SOLAR ENERGY

CONTENTS

1 INTRODUCTION
2 CLASSIFICATION OF SOLAR POWER
(a) PASSIVE OR ACTIVE
(b) FOCUS TYPE
3 TYPES OF SOLAR POWER TECHNOLOGIES
(a) SOLAR DESIGN IN ARCHITECTURE
(b) SOLAR HEATING SYSTEMS
(i) compact system
(ii) pumped system
(iii) solar heating thermal collectors
(iv) solar thermal cooling
(c) PHOTO VOLTAIC CELLS
(d) SOLAR THERMAL ELECTRIC POWER PLANTS
(i) concentrating solar power (csp) plants
(ii) solar chimney
(e) SOLAR CHEMICAL
(f) SOLAR COOKING
(g) SOLAR LIGHTING

4 ENERGY STORAGE
5 CONCLUSION





INTRODUCTION

Solar power describes a number of methods of harnessing energy from the light of the Sun. It has been present in many traditional building methods for centuries, but has become of increasing interest in developed countries as the environmental costs and limited supply of other power sources such as fossil fuels are realized. It is already in widespread use where other supplies of power are absent such as in remote locations and in space.
As the Earth orbits the Sun, it receives approximately 1,400 W / m² of energy, as measured upon a surface kept normal (at a right angle) to the Sun (this number is referred to as the solar constant). Of the energy received, roughly 19% is absorbed by the atmosphere, while clouds on average reflect a further 35% of the total energy. The generally accepted standard is 1020 watts per square meter at sea level.
After passing through the Earth's atmosphere, most of the sun's energy is in the form of visible and ultraviolet light. Plants use solar energy to create chemical energy through photosynthesis. We use this energy when we burn wood or fossil fuels or when we consume the plants as a source of food.

















CLASSIFICATION OF SOLAR POWER



Solar power can be classified as :
(a) direct or indirect.
(b) Passive or active type
(c) Focus type
Direct solar power involves only one transformation into a usable form. For example:
• Sunlight hits a photovoltaic cell (also called a photoelectric cell) creating electricity.
• Sunlight hits the dark absorber surface of a solar thermal collector and the surface warms. The heat energy is carried away by a fluid circuit.
• Sunlight strikes a solar sail on a space craft and is converted directly into a force on the sail which causes motion of the craft.
• Sunlight strikes a light mill and causes the vanes to rotate, although little practical application has yet been found for this effect.
• Sunlight is focused on an externally mounted fibre optic cable which conducts sunlight into building interiors to supplement lighting.









Indirect solar power involves more than one transformation to reach a usable form. For example: Systems which close insulating shutters or move shades. Many other types of power generation are indirectly solar-powered. Some of these are so indirect that they are often excluded from discussion of solar power:
• Vegetation use photosynthesis to convert solar energy to chemical energy, which can later be burned as fuel to generate electricity, see biofuel.
• Energy obtained from oil, coal and peat originated as solar energy captured by vegetation in the remote geological past and fossilised. Hence the term Fossil fuel. Though strictly solar power, the great time delay between the input of the solar energy and its recovery means these are not normally classified as such.
• Hydroelectric dams and wind turbines are indirectly powered by solar energy through its interaction with the Earth's atmosphere and the resulting weather phenomena.
• Energy obtained from methane (natural gas) may be derived from solar energy either as a biofuel or fossil fuel , but some methane derives from the primeval gas cloud which formed the Solar system and is therefore not solar in origin.
• Ocean thermal energy production uses the thermal gradients that are present across ocean depths to generate power. These temperature differences are ultimately due to the energy of the sun.












