A Stand-Alone Hybrid Generation System Combining Solar Photovoltaic and Wind Turbine with Simple Maximum Power Point Tracking Control Nabil A. Ahmed and Masafumi Miyatake Sophia University, Tokyo, Japan Email:
[email protected] Abstract—This paper proposes a hybrid energy system combing solar photovoltaic and wind turbine as a small-scale alternative source of electrical energy where conventional generation is not practical. A simple and cost effective control technique has been proposed for maximum power point tracking from the photovoltaic array and wind turbine under varying climatic conditions without measuring the irradiance of the photovoltaic or the wind speed. The proposed system is attractive owing to its simplicity, ease of control and low cost. A complete description of the proposed hybrid system along with detailed simulation results which ascertain its feasibility are given to demonstrate the availability of the proposed system in this paper. Simulation of the hybrid system under investigation was carried out using PSIM software. Keywords-Hybrid energy system, solar photovoltaic, wind turbine, stand alone applications, boost dc-dc converter, maximum power point trackin.
I.
INTRODUCTION
Renewable energy from wind turbine and solar photovoltaic are the most environment-friendly type of energy to use. They have come of age and are global phenomenon, the world's fastest growing energy resources, a clean and effective modern technology that provides a beacon of hope for a future based on sustainable, pollutionfree technology. Today's wind turbines are state-of-the-artof modern technology-modular and very quick to install. The importance of utilizing the renewable energy system, including solar photovoltaic (PV) and wind turbine (WT) generation systems have been attracted greatly in these days because the electricity demand is growing rapidly all over the world. Therefore, there is an urgent need for the renewable energy resources and it has formulated as a national strategy for the development of renewable energy applications and energy conservation measures. For this purpose, continuous effort to develop more attracting systems with lower-cost, higher-performance and multifunctions are required. Sensor-less approaches and combined generators explained in this paper are one of such key aspects. Small-scale stand-alone power generation systems are an important alternative source of electrical energy, finding applications in locations where conventional generation is not practical. Consider, for example, remote villages in developing countries or ranches located far away from main power lines. It has been shown that a remote load has only to be a matter of a few miles away from a main power line for a stand-alone wind generator to be cost-effective [1]-[3].
The certainty of load demands at all times is greatly enhanced by hybrid generation systems, which use more than one power source. It is possible to achieve much higher generating capacity factors by combining wind turbine and photovoltaic generators with a storage technology to overcome the fluctuations in plant output. An efficient energy storage system is required, to get constant power and the electrical energy delivered by the wind turbine and photovoltaic has to be easy converted into storage energy. This conversion might be realized by a battery bank or energy capacitor system (ECS). The battery bank or ECS meets the daily load fluctuations [4]-[5]. In this paper a hybrid energy system combining variable speed WT and PV array generating system is presented to supply continuous power to the stand-alone load. The wind and PV are used as main energy sources, while the battery is used as back-up energy source. Two individual dc-dc boost converters are used to control the power flow to the load. A simple and cost effective control with dc-dc converter is used for maximum power point tracking (MPPT) and hence maximum power extracting from the WT and the PV array. II.
PROPOSED HYBRID ENERGY SYSTEM
Fig. 1 depicts the topology of hybrid energy system consisting of variable speed WT coupled to a permanent magnet generator (PMG) and PV array. The two energy sources are connected in parallel to a common dc bus line through their individual dc-dc converters. The load may be dc connected to the dc bus line or may include a PWM voltage source inverter to convert the dc power into ac at 50 or 60 Hz. The load configuration is beyond the scope of this paper. Each source has its individual control. The Diodes D1 and D2 allow only unidirectional current flow from the source to the dc bus line, thus keeping each source from acting as a load on each other or on the battery. Therefore in the event of malfunctioning of any of the energy sources, the respective diode will automatically disconnect that source from the system. The output of the hybrid generating system goes to the dc bus line to feed the isolating dc load or to the inverter, which converts the dc into ac. A battery charger is used to keep the battery fully charged at a constant dc bus line voltage. When the output of the system is not available, the battery powers the dc load or discharged to the inverter to
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power ac loads, through a discharge diode Db. A battery discharge diode Db is to prevent the battery from being charged when the charger is opened after a full charge. A dump load may be required, if excessive power is still available after fully charging the battery. As depicted in the system configuration represented in Fig. 1, the Vdc is st to a fixed dc bus line voltage and the output dc voltage from each source is controlled independently for both generation systems to get maximum power point tracking.
