| Liquid air energy storage(LAES)is becoming more and more popular due to its advantages such as long life,environmental friendliness,no geographical constraints,and flexible layouts.However,the lack of knowledge about heat/mass circulation key components and systematic theoretical research leads to the poor performance of LAES,the development of LAES technology is therefore limited.To address these issues,this thesis introduces the basic principle of LAES and establishes the overall models,investigates the effects of the air purification unit and thermal energy storage on the system,along with the approaches to the energy density improvement and multi-supplies of LAES.Theoretical,experimental,and simulation methods are adopted in this research,which provides the research basis and improvement methods for the thermodynamic and economic analysis of LAES systems.Firstly,this thesis studies the performance characteristics of key components in the LAES system,and focuses on building a cold storage packed bed testing system.A kind of white cobbles is selected as the cold storage material and a set of characterization methods are proposed for theoretical model calculation and prediction.Its specific heat increases with the increase of temperature,while the thermal conductivity decreases with the increase of temperature.The experimental results show that the average errors between the experimental data and the simulated data in the charge/discharge processes at both supercritical pressure and non-supercritical pressure are less than 1.54%,indicating the accuracy and adaptability of the revised models.It provides the base for the following theoretical simulation research.Secondly,this thesis comprehensively studies the dynamic characteristics of the air purification unit and the effects on the system from the molecular scale to the system scale.It is found that the exhaust gas from air turbine can completely desorb the adsorbents in the adsorber bed without external heat sources or power inputs.Compared with the traditional regeneration methods that consume heat or electricity,the round trip efficiency of LAES can be improved by 2.3%;meanwhile,the compression heat for bed regeneration can significantly shorten the regeneration time of the air purification unit.Thermodynamic analysis shows that the LAES system generally has high combined heat and power efficiency(82.5%-86.7%),round trip efficiency(47.9%-59.6%),and exergy efficiency(58.4%-68%).In addition,the payback period of LAES systems(10～200MW)is generally 8.4-27.9 years.Thirdly,this thesis studies the dynamic characteristics of the thermal energy storage units and the effects on the system.It is found that the cold storage packed bed experiences continuous dynamic changes in daily operation.With the increase of operating days,the cold energy is accumulated in the cold storage packed bed;to combat the dynamic performance of the cold storage packed bed and the negative impact on the air liquefaction process,it is proposed that the air liquefaction process preferentially uses the cold contribution from the cold storage packed bed,once the remaining cold energy in the packed bed is not enough to cool down the air to the determined low temperature,it completely relies on the cryo-expander/throttling valve for refrigeration and rejects the cold contribution from the cold storage unit.The method to select the optimal cold/heat recovery fluid is proposed to improve the system performance,and it is pointed out that pressurized air and methanol/propane are both good cold recovery fluids in terms of heat transfer inside the low-temperature heat exchangers,and can achieve～68%liquid yield with cold contribution from cold storage unit;it is also showed that using the pressurized air as the optimal cold recovery fluid is optimal,which results in an average liquid yield of 56%when the system is stable.Moreover,the compression heat is enough to reheat the air before air turbine,and the remaining compression heat stored in the heat storage packed bed can be used for other occasions.Fourthly,this thesis studies methods to improve the energy density of LAES.It is pointed out that the energy density of current LAES is generally overrated,and the internal relationships between the energy densities of the cold/heat storage unit and the overall system are explored,the pressurized propane of 1 MPa for cold storage is taken as an example to recover the cold energy of liquid air vaporization.Compared to the traditional cold storage unit using methanol/propane for cold recovery,the volume energy density of cold storage unit using pressurized propane for cold recovery is increased by～52%,while the investment cost is reduced by 37%,and its system energy storage density is 9.16 k Wh/m~3(16.69%higher than traditional LAES).In addition,the overall investment cost of the LAES system with pressurized cold storage is also 6-7%lower than that of the traditional LAES system.Finally,this thesis studies multi-supplies methods to improve the economic benefits of LAES.The multifunctional LAES system shows better economic performance than the traditional LAES system,which is mainly due to the additional benefits of pure oxygen production and heating output except for electrical output,the savings to investment ratio of the multifunctional LAES system(10 MW/80 MWh)is 3.12,which is 153%higher than that of the traditional LAES system,and the payback period is greatly shortened.Although the multifunctional LAES system is more complex with a relatively low round trip efficiency(～39%),it has great commercial prospects because of its unique cold recovery mechanism and diversified output forms. |
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