SOLAR Energy

The visible and near visible (Ultra-Violet and near Infrared) electromagnetic radiation that is emitted by the Sun is termed as Solar Radiation. The solar radiation has a spectral or wavelength, distribution that corresponds to different energy levels, short wavelength radiation has a higher energy than long wave length radiation.

These solar radiation convert into useful forms of energy such as "heat and electricity", using a variety of technologies. The technical feasibility and economical operation of these technologies at a specific location depends on the available solar radiation or solar resource.

Photovoltaic Effect:-

Photo-voltaics are the technology for conversion of solar radiation (sunlight) directly into electricity. A photo-voltaic cell also called a PV or a solar cell is the device used for this purpose. The PV technology has emerged as a promising technology to generate electricity for small applications like lighting and meeting other electrical needs of households in un-electrified areas.

Solar cells are generally made of high purity silicon wafers. Several materials such as silicon thin films (poly-crystaline and amorphous), gallium arsenide (GaAs), cadmium telluride (CdTe), Copper indium diselenide (CIS) etc. are also used to make solar cells.

The photoelectric effect was first observed in 1839 by a French pysicist, Edmund Bequerel, who found that certain materials are capable of producing small amount of electric current when exposed to light. In 1873 British scientist Willoughby Smith observed that ability of selenium to conduct electricity increased in direct proportion to the amount of light falling on it. In 1880 Charles Fritts made the first selenium based solar cell. In 1905 Albert Einstein described the nature of light and the photoelectric effect on which photovoltaic technology is based. He won a Nobel Prizes in Physics for this.

The first silicon based photovoltaic module was built by Bell Laboraties in 1954 which had an efficiency of about 6% It was too expensive to gain widespread use. In the 1960s, space industry began to make the first serious use of the PV technology to provide power aboard spacecraft. During the energy crisis in the 1970s, PV technology gained recognition as a reliable and potential source of power for domestic and industrial applications.

PHOTOVOLTAIC EFFECT

You may understand photovoltaic effect by observing the following:

1). Solar radiation (sunlight) are made of bundles of light energy called photons or light quanta. Photo behave like particles having zero mass and travel with the speed of light. Each photon is associated with a particular wavelength of solar spectrum and has energy defined by that wavelength.

2). Photons associated with different wavelengths have different energies.

3). Solar Cell is made of semiconductor material. When a photon strikes a solar cell, it may be either reflected, absorbed or pass through. Photons which get absorbed transfer their energy to the electrons in the semiconductor enabling them to generate electricity.

4). When enough photons are absorbed by a solar cell, electrons get detached from the material's atoms and move to the surface. The energy of a photon is transferred to an electron in an atom of the semiconductor device. The electron is able to escape from its normal position associated with a single atom in the semiconductor and is free to move and create to a current in an electrical circuit.

5). When electrons (-ve charge) are able to leave their position, holes (+ve charge) are created.

6). When large number of electrons travel towards the front surface of the cell, an imbalance of charge between the cell's front and back surfaces creates a voltage potential similar to the negative and positive terminals of a battery.

7). The electricity flows when two surfaces are connect through an electrical load.

Conditions for Occurance of Photovoltaic Effect:-

The basic conditions for the photovoltaic effect to occur are:-

1). Absorption of photons creating electrons-hole pairs in a semi-conductor.

2). Separation of the electrons and holes so that their recombination

3). Collection of the electrons and holes, separately by each of two current-collecting electrodes so that current can be induced to flow in a circuit external to the semi-conductor itself.

Solar Cell :-

There are basic types of solar cells made from silicon. These are:-

1). Single Crystal

2). Poly crystalline

3). Amorphous

1). Single Crystal Silicon:- They are sliced into round or hexagonal wafers form in long cylinders. The process is highly energy intensive and also results in material waste but gives the highest efficiency cells of the order of 25% which could be further enhanced to about 30% when used in combination with concentrators.

2). Polycrystalline Silicon:- They are made of molten silicon cast into ingots or drawn into sheets and thereafter sliced into squares. The efficiency of these cells is about 15% but cost is relatively less as compared to single-crystal cells.

3). Amorphous Silicon:- The silicon is sprayed onto a glass or metal surface in thin films. This results in making the module in one step. The production approach is least expensive but the efficiency of these cells is about 5%.

Sizing of PHOTOVOLTAIC SYSTEMS:-

The applications of solar PV technology are increasing very rapidly. At present, the following three areas are gaining importance:-

1). Domestic applications (home light, street light, community light etc.).

2). In agriculture sector for lift irrigation.

3). Power generation.

For making solar PV system, you need to know the following things:-

1). The energy to be used by the appliances under consideration.

2). The energy storage capacity of the battery.

3). The energy to be used by the appliances under consideration.

4). The size of solar PV system.

Classification of PV System:-

Photovoltaic power systems are generally classified according to the functional and operational requirements, the component configurations and how these are to be connected to other power sources and electrical loads. The two principle classification are:-

1). GRID - CONNECTED or UTILITY-INTERACTIVE PV SYSTEMS

2). STAND-ALONE PV SYSTEMS

Photovoltaic systems can be designed to provide DC and/or AC power applications, can be operate interconnected with or independent of the utility grid and can be connected with other energy sources and energy storage systems.