: Boltzmann constant (1.38e-23 J/oK) : pn junction material factor : Temperature (oK) : Series resistance.
k A T Rs
Rs
IL
I
λ
Rsh
DC/DC Converter
ID
D1
V
I sh
Figure 2. Equivalent circuit of PV module.
WG
V
PV
dc
Load Db Battery Charger
Battery
Figure 1. Equivalent circuit of PV module.
III.
SOLAR PHOTOVOLTAIC SYSTEM
The European PV industry Association reported that the total global PV cell production world wide in 2002 was over 560 MW and has been growing about 30% annually in recent years. The physical of PV cell is very similar to that of the classical diode with a pn junction formed by semiconductor material. When the junction absorbs light, the energy of absorbed photon is transferred to the electron-proton system of the material, creating charge carriers that are separated at the junction. The charge carriers in the junction region create a potential gradient, get accelerated under the electric field, and circulate as current through an external circuit. The solar cell is the basic building of the PV power system it produces about 1 W of power. To obtain high power, numerous such cell are connected in series and parallel circuits on a panel (module), The solar array or panel is a group of a several modules electrically connected in seriesparallel combination to generate the required current and voltage. The electrical characteristics of the PV module are generally represented by the current vs. voltage (I-V) and the current vs. power (P-V) curves. Figs. and show the (IV) and (P-V) characteristics of the used photovoltaic module at different solar illumination intensities. Using the equivalent circuit of solar cells shown in Fig. 2, the radiation dependent V-I characteristic of ns series cell and np parallel modules can be represented by:
n p I sc − I + np I D ns V = ns ( AkT ] − n p IRs q ) lin[ np I D where I sc : Short circuit current per cell (A)
I D : Diode saturation current (A) q : Electron charge (1.6e-19 C)
For an ELR615 160Z, 750 W, Fuji Electric solar panel (ns =3, np =5) used in this paper and neglecting the series and shun resistances, Eq. (1) can be written as: 3 lin[ 5*2.281− I + 2*8.66e− 5 ] (2) V = 0.482 5*8.66e− 5 Fig. 3 and 4 shows the strong non linearity of the I-V and P-V characteristics of the used solar wt different insolation levels. The I-V characteristic of the solar PV decreases gradually as the voltage goes up and when the voltage is low the current is almost constant. The power output of the panel is the product of the voltage and current outputs. The PV module must operate electrically at a certain voltage that corresponds to the peak power point under a given operation conditions. 18 1.0 KW/m2
16 14
0.75 KW/m2
12 Current [A]
D2
10 0.5 KW/m2
8 6
0.25 KW/m2
4
0.1 KW/m2
2 0
0
10
20
30 40 Voltage [V]
50
60
70
Figure 3. I-V characteristics of PV module. 800
1.0 KW/m2
700 600
0.75 KW/m2
500 power [W]
DC/AC Inverter
400
0.5 KW/m2
300
(1)
200
0.25 KW/m2
100 0
0.1 KW/m2 0
10
20
30 40 Voltage [V]
50
60
70
Figure 4. P-V characteristics of PV module
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Various techniques of maximum power tracking have been considered in PV power applications. Among these, the perturbation and observation (P&O) method, which moves the operation point toward the maximum power point by periodically increasing or decreasing the array voltage, is often used in many PV systems. The advantage of this method is that it works well when the irradiation does not vary quickly with time, however, the P&O method fails to quickly track the maximum power points [6]. The incremental conductance (IncCond) method is also often used in PV systems. The IncCond method tracks the maximum power points by comparing the incremental and instantaneous conductance of the PV array. This incremental conductance method offers good performance under rapidly changing atmospheric conditions [7]. However, it has two divisions and the structure is similar with P&O algorithm because the condition, dP/dV=0, is rarely happen. For most PV modules, the ratio of the voltage at the maximum power point for different insolation levels to the open circuit voltage (Vmp/Voc) is approximately constant. Also, the ratio of the current at the maximum power point for different insolation levels to the short circuit current (Imp/Isc) is constant [8], [9]. Figs. 5 and 6 indicate the linear relation Vmp=0.77Voc and Imp = 0.89Isc with the computed (almost linear) dependency shown by “*” signs. Therefore, if unloaded cell is installed on the array and kept in the same environment as the power producing cells, and its open circuit voltage or short circuit current are periodically measured. The operating voltage or the current of the power producing array are then set to the required values, which corresponding to maximum power as shown in Figs. 5 and 6. The MPPT technique proposed in this work makes use of a predetermined relationship between the operating voltage or current and the open circuit voltage/short circuit current to obtain MPPT at any operating conditions.
