Microprocessor – controlled charging modes, Under independent charging condition, the solar energy and wind energy charges respectively the corresponding battery. Under hybrid charging condition, both energy sources charge the two batteries simultaneously.
Large wind turbines use mechanical systems, such as geared or gearless devices to increase the speed of the generator. In addition, an inverter is employed to adjust the output voltage to exceed the grid value. With its phase leading the bus phase, wind power can be integrated into the grid bus. The integration can be easily realized owing to negligible impedance of the utility bus. The main issues for wind-power generating systems include fluctuations in output voltage and quality of power supplied to the utility power system. In small renewable energy systems, wind power and solar energy are integrated to improve the reliability of the individual power system. Conventionally, the AC output voltage of the wind turbine is rectified, and then combined with the output voltage of the solar cell to charge the battery and provide power supply to the load. To characterize the battery, the Thevenin battery model considering the nonlinear effect of source impedances was proposed. A battery evaluation test system was employed to validate this model. The curve of test results follows entirely the theoretical calculation -. For photovoltaic application, the inner resistance of solar panel was also included in the theoretical analysis. The maximum power point tracking (MPPT) of solar panel for different insolation levels verified the proposed solar cell model. The MPPT technique adjusted continuously the battery-charging rate and obtained shorter charge time . It was reported that a dual battery configuration with deep-cycle batteries can increase the available capacity. Moreover, the system may achieve optimum utilization of the PV array and proper maintenance of the storage battery . In another research, a microcontroller was employed to adjust the maximum charging current according to the PV power production and battery voltage level. Using this method, better exploitation of the power produced by the PV power source can be achieved. Moreover, battery lifetime can be increased by restoring high state of charge (SOC) in short charging time . Another study compared the performance of equal rate charging, proportional rate and pulse current charging in charging multiple batteries. The total charging time is shortest when using pulse current charging strategy. All the batteries become fully charged almost simultaneously when they are charged with proportional rate or pulse current method . The optimum size of the PV module for a specific wind turbine to meet the load requirement for the hybrid wind/PV system was investigated in order to minimize the overall cost of the system  - . Nevertheless, the effect of source impedance in a small hybrid wind/PV system has not yet been investigated. In this study, theoretical analysis shows that it is difficult to obtain both wind power and solar energy at the same time by traditional methods, which is verified by field test. To overcome such problem, a microprocessor-based controller design for detecting instantaneous voltage variations of both energy sources is proposed, and a charge controller is employed to optimize the charging operation.
2. Theory and analysis
Owing to the large variation in the wind and solar energy, the converter is employed to provide the stable power for normal application. When only one energy source supplies the load, as shown in Fig.1(a), the voltage and frequency of the converter output is adjusted to meet the load specification. In Fig. 1(b), both wind and solar energy supply the same load simultaneously. In addition to the load requirement, the voltage and frequency of both converter outputs are adjusted such as the two energy sources can supply the load at the same time. In case of the DC-DC converter, only the output voltage of both converters should be adjusted to charge the same load.
In a small hybrid power system, a battery is usually utilized to store the renewable energy to improve the reliability of the system. Moreover, to simplify the power system, the power source charges directly the battery. Figure 2 shows the conventional charging system, in which the rectified DC voltage charges two batteries. In addition to source voltages Ew, E1 and E2, charging currents I1 and I2 are also determined by source resistances rw, r1 and r2 for the wind power and the two batteries, respectively. The output voltage V0 is the summation of V0w, V01 and V02 from wind power and batteries E1 and E2, respectively. According to the circuit theory, the equations for V0w, V01 and V02 are as follows:
As mentioned, Vow, Vo1 and Vo2 are voltages from wind power and the two batteries, respectively, which contribute to the output voltage Vo independently.
The following are possible charging situations.
When V0w > V01 = V02, the charging current from wind power charges simultaneously both batteries E1 and E2.
When V0w > V01 > V02, the charging currents from both wind power and battery E1 charge battery E2.
When V0w > V02 >V01, similar to case (b), the charging currents from both wind power and battery E2 charge battery E1.
When V01 > V0w >V02, wind power has no effect on the circuit, only battery E1 charges battery E2.
When V02> V0w >V01, similar to case (d), wind power has no effect on the circuit, only E2 charges battery E1.
As seen in above analysis, it is only in case (a) that wind power can charge both batteries at the same time. However, in case (b), when battery E1 also charges battery E2, the voltage drop of I1 r1 and I2 r2 cause increase in V02 and decrease in V01 respectively. Finally, when V01 = V02, the charging condition returns to case (a). Case (c) is similar to case (b), so it exhibits self-regulating behavior during the charging process. Cases (d) and (e) are not normal charging conditions.
Figure 3(a) shows a hybrid wind and PV power generating system. Ew, Ep, Eb, rw, rp and rb are factors that determine the charging current. Similar to the above conventional charging conditions, the output voltage V0 is made up of Vow, Vop and Vob from the wind generator, solar panel and battery, respectively. The related equations are listed below.
