| Adsorption thermal energy storage(TES),which performs heat storage through adsorption/desorption process of adsorent-adsorbare working pair,is regarded as a promising alternative to conventional technologies such as sensible heat storage and latent heat storage with the advantages of higher energy storage density,lower storage volume,near-zero heat loss,and less moving parts.Adsorption TES can efficiently recover and store lower temperature heat such as solar thermal energy,geothermal energy and industrial waste heat for residential heating or domestic hot water application.However,since the charging/discharging time during sorption process of adsorber is longer and higher heat storage density is required,to improve energy storage density and charging/discharging performance of hot-liquid tank,immersing an adsorption bed(adsorber)into stored hot liquid as thermal energy storage component is an attractive solution.In this study,a configuration of immersed annular adsorber with embedded heat pipes(HP),called IAA+HPs,was proposed and investigated.CFD based numerical method was developed to simulate the charging/discharging process of the heat pipe-assisted adsorber,coupling porous medium model with thermal resistant network.Silica gel/water was selected as the adsorbent/adsorbate working pair,and liquid water as HTF.The charging and discharging process were both investigated considering the influences of with and without heat pipes,thermal conductivity of adsorbent,the embedding length of heat pipes,and the amount of heat pipes in adsorber,respectively.In the desorption process,IAA+HPs can speed up the heat transfer rate,and the adsorbate vapor can be quickly desorbed from the silica gel particles.During desorption,it can obtain larger charged energy and higher energy density in a short time.Compared with IAA,the charging time of IAA+HPs can be shortened by 67.58%,which is beneficial to promote the charging process.When the conductivity increases,the charging time of the desorption process is shortened by 33.33%,58.33%,and 75.00%,respectively.Increasing the embedding length of the heat pipe reduces the charging time by 57.70%,67.58%,and 75.68%,respectively.Increasing the amount of heat pipes can also increase the heat storage and energy density,but the effect is small.The difference between the average desorption temperature and the charged energy under the three heat pipe arrangements is small.To promote the charging process,the method of changing the heat pipe arrangement is not ideal.The time for IAA+HPs to reach the same average desorption temperature decreases with the increase of the length of IAA,and the charged energy increases with the increase of the length of IAA.Choosing an IAA with large length and small diameter can achieve greater charged energy and higher energy density in a short time.In the adsorption process,the embedding heat pipes accelerate the heat transfer rate.The discharged energy by the adsorption of IAA+HPs can be quickly transferred to maintain a lower bed temperature and a large unit adsorption capacity,which promotes the discharging process.As the thermal conductivity increased from 0.175W/(m·K)to1.400W/(m·K),the energy density increased by 12.54%,18.40%,24.13% when the adsorption progressed to 3600 s.Increasing the thermal conductivity of the adsorbent can not only promote the adsorption of water vapor by the adsorbent silica gel,but also enable the rapid transfer of heat,which is beneficial to the discharging process.As the embedding length of the heat pipes increases,the energy density also increases.The increase of the embedding length of the heat pipes is beneficial to promote the heat transfer and the discharging process.Increasing the amount of heat pipes can also promote the heat transfer and adsorption process,but the effect is limited.The difference between the average temperature and the amount of discharged energy under the three heat pipe arrangements is small,that is,if you want to promote the discharing process and transfer more heat,the method of changing the heat pipe arrangement is not ideal.As the length of IAA increases,the average adsorption temperature of IAA+HPs first decreases and then increases,and the discharged energy first increases and then decreases.Therefore,it is necessary to find a set of optimal structure size parameters for IAA,while promoting the charging as well as discharging process. |
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