(a) Irradiation
(b) PV generated power and maximum power
(c) PV voltage and reference voltage
60 Computed Simplified
50
Vmp [V]
40
30
20
(d) Duty cycle
10
0
0
10
20
30 40 Voc [V]
50
60
70
Figure 5. Vmp and Voc of PV module. 18 16 Computed Simplified
14
Imp [A]
12 10 8 6 4 2 0
0
2
4
6
8 10 Isc [A]
12
Figure 6. Imp and I sc of PV module.
14
16
18
Figure 7. PV generation system characteristics under MPPT.
Simulation of the PV system under investigation was carried out using PSIM software. The simulation results of the dynamic performance, which validates the efficient MPPT of PV generation system when the irradiance changes dramatically are presented. Fig. 7 shows the irradiation, the power and maximum power, PV voltage and reference voltage and the PV DC-DC boost converter duty cycle, respectively of the voltage-based maximum power point tracking technique when the irradiation changes dramatically from 1 kW/m2 to 0.25 kW/m2 and again to 1 kW/m2 at a step of 0.25 kW/m2 and at a time step of 1s. The proposed simple MPPT is efficiently able to capture
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the maximum power corresponds to each irradiance. The PV generated power is not constant and it depends on the irradiance conditions. IV.
WIND ENERGY SYSTEM
Because wind energy has become the least expensive source of new renewable energy that is also compatible with environment preservation programs, many countries promote wind power technology by means of national programs and market incentives. The wind turbine captures the wind’s kinetic energy in a rotor consisting of two or more blades mechanically coupled to an electrical generator. The fundamental equation governing the mechanical power capture of the wind turbine rotor blades, which drives the electrical generator, is given by:
P=
1 2
ρAC pV 3
controlled and the coefficient of performance to be maximized. Thus, in turn, the generated electrical energy may be maximized. Unfortunately, accurate wind speed measurement in the rotor of the turbine is difficult and requires the use of a relatively expensive anemometer if it is to be used for system control. Based on Eq. (4), the optimum speed of the rotor can be estimated as:
ωopt =
TSRoptV
(5)
R
Combining Eqs. (3) and (5), the output torque of the turbine can be written as:
T=
1 ρAC p max Rωopt 3 { } TSR 2 ω opt opt
(6)
(3)
where ρ : Air density (kg/m3) A : Area swept by the rotor blades V : Velocity of air (m/sec), C p : Power coefficient of the wind turbine. The theoretical maximum value of the power coefficient C p is 0.59 and it is often expressed as function of the rotor tip-speed to wind-speed ratio (TSR). TSR is defined as the linear speed of the rotor to the wind speed.
ωR V
(4)
where R and ω are the turbine radius and the angular speed, respectively. In practical designs, the maximum achievable C p ranges between 0.4 to 0.5 for modern high speed turbines and between 0.2 to 0.4 for slow speed turbines. Attaining C p above 0.4 is considered good. Whatever maximum value is attainable with a given wind turbine, it must be maintained constant at that value for the efficient capture of maximum wind power. A relatively small deviation on either side of the TSR will result in a significant reduction of the power available for conversion Cp to electrical energy. Fig. 8 exhibits the poor performance at different TSR for various types of wind turbines [10]. Fig. 9 illustrates the typical power coefficient C p curve for a 503 series WINDSEEKER by Southwest wind power, which is used for the analysis and simulations discussed in this paper. Fig. 9 shows that C p has its maximum value ( C p max ) at a certain optimum value of tipspeed to wind-speed ratio called TSRopt. It is clear that (for this case) the maximum power captured by the wind turbine will occur when TSR is approximately 9. The typical turbine torque and power vs. rotor speed are plotted in Figs.10 and 11. The maximum power for different wind speeds is generated at a different rotor speeds. Therefore, the turbine speed should be controlled to follow the ideal TSR, with an optimal operating point which is different for every wind speed. This is achieved by incorporating a speed control in the system design to run the rotor at high speed in high wind and at low speed in low wind. Employing control of the rotational speed of the turbine allows the TSR to be
Cp
Figure 8.
vs. TSR for various types of wind turbines.
