INTRODUCTION Photovoltaic (PV) cells generate electricity by capturing the sun's energy - the light not its heat. PV systems are easy to operate, rarely need maintenance and do not pollute the environment. And best of all, the fuel ?sunlight - is free and inexhaustable. Quick Facts About PV
PHOTOVOLTAICS PHYSICS
The following paragraphs discuss how solar cells capture energy from sunlight and power electric devices. Crystalline silicon cells are used to explain the process, although there are non-crystalline cells and other materials used besides silicon. The basic principles, however, are the same.
PV Cell Basics
A conventional solar cell consists of a wafer of silicon that is about 1/50th of 1" thick. Typical cells that are four inches in diameter produce about 1W of power, and are grouped into modules of dozens of cells. Modules are further grouped into panels and then arrays, which may produce several kW of power.
Pics: array, cell, module, panel
When Light Strikes Silicon
Pic: light
How a Silicon Semiconductor is Created
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To create a semiconductor, two halves of a crystal of pure silicon are contaminated, or "doped", with two different types of material called "dopants": one that contains excess electrons, and one that is electron deficient. The junction between the halves is critical to the operation of the cell.
The Effects of the Junction
Pic: junction1
Because of the presence of the dopants, an "electric field" exists across the junction of the two halves of the crystal that sweeps free electrons across the junction in
one direction only. It is this property of the junction that causes current flow in a solar cell.
If an electron is freed in the half of the cell that has excess electrons, the junction prevents the electron from drifting into the other half, recombining with a hole, and losing its energy. If an electron is freed in the half of the cell with excess holes, the electric field sweeps the electron into the other half. These effects induce electrons to flow in only one direction across the junction.
How a Cell Powers an External Load
Use also junciton1, so it should be placed on top of these 2 topics
An electric "load" is any device that uses electricity such as an electric motor or a lamp. A load is connected to a power supply by two wires called "leads". When a load is connected to silicon cell, one lead is connected to one half of the doped crystal, and one lead to the other half.
When light shines on the crystal and electron-hole pairs are created, the electrons travel through the load to recombine with the holes. The electrons moving through the load are what cause the motor to spin or the lamp to shine. Moving electrons are also known as electric "current".
As long as light is shining on the crystal, the process is repeated:
Efficiency of Photovoltaic Cells
Conventional PV cells today convert 5-15% of the energy in sunlight into usable energy. Experimental cells have achieved about double that efficiency, but only under carefully controlled laboratory conditions and with expensive materials and high production cost. Efficiency is constantly increasing, however, as new materials and manufacturing processes are developed.
APPLICATIONS
On a small scale, PV provides electricity for lighting, refrigeration, and other services to households and businesses in many countries. PV is especially appealing in remote areas where extending the utility's electricity grid is expensive or impossible. On a larger scale, utilities have used PV to provide power for centralized grid systems.
Why Use PV?
PV-generated power offers advantages over diesel generators, primary (one-time use) batteries, and even conventional utility power. These benefits make PV the power of choice in more and more cases every day:
High Reliability
PV cells were originally developed for use in space, where repair is extremely expensive, if not impossible. PV still powers nearly every satellite circling the earth because it operates reliably for long periods of time with virtually no maintenance.
This PV-powered water-level monitor on the Laramie River in Wyoming will operate reliably for several years with little or no maintenance.
Low Operating Costs
PV cells use the energy from sunlight to produce electricity—the fuel is free. With no moving parts, the cells require little upkeep. These low-maintenance, cost-effective PV systems are ideal for supplying power to communications stations on mountain tops, navigational buoys at sea, or homes far from utility power lines.
Pic: telecom
Once installed, PV power systems can operate continuously with little upkeep and minimal operating costs—a great benefit for this telecommunications station in a remote area of California's Inyo National Forest.
Environmental Benefits
Because they burn no fuel and have no moving parts, PV systems are clean and silent. This is especially important where the main alternatives for obtaining power and light are from diesel generators and kerosene lanterns.
As we become more aware of "greenhouse gases" and their detrimental effects on our planet, clean energy alternatives like PV become more important than ever.