Possible charging conditions are as follows:
When Vow > Vop > Eb, both wind and solar energies charge the battery.
When Vop > Vow > Eb, both wind and solar energies charge the battery.
When Vow > Vop, only wind energy charges the battery.
When Vop > Vow, only solar energy charges the battery.
From the above analysis, in cases (a) and (b), there are two energy sources charging a battery at the same time. However, in case (a), the larger current Iw from wind energy may result in a larger internal voltage drop Ib rb of the battery. Therefore, when Vow > Vop, the charging condition becomes case (c), and solar energy cannot be utilized to charge the battery. Case (b) shows the same behavior. Contrary to the conventional charging system, the hybrid charging system exhibits a competition effect, meaning that only the larger power source can dominate the charging system.
Figure 3(b) shows the I-V curves of the charging system. Because the source resistance of the wind generator is much smaller than that of the solar panel, the wind I-V curve reduces slowly with increase in charging current. The source resistance of the battery is also much smaller than that of the wind generator. Therefore, the terminal voltage of the battery Vo increases only slightly with increase in charging current. When the solar I-V curve drops to P, which is equal to the terminal voltage of the battery, the solar energy stops charging the battery.
Figure 3(c) shows the V-T curves of the charging system. Before time To, the battery is in under-charge condition, both power sources behave as the current sources with their ratio of output currents proportional to that of generated power levels. For time To to Tf, the source resistance of the solar panel lowers gradually the solar charging current and the battery terminal voltage Vo increases slowly. Finally, at time Tf, only wind power can charge the battery.
To improve the performance of the hybrid power generating system shown above, a switch control is employed. It is connected to the battery circuit as shown in Fig. 4. In this operation mode, both wind and solar energy can be utilized, although only one energy source can charge the battery at any time. Owing to the different characteristics of wind and solar energy, as shown in Eq. (10), we can adjust the charging duty cycle ratio k of the two energy sources to obtain maximum energy in the battery. The equations are listed below.
To overcome the drawbacks of the hybrid wind and PV charging system shown above, a microprocessor-controlled power generating system, as shown in Fig.5, is proposed. The different charging modes, which vary with the weather conditions to obtain the maximum energy, are shown in Table 1. With both energy sources, the system operates in the independent charging mode. The wind and solar energy charge batteries Ebw and Ebp, respectively. If there is only one energy source, the system runs in the hybrid charging mode. Either energy source can charge batteries Ebw and Ebp simultaneously. Owing to the instability of wind energy, if both energy sources co-exist, wind energy exceeds the threshold value, the charging system runs in the wind-enhanced mode. In this case, not only can both energy sources be employed to improve the reliability of power system, the fluctuations in the small wind power generating system can also be reduced.
3. Results and discussion
3.1. Source resistance measurements
To measure battery source resistance, as shown in Fig. 6(a), a 12-V/75-AH battery supplied the load through a switch. We adjusted the load resistance RL to change the battery discharging current Ib. From the voltage difference Eb – Vo and Ib, the source resistance rb can be determined.
Because the solar energy is much smaller than the battery energy and the solar cell internal resistance rp is much larger than the battery source resistance rb, as shown in Fig. 6(b), the output terminal of the solar panel is directly grounded to measure the solar charging current Ip. A 75-W solar cell panel was used in the experiment performed outdoors. From the solar voltage Ep and the current Ip, the solar internal resistance can be determined.
As shown in Fig. 7(a), the source resistance of battery decreases with increasing discharge current. The battery source resistance is about 0.02-0.12 Ω for discharging current 1-13 A. The power loss of the source resistance results in temperature rise of the battery. Hence, chemical reaction proceeds more easily with increasing charging current and the resistance to battery charging is reduced.
The source resistance of solar cell panel, as shown in Fig. 7(b), also decreases with increasing short-circuit current However; the source resistance of the solar cell panel is much larger than that of battery. Even though the area of the solar cell panel is large, the thickness of the solar cell structure is too small to increase the efficiency of optical absorption. Conventionally, the thickness of the solar cell active layer is in the micrometer range. Moreover, the resistivity of solar cell is large, which results in high source resistance.
The stator of PMG has 36 slots wound with 40 turns of 0.8-Φ copper wires. The source resistance (one phase) of PMG is found to be 0.5 Ω. It is much smaller than the source resistance of the solar cell panel (about 6-18 Ω). In general, the difference in voltage between wind energy source Ew and solar energy source Ep is not large. According to Eqs. (5) and (6), when the wind turbine is in operation, the wind output Vow is much larger than the solar output Vop. Therefore, the wind turbine dominates the battery charging behavior.
