# Operational Experience of 5 kW/5 kWh All-Vanadium Flow Batteries in Photovoltaic Grid Applications

^{1}

^{2}

^{*}

## Abstract

**:**

## 1. Introduction

## 2. Methodology

_{out}/(Wh

_{in}+ Wh

_{pumping}) considering Wh

_{out}as the energy provided by the battery in discharge. The battery was then operated using different current levels to measure the power requirements for the centrifugal pumps.

## 3. Battery Characteristics

^{−2}(equivalent to 100 A). Auxiliary equipment and the hydraulic system have been designed and installed allowing us to achieve the most adequate battery operative conditions (i.e., variable flow rate, electrolyte conductivities and redox potential monitoring…). Attached to these current collectors were porous graphite felts with an active area of 1800 cm

^{2}(36 cm in width and 50 cm in length) for each felt electrode with 7.0 mm thickness (20% compression) and a stated surface area of 0.4 m

^{2}g

^{−1}. The membrane was Nafion type (Model 117, DuPont, Midland, MI, USA) meanwhile the flow frame used in the stack consists in a zigzag configuration with primary and secondary paths constituting multi-distribution channels. The design details for this stack have been extensively outlined by Wu et al. [11].

_{2}or O

_{2}evolution, or vanadium precipitation arising from a drop in electrolyte acidity or damage to the electrode materials [12].

## 4. System Cyclability

_{OCV}− V

_{DC})/I. The internal resistance is primarily due to Ohmic resistance in the cells while activation losses and other effects play a minor role. The resistance can also change by a few percent with changes in temperature. In general, the resistance is expected to increase as the cells degrade, leading to lower efficiencies. Figure 5 shows the resistance (and capacity) as function of the number of cycles. Results indicate that the resistance does not seem to increase over the 28 cycles of operation. Therefore, the capacity loss can only be due to the cross mixing of species through the membrane during the 3 months of operation.

## 5. Polarization Tests

^{−1}(equivalent to a flow factor of 2 compared with 100 A-20–80% SOC). Figure 6 and Figure 7 show the IR-free kinetics of the RFB under current density and power pulses. This stack exhibits low Ohmic resistance; 1.5–1.7 ohm cm

^{2}measured in terms of the voltage drop at different current densities, and 1.7–1.8 ohm cm

^{2}in terms of the voltage vs. current slope.

^{−2}/1.1 kW m

^{−2}.

## 6. Peak Power Tests

^{−2}. Figure 8 illustrates this sequence of steps. The same current values are used at all SOC levels.

_{IRFree}= V − IR

_{IRFree}

^{2})/R

_{IRFree}− Discharge Voltage Limit)/R

_{MAX}× (V

_{IRFree}+ R × I

_{MAX})

^{−2}as current density. This reactor showed a ohmic resistance increase of 25% when the electrolytes were at 13% SOC compared with the highest SOC, probably due to the limited active species availability at such a low state of charge.

_{0}is standard reduction potential, R is the gas constant, T is the temperature and F is the Faraday constant.

^{−2}maintaining a reasonable voltage during the 30 s, performance a bit beyond the typical values of Vanadium redox flow batteries (1 kW m

^{−2}) [17], although commercial systems utilizing high performance electrodes reported values of 1.5 kW m

^{−2}[18] and achieving 5.57 kW m

^{−2}in the case of the highest instantaneous value at 60% SOC obtained so far [19].

## 7. Fast Charge Tests

- 1)
- After fully charging of the battery (100 A CC-CV up to 64 V, cut-off current of 50 A), it was discharged to 35% SOC at a nominal constant current rate (800 A m
^{−2}). - 2)
- Charge the battery with constant current at the selected charge rate until the maximum voltage of 1.6 V
_{cell}is reached. - 3)
- Fully discharge the battery at a nominal rate to determine the amount of the recharge (Ah) available for use.

^{−2}. As the charge rate was increased, the amount of real charge available for use was progressively lower from 42% SOC at 0.3 kW m

^{−2}to 21% SOC at the highest charge rate 1.6 kW m

^{−2}.

## 8. Electrochemical Energy Efficiency and Coulombic Efficiency Optimizing the Flow Rate

^{−1}negative electrolyte).

_{reactant}) and, thus, the pumps can work below the standard flow rate defined by the manufacturer (900 L h

^{−1}). Meanwhile, at the top of charge, the vanadium species that act as the active material are exhausted and the pumps should be forced to work above the standard flow rate to continue supplying reactant electrolyte to the circuit. Presumably this operational mode saves overall energy.

#### 8.1. Charge-Discharge Cycles at Constant Flow Rate

_{global}) is observed at low current densities. This is due to the fact that at lower currents, excessive pumping (i.e., flow rates far above the stoichiometric) is applied which increases the energy required to operate the pumps—a parasitic loss for the battery. The global efficiency (EE

_{global}) also shows low efficiency at high current densities, following the voltage efficiency (VE) trend, due to concentration overpotentials which consequently lower system efficiency.

