Translated Abstract
With the increasing severe energy crisis and environmental pollution, the development of new energy has attracted the extensive attention. Because of the characteristics of the wide collection, clean and environmental-friendly, solar energy is increasing of interest. The related research on solar thermal utilization has been rapidly developed. In the field of the concentrating solar power plants(CSP), a thermal energy storage system is usually equipped to solve the fluctuation and discontinuity issues of solar energy. The thermal energy storage system can significantly solve the spacial unmatched problem between the thermal energy supply and demand of the solar thermal power generation system.To further relieve the influence of weather change on the performance of CSP system and improve the stability of electricity generation and utilization efficiency of energy. The common thermal energy storage technology is consist of sensible thermal energy storage, latent heat thermal energy storage and chemical energy storage. Among these three thermal energy storage techonology, the packed-bed thermal energy storage(PBTES) system adopting molten salt as phase change material has the advantage of high thermal energy storage density, constant temperature output and low system cost that leads it become a frontier study nowadays.
Because of the complexity of heat transfer process and energy storage characteristics, according to our acknowledgment, there is few experimental studies on the performance of PBTES. It is critical to observe the temperature evolutions of the PBTES and further investigate the effects of different parameters on the thermal performance that results in an enhanced method to further improve the thermal performance. Being motivated by these issues, a high temperature PBTES is studied experimentally and numerically. At first, the thermophysical properties of PCM is tested and a new phase change material (PCM) capsule is fabricated. Then, a series of experimental studies are carried out to investigate the charging and discharging characterisics with the different operating condition. Moreover, a new two-layered PBTES is established. The thermal performance of two-layered PBTES is numerically optimized and experimentally investigated. Finally, a 50 MWht PBTES system is designed and an economic model is developed. The charging-discharging cyclic behaviors are investigated and the investment cost of the whole PBTES system was analyzed as well. The detailed contents are as follows.
1) A new PCM capsule is fabricated with the properties of high melting point, high thermal heat storage density and good sealing. The thermophysical properties of the molten salt is tested. The ternary carbonate Li2CO3-K2CO3-Na2CO3 (32-35-33 wt%) has been selected as the optimal option of PCMs. The melting point is 395.1℃ and the energy storage density is 174.7 kJ•kg-1. The PCM capsules are encapsulated by welding procedure including wire-electrode cutting, filling, heating and welding. The Allowable stress of the stainless steel is tested and the result is satisfied with the experimental environment. The PCM capsule has the characteristics such as high thermal heat storage density, good thermal conductivity, high mechanical strength, good sealing property and easy-fabrication. That make this new capsule be a good thermal energy storage material for high temperature PBTES.
2) A new high-temperature PBTES with PCM capsules is designed. The temperature evolution of heat transfer fluid and that of PCM capsule are obtained. The temperature difference between the capsule and the heat transfer fluid is further analyzed. The influence of inlet temperature and mass flow rate are studied and compared with that of shell and tube thermal energy storage. In the experimental study, the air is selected as the heat transfer fluid and the temperature of air and PCM capsules are tested as well. The results are concluded as follows. (1) The temperature evolutions of the heat transfer fluid and that of capsule in the PBTES are obtained. There is a certain temperature difference between the capsule and the heat transfer fluid. Moreover, the convection heat transfer resistances of the capsule and the heat transfer fluid are the main influential factors in the heat exchange process. It can be improved by the inlet temperature and mass flow rate of fluid. (2) The improvement of both inlet temperature and mass flow rate in PBTES can increase its charging and discharging efficiencies. The overall efficiency of the system can be increased to 86.1% from 77.4% through increasing inlet temperature from 425℃ to 465℃. The efficiency will up to 83.6% from the level of 80.6% by the rise of mass flow rate. (3) To Compare to the shell and tube thermal storage system, the charging and discharging rate of PBTES are 1.8-3.2 times higher than the former one. The overall efficiency of PBTES is 1.9-2.4 times greater than that of shell and tube thermal storage system.
3) A new two-layered packed-bed thermal energy storage system is developed and the thermal performance of the PBTES is experimentally investigated either. First, the effects of different diameters on thermal energy storage (TES) charging rate and TES density are numerically analyzed. Second, a new two-layered PBTES is proposed with the diameter of the capsule changed along the flow direction. The optimized hierarchical structure of PBTES is further constructed with the optimal comprehensive index called TES rate density. Finally, thermal performance of two-layered PBTES is experimentally examined. The influence of different inlet temperature and mass flow on the thermal performance of system are investigated and compared with that of single-layered PBTES. The results are concluded as follows. (1) The PCM capsules of lower layer are melted faster by adopting the two-layered PBTES system. The improvement of heat transfer performance and enhancement of uneven heat transfer temperature can further increase the thermal performance of system. (2) When the air inlet temperature increases from 425℃ to 465℃, the TES charging rate can be increased by 60.5% and the heat storage efficiency rises from 84.8% to 91.1%. The TES charging rate is grew by 23.5% when the mass flow rate increases from 180 kg•h-1 to 260 kg•h-1, and the heat storage efficiency is decreased slightly. (3) With the same inlet temperature and mass flow, the TES charging rate of diameter-changed two-layered PBTES is better than that of single-layered PBTES. The maximum TES charging rate of former system can be increased by 12.4% comparing to that of latter system, and TES rate density of two-layered PBTES can be improved by 13% as well.
4) A 50 MWht PBTES system is designed. The influence of different parameters on the thermal performance of discharging process is investigated and the cyclic behavior is analyzed to obtain the total performance in the charging/dicharging processes. An economic model is established to investigate the investment of whole PBTES system. The results are concluded as follows. (1) In the discharging process, with the reduction of heat transfer fluid velocity, the discharging rate is obviously improved and the disharging efficiency is decreased, the effective disharging ratio decreases slightly. With the increase of PCM capsule diameter, the discharing rate is continuously decreased. The disharging efficiency and the effective disharging ratio are declined as well. When the PCM melting temperature decreases, the discharing rate, the disharging efficiency and the effective disharging ratio are all reduced. Particularly, when the melting temperature below the cut-off temperature, the effective disharging ratio decreases obviously. (2)After 6 times charging/discharging processes, the outlet temperature keeps constant and the system reaches a stable state. (3) The economic analysis presents that the investment of PCM materials takes 27.0% of total system. The cost of stainless steel possesses 21.1% of total system and the cost of auxiliary equipment takes 9.1% of total system. The cost of encapsulation is 43.8% of total system that becomes the main element of investment cost of the system. And the fluctuation of encapsulation cost has the prominently influence on the investment cost.
Translated Keyword
[Experiment study, High-temperature latent heat thermal energy storage, Numerical analysis, Packed-bed thermal energy storage, Solar energy]
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