## 8 Conclusions  
This paper has presented a methodology for modelling an electrically driven flywheel based on a PMSM controlled by back to back converter topology. The components and structure of the FESS are introduced, and its characteristics and main applications are briefly discussed. Flywheels offer the unique characteristics of very high cycle and calendar life, and are the best storage technology for applications demanding these requirements. High power capability, instant response with high round trip efficiency, and ease of recycling, are its main attributes.  
In this paper, calculation of the losses and efficiency of a proposed FESS is presented. The system losses were calculated by deriving the mathematical loss model of the FESS for extreme temperature and pressure conditions, which are hardly ever reached as the flywheel is operated in vacuum. The loss model included windage loss, bearing loss, copper loss, and stray loss, which is a combination of eddy current loss, iron core loss, and other metallic parts, due to flux leakages. It was shown that the mechanical losses account for 75% of the total losses, and to limit the losses to an acceptable level, the pressure needs to decrease below 75 Pa for 20,000 rpm. The results of the analysis also showed that the run-down energy loss of the FESS is approximately 37 Wh during 15 min of standby mode. It will lose about 5% of the total stored energy per hour if left in idle mode. Neglecting switching and conduction losses in power converters and windage losses on the motor-generator, the total efficiency of the system was more than 88%. The system under consideration was a high inertia flywheel for longer duration storage to accumulate 5 kWh within a speed range of 10,000-20,000 rpm. This is contrary to most of the flywheel systems in industry, usually performing short term high power applications. The choice of ratio of power to energy is a free design choice, one other benefit of the flywheels.  
The dynamic performance and response of the system was tested when simulating the FESS for real life scenarios of charging and discharging states. The analysis results showed the characteristic performance and compliance of the load torque, SOC, input-output power, and three-phase current and voltage of the PMSM-Flywheel. The no-load speed of the PMSM was also plotted to show the system response when the flywheel rotor is not connected. It was shown that sudden step changes in torque develops corresponding changes in flywheel speed, as well as its SOC and stored energy. At the instant of load change, the DC-bus voltage fluctuations were within \(\pm\)3% of the rated voltage, which was controlled and recovered within 40-45 ms.  
Conformation of the simulated results implies responsiveness and validity of the software model. Modelling and simulation of a higher inertia flywheel for ground power applications is a new area which has not been widely explored due to unavailability of the experimental data. This software tool can be developed to analyse different scenarios for such applications. It can be considered as an initiative for future researchers exploring models to analyse PMSM assisted FESS for energy storage applications.  
Future work will include modelling and analysis of an isolated solar photovoltaic system with a FESS providing backup power for islanded remote areas.  
All authors have contributed to the idea behind the work. [PERSON] and [PERSON] conceived and designed the experiments and [PERSON] performed the experiments. [PERSON] and [PERSON] analysed the data, [PERSON] contributed his expertise in FESS as head of research group and [PERSON] contributed his expertise in Power Electronics. [PERSON] wrote the majority of the paper, whereas [PERSON] and [PERSON] made small contributions.  
The authors declare no conflict of interest.