Design and optimization of a small compressed air energy storage system for isolated applications

In this study, a variety of practical and theoretical CAES systems are reviewed in order to show the current status of the available CAES systems. Then a small scale compressed air energy storage for small isolated wind based hybrid energy system is introduced and discussed. In order to develop a cost-effective renewable based hybrid energy system, this research investigates the optimization of each component of the system from the wind turbine to the load to deliver the required energy in the most efficient way. After finding the general control strategy for energy harvesting from the wind, the control strategy based on predictive initial working condition of the air vane motor is investigated through practical tests. A control path is developed using the implemented air motor steady state operation based on its manufacturer datasheet, and it is used to supply a fixed amount of power to the grid. A new criterion for evaluation of different energy storage system with identical power rating and storage capacity is proposed and examined during a case study and the performance of pumped hydro, battery and compressed air energy systems are compared based on the total shortage time and total fuel consumption of backup diesel generators. The Monte Carlo simulation was used in order to regenerate the wind speed data with 10 minute resolution to represent more accurate variable wind speed. The proposed Harvested Energy Index (HEI) showed the ability of energy storage systems with low efficiency in utilizing excess wind energy and reach their storage capacity. Based on the obtained results, a novel general control concept for such systems is proposed and its steady state simulation results are discussed

Design and Operation of Compressed Air Energy Storage (CAES) for Wind Power through Process Modelling and Simulation

The compressed air energy storage (CAES) system, one of the grid-scale (>100MW) energy storage technologies, has been deployed in Germany and the USA. The round-trip efficiency of current commercial CAES plants is still low and needs to be improved. The CAES system will also play an important role in balancing electricity supply and demand because it can be integrated with renewable energy sources to overcome the intermittency problem. One unique feature of a CAES system integrated with wind power is that it is difficult to maintain constant operating conditions for CAES compression system due to fluctuating wind power output. The aim of this thesis is to study the approaches to improve the round-trip efficiency of CAES system, design and operation of the CAES system in the context of wind power and cost reduction when implementing the CAES system. This study is performed through process modelling, simulation and analysis. In this study, an integrated system consisting of a CAES system and an organic Rankine cycle (ORC) was proposed to recover waste heat from the CAES system for improving the round-trip efficiency of the CAES system. Steady-state process models of the CAES system and the ORC were developed in Aspen Plus®. These models were validated using data from the literature. Process analysis was carried out using validated models regarding the impact of different organic working fluids (e.g. R123, R134a, R152a, R245fa, R600a) of ORC and expander inlet pressures of the ORC on system performance. It was found that integrating ORC with CAES system can be an effective approach to improve the performance of the CAES system. The round-trip efficiency was improved by 3.32-3.95%, compared to that of a CAES system without ORC. The performance analysis of a CAES system in the context of wind power at design and off-design conditions were investigated through process simulation. Different operation strategies for fluctuating wind power outputs were proposed. Steady-state models for charging and discharging processes of the CAES system were developed in Aspen Plus® and validated. To enable off-design performance analysis, compressor and turbine characteristic curves were used during model development. A pseudo-dynamic model for cavern was developed in Excel. It was found that the CAES system at variable shaft speed mode has better performance than that at constant shaft speed mode because at variable shaft speed mode, it can utilise more excess wind energy (49.25 MWh), store more compressed air (51.55×103 kg), generate more electricity (76.00 MWh) and provide longer discharging time than at constant shaft speed mode. Economic evaluation based on levelized cost of electricity (LCOE) was performed using Aspen Process Economic Analyser® (APEA). In terms of CAES system integrated with ORC, different working fluids in ORC and different power sources (e.g. wind and solar) associated with the CAES system were considered to estimate the LCOEs. It was found that the LCOEs for the integrated system were competitive with fossil-fuel fired power and even lower than offshore wind power and solar power. As for the CAES system in the context of wind power, it was found that the LCOE for the CAES system for wind power at variable shaft speed mode is lower than at constant shaft speed mode and the LCOEs at both modes are lower than solar and offshore wind power, conventional powers (e.g. natural gas combustion turbine) and especially the residential electricity price. Results and insights presented in this PhD thesis will help to promote the commercial development of CAES technology