Compressed Air Energy Storage (CAES), a technology capable of large-scale energy storage (>100MW), has already been implemented commercially in industry. However, the round-trip efficiency (RTE) of existing commercial CAES plants leaves room for significant enhancement. CAES systems hold an important role in balancing electricity supply and demand, especially as they can be integrated with renewable energy sources to overcome their inherent intermittency. This thesis aims to explore strategies to enhance the RTE of CAES, and to examine its design, operation and cost reduction. This investigation was achieved through process simulation and optimisation.
Firstly, a newly combined cooling heating and power (CCHP) system that integrates a CAES system, organic Rankine cycle (ORC) and single-effect absorption refrigeration system (ARS) using LiBr/H2O is proposed in this study. The ORC can operate during both the charging and discharging process. The waste heat was recovered from the recuperator in CAES, thereby improving its RTE. Steady-state process models of the CCHP system were developed in Aspen Plus® V12 and validated individually. Process analysis was undertaken using the validated models to assess the effect of six organic working fluids of the ORC and several key parameters (inlet mass flowrate of pump in ARS, inlet temperature of combustion chamber 1, different working fluids for ORC, ORC turbine inlet pressure, inter-cooler temperatures, and compressor’s inlet temperatures) on system performance. Results show that the RTE of CCHP (using R290) and overall exergy efficiency of the CCHP system are 67.6% and 51.21%, respectively. Among the factors examined, the inlet temperature of the compressor and the inlet temperature of the combustion chamber are the most decisive parameters influencing the system's performance.
Additionally, a multi-objective optimisation was conducted to maximise the RTE and minimise the total investment cost per output power (ICPP) of the CCHP (including CAES, ORC and single-effect ARS using LiBr/H2O). The findings reveal that the optimised CCHP system has advantages with a significantly enhanced efficiency reaching 68.38% RTE (increased by 0.78%) and a cost-efficiency improved to 0.20 $/kWh ICPP (decreased by 2.68%).
In another configuration of CCHP (including CAES, ORC and different types of ARS), the ORC was designed to operate exclusively during the charging process, while the ARS functioned solely during the discharging phase. Under these design conditions, the low-grade waste heat from the flue gas was completely supplied to ARS. The CCHP system integrated with different types of ARS (i.e. different effect and different working medium) was compared. The results indicate that the double-effect ARS performs better than the single-effect ARS with the same working medium. The lithium bromide/water (LiBr/H2O) is more suitable than ammonia/water (NH3/H2O) as a working medium. The double-effect ARS LiBr/H2O has the better performance, which can produce 206 MW of electrical energy, 7.26 MW of heating, and 27.28 MW of cooling capacity (COP 1.36). The levelized cost of electricity (LCOE) of the CCHP-3 system is 31.01 $/MWh, and the payback period is 12.9 years.
