DEVELOPMENT OF A PHOTOVOLTAIC PANEL EMULATOR USING LABVIEW

Dale S.L. Dolan1, Joseph Durago

1, and Taufik

1

1California Polytechnic State University, San Luis Obispo, CA, USA

ABSTRACT

With increasing interest in renewable energies, large amounts of money and effort have been put into research and development for photovoltaic systems. The larger interest in PV systems has increased demand for appropriate equipment to test PV systems and for teaching and training the next generation of workers for the sector. A photovoltaic emulator is a device that satisfies a portion of these needs. A photovoltaic (PV) emulator was created using a programmable DC power supply and a developed GUI using Labview. The photovoltaic emulator provides the same current and voltage characteristics as any desired PV panel under various sets of environmental conditions. A PV emulator allows the remaining system to be analyzed in a controlled environment with control over the inherent variability of outside temperature and weather conditions and allows repeatable conditions to test PV equipment such as inverters and MPPT algorithms. The emulator provides an ideal tool and environment for both teaching and research at the University level without the costly investment of a set of commercial PV emulators. An existing power supply that can be used for a variety of other purposes is controlled via Labview to emulate a photovoltaic panel. The power supply can provide an open circuit voltage of 60V and short circuit current of 9A, such that any panel with parameters within this range may be emulated. Thus an extensive variety of panels may be emulated and the majority of panels from the Sandia database are within the capabilities of the available power supply.

DEVELOPMENT OF PV EMULATOR

Having a controlled environment to test PV equipment is difficult since consistent repeatable conditions are impossible to replicate outdoors. This approach has been taken by others on various platforms [1-5]. A PV panel’s electrical characteristics will change based on a variety of factors including the amount of irradiance received, temperature of the panel, and the material used to make the PV panel. The PV emulator will simulate the current and voltage characteristics of a photovoltaic panel under these various conditions. Having consistent electrical characteristics will allow easier analysis and optimization of PV systems.

A diagram for the hardware implementation for the PV emulator can be seen in Figure 1. It consists of a computer running Labview 8.6 and a programmable DC power supply. Labview will receive user specified parameters such as panel type, irradiance, and temperature, and will calculate the solar panel I-V curve for the power supply to emulate. Labview is used to control the power supply for its relative ease to communicate with hardware devices, ease of constructing sophisticated graphical user-interfaces, and extensive library for performing specific array manipulations and mathematical calculations.

Figure 1: PV Emulator Hardware Setup.

PV panel manufacturer I-V curves will be compared to those created by the emulator to evaluate the accuracy of the calculated curves. To test that the power supply is able to follow the I-V curve correctly, a power decade resistor box is connected to the power supply. The resistance will be varied from a large resistance to a small resistance to have the PV emulator sweep through the I-V curve. The speed at which the emulator is able to change its current and voltage, based on a change in load, will need to be minimized as much as possible to emulate the real-time response of an actual solar panel. The set of equations to model a photovoltaic module from its manufacturer parameters were obtained from [6]. The set of equations obtained from the Matlab code were modified to allow the emulation of various solar panels from the Sandia PV database in Labview. The Labview model currently has the ability to change the IV curve for each panel, based on the simulated irradiance and temperature. These may be set at the beginning of the emulation or dynamically throughout the emulation if desired. Figure 2, Figure 3 and Figure 4 show the Labview virtual instrument block diagrams developed for the PV emulator.

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Figure 2 shows the block diagram for obtaining an output current based on the generated voltage of the panel. This is the precursor for creating the entire IV curve for a given set of environmental conditions.

Figure 2: Block diagram for obtaining output current

based on voltage of panel.

Figure 3: Block diagram for creating entire IV curve.

Figure 3 shows the block diagram for creating the entire curve based on a particular panel at a specified set of environmental conditions. As load is varied the IV curve generated here is reproduced by the power supply through the control implemented in Labview as demonstrated in Figure 4.

Figure 4: Block diagram for power supply control based on IV curve and load.