^{−1}. The highest values of energy density (18.0–18.9 Wh L

^{−1}) were obtained at relatively low current densities; 37.5–50 A (300–400 A m

^{−2}).

^{−2}. From Table 2, it can be seen that in this zone, the coulombic, voltage, energy and global energy efficiency was improved to 91.1%, 80.9%, 73.3% and 64.0%, respectively, at a current intensity of 75 A (600 A m

^{−2}), and a maximum average output power of 3.7 kW (0.7 kW m

^{−2}) was determined.

#### 8.2. Charge-Discharge Cycles at Variable Flow Rate

_{reactant}) in such a way that the following flow rate vs SOC curves at different currents can be devised when a flow factor equivalent to 5 is used.

^{−1}flow rate equivalent to constant flow procedure is also shown.

^{−1}flow rate equivalent to the constant flow procedure is also included for purposes of comparison. The plot in Figure 13, describes a model of system behavior. There is a maximum operable flow rate (plotted at 1200 L h

^{−1}) determined by the manufacturer which the model can surpass but along which experimental flow rates may only taper. Therefore, the high output capability at low SOC suggested in the model below has not yet been achieved.

_{reactant}concentrations begin to run low, i.e., from the second half of the process (charge or discharge steps) until the voltage cut-off. When compared to the constant flow rate cases, adapting the flow rate reduces concentration overpotential, thus allowing lower voltage in operation in charge and higher voltage output in discharge, which increases the VE. This effect is particularly significant at high currents, when the algorithm proposed is demanding large flow rates, and this is consistent with the evolution in voltaic efficiency.

_{global}is observed over the entire range of current intensity, which is an indication of the flexibility provided by this mode of operation.

^{−1}) indicating that under this control configuration the system is versatile, charging and discharging in a wide range of current without compromising performance. A similar approach was followed by Li et al. [21] However, energy densities slightly below those of the constant flow rate profile were obtained. Under the conditions studied, the energy density obtained is 30% lower when compared with constant flow rate profile.

^{−2}current densities, a relatively high global energy efficiency can be maintained. From Table 4, it can be seen that in this zone the coulombic, voltage, energy and global energy efficiency was on average 91.6%, 79.8%, 73.1% and 65.0%, respectively, with a maximum average output power between 3.0 and 5.4 kW (0.6–1.1 kW m

^{−2}).

#### 8.3. Comparison Constant Versus Variable Flow Rate

^{−1}labels indicate the output energy density at a given applied current in order to keep track of output energy as it compares with efficiency.

^{−1}one may cause a slight over pumping of the system, where the extra reactant does not need to be employed. By utilizing a variable flow rate method, the system can improve slightly the Coloumbic efficiency and save energy by applying an adequate lower flow rate at lower current densities.

^{−1}) as they were in both of these trials, the high current density phase could have used even higher flow rates to further adapt flow rates to increased efficiency.

_{global}efficiencies. Additionally, the data for energy values show that the variable test energies supplied did not lag far behind those of the constant flow rate test. However, a trade-off between pumping consumption and concentration overpotential in the stack needs to be adjusted further in order to optimize the battery performance.

## 9. Conclusions

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

## References

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**Figure 2.**5 kW all-vanadium RFB demonstrator with the specifications given by the supplier Golden Energy Fuel Cell Co.,Ltd.

**Figure 3.**Voltage and current profiles on charging and discharging across the charge levels 0–100% at 100 A.

**Figure 6.**The effects of charge–discharge current pulses on the IR-free kinetic region for the RFB in charge at (

**a**) 25% SOC, (

**b**) 50% SOC and (

**c**) 75% SOC, and in discharge at (

**d**) 25% SOC, (

**e**) 50% SOC and (

**f**) 75% SOC.

**Figure 7.**The effects of charge– discharge power pulses on the IR-free kinetic region for the RFB in charge at (

**a**) 25% SOC, (

**b**) 50% SOC and (

**c**) 75% SOC, and in discharge at (

**d**) 25% SOC, (

**e**) 50% SOC and (

**f**) 75% SOC.

**Figure 9.**Peak power capability test. (

**a**) Open circuit voltage vs. SOC and (

**b**) power capability vs SOC.

**Figure 11.**Efficiencies vs current: coulombic efficiency; voltage efficiency; stack energy efficiency; battery energy efficiency including pumping energy losses, at constant flow rate (900 L h

^{−1}).

**Figure 13.**Variable flow rate profiles during 100 A discharge applying; Constant Flow Factor (FF = 5) and variable Flow Factor (each Flow Factor is labelled next to the points).

**Figure 14.**Efficiencies vs current: coulombic efficiency; voltaic efficiency; stack energy efficiency; battery energy efficiency including pumping energy losses, at variable flow rate.

**Figure 18.**Battery energy efficiencies including pumping energy losses vs current at variable flow rate and constant flow rate.

