# Flywheel vs. Supercapacitor as Wayside Energy Storage for Electric Rail Transit Systems

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## Abstract

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## 1. Introduction

## 2. Energy Storage Technologies

#### 2.1. Flywheel

_{min}and ω

_{max}to avoid excessive voltage variation, and to limit the maximum torque applied to the electric machine [9]. The power of a flywheel is presented as follows:

#### 2.2. Supercapacitor

#### 2.3. Characteristic Comparisons

## 3. System under Study

_{R}, F

_{W}, and F

_{g}are the friction force, the force due to the wind, and the gravity force, respectively; which could be overcome by the tractive effort (F

_{T}) produced by the electric drives in each car. T

_{w}and ω

_{w}are the required torque and angular speed of the wheel. M

_{metro}is the weight of the train; f

_{R}is the friction factor; g is the acceleration gravity; and C

_{w}, A, and γ are drag coefficient, the front area of the train, and air density, respectively. The velocity dependent part of the running resistance was considered in the rolling resistance coefficient. The number of cars and the radius of the wheels are presented by n and r, respectively. T

_{G}and ω

_{G}are the torque and the speed of the gearbox. η

_{G}is the gearbox efficiency, γ

_{G}is the gearbox ratio, and B represents the vehicle losses.

_{s}and N

_{p}are the number of series and parallel capacitors, N

_{e}is the number of electrode layers, and ε and ε

_{0}are permittivity of material and air, respectively. R and T are ideal gas constant and operating temperature, respectively, and A

_{i}is the interfacial area between electrodes and electrolyte. To connect SESS to the third rail, a bidirectional DC/DC converter was used. The schematic and the control circuit of this converter are presented in Figure 9 [16].

_{PCC}) of the ESS was greater than the voltage set for triggering the charging process, it would charge until the state of charge ESS reached an upper limit. Similarly, if the V

_{PCC}was smaller than a specific limit set for discharging, the ESS would discharge until the state of charge reached a lower limit. For safe operation, the upper and lower limits were considered in this paper to be 90% and 40%, respectively. Other than these two conditions, the ESS remained idle.

## 4. Case Study

#### 4.1. Peak Demand Reduction

#### 4.2. Voltage Regulation

#### 4.3. Cost Analysis

_{$/kW}and C

_{$/kWh}are the cost per kW and kWh, respectively. The costs for SESS range from 100–300 $/kW and 300–2000 $/kWh. However, for FESS, costs range from 250–350 $/kW and 1000–5000 $/kWh. In this study, we considered the average value for each cost and each technology [33]. The cost of energy conversion and balance of plant were 153 $/kW and 100 $/kW, respectively. Operation and maintenance costs were classified into fixed cost and variable cost. Fixed cost was considered 2% of the initial cost and included the cost of annual tax and insurance. The variable cost for the supercapacitor was 6.7 $/kW per year and included the cost of periodical inspection of the cells and interconnection cable and fixing, if there was a problem confirming the DC voltage and current. The variable cost of FESS was 9.1 $/kW per year and included the service cost for changing the air tank filter, oil, and bearing [10]. The replacement cost was associated with the life of the energy storage system. For the supercapacitor, the end of life is the moment when capacity goes down to 80% of its initial capacity, or when its ESR doubles. A supercapacitor is claimed by manufacturers to have a 10 year life span, while a recent study showed that after almost 5 years the cells need to be replaced [34]. To the best of the authors’ knowledge, there is no credible information available for the flywheel from real-world implementation; it is claimed by the manufacturer to have a 15–20 year life span. In this study, we considered the life span of the supercapacitor and flywheel, respectively, as 5 and 15 years. The results of the cost analysis for the application of peak demand reduction are presented in Table 5.

