with the number of fins.It is suggested that the optimal number of fins in the whole energy storage and release stage should be the optimal value corresponding to the melting process.On the best fin number working conditions,the heat transfer temperature difference is 15 oC,25oC and 35 oC,the melting time shorten 76.7%,76.3%and 76.1%respectively,and the solidification time by 91.1%,84.6%and 83.8%respectively.The interaction between Ra number and eccentricity on heat transfer enhancement was revealed.In the process of melting,Ra number affect PCM melting rate under different eccentricity and varied with its value when the Ra number is small,due to natural convection strength is weak,and melting rate would appear to rise with the increase of eccentricity and then decrease.When the Ra is greater than a certain value,PCM melting rate rising with the increase of eccentricity monotonously.The optimal eccentricity values under different Ra number was studied in the melting process,and calculation formula of fitting was obtained as:In the process of solidification,the solidification rate of PCM decreased with the increase of eccentricity.So total melting and solidification time was chosen as a standard to evaluate optimal eccentricity,and the optimal eccentricity values were calculated under different Ra number.When the Ra number was 1.63×10~6?5.52×10~6?2.17×10~6 and 4.2×10~7,the corresponding optimal eccentricity was 0.3,0.4,0.7 and 0.8,respectively.The heat transfer model for single shell and tube with PCM was established to realize the rapid numerical solution of latent thermal energy storage,and the influences of fluid operation parameters,such as inlet temperature,flow rate to the thermal storage heat storage performance were investigated.With the increase of heat transfer temperature difference between HTF and PCM,melting or solidification rate of PCM are enhanced and it is more significant to melting rate,but improving heat transfer temperature difference blindly leaded to a too high or too low outlet fluid temperature.The increase of flow rate will also increase the heat storage and release rate of phase change,but the increase of flow rate will lead to a higher pump power consumption.A smaller flow rate can improve energy utilization efficiency.Therefore,the heat storage rate and pump power consumption should be considered when we decided the value of flow rate.Finally,latent thermal energy storage performance under different geometry parameters was studied.L,L/do,and do/di were chosen for geometry control parameters while the PCM volume kept the same,and the heat transfer process was simulated.Heat storage rate,heat storage efficiency were defined to evaluate thermal storage performance,and the influences of parameters on the heat storage performance were analyzed.On this basis of that,the optimal geometric parameters under specific design constraints or requirements were determined with the maximum energy storage efficiency under the minimum heat transfer surface area as the objective function.