## 6 Discussion  
Real-time simulations carried out on the proposed FESS at different operating conditions give a generally positive feedback, although they highlight some potential criticalities that should be considered carefully.  
Focusing on round trip tests at first (cases A1 and A2), round trip efficiency is high, especially that achieved at constant-power operation (approximately 92.5%), consistent with that of most of the electrochemical batteries on the market. However, it is worth pointing out that it does not account for converter losses (both M/G-side and grid-side), as well as for all the auxiliaries (control boards, cooling system, etc.). Consequently, a more accurate efficiency assessment would lead to values around 90%, meaning that FESS design should be improved further to make it competitive on the market versus other energy storage solutions. In this regard, a first and viable solution surely consists of employing magnetic bearings, by decreasing mechanical losses significantly, as electromagnetic losses are already quite low due to the large airgap and suitable PM materials. In addition, more advanced control strategies could be employed, such as Maximum Efficiency, thus benefiting from some flux-weakening even at low speed operation for decreasing core losses suitably. On the other hand, the very similar results achieved at both 360 V and 720 V reveal the high flexibility of the proposed FESS, which could be connected to either European 230/400 V\({}_{\text{RMS}}\) or US 120/208 V\({}_{\text{RMS}}\) grids indifferently (both single-phase and three-phase connections).  
Considering the self-discharge test (case B1), no particular criticalities arise as long as the proposed FESS is not intended to store energy for long periods of time (several hours/days). Consequently, although FESS self-discharge rate is surely much higher than those of electrochemical batteries, it does not jeopardize its daily employment on condition that long idle periods are avoided.  
Considering the real operating scenarios (cases C1 and C2), the corresponding results reveal good FESS operation, particularly FESS is able to guarantee no energy exchange with the main grid for relatively long periods of time, especially during cloudy days. However, daily efficiencies are relatively low if compared to those achieved in cases A1 and A2 (approximately 71% instead of 92.5%): this is due to the fact that FESS power is generally low in cases C1 and C2, particularly its peak value is approximately half of the rated value (4 kW), as highlighted in Figure 13. As FESS losses mostly depends on its rotating speed, such a weak FESS power exploitation determines reduced efficiencies compared to round-trip tests (cases A1 and A2). Better results could be thus achieved by considering increased PV size and/or load demand, such those occurring for a large house or a building rather than for small-medium size houses.  
In conclusion, FESS operation can be improved further by, for example, a better time coordination between FESS _pre-charging/shut down_ and user's residual power profile, namely the difference between PV production and load demand. In particular, FESS operating modes now occur at fixed time slots (8:00 and 21:00, respectively), whereas they could be defined and updated adaptively based on historical data. Similarly, these starting/ending stages could last more or less than 1 h in accordance with user's preference or energy optimization criteria. Last but not least, more advanced energy management systems can be employed, by preventing or optimally determining the achievement of its maximum/minimum energy content [38; 39; 40]. Moreover, coordination with other energy storage systems and/or deferrable loads could be carried out to optimize system performance further.