SIMULATION AND ANALYSIS ON THE EFFECTS OF HEAT EXCHANGERS IN A LIQUID AIR ENERGY STORAGE SYSTEM

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1 Proceedings of the Asian Conference on Thermal Sciences 207, st ACTS March 26-0, 207, Jeju Island, Korea ACTS-P00060 SIMULATION AND ANALYSIS ON THE EFFECTS OF HEAT EXCHANGERS IN A LIQUID AIR ENERGY STORAGE SYSTEM Xiang Li, Jingchao Wang, Qiuwang Wang, Min Zeng * Key Laboratory of Thermo-Fluid Science and Engineering, Ministry of Education, Xi an Jiaotong University, No. 28, Xianning West Road, Xi an, Shaanxi 70049, P. R. China Presenting Author: larrylee@yeah.net * Corresponding Author: zengmin@mail.xjtu.edu.cn ABSTRACT Liquid air energy storage (LAES), one of the cryogenic energy storage (CES), has a great potential among the innovative proposals for electric energy storage. Compared with compressed air energy storage (CAES) systems, LAES systems can integrate and accommodate wind and solar energy with much smaller volume. Heat exchanger parameters obviously affect the energy efficiency of LAES systems. In this paper, simulation using Aspen Plus, a commercial process simulation software, is established to investigate the effects of heat exchanger parameters on the performance of a large scale LAES system. Effective UA, where U is the overall heat transfer coefficient and A is the heat transfer area, is taken as an independent parameter, which takes into account all thermal irreversibility and configuration effects. The simulation results show that heat exchanger effectiveness has a linear relationship with variation of UA; HX is more sensitive to the increase of UA while all heat exchangers considered together; the maximum UAs of all heat exchangers have been found to be HX: 240 J/s K, HX2:990 J/s K and HX: 90 J/s K; and HX2 has the best performance among HX, HX2, and HX, both in fraction of air liquefied and in power produced by expander. It can be also concluded that, for different heat exchangers, tendency of power produced by expander with the variance of UA is different. Conclusions from this study may be significant for the heat exchanger design in LAES systems. KEYWORDS: Cryogenic energy storage, Liquid air, Heat exchanger, Aspen Plus simulation. INTRODUCTION With the development of smart grid in the future, energy storage technologies will play a key role. Liquid air energy storage (LAES), one of the cryogenic energy storage (CES), has a great potential among the innovative proposals for electric energy storage. Compared with compressed air energy storage (CAES) or Pumped Hydro Storage (PHS), LAES has few geological constraints, meanwhile, it can integrate and accommodate wind and solar energy with much smaller volume. Many researchers have studied LAES to improve performance of LAES. Based on the both cold and heat storage, Guizzi et al. [] obtained the round-trip efficiency of a LAES system in the range 4%-%. Li et al. [2] studied a LAES system integrated with a nuclear power plant, and reached a round-trip efficiency higher than 70%. Kantharaj et al. [] established a CA-LA hybrid energy storage system, which has greater Return On Investment (ROI) than the equivalent LAES plant or CAES plant. Performance of LAES plants is dependent on a number of geometric and operating parameters of its constituting components such as compressors, heat exchangers, expanders, valves, etc. Thomas et al.[4][] studied the role of heat exchangers and expanders in the helium liquefaction cycles using Collins cycle, and obtained the detailed results about the parametric effects of the two components. This paper performs a study to analyze effects of variation of heat exchanger parameters on the air liquefaction cycles. Effectiveness of heat exchangers and the effective thermal sizes of heat exchangers have been varied to determine their effects on the cycle performance. A Claude cycle, the

2 basic cycle for an air liquefaction cycle, is established to study the performance of heat exchangers. This may provide a clear understanding on the role of the heat exchangers and help in deciding the effective size of heat exchangers for larger systems. 2. METHODOLOGY Fig. shows the schematic of a CES-Claude cycle with recovering system [6]. In the LAES system, Renewable Energy Sources, such as wind, solar, can be integrated. Cold TES HX HX HX2 HX JT Ex S Cr C G T HX HX SH RES E TES Hot Fig. Schematic of a CES-Claude cycle with recovering system [6]. P To focus on the effects of heat exchangers, the cycle in the red dashed line are analyzed, which omits the air compression process and the recovering system. Robert Morgan et al. [7] suggested that 0 bar and bar are the optimal working pressures of a mature LAES system. 0 bar is comparable with the peak operating pressures of modern steam plant expanders. However, for the air liquefaction cycle investigated in this paper, separated gaseous air is the only source of cold energy, so that 0 bar is chosen to be the compression pressure instead of bar. The schematic and temperaturespecific entropy (T-s) diagrams of Claude air liquefaction cycle are shown in Fig. 2. W Q AIR OUT COMPRESSOR AIR IN T AIR IN HX AIR OUT 0 2 P HX W EXPANDER 9 P 2 HX SEP 7 6 VAVLE 6 7 S Fig. 2 Schematic and T-s diagrams of Claude air liquefaction cycle. 2

