3. Results

prevent overcharging. A signal "HIGH" which equals to 5 V was sent to Arduino digital pin "MOSFET 2" at the same time, and the rechargeable battery then was charged by the supercapacitor bank. Now, a signal "HIGH" was sent to one of the Arduino digital pins assigned "MOSFET 1" as soon as the voltage across supercapacitor bank was lesser than 4 V. A signal "LOW" equivalent to 0 V was sent to Arduino digital pin assigned to "MOSFET 2" at the same time. This charging and discharging of supercapacitor bank algorithm repeated

With NI-6212 device, data acquisition was implemented. A graphical user interface (GUI) was developed using LabVIEW. This GUI enables the user to easily monitor and analyze data. The LabVIEW interface is shown in Figure 7. This GUI displays supercapacitor and battery's

simultaneously until battery was fully charged.

10 Supercapacitors - Theoretical and Practical Solutions

Figure 8. Data exported to excel spreadsheet from LabVIEW.

Figure 9. Experimental setup for the integrated system.

2.2.5.1. DAQ and Labview

This section gives a performance analysis of a Supercap (supercapacitor)-based energy harvesting battery charging device operated by the Maglev VAWT adopted to a 200 W PMSG as per the configuration discussed previously which was sent for fabrication. Upon arrival of the turbine, the system was set up in the laboratory, and field testing was performed to tabulate the data.

This subchapter has two parts. First part includes one of the three cases in detail which has been compared for performance analysis. "Case A" showed a battery of 6 V, 3.2 AH, which was charged from 4.2 to 5 V through a DC/DC converter followed by a series of four supercapacitors (2.7 V, 35 F). "Case B" and "Case C" demonstrated the direct charging of the battery where "Case B" was experimented with the converter and "Case C" was without converter. All the three cases were experimented in low wind speed that ranges between 6 and 3 m/s. To keep it short, only results from wind speed 4 m/s will be discussed in detail. The remaining results have been given in a tabularized form to compare and find out the efficiency of the EHC.

#### 3.1. At wind speed = 4 m/s

#### Case A: Energy harvesting through supercapacitor.

The same procedure from the earlier section was followed, and results were graphically plotted for analysis. Following figures are the details of the charging process. It is noteworthy mentioning that both the Supercap discharge voltage and discharge current were the same as the previous value. This is because while Supercap bank discharged its charge to the battery, the turbine system was isolated through the MOSFET switch. Therefore, wind speed cannot make any impact on the discharging half cycle. Consequently, in all the three cases, the discharge voltage and current amount with respect to time were the same. Here, Figures 10 and 11 show the charging voltage and current graph with respect to time. For the discharging details, Section 4.5.1 may be reviewed as in both of the cases, the data will be the same.

At this point, 35 min were required to charge up the Supercap bank. Adding the discharging cycle time which was 2 min, the complete cycle duration was then 37 min. The starting current was 145 mA which took a rapid fall in the next second, bringing the current down to 22 mA. As for the inertia of the turbine, understandably, the charging current at first was very high but that could not be misinterpreted as the actual current. The real current started from 22 mA followed by a gradual decrease that ended up at 2.5 mA. Therefore, the pick current could be considered as 22 mA. Figure 12 displays the complete cycle process, which basically was the charging and discharging cycle of 37 min.

Again 18 cycles were needed to charge up the Supercap bank from 4.8 to 5 V, but in this time, one cycle consisted of 37 min which in total made the system take 10.4 h of charging time. Figure 13 represents the battery voltage charging up to 5 V in 10.4 h.

Case B: Energy harvesting without Supercap (with converter).

According to Figure 14, it took 18.75 h to reach its maximum value of 4.54 V. After that the increase of the voltage was so less with respect to time, the value was not taken into consideration. Therefore, this charging system was incapable to charge up the device at 4 m/s.

#### Case C: Energy harvesting without Supercap (without converter).

"Case C" took 15 h to finish the task. Figure 15 shows the battery charging voltage with respect to time.

Figure 11. "Supercap charging current" vs. "time" at wind speed 4 m/s.

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Figure 12. Supercap charging and discharging voltage with respect to time at wind speed 4 m/s.

Table 2 recapitulates the result of this section in brief.

Figure 10. "Supercap charging voltage" vs. "time" at wind speed 4 m/s.

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Figure 11. "Supercap charging current" vs. "time" at wind speed 4 m/s.

was 145 mA which took a rapid fall in the next second, bringing the current down to 22 mA. As for the inertia of the turbine, understandably, the charging current at first was very high but that could not be misinterpreted as the actual current. The real current started from 22 mA followed by a gradual decrease that ended up at 2.5 mA. Therefore, the pick current could be considered as 22 mA. Figure 12 displays the complete cycle process, which basically was the

Again 18 cycles were needed to charge up the Supercap bank from 4.8 to 5 V, but in this time, one cycle consisted of 37 min which in total made the system take 10.4 h of charging time.

