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SOLA2540 Applied Photovoltaics/ SOLA9001 Photovoltaics
Lab 1: Thermal & Electrical properties and modelling of PV modules
REAL LIFE SCENARIO
A major Australian Research and Development Facility has contracted by a PV installation company to estimate power generation by a photovoltaic system at different conditions. As an intern PV Engineer, the company has tasked you create a model of the solar model to be used for the PV system and study the effect of temperature and intensity of illumination on the power generation.
OBJECTIVES
In this lab, you will:
1. Determine temperature coefficients of a PV module.
2. Measure IV and Power-V curves and determine short circuit current, open circuit voltage, current at maximum powerpoint, voltage at maximum powerpoint, fill factor and efficiency of a solar module.
3. Extract series resistance, shunt resistance, and dark saturation current from IV curve and simulation.
4. Develop an electrical model of the solar module in LTSpice and use it to predict PV generation at a given solar insolation and temperature.
BACKGROUND
A solar cell can be modelled by the lumped parameter model. The equivalent of a solar cells circuit is shown in Fig. 1. The current source IL represents the current due to light generated carriers (electrons and holes), which are collected. Therefore, this is the largest possible current (ISC) from the solar cell. The diode represents the p-n junction itself.
The resistances RSH and RS are the shunt and series resistance, respectively. The series resistance RS models the resistive drops that carriers encounter by moving from where they are generated to the point of use. The shunt resistance RSH, models any unwanted conduction pathways that exist between the n and p contacts (usually due to manufacturing flaws).
Fig. 1: An equivalent circuit of a solar cell.
The current-voltage (I-V) relationship for such a device can be expressed as:
where:
IL = photo-generated current k = Boltzmann's constant = 1.38 x 10-23 JK-1
I0 = dark saturation current q = charge of electron = 1.6 x 10-19 C
n = diode ideality factor T = cell temperature (K)
RS = series resistance RSH = shunt resistance.
The dark saturation current I0 is also commonly represented by Is.
A solar module consist of a number of similar solar cells -typically connected in series (e.g. 60 cells or 72 cells). In a solar module with N identical series-connected cells, the current through each cell is identical and thus the module current IM is the same as the current through a single cell. However, the voltage of the module is given by, VM = N × V, where Vis the voltage of a single cell. The I-Vcurve of the module can therefore simply be obtained by scaling the voltage axis. That is by replacing V in the one-cell I-V curve by VM/N. The I-V relationship of N identical cells in series can be expressed as:
The term (VM/N + IMRS) is the voltage across D1 (and also across RSH) for a single cell. Eqn. (2) can be written as:
Where, Rs(m) = NRS (series resistance of the module), Rsh(m) = NRSh (shunt resistance of the module), and Vt = kT/q (thermal voltage).
The power generated by a solar module is given by the product of current and voltage:
PM = IM × VM (4)
Typical I-V curve (blue) and Power - V curve (red) of a solar module are shown in Fig. 2.
The voltage when the current IM = 0 is called the open-circuit voltage, VOC and the current when the voltage VM = 0 is called the short-circuit current, ISC. These are the maximum voltage and current generated by a solar module (at a given solar irradiation and temperature), although power generated at these points is zero. The operating point at which the power generation is maximum is called the maximum power point (MPP). The current, voltage and power corresponding to MPP are defined as Imp (current at MPP), Vmp (voltage at MPP) and Pmp (power at MPP), respectively. At short-circuit condition [VM=0], Eqn. (3) can be expressed as:
For modern solar module Rs ~ 0 and Rshis very large, therefore Eqn. (5) can be approximated as:
ISC ~IL (6)
Applying open-circuit voltage condition [IM=0] and setting IL = ISC, Eqn. (3) can be expressed as:
As shown by equations (6) and (7), the current ISC is proportional to IL which depend on the intensity of light, and VOC has logarithmic dependence on the intensity of light.
Module manufacturers provides performance of solar modules such as power output, ISC, VOC and VMP, at Standard Test Conditions (STC) [i.e. cell temperature = 25 。C and solar irradiance = 1000 W/m2 with AM1.5 spectrum]. However, the solar modules in the field are unlikely to be operating at 25 。C. When a solar module operates (under illumination), the temperature of the cell increases. This increase in the temperature depends on factors such as intensity of the illumination, ambient temperature, power conversion efficiency of the cell, thermal properties of the module, module mounting method and wind speed. There are different models to estimate solar module temperature. This paper details different methods to determine solar module temperature.
Current, voltage and power output of a solar module change with the cell temperature and their temperature dependence are given by:
Where,
• ISC(STC), VOC(STC), Vmp(STC) and Pmp(STC) are short-circuit current, open-circuit voltage, voltage at MPP and power at MPP at STC, respectively;
• tc(Isc), tc(Voc) and tc(Pmp) are temperature coefficient of ISC, VOC and Pmp, respectively; and
• ISC(T), VOC(T), Vmp(T) and Pmp(T ) are short-circuit current, open-circuit voltage, voltage at MPP and power at MPP atcell temperature T, respectively.
In PartI and Part II of this lab, you will determine thermal and electrical parameters of a solar module. In Part III, you will create an electrical model for the given solar module using the optimised value of these parameters and parameters provided by the solar module manufacturer. You will then use this model to predict the short circuit current, open circuit voltage and power generation at a given illumination intensity and temperature.
LABORATORY WORK
Location: Meet in LG10 TETB Building. Your demonstrator will lead you to outdoor measurement site. In case of a bad weather, you will perform the lab indoor. See instruction below for indoor measurement.
