Application of Phase Change Material to Improve Adiabatic Compressed Air Energy Storage System

Abstract

The use of renewable energy, such as wind and solar, has significantly increased in the last decade. However, these renewable technologies have the limitations of being intermittent; thus,storing energy in the form of compressed air is a promising option. In compressed air energy storage (CAES), the electrical energy from the power network is transformed into a high pressure energy through a compressor. When the demand for electricity is high, the stored high pressure air is used to drive a turbine to generate electricity. The advantages of CAES include high energy density and quality, but the efficiency is relatively low (about 50%) since a significant amount of the compression energy is lost as heat. Additionally, in the expansion process, this technology would require a non-renewable source of energy for heating the air to prevent frosting. To overcome this drawback, an adiabatic CAES (ACAES) system has been proposed by applying methods of storing the generated heat during compression. The generated heat during compression is stored in the specific thermal storage system and is utilised to heat up the air during the expansion process. This method eliminates or limits the use of extra energy to heat the expanded air, usually needed in CAES system, which enhances the efficiency of the system by up to 70%. However, there are still challenges related to the selection of the thermal energy storage (TES) system needed in this application.The thermal storage material should have large storage capacity and should be able to store/release the heat rapidly during compression and expansion. For that reason, this thesis aims to develop a new method for the ACAES system using microcapsule of phase change material (PCM) for thermal storage. The use of PCM is selected since it has high latent heat of melting and hence is able to store a large amount of heat within a narrow change of temperature.The microcapsules are not only needed to contain the PCM but also to provide the large surface Philoarea needed for the heat to be stored in or released from it at a very high rate. In addition, a specific goal of this research is to develop a model for a small ACAES, which requires solving energy equations in both air and container wall and validate the model experimentally. A small CAES system has been designed for experimental purposes to validate the conceptual model. During the compression stage, the compressed air is stored into a 2L cylinder at 200 bar, while during the expansion stage, the compressed air is released to the environment. The results show that at the beginning of compression the air temperature rises from approximately 17°C to over 60°C, while it drops to -20°C during expansion. The previous model is further improved to account for the presence of PCM microcapsules and then validated experimentally. In the presence of PCM microcapsules (Micronal® DS 5038X), the air temperature rises from 24°C to around 50°C during compression, which is lower than without PCM, since PCM absorbs some of the heat and stores it in the form of latent heat. While in expansion, the minimum temperature drops to only -2 °C compared to -20°C when operated without PCM, which indicates that PCM has efficiently transferred its stored heat to the air. The effect of compression on physical and thermal properties of PCM microcapsules are investigated by comparing their characteristics before and after compression and for a number of cycles. Since air compression could crack the shell of the microcapsule, a metal-coating process, well-described in the thesis, is applied to prevent cracking of the polymer shell of the microcapsules and to improve their stability. Also to have a better understanding, two different PCMs are applied in this research: Micronal® DS 5038X and Microtek 24D, together with Microtek 24D metal-coated. All PCM microcapsules used in this research are analysed using differential scanning calorimetry (DSC), thermal gravimetric analysis (TGA) and scanning electron microscope (SEM), before and after 20 compression-expansion cycles. The results show that Micronal® DS 5038X has a better stability than Microtek 24D since these microcapsules are lumps of very small capsules. The performance of Microtek 24D is improved when metal coating is applied to the capsule. The results disclosed in this thesis indicate that PCM microcapsules are able to successfully store the heat generated during compression and release it during expansion at a very high rate due to their large surface area. The developed model has successfully predicted both air and cylinder’s wall temperature during compression and expansion processes.