**4. Experimental Validations**

To validate the proposed SC-ESS design, a test setup based on the manufactured four modules has been constructed. The test setup as shown in Figure 8 consists of a power supply that acts as a charger, the electronic load that acts as the system load, and the Arduino Mega 2560 board that acts as the SC-ESS controller. The Arduino board is interfaced with the PC to perform data-logging as well.

The evaluation of the proposed SC-ESS design with associated management electronics is achieved by three tests as detailed below.

**Figure 8.** Experimental setup to validate the proposed design for SC-ESS: (**a**) test setup; (**b**) assembled SC-ESS.

The first test is reported in Figure 9, in which the operation and performance of the protective action of the OVP subsystem are evaluated. It is conducted under continuous charge–rest–discharge cycling with 15 A constant current charge and 1.8 kW constant power discharge whilst voltages of the SC cells are measured by the voltage monitoring subsystem and the stack current (SC-ESS current) are measured by a current prob. The test started with forced voltages mismatches between the SC cells of 0.25 V to emulate a capacity mismatch of 12.5%, which caused the cells that were initially at higher voltages to hit the maximum voltage limit (2.5 V) at t = 38 s, and hence the OVP subsystem activated the shunt circuits associated with those cells. Accordingly, their voltages were kept at the limit whilst other cells continued charging until they hit the limit as well (at t = 47 s in this experiment) and the shunt circuits for these cells were also activated to maintain their voltages at the limit until the charging process stopped (at t = 62 s). The overshoots of cell voltage due to the switching of the shunt circuit were maintained at ≤10 mV to ensure the maximum utilization of cells' capacity. Hence, the test confirmed that the OVP subsystem enabled safe operation for SC-ESS that was able to complete the charge cycles and protect the SC cells that were at the risk of overvoltage under the imposed voltages mismatching that emulates SC cell capacity fade due to degradation.

**Figure 9.** Performing of protective action to protect the SC cells by OVP subsystem during continuous charge/discharge cycling: (**a**) SC cells' voltages; (**b**) charge/discharge power and current.

The second test, which is reported in Figure 10, evaluates the operation of the OVP subsystem under given discharging commands from the central controller to demonstrate its ability to execute these commands when required. These may happen under some conditions like a safe discharge of the ESS. The test starts with SC cells having unequal voltages, and because of applying multiple charging cycles (at t = 49 s and 130 s) with a current of 15 A, the voltage raises to the overvoltage limit; hence, activation of the associated shunt circuits by the OVP preventive action was required at t = 160 s, which continued in operation until the charging cycle stopped at t = 172 s. Accordingly, as the shunt resistors were thermally coupled with the modules' enclosures, the power dissipated in these resistors causes temperatures (temp.) of the SC-modules' enclosures to be increased, as can be seen for modules 1 and 2 recorded temps. (M1 and M2 case temp.) in Figure 10. At t = 360 s, a controlled discharge for the cells of modules 1 and 3 with a duty cycle of 20% followed by faster discharging with larger duty cycles (50% and 75%) based on the received commands from the central controller were reported. Accordingly, the energy stored in these modules dissipated in the shunt circuits, causing temp. increase, as can

be seen for M1 case temp. recorded. When the voltage reached 0.4 V, the discharge was stopped. At t = 580 s, the remaining cells at modules 2 and 4 were also commanded to discharge with a 75% duty cycle and this caused a fast discharge to the same minimum cell voltage as the other cells (0.4 V) to achieve a balanced state for all cells while the energy dissipated in these modules also caused its temp. to be increased, as can be seen for M2 case temp. recorded. This test confirmed the capability of the OVP subsystem to perform a discharge cycle for specific SC cells by a specific discharging rate controlled by the duty cycle that is selected by the central controller and commanded via the communication bus.

**Figure 10.** Performing of protective action to protect the SC cells by OVP subsystem during charge/discharge as well as responding to discharge commands from the central controller: (**a**) SC cells' voltages; (**b**) SC-modules 1 and 2 enclosures' temperatures; (**c**) Discharge power and current.

The third test reported in Figure 11 evaluates the capability of the OVP subsystem to perform active dissipative SC cells' voltage balancing as a preventive action to protect the cells from overvoltage. The test started with imposed mismatching between the SC cells, then a discharging process by a constant power of 1.3 kW was introduced at t = 15 until t = 25 s to allow for sufficient charging time for demonstrating the balancing mechanism. After a rest time provided to the system, the charging of the SC-ESS started by a constant current of 5 A at t = 47 s, where the mismatching in the SC cells' voltages was detected by the system controller. Following this, it activates the balancing mechanism in which the SC cells' shunt circuits equalize the cells' voltages in a PWM manner. As can be seen, the balancing mechanism succeeded to achieve balanced cells' voltages in a considerable short time due to the ability of the shunt circuit to handle a high current.

**Figure 11.** Performing preventive action by OVP subsystem via active dissipative balancing during the continuous charge cycle: (**a**) SC cells' voltages; (**b**) Charge/discharge power and current.

In the three experimental tests, the results confirmed the proposed functionalities of the SC-ESS in terms of cells' voltage monitoring, fast balancing, and overvoltage protection.

#### **5. Conclusions**

In this paper, the design and implementation of a smart super-capacitors-based energy storage system are proposed. The SC-ESS sizing was optimized for minimum weight by utilizing the highest energy density cells of the available range and adjusting the operating voltage range, accordingly. In addition, the proposed design minimizes the wirings for cells' voltage monitoring and overvoltage protections by employing a distributed modular architecture coordinated via standard communication buses. SC cells overvoltage protection subsystem in the proposed design implements two independent preventive and protective actions for the highest reliability to ensure safe operation of the system. Finally, the design is validated based on experimental testing of the manufactured modules and the results showed the achievement of the proposed functionalities.

**Author Contributions:** Conceptualization, A.M.F., M.R. and C.K.; methodology, A.M.F.; software, A.M.F. and M.K.; validation, A.M.F.; formal analysis, A.M.F., M.R. and C.K.; investigation, A.M.F. and C.K.; resources, S.B. and C.K.; data curation, A.M.F.; writing—original draft preparation, A.M.F.; writing—review and editing, A.M.F., C.K. and S.B.; visualization, A.M.F.; supervision, C.K. and S.B.; funding acquisition, S.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** The project received funding from the Clean Sky 2 Joint Undertaking under the European Union's Horizon 2020 research and innovation program under grant agreement No 755485.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

#### **References**

