Technology And Sunlight Electricity Engineering Essay

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Photovoltaics is the field of technology and research related to the devices which directly convert sunlight into electricity. The solar cell is the elementary building block of the photovoltaic technology. Solar cells are made of semiconductor materials, such as silicon. One of the properties of semiconductors that makes them most useful is that their conductivity may easily be modified by introducing impurities into their crystal lattice. For instance, in the fabrication of a photovoltaic solar cell, silicon, which has four valence electrons, is treated to increase its conductivity. On one side of the cell, the impurities, which are phosphorus atoms with five valence electrons (n-donor), donate weakly bound valence electrons to the silicon material, creating excess negative charge carriers. On the other side, atoms of boron with three valence electrons (p-donor) create a greater affinity than silicon to attract electrons. Because the p-type silicon is in intimate contact with the n-type silicon a p-n junction is established and a diffusion of electrons occurs from the region of high electron concentration (the n-type side) into the region of low electron concentration (p-type side). When the electrons diffuse across the p-n junction, they recombine with holes on the p-type side. However, the diffusion of carriers does not occur indefinitely, because the imbalance of charge immediately on either sides of the junction originates an electric field. This electric field forms a diode that promotes current to flow in only one direction. Ohmic metal-semiconductor contacts are made to both the n-type and p-type sides of the solar cell, and the electrodes are ready to be connected to an external load. When photons of light fall on the cell, they transfer their energy to the charge carriers. The electric field across the junction separates photo generated positive charge carriers (holes) from their negative counterpart (electrons). In this way an electrical current is extracted once the circuit is closed on an external load. There are several types of solar cells. However, more than 90 % of the solar cells currently made worldwide consist of wafer-based silicon cells. They are either cut from a single crystal rod or from a block composed of many crystals and are correspondingly called mono crystalline or multi-crystalline silicon solar cells. Wafer-based silicon solar cells are approximately 200 μm thick. Another important family of solar cells is based on thin-films, which are approximately 1-2 μm thick and therefore require significantly less active, semiconducting material. Thin-film solar cells can be manufactured at lower cost in large production quantities; hence their market share will likely increase in the future. However, they indicate lower efficiencies than wafer-based silicon solar cells, which means that more exposure surface and material for the installation is required for a similar performance.

Solar Cell

A number of solar cells electrically connected to each other and mounted in a single support structure or frame is called a 'photovoltaic module'. Modules are designed to supply electricity at a certain voltage, such as a common 12 volt system. The current produced is directly dependent on the intensity of light reaching the module. Several modules can be wired together to form an array. Photovoltaic modules and arrays produce direct-current electricity. They can be connected in both series and parallel electrical arrangements to produce any required voltage and current combination.

There are two main types of photovoltaic system. Gridconnected systems (on-grid systems) are connected to the grid and inject the electricity into the grid. For this reason, the direct current produced by the solar modules is converted into a grid-compatible alternating current. However, solar power plants can also be operated without the grid and are then called autonomous systems (off-grid systems). More than 90 % of photovoltaic systems worldwide are currently implemented as grid-connected systems. The power conditioning unit also monitors the functioning of the system and the grid and switches off the system in case of faults.

Type of phovoltaic

Photovoltaic power systems are generally classified according to their functional and operational requirements, their component configurations, and how the equipment is connected to other power sources and electrical loads. The two principal classifications are grid-connected or utility-interactive systems and stand-alone systems. Photovoltaic systems can be designed to provide DC and/or AC power service, can operate interconnected with or independent of the utility grid, and can be connected with other energy sources and energy storage systems.

Grid-connected or utility-interactive PV systems are designed to operate in parallel with and interconnected with the electric utility grid. The primary component in grid-connected PV systems is the inverter, or power conditioning unit (PCU). The PCU converts the DC power produced by the PV array into AC power consistent with the voltage and power quality requirements of the utility grid, and automatically stops supplying power to the grid when the utility grid is not energized. A bi-directional interface is made between the PV system AC output circuits and the electric utility network, typically at an on-site distribution panel or service entrance. This allows the AC power produced by the PV system to either supply on-site electrical loads, or to back-feed the grid when the PV system output is greater than the on-site load demand. At night 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 feed back into the utility grid when the grid is down for service or repair.