## 5. Conclusions

## Author Contributions

## Funding

## Conflicts of Interest

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Location | Company | Size | Purpose | Results/Comment | Reference |
---|---|---|---|---|---|

Los Angeles Metro | VYCON | 2 MW, 8.33 kWh | Energy saving | The total weekly saving reported as 10.5 MWh (11.5%) | [13] |

Hanover (Germany) | Pillar | 0.2 MW, 1.5 kWh | Energy saving | Tested in 2004 and showed energy saving of 462 kwh/year | [7,14] |

London Underground | Urenco Power Technology | 3 units of 100 kW | Energy saving | This was a testing trial done in 2000 | [15] |

Keihin Electric Railway (Japan) | - | 3 MW, 25 kWh | Voltage regulation | 12% energy saving was reported | [16] |

Far Rockaway (NY) | Urenco Power Technology | 1 MW | Peak demand | Energy saving of 7–25% was reported | [7,14] |

Location | Voltage | Purpose | Comment | Ref |
---|---|---|---|---|

Seibu | 1500 V | Energy Saving | - | [15] |

Columbia | 1650 V | Energy Saving | Maxwell 125 V modules were used | [15] |

Tehran | - | Energy Saving | 25% energy saving achieved | [21] |

Brussels | 850 V | Energy Saving | 37% energy saving achieved | [22] |

Toronto | 600 V | Energy Saving | Sitra SESS by simense was used | [23] |

Madrid | 750 V | Voltage Improvement | Sitra SESS by simense was used | [23] |

Beijing | 750 V | Energy Saving | Sitra SESS by simense was used | [23] |

Naples | 750 V | Energy Saving | Integration of PV farm and SESS | [24] |

Cases | kW/kg | MW/m3 | Wh/kg | kWh/m3 |
---|---|---|---|---|

Supercapacitor | 0.5–5 | 0.4–10 | 2.5–15 | 150–500 |

Flywheel | 1–5 | 1–2.5 | 10–50 | 20–80 |

Supercapacitor Cell | C = 3000 F, Vmax = 3, ESRDC = 0.27 mΩ | Flywheel Module | 125 kW, 750 Vdc |
---|---|---|---|

Configuration | 2 string*180 cells in series | Configuration | 3 modules in parallel |

Max Voltage | 500 | Energy Storage | 1875 kW/sec |

Stored Energy | 3.04 Wh | Speed | 10,000 to 20,000 rpm |

Specific Power | 5.9 kW/kg | DC Current | 167 Adc |

I_{max} Discharge | 500 A in 20 seconds | Recharge Time | 15 seconds |

Cost Options | Supercapacitor ($) | Flywheel ($) |
---|---|---|

Initial | 433,258.10 | 117,180.00 |

Energy Conversion System | 330,480.00 | 57,375.00 |

Balance of Plant | 216,000.00 | 37,500.00 |

Annual Operation and Maintenance Cost (O & M) | 23,137.16 | 5756.10 |

Replacement (every 5 years) | 763,738.10 | - |

Replacement (in 10 years) | 1,527,476.20 | - |

O & M (in 10 years) | 231,371.60 | 57,560.00 |

Total cost (in 10 years) | 2,738,585.90 | 384,365.00 |

Cost Options | Supercapacitor ($) | Flywheel ($) |
---|---|---|

Initial | 1,949,658.00 | 937,440.00 |

Energy Conversion System | 1,487,160.00 | 459,000.00 |

Balance of Plant | 972,000.00 | 300,000.00 |

Annual O & M | 133,860.36 | 55,228.80 |

Replacement (every 5 years) | 3,436,818.00 | - |

Replacement (in 10 years) | 6,873,636.00 | - |

Maintenance (in 10 years) | 1,338,603.60 | 552,288.00 |

Total cost (in 10 years) | 12,621,057.60 | 2,248,728.00 |

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**MDPI and ACS Style**

Khodaparastan, M.; Mohamed, A. Flywheel vs. Supercapacitor as Wayside Energy Storage for Electric Rail Transit Systems. *Inventions* **2019**, *4*, 62.
https://doi.org/10.3390/inventions4040062

**AMA Style**

Khodaparastan M, Mohamed A. Flywheel vs. Supercapacitor as Wayside Energy Storage for Electric Rail Transit Systems. *Inventions*. 2019; 4(4):62.
https://doi.org/10.3390/inventions4040062

**Chicago/Turabian Style**

Khodaparastan, Mahdiyeh, and Ahmed Mohamed. 2019. "Flywheel vs. Supercapacitor as Wayside Energy Storage for Electric Rail Transit Systems" *Inventions* 4, no. 4: 62.
https://doi.org/10.3390/inventions4040062