Cryogen based energy storage: process modelling and optimisation

Reliable operation of large scale electric power networks requires a balance of generation and end-user. The electricity markets mainly depend on the real-time balance of supply and demand because no sufficient power storage is available at present. As the difference between the peak and off-peak loads is significant, it is very expensive for the power companies to deal with the demand-supply mismatch. The situation is getting more challenging with the increasing use of renewable energy sources particularly wind and solar, which are intermittent and do not match the actual energy demand. This makes the large scale energy storage and power management increasingly important. This thesis studies a Cryogen based Energy Storage (CES) technology which uses cryogen (or more specifically liquid air/nitrogen) as an energy carrier for large scale applications in Supply Side Management (SSM). The aim of this research is to seek the best routes and optimal operation conditions for the use of the CES technology. A systematic optimisation strategy is established by extending the concept of ‘superstructure’ and combining with Pinch Technology and Genetic Algorithm. Based on this strategy a program named Thermal System Optimal Designer (TSOD) is developed to evaluate or optimise both the thermodynamic and economic performances of thermal systems. Three types of CES systems are proposed and optimised for the applications of load levelling, peak-shaving and cryogenic energy extraction. In the load levelling system it is found that the integration of air liquefaction and energy releasing process gives a remarkable improvement of the round trip efficiency. If the expander cycle is used to supply cold energy and the waste heat with a temperature higher than 600K is available, the round trip efficiency attains to 80 - 90% under rather reasonable conditions. Economic analyses reveal that such a CES system is very competitive with the current energy storage technologies if the operation period of the energy releasing unit is longer than 4 hours a day. In the peak-shaving system CES is integrated with Natural Gas Combined Cycle (NGCC) to form oxy-fuel combustion for CO2 capture. The optimisation of such systems gives an exergy efficiency of 70% and electricity storage efficiency of 67% while using helium or oxygen as the blending gas. Economic analyses show that both the capital and peak electricity costs of the peak-shaving systems are comparable with the NGCC which are much lower than the oxy-NGCC if the operation period is relatively short. And the use of helium as the blending gas gives the lowest costs due to the lowest combustion pressure and mass flowrate. A new solar-cryogen hybrid power system is proposed to extract cryogenic energy and to make use of solar radiation for power generation. The system is compared with a solar thermal power system and a cryogen fuelled power system. Thermodynamic analyses and optimisation show that the hybrid system provides over 30% more power than the summation of the power outputs of the other two systems. The results also suggest that the optimal hot end temperature of the heat carrier heated by the solar collectors be about 600K for the hybrid system. Although interesting and promising results are obtained in this study, practical applications of CES technology meet a number of challenges including the dynamic operation and economic optimisation of the system in the simulation aspect and related experimental demonstration for both the key components and the integrated systems.