1. 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*
State Key Laboratory of Engines (SKLE)
Tianjin University, China

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

3. 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.

4. 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

5. 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

6. 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.

7. 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

8. CTPC model for waste heat recovery7
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

9. 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)

10. 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。

11. 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。

12. 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

13. 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

14. 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

15. Control strategy for PR-CTPC14
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

16. 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.

17. Please criticize and correct!
Thank you!
Please criticize and correct!