Passive or active :
Solar power can also be classified as passive or active:
• Passive solar systems are systems that do not involve the input of any other forms of energy apart from the incoming sunlight, although (in the case of solar heat through windows) there may be draperies or panels used to reduce nighttime heat losses and thermostatically or manually operated vents (but not fans) to prevent overheating. Some passive solar water heating systems use a thermosiphon to reduce nighttime heat loss and have no pumps. Other space heating systems use a thermal diode to similar effect.
• Active solar This usually refers to system which use additional mechanisms such as circulation pumps, air blowers or automatic systems which aim collectors at the sun.
Focus type :
Effective use of solar radiation often requires the radiation (light) to be focussed to give a higher intensity beam. Consequently, another scheme for classifying solar power systems is
• Point focus. A parabolic dish or a series of heliostats are used to concentrate light at a point (the focus). At the focus you might place high-concentration photovoltaic cells (solar cells) or a thermal energy 'receiver'. Solar One was an example of the latter.
• Line focus. A parabolic trough or a series of long narrow mirrors are used to concentrate light along a line. The SEGS systems in California are an example of this type of system.
• Non-focussing systems include solar domestic hot water systems and most photovoltaic cells. These systems have the advantage that they can make use of diffuse solar radiation (which can not be focussed). However, if high temperatures are required, this type of system is usually not suitable, because of the lower radiation intensity.
Types of solar power technologies
Most solar energy used today is harnessed as heat or electricity.
Solar design in architecture
Solar design is the use of architectural features to replace the use of grid electricity and fossil fuels with the use of solar energy and decrease the energy needed in a home or building with insulation and efficient lighting and appliances.
Architectural features used in solar design:
• South-facing (for the Northern Hemisphere) or north-facing (for the Southern Hemisphere) windows with insulated glazing that has high ultraviolet transmittance.
• Thermal masses -- any masses such as walls or roofs that absorb and hold the sun's heat. Materials with high specific heat like stone, concrete, adobe or water work best. See Trombe walls.
• Insulating shutters for windows to be closed at night and on overcast days. These trap solar heat in the building.
• Fixed awnings positioned to create shade in the summer and exposure to the sun in the winter.
• Movable awnings to be repositioned seasonally.
• A well insulated and sealed building envelope.
• Exhaust fans in high humidity areas.
• Passive or active warm air solar panels. Pass air over black surfaces fixed behind a glass pane. The air is heated by the sun and flows into the building.
• Active thermal solar panels using a heat transfer fluid (water or antifreeze solution). These are heated by the sun and the heat is carried away by circulation of the fluid for domestic hot water or building heating or other uses.
• Passive thermal solar panels for preheating domestic hot water.
• Photovoltaic systems to provide electricity.
• Solar chimneys for cooling.
• Planting deciduous trees near the windows. The leaves will give shade in summer but fall in winter to let the sunlight enter the building.

Solar heating systems
Solar heating systems are generally composed of solar thermal collectors, a fluid system to move the heat from the collector to its point of usage, and usually a reservoir to stock the heat for subsequent use. The systems may be used to heat domestic hot water, to heat a swimming pool, to provide heat for a heating circuit (usually radiators or floor heating coils). The heat can also be used for industrial applications or as an energy input to other uses (such as cooling equipment).
In many climates, a solar heating system can provide a very high percentage of domestic hot water energy. In many northern European countries, combined systems (hot water and space heating) are used to provide 15 to 25% of home heating energy.
Residential solar thermal installations can be subdivided in two kind of systems: compact and pumped systems. Both include typically an auxiliary energy source (electric heating element or connection to a gas or fuel oil central heating system) that is activated when the water in the tank falls below a minimum temperature setting(i. e. 50 ºC), so hot water is available always, even in rainy days.









Compact systems
Consist of a tank for the heated water, a few panels and pipes. Based on the thermo siphon principle, the water flows upwards when heated in the panel. When this water enters the tank (placed in the upper part) it expels some cold water from inside, so there is no need for pumps. A typical system for a 4 members home in a sunny region consists of a 300 liters tank and 2 panels (2 square meters each).
"Direct" compact systems are not suitable for cold climates, because at nighttime the remaining water in the panels can freeze and damage them. Besides, the tank is placed together with the panels, generally outside the house (even if the can be hidden beneath the tiles). Some compact systems have a “primary circuit”. This primary circuit includes the collectors and the external part of the tank. A graphical explanation of the thermosyphon principle can be found at this site Instead of water, some non-toxic antifreezing liquid is used. When this liquid is heated up, it flows to the external part of the tank, transferring the heat to the water placed inside. However, direct systems are slightly cheaper and more efficient.
A compact system can save up to 4.5 tonnes per annum of gas emissions. So, in order to achieve the aims of the Kyoto Protocol, several countries are offering subsidies to the end user. Some systems can work for up to 25 years with minimum maintenance. These kinds of systems can be redeemed in 6 years, and they achieve a positive balance of energy (energy used to build them minus energy they save) of 1.5 years. Most part of the year, when the electric heating element is not working, these systems don't use any external source for power (as water flows due to thermosyphon principle).
Usually flat solar thermal collectors are used, but a few compact systems with vacuum tubes can be found.

