Rui Wang, Gequn Shu, Hua Tian, Xuan Wang* 2019.09.11 THE DYNAMIC PERFORMANCE OF A CO2 MIXTURE TRANSCRITICAL POWER CYCLE FOR WASTE HEAT RECOVERY OF INTERNAL COMBUSTION ENGINE Rui Wang, Gequn Shu, Hua Tian, Xuan Wang* 2019.09.11 State Key Laboratory of Engines (SKLE) Tianjin University, China

Outline 1. Background 2. Dynamic model 3. Dynamic performance and control strategy 4. Conclusion and future work

Background 2 Internal combustion engine plays an important role in heavy-duty trucks. BTE: 40% Friction: 5% Cooling: 25% Exhaust: 30% Total energy:100% Fuel Fuel Waste heat energy Waste heat recovery is an important way to improve thermal efficiency and reduce oil consumption.

CO2 transcritical power cycle Background 3 Character of ICE waste heat in truck Waste heat of ICE Cooling water temperature 90˚C± Exhaust temperature 200˚~700˚C（1-2s） Exhaust pressure 0.1~0.5MPa（1-2s） Multiple energy quality High exhaust temperature Transient CO2 transcritical power cycle (CTPC) Advantages of CTPC Recover waste heat of exhaust and cooling water at the same time Direct heat transfer with exhaust, owing to high decomposition temperature Quick dynamic response speed natural, non-toxic, harmless, clean, and easy to produce

CO2 mixtures is worth of studying Background 4 Disadvantages of CO2 owing to high critical temperature High pressure Low condensation temp Low efficiency Advantages of CO2 mixtures 1. Reduce operating pressure and potential safety hazards 2. Reduce the requirement of cold source, owing to raising condensing temp 3. Improve cycle efficiency CO2 mixtures is worth of studying Picture comes from: Thermodynamic analysis and performance optimization of transcritical power cycles using CO2-based binary zeotropic mixtures as working fluids for geothermal power plants[J]. Applied Thermal Engineering

Background 5 The working condition of engine changes frequently In order to insure the safety and efficiency of system under all conditions, it is significance to study dynamic performance and control strategy of CO2 mixture CTPC system.

Dynamic CTPC model 6 Basic CTPC: Preheater and Reheater-CTPC: Only recover energy of exhaust Basic dynamic performance research Preheater and Reheater-CTPC: Recover the energy of exhaust and jacket water Control strategy comparison

CTPC model for waste heat recovery 7 The heat exchanger models of preheater, gas heater, and reheater are built with Finite volume (FV) method. The condenser model is built with Moving boundary (MB) method and coupled with a receiver. The static models are used for pump and expander due to the fast response speed. Pump Expander

CTPC model validation 8 Experiment bench built by our group B-CTPC with CO2 as working fluid B-CTPC with CO2 mixture (CO2/R123a 0.7/0.3) PR-CTPC with CO2 mixture (CO2/R123a 0.7/0.3)

Dynamic performance of B-CTPC system 9 Dynamic response speed of different proportion of mixtures Dynamic performance of B-CTPC system 9 Step change in exhaust temperature The dynamic response speed of the system increases as the proportion of refrigerant decreases. Step change in exhaust mass flow rate The dynamic response speed is valued by settling time, constant time and peak time 详细描述了一次能源通过原动机和余热回收技术向各类负荷的流动，并且能流中有详细的量化数值。同时也有描述和其他能量来源的交互，比如从电网买和卖多少电，从光伏获得多少电。

lower pump speed as the proportion of refrigerant increases. The offset of the optimal pump speed Dynamic performance of B-CTPC system 10 The optimal net power output and thermal efficiency offset to the direction of the lower pump speed as the proportion of refrigerant increases. 详细描述了一次能源通过原动机和余热回收技术向各类负荷的流动，并且能流中有详细的量化数值。同时也有描述和其他能量来源的交互，比如从电网买和卖多少电，从光伏获得多少电。

for designing the PR-CTPC system Design of the PR-CTPC system 11 Three-step work for designing the PR-CTPC system Typical high way speed and torque time percentage distribution Design point of engine working condition Most frequent running The relationship between the optimal variables and targets Optimization of the system design parameters Variables: operating pressure, turbine inlet temp, proportion Obj: output power The model validation against experimental data Validation against the experimental data

Optimal points of net power output Control strategies Control strategy for PR-CTPC 12 Constant turbine inlet temperature by pump control Optimal points of net power output Constant pressure by valve control Optimal pump speed control In each region the optimal pump speed is similar The power slightly changes around the optimal pump speed Engine operating conditions in the same region can correspond to the same optimal pump speed Design point

Dynamic performance under slow step change conditions Control strategy for PR-CTPC 13 Dynamic performance under slow step change conditions Turbine inlet temperature Operating pressure Net power output The optimal control strategy always keeps the largest output The constant temperature control outputs the least power above the design conditions, while the constant pressure control outputs the least power below the design conditions

Control strategy for PR-CTPC 14 Dynamic performance under transient change conditions Turbine inlet temperature Operating pressure Net power output The optimal control is failed! The variation of the engine working condition is too fast so that the system basically works in unsteady state, so the controller cannot make the system track the optimal steady state at any time

Conclusion 15 The FV method and MB method are suitable for modelling a power cycle with CO2 mixtures as the working fluid, the simulation results are in good agreement with the measurement. The dynamic response time of CTPC system increase as the proportion of refrigerant increases. The optimal net power output offset to the direction of the lower pump speed as the proportion of refrigerant increases. The optimal and constant temperature control strategy works well under slow step change conditions, while the control strategy is failed under transient change conditions.

Please criticize and correct! Thank you! Please criticize and correct! wangruier@tju.edu.cn.