0.5 0.45 0.4 Actual Simplified
0.35 0.3 Cp
TSR =
0.25 0.2 0.15 0.1 0.05 0
0
5
Figure 9. Typical
10
15
20 TSR]
25
30
35
40
C p curve used for the analysis and simulation.
A typical, small-scale, stand-alone, wind electric system is composed of a variable speed wind turbine, a permanentmagnet generator (PMG) and a diode bridge rectifier. In many small-scale systems, the dc system is set at a constant dc voltage and is usually comprised of a battery bank, allowing energy storage; a controller to keep the batteries from overcharging; and a load. The load may be dc or may include an inverter to an ac system. Connecting a wind generator to a constant dc voltage has significant problems
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due to the mismatching the poor impedance matching between the generator and the constant dc voltage (battery), which will limit power transfer to the dc system. In response to these problems, researchers have investigated incorporating a dc–dc converter in the dc link [11], [12].Adjusting the voltage on the dc rectifier will change the generator terminal voltage and thereby provide control over the current flowing out of the generator. Since the current is proportional to torque, the dc–dc converter will provide control over the speed of the turbine. Control of the dc–dc converter can be achieved by means of a predetermined relationship between rotor speed and rectifier dc voltage to achieve maximum power point tracking or by means of a predetermined relationship between generator electrical frequency and dc-link voltage [13].
terminal voltage of the generator can be determined for a machine with negligible saliency can be expressed as:
Va = E 2 − ( I a X s cosφ + I a Ra sin φ ) 2 + I a X s sin φ − I a Ra cosφ
It is assumed that the generator is connected to a diode rectifier and assumed that the phase voltage and fundamental component of the armature current of the generator are in phase. Then Eq. 9 may be written as
Va = E 2 − ( I a ωLs ) 2 − I a Ra
Figure 11. Wind turbine power vs. rotor speed.
A.
Permanent Magnet Synchronous Generator An analytical model of a small PMSM is used to investigate the effect of controlling the dc link voltage on the capture of maximum power. The model relates the dc link voltage of the machine to its rotor speed. It neglects magnetic saturation. The effective air gap in a PMSM with magnets mounted on the rotor surface can be considered constant and relatively large. This is due to the relative permeability of the PM material being close to unity. The d and q-axis synchronous reactances are consequently identical. The generator armature current can be related to the torque and induced voltage as follows:
T = Kt I a E = Ke I a
(7) (8)
Control over the rotor speed can be achieved simply by varying the generator terminal voltage. The steady state 1-4244-0449-5/06/$20.00 ©2006 IEEE
(10)
The rectified dc-link voltage may be obtained using the standard equations for a three-phase full-bridge diode rectifier taking the effect of commutation overlap into account as [14]
VDC = 3π 6 V − 2Vdiode − 3 ωLs I a 6
Figure 10. Wind turbine torque vs. rotor speed.