Pumped systems
They are commonly used in bigger installations (hotels, gyms, and so forth) and the main difference is that the storage tank is placed inside the building, and thus require a controller that measures when the water is hotter in the panels than in the tank, and a pump for transferring water between the two. Most controllers also activate the pump when the outside temperature gets close to 0º C, in order to prevent the water from freezing and thus damaging the panels.
These systems can be controlled remotely, by means of the data logger and a modem-connection.
The most commonly used panel is the flat panel, but sometimes cheaper ones, like polypropylene panels (for swimming pools), or higher-performing ones like vacuum tubes are used.

Solar heating thermal collectors
There are three main kind of solar thermal collectors in common use:
• Formed Plastic Collectors (such as polypropelene, EPDM or PET plastics). These consist of tubes or formed panels through which water is circulated and heated by the sun's radiation. Used for extending the swiming season in swiming pools. In some countries heating a open-air swiming pool with non-renewable energy sources is not allowed, and then these cheap systems offer a good solution. This panel is not suitable for year round uses like providing hot water for home use, mainly due to its lack of insulation which reduces its effectiveness greatly when the ambient air temperature is lower than than temperature of the fluid being heated.
• Flat Collector. It consists of a thin absorber sheet (usually copper, to which a selective coating is applied) backed by a grid or coil of fluid tubing and placed in an insulated casing with (usually) a glass cover. Fluid is circulated through the tubing to remove the heat from the absorber and transport it to an insulated water tank, to a heat exchanger, or to some other device for using the heated fluid. Flat-plate collectors for solar water heating had a popularity in Florida and Southern California in the 1920s. There was a resurgence of interest in them in North America in the 1970s. With various improvements, the collectors of this basic design have frequently been used in "off-grid" home situations (or in other sorts of buildings), but now they present in all most every city in the world. Naturally, like a lot of solar-heating strategies that have been available until recently, conventional flat-plate solar collectors were originally developed for use in sunny, warm climates. Benefits from this kind of collector are considerably diminished when colder or cloudy days present unfavorable conditions

Evacuated (or vacuum) tubes panel
• Evacuated tube collectors are made of a series of modular tubes, mounted parallel, whose number can be added to or reduced as hot-water-delivery needs change. This type of collector consists of rows of parallel transparent glass tubes, each of which contains an absorber tube (in place of the absorber plate to which metal tubes are attached in a flat-plate collector). The tubes are covered with a special light-modulating coating. In an evacuated-tube collector, sunlight passing through an outer glass tube heats the absorber tube contained within it, and in doing so the heat is transferred to a liquid flowing through the tube. The heated liquid circulates through a heat exchanger and gives off its heat to water that is stored in a storage tank (which itself may be kept warm partially by sunlight). Evacuated-tube collectors heat to higher temperatures. Even in some northern climates, this sort of system may capture excess heat which can also be used to supply room heat in winter. However they are more expensive and fragile than flat panels.
Solar thermal cooling
There are some new applications of thermal hot water, like air cooling, currently under development. The absorber machine works basically as a fridge; it uses hot water to compress a gas that once expanded will produce an endothermic reaction, cooling the air. The main problem right now is that the absorber machine works with liquid at 90ºC, a pretty high temperature to be reached with pumped solar pannels with no auxiliary power supply. Some commercial systems are expected to be relased soon.
The same pumped solar thermal installation can be used for producing hot water the whole year, cooling in summertime and partially heating the building in wintertime.


