Principle of Photovoltaics

Photovoltaic modules and arrays have proven to be dependable electrical energy sources, but they must be correctly built for best efficacy. The photovoltaic cell is the fundamental unit of a solar system. Cells are electrical devices that use the photovoltaic effect to convert sunlight into direct current electricity. To generate electrical energy from the sun’s energy, photovoltaic cells do not require any moving parts. When sunlight strikes a cell, electrons are energized and an electric current is generated, which is transferred through wires within the cell to an electrical circuit. They need no fuel and have a life expectancy of at least 25 years. PV cells have the ability to meet a considerable portion of our electrical energy requirements.

A module is a grouping of solar cells that are linked in series or parallel to provide the desired voltage and current. When PV cells are wired in series, the voltage is added while the current remains constant, just like batteries. When they are connected in parallel, the current increases while the voltage remains constant.

To protect the PV cells from weather and other environmental conditions, they are enclosed within the module. Modules come in a variety of sizes and forms. They are typically flat rectangular panels that output ranging from 5 to 200 watts. The terms "module" and "panel" are frequently used interchangeably, although a panel is actually a series of modules linked together to generate a particular voltage. An array is a collection of panels that are linked together to provide the desired voltage and current.

Types of Modules

The following photovoltaic module components distinguish the various types of modules:

·         Cellular material

·         Glazing substance

·         Frame, hardware, and electrical connections

The cell material, which is the composition of the silicon crystalline structure, is the most significant component. The crystalline material can be formed as a single crystal (single-crystalline), cast as a poly-crystalline ingot, or deposited as a thin film (amorphous silicon). Although single crystalline cells are slightly more efficient than polycrystalline, the two forms of crystalline silicon cells operate similarly. Thin film or amorphous silicon is far less expensive to produce, although it is only approximately half as efficient as crystalline silicon cells.

Connecting PV Modules

PV modules are the building blocks of a solar system. Each module has a rated voltage or current, but they are linked together to achieve the desired system voltage.

Circuits in Series

Voltage increases when modules are joined in series. The current is kept constant by using series wiring. Figure 3.3 depicts two modules connected in series. It is important to note that series wiring connections are established from one module’s positive (+) end to another module’s negative (-) end.

Circuits in parallel

When modules are connected in parallel, currents are added while voltage remains constant. To boost a system’s current, the modules must be wired in parallel. Figure 3.4 depicts PV modules connected in parallel to create a 12V, 6amp system. It is important to note that parallel wiring adds currents while keeping the voltage constant. In addition, positive (+) to positive (+) and negative (-) to negative (-) parallel wiring connections are formed.

Circuits in Series and Parallel

To obtain the needed voltages and currents, systems may employ a combination of series and parallel wiring. Figure 3.5 depicts the wiring of four 12V, 3amp modules. Strings of two modules are connected in series to boost the voltage to 24V. The current is then increased to 6 amps by wiring each of these strings in parallel. As a result, the system is 24V, 6amp.

Performance of Module
A photovoltaic module’s total electrical output (wattage) is equal to its output voltage multiplied by its operating current. Any specific module’s output characteristics are defined by a performance curve known as an I-V curve, which depicts the relationship between current and voltage. The figure above is a typical I-V curve. The horizontal axis is used to plot voltage (V). The vertical axis is used to plot current (I). Most I-V curves are published using standard test conditions (STC) of 1000 watts per square meter sunlight (commonly referred to as one peak sun) and cell temperature of 25 degrees Celsius (77 degrees Fahrenheit). The maximum power point (representing both Vmp and Imp), the open circuit voltage (Voc), and the short circuit current (Isc) are all critical locations on the I-V curve.

Maximum Power Point

This point, labeled Vmp and Imp, is the operating point at which the module will provide the most output under the operating parameters specified for that curve. By extending a vertical line from the curve downward to read a value on the horizontal voltage scale, the voltage at the highest power point can be computed. At maximum power, the example in Figure 3.6 shows a voltage of around 17.3 volts.

The maximum power point current can be calculated by extending a horizontal line from the curve to the left and reading a value on the vertical current scale. At the maximum power point, the current in Figure 3.6 is roughly 2.5 amps.

The watts at the maximum power point is calculated by multiplying the maximum power point voltage by the maximum power point current. As the voltage drops, so does the power output. Most modules’ current and power output decrease when the voltage rises above the maximum power point.

