*2.4. Compressed Air Storage*

The basic idea on which the proposed approach is based is the shift of the peak aeration load to low energy cost periods, introducing a storage system able to sustain the aeration during peak energy cost periods. In order to evaluate the economic convenience of the proposed technology, the energy required to compress the air in the tank, *Ecompr* (kJ), is calculated as follows (Equation (2)):

$$E\_{\text{cumpr}} = P\_{\text{starge}} \times V \times \ln{\frac{P\_{\text{@}}}{P\_{\text{starge}}}} + \left(P\_{\text{starge}} - P\_{\text{@}}\right) \times V \tag{2}$$

where *V* represents the storage tank volume (m3), *Pstorage* is the pressure inside the tank (MPa), and *P@* (MPa) is atmospheric pressure.


**Robust Design Optimization of Electrical Machines and Devices**

Editors

**Tam ´as Orosz David P ´anek Anton Rass ˜olkin Miklos Kuczmann**

MDPI • Basel • Beijing • Wuhan • Barcelona • Belgrade • Manchester • Tokyo • Cluj • Tianjin

*Editors* Tamas Orosz ´ Szechenyi Istv ´ an University ´ Hungary

David Panek ´ University of West Bohemia Czech Republic

Anton Rassolkin ˜ Tallinn University of Technology Estonia

Miklos Kuczmann Szechenyi Istv ´ an University ´ Hungary

*Editorial Office* MDPI St. Alban-Anlage 66 4052 Basel, Switzerland

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*2.1. Catalytic Activity*

*Scapharca Broughtonii* shell (S-shell), conch shell (C-shell), and oyster shell (O-shell) are three typical waste seashells in Dalian, China, which were chosen as raw materials. After simple pretreatment (calcined at 950 ◦C for 4 h), three kinds of seashell-derived catalysts were prepared and named as S-shell-950, C-shell-950, and O-shell-950, respectively.

Figure 1 demonstrates the conversion of cyclopentanone and carbon yield of dimer obtained under di fferent catalysts. Among the investigated waste seashell catalysts, S-shell-950 exhibited the best catalytic activity.

From S-shell-950, an 82.2% yield of dimer was achieved, which was comparable with commercial CaO catalyst (88.2%). C-shell-950 also achieved relatively high activity (dimer yield: 82.1%). However, dimer yield of O-shell-950 catalyst was not desirable, which was only 4.6%.

Subsequently, the e ffect of the S-shell calcination temperature on dimer yield was also investigated (as shown in Figure 2). In the absence of catalyst, no conversion of cyclopentanone was observed. Likewise, no conversion of cyclopentanone was observed if S-shell was used as a catalyst without calcination. However, if we calcined S-shell at di fferent temperatures (e.g., S-shell-550 represents S-shell calcined at 550 ◦C for 4 h), the catalytic activities showed obviously di fferent: the dimer yield increased significantly when the calcination temperature reached 750 ◦C, then decreased slightly with the further increment of the calcination temperature. In contrast, trimer can only be detected above 850 ◦C and the yield of trimer was increased with the further increment of the calcination temperature. Among the investigated catalysts, S-shell-750, S-shell-850 and S-shell-950 exhibited a relatively good catalytic performance. In particular, S-shell-750 catalyst exhibited the best performance; a 92.1% yield of dimer was obtained with nearly 100% selectivity. Moreover, catalyst reusability of commercial CaO

and S-shell-950 in this reaction was comparable (as shown in Figure S1). The evidence above indicates that commercial CaO can be substituted directly with S-shell catalysts in this condensation reaction.

**Figure 1.** Effect of CaO source on carbon yield of dimer. Reaction conditions: 180 ◦C; 4.0 g cyclopentanone; 1.0 g catalyst; 2 h.

**Figure 2.** Effect of S-shell calcination temperature on carbon yield of dimer. Reaction conditions: 180 ◦C; 4.0 g cyclopentanone; 1.0 g S-shell catalyst; 2 h.