3.2. Conventional hybrid wind and PV power generating system
In this study, a 250-W permanent magnet alternator driven by wind turbine and a 12-V/75-W solar cell panel were used as the wind and solar energy source, respectively. Both energy sources were output to 12-V/75-AH lead batteries, which were kept in under-charged condition before test. In the experiment, a 100- MHz scope was employed to measure the charging current and battery voltage. A current probe set at 100 mV/A was utilized to sense the charging current. The alternator outputs were converted into DC output by a rectifier module to charge the batteries. As shown in Fig. 8, there was a large variation in current and voltage because of the unstable wind speed. The fluctuations in amplitude of the charging current were attributed to the conventional AC-DC rectification effect. Increase in wind speed also led to increase in charging current. The variation in battery voltage as a result of internal impedance is above 1V.
A battery was charged by wind and solar energy sources using the conventional wind and PV charging system, as shown in Fig. 3. In the first 30 minutes, the battery remained in an under-charged condition. As shown in Fig. 9(a), the stable 0.8-A PV charging current is not affected by the large variation in wind charging current. After 1 hour, as seen in Fig. 9(b), the charging curve of solar energy shows an opposite trend as that of wind power. The solar charging current decreases as the wind charging current increases. After 2 hours, as shown in Fig. 9(c), either solar energy or wind power dominates the charging behavior. When the wind charging current exceeds 3 A, the solar energy source does not output any power. Therefore, the system cannot get both wind and solar energy at the same time.
3.3. Switch- control hybrid wind and PV power generating system
In order to acquire both wind and solar energy at the same time, the system is configured as in Fig. 4. As seen in Fig. 10(a)-(c), the wind to solar charging duty cycle ratio is changed to examine the charging behavior at three wind speeds. Owing to fluctuations in wind speed at 3 m/s, the system sometimes stops outputting the wind charging current, as seen in Fig. 10(a), while the charging of battery by solar energy remains very stable. When wind speed increases to 4 m/s, as shown in Fig. 10(b), the wind charging current continues to charge the battery during its duty cycle, but the current decreases during charging. When the solar charging duration is increased to 3.2 seconds, the wind charging current drops to 2 A, as seen in Fig. 10(c) prior to solar charging. Upon completion of solar charging, i.e. after 3.2 seconds, the wind charging current increases to 4 A and falls gradually back to 2 A This phenomenon can be explained as follows. During solar energy charging, the wind turbine is in a no-load condition. Wind energy is thus converted into mechanical energy, which speeds up the alternator. In other words, owing to the inertial momentum, the wind turbine can store mechanical energy, and solar energy is not best utilized or lost during wind charging. Therefore, the charging duty cycle ratio between wind and solar energy can be adjusted to obtain the maximum energy source.
3.4. Microprocessor- controlled hybrid wind and PV power generating system
In the above two cases, there is always some energy loss during the power generating system operation. To obtain both wind and solar energy at the same time, a microprocessor and two batteries as shown in Fig. 5 are employed to control the charging operation from both energy sources. Figure 11 displays the circuit in detail, in which controller IC1 and comparator IC2 control the system operation. Depending on the weather condition, wind energy can charge the wind battery Ebw directly or the solar battery Ebp indirectly through Qws. Both wind and solar energy are sensed by two comparators of IC2. One senses the sunlight to control the load while the other monitors the charging condition of the wind battery. If there has been no wind or sun for some days and both wind and solar energies remain insufficient, the two batteries will be in under-charged condition and utility power supply will be used instead. In view of large variations in wind energy, a current transformer CT is employed to detect the charging current of the wind battery. If the charging current exceeds the specification of the battery, Qw runs in PWM mode to protect the battery. All functions are controlled by the software of controller IC1.
As shown in Fig. 12, when wind speed is low, about 3 m/s, the system runs in the independent charging mode and the solar battery charging current remains constant at 2A, although there are slight variations in the charging current of the wind battery.
If there is only wind power, as shown in Fig. 14(a), the system runs in the wind- hybrid charging mode, so the unstable wind charging current charges both wind and solar batteries. When the charging current is less than 2 A, as shown in Fig. 14(b), the system remains in the wind- hybrid charging mode and only the wind battery is being charged.
When there is solar energy and a high wind speed at the same time, as shown in Fig. 15, the system runs in the wind-enhanced charging mode. In order to benefit from both energy sources and reduce fluctuations from the wind source, the system runs in the wind-enhanced charging mode when the wind charging current is above 3A. The wind charging current charges the solar battery in addition to the original wind battery, leading to variations in the solar charging current. When the wind charging current is below 3A, the system runs in the independent charging mode. That is, the stable solar charging current of 2A and the fluctuating wind charging current charge the solar battery and the wind battery, respectively.
In this study, theoretical investigations are performed to examine the effect of source impedance on a small hybrid wind and PV power system. Because of voltage drop in power sources, both energy sources cannot charge a battery simultaneously after initial charging. This study proposed using a switch circuit to increase the utilization of both energy sources. There is only slight solar energy loss when wind power is in operation. To increase energy efficiency by gaining both wind and solar energy, a microprocessor-based hybrid charging system is proposed. Results show that besides increasing the reliability of the power system, the fluctuations in wind energy source are also reduced.