04.03.2013 Aditya Poudyal

1. Electrical equivalent modelling Electrochemistry Redox couples Stack design Elecrolyte flow circuit optimization Simulation Electrode and membrane design Electrical interfacing Basic chemistry and material science Redox flow battery Scale up, structural and operation optimization of flow geometries Renewable energy Distributed generation Cost Modlellng optimization , and simualtion Grid System science Energy storage Electric vehicles Trasients phenomena 04.03.2013 Aditya Poudyal

2. Electricity value chain Fuel/Energy Source Generation Transmission Distribution Coal Nuclear Hydro Industry households Office buldings ..... Traditional way: Regulated utility, bundled functions. Energy Storage Renewables (Wind, Solar, ) Fuel/Energy Source Generation Transmission Distribution Coal Nuclear Hydro Industry households Office buldings ..... • Unbundled services • Unbundled prices • New service strategies • Privatized services Distributed Generation

3. Electricalenergystoragealongelectricityvaluechain 04.03.2013 Aditya Poudyal

4. Energy storage 04.03.2013 Aditya Poudyal

5. Candidates for grid storage (Electrochemical) • Liquid metal battery • Lithium Ion Battery • Lithium Ion Battery • Sodium sulfur • Sodium metal chloride • Nickel cadmium • Flow batteries • But they are not meeting the following challenges: • Un commonly high power • Long service lifetime and • Super low cost 04.03.2013 Aditya Poudyal

6. Comparison table for various storage systems

7. Vanadium Table: Stadard potential of vanadium couple s at in aqueous solution at 250O • Discovered in 1801 by a Spanish minerologist Andres Manuel del Rio • Named it after the Scandinavian goddess of beauty Vanadis. • Rediscovered in 1830 by Swedish chemist Nils Gabriel Sefstrom • In 1867 isolated in nearly pure form by Roose by reducing its chloride with hydrogen. • Steel grey metal which exists in number of different oxidation states i.e. -1, 0, +1, +2, +3, +4, and +5 04.03.2013 Aditya Poudyal

8. 1. No problems of cross contamination. 2. High charge and voltage efficiency >> fast kinetics of the vanadium redox couples. 3. Low rate of gas evolution during charge rates associated with rapid charging cycles. 4. ”No memory effect” & ”Can be over charged and deeply discharged” without doing permanent damage to the electrolyte and the cells. Why all Vanadium? 5. Reusability of electrolyte >> Long cycle life 5. Fast response 6. Modularity 6. Safe operation Challenges 1. Specific energy density 2. At high molar concentration precipitation occurs in th V5+ electrolyte at tempertaure above 40oC and solid vanadium oxides in V2+ or V3+ solution below 10oC. 04.03.2013 Aditya Poudyal

9. Components of cell stack OUT End plate electrode Bipolar electrode Membrane End plate electrode Positive electrolyte Negative electrolyte IN 04.03.2013 Aditya Poudyal

10. ELECTROCHEMISTRY Rxn occurs between electrolytes No electrodeposition Electrolytes are stored in external tanks and circulated through the stack. Simultaeneous reaction occues at the both side of electrolyte Electrical balance is maintained by proton migration across membranes. Can be operated under the temperature range of 10-(35)40oC. Discharge: Electrons are removed from Anolyte and trasnferred to the Catholyte via external circuit. 04.03.2013 Aditya Poudyal

11. Vanadium concentrations during battery operation 04.03.2013 Aditya Poudyal

12. Electrolyte preparation • Based on the the electrolysis of Vanadyl Sulphate. • Catholyte is obtained from the electrolytic oxidation of VOSO4 solution and anolyte from the elecrolytic reduction. 04.03.2013 Aditya Poudyal

13. Electrolyte stability • Depends upon • temperature, • the vanadium concentration, • the suplphric acid concentration and • on the SOC. • At higher temperature • Catholyte precipitaion at fully charged state. • But not irreversible >> dissloves when discharging • Lower temperature • V4+, V3+ and V2+ start to precipitate. • Slows the rates of the reactions at the electrodes; operation at 0oC could result iin significantly slower reaction rates. • Increasing the stability • Use of inhibitors • Dispersion: decrease the strength of attraction forces betn the particles • Comlexing: forms new complexes with one of the ion involved in precipitation • Threshold: inhibit the precipitation of certain compunds • Use of heat treatment. • Boil the electrolyte for few hours to remove the precipation process. 04.03.2013 Aditya Poudyal

