Solar Cells (Photovoltaic Cells)
A Solar Cell is an electrical device that converts light energy to electrical energy.
General Theory:
Photons from a light source are absorbed by the photovoltaic materials within the photovoltaic panel. Once the energy absorption reaches a critical point, electrons are released from the semiconductor. Due to certain properties of the cell configuration within the panel, electrons are forced to go a single direction. Therefore from this effect we can achieve a substantial current from the photovoltaic panel
Materials:
The most prevalent semiconductor material for solar cells is crystalline silicon. This is due to its properties in electron and hole mobility. In addition, the optical properties of the crystalline silicon enable the electrons to be released with minimal electron volts of photon energy. However, crystalline silicon inherently does not have as good conductivity as copper. To improve this impurities are added to the crystals to not only improve conductivity and electron mobility, but may also change the cells photovoltaic effects.
For instance, dope crystalline silicon with phosphorous to increase the amount of free electrons available. By doing this we can drastically decrease the ionization energy required to free the valence electrons of the silicon-phosphorous molecule. This Si-P layer creates a N-type material. However, this is electrically neutral material. To accomplish this the crystalline silicon is doped with boron to create a P-type layer. The final result is a P-N Junction in which the electrons can flow from the N layer to the P layer.
Figure 1 - Solar Panel Cross-Section
If we take a look at the silicon atom, we can see that it has four valence electrons, which are free to move about the P-N junction. By doping the silicon with phosphorous atoms we can increase this number to 12 valence electrons within the c-Si-P layer.
With the P-N junction, electrons can only flow in a single direction. If we connect a load across the P-N junction, we can create a current flow.
Pulse Width Modulation
Pulse Width Modulation is a technique to convert the information stored within the amplitude of the input signal to the duty cycle of a rectangular pulse. The output waveform with varying duty cycles can then be used as an input to voltage switches as well as servos.
Method:
A simple way to implement a PWM is using a sawtooth reference signal and a comparator. The comparator compares the voltage difference between the input signal and the reference sawtooth signal. If, for example, voltage of the input signal is larger than the voltage of the reference signal, the comparator will output a High for the duration in which the input signal has a larger voltage than the reference signal. On the other hand if the input voltage is less than the reference voltage the comparator will have an output Low.
Using a PWM microcontroller we can achieve an efficient PWM. The type of PWM that most microcontrollers use is called Time Proportioning. Time Proportioning, is practical in switch use. For example, Time Proportion Control can be used as a proportional controller for a heater/cooler.
Maximum Power Point Tracking
The Maximum Power Point Tracking System is used as a mechanism, which extracts the most optimum power from the solar panel. It does this by sampling the output of the solar panel and apply a corresponding resistance (load) to properly achieve the maximum power output.
The I-V curve above describes the current and voltage level outputs. The shaded region represents the power at that given point PMAX. This I-V curve changes at different times of the day when the solar panel gets more or less sun.
The above curves represent various irradiation (sunlight) levels and their corresponding PMAX points. It can be seen that the PMAX point occurs at the ‘knee’ of the I-V curve of the solar cells.
Using this information the MPPT calculates a precise resistive load to achieve optimum power. This effect can be described by using Ohm’s Law:
Therefore, for a given resistor value we can achieve optimum Power using the output current and voltage of the solar cell.
Methods:
- The most common method of MPPT is the perturb and observe method. This method observes the output power and perturbs the current in a given direction. The change in direction occurs when the MPPT senses a drop in power.
The above flow diagram describes a software implementation of the MPPT. When a disturbance or change occurs within the conditions of the charge circuitry, power will be calculated at various voltages and compared to past values. If power increases with an incremented voltage, it will continue to increase voltage until the corresponding power values start to decrease. Voltage will then be decremented to again find the maximum power point before reaching a lower power value.
2. Incremental Conductance: uses the fact that the slope of the power-voltage curve should be approximately zero at the maximum power point. This implies that a negative slope should be to the left of the max power point, and at the right of the maximum power point. Maintaining this balance of negative slopes ensures operation at local maxima of the I-V curve.
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