Grid-Connected PV Systems:-

A grid-connected PV system is shown in figure. The primary component of such a system is an inverter. It is also called power conditioning unit (PCU). The PCU converts the DC power produced by the PV array into AC electricity of the same quality that comes from the power grid. Any energy that is not used is fed back into the power grid for others to use.

The PCU automatically stops supplying power to the grid when the utility grid is not energized. During off sunshine hours and during other periods when the electrical loads are greater than the PV system output, the balance of power required by the loads is received from the electric utility. This safety feature is required in all grid-connected PV systems and ensures that the PV system will not continue to operate and feedback onto the utility grid when the grid is down for service or repair.

GRID-CONNECTED PV SYSTEMS

Stand Alone PV Systems:-

Stand-alone PV systems, as the name indicates, are designed to operates independent of the electric utility grid. Such systems are sized to supply certain DC and/or AC electrical loads. These types of systems may be powered by a PV array only or may use biomass/biogas based power generation system. Such types of systems are called a PV-hybrid system. The simplest type of stand-alone PV system is shown in figure.

STAND-ALONE PV SYSTEM

SOLAR PV POWER PLANTS:-

In SPV power plant, electricity is centrally generated. The electricity is either made available to users through a local grid in a "stand alone" mode or connected to the conventional power grid in a "grid interactive" mode.

Stand-alone power plants provide grid-quality power locally to people to meet their requirements for lighting and other needs. Power plants are preferred over individual SPV systems if a number of uses are in close proximity.

STAND ALONE SOLAR POWER PLANT:-

A stand alone SPV power plant is typically designed for specific requirements. The capacity of a stand-alone power plant usually varies from 1 KWp to 25 KWp. These systems are used where conventional grid supply is not available or is erratic or irregular. A stand-alone power plant function like an uninterrupted power supply (UPS) and provide a constant, stable and reliable supply is available; in such places the power plants operate like a hybrid power plant, working with grid as well as with SPV. The capacity of its battery bank depends on user requirements. The most common use for such plants is the electrification of remote villages. Other uses includes power for hospitals, hotels, communications equipment, railway stations, broder outposts etc.

SPV POWER PLANT

Stand alone SPV power plant comprise PV array, battery bank, inverter and charge controller. Depending on the system voltage, SPV modules are arranged in series and parallel combinations. The size of the battery bank is determined by the system voltage and ampere-hour requirements of the load. The inverter is selected based on the system voltage and peak load capacities. Other components such as junction boxes, distribution boxes and cables are selected according to the maximum amount of current to be handled by them.

Solar Generators:-

A solar generator is a small capacity, stand alone SPV power system based on a PV array, connected to a battery bank and an inverter of appropriate size. This system is designed to limited loads for a period of two to three hours daily in situations such as conventional power failure or load-shedding. The MNRE, Govt. of India is promoting four models of solar generators with capacities of 150, 350, 450 and 600 Wp. These solar generators are mainly meant to replace the conventional small-capacity petrol based generators that are used during routine load-shedding periods in urban areas by shops, clinics and other small establishments.

The components of a typical solar generator are a small SPV array connected to a battery bank of appropriate size and an inverter based on 12, 24,or 48 V. The system is designed to supply power to loads such as lights, fans, credit card, operating machines and personal computer for a period of two to three hours.

SOLAR GENERATOR

Building-integrated PV system:-

In a building-integrated photovoltaic (BIPV) system, PV panels are integrated into the roof or facade of a building. BIPV system are becoming common in Europe, USA and Japan. The SPV panels generate electricity during the daytime, which is used to meet a part of electrical energy needs of nthe building. BIPV systems have significant potential in INDIA, where a large number of building are constructed every year for different purposes and where energy consumption in buildings is growing at a rapid rate. Although the initial costs of a BIPV system are high, long term savings result from a reduction in electricity consumption.

India needs more experience in the field of BIPV technology. In order to encourage this application and to prepare manufacturers and users, the Ministry support BIPV project by meeting 80% of the cost of PV modules installed in the systems on government and semi-government buildings.

BIPV SYSTEM

SPV Pumping System :-

Water pumping is one of the most important applications of PV in INDIA. A SPV water pump is DC or AC, surface-mounted or submersible or floating pump that runs on power from an SPV array is mounted on a suitable structure and placed in a shadow free open space with its modules facing south and inclined at local latitude.

A typical SPV water-pumping system consists of an SPV array of 200-3000Wp capacity, mounted on a tracking type of structure. The array is mounted, submersible or floating type. Interconnecting cables and electronics irrigation as well as for drinking. The normal pumping heads are in the range of 10 metre for irrigation and 30 metre for drinking water.

The SPV array converts sunlight into electricity an ddelivers it to run the motor to pump up water. The water can be stored in tanks for use during non-sunny hours. For maximum power output from the SPV array, the structure on which it is mounted should track the sun. Electronic devices are used to do this in some models, thereby enabling the systems to operate at maximum power output.