Figure 1. Diagram of grid-connected photovoltaic system.

Stand-Alone Photovoltaic Systems

Stand-alone PV systems are designed to operate independent of the electric utility grid, and are generally designed and 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 wind, an engine-generator or utility power as an auxiliary power source in what is called a PV-hybrid system. The simplest type of stand-alone PV system is a direct-coupled system, where the DC output of a PV module or array is directly connected to a DC load (Figure 5). Since there is no electrical energy storage (batteries) in direct-coupled systems, the load only operates during sunlight hours, making these designs suitable for common applications such as ventilation fans, water pumps, and small circulation pumps for solar thermal water heating systems. Matching the impedance of the electrical load to the maximum power output of the PV array is a critical part of designing well-performing direct-coupled system. For certain loads such as positive-displacement water pumps, a type of electronic DC-DC converter, called a maximum power point tracker (MPPT), is used between the array and load to help better utilize the available array maximum power output.

Figure 2. Direct-coupled PV system.

In many stand-alone PV systems, batteries are used for energy storage. Figure 6 shows a diagram of a typical stand-alone PV system powering DC and AC loads. Figure 7 shows how a typical PV hybrid system might be configured.

Figure 3. Diagram of stand-alone PV system with battery storage powering DC and AC loads.

 

Figure 4. Diagram of photovoltaic hybrid system.

What is photovoltaic?[ http://www.technologystudent.com/energy1/solar5.htm]

Photovoltaic cells look similar to solar panels but they work in a different way. Solar panels are use to produce hot water or even steam. Photovoltaic panels convert the sunlight directly into electricity. A typical example of a device powered by photovoltaic cells is a solar powered calculator. This type of device only needs a small amount of electrical power to work and can even be used in a room with artificial light (bulbs / fluorescent light).

Although we see photovoltaic cells powering small devices such as calculators they have a more practical application especially in the third world. Photovoltaic cells have been developed that will provide electrical power to pump drinking water from wells in remote villages. British Telecom have developed a system that can be used to power a radio telephone system. During the day the cells power the phone and also charge batteries. The batteries power the phone during the night. Often photovoltaic cells are used as a backup to conventional energy. If conventional fails the cells are used to produce electricity.

 

A TYPICAL PHOTOVOLTAIC CELL

Silicon is a material known as a 'semiconductor' as it conducts electricity and it is the main material for photovoltaic cells. Impurities such as boron or phosphorus are added to this base material. These impurities create the environment for electrons to be freed when sunlight hits the photovoltaic panel. The freeing of electrons leads to the production of electricity.

The diagram above shows a basic photovoltaic cell. The blue represents the main material, silicon. The black round and irregular shapes represent the impurities of boron or phosphorous. As the sun/light strikes the cell the impurities free up electrons which 'bounce' around at incredible speeds. This creates an electrical charge.

What is solar energy[http://www.technologystudent.com/energy1/solar1.htm]

Solar power is energy from the sun and without its presence all life on earth would end. Solar energy has been looked upon as a serious source of energy for many years because of the vast amounts of energy that are made freely available, if harnessed by modern technology.

A simple example of the power of the sun can be seen by using a magnifying glass to focus the suns rays on a piece of paper. Before long the paper ignites into flames.

This is one way of using the suns energy, but flames are dangerous and difficult to control. A much safer and practical way of harnessing the suns energy is to use the suns power to heat up water.

A magnifying glass can be used to heat up a small amount of water. A short piece of copper tube is sealed at one end and filled with water. A magnifying glass is then used to warm up the pipe. Using more than one magnifying glass will increase the temperature more rapidly. After a relatively short time the temperature of the water increases. Continuing to heat the water will cause water vapour to appear at the top of the tube. In theory, with enough patience, several magnifying glasses and very strong sun light enough heat should be generated to boil the water, producing steam. This is one way of harnessing solar power.