Modularity
A PV system can be constructed to any size based on energy requirements. Furthermore, the owner of a PV system can enlarge or move it if his or her energy needs change. For instance, homeowners can add modules every few years as their energy usage and financial resources grow. Ranchers can use mobile trailer-mounted pumping systems to water cattle as the cattle are rotated to different fields.
Low Construction Costs
PV systems are usually placed close to where the electricity is used, requiring much shorter power lines than if power is brought in from the utility grid. In addition, using PV eliminates the need for a step-down transformer from the utility line. Less wiring means lower costs, shorter construction time, and reduced permitting paperwork, particularly in urban areas.
Pic:brazil
In the village of Cacimbos of Brazil, people are several miles from a utility grid. So, each of these homes has a 50W PV system to provide enough power for 2 fluorescent lights. (Photo: R. Taylor, NREL)
CASE STUDIES
Hurricane Andrew
Pics: ha1, ha2, ha3 (1@paragraph)
Photovoltaics have the valuable characteristic of being able to provide small, practical amounts of power in a relatively short period of time to support humanitarian efforts aimed at alleviating the suffering and hardship of a community or region after a natural disaster.
Such was the case in Southern Florida after Hurricane Andrew swept through the region, destroying the utility-based electricity supply. A few solar companies along
with other interested agencies and individuals helped provide photovoltaic powered systems for security, lighting, and mobile medical units.
Photovoltaics were also used to power message boards that dotted Florida's highways. Such mobility greatly enhances the applicability of photovoltaics while offering government and volunteer agencies a cost-effective, low maintenance means of mobile power supply.
Mali
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In Mali, more than 100 PV-powered pumping systems have been installed in the past 11 years. Many of the local people have been involved with the construction of water tanks, foundations, and access that lowered the initial capital costs and generated a level of local enthusiasm. Equipment reliability in the harsh Sahelian environment has been excellent.
Sainsbury Solar Powered Refrigeration
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The Sustainable Energy Research Group (SERG), Civil & Environmental Engineering Department, Southampton University (UK) in conjunction with Low Energy Refrigeration have developed what is believed to be the world's first solar powered refrigeration unit installed on a working articulated vehicle for Sainsbury's.
Ample power for the refrigeration unit is generated for photovoltaic panels mounted on the trailer's roof. An on-board battery stores the excess power for use during the hours of darkness.
The solar powered refrigeration unit uses considerably less energy and produces less noise pollution than conventional refrigeration equipment powered by diesel generators. Emissions associated with diesel exhausts are totally eliminated. The refrigeration system is capable of operating at temperatures down to + 3
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PV in Hong Kong
Hong Kong (HK) has little knowledge and little real experience of photovoltaics.
Applied to buildings, PV is normally associated with rooftop arrays, at almost horizontal angles to capture midday sun - Because HK's high levels of autumn and winter sunshine, when the sun is lower in the sky, PV panels are also effective on vertical faces which at the same time modify solar heat gain factors reducing the internal cooling load.
PV panels are already modular and so well suited to building systems. Well-suited to the high-rise developments of Hong Kong and particularly curtain walling which is already accepted as a prestigious building form. Properly incorporated in the architectural design from the earliest stages, Building Integrated Photovoltaics (BIPV) can be stylish and contemporary.
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The University of Hong Kong has been undergoing a study on monitoring the performance of a wide range of PV module types positioned to simulate their use as building components, as cladding panels or window shading devices. The intention is to build up a performance database that can be accessed by the community's developers and building professionals to assist in the application of PV to future developments.
References & Image Sources
For more up-to-date information about photovoltaics, please visit the following websites:
http://arch.hku.hk/research/photovoltaic/index.html
ftp://ftp.hq.nasa.gov/pub/pao/images/index/photoindex/indexm.htm
http://www.crest.org/renewables/re-kiosk/solar/pv/index.shtml
http://www.soton.ac.uk/~serg/solar/solar.htm#arts2
http://www.sandia.gov/Renewable_Energy/PV_NOW/FR_contents.html