Open Circuit Voltage

This point, labeled Voc, represents the highest voltage obtained when no current is pulled from the module. Because no current is flowing, the module has the highest electrical potential. Figure 3.6 depicts an open circuit value of approximately 21.4 volts. Because there is no current, the power output at Voc is zero watts.

Short Circuit Current

This is the highest current output that the module can achieve under the conditions of a circuit with zero resistance or a short circuit. The current in the example in Figure 3.6 is roughly 2.65 amps. Because the voltage is zero, the power output at Isc is zero watts.

Module Performance Factors

The following are the major factors that influence the performance of photovoltaic modules:

Load Resistance

The voltage at which the module will run is determined by a load or battery. In a nominal 12-volt battery system, for example, the battery voltage is often between 11.5 and 14 volts. The modules must operate at a little greater voltage than the battery bank voltage in order for the batteries to charge.

When at all possible, system designers should guarantee that the PV system works at voltages close to the array’s maximum power point. If the resistance of a load is well matched to the I-V curve of a module, the module will function at or near its maximum power point, resulting in the highest potential efficiency. As the resistance of the load grows, the module will operate at voltages higher than the maximum power point, reducing efficiency and current output. As the voltage falls below the maximum power point, efficiency falls as well.

When an inductive load, such as a pump or motor, is driven directly by the array, this interaction between the load and photovoltaic array is especially important. For best efficiency, a control device that tracks the maximum power point can be employed to continuously match the voltage and current operating needs of the load to the photovoltaic array.

Intensity of Sunlight

The current output of a module is proportional to the amount of solar energy to which it is exposed. More intense sunshine means higher module output.

Cell Temperature

The module performs less effectively and the voltage falls as the cell temperature increases above the recommended working temperature of 25 degrees C. As seen in Figure 3.8, as cell temperature goes above 25 degrees C (cell temperature, not ambient air temperature), the form of the I-V curve remains constant, but it swings to the left, suggesting reduced voltage output. To avoid high cell temperatures, airflow under and over the modules is crucial. Lower cell temperatures can be maintained by using a mounting method that allows for adequate ventilation, such as a stand-off or rack mount.

Shading

Photovoltaic modules will experience a significant output loss even with slight shade. Shading has a greater impact on some modules than others. The extreme effect of darkening on one cell of a crystalline cell module. One completely shaded cell can cut the module’s output by as much as 75%. Compared to this example, some modules are less impacted by shading.

Finding shading barriers on the site is a crucial component of evaluating the site. Underestimating the impacts of shading, even partial shading, might result in a reduction in the performance of the entire system. Some manufacturers utilize bypass diodes inside the module to let current travel over shaded cells, reducing the impact of shading covering the solar panel with any opaque or semi-transparent covering may result in a loss of current and your appliances turning off. The installation location should be such that the array is not shadowed between 9 a.m. to 3 p.m., when the sun is at its brightest. If there is any shadowing during this time, more modules will be required to create enough
electricity.

Table-1: Effects of shading on module power

Diodes

A diode is a semiconductor device that enables just one direction of electric current to flow. Diodes can be used for a variety of purposes in solar systems, including the following:

Diodes for Blocking

This type of diode is installed between the modules and the battery to prevent reverse current flow from the batteries to the array at night or during cloudy conditions. Some controllers include a diode or execute this function by closing the circuit. Never remove the blocking diode between the solar panel and the battery; otherwise, the reverse current flow will harm your panel.

Diodes with Bypass

In the event of shading, this sort of diode is linked within a module to deflect current around a few cells. Several bypass diodes are often pre-wired in a module, each in parallel with a group of cells.

Important Points

Ø  Photovoltaic cells use the photovoltaic effect to convert sunlight into direct current. They have no moving parts and do not require any fuel.

Ø  Although single-crystalline silicon cells are marginally more effective than polycrystalline silicon cells, amorphous silicon cells are only half as efficient as crystalline silicon cells. Amorphous silicon cells are significantly less expensive to produce than crystalline silicon cells.

Ø  When modules are joined in series, voltage adds and current remains constant, but current adds and voltage maintains constant when modules are connected in parallel.

Ø  The key elements influencing module performance are load resistance, sunshine intensity, cell temperature, and shading.

Ø  A blocking diode is installed between the modules and the battery to prevent battery current from reversing direction from the batteries to the array at night or during cloudy weather.

Ø  In the event of shading, a bypass diode is coupled in parallel with a few cells within a module to deflect current around the cells.