Important: Only expose the PV module to light when you are ready to take measurements! Note for indoor measurement (in case outdoor measurement is not possible):
Important: Only expose the PV module to light when you are ready to take measurements!
Use the halogen lights supplied. Adjust the distance between the solar module and the lights to get uniform illumination on the module. For Part II, adjust the distance between the solar module and the lights to vary the illumination intensity on the module.
Equipment
You will receive the equipment in a trolley (from LG10, Tyree Energy Technology Building). Please return the trolly with all equipment after your lab session.
|
• Solar Module with a frame |
• Power supply |
• Cables |
|
• Electronic Load |
• Laptop |
|
Equipment Set-Up
1. Takeout the solar module and a frame from the trolley and place it in anunshaded place. Point the solar module to the Sun by adjusting the orientation and tilt angle of the module. Solar radiation received by the solar module is maximum when the azimuth of the module = the azimuth of the Sun, and the tilt angle of the module = 90 degree – altitude of the Sun;
2. Connect the solar module to the electronic load using two MC4 extension cables. Make sure that the positive terminal of the MC4 cable is connected to the positive terminal in the electronic load and similarly for the negative terminal.
3. Connect the electronic load to the power supply. Leave the electronic load and power supply turned OFF.
4. Connect the leads from temperature sensor (at the back of the solar module) to the DAQ unit, and then the DAQ unit to the laptop.
5. Connect the USB cable from the electronic load (at the back) to the laptop.
6. Turn ON the laptop. (Password is the last four digits of the username).
7. Turn ON the power supply and the electronic load.
LabView Program
You can control the lab equipment and record data with “SOLA2540Lab1” LabView program. Shortcut to this program is available on the Desktop. Animage of the user interface is shown in Fig. 2.
Figure 2: Animage of SOLA2540Lab1 LabView user interface.
PART I: MEASUREMENT OF THERMAL COEFFICIENT OF SOLAR MODULE
PROGRAM INITIALISATION: Click the “Connect” button and then click “Run” . DO NOT create a new file. STOP the program. In the LabView program:
1. Choose the “VISA resource name” (from the dropdown menu, choose the connected COM port).
2. Turn ON the “Connect” button.
3. Turn ON the “Input ON/OFF” button.
4. Click “Run” (block arrow at the top-left corner). It will prompt a message to create a CSV file to save the data.
5. Click the “Temp” button.
6. Click the “ISC” button and thenunclick “ISC” button.
7. Click the “VOC” button and thenunclick then “VOC” button.
8. Type/ select 1sfor the “Interval”
9. Click the “VOC ISC” button.
Observe the temperature dependence of current and voltage, and STOP the program when VOC has stabilised (typically it take about 25 minutes). These measurements are displayed in the current and voltage readings toward the top of the LabView user interface and on the top graph.
PART II: MEASUREMENT OF THE SOLAR MODULE I-V CURVE
You will measure the solar module I-V curve at different illumination conditions- first using voltage, and then current sweep.
I-V measurement (voltage sweep)
In the LabView program:
1. Make sure the “VISA resource name” is selected (see STEP 1 in PART I).
2. Make sure the “Connect” button is tuned on (see STEP 2 in PART I).
3. Make sure the “Input ON/OFF” button is turned on (see STEP 3 in PART I).
4. Click “Run” (block arrow at the top-left corner). It will prompt a message to create a CSV file to save the data.
5. Select 100 for the “No. steps” and 0.5sfor the “Interval”
6. Make sure the “Temp” button is ON.
7. Click the “ISC” button and thenunclick it
8. Click the “VOC” button and thenunclick it
9. Click the “ IV(V)” button.
10. STOP the program when the I-V is over.
I-V measurement (current sweep)
In the LabView program:
1. Copy ISC data to the “offset value” box.
2. Click “Run” (block arrow at the top-left corner). It will prompt a message to create a CSV file to save the data. Create a new file.
3. Click the “IV(I)” button.
4. STOP the program when the I-V is over.
Measure I-V curve at two other solar irradiations (by changing tilt angle and/ or azimuth of the solar module). Use tilt angles and or azimuth angle that would give a wide range of ISC. After changing the tilt angle and/ or azimuth angle wait for about 10 minutes to let the module temperature stabilise.
PART III: MODELLING OF SOLAR MODULE
In this part you will first create a model of the solar module using the values supplied by the manufacturer and the measured values, and then use it to predict module parameter at different illumination intensity and temperature.
Fig. 3: An example of LTspice circuit simulation for a solar cell/ module.
A LTspice model of a solar module is shown in Fig. 3. Use the values for series resistance (Rs), shunt resistance (Rsh) and ideality factor (n) from the fit (Part II, Question 6). The diode (D1) can be modelled as [Is = Io(t) N = nNc Tnom = temp], where Is = Io(t) represents the temperature dependent diode ideality factor and is given as [see Eqn. (7)] :
Where,
• Isc(t) = Isc ( 1 + tc(Isc)(T − 25)) [Temperature dependence of Isc]
• Voc(t) = Voc ( 1 + tc(voc)(T − 25)) [Temperature dependence of Voc]
• Vt(t) = q/kT = 1.602e-19/1.38e-23(T+273.15)
• Nc is the number of series connected cells in the solar module.
Note that LTspice (and other SPICE simulations) default temperature is 27 ºC whereas cell temperature at STC is 25 ºC. Temperature coefficients of solar modules changes with illumination intensity, especially at low illumination but we will ignore this.
For DC analysis, goto Simulate > Edit Simulation Cmd, select DC sweep and set the conditions. To save the result data,double-click on the result window and select file > export data as text.