Storing energy is important since electricity should be available whenever needed and must be used or stored immediately after being generated. Many renewable energy technologies such as solar and wind energy, provide intermittent power generation and sometimes produce surplus electricity when demand is low. Current growth of renewable energy systems are subjected to the issues of higher costs and power instability which makes energy storage systems essential.
This research work focuses on a mechanical hybrid energy storage system which uses the concepts of combined pumped hydro storage (PHS) and compressed air energy storage (CAES). The system consists of one open tank to the air and one closed tank which stores water and compressed air. The multistage pump and hydro turbine are used for the charging and discharging process respectively, similar to pumped hydro storage operation in hydropower plants. When the grid power is at surplus, the unused power can be utilized to operate the multistage pump and store water and compressed air in the pressure vessel. The energy of compressed air can be released to drive water which passes through the hydro turbine, resulting in the generation of electricity when the grid power is insufficient. A major disadvantage of the conventional PHS and CAES is that the site where the systems can be installed is rare and have environmental side effects. As an alternative, this energy storage system is capable of overcoming the difficulties posed by PHS and CAES. This system can be used regardless of site conditions, since it uses a pressure vessel instead of two reservoirs of pumped hydro storage.
This study was carried out to verify the operating principle and analyze the characteristics of energy charging and discharging of the mechanical hybrid energy storage system. Firstly, the characteristics of energy charging and discharging of lab-scale model was analyzed and additionally CFD analysis and experimental test were performed on the charging and discharging process. It was found that the pressure in the vessel depends only on stored volume and air compression ratio of water at isothermal state without the loss to outside. Therefore, it is more effective to control the discharging flow rate from the pressure vessel in the operation. Secondly, the characteristics study of the charging process by multistage pump depends on variable speed which was carried out. In addition, a numerical model of the multistage pump was made and analysed using CFD and the performance and characteristics of the pump were determined and plotted. By using the plotted data and related formulas, a more efficient charging process by pump operation was found. In this thesis, the submerged floating-type mechanical hybrid energy storage system that can minimize the pressure differences between inside and outside of pressure vessel, by installing it in the sea was suggested. The submerged floating-type mechanical hybrid energy storage system has advantages such as the size reduction of pressure vessel and ensuring stability of the pressure vessel. Regardless, it should be further investigated for stability of mooring line considering effect of ocean current.