1.3 Advantages of

Photovoltaic Technology

Photovoltaic systems offer substantial advantages

over conventional power sources:

• Reliability. Even in harsh conditions, photovoltaic systems have proven their reliability. PV arrays prevent costly power failures in situations where continuous operation is critical.

• Durability. Most PV modules available today show no degradation after ten years of use. It is likely that future modules will produce power for 25 years or more.

• Low Maintenance Cost. Transporting materials and personnel to remote areas for equipment maintenance or service work is expensive. Since PV systems require only periodic inspection and occasional maintenance, these costs are usually less than with conventionally fueled systems.

• No Fuel Cost. Since no fuel source is required, there are no costs associated with purchasing, storing, or transporting fuel.

• Reduced Sound Pollution. Photovoltaic systems operate silently and with minimal movement.

• Photovoltaic Modularity. PV systems are more cost effective than bulky conventional systems. Modules may be added incrementally to a photovoltaic system to increase available power.

• Safety. PV systems do not require the use of combustible fuels and are very safe when properly designed and installed.

• Independence. Many residential PV users cite energy independence from utilities as their primary motivation for adopting the new technology.

• Electrical Grid Decentralization. Small-scale decentralized power stations reduce the possibility of outages on the electric grid.

• High Altitude Performance. Increased insolation at high altitudes makes using photovoltaics advantageous, since power output is optimized. In contrast, a diesel generator at higher altitudes must be de-rated because of losses in efficiency and power output.

1.4 Disadvantages of

Photovoltaic Technology

Photovoltaics have some disadvantages when compared to conventional power systems:

• Initial Cost. Each PV installation must be evaluated from an economic perspective and compared to existing alternatives. As the initial cost of PV systems decreases and the cost of conventional fuel sources increases, these systems will become more economically competitive.

• Variability of Available Solar Radiation. Weather can greatly affect the power output of any solar-based energy system. Variations in climate or site conditions require modifications in system design.

• Energy Storage. Some PV systems use batteries for storing energy, increasing the size, cost, and complexity of a system.

• Efficiency Improvements. A cost-effective use of photovoltaics requires a high-efficiency approach to energy consumption. This often dictates replacing inefficient appliances.

• Education. PV systems present a new and unfamiliar technology: Few people understand their value and feasibility. This lack of information slows market and technological growth.

2.1 Photovoltaic System

Solar energy has been used around the world for powering numerous applications. It works by converting energy from the sunlight directly into electricity (DC). The smallest part of a photovoltaic panel is called photovoltaic cell. Multiple solar or photovoltaic cells are connected to form a solar module and combination of solar module by series or parallel is called the solar array. The electricity from the solar cells is stored in the battery for immediate or later use. The role of the charge controller is to regulate the voltage and current from the solar cells before it is stored in the battery. It monitors the condition of the battery state of charge and protects the battery from being over-charged. The charge controller will also protect the battery from discharging below its lowest acceptable voltage. Where required, an inverter is used to change the Direct Current (DC) to Alternating Current (AC) to power most AC appliances. Due to the low voltage of an individual solar cell (typically 0.5V), several cells are combined into photovoltaic modules, which are in turn connected together into an array. The electricity generated can be either stored, used directly (island/standalone plant) or fed into a large electricity grid powered by central generation plants (gridconnected/ grid-tied plant) or combined with one or many domestic electricity generators to feed into a small grid (hybrid plant) [2].

2.1.1 Photovoltaic Stand-Alone Systems

A standalone system does not connect to the electricity mains. This system is designed to operate independently to supply DC/AC electrical loads. The system is one that is carefully sized to cope with the total load demand throughout the day with no additional supplement source. The solar modules forming the array generate the power requirement for all loads. The battery bank is sized to meet all situations of low sunshine period and keeps all loads running continuously [3]. Figure 2.1 shows the typical block diagram of photovoltaic stand-alone system.

