The key component of PV system is PV panels. PV panels convert available solar energy into usable electrical energy. Photovoltaic panel requires various other components to complete the process of energy generation, storage and transmission depending on the PV technology being used and the function of the PV. They typically include an AC-DC converter, a battery bank for remote systems, auxiliary energy supply, charge controller, rectifier, and energy distribution to appliances or to the grid. The system also includes a series of electrical wiring for connection and controls panels at various levels.
A photovoltaic panel is made up of basic element called a photovoltaic cell. It works on the principle of absorbing photons in light to generate electricity. Various types of photovoltaic materials are available today such as mono/poly/silicon and thin film technologies such as Cadmium telluride (CdTe), copper indium gallium selenide (CIGS) and amorphous silicon. Several photovoltaic cells are grouped together to form a photovoltaic module and a solar PV panel consists of a number of such modules in arrays. The electricity generation is clean and silent since there is no burning of fuel involved or does not involve any moving parts.
Traditional PV cells are made up of crystalline silicon. Two layers of crystalline silicon are attached together to form a diode. One layer is positively doped to form a P-layer and the other layer is negatively doped to form an N-layer. The junction thus forms a PV diode junction capable of transferring electrons. When a P-layer with excess electrons absorbs photons from light, the extra electrons are released and are then absorbed on the N-layer, creating a potential difference in the process. This potential difference can be tapped and stored or used as electrical energy in various forms.
The basic element of a photovoltaic panel is a solar photovoltaic cell. Several cells form a module. A single photovoltaic panel consists of an array of such modules. Often sets of four or more smaller modules are framed or attached together by struts in what is called a panel.
Photovoltaic panels have long been using various forms of crystalline silicon as the basic element for making cells and it represents 85-90% of the global annual market (IEA, 2010). Various other types of materials used in the process of making PV cells are Cadmium telluride (CdTe), copper indium gallium selenide (CIGS) and are referred to as thin film technologies. PV technologies can be broadly classified into two types crystalline silicon and thin film technologies. Crystalline cells are made from pure silicon and are typically 15-200 microns thick where as think film is made by depositing layers of semiconductor material barely 0,3-2 micrometres thick on to glass or steel substrates. Various technologies currently available are listed in figure below.
The performance of PV modules and arrays is generally rated according to their maximum DC power output (watts) under Standard Test Conditions (STC). Standard Test Conditions are defined by a module (cell) operating temperature of 25 °C, and incident solar irradiance level of 1000 W/m2 and under Air Mass 1.5 spectral distribution. Since these conditions are not always typical of how PV modules and arrays operate in the field, actual performance is usually 85 to 90 % of the STC rating (Florida Solar Energy Centre, 2013).
A graphical representation of current Vs. voltage (as shown in the figure below) commonly known as i-v curve that is representative of a Photovoltaic panel is used to understand the performance characteris-tics of a photovoltaic panel.
The i-v curve represents the current (i) in Amperes and voltage (v) in Volts for a given solar panel under various levels of irradiation. This helps in understanding how the solar PV panel performs under various levels of irradiance for a given kWp capacity. Other critical information that this graph gives is the power that is generated under different levels of irradiance.
Efficiency of various types of PV panels
The efficiency of the solar PV cell depends on the capacity of the PV cell to convert sunlight into electrical power. Conversion efficiency can be defined as the ratio between the produced electrical power and the amount of incident solar energy per second and is one of the main performance indicators of PV cells and modules (IEA, 2010). Efficiencies and approximate cost range of different PV module technologies can be seen in the figure below.
Orientation
For any given location the solar panel should face equator with little or no deviations towards East or West. The more the deviation from the Equator the more is the loss in harnessing solar radiation (see figure below).
Cell temperature
The temperature of the PV cell critically effects it’s performance. The standard power output of a PV cell is measured at STC when the temperature of the cell is 25 C. As the temperature of the cell increases it’s efficiency decreases. The performance depreciation is different for different types of PV cells such as crystalline silicon and thin films etc. A typical drop in performance relative to STC is shown in the figure below. Hence it is always recommended to ensure the cell temperature is maintained at STC conditions and at the same time not affecting the amount of solar radiation falling onto it.
