How to keep your roofs cool?

White painted roofs have been popular since ancient times in places like Greece. Similar technology can be easy to adapt to modern homes and other buildings. | Credit: ©iStockphoto/PhotoTalk
A cool roof is one that has been designed to reflect more sunlight and absorb less heat than a standard roof. Cool roofs can be made of a highly reflective type of paint, a sheet covering, or highly reflective tiles or shingles. Nearly any type of building can benefit from a cool roof, but consider the climate and other factors before deciding to install one.

Just as wearing light-colored clothing can help keep you cool on a sunny day, cool roofs use solar-reflective surfaces to maintain lower roof temperatures. Standard or dark roofs can reach temperatures of 150°F or more in the summer sun. A cool roof under the same conditions could stay more than 50°F cooler

BENEFITS OF COOL ROOFS

A cool roof can benefit a building and its occupants by:

Reducing energy bills by decreasing air conditioning needs
Improving indoor comfort for spaces that are not air conditioned
Decreasing roof temperature, which may extend roof service life.
Beyond the building itself, cool roofs can also benefit the environment, especially when many buildings in a community have them. Cool roofs can:

Reduce local air temperatures (sometimes referred to as the urban heat island effect)
Lower peak electricity demand, which can help prevent power outages
Reduce power plant emissions, including carbon dioxide, sulfur dioxide, nitrous oxides, and mercury, by reducing cooling energy use in buildings.

TYPES OF ROOFS AND HOW THEY CAN BE MADE COOL

There are many types of roof systems available, but the surface exposed to the sun is the one that determines if a roof is cool or not. You can usually make a new or existing roof cool by selecting the appropriate surface.

Cool roof coatings are white or special reflective pigments that reflect sunlight. Coatings are like very thick paints that can protect the roof surface from ultra-violet (UV) light and chemical damage, and some offer water protection and restorative features. Products are available for most roof types.

LOW SLOPED ROOFS

Single-ply membranes are pre-fabricated sheets rolled onto the roof and attached with mechanical fasteners, adhered with chemical adhesives, or held in place with ballast (gravel, stones, or pavers).

How they can be made cool: Reformulate or coat black membranes to make them reflective.

Built-up roofs consist of a base sheet, fabric reinforcement layers, and (usually) a dark protective surface layer.

How they can be made cool: The surface layer can be made different ways, and each has cool options:

Substitute reflective marble chips or gray slag for dark gravel in a flood coat of asphalt
Use reflective mineral granules or a factory-applied coating rather than a dark coating on a mineral surfaced sheet
Apply a cool coating directly on top of a dark asphaltic emulsion coating.
Modified bitumen sheet membranes have one or more layers of plastic or rubber material with reinforcing fabrics, and are surfaced with mineral granules or a smooth finish. These can also be used to surface a built-up roof—known as a "hybrid" roof. 


How they can be made cool: Pre-coat with a cool roof coating at the factory.

Spray polyurethane foam roofs are constructed by mixing two liquid chemicals together that react and expand to form one solid piece that adheres to the roof. Foams are highly susceptible to mechanical, moisture, and UV damage, and rely on a protective coating. 


How they can be made cool: The protective coatings are usually already reflective, and offer cool roof performance.

STEEP SLOPED ROOFS

Shingle roofs consist of overlapping panels made from a variety of materials such as fiberglass asphalt, wood, polymers, or metals.

How they can be made cool: Buy cool asphalt shingles, which use specially coated granules that provide better solar reflectance. (Coating existing asphalt shingles to make them cool, however, is not normally recommended or approved by shingle manufacturers.) Other roof shingles can be coated at the factory or in the field to make them more reflective.

Tile roofs can be made of clay, slate, or concrete. Tiles can be glazed to provide waterproofing or coated to provide customized colors and surface properties.

How they can be made cool: Some are naturally reflective enough to achieve cool roof standards, and surface treatments can transform tiles with low solar reflectance into cool roof tiles.

LOW AND STEEP SLOPED ROOFS

Metal roofs are available with natural metallic finishes, oven-baked paint finishes, or granular coated surfaces.