The basic algorithm to model a photovoltaic module was obtained from an existing model that had been developed in Matlab [6]. The algorithm obtained from the Matlab code was modified to allow the emulation of various solar panels from the Sandia PV database in Labview. It retrieves specific PV panel parameters such as the panel material, Voc, Isc, and diode ideality. These parameters are used by the mathematical model to create the I-V curve for the PV emulator. The Labview model has the ability to change the I-V curve based on the simulated irradiance and temperature as well as the specific PV model type. Below are flowcharts showing how the algorithm of the PV emulator operates. Figure 5 shows the basic algorithm used in calculating the I-V curve. The inclusion of the series resistance parameter into the diode model makes the solution to the panel current iterative. Newton’s method is used for its fast convergence to iteratively solve for the output current [7]. The algorithm calculates a single output current based on a desired voltage. The full I-V curve is then created by creating a voltage array which contains elements from 0 Volts to the open-circuit voltage and the PV model is then used to calculate an array of corresponding currents. The calculated I-V curve, which is an array composed of discrete voltages and their corresponding current values, will be used by the main PV emulator algorithm to control the power supply’s output characteristics

Figure 5: Algorithm to Calculate I-V Curve.

Figure 6 shows the main algorithm of the PV emulator. It initializes the power supply at the beginning, by choosing the communication port that the computer and power supply will be communicating to one another with. Then the algorithm determines if it is the first time running through the loop. If it is the first time, then an initial I-V curve is calculated and the power supply’s maximum supply current is set to 10% above the short circuit current. The maximum supply current is set to this value so that the current does not swing wildly and output the absolute maximum rated current of the power supply.

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After creating the initial I-V curve the voltage and current output of the power supply is measured. From these measurements a new voltage and current operating point is found using a resistance line method. Finally the I-V curve and operating point are displayed in a graph. The process then repeats. If the panel type, irradiance, or temperature is changed by the user, a new I-V curve is calculated. This is done to minimize the amount of times the computer has to calculate an I-V curve and communicate with the power supply to improve the speed of the PV emulator.

Figure 6: Main PV Emulator Algorithm.

Figure 7 shows the graphical user interface (GUI) of the emulator operating at a single load point. The GUI displays the calculated I-V curve and its operating point, a resistance line, and power curve along with its operating point. Each of these curves can be turned on or off depending on what the user would like to be displayed on the graph by pressing the buttons below the graph. The

sliders to the right of the graph allow the user to change the temperature and irradiance. Above the sliders is a drop down menu where the user can select up to 284 different solar panels to emulate. Numeric values for the temperature, irradiance, output voltage, output current, and output power are displayed to the user. The emulator tracks the ideal IV curve quickly and accurately. As the user moves the temperature or irradiance slider up or down, the curves will change in real time to reflect the changes specified by the user. The GUI is intended to give the user an intuitive feel as to how a real solar panel would react to changes in temperature, irradiance and loads. It will also be possible in the future to program a time series of temperature and irradiance conditions to allow for repeatable testing conditions for balance of PV system components. For demonstration purposes a SunPower 205-BLK was emulated.

Figure 7. PV emulator GUI displaying IV curve, load operating point and power curve for SunPower SPR-205-BLK panel at 25

oC and 500 W/m

2. The panel is

operating at the maximum power point for this reduced irradiance level.

EMULATION RESULTS

Two power resistor decade boxes in parallel were used to determine how effective the PV emulator was in mimicking a commercially available solar panel. I-V curves generated by the PV emulator were compared to those given from a commercially available data sheet. By changing the resistance of the paralleled power resistors, a fine resolution sweep was made from the open-circuit voltage to the short-circuit current of the I-V curve. The operating point of the emulator was recorded to determine the accuracy with which the output voltage and current matched the generated I-V curve. Curves provided by solar panel manufactures were compared to curves calculated by the PV emulator. Figure 8 shows emulated I-V curves for a Sunpower 205-BLK panel. The curves shown are for varying irradiances of 1000W/m

2 ,800W/m

2 , and 500W/m

2 at 25°C and for

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50°C at 1000W/m2. Figure 9 shows the manufacturers I-V

curves for the same panel. It can be seen that the calculated I-V curves are nearly identical to the manufacturer supplied I-V curves and the steady-state operating points of the power supply fall right on top of the calculated curves, thus showing that the PV emulator is able to accurately emulate a solar panel at various irradiances. Figure 10 shows I-V curves for an emulated Solarex MSX-60 panel. The curves shown are for temperatures of 0°C, 25°C, 50°C, and 75°C at an irradiance of 1000W/m

2.

