| dc.description.abstract | Due to the threat of climate change, our energy systems tend to adopt lower-carbon energy sources, such as renewables. In addition, our energy systems are likely to follow another trend from very large centralized industrial energy systems to decentralized energy hubs serving both industry and the domestic sector in the coming years. With time variations (daily and seasonal) both on the supply side (intermittency of renewables) and the demand side, energy storage technologies will play a very important role. Liquid air energy storage (LAES) is a promising electricity storage technology that has certain advantages, such as being geographically unconstrained, having high energy density and low maintenance and operational costs. The investigation of the LAES system is in the initial stages of development, thus, a proper evaluation and improvement of the system are important to enhance the competitiveness of this technology. In this thesis, the optimization of various LAES systems is performed by a stochastic search optimization method (Particle Swarm Optimization) to improve their thermodynamic performance, and an economic evaluation of the proposed systems is carried out to test their feasibility.
In a stand-alone LAES system, the cold energy from liquid air regasification is insufficient to liquefy air in the charging part. Thus, the heat transfer efficiency of cold thermal energy recovery cycles is important for the performance of the overall system. In order to explore the improvement potentials for the LAES, different working fluids and configurations are proposed and compared as promising cold thermal energy recovery cycles. The results prove that a dual multi-component fluid cycle has the best performance in terms of exergy efficiency and round-trip efficiency (RTE). The system with only one cold energy recovery cycle has lower RTE due to the large exergy destructions in low-temperature heat exchangers caused by the large temperature differences between the working fluid and air. Organic Rankine Cycles (ORCs) have also been tested as cold thermal energy recovery cycles. However, optimization results indicate that ORCs used in the cold thermal energy recovery system are not producing any work, and only phase change of the working fluid takes place, thus they should not be used.
The compression and expansion sections directly affect the power consumption and production and are therefore critical to the RTE of the system. To improve the performance of the LAES, systems with different number of compression and expansion stages are studied. It is found that the best performance of the LAES is achieved in a process where hot and cold streams have close to parallel temperature profiles in the preheaters of the expansion section. The optimal results show that the highest RTE of 66.7% is obtained when there is a 2-stage compressor and a 3-stage expander in the LAES system.
The incomplete utilization of compression heat from the charging part is another factor that limits the RTE of the LAES system in some configurations. Organic Rankine Cycle (ORC), Absorption Refrigeration Cycle (ARC) and High Temperature Heat Pump (HTHP) are considered to utilize the surplus compression heat in the LAES system. Optimal results indicate that the ORC, the ARC and the HTHP can effectively improve the performance of the LAES system with the available surplus compression heat. The RTE of the LAES-ORC system is increased by 4% with R600a as working fluid. For the optimized LAES-ARC system, the RTE reaches 63.5% with an increased liquid yield of air of 89.6%. In the LAES-HTHP system, the RTE is increased by 7% when the HTHP uses R1233zd as working fluid.
To study the feasibility of the proposed LAES system, an economic comparison of four LAES configurations with a storage capacity of 10 MW / 80 MWh is carried out. The results show that the LAES system with a 3-stage compressor and a 4-stage expander has a lower Levelized Cost of Storage (LCOS) compared to other LAES configurations. However, the system with a 2-stage compressor and 3-stage expander is the most profitable layout for the LAES when the income of the LAES is considered. The comparison between the LAES and other energy storage technologies indicate that the LAES system has a better economic performance than Li-ion and Pb batteries, but LAES cannot compete with Pumped Hydroelectric Energy Storage (PHES) and Compressed Air Energy Storage (CAES). | en_US |
