Application Study And Dynamic Analysis Of Compressed CO₂ Energy Storage System

Excessive use of fossil fuels emits a large amount of CO₂in the environment,and excessive emission of CO₂ has led to global climate change.In order to mitigate the climate change,reducing carbon emissions is urgent,and the use of environmentally friendly renewable energy is one of the key ways.However,due to the instability and unpredictability,such intermittent renewable energy poses serious security and reliability problems to the power grid.To tackle such challenges,energy storage technologies（ESTs）are playing key roles,such as the compressed CO₂energy storage system（CCES）technology,which can be used for large scale energy storage.However,the high cost is still the main barrier to the wide application of large-scale ESTs.This thesis proposed to integrate CCES with oxy-coal combustion power plants.CO₂captured from the power plant can be used as working fluid in CCES,which can eliminate the need of low-pressure CO₂ storage.Consequently,the cost of CCES can be reduced.To investigate the techno-economic feasibility of the oxy-coal combustion power plant integrated with CCES（Oxy＿CCES）,a simulation model was developed in Aspen Plus,and the electricity tariff of Time of use（TOU）was used for assessing the economic benefit.In order to demonstrate its competence,Oxy＿CCES was also compared with the oxy-coal combustion power plant without energy storage（Ref）,and the oxy-coal combustion power plant integrated with liquefied O₂ energy storage（Oxy＿O₂）.The simulation results show that,Oxy＿O₂ consumed more electricity in the off-peak price period but generated more electricity in the peak price period than Oxy＿CCES,both led to lower net electricity efficiencies,which were 30.1%and 31.7%respectively,compared to that of Ref,which was 34.4%.Two cases were considered for the economic evaluation:Case 1-retrofitting an existing oxy-coal combustion power plant and Case 2-building a new oxy-coal combustion power plant.For Case 1,due to the investment cost of CCES was lower than liquefied oxygen energy storage system,within 25 years of lifecycle,Oxy＿CCES of net present value will be higher than that of the Oxy＿O₂,at the same time,payback time of investment cost was less.Under the 10%discount rate,the investment payback time was4 years.For Case 2,the electricity cost was lower due to the integrated of CCES,compared to Oxy＿O₂.As the operating conditions vary during the charging and discharging processes of CCES,it is of great importance to study the dynamic characteristics of the operation,which can clearly affect the energy efficiency of CCES and the economic performance.A dynamic model was developed by using Matlab and Aspen Hysys to study the dynamic operation of CCES.To validate the model,a testing system of CCES was built.After the model was validated,the influences of key operating parameters,such as pressure and pressure ratio,were investigated.The results show that the change of inlet and outlet pressures of the compressor and expander caused by the fixed volumes of storage tanks can have a great influence on the power consumption in charging and generation in discharging.As the charging process went on,the pressure ratio of the compressor gradually increased,leading to decreasing volumetric efficiency of the compressor.In the initial stage of charging,the mass flow of CO₂ was controlled through the valve to keep stable,while the change of pressure ratio led to the change of compressor inlet CO₂mass flow,which decreased in the later stage significantly.the power consumption of the compressor decreased gradually with the decrease of mass flows.During the discharge process,the output power of the expander decreased obviously due to the gradual decrease of the pressure ratio.The round-trip efficiency of the system was 25.7%,the round-trip efficiency of the steady-state system is20.4%.