According to Figure 14, it took 18.75 h to reach its maximum value of 4.54 V. After that the increase of the voltage was so less with respect to time, the value was not taken into consideration. Therefore, this charging system was incapable to charge up the device at

"Case C" took 15 h to finish the task. Figure 15 shows the battery charging voltage with respect

Figure 13 represents the battery voltage charging up to 5 V in 10.4 h.

Case B: Energy harvesting without Supercap (with converter).

Case C: Energy harvesting without Supercap (without converter).

Table 2 recapitulates the result of this section in brief.

Figure 10. "Supercap charging voltage" vs. "time" at wind speed 4 m/s.

charging and discharging cycle of 37 min.

12 Supercapacitors - Theoretical and Practical Solutions

4 m/s.

to time.

Figure 12. Supercap charging and discharging voltage with respect to time at wind speed 4 m/s.

Figure 13. Battery charging voltage with respect to time for 4 m/s wind speed.

3.2. At wind speed = 3 m/s

Table 3 recapitulates the result of this section in brief.

Efficiency comparison among Case A, Case B and Case C at 4 m/s

Case B (Charging with converter): Incompetent

Figure 15. Battery charging voltage with respect to time for 4 m/s wind speed (without converter).

Battery charging voltage (4.2–5 V) Efficiency (%) Reference point:

Table 2. Charging battery (from 4.2 to 5 V) through Supercap at 4 m/s wind speed.

Supercap charging cycle: 35 min Supercap discharging cycle: 2 min Number of complete cycle: 18 Maximum Supercap charging current: 22 mA Maximum Supercap discharging current: 18.5 mA Time duration: 10.4 h

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Case A (Energy harvesting): 31 Case C – Direct charging without converter

Figure 14. Battery charging voltage with respect to time for 4 m/s wind speed (with converter).

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Figure 15. Battery charging voltage with respect to time for 4 m/s wind speed (without converter).


Efficiency comparison among Case A, Case B and Case C at 4 m/s


Table 2. Charging battery (from 4.2 to 5 V) through Supercap at 4 m/s wind speed.

#### 3.2. At wind speed = 3 m/s

Figure 13. Battery charging voltage with respect to time for 4 m/s wind speed.

14 Supercapacitors - Theoretical and Practical Solutions

Figure 14. Battery charging voltage with respect to time for 4 m/s wind speed (with converter).

Table 3 recapitulates the result of this section in brief.


Worthington's work of pulling off 300% more efficiency with hybrid energy harvesting, it is drastically low. However, his storage system was implemented to a pump tire circuit, whereas our circuit was designed for a low wind application. As an off-grid stand-alone low voltage energy harvesting system, the EHC was able to provide, noteworthy, better efficiency in all

Wind speed (m/s) Battery charging via Supercap (h) Direct battery charging time (h) Efficiency (%)

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5 8.1 10 19 4 10.4 15 31 3 38.4 53 28

Table 5. Summary of energy harvesting circuit result for charging a 6 V lead acid battery from 4.2 to 5 V.

An important observation had been made in this experiment. At low wind speed, the turbine tends to slow down and stop if there is a heavy load. This is because a permanent magnet synchronous generator has an output frequency, which is proportional to its armature speed. The required torque to rotate the PMSG is proportional to the electrical load. Therefore, at low wind speed, with the increase of the electric load, there is always a tendency to slow down while the mechanical input coming from the VAWT restores it. However, if the load is too much to handle, the mechanical speed from the turbine becomes very slow and eventually the

2

CV<sup>22</sup> <sup>¼</sup> <sup>1</sup> 2

<sup>8</sup>:<sup>76</sup> � <sup>10</sup>:8<sup>2</sup> � 42 <sup>¼</sup> <sup>440</sup> <sup>J</sup>

However, the boost converter cannot step up voltage less than 4 V. Therefore, usable energy in

CV<sup>12</sup> � <sup>1</sup> 2

Battery rating, 6 V, 3.2 AH which is equivalent to 19.2 Wh [A 6 V 1 AH can store 12 Wh].

2

¼ 1 2

From one cycle of supercapacitor bank, battery can store energy up to 440 J.

CV<sup>2</sup> <sup>¼</sup> <sup>1</sup> 2

C V12 � <sup>V</sup><sup>2</sup>

<sup>8</sup>:<sup>76</sup> � <sup>10</sup>:82 <sup>¼</sup> <sup>511</sup> <sup>J</sup> (1)

(2)

3.5. Theoretical analysis of battery charging via supercapacitor cycle

Energy in supercapacitor bank, ESupercap\_Bank <sup>¼</sup> <sup>1</sup>

ESupercap\_Bank\_Effective <sup>¼</sup> <sup>1</sup>

The peak voltage of Supercap bank cycle = 10.8 V,

Here, 19.2 Wh ~ (19.2 � 3600 J) ~ 69,120 J.

three low wind speeds.

turbine stops.

For wind speed = 5 m/s. Theoretical calculation:

the supercapacitor bank is

Table 3. Charging battery (from 4.2 to 5 V) through Supercap at 3 m/s wind speed.