**Figure 19.**Battery energy inefficiencies including pumping energy losses vs current at variable flow rate and constant flow rate.

Days | Capacity (Ah) | Energy (Wh) | Pumping (Wh) | Wh L^{−1} | R (Ω cm ^{2}) | η_{Ah} (%) | η_{Wh} (%) | η_{V} (%) | η_{Total} (%) | V_{Ch} (V) | V_{Dch} (V) | Power Output (kW) | ||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|

Ch | Dch | Ch | Dch | |||||||||||

Day 1 | 115.5 | 107.1 | 7177 | 5333 | 1015 | 17.8 | 1.69 | 92.7 | 74.3 | 80.2 | 65.1 | 62.1 | 49.8 | 5.0 |

Day 88 | 86.5 | 79.3 | 5405 | 3982 | 621 | 13.3 | 1.41 | 91.7 | 73.7 | 80.3 | 66.1 | 62.5 | 50.2 | 5.0 |

Current Intensity (A) | Average Output Voltage (V) | Average Output Power (kW) | CE (%) | VE (%) | EE (%) | EE_{global} (%) | EE Losses to Pumping Consumption (%) |
---|---|---|---|---|---|---|---|

25 | 53.2 | 1.3 | 85.7 | 91.3 | 78.2 | 56.6 | 21.6 |

37.5 | 52.2 | 2.0 | 87.9 | 88.1 | 77.5 | 61.2 | 16.3 |

50 | 51.3 | 2.6 | 89.5 | 85.6 | 76.6 | 63.6 | 13.0 |

62.5 | 50.2 | 3.1 | 90.7 | 82.7 | 75.0 | 64.0 | 11.0 |

75 | 49.5 | 3.7 | 91.1 | 80.9 | 73.7 | 64.0 | 9.7 |

87.5 | 48.5 | 4.2 | 91.4 | 78.6 | 71.8 | 63.6 | 8.2 |

100 | 47.8 | 4.8 | 91.5 | 76.8 | 70.3 | 62.4 | 7.9 |

112.5 | 45.3 | 5.1 | 92.7 | 72.0 | 66.7 | 59.7 | 7.0 |

SOC Range (%) | Flow Factor in Charge | Flow Factor in Discharge |
---|---|---|

100–90 | 0.2 | 6.6 |

90–80 | 6.0 | 6.6 |

80–70 | 6.0 | 5.8 |

70–60 | 6.0 | 4.8 |

60–50 | 6.4 | 4.8 |

50–40 | 6.4 | 4.8 |

40–30 | 6.4 | 4.6 |

30–20 | 6.8 | 3.8 |

20–10 | 7.8 | 0.4 |

10–0 | 8.8 | 0.2 |

Current Intensity (A) | Average Output Voltage (V) | Average Output Power (kW) | CE (%) | VE (%) | EE (%) | EE_{global} (%) | EE Losses from Pump’s Consumption (%) |
---|---|---|---|---|---|---|---|

50 | 51.5 | 2.6 | 90.1 | 85.4 | 76.9 | 66.4 | 10.5 |

62.5 | 48.7 | 3.0 | 91.1 | 79.7 | 72.6 | 64.6 | 8.0 |

75 | 49.6 | 3.7 | 91.4 | 80.6 | 73.7 | 65.4 | 8.3 |

87.5 | 49.5 | 4.3 | 91.7 | 80.1 | 73.5 | 65.4 | 8.1 |

100 | 48.9 | 4.9 | 92.1 | 78.8 | 72.6 | 64.7 | 7.9 |

112.5 | 48.8 | 5.4 | 91.1 | 77.4 | 77.4 | 63.3 | 7.2 |

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**MDPI and ACS Style**

García-Quismondo, E.; Almonacid, I.; Cabañero Martínez, M.Á.; Miroslavov, V.; Serrano, E.; Palma, J.; Alonso Salmerón, J.P.
Operational Experience of 5 kW/5 kWh All-Vanadium Flow Batteries in Photovoltaic Grid Applications. *Batteries* **2019**, *5*, 52.
https://doi.org/10.3390/batteries5030052

**AMA Style**

García-Quismondo E, Almonacid I, Cabañero Martínez MÁ, Miroslavov V, Serrano E, Palma J, Alonso Salmerón JP.
Operational Experience of 5 kW/5 kWh All-Vanadium Flow Batteries in Photovoltaic Grid Applications. *Batteries*. 2019; 5(3):52.
https://doi.org/10.3390/batteries5030052

**Chicago/Turabian Style**

García-Quismondo, Enrique, Ignacio Almonacid, Maria Ángeles Cabañero Martínez, Veselin Miroslavov, Enrique Serrano, Jesús Palma, and Juan Pedro Alonso Salmerón.
2019. "Operational Experience of 5 kW/5 kWh All-Vanadium Flow Batteries in Photovoltaic Grid Applications" *Batteries* 5, no. 3: 52.
https://doi.org/10.3390/batteries5030052