Essay: Photovoltaic systems

HISTORY OF PHOTOVOLTAIC
Photovoltaic effect was discovered by a French physicist named Edmund Becquerel in 1839 who had conducted an experiment with an electrolytic cell made up of two metal electrodes. Based on this experiment, Becquerel discovered that with the exposure of light, certain materials produced small amounts of electric current flow. Explanation of photoelectric effect by Einstein in 1905 established the foundation for the theoretical understanding of photovoltaic effect (Stapleton & Neill, 2012).
TYPES OF PHOTOVOLTAIC SYSTEMS
Photovoltaic usually categorise into their functional and operational requirement, their component configurations and how the equipment is connected to the other power sources and electrical loads (“Types of PV Systems,”). There are two types of the PV system which is grid-connected system and stand-alone system. These systems can be used to operate the interconnected with or independent of the utility grid and also connect with other energy sources and energy storage systems.
GRID-CONNECTED PHOTOVOLTAIC SYSTEM
Grid-connected systems could categorised by system configuration type which are distributed system and centralised system. The capacities vary widely for distributed systems especially for large buildings such as factories or stores and also in residential systems as shown in Figure 2.1. The capacity may be several MWp for centralised systems and this is also known as a solar form.
Figure 2.1: Example of residential grid connected PV system (Source: MathWorks)
STAND-ALONE PHOTOVOLTAIC SYSTEM
System that not connect to the local utility’s grid system is called stand-alone photovoltaic system as shown in Figure 2.2. Stand-alone photovoltaic system could be categorized by system configuration as follows:
PV direct – PV used to supply DC power directly to DC load.
PV battery – PV used to supply power to load during the day time or limited number of hours per day. Charge storage such as battery is used to store excess power for night usage.
PV hybrid system – PV used to supply power to the load via storage system and increased by the supplementary power source.
Figure 2.2: Example of stand-alone photovoltaic system (Source: Green Energy Innovation)
PHOTOVOLTAIC MODULE
A photovoltaic module is characterized based on current-voltage performance of the string of solar cells. By made of multiple interconnection of solar cells in turn e photovoltaic array is formed due to linked collection of photovoltaic modules (Tiwari & Dubey, 2009).
PHOTOVOLTAIC MODULE STRUCTURE
The structure of the photovoltaic module is determined by some requirements. Photovoltaic module structure made up of several layers of materials as shown in
Figure 2.3.
Figure 2.3: The layer of photovoltaic module structures (Source: Clean Electricity from Photovoltaics)
Figure 2.4: Structure of front and back of a photovoltaic module. (Source: Solar Panel Anatomy)
TYPES OF PHOTOVOLTAIC MODULE
There are several types of the photovoltaic module known which are mono-crystalline silicon, poly-crystalline silicon and thin films. These modules have different type of materials that make them different from each other.
Mono-crystalline Silicon Photovoltaic Module
Mono-crystalline silicon is also called single-crystalline silicon. Mono-crystalline silicon (mono-si) photovoltaic modules are made from highly purity silicon. Mono-crystalline or single-crystal silicon photovoltaic modules are widely used as the photovoltaic module as it is most efficient and effective (El Chaar, lamont, & El Zein, 2011). It is also relatively expensive because of manufacturing of the crystalline silicon take deliberate and cautious process to make the product.
Figure 2.5: Solar panel and solar cell of mono-crystalline silicon photovoltaic module.
Poly-crystalline Silicon Photovoltaic Module
Poly-crystalline silicon or multi-crystalline silicon photovoltaic module is formed when it cast into an ingot of multiple crystals (Fletcher, 2014). Poly-crystalline are cheaper than mono-crystalline silicon because of its simpler manufacturing process. Even the poly-crystalline silicon is cheaper, the efficiency of the material is 15% lower than mono-crystalline due to the existence of the grain boundaries (El Chaar et al., 2011).