Photovoltaic cells

The solar panels (photovoltaic arrays) on this small yacht at sea can charge the 12 V batteries at up to 9 Amps in full, direct sunlight
Solar cells (also referred to as photovoltaic cells) are devices or banks of devices that use the photovoltaic effect of semiconductors to generate electricity directly from the sunlight. Because of high manufacturing costs, their use has been limited until recently. One cost-effective use has been in very low-power devices such as calculators with LCDs. Another has been remote applications such as roadside emergency telephones, remote sensing, cathodic protection of pipe lines, and limited "off grid" home power applications. A third has been to power orbiting satellites and other spacecraft.
However, the continual decline of manufacturing costs (dropping at 3% to 5% a year in recent years) is expanding the range of cost-effective uses. The average retail cost of a large solar panel declined from $7.50 to $4 per watt between 1990 and 2005. With many jurisdictions now giving tax and rebate incentives, solar electric power can now pay for itself in five to ten years in many places. "Grid-connected" systems - that is, systems with no battery that connect to the utility grid through a special inverter - now make up the largest part of the market. In 2004 the worldwide production of solar cells increased by 60%. 2005 is expected to see large growth again, but shortages of refined silicon have been hampering production worldwide since late 2004.
Solar thermal electric power plants
The two main types of solar thermal power plants are Solar Chimneys and Concentrating Solar Power (CSP) plants.
Concentrating solar power (CSP) plants


Solar Two, a concentrating solar power plant
Solar thermal power plants generally use reflectors to concentrate sunlight into a heat absorber. Such powerplants are known as Concentrating Solar Power (CSP) plants.
• Heliostat mirror power plants (power towers) use an array of flat, moveable mirrors to focus the sun's rays upon a collector tower (the target). The high energy at this point of concentrated sunlight is transferred to a substance that can store the heat for later use. The more recent heat transfer material that has been successfully demonstrated is liquid sodium. Sodium is a metal with a high heat capacity, allowing that energy to be stored and drawn off throughout the evening. That energy can, in turn, be used to boil water for use in steam turbines. Water had originally been used as a heat transfer medium in earlier power tower versions (where the resultant steam was used to power a turbine). This system did not allow for power generation during the evening. Examples of heliostat based power plants are the 10 MWe Solar One, Solar Two and the 15 MW Solar Tres plants. In South Africa a solar power plant is planned with 4000 to 5000 heliostat mirrors, each having an area of 140 m².
• A parabolic trough power plant is another type of solar thermal collector. It consists of a series of troughs rather like rainwater guttering with a hollow tube running its length. Sunlight is reflected by the mirror and concentrated on the tube. Heat transfer fluid, oil in the Luz systems, runs through the tube to absorb heat from the concentrated sunlight and is used to power a steam turbine.
• A Parabolic Reflector power plant is rather like a large satellite dish but with the inside surface made of mirror material. It focuses all the sun's energy to a single point and can achieve very high temperatures. Typically the dish is coupled with a Stirling engine in a Dish-Stirling System, but also sometimes a steam engine is used. These create rotational kinetic energy that can be converted to electricity using an electric generator. Planned 850 megawatt Solar Stirling Condenser array [1] [2].
• A linear Fresnel reflector power plant uses a series of carefully angled plane mirrors to focus light onto a linear absorber. Recent prototypes of these types of systems have been built in Australia (CLFR) and Belgium (SolarMundo).

solar chimney
A solar chimney is an apparatus for harnessing solar energy by convection of heated air.
In its simplest form, it consists of a black-painted chimney. During the daytime, solar energy heats the chimney, thereby heating the air within it, resulting in an updraft of air within the chimney. The suction this creates at the chimney base can be used to ventilate, and thereby cool the building below. In most parts of the world, it is easier to harness wind power for such ventilation, but on hot windless days such a chimney can provide ventilation where there would otherwise be none.


