3.4.3. Supercapacitor

Supercapacitors can produce much higher specific powers (multiple kW/kg) but have lower specific energy capacities (currently only a few Wh/kg) than batteries and HFC. That is, SCs cannot be used alone or in combination with an HFC. For this component, unlike the batteries, the power limitations are less constraining, since the limitations are much higher than what the load requires. The limitations are mainly related to energy. In addition, they indirectly protect the fuel cell, batteries, and DC bus. Indeed, they absorb the DC bus voltage fluctuations and can extend the battery's lifetime [34,35]. Maxwell 350F/2.7V supercapacitor technology was considered in this paper [36]. The cell characteristics are presented in Table A2. The useful energy *ESC* available in a pack of *NSSC* elements in series and *NPSC* branches is calculated as follows:

$$E\_{\rm SC} = \frac{3}{8} \cdot \frac{N P\_{\rm SC}}{N S\_{\rm SC}} \cdot \mathbb{C}\_{\rm cell} \cdot (\mathcal{U}\_{\rm SC})^2,\tag{24}$$

where *Ccell* and *USC* are, respectively, the nominal capacity and the maximum voltage of a supercapacitor element. The flight time *tflight* (min) of the eVTOL aerial vehicle (supercapacitor discharging) is given by

$$t\_{flight} = \frac{60 \cdot \rho\_{SC} \cdot m\_{SC}}{P\_{SC}} \,\text{},\tag{25}$$

where *ρSC*, *mSC*, and *PSC* are, respectively, the energy density, and the mass and the power of a supercapacitor element. The mass of this component is directly estimated using data from Table A2.

**Figure 7.** Regression model for each component.