3 2. SOLUTION PROCEDURE The Claude air liquefaction cycle has been established using a commercial process simulation software, Aspen plus V7.2. PENG-ROB equation of state is utilized for generating the thermophysical properties of air. Parameters for the liquefaction cycle are listed in Table. Table Parameters for the Claude air liquefaction cycle Component Ambient pressure (kpa) Ambient temperature (K) Liquid product pressure (kpa) Inlet pressure of compressed air (kpa) Inlet temperature of compressed air (K) Split fraction of expander stream Outlet pressure of expander (kpa) Isentropic efficiency of expander Temperature approach of heat exchanger T approach (K) Value NONDIMENTIONALIZATION Both effectiveness and NTU are taken as the thermal parameters to represent heat exchanger, which takes into account all thermal irreversibility and configuration effects. The energy equilibrium between hot and cold streams in a heat exchanger can be written as: q c ( T T ) q c ( t t ) () m 2 m2 2 2 ( T t2) ( T2 t ) qm c ( T T2 ) UA T t2 ln T t 2 (2) Where U is the overall heat transfer coefficient, A is the heat transfer area, q m is the mass flow rate, and c p is the specific heat of heat exchanger steams. Then effectiveness and NTU can be obtained as: UA NTU () ( qc m ) min exp[ NTU ( R)] (4) Rexp[ NTU ( R)] Where R=(q m c) min /(q m c) max is the ratio of lower to higher heat capacity rates of heat exchanger steams.. RESULTS AND DISCUSSION

4 Power Output (W) Fraction of Air Liquefied HX-All HX2-All HX-All HX Alone ε Fig. Effects of UA variation on heat exchanger effectiveness. Fig. shows that heat exchanger effectiveness has a linear relationship with the variation of UA. For stream specific heat changes with temperature, it is feasible to use UA to replace NTU as analysis parameters at the constant mass flow rate (kg/s). The gradient of the curve HX-ALL is different from the other curves, it implies that HX is more easily affected by the increase of UA while all heat exchangers considered together. What s more, a Claude liquefaction cycle with heat exchangers having limiting or saturation UAs satisfy the utilization of cold energy. It can be found that all the heat exchangers in the cycle have different limiting UA, and they are HX: 240 J/s K, HX2:990 J/s K and HX: 90 J/s K All HXs HX Alone Fig. 4 Effects of UA variation on liquid production All HXs HX Alone Fig. Effects of UA variation on expander power output. 4

5 Figs. 4 and shows effects of UA variation on liquid production and expander power output, respectively. It can be observed that UA variations of HX and HX have almost similar impact on liquid production while the impact on power output is totally different. For cold energy is limited in this studied Claude cycle, HX2 has the best performance among HX, HX2, and HX, both in fraction of air liquefied and in power produced by expander. Increasing UA for HX lowers the temperature at the inlet of expander and the enthalpy of air, so that the expander power output decreases with the increase of UA for HX.For HX2 and HX, increasing UA lowers the temperature at the outlet of expander, which means the greater enthalpy difference and power output. 4. CONCLUSIONS According to the parametric study, the effects of variation of heat exchanger performance in a Claude air liquefaction cycle are obtained. The major conclusions are: ) Heat exchanger effectiveness has a linear relationship with variation of UA, however, HX is more easily affected by the increase of UA while all heat exchangers considered together. 2) The maximum UAs of all heat exchangers in the Claude liquefaction cycle have been found to be HX: 240 J/s K, HX2:990 J/s K and HX: 90 J/s K. ) HX2 has the best performance among HX, HX2, and HX, both in fraction of air liquefied and in power produced by expander. 4) Power output is mainly affected by the air enthalpy difference between inlet and outlet of an expander. For HX2 and HX considered alone, Power produced by expander increases with the increase of UA. For HX considered alone, it decreases. For all heat exchangers considered together, it keeps constant. REFERENCES [] G. L. Guizzi, M. Manno, L. M. Tolomei, and R. M. Vitali, Thermodynamic analysis of a liquid air energy storage system, Energy. 9 (20) [2] Y. Li et al., Load shifting of nuclear power plants using cryogenic energy storage technology, Applied Energy. (204) [] B. Kantharaj, S. Garvey, and A. Pimm, Compressed air energy storage with liquid air capacity extension, Applied Energy. 7 (20) [4] R. J. Thomas, P. Ghosh, and K. Chowdhury, Role of expanders in helium liquefaction cycles: Parametric studies using Collins cycle, Fusion Engineering and Design. 86 (20) [] R. J. Thomas, P. Ghosh, and K. Chowdhury, Role of heat exchangers in helium liquefaction cycles: Simulation studies using Collins cycle, Fusion Engineering and Design. 87 (202) [6] R. F. Abdo, H. T. C. Pedro, R. N. N. Koury, L. Machado, C. F. M. Coimbra, and M. P. Porto, Performance evaluation of various cryogenic energy storage systems, Energy. 90 (20) [7] R. Morgan, S. Nelmes, E. Gibson, and G. Brett, Liquid air energy storage Analysis and first results from a pilot scale demonstration plant, Applied Energy. 7 (20) 84 8.