Figure 2.1: Photovoltaic Stand-Alone System [3].

2.1.2 Photovoltaic Grid Connected System

A grid connected system is connected to a large independent grid (typically the public electricity grid) and feeds power into the grid. The grid tie photovoltaic system is designed to work with the utility grid power. The inverter converts the DC power from the PV array into AC power. The AC power can be used directly by your AC appliances or sold back to your local utility company [3]. Figure 2.2 shows the typical block diagram of photovoltaic grid tie system. 6

Figure 2.2: Photovoltaic Grid Connected System [3].

2.1.3 Photovoltaic Hybrid System

The hybrid system is one that integrates more than one generating source, for instance fuel or wind generator, which will supplement the existing photovoltaic system. The battery bank is recharged firstly by the photovoltaic system and when insufficient, the generator charges the batteries [3]. Figure 2.3 shows the typical block diagram of photovoltaic hybrid systems.

Figure 2.3: Photovoltaic Hybrid System [3].

[2] http:// Solar Power System - Basic Information.htm.

[3] http:// PV resources website, Hybrid power station accessed 10 Feb 08

Solar Photovoltaic System uses solar cells to convert light into electricity. A PV system consists of PV modules and balance of systems (BOS). Balance of systems includes module support structure, storage, wiring, power electronics, etc.

DC (direct current) electricity is generated when solar radiation strikes the PV module. Power can be used in any DC load directly during this generation. But the generation exists during daytime. So, some storage device is needed to run the system at night or in low sunshine hour. Again this power cannot be used to run any AC (alternate current) load. Inverter has to be used to convert DC into AC.

Solar PV systems are categories into

Stand-alone PV systems (also called off-grid systems)

Grid connected PV systems (also called on-grid systems)

Hybrid systems

Stand-alone PV systems

Stand-alone systems are not connected with utility power lines and these are self sufficient systems. These systems could either be used to charge the batteries that serve as an energy storage device or could work directly using the solar energy available in the daytimes. These systems consist of the following:

Solar panels mounted on the roof or in open spaces. Photovoltaic modules produce direct current (DC) electrical power.

Batteries to store DC energy generated by the solar panels.

Charge controller to prevent overcharging the battery.

Inverter to convert electricity produced by the system from DC to AC power.

The following diagram shows PV system powering AC loads with battery bank. DC loads can also be connected directly to the battery bank. It is also possible to power the AC load without battery, but in that case it would be confined only to daytime when solar radiation is sufficient to generate required electricity.

Grid connected PV systems

A grid connected photovoltaic system will be interacted with utility grid. The main advantage of this system is that power can be drawn from the utility grid and when power is not available from grid, PV system can supplement that power. These grid connected systems are designed with battery or without battery storage. These systems consist of the following:

Solar panels mounted on the roof or in open spaces. Photovoltaic modules produce direct current (DC) electrical power.

Batteries to store DC energy generated by the solar panels.

Charge controller to prevent overcharging the battery.

Specially designed inverter to transform the PV generated DC electricity to the grid electricity (which is of AC) at the grid voltage.

The following diagram shows PV system powering AC loads. This system is connected to utility power supply and having battery storage for backup.

Hybrid systems

System with more than one source of power is called Hybrid system. It is often desirable to design a system with additional source of power. The most common type of hybrid system contains a gas or diesel powered engine generator. Another hybrid approach is a PV/Wind system. Adding a wind turbine to a PV system provides complementary power generation. These systems consist of the following:

Solar panels mounted on the roof or in open spaces. Photovoltaic modules produce direct current (DC) electrical power.

Batteries to store DC energy generated by the solar panels.

Charge controller to prevent overcharging the battery.

Specially designed inverter to transform the PV generated DC electricity to the grid electricity (which is of AC) at the grid voltage.

The following diagram shows PV system powering AC loads. This system is connected to utility power supply & diesel generator and having battery storage for backup.

http://www.synergyenviron.com/resources/solar_photovoltaic_systems.asp

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