Holistic modeling and optimization of vanadium redox flow batteries

Transcription

1 Holistic modeling and optimization of vanadium redox flow batteries Sebastian König Kraftwerk Batterie Münster, March 25th 214 INSTITUTE OF ELECTRICAL POWER SYSTEMS AND HIGHVOLTAGE TECHNOLOGY (IEH) KIT University of the State of BadenWuerttemberg and National Research Center of the Helmholtz Association

2 Why model based system analysis? Vanadium Redox Flow (VRF) technology is very promising for large energy storage systems Grid Power and energy independently scalable. Excellent cycle stability. No self discharge. But: VRF battery systems are very complex. DCbus Power transformer AC/DCconverter DC/DCconverter Power conditioning System (PCS) I stack (t) U stack (t) Pump 1 Q(t) e (discharge) e (discharge) Q(t) Pump 2 e (charge) e (charge) V 5 /V 4 KatholyteTank V 5 discharge V 4 charge V 5 V 2 discharge V 3 charge V 2 Battery cell/stack V 2 /V 3 AnolyteTank 2 March 25th 214 S. König Holistic modeling and optimization of vanadium redox flow batteries

3 Aims of model based system analysis Holistic modeling approach to include influences, the subsystems have on each other. Identify optimal hydraulical and electrical system design Identify optimal system operation mode Suppress shunt currents 3 March 25th 214 S. König Holistic modeling and optimization of vanadium redox flow batteries

4 Introduction of holistic modeling approach Electrochemical model: Implementation of Nernst equation with Simulink blocks Hydraulical model: Implementation with Simscape elements Power conditioning system: Separate simulation with high resolution Efficiency considered using lookup tables in holistic model Control system: Implementation of charge/discharge controller Implementation of electrolyte flow controller 4 March 25th 214 S. König Holistic modeling and optimization of vanadium redox flow batteries

5 Flow rate / l/s Concentration / mol/l Voltage / V Current / A SOC / 1 Power / kw Simulation results 5 P Bat P PCS P Sys Voltage Current Q ges Q stack c V Time / h 5 March 25th 214 S. König Holistic modeling and optimization of vanadium redox flow batteries

6 Model based optimization: PCS Question: How many battery stacks should be connected in series? More stacks in series: Higher system voltage Higher efficiency of PCS But: Higher losses caused by shunt currents. 6 March 25th 214 S. König Holistic modeling and optimization of vanadium redox flow batteries

7 Excursion: Shunt currents All battery electrodes are connected by the liquid electrolyte Electrolyte is a bad insulator (approx. 5 S/m) Shunt currents between different cells and stacks Electrical connection Cell 2 Cell 2 Shunt currents Cell 1 Stack 1 Cell 1 Stack 2 Elektrolyte piping Tank 7 March 25th 214 S. König Holistic modeling and optimization of vanadium redox flow batteries

8 Efficiency PCS / % Round trip efficiency / % Model based optimization: PCS Question: How many battery stacks should be connected in series? More stacks in series: Higher system voltage Higher efficiency of PCS But: Higher losses caused by shunt currents 95 Efficiency PCS 75 System efficiency stack in series 2 stacks in series 3 stacks in series 6 stacks in series P PCS /P r / % 5 1 stack in series 2 stacks in series 45 3 stacks in series 6 stacks in series P bat /P r / % In this case: Series connection of two stacks is optimal March 25th 214 S. König Holistic modeling and optimization of vanadium redox flow batteries

9 Model based optimization: Shunt currents R C UC2 UC1 Question: How can system design variations lower shunt currents? Shunt current model: R C UC2 R C R C R C R M R M R C R C UC1 Cell 2 Cell 1 Stack 1 R C R C R C R M R M R C R C Cell 2 Cell 1 Stack 2 R Pipe R Pipe R Outlet R Tank R Inlet 9 March 25th 214 S. König Holistic modeling and optimization of vanadium redox flow batteries

10 Shunt currents: Possible approaches Design variations in hydraulic circuitry Narrower pipes reduce shunt currents but increase pump losses Assignment of stacks to different tanks More complicated piping, higher pump losses Bigger cell membrane area Problems regarding manufacturing Hydraulical series connection of cells and/or stacks Proper control of electrolyte flow more difficult 1 March 25th 214 S. König Holistic modeling and optimization of vanadium redox flow batteries

11 Roundtrip efficiency / % Energy for pumps / Wh Maximal pump power / kw Shunt current / A Mean power loss / W Optimization results for main pipe diameter mm 1mm 4mm Pump Energy Max. pump power Time / s Main pipe diameter / mm Optimal main pipe diameter: d Main = 5 mm 11 March 25th 214 S. König Holistic modeling and optimization of vanadium redox flow batteries

12 Summary Model based system analysis can help to decrease the costs of VRFbatteries by predicting the optimal system design. Shunt currents are a technical issue that prevent higher system voltages and therefore require more expensive special PCS. Outlook Presented approaches for shunt currents suppression are currently evaluated. Model validation using a prototype would be desirable. 12 March 25th 214 S. König Holistic modeling and optimization of vanadium redox flow batteries