Investigation On Compressed Air Energy Storage System

Nowadays, there is a growing demand in electricity, while the corresponding request of electrical production and the transmission grid construction are lagged far behind. The percentage of the renewable energy (solar energy, wind energy, etc.) in the total power production is growing, but it comes along with inherent drawbacks, such as fluctuation and intermittence. On the customer side, the electricity consumption is not constant within a day or a week, in which there are peak and off-peak durations. All these problems make current electricity system complicated and unsafe. It is in great demand to introduce Electrical Energy Storage (EES) technology to solve the problems generating in the processes of electricity production, transmission and consumption. EES refers to a process of converting electrical energy from a power network into a form that can be stored for converting back to electrical energy when needed. The development of EES is a key way to break the bottleneck of the use of renewable energy in a large scale, and it also could increase the traditional power system efficiency, security and economy, and enhance the utility of distributed generation system.In this dissertation, the performance of the energy storage system based on compressed air is studied. With this system, the off-peak electricity from either traditional power plant or renewable energy plant is stored in a form of compressed air by high pressure compressor. After a certain time, when the electricity is in peak demand, the compressed air is released, then absorbs some heat, and drives an expander to generate power, which will feedback to the grid. Firstly, the research progress of Compressed Air Energy Storage (CAES) system is reviewed. Some research topics are highlighted, such as high pressure, thermal energy storage. And then, the thermodynamic analysis and comparisons are made among three configurations of CAES system, which provides a reference for CAES development.An experimental system is set up to explore further the performance of the CAES system, especially on high pressure, thermal storage etc.. The rig is made up of a high pressure compressor, high pressure cylinders, an electrical heater, an expander, a power measurement system and data acquisition system. This system was run successfully in a laboratory, which indicates the feasibility of lab-scale CAES system with high pressure. In experiments, the air could be compressed to be as high as260bar. The efficiency is in the range of30%～43%without compressed heat storage, while it could be35%～63%if the heat is taken into account. The pressure during energy storage process is changeable in cylinders, and the process efficiency is about25～35% (without compressed heat storage) or35～60% (with compressed heat storage). The power output of the expander is in the range of200-1650W, and the efficiency is about10～35%. The total system efficiency is not high due to compressor and expander efficiencies are not very good, and as a result the storage and expansion pressures do not affect the whole system too much.Based on the experimental analysis, some solutions to improve the whole system efficiency are discussed,(i) It could increase the storage system performance significantly by using more suitable components such as compressor and expander with high efficiencies. The round trip efficiency of the storage system can be above65%.(ii) During the energy storage process, the compressor chain works in a ramping pressure mode instead of the normal one in series mode with constant pressure output could decrease total relative compression energy consumption. During the energy discharging process, the expander chain works in a sliding mode instead of the normal one in series mode with constant pressure inlet could increase relative total energy output.(iii) It can also increase the power output and efficiency to couple the storage system with traditional power systems such as diesel engine as a hybrid power configuration. The maximum efficiency of the hybrid systems could be increased more than10%. The relative Greenhouse Gas (GHG) emission of these two hybrid system are quite lower, which could be40%less than that of diesel engine only. This research indicates the hybrid systems enjoy better performances and flexibility, and present the promising development.In the final part, the conclusion and future research prospects are drawn.