(9)
(11)
Using Eqs. (6)-(11), it is possible to obtain a prediction for the dc-link voltage as a function of the terminal phase voltage or mechanical speed and TSR. Fig. 12 shows the optimum relation between the dc voltage and the rotor speed for the capture of maximum power when the generator operates at the peak power coefficient C p max and TSRopt. Considering Eq. (4)-(6) and Fig. 12, a sudden increase in wind speed will decrease both TSR and C p . According to Eq. (6), an increase in the wind speed will result an increase in the torque transmitted from the turbine to the generator. Then, the turbine will try to accelerate in response to an increase in wind speed. An acceleration of the turbine will result in an increase in the commanded dc-link voltage (i.e., dc-link voltage will increase in response to an increase in wind speed). Increasing the dc link voltage increases the difference between the generated voltage and he dc-link voltage. Thus, the armature current decreases which decreases the braking torque. This will continue until the speed is increased such that torque is balanced. When the wind speed falls rapidly, a sudden decrease in wind speed will result in a high TSR and C p will decrease, decreasing the torque. With low applied torque to the generator, the inductance and inertia of the system will result in a braking torque being applied, slowing the generator and turbine. The reduction in speed will lower the dc–link voltage. As the dc voltage falls, the difference between generated voltage and dc-link voltage will remain high, maintaining current flow and applied braking torque. This process will continue until the speed is reduced such that the TSR is low enough that the turbine increases and torque is balanced. In order to evaluate the dynamic performance of the wind generation system, an example wind speed variation was developed, defined as
v w = 9 + 6 sin(4t ) + 0.6 sin(36t )
(12)
The choice of (12) allows the investigation of the system response to a fast and continuous change in wind speed. The development of the control relationship is based on the ideal steady-state relationship of the wind speed and rotational (turbine) speed given by Eq. 4. In case where the IPEMC 2006
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wind speed is continuously changing, the system inertia will introduce a time lag between a change in wind speed and a noticeable change in rotational speed. This time lag is neglected in this study. Fig. 13 depicts the simulation results of the dynamic performance which validates the efficient MPPT of WT generation system when the wind speed changes rapidly and continuously. Fig. 12 plots the variation in wind speed, power coefficient C p , tip-to-speed ratio TSR, rectified dclink voltage, wind power, generated power, turbine speed and dc-dc converter duty cycle. By controlling the dc-link link voltage according to Fig. 11, the TSR can be kept closer to the ideal value of 9 and the power coefficient is almost constant at its maximum value of 0.42. Therefore, the wind turbine generated power increases with wind speed. The output power from the wind system is not constant and varies with wind speed.
(c) Power coefficient
160 140 Computed Simulated Simplified
DC voltage [V]
120
(d) DC voltage and reference voltage
100 80 60 40 20 0
0
20
40
60
80 100 120 Rotor Speed [rad/s]
140
160
Figure 12. Optimum dc voltage vs. rotor speed characteristic.
180
(e) Wind power and generated power
(f) Turbine speed (a) Irradiation
(g) Duty cycle (b) Tip-speed-ratio
Figure 13. Wind generation system characteristics under MPPT.
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V.
Figure 14. Generated power of hybrid system.
CONCLUSIONS
This paper describes a renewable energy hybrid generation system combining solar photovoltaic and variable speed wind turbine. A simple and cost effective maximum power point tracking technique is proposed for the photovoltaic and wind turbine without measuring the environmental conditions. This is based on controlling the photovoltaic terminal voltage or current according to the open circuit voltage or short circuit current and the control relationship between the turbine speed and the dc-link voltage is obtained using simple calculations. More expensive and complex control algorithms are not required. A complete description of the hybrid system has been presented along with its detailed simulation results which ascertain its feasibility. The power fluctuation of the hybrid system is less dependent on the environmental conditions as compared to the power generated of individual PV and WG systems. This power fluctuation has been suppressed using a battery in this paper and it will be the subject of future work. REFERENCES [1]
Figure 15. Power supplied by battery. [2]
[3]
[4] [5] Figure 16. Load power.
Fig. 14 illustrates the total generated power of the hybrid system. The output power of hybrid system is mostly fluctuating and the fluctuation has an effect on system frequency. From Fig. 13, it is clear to note that the power fluctuation of the hybrid system is less dependent on the irradiance conditions and wind speed variations as compared to the power generated of individual PV and WG systems shown in Figs. 7(b) and 13(e). However, this fluctuation must be suppressed. One existing method to solve these issues is to install batteries which absorb power from the system as shown in Fig. 1. The other method is to install a dump load, which dissipates fluctuating power. Using these methods the PV/WT hybrid generation system can supply almost good quality power as shown in Fig. 15. Fig. 16 shows the power supplied by the battery. However, these methods have disadvantages that they require batteries, which are costly and the installation of dump load is not an efficient method to dissipate fluctuating power. Moreover, they can not guarantee certainty of load demands at all times especially at bad environmental conditions, where there is no power from the PV and WG systems. In the future work, the authors suggest a new hybrid generation system which combining Solar PV, WG and fuel cell generation systems. 1-4244-0449-5/06/$20.00 ©2006 IEEE
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