Figure 2.6: Solar panel and solar cell of poly-cristalline silicon photovoltaic module
Thin-film Photovoltaic Module
Thin-film photovoltaic modules can be found in various of shapes and styles. Thin-film also known as amorphous silicon which means no definite shape that the module form (Fletcher, 2014).
Figure 2.7: Example of thin-film photovoltaic module.
INVERTERS IN GRID-CONNECTED PHOTOVOLTAIC SYSTEM
Inverter technology has been slow to advance as related to the capacity of handling power because it is an electronic technology. Inverters are the brain of the system because they are an important part of any solar installation.
There are some different types of common inverters use in grid-connected photovoltaic system nowadays (Zipp, 2016):
String inverters
Central inverters
Microinverters
Battery based inverters
TRANSFORMER INVERTER
The transformer in power inverter refers to a device which has two circuits partnered by a magnetic field that linked each other. Any changes in any circuit affects the other. The transformer inverter are used for the conversion of big primary currents into smaller and also ease the measurement of secondary currents (Watkins, 2013).
TRANSFORMERLESS INVERTER
Transformerless inverter is more advanced and more current innovation. The transformerless inverter can maximizes power distribution while simplifying the system for installation and also independent power-procedures (Watkins, 2013).
TYPES OF FAULTS IN GRID CONNECTED PHOTOVOLTAIC SYSTEM
Faults occur in grid connected photovoltaic system is divided into two main faults which are direct current (DC) grid side and photovoltaic (PV) system side or also known as alternating current (AC) side. Classification of possible faults may occur in grid connected photovoltaic (GCPV) system is shown in Figure 2.10.
Figure 2.10: Classifiction of faults in photovoltaic system
MAXIMUM POWER POINT TRACKER (MPPT) FAULT
MPPT increases the power fed to the inverter from photovoltaic (PV) array. The performance of maximum power point tracker (MPPT) reduces when the failure occurs in the charge regulators. The output voltage and the output power reduces when fault occur in MPPT (Mahalakshmi, Karuppasamypandiyan, Bhuvanesh, & Ganesh, 2016). Therefore, the ratio of output voltage and output power increase.
INVERTER FAULT
The AC output power will become low and DC output power remains the same, when there is a fault in the inverter. This details confirms that there is no possibility that a wire between modules or strings and inverter was broken or a breakdown occurs in strings or modules. Faults in the inverter are the reason for power loss (Mahalakshmi et al., 2016). To confirm whether a failure which happens in the inverter could be detected by this method, an inverter disconnection fault is generated on the GCPV system (Chine, Mellit, Pavan, & Kalogirou, 2014).
I-V CURVE CHARACTERISTICS
Output characteristics for a photovoltaic module can be obtained in an I-V curve graph (Figure 2.8). According to Fletcher (2014), the values of different voltage and current that represented by an I-V curve are usually based on standard operating conditions of a solar irradiance of 1000 Watts per square metre and cell temperature of 77°F (25°C) and also determine the size of photovoltaic array.
Figure 2.8: I-V curve for a common module size (Source: (Fletcher, 2014))
FILL FACTOR
The fill factor is a parameter which, determines the maximum power from a solar cell/module in conjunction with Voc and Isc. As shown in Figure 2.9 graphically, the fill factor is a measure of the “squareness” of the solar cell and is also the area of the largest rectangle which will fit in the IV curve (Solmetric Corporation, 2011).
Therefore, a higher fill factor means a higher module performance. The fill factor is the parameter of PV modules that used for the evaluation of the PV module performance (Azim, 2013):
= Vmp*Imp /Voc*Isc   (1)
The fill factor is defined as the ratio of the maximum power from the solar cell to the product of Voc and Isc, where;
FF= fill factor
= open circuit voltage
= short circuit voltage
∗= Pmax
Pmax = maximum power point
Figure 2.9: The “squareness” of the solar cell and the area of the largest rectangle which will fit in the IV curve (Source: (Solmetric Corporation, 2011)).