Study On The Ice Storage Performance Of Ice-tank Combined With Finned Tube

Cooling energy generated from off-peak electric power can be stored by latent heat storage materials and released subsequently to meet the on-peak demand when the cooling load is increased.Latent heat storage techniques are of significance especially in energy conservation and emission reduction.Ice storage systems are most commonly used in HVAC systems to enhance the reliability as well as peak-load shifting of the urban electricity consumption within reduced install capability.Hence,ice storage system has incurred major international research interest.Characteristics during water freezing to ice are essential for the performance of the ice storage system.Arrays of smooth tubes are utilized as the ice storage units in traditional ice storage systems with ice freezing on the outside of the tube walls.However,distinct increment of thermal resistance due to the thickening of ice is the main reason of performance degradation of the freezing processes which limit the applications of ice storage systems severely.It's worth noting that,new types of ice storage systems with high storage performance using finned tubes have attracted wide attentions.The metal fins with high thermal conductivity are beneficial for the diffusing of energy along the radial directions during the freezing processes.Additional expandability and flexibility provided by ice storage systems with finned tubes can be desirable for applications requiring large amount of energy storage.To optimally design the ice storage systems as well as enhance the performance of the ice storage system,it is of significance to fully explore the freezing processes in the enclosed type ice storage system with finned tubes through both numerical and experimental approaches.Nowadays,researches on the ice storage characteristics outside the finned tube of chilly mediums in ice storage system have been extensively conducted.However,the underlying mechanisms of cooling energy transfer during the freezing processes with presence of natural convection are waiting to be revealed.Especially,the effects of fin parameters on the efficiency of energy storage are still not completely understood.In addition,effective methods enhancing the performance of ice storage systems are required.Therefore,a two dimensional transit model of freezing process in ice storage system with finned tube is developed and solved numerically.Enthalpy-porosity technique is adopted with freezing interface tracked by the liquid fraction.Morphologies of the ice outside the finned tubes are reconstructed from the and the temperature contours through out the simulation domain are investigated.Based on the simulation results,the effects of natural convection on the ice storage performance are revealed.In addition,various parameters that describe the fins including height,thickness and number of the fins have been discussed quantitatively to determine the influence.Meanwhile,visualization experiment on freezing inside an ice storage unit is conducted to observe the behaviors of the liquid and solid during freezing.Variation of the temperature in the region of freezing under wide ranges of operating conditions are obtained.And the mathematical model is validated using the experimental results.Furthermore,experiments on the ice storage efficiency with foam metals induced between the fins are carried out.Both experimental results and numerical simulations verified the enhancement of ice storage performance with the implementation of foam metals.In summery,series of important results and conclusions are obtained as follows;(1)Density differences originated from the temperate differences in the freezing region give rise to natural convection and local vortex motions are aroused.Natural convection inducing mixing within the fluid regions and variations of the temperatures.The efficiency of cooling energy transfer is affected and the performance of the ice storage system is weakened evidently.(2)Increasing the length,thickness and numbers of the fins are valid approaches to increase the energy transfer areas between the fins and water.Transfer of cooling energy along the radial directions can be enhanced contributing to intensified ice storage performance.Under the condition of same increment on the quality of fins,increasing the thickness of fins plays the minimum role in improving the performance of ice storage.At the initial stage of ice storage,increasing the fins number contributes most to enhancing the ice storage performance while extending the fins does better at the later stage.(3)Numerical results on the morphologies of freezing ice and the characteristic temperature variations agreed well with experiments which validate the rationality of the numerical model.The results indicate that,efficiency of ice storage system varies positively with the initial temperature difference between the chilly media and water that storing the cooling energy.Higher temperature differences lead to greater temperate gradient between chilly media and water that the cooling energy diffusion efficiency increases accompanied with faster freezing.On the other hand,when the temperature of the chilly medium are kept constant,less sensible heat is required to cool the water to the freezing point under the condition of lower initial water temperature.And the freezing process can be accelerated.(4)The rate of freezing presents negative correlation with the porosity of foam metal under the same equivalent pore diameter.In other word,the freezing process is fast with smaller porosity.The fins outside the chilly medium tube are of significance in enhancing the cooling energy transfer along the radial directions.The ice storage performance of systems with foam metals combined with fins are higher than systems using foam metals solely.