Figure 11 shows the manufacturers I-V curves for the same panel. It can be clearly seen that the PV emulator is able to accurately calculate an I-V curve that is nearly identical to the manufacturer I-V curves. The steady-state operating points of the emulator, shown as black circles in Fig. 10, are nearly identical to the calculated I-V curve and therefore the emulator is shown to be able to effectively emulate a solar panel at various temperatures.

Figure 8. SunPower 205-BLK PV Emulator IV-Curves for 1000 W/m

2, 800 W/m

2, and 500 W/m

2 at 25

oC and for

50 o

C at 1000 W/m2.

Figure 9. SunPower 205-BLK Manufacturer Supplied IV-Curves for 1000 W/m

2, 800 W/m

2, and 500 W/m

2 at

25oC and for 50

oC at 1000 W/m

2.

Figure 10. Solarex MSX-60 PV Emulator IV-Curves for 0

oC, 25

oC, 50

oC and 75

oC at an irradiance of 1000

W/m2

Figure 11. Solarex MSX-60 Manufacturer Supplied IV-Curves for 0

oC, 25

oC, 50

oC and 75

oC at an irradiance

of 1000 W/m2.

Figures 12-14 show the emulator operating at 3 distinct operating points for a SunPower 205-BLK panel, to illustrate the versatility of the emulator. Figure 12 shows an operating point to the left of the maximum power point. As resistance is further decreased the emulated panel would reach the short circuit current. Figure 13 shows an operating point at the maximum power point. As resistance is either increased or decreased it is seen that the emulated panel would produce decreased power. Figure 14 shows the emulated panel at an operating point to the right of the maximum power point. As resistance is further increased the emulated panel would reach its open circuit voltage and produce no power.

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Figure 12. PV emulator GUI displaying IV curve, load operating point and power curve for SunPower SPR-205-BLK panel at 25

oC and 1000 W/m

2. The operating

point is to the left of the maximum power point and is approaching short circuit current.

Figure 13. PV emulator GUI displaying IV curve, load operating point and power curve for SunPower SPR-205-BLK panel at 25

oC and 1000 W/m

2. The panel is

operating at the maximum power point.

Figure 7 shows an emulated SunPower SPR-205-BLK panel operating at its maximum power point when exposed to an irradiance of only 500 W/m

2. This can be

compared to Figure 13 where the emulated panel is exposed to an irradiance of 1000 W/m

2. This shows that

the emulator can reflect varying maximum power points as irradiance is altered. Figure 15 shows an emulated Solarex MSX-60 panel panel operating at its maximum power point when exposed to an irradiance of 1000 W/m

2. This can be

compared to Figure 13 where an emulated SunPower SPR-205-BLK panel is exposed to an irradiance of 1000 W/m

2. This shows the variation that is seen at the same

environmental conditions with different panels.

Figure 14. PV emulator GUI displaying IV curve, load operating point and power curve for SunPower SPR-205-BLK panel at 25

oC and 1000 W/m

2. The operating

point is to the right of the maximum power point and is approaching open circuit voltage.

Figure 15. PV emulator GUI displaying IV curve, load operating point and power curve for Solarex MSX-60 at 25

oC and 1000 W/m

2. The panel is operating at the

maximum power point. This demonstrates the flexibility of the PV emulator to emulate any of the hundreds of panels within the database at various irradiance and temperature conditions.

CONCLUSIONS

A PV emulator was developed using Labview and a programmable DC power supply. The emulator was shown to accurately emulate a variety of PV panels over a wide range of environmental conditions. The power supply can provide an open circuit voltage of 60V and short circuit current of 9A, such that any panel with this range of parameters may be emulated. The current implementation of the PV emulator thus allows for 284 different panels to be emulated accurately over a wide range of irradiances and temperatures. An extensive array of panels can be emulated and used for various research and educational purposes.

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