# Li-Ion Battery-Flywheel Hybrid Storage System: Countering Battery Aging During a Grid Frequency Regulation Service

^{*}

## Abstract

**:**

## 1. Introduction

## 2. Battery Aging Following Frequency Regulation Cycles

_{0}to t

_{2}can be represented by means of two R–C circuits in series. The voltage behaviour in t

_{0}can be modelled by means of a resistance R

_{0}.

_{0}(SoCi,ϑ), C

_{n}(SoC,i,ϑ), R

_{n}(SoC,i,ϑ) with n = 1,2, are automatically computed by means of advanced interpolation functions [19]. The instantaneous battery SoC during the simulation is computed as:

_{b}can be computed as:

#### 2.1. Battery Electric Model Validation

_{loss}was computed by means of the approach suggested in [22] by J. Wanga et al.:

_{loss}computation has been implemented in the model, the battery SoC computation is updated by means of the following equation:

## 3. Flywheel Electrical Model

#### 3.1. Flywheel System

#### 3.2 Formulation of the Model Parameters

#### 3.3. Simulation Scheme

^{®}code using the parameter values provided in Table 3. It is worth noticing that at ${\mathsf{\Omega}}_{max}$ the mechanical losses have the most relevant contribution (≈1530 W); on the contrary, the ohmic losses are the most important item at ${\mathsf{\Omega}}_{min}$, ranging from about 435 W to 610 W according to the temperature value.

- the power error is very low, confirming the current control effectiveness for the output electrical power regulation;
- the speed suitably tracks the requested accelerating and decelerating conditions; however, the prevalent output power requirement during this test results in a temporarily switch off of the flywheel system to recharge it;
- the temperature increase is more significant during the first part of the test because of the higher losses, particularly the mechanical ones; after that, it stabilizes to values compatible with short operating cycles.

## 4. The Hybrid Storage System Simulation

## 5. Conclusions

## Author Contributions

## Funding

## Conflicts of Interest

## References

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**Figure 4.**Battery voltage measurement (left) from which to derive the battery equivalent electrical model (right).

**Figure 6.**(

**a**) Comparison between measurements and model results for the 15th and 16th aging cycles; (

**b**) Model percentage error.

**Figure 9.**Comparison between model results and experimental measurements once the aging effect has been taken into account in the model.

**Figure 12.**Simulation results related to the application of a test reference power; (

**a**): output power reference and FWR speed; (

**b**): error between reference and actual output power; (

**c**): EM internal temperature; (

**d**): EM losses.

**Figure 14.**Hybrid system aging test simulation: (

**a**) Total power; (

**b**) battery power; (

**c**) flywheel power.

**Figure 15.**Comparison between the battery residual capacity after 1424 aging cycles for the two systems.

Rank Characteristics | Module Characteristics | ||
---|---|---|---|

Number of modules | 16 module + 1 BMS | Number of cells | 16 + 1 BMS |

Dimensions | 1049 × 549 × 1851 mm | Dimensions | 485 × 510 × 162.5 mm |

Weight | 1000 kg | Weight | 48 kg |

Capacity | 60 Ah | Capacity | 60 Ah |

Rated voltage | 947.2 V | Rated voltage | 59.2 V |

Nominal Energy | 47.7 KWh | Nominal Energy | 2.98 kWh |

Operating voltage range | 768–1054.7 V | Operating voltage range | 48–65.92 V |

Rated discharge time | 1 h | Rated discharge time | 1 h |

Operational Data | Sizes | ||
---|---|---|---|

Rated power | 20 kW | Active length | 120 mm (EM) |

Speed range | 10,833 rpm÷32,500 rpm | Airgap width | 2 mm (EM) |

Max current rms value | 223 A | Outer diameter | 400 mm (EM) 500 mm (FWR) |

Max torque (@40 °C) | 17.62 Nm | Flywheel disc height | 91 mm |

Max phase voltage (@40 °C) | 90 V | Rotating mass | 9 kg (EM) 73 kg (FWR) |

Temperature range | 25 °C ÷ 130 °C | Rotational inertia | 1.17 kgm^{2} |

Quantity | Value | Quantity | Value |
---|---|---|---|

Phase resistance | ${R}_{s,,40\xb0}$ = 0.3 mΩ | Thermal time constant | ${\mathsf{\tau}}_{th}$ = 60 min |

Torque constant | ${k}_{T,40\xb0}$ = 0.079 Nm/A | Thermal capacitance | ${C}_{th}$ = 4 10^{4} J/°K |

PM temperature coefficient | ${k}_{Br}$ = −0.2 %/°C | Reference speed | ${\mathsf{\Omega}}^{*}$ = 32,500 rpm |

Mechanical loss coefficients | ${a}_{fr}$ = 0.0272 Ws/rad ${a}_{w}$ = 3.63∙10 ^{−8} Ws^{3}/rad^{3} | Core loss coefficients | ${b}_{hys,40\xb0}$ = 0.081 Ws/rad ${b}_{ec,40\xb0}$ = 1.3∙10 ^{−8} Ws^{2}/rad^{2} |

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**MDPI and ACS Style**

Dambone Sessa, S.; Tortella, A.; Andriollo, M.; Benato, R.
Li-Ion Battery-Flywheel Hybrid Storage System: Countering Battery Aging During a Grid Frequency Regulation Service. *Appl. Sci.* **2018**, *8*, 2330.
https://doi.org/10.3390/app8112330

**AMA Style**

Dambone Sessa S, Tortella A, Andriollo M, Benato R.
Li-Ion Battery-Flywheel Hybrid Storage System: Countering Battery Aging During a Grid Frequency Regulation Service. *Applied Sciences*. 2018; 8(11):2330.
https://doi.org/10.3390/app8112330

**Chicago/Turabian Style**

Dambone Sessa, Sebastian, Andrea Tortella, Mauro Andriollo, and Roberto Benato.
2018. "Li-Ion Battery-Flywheel Hybrid Storage System: Countering Battery Aging During a Grid Frequency Regulation Service" *Applied Sciences* 8, no. 11: 2330.
https://doi.org/10.3390/app8112330