Solar energy is the main prerequisite of the life on the Earth and it is the most readily available source of energy. India has incredible potential and promising as one of the leaders in solar power generation. The geographical location of India is favorable for solar energy implementation due to an average annual temperature that ranges from 25 °C –27.5 °C and its location is between the Tropic of Cancer and the Equator. Most parts of India get 300 days of sunshine a year, which make the country a very promising place for solar energy utilization. The daily average solar energy incident over India varies from 4 to 7 kWh/m2 with the sunshine hours ranging between 2300 and 3200 per year, depending upon a location.
Solar energy is a clean renewable energy with zero emission and recent developments in solar energy systems are easily available for industrial and domestic use with the added advantage of minimum maintenance.
Chemistry of Electricity Generation with Solar Cells
Sunlight is composed of photons which contain various amounts of energy corresponds to different wavelengths of solar spectrum. When photons hit PV cell, some of the photos reflected. Electricity will be generated by only absorbed photons. Energy of the absorbed photons will be transferred to an electron in an atom of the cell. The electron by acquiring the energy is able to escape from its bonding associated with the atom to constitute current in the electrical circuit.
To produce electricity within a PV cell, a junction of two different semiconductors P type or N type is to be created. The common way of creating a P or N type Silicon is adding an element of excess of electron or deficit of electron by the process of doping. The most common material used in manufacturing of PV cell is Silicon. N type, P type materials can be achieved by doping V group elements and III group elements in the periodic table to Silicon which belongs to IV group, atomic number of Silicon is 14 and has 4 valance electrons 1s2 2s2 2p6 3s2 3p2 respectively. The most commonly used V group element is Phosphorous and its atomic number is 15 and has 5 valance electrons 1s2 2s2 2p6 3s2 3p3 and III group element is Boron and its atomic number is 5 and has 3 valance electrons, 1s2 2s2 2p1.
The photovoltaic module compromises of photovoltaic cells which convert solar energy into electric energy. The power output of the cell depends upon two factors i.e. amount of energy incident on it and ts operating temperature. The power output of single cell can supply small loads like calculator, watches. In order to generate large amounts of power cells, it can be arranged in series and parallel connections. The photovoltaic module is an array of photovoltaic cells arranged in single mounting mould.
Types of Solar Cells
There are three main type of commercially available PV cells and are categorized namely 1) Mono Crystalline Silicon PV 2) Polycrystalline Silicon PV 3). Thin film amorphous Silicon PV. Presently, the first two categories dominate world markets constituting 93% and third category for 7% of the market.
Monocrystalline Cell Technology
Monocrystalline Silicon is used in the manufacturing of high performance solar cells, as the name reveals these are made of single crystal. Its advantages and disadvantages are mentioned below:
- Well established and proven technology
- Relatively efficient
- These have longer life time
- Expensive material to manufacture
- Lot of waste in slicing wafers
- Round cells cannot be spaced in modules efficiently
Multicrystalline Cell Technology
The production process allows multiple crystalline structures to develop within cell. It is easier to implement in production line. It is relatively cheaper than Monocrystalline at the expense of lower efficiency.
- Well established and tested technology
- Relatively efficient (typically 14-16% efficient )
- Square cells for more efficient spacing.
- Expensive material and fairly costly to manufacture
- Lot of waste in slicing wafers
- Slightly less efficient than single crystalline Silicon (Due to lower Silicon purity)
- Polycrystalline solar panels tend to have slightly lower heat tolerance than Monocrystalline solar panels.
Thin Film Technology
Thin-film solar cell is a second generation solar cell that is made by depositing one or more thin layers, or thin film of photovoltaic material on a substrate, such as glass, plastic or metal. Thin-film solar cells are commercially used in several technologies and this technology uses less Silicon to develop the cell allowing for cheaper production costs. It tends to be less expensive but also lower efficiency.
- Very low material use
- Potential for highly automated and very rapid production
- Very low cost
- High temperatures and shading have less impact on solar panel performance
- These panels tend to degrade faster than mono crystalline and poly crystalline panels.
- Low efficiency.