General concept of proposed solar chimney power station
This principle has been proposed for electric power generation, using a large greenhouse at the base rather than relying on heating of the chimney itself.
The main problem with this approach is the relatively small difference in temperature between the highest and lowest temperatures in the system. Carnot's theorem greatly restricts the efficiency of conversion in these circumstances.
Solar chemical
There have been experiments to harness energy by absorbing sunlight in a chemical reaction in a way similar to photosynthesis without using living organisms but no practical process has yet emerged.
A promising approach is to use focussed sunlight to provide the energy needed to split water into its constituent hydrogen and oxygen in the presence of metalic zinc.
Solar cooking
A solar box cooker traps the Sun's power in an insulated box; these have been successfully used for cooking, pasteurization and fruit canning. Solar cooking is helping many developing countries, both reducing the demands for local firewood and maintaining a cleaner environment for the cooks. The first known western solar oven is attributed to Horace de Saussure
Solar lighting
The interior of a building can be lit during daylight hours using fibre optic light pipes connected to a parabolic collector mounted on the roof. The manufacturer claim this gives a more natural interior light and can be used to reduce the energy demands of electric lighting.
Energy storage
See main article at Grid energy storage
For a stand-alone system, some means must be employed to store the collected energy for use during hours of darkness or cloud cover. The following list includes both mature and immature techniques: -
• Electrochemically in batteries,
• Hydrogen produced by electrolysis of water and then available for pollution free combustion (see direct solar thermal water splitting),
• Compressed air in a cylinder,
• Pumped-storage hydroelectricity
• Flywheel energy storage,
• Molten salt
• Superconducting magnetic energy storages.
• Cryogenic liquid air or nitrogen
Storage always has an extra stage of energy conversion, with consequent energy losses, greatly increasing capital costs. One way around this is to export excess power to the power grid, drawing it back when needed. This appears to use the power grid as a battery but in fact is relying on conventional energy production through the grid during the night.

ELECTRIC CAR

INTRODUCTION:
Ironically, one of the hottest fields of research for alternative-fuel automobiles is not a new development. Electric automobiles, believed by many to offer the best hope for an emission-free automobile in the near future, have been around as long as their gasoline-powered counterparts.
The first automobiles were powered by steam engines, similar to those seen on steam locomotives. They relied on coals or a fire to heat water and create compressed steam, which was then used to push a cylinder and move the car. Steam was a cumbersome technology for automobiles, however. There was a widespread fear of boiler explosions (although these fears were, quite likely, unfounded). In addition, a lightweight steam engine ... required constant maintenance beyond the skill of most casual owners. The water necessary to run a steam engine also presented a problem; in remote locations where there was a small or nonexistent supply of soft water, water would have to be pumped in to service the cars. Perhaps the most basic factor working against the steam engine was that the gasoline was much more thermally efficient; that is, more of its energy was converted to useful work, rather than waste heat. Because these early vehicles were cumbersome and inconvenient, inventors quickly began to search for other methods of propulsion that would be more flexible.
It soon became evident that the future of the automobile lay in one of two directions: electric, or gasoline-powered. Both of these technologies offered a convenient, portable fuel source which could easily be converted to motion through the use of a simple motor or engine. Thus the race was on between the two technologies. However the electric powered vehicles have an upper hand over the gasoline powered automobiles as far as maintenance, efficiency and life span is concerned.

ELECTRIC VEHICLE (cars)
Electric Car, automobile propelled by one or more electric motors, drawing power from an onboard source of electricity. Electric cars are mechanically simpler and more durable than gasoline-powered cars. They produce less pollution than do gasoline-powered cars. Electric vehicles (EVs) are cars that run on electricity stored in batteries. EVs are often confuse;d with hybrid electric vehicles which combine an internal combustion engine with a battery. EVs are the only truly zero emission car available today because they have no tailpipe exhaust and no evaporative emissions from fuel systems. Manufacturers have developed a broad spectrum of EVs - from neighborhood electric cars which can be used for short trips around town to full function electric cars which can be used for longer trips and have the body of conventional cars. The availability and styles of these vehicles vary from year to year, but with battery technology getting more sophisticated, manufacturers will have the ability to design electric vehicles with extended range, faster charging and more power.
From the outside, it is very difficult to guess that a car is electric. In most cases, electric cars are created by converting a gasoline-powered car, and in that case it is impossible to tell. When you drive an electric car, often the only thing that clues you in to its true nature is the fact that it is nearly silent.