14. Electrical equivalent circuit • R reactionand R resistivecompise the internal losses, reaction kinetics, mass transport reisistance, membrane resistacne, solution resistance, electrode resistance and bipolar resistance. • Rfixedloss represent the parasitic losses • Ipump stands for the power consumption by recirculation pump, system controller, and power loss from cell-stack-by pass. • Celectrodes represnet the transie component associated with the electrode capacitance. 04.03.2013 Aditya Poudyal

15. VRB discharging and charging cycles: Charging take longer time than to discharge it. • Ipump soars dramatically as the SOC drops >> more catholyte and anolyte are required to provide the same power when the SOC lowers • Stack voltage is higher than the output volatge when dscharging , stack volatge is smaller when charging and it implies internal losses. • Efficiency decreases by 5 % when SOC is 0.2%. 04.03.2013 Aditya Poudyal

16. Trasients and response time Transients are essential because of the importance of the system ability to respond to the fast change. Trasient behaviour is related to the electrode capacitance as well as concentration depletion close to electrodes. Worst case transients were considered the operation is switched from -65A and then back.Figure shows that it takes 0.045 seconds for battery voltag e to reach steady state 04.03.2013 Aditya Poudyal

17. Equilibrium Potential The equilibrium voltage corresponds to the sum of equlibrium potential of each cell in stack. Equilibrium potential is given by the Nernst equation and depends upon vanadium species concentration and the proton concentrations. is standard potentials and it is important parameter in nersnt equation as it expresses the reaction potentials at standard conditions. R is gas constant T is temperature F is Faraday constant 04.03.2013 Aditya Poudyal

18. Standard potential Standard Gibbs free enthalpy of reaction which represents the change of free energy that accompanies the formation of 1M of a substance from its component elements at their standard states: 250C, 100kPa, and 1M Where the standard reaction enthalpy is the difference of molar formation enthalpies between the products and reagents and the standard reaction of entropy is the differnce of molar formation entropies between the products and the reagents An ideal state where the battery is at standard conditions: Vanadium species at a concentration of 1 All acticity coefficients equal to 1 and temperature 250C. Can be detemrined from the thermodynamic principle called the Gibbbs free enthalpy, the conservation of energy and empirical parameter that can be found in electrochemical tables. 04.03.2013 Aditya Poudyal

19. Standard potential = -155.6kJ/mol =-121.7 J7mol.K Inserting thermodynamical data the standard reaction enthalpy ∆H0r becomes : The conservation of energy relates the change in free energy resulting from the transfer of n moles of electrons to the difference of potential E: Therefore standard potential can e written as ”The standrad potentia is 1.23V at 250C.” and similarly the standard reaction entropy ∆S0r 04.03.2013 Aditya Poudyal

20. Characteristic curve of the equilibrium potential E for a single cell 04.03.2013 Aditya Poudyal

21. Electron exchange rate 04.03.2013 Aditya Poudyal

22. Proton concentrations 04.03.2013 Aditya Poudyal

23. Internal loss • When current starts to flow • Cell Voltage ≠ Nernst Voltage • The losses are called overpotentials • represents the energy required to force the redox reaction to proceed at required rate • Ohmic loss occurs in electrodes, the bipolar plates and the collector plates. • Ionic loss occurs in electrolytes and membranes • Electrode phenomena and are associated with • the energy required to initiate the charge transfer and • Concentration difference between bulk solution and electrode surface 04.03.2013 Aditya Poudyal

24. Efficiencies Enegy released during discharge and energy supplied during charge • Defined for charge and discharge cycle for constant currnet. • Is meaure of ohmic and plarization losses during the cycling. • Can be maximized by contact, electrode, electrolyte and membrane resistance • By using an electrode material with good electro-catalytic properties for the reactions. • Ratio of the charge withdrawn from the system during the discharge to the charge supplied • Can be caused by side reaction such as oxygen and hydrogen evolution • Cross mixing of electrolyte through membrane due to ion transfer • Unbalanced flowrates of the electroly 04.03.2013 Aditya Poudyal

25. Charge and discharge at costant currnet Efficiencies at various currents. The cycle starts at 2.5% SOC, and charged upto 97.5% SOC and again discharged to 2.5%

26. Cost breakdown 04.03.2013 Aditya Poudyal

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28. Thank you for the attention !!! 04.03.2013 Aditya Poudyal