## 4 Innovative Micro-Grid with Bi-Directional Flows for RES and EV Charging  
As anticipated above, a new micro-grid architecture is described in this Section (see Figure 6), which exploits bi-directional EV charger plus RES to overcome the flexibility limit of the architecture, as shown in Figure 1. Section 3 has shown why Li-ion batteries are being widely used for the high power and energy densities. Thus, they serve as energy storage unit connecting to the smart grid and to a RES such as PV panels in our focused application. This leads to a hub connection discussion: The current industry devices lack an efficient integrated connection system, since PV panels are not directly connected to the EV charger. Typically, they are first connected to the grid via a unidirectional DC/AC solar inverter, then a traditional unidirectional EV charger is used to connect the grid with the EV implementing an AC/DC conversion. This means that energy flows over two paths in a conventional solar-powered EV charging system, resulting in extra power losses and higher system cost. Instead, a novel bidirectional EV charger system is proposed in Figure 6 to build a direct connection between PV panels and the EV, and to create a V2G path. Therefore, reduced power loss and lower system cost features are achieved from a highly integrated power electronics system with high efficiency and high-power density. The idea of the system scheme in Figure 6, as further detailed in the circuit scheme of Figure 7, is having a central DC power bus, plus a bidirectional flow between the AC grid and the power DC bus thanks to a 3-phase bidirectional converter, and a bidirectional flow between the DC battery on-board the EV and the DC power bus thanks to a bidirectional DC/DC converter. A unidirectional flow is still foreseen from the PV solar panel sub-system towards the DC power bus, using a boost DC/DC converter to adapt the output solar PV level to the DC power bus level. The DC power bus voltage is typically in the range from 250 V to 600 V, e.g. it has been sized at 400 V in [3] in case of a 10 kW bidirectional EV charger and at 450 V in case of a 1.65 kW bidirectional EV charger in [71]. In [72; 73; 74], as special optimization case, a bidirectional EV charger is proposed where the value of the DC power bus can be sized from 500V to 840V. The AC grid is typically a 3-phase 380 Vac one. In the detailed circuit schematic of the EV bidirectional charger in Figure 7 the bidirectional 3-phase AC/DC converter and the boost DC/DC converter follow classic circuit solutions. For the isolated bi-directional DC/DC converter in Figure 7, instead of using a dual-active bridge topology as  
Figure 7: Circuit schematic of the bi-directional converter (1 AC grid port and 1 DC EV battery bidirectional ports; 1 DC unidirectional solar power port). in [71; 72; 73; 74] (see Figure 8B), a half-bridge series resonant LLC topology is proposed (that of Figure 8A). This approach allows reducing the number of active switches to be used and hence it makes more convenient the adoption of SiC power Mosfets (e.g., Cree C2M0040120D adopted in [3]) instead of classic Si power Mosfets (e.g. Infineon IPW60R045 CP adopted for the primary stage in [71]). Indeed, SiC power Mosfets are more expensive than Si power Mosfets: a market analysis on stocks from 100 to 1000 devices for the power mosfets in Table 1 has shown that the selling price for each SiC power device is 1.7 to 3.5 times higher than that of Si power device. Hence, a circuit solution like that in Figure 8A that minimizes the use of active devices make more convenient the adoption of SiC power Mosfets.