Types of Solar Power Systems
Photovoltaic (PV) system converts solar energy into electric energy through photovoltaic effect. PV modules are main building blocks of the system. These modules are arranged into arrays to produce more electrical energy. PV system broadly classified into two major groups:
- Stand-Alone PV System
- Grid Tied PV System
Stand-Alone PV System
Stand-alone systems rely on solar power only. The system consists of the PV modules and a load only or they can include batteries for energy storage. When using batteries charge regulators are included, which switch off the PV modules when batteries are fully charged, and may switch off the load to prevent the batteries from being discharged below a certain limit. The batteries must have enough capacity to store the energy produced during the day to be used at night and during periods of poor weather. Figure 1 shows schematically a stand-alone system.
Grid Tied PV Systems
Grid-connected PV systems have become increasingly popular for building integrated applications. As illustrated in figure 2, they are connected to the grid through inverters, which convert the DC power into AC power. In small systems, as they are installed in residential homes, the inverter is connected to the distribution board, from where the PV-generated power is transferred into the electricity grid or to AC appliances in the house.
These systems do not require batteries, since they are connected to the grid, which acts as a buffer into that an oversupply of PV electricity is transported while the grid also supplies the house with electricity in times of insufficient PV power generation.
Electrical System Components for PV Solar System
In this section, the major electrical components require to function and operate the PV system is presented. It includes major components such as DC-AC inverter, battery bank and battery controller.
Inverters are used to transform DC current to AC current. Solar inverters may be classified into three broad types such as 1) Stand-alone inverters, 2) Grid-tie inverters and 3) Battery backup inverters.
These are used in isolated systems where the inverter draws its DC energy from batteries charged by photovoltaic arrays. Many stand-alone inverters also incorporate integral battery chargers to refill the battery from an AC source, when available.
These inverters match phase with a utility-supplied sine wave. Grid-tie inverters are designed to shut down automatically upon loss of utility supply, for safety reasons. They do not provide backup power during utility outages.
Battery Backup inverters
This type of inverters are designed to draw energy from a battery, manage the battery charge via an onboard charger, and export excess energy to the utility grid. These inverters are capable of supplying AC energy to selected loads during a utility outage, and are required to have anti-islanding protection.
These are most commonly used to store energy in standalone applications for use at times when no irradiance available at night times, rainy seasons. Batteries are also used for diverse number of applications including standby power and utility interactive schemes. PV batteries should have deep tolerance to deep discharges and irregular charging patterns. Some application may require the batteries to remain at a random state of charge for prolonged time. The most commonly used batteries are Lead-acid batteries. These batteries are available in two major categories:
Flooded batteries are the most common type of lead acid battery and one of the cheapest. In this batteries, over charging results in conversion of water into hydrogen and oxygen gases. These gases are released into atmosphere. Hence, the batteries that require water is to be replaced adding maintenance cost to the system.
Valve Regulated Lead Acid Batteries
A valve-regulated lead-acid battery (VRLA battery) sometimes called sealed lead-acid or maintenance free battery. The chemical characteristics of these batteries allow for maintenance free operation because the oxygen is allowed to recombine with hydrogen within the battery. The recombination has a maximum rate which depends on the charging current if excess pressure builds up, it is vented through valves to the atmosphere, proper charge control can limit this effect. These batteries tend to allow deeper discharge cycles, resulting in smaller battery banks and are expected to have longer life times.
The charge controller is a DC to DC converter whose main function is to control the current flow from the PV modules array with the purpose of charging batteries. Most of the devices can maintain the maximum charge of the battery without overcharging or reaching the minimum design cycle. The main function of charge controller is to protect batteries from overcharging by interrupting the supply of current from PV modules and also regulates the battery voltage.
The global energy consumption is rising rapidly and it is vital to harness the solar energy resources. The solar energy is no longer an alternate energy and it became an important part of the solution to nations energy needs. Photovoltaic systems have advantages for low-power demand, stand-alone systems and building-integrated grid-connected systems. The solar energy based power generating systems can play a major role towards the fulfillment of energy requirements of industry. The economically exploitable potential of the solar power technology of India is quite high. This study reveals an improvement in technology in the fields of manufacture, supply and installation leading to reduction in cost in comparison with conventional power.
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