A typical electric car, this one has some particularly snazzy decals. This vehicle is owned by Jon Mauney.

HISTORY
Electric motive power started with a small railway operated by a miniature electric motor, built by Thomas Davenport in 1835. In 1838, a Scotsman named Robert Davidson built an electric locomotive that attained a speed of four miles an hour. In England a patent was granted in 1840 for the use of rails as conductors of electric current, and similar American patents were issued to Lilley and Colten in 1847.
Between 1832 and 1839 (the exact year is uncertain), Robert Anderson of Scotland invented the first crude electric carriage, powered by non-rechargable Primary cells.
By the 20th century, electric cars and rail transport were commonplace, with commercial electric automobiles having the majority of the market. Electrified trains were used for coal transport as the motors did not use precious oxygen in the mines. Switzerland's lack of natural fossil resources forced the rapid electrification of their rail network.
Electric vehicles were among the earliest automobiles, and before the preeminence of light, powerful internal combustion engines, electric automobiles held many vehicle land speed and distance records in the early 1900s. They were produced by Anthony Electric, Baker Electric, Detroit Electric, and others and at one point in history out-sold gasoline-powered vehicles.



Edison and an electric car, 1913 (courtesy of the National Museum of American History)
ELECTRIC CARS
An electric car is a car powered by an electric motor which is controlled by several components which make it function much like a standard gasoline powered car. The only difference is when you step on the accelerator the electronic "brain" tells the motor how fast to revolve. Instead of ann explosion making pistons turn and electric car uses clean electromagnetic forces created by electrical current. In most cases, electric cars are created by converting a gasoline-powered car. When you drive an electric car, often the only thing that clues you in to its true nature is the fact that it is nearly silent.
Everything else about the car is stock. When you get in to drive the car, you put the key in the ignition and turn it to the "on" position to turn the car on. You shift into "Drive" with the shifter, push on the accelerator pedal and go. It performs like a normal gasoline car. Here are some interesting statistics:
• The range of this car is about 50 miles (80 km).
• The 0-to-60 mph time is about 15 seconds.
• It takes about 12 kilowatt-hours of electricity to charge the car
after a 50-mile trip
• The batteries weigh about 1,100 pounds (500 kg).
• The batteries last three to four years.

MECHANISM
Mainly its motion is provided by electric motors. The motion may be provided either by wheels or propellors driven by rotary motors, or in the case of tracked vehicles, by linear motors. The electrical energy used to power the motors may be obtained from a direct connection to land-based generation plants, as is common in electric trains; from chemical energy stored on the vehicle in batteries or diesel fuel; from nuclear energy, on nuclear submarines and aircraft carriers; or more esoteric sources such as flywheels, wind and solar.

Under the hood, there are a lot of differences between gasoline and electric cars:
• The gasoline engine is replaced by an electric motor.
• The electric motor gets its power from a controller.
• The controller gets its power from rechargeable batteries.

The Controller
The heart of an electric car is the combination of:
• The electric motor
• The motor's controller
• The batteries
The controller takes power from the batteries and delivers it to the motor. The accelerator pedal hooks to a pair of potentiometers (variable resistors), and these potentiometers provide the signal that tells the controller how much power it is supposed to deliver. The controller can deliver zero power (when the car is stopped), full power (when the driver floors the accelerator pedal), or any power level in between.
The controller normally dominates the scene when you open the hood, as you can see here:

The 300-volt, 50-kilowatt controller for this electric car is the box marked "U.S. Electricar."
When you push on the gas pedal, a cable from the pedal connects to these two potentiometers:

The potentiometers hook to the gas pedal and send a signal to the controller.
The signal from the potentiometers tells the controller how much power to deliver to the electric car's motor. There are two potentiometers for safety's sake. The controller reads both potentiometers and makes sure that their signals are equal. If they are not, then the controller does not operate. This arrangement guards against a situation where a potentiometer fails in the full-on position.
DC CONTROLING



A simple DC controller connected to the batteries and the DC motor. If the driver floors the accelerator pedal, the controller delivers the full 96 volts from the batteries to the motor. If the driver take his/her foot off the accelerator, the controller delivers zero volts to the motor.

