Solar power is a useful form of energy in that it can be used to dry cereals, to boil water or it creates winds by heating up the atmosphere. This is direct use of solar energy. This form of using solar energy directly limits its use. However, if the solar can be converted to electrical energy, its use to be expanded to different forms. Solar PV arrays are used to convert solar energy to electrical power if form of DC voltage and current. The volage is then boasted to higher DC voltage using DC to DC boast converter as shown in the diagram below.
Figure 1:MMPT and PI controller of solar system
The solar power is normally not constant but keeps on fluctuating depending on the time of the day. In the mornings and in the evenings the solar power reaching the PV arrays is low which during midday the solar power is higher. Therefore, the electrical output if capture directly will keep on fluctuating.
Due this a controller is designed to keep to power output of the solar system fairly constant. This controller is based on two parts. The first part is an MPPT controller which monitors the current and voltage output of a solar PV array. This translates directly to the power. The output of the MPPT system is fed to PI controller which feeds to a pwm generator. The output of a pwm generator are pulses whose width is varied by a PI controller. These pulses are fed to the gate of a boast converter to keep the voltage and current of the system fairly constant hence mitigate on the power fluctuation.
The MPPT system-maximum power point tracking algorithm tracks the solar PV array output in terms of voltage and current in order to extract maximum power from it.
This controller was designed and simulated as a Simulink model in Matlab. The results were analyzed in order to evaluate the effectiveness of the controller on the system.
The system has three main parts. The solar PV array, the controller and the boost converter. The solar PV array system is made of modules arrange in series and parallel arrangement. The series arrangement increases the volage output while the parallel arrangement increases the current output from the solar array. The solar PV array module chosen for this design was 1-Soltech module which has the following specifications.
- STC power rating : 215
- PTC Power rating 4
- PTC/STC Power Ratio 88.1%
- Open circuit voltage 3
- Short Circuit Current 84%
- Voltage at maximum power0
- Current at maximum power 35
For the for it to output 250-350v to there should be a series connection of 10+ modules at maximum power. To increase current hence power output from the modules, several of these series modules have connected in parallel. For this, 47 modules were connected in parallel. At 300 volts, the maximum power is 100kW when the solar radiation power is 1kW/m^2. The maximum current output the modules can produce is 290 Amps under this condition.
The boost converter is designed to have the input voltage of 250-350v and output 500v. The power output of the converter is 100w. The current and voltage ripples of 1% and 5% respectively. The switching frequency of the converter is 5kHz. the circuit of the converter is as shown in the diagram below.
The inputs of the model of solar PV array are: irradiance which is set to 1000watts/m^2 and temperature of 25 degrees Celsius. The output of the array is DC voltage which is connected to boost converter. The boost converter is made using IGBT transistors as a switch and the diode. The load of the converter is a parallel connection of a capacitor and a resistor. This is a low pass filter. DC output of solar PV array is connected to the converter whose gate is switched by pulses. Voltage and current measurement blocks measure the output voltage and current and display on the scope.
An MPPT controller is added. Current and voltage measurement from the solar PV array is fed to the MPPT controller whose output serves as the input of the PID controller.
Figure 2:graphs of power, current and voltage output from the system
1000 watts/m^2 irradiance power output from the converter is 100kW. The voltage output is 300volts while the current is 350 Amperes.
At 500 watts/m^2 irradiance the power output from the converter and solar PV array is half the maximum. The output is 50 kW while the voltage output is 300volts while the current is 180 amperes. Whenever the irradiance drops the current output converter increases its current output to maintain power around the maximum power while the voltage is constant
The controller works by maintaining constant voltage output of the converter while the current changes. The current change either reduces power output or increases it. The main goal of maintaining fairly constant voltage output around 300 volts has been achieved.