Thermodynamic System Research And Optimization Based On Supercritical CO₂ Power Cycle For Gas Turbine Waste Heat Recovery

The thermodynamic cycle with carbon dioxide as the work substance is one of the important technical methods to achieve the thermal to power conversion.Supercritical CO₂（S-CO₂）power cycle has the advantages of high efficiency,compactness,flexibility and low cost,etc.The efficiency of S-CO₂power cycle is high,and the heat source temperature is mainly concentrated in the middle and high temperature range.Gas turbine（GT）,as one of the important equipment in the power field,has the high exhaust temperature at the turbine outlet,which contains part of the available exergy energy.Using S-CO₂power cycle can well recover the exhaust waste heat of gas turbine and realize the conversion of thermal energy to electrical energy.Based on the S-CO₂power cycle,this thesis conducts the study of the combined cycle system with the objectives of improving the waste heat utilization of the gas turbine,increasing the net output power,improving the efficiency and reducing the cost.The main contents are as follows:First,the combined SCRC-ORC/TCO₂and SCRC-ARC cycle systems were constructed by using the supercritical CO₂recompression cycle（SCRC）configuration as the topping cycle of the system,and the organic Rankine cycle（ORC）,the transcritical CO₂（T-CO₂）cycle and the absorption refrigeration cycle（ARC）as the bottoming cycle of the system to absorb the low-temperature waste heat in the SCRC,respectively,and the parameters of the systems were analyzed and the optimized performance was compared.Second,based on the single-stage cycle study,a new triple cascade cycle（TC）power generation system consisting of an S-CO₂power cycle coupled with a transcritical CO₂regenerative heat cycle（TCRC）is proposed.Compared with the gas turbine alone,the SCRC-TCRC cascade cycle increases the net output power by 5.67 MW and the thermal efficiency of the integrated GT-TC system is improved by 12.2%.The analysis of the component’s exergy and cost shows that the heat exchanger has a large proportion of the exergy destruction,and the investment cost rate of the turbine,compressor and other power making components is high.Parameter sensitivity analysis shows that both the inlet temperature and pressure of the main compressor have a large impact on the system performance,and CO₂near the critical point can improve the thermal performance of the cycle.The net output power,exergy efficiency and unit energy cost rate were selected as the objective functions for multi-objective optimization of the system,and the optimization results showed that the economic performance was better with the combination of exergy efficiency and unit energy cost rate,and the exergy efficiency was 50.37%and the unitized energy cost was 4.66$/GJ.By comparing with the other two configurations of the system,the SCRC-TCRC cycle system proved to be superiority.Third,a power generation system combining SCRC/TCO₂combined cycle（CC）,compressed CO₂energy storage（CCES）and thermal storage system（CC-CCES）is constructed based on changing the fuel quantity to regulate the operating load of the gas turbine.The effects of different load operating conditions on the performance of the gas turbine under variable flow regulation were investigated.The results show that the thermal performance of the gas turbine decreases at low load operation and the fuel consumption rate increases,leading to a decrease in the economics of the unit as well.The thermodynamic,exergoeconomic performance models and evaluation indexes of the CC-CCES system are established,and the effects of key operating parameters on the system performance are explored.The optimization results showed that the thermal efficiency of the system reached more than 40%,and the exergy efficiency reached more than 65%,with the lowest total product unit cost of 12.45$/GJ.The performance of the system under different operating loads is analyzed and compared.Compared with the SCRC/TCO₂combined cycle system without CCES coupling,the CC-CCES system uses the thermal storage system to store the excess secondary waste heat at high loads,and at low loads,the heat deficit required for the energy release condition is then provided by the thermal storage tank,which improves the performance of the waste heat recovery system at low loads and flexibly balances the energy supply and demand.Finally,based on the way of bypass extraction of gas turbine units to regulate the operating load,a cooling,heating and power cogeneration system（CCHP）consisting of gas turbine cycle（GTC）,compressed air energy storage（CAES）,SCRC,ORC and ARC is proposed.Based on the traditional cascade waste heat recovery system,CAES is used not only to achieve flexible waste heat recovery and load demand regulation,but also CAES effectively stores and makes secondary use of the bypass extracted air from the compressor.The expanded air exhaust temperature is lowered and returned back to the gas turbine unit,improving the thermal performance of the unit.The energy,exergy,economic and environmental analyses of the CCHP system were performed.The parametric study shows that changing the split ratio can realize the distribution regulation of different energy forms and the energy storage pressure ratio has an important effect on the performance of CAES subsystem.The exergy and economic analysis show that the GTC subsystem has the largest percentage of exergy destruction and investment cost ratio.At the lowest operating load,the energy efficiency of the system can reach 75.99%,the exergy efficiency is 45.89%,the unit energy cost is 0.032$/k W-h,and the greenhouse gas equivalent emission is 0.313 kg/k W-h.Compared with the other four conventional cascade waste heat recovery,the proposed CCHP system greatly improves the thermodynamic performance of the system at low load operation and reduces the unit energy cost rate and GHG-equivalent emissions.