#### 3.3. At wind speed = 5 m/s

Table 4 recapitulates the result of this section in brief.

#### 3.4. Efficiency comparison

As shown in Table 5, the energy harvesting circuit data show excellent values for all the results with very good performance overall. Change in the wind speed from 5 to 4 m/s produces better efficiency as it goes to 31% from 19%. For a low speed of 3 m/s, where direct charging displays a poor performance, the energy harvesting circuit, even though it took a long time of 38.4 h to charge up the battery, still maintains its productivity by producing 28% efficiency. Here, the highest amount of efficiency drawn from the system was 31%. Comparing to


Table 4. Charging battery (from 4.2 to 5 V) through Supercap at 5 m/s wind speed.


Table 5. Summary of energy harvesting circuit result for charging a 6 V lead acid battery from 4.2 to 5 V.

Worthington's work of pulling off 300% more efficiency with hybrid energy harvesting, it is drastically low. However, his storage system was implemented to a pump tire circuit, whereas our circuit was designed for a low wind application. As an off-grid stand-alone low voltage energy harvesting system, the EHC was able to provide, noteworthy, better efficiency in all three low wind speeds.

An important observation had been made in this experiment. At low wind speed, the turbine tends to slow down and stop if there is a heavy load. This is because a permanent magnet synchronous generator has an output frequency, which is proportional to its armature speed. The required torque to rotate the PMSG is proportional to the electrical load. Therefore, at low wind speed, with the increase of the electric load, there is always a tendency to slow down while the mechanical input coming from the VAWT restores it. However, if the load is too much to handle, the mechanical speed from the turbine becomes very slow and eventually the turbine stops.

#### 3.5. Theoretical analysis of battery charging via supercapacitor cycle

#### For wind speed = 5 m/s.

3.3. At wind speed = 5 m/s

3.4. Efficiency comparison

Table 4 recapitulates the result of this section in brief.

Efficiency comparison among Case A, Case B and Case C at 5 m/s

Case B (Charging with converter): Incompetent

Efficiency comparison among 'Case A', 'Case B' and 'Case C' at 3 m/s

16 Supercapacitors - Theoretical and Practical Solutions

Case B (Charging with converter): Incompetent

Battery charging voltage (4.2–5 V) Efficiency (%) Reference point:

Table 3. Charging battery (from 4.2 to 5 V) through Supercap at 3 m/s wind speed.

As shown in Table 5, the energy harvesting circuit data show excellent values for all the results with very good performance overall. Change in the wind speed from 5 to 4 m/s produces better efficiency as it goes to 31% from 19%. For a low speed of 3 m/s, where direct charging displays a poor performance, the energy harvesting circuit, even though it took a long time of 38.4 h to charge up the battery, still maintains its productivity by producing 28% efficiency. Here, the highest amount of efficiency drawn from the system was 31%. Comparing to

Supercap charging cycle: 25 min Supercap discharging cycle: 2 min Number of complete cycles: 18 Maximum Supercap charging current: 30 mA Maximum Supercap discharging current: 18.5 mA Time duration: 8.1 h

Case A (Energy harvesting): 19% Case C – Direct charging without converter

Battery charging voltage (4.2–5 V) Efficiency (%) Reference point:

Table 4. Charging battery (from 4.2 to 5 V) through Supercap at 5 m/s wind speed.

Case A (Energy harvesting): 28 Case C – Direct charging without converter

Supercap charging cycle: 95 min Supercap discharging cycle: 1 min Number of complete cycle: 25 Maximum Supercap charging current: 18 mA Maximum Supercap discharging current: 18.5 mA Time duration: 38.4 h

Theoretical calculation:

$$\text{Energy in supercapacitor bank, } E\_{\text{Superap\\_Rank}} = \frac{1}{2}CV^2 = \frac{1}{2}8.76 \times 10.8^2 = 511 \text{ J} \tag{1}$$

The peak voltage of Supercap bank cycle = 10.8 V,

However, the boost converter cannot step up voltage less than 4 V. Therefore, usable energy in the supercapacitor bank is

$$\begin{aligned} E\_{\text{Supercap\\_Bank\\_Effective}} &= \frac{1}{2} \text{CV1}^2 - \frac{1}{2} \text{CV2}^2 = \frac{1}{2} \text{C} (V1^2 - V^2) \\ &= \frac{1}{2} 8.76 \times \left( 10.8^2 - 4^2 \right) = 440 \text{ J} \end{aligned} \tag{2}$$

Battery rating, 6 V, 3.2 AH which is equivalent to 19.2 Wh [A 6 V 1 AH can store 12 Wh]. Here, 19.2 Wh ~ (19.2 � 3600 J) ~ 69,120 J.

From one cycle of supercapacitor bank, battery can store energy up to 440 J.

Again, 440 J is stored into a 6 V 3.2 AH battery in one cycle.

Therefore, 69,120 J can be stored into a 6 V 3.2 AH battery in (69,120/440) or 157 cycles.

Note: one charging cycle of supercapacitor bank takes 27 min on average.