The very simplest DC controller would be a big on/off switch wired to the accelerator pedal. When you push the pedal, it would turn the switch on, and when you take your foot off the pedal, it would turn it off. As the driver, you would have to push and release the accelerator to pulse the motor on and off to maintain a given speed.
Obviously, that sort of on/off approach would work but it would be a pain to drive, so the controller does the pulsing for you. The controller reads the setting of the accelerator pedal from the potentiometers and regulates the power accordingly. Let's say that you have the accelerator pushed halfway down. The controller reads that setting from the potentiometer and rapidly switches the power to the motor on and off so that it is on half the time and off half the time. If you have the accelerator pedal 25 percent of the way down, the controller pulses the power so it is on 25 percent of the time and off 75 percent of the time.
Most controllers pulse the power more than 15,000 times per second, in order to keep the pulsation outside the range of human hearing. The pulsed current causes the motor housing to vibrate at that frequency, so by pulsing at more than 15,000 cycles per second, the controller and motor are silent to human ears.

AC CONTROLING



An AC controller hooks to an AC motor. Using six sets of power transistors, the controller takes in 300 volts DC and produces 240 volts AC, 3-phase. The controller additionally provides a charging system for the batteries, and a DC-to-DC converter to recharge the 12-volt accessory battery.

In an AC controller, the job is a little more complicated, but it is the same idea. The controller creates three pseudo-sine waves. It does this by taking the DC voltage from the batteries and pulsing it on and off. In an AC controller, there is the additional need to reverse the polarity of the voltage 60 times a second. Therefore, you actually need six sets of transistors in an AC controller, while you need only one set in a DC controller. In the AC controller, for each phase you need one set of transistors to pulse the voltage and another set to reverse the polarity. You replicate that three times for the three phases -- six total sets of transistors.
Most DC controllers used in electric cars come from the electric forklift industry. The Hughes AC controller seen in the photo above is the same sort of AC controller used in the GM/Saturn EV-1 electric vehicle. It can deliver a maximum of 50,000 watts to the motor.
The Motor
Electric cars can use AC or DC motors:
• If the motor is a DC motor, then it may run on anything from 96 to 192 volts. Many of the DC motors used in electric cars come from the electric forklift industry.
• If it is an AC motor, then it probably is a three-phase AC motor running at 240 volts AC with a 300 volt battery pack.
DC installations tend to be simpler and less expensive. A typical motor will be in the 20,000-watt to 30,000-watt range. A typical controller will be in the 40,000-watt to 60,000-watt range (for example, a 96-volt controller will deliver a maximum of 400 or 600 amps). DC motors have the nice feature that you can overdrive them (up to a factor of 10-to-1) for short periods of time. That is, a 20,000-watt motor will accept 100,000 watts for a short period of time and deliver 5 times its rated horsepower. This is great for short bursts of acceleration. The only limitation is heat build-up in the motor. Too much overdriving and the motor heats up to the point where it self-destructs.
AC installations allow the use of almost any industrial three-phase AC motor, and that can make finding a motor with a specific size, shape or power rating easier. AC motors and controllers often have a regen feature. During braking, the motor turns into a generator and delivers power back to the batteries.
The Batteries
Right now, the weak link in any electric car is the batteries. There are at least six significant problems with current lead-acid battery technology:
• They are heavy (a typical lead-acid battery pack weighs 1,000 pounds or more).
• They are bulky (the car we are examining here has 50 lead-acid batteries, each measuring roughly 6" x 8" by 6").
• They have a limited capacity (a typical lead-acid battery pack might hold 12 to 15 kilowatt-hours of electricity, giving a car a range of only 50 miles or so).
• They are slow to charge (typical recharge times for a lead-acid pack range between four to 10 hours for full charge, depending on the battery technology and the charger).
• They have a short life (three to four years, perhaps 200 full charge/discharge cycles).
• They are expensive (perhaps $2,000 for the battery pack shown in the sample car).
