## 4 Innovative Micro-Grid with Bi-Directional Flows for RES and EV Charging
Its flow chart is viewed in Figure 6, in contrast to the traditional way shown in Figure 1. In the bi-directional power flow interaction, the solar power can flow either from the PV panel to the grid or to the EV, and the battery charging power can flow from grid to vehicle (G2V) or vice versa (V2G). Two additional power flows are provided and managed in Figure 6 if compared to the unidirectional mode in Figure 1. Moreover, a further partner of the new architecture can also be the ESS, discussed in Section 3, located inside the residential installation that may or may not be included in the above described scenario. The proposed approach in Figure 6 has several main advantages. First, there is an increase in system flexibility. V2G power flow implies the power can be drawn from the battery serving as the electric source in [4], which has significant benefits in the future applications such as powering house during the electricity shutdown. Moreover, the system becomes quite simpler after applying the bi-directional power flow method, and its benefits can be summarized in these two points: (i) Direct access to solar generation and EV battery; (ii) efficiency maximization. Concerning the direct access to solar generation and EV battery, thanks to the two additional power flow path--solar power to EV charging and EV back-feed to the grid-(see Figure 6 compared to Figure 1), the system becomes more valuable in the energy usage and transfer. The system integrates the functionality of solar inverter and battery charging into one system, by providing direct access between each power element, which significantly reduces the system size. Concerning efficiency maximization, the overall system efficiency can be improved of about 4% by comparing the bi-directional power flow method with the traditional unidirectional power flow method. From the perspective of fairness, their measured efficiency data are based on same metrics of power electronics system experiment: EV charger data are taken from [3], and solar inverter data are taken from [5]. If for example the power converters (solar inverter and unidirectional converter in Figure 1, and bi-directional converter in Figure 6) have an efficiency of 96%, the scheme in Figure 1 will result in an overall efficiency of 92% less than the efficiency of 96% achieved with the scheme in Figure 6.  
The 4% improvement is a substantial saving on the economics of electricity usage, besides the materials cost saving from one compact integrated system, compared to two bulky systems separated systems. The only hardware requirement to implement the circuits and systems proposed in Figures 6 and 7 is the use of bi-directional capable semiconductor switches in [3], based on the today available SiC technology. Since SiC is becoming a mature technology, the proposed approach adds trivial circuit complexity as shown in the circuit in Figure 7, which has been implemented and characterized in [3; 21; 22; 23]. The bi-directional converter in Figure 7 has been tested with battery packs of 400 V and charging/discharging power levels in the range from 1 to 10 kW. It is worth noting that the scheme in Figure 6, and hence the circuit in Figure 7, can further be improved in terms of flexibility since it is possible to exploit as ESS, not only where available in the EV side, but also that available at RES side. Indeed, new solar energy sources are available in the market [2], where a high-voltage Li-ion battery energy storage is integrated with capacity scalable from 4 kWh up to 12 kWh. The energy storage in [2] can sustain DC voltage inputs from 170 V to 575 V at the battery port side, where each basic battery module of 4 kWh can be charged/discharged at maximum charge power of 1.6 kW, and maximum discharge power of 2 kW. By parallelizing the modules, we obtained an overall energy storage up to 12 kWh with maximum charge power of 4.8 kW and maximum discharge power of 6 kW.