Shading
Typical crystalline PV modules contained a group of 12-18 cells a group. Bypass diodes are placed across a group of cells to prevent damage from shading of individual cells. Thus, even if a single cell is shaded from a particular group the generation from the entire group is bypassed. Random shading should be kept away from any elements that cast a shadow on them and will drastically reduce the output of the entire PV module. Partial shading of a portion of PV panel of crystalline silicon PV panel however, will still produce electricity but at a lower rate (as shown in the figure below). Shaded PV modules can still be designed to generate partial electricity for a portion of the day although it has to be ensured that any shading that is not evaluated in the design such as from the growth of trees etc. should be checked regularly. A site shade survey should be thoroughly done before the designing of a PV system.
Other factors
Besides cell temperature and shading other important factors that effect the performance and efficien-cy of the photovoltaic system are dirt and dust which can reduce the efficiency by approximately 7%, module mismatch and wiring losses which account up to 5% system power losses and also DC to AC conversion losses which are anywhere between 8-12%.
Photovoltaic system can either be a grid connected or a stand-alone system or a hybrid one combining both the features. The configuration depends primarily on the purpose of the PV system and its capacity. A typical solar photovoltaic installation consists of the following components:
Sufficient space has to be provided within the building to house components other than the PV panels like battery bank, inverter etc.
Hybrid system
A hybrid system consists of provision for both onsite use with battery storage and auxiliary backup or integration with other power systems such as wind turbine, energy generator or grid supply.
Stand alone system
A standalone PV system typically serves onsite energy requirement and is operated independent of the grid. It provides for storage of excess energy generated. In this configuration, the PV system is either connected directly to a DC load or it is connected to a battery or a power inverter. Loads needing AC power can be served through the power inverter and conditioner. The excess energy generated is stored in the batteries for later use.
Grid connected system
A grid connected PV system consists of a PV panel array connected to a power inverter and conditioner, which are then connected to distribution panel. The power from the distribution panel is either used by onsite AC loads or directly fed into the electrical grid. During the day when energy generation exceeds the required onsite capacity the excess can be backfed into the grid. During nights and cloudy days when the power generation through the PV system falls short of required onsite capacity power can be drawn from the utility grid to meet the demand. This configuration is used when there is no pro-vision to store the energy generated through the PV system.
Total annual irradiance available on the site, area available for the installation of solar panels, type and capacity of each solar panel would determine the maximum potential of harnessing the solar energy. An assessment of the load that is to be catered by the PV system also needs to be done. This is important to determine the kind of PV system and other components to be installed. After an initial assessment the following configurations could be considered. In addition, the area available for the installation of PV system and cost benefit analysis are the key factors in determining the type of PV systems to be installed.
| Case | Configuration |
|---|---|
| System capacity > load | Standalone system with provision for back feeding to the grid |
| System capacity < load | Standalone system with auxiliary power backup from the utility/grid/other power source |
Location, area available for the installation, technology available, cost benefit analysis are the key factors that influence the installation and performance of solar PV system.
Freestanding systems
Freestanding PV systems can typically be a standalone, grid connected or
hybrid system. The PV system is placed on site in a shade free region
aloof from the building. This is typically done when there is free space
available in the building precincts.
Roof top systems
PV systems are located on the rooftop of the buildings when the available free area for on site PV installation is less and the building’s footprint occupies significant area of the site. The roof top area free from strong shading from other service installation on the roof is selected for PV system installation.
Integration as façade, shading elements etc.
Modern PV systems offer customization to be integrated into building façade as an external finish. Building elements such as shading devices like fixed shades and louvers could be replaced with PV panels to serve dual purpose of shading and energy generation. Sunspaces and sky roofs can be replaced with modern transparent/translucent PV panels instead of plain glass.
Commissioning, installation of key components, calibration and maintenance
The following steps should be followed while installing a solar PV system
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