You can replace lead-acid batteries with NiMH batteries. The range of the car will double and the batteries will last 10 years (thousands of charge/discharge cycles), but the cost of the batteries today is 10 to 15 times greater than lead-acid. In other words, an NiMH battery pack will cost $20,000 to $30,000 (today) instead of $2,000. Prices for advanced batteries fall as they become mainstream, so over the next several years it is likely that NiMH and lithium-ion battery packs will become competitive with lead-acid battery prices. Electric cars will have significantly better range at that point.
When you look at the problems associated with batteries, you gain a different perspective on gasoline. Two gallons of gasoline, which weighs 15 pounds, costs $3.00 and takes 30 seconds to pour into the tank, is equivalent to 1,000 pounds of lead-acid batteries that cost $2,000 and take four hours to recharge.
The problems with battery technology explain why there is so much excitement around fuel cells today. Compared to batteries, fuel cells will be smaller, much lighter and instantly rechargeable. When powered by pure hydrogen, fuel cells have none of the environmental problems associated with gasoline. It is very likely that the car of the future will be an electric car that gets its electricity from a fuel cell. There is still a lot of research and development that will have to occur, however, before inexpensive, reliable fuel cells can power automobiles.
Accessory Battery
Just about any electric car has one other battery on board. This is the normal 12-volt lead-acid battery that every car has. The 12-volt battery provides power for accessories -- things like headlights, radios, fans, computers, air bags, wipers, power windows and instruments inside the car. Since all of these devices are readily available and standardized at 12 volts, it makes sense from an economic standpoint for an electric car to use them.
Therefore, an electric car has a normal 12-volt lead-acid battery to power all of the accessories. To keep the battery charged, an electric car needs a DC-to-DC converter. This converter takes in the DC power from the main battery array (at, for example, 300 volts DC) and converts it down to 12 volts to recharge the accessory battery. When the car is on, the accessories get their power from the DC-to-DC converter. When the car is off, they get their power from the 12-volt battery as in any gasoline-powered vehicle.
The DC-to-DC converter is normally a separate box under the hood, but sometimes this box is built into the controller.
Regenerative Braking
The electric motor applies resistance to the drivetrain causing the wheels to slow down. In return, the energy from the wheels turns the motor, which functions as a generator, converting energy normally wasted during coasting and braking into electricity, which is stored in a battery until needed by the electric motor.
Regenerative braking converts otherwise wasted energy from braking into electricity and stores it in the battery.
In regenerative braking, the electric motor is reversed so that, instead of using electricity to turn the wheels, the rotating wheels turn the motor and create electricity. Using energy from the wheels to turn the motor slows the vehicle down.
If additional stopping power is needed, conventional friction brakes (e.g., disc brakes) are also applied automatically.
ADVANTAGES
1. no engine.
2. no gas/petrol tank.
3. no emission.
4. no noise.
5. no apparent pollution.
DISADVANTAGE
1. high price.
2. efficiency of battery is less.

CONCLUSION
The future was unclear because of the low range and small lifespan of the batteries. But there are several developments which could bring back electric vehicles outside of their current field of application -- namely operational yards and indoor operation. The first improvement[1] was to decouple the electric motor from the battery through electronic control while employing ultra-capacitors to buffer large but short power demands and recuperable braking energy. The development of new cell types compared with intelligent cell management improved both weak points mentioned above. The cell management is not only able to monitor the health of the cells but by having a redundant cell configuration (one cell more than needed) and a sophisticated switched wiring it is possible to condition one cell after the other while the rest are on duty. Perhaps the most important point is that a monovalent operation (electric only) is no longer considered dogma. The use of fuel cells instead of internal combustion engines can create propulsion systems that are nearly emissions-free (regarding local emissions).


REFERENCES

1. http://www.wickipedia.com/
2. http://www.howstuffworks.com/
3. http://www.google.com/
4. http://www.answers.com/
5. http://www.electriccars.com/