Recently, energy storage system (ESS) with carbon dioxide (CO2) as working fluid has been proposed as a new method to deal with the application restrictions of Compressed Air Energy Storage (CAES) technology, such as dependence on geological formations and low energy storage density. A novel ESS named as Compressed CO2 Energy Storage (CCES) based on transcritical CO2 Brayton cycle is presented in this paper. The working principle of CCES system is introduced and thermodynamic model is established to assess the system performance. Parametric analysis is carried out to study the effect of some key parameters on system performance. Results show that the increase of turbine efficiency is more favorable for system optimization and the effect of minimum pressures on system performance is more significant compared with maximum pressures. A simple comparison of CCES system, liquid CO2 system and Advanced Adiabatic Compressed Air Energy Storage (AA-CAES) system is conducted. It is shown that the system efficiency of CCES is lower than that of AA-CAES system but 4.05% higher than that of liquid CO2 system, while the energy density of CCES system is 2.8 times the value of AA-CAES system, which makes CCES a novel ESS with potential application.
Conclusions
It is a new thought to use CO2 to replace air as working fluid of energy storage system. In the present study, a novel system named as Compressed Carbon-dioxide Energy Storage (CCES) is proposed. The system model is established and system performance is investigated based on thermodynamic analysis. The effects of some key parameters on system performance are also evaluated. The main conclusions are summarized as follows:
(1) With appropriate settings of system parameters, CCES system can convert electrical power to heat energy, cold energy and potential energy of CO2 during energy storage process. Thus the system can provide electrical power efficiently without supplements of heat energy and cold energy from the outside during energy recovery process.
(2) The round-trip efficiency of CCES system is 4.05% higher than that of liquid CO2 system, while the energy density of CCES system is 2.8 times the value of AA-CAES system. This implies the application advantages of CCES system to some extent.
(3) Increasing the efficiencies of turbomachinery components can improve system performance and decrease exergy destruction. As the increase in turbine efficiency can also increase energy density of system, the optimization of turbine should be paid more attention.
(4) Compared with maximum pressures, the effect of minimum pressures on system performance is more significant. Meanwhile, both the increase of maximum pressure (pmax,er) and the decrease of minimum pressure (pmin,er) during energy recovery process can improve the system overall performance. Thus pressures during energy recovery process, especially minimum pressure, deserve more attention.
For the moment, this novel CCES system has acceptable RTE and energy density. Parametric analysis signifies that a better system performance could be achieved after further optimization. Besides system efficiency and energy density, economics of energy storage system is also a significant criterion to evaluate system performance. In future studies, thermo economic analysis and related experiments will be carried out to validate the feasibility of this system.