How they can be made cool: Unpainted metals are typically good solar reflectors but poor thermal emitters, so they rarely satisfy low slope cool roof requirements. Painting a metal roof can increase its solar reflectance and thermal emittance, allowing it to achieve cool roof status. Alternatively, you can apply cool reflective coatings.

DECIDING WHETHER TO INSTALL A COOL ROOF

When deciding whether to install a cool roof, you’ll need to determine whether the cost will justify the energy savings. How much energy you will save depends on several factors such as your home's climate and environment, how well insulated your current roof is, the type of roof you have, and the efficiency of your heating and cooling system.

If you are building a new home, you can decide during the planning phase what type of roof to install and whether it should be a cool roof. If you want to convert an existing roof into a cool roof, you have three basic options:

Coat the roof

Re-cover it with a new waterproofing surface
Tear off the existing roof and replace it with a new one.
If your roof is in poor condition or near the end of its life, it is usually best to re-cover, replace, or retrofit the roof.

COST AND ENERGY SAVINGS

A cool roof does not necessarily cost more than a non-cool roof, especially if you are installing a new roof or replacing an existing one. However, converting a standard roof that's in good condition into a cool roof can be expensive. Major roof costs include upfront installation (materials and labor) and ongoing maintenance (repair, recoating, and cleaning). Additional cool roof costs include specialized materials and labor.

Cool roofs can save money several ways, including energy savings, rebates and incentives, HVAC equipment downsizing, and extended roof lifetime. One way to estimate how much energy you would save by installing a cool roof is by using a cool roof calculator, available at DOE Cool Roof Calculator.

CLIMATE AND ENVIRONMENT

Your climate is an important consideration when deciding whether to install a cool roof. Cool roofs achieve the greatest cooling savings in hot climates, but can increase energy costs in colder climates due to reduced beneficial wintertime heat gains.

MOISTURE CONTROL

In warm, moist locations, cool roof surfaces can be more susceptible to algae or mold growth than hot roofs. Some roof coatings include special chemicals that prevent mold or algae growth for a few years.

In cold climates, roofs can accumulate moisture through condensation, and it is possible that cool roofs might be more susceptible to accumulating moisture than dark roofs of the same design. Condensation can be avoided using proper design techniques.

sources:http://energy.gov/energysaver/cool-roofs

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PASSIVE SOLAR HOME DESIGN

This North Carolina home gets most of its space heating from the passive solar design, but the solar thermal system (top of roof) supplies both domestic hot water and a secondary radiant floor heating system. | Photo courtesy of Jim Schmid Photography.

Passive solar design takes advantage of a building’s site, climate, and materials to minimize energy use. A well-designed passive solar home first reduces heating and cooling loads through energy-efficiency strategies and then meets those reduced loads in whole or part with solar energy. Because of the small heating loads of modern homes it is very important to avoid oversizing  south-facing  glass and ensure that south-facing glass is properly shaded to prevent overheating and increased cooling loads in the spring and fall.

ENERGY EFFICIENCY FIRST

Before you add solar features to your new home design or existing house, remember that energy efficiency is the most cost-effective strategy for reducing heating and cooling bills. Choose building professionals experienced in energy-efficient house design and construction and work with them to optimize your home’s energy efficiency. If you’re remodeling an existing home, the first step is to have a home energy audit to prioritize the most cost-effective energy efficiency improvements.

SITE SELECTION

If you’re planning a new passive solar home, a portion of the south side of your house must have an unobstructed “view” of the sun. Consider possible future uses of the land to the south of your site—small trees become tall trees, and a future multi-story building can block your home’s access to the sun. In some areas, zoning or other land use regulations protect landowners’ solar access. If solar access isn’t protected in your region, look for a lot that is deep from north to south and place the house on the north end of the lot.

HOW A PASSIVE SOLAR HOME DESIGN WORKS

In simple terms, a passive solar home collects heat as the sun shines through south-facing windows and retains it in materials that store heat, known as thermal mass. The share of the home’s heating load that the passive solar design can meet is called the passive solar fraction, and depends on the area of glazing and the amount of thermal mass. The ideal ratio of thermal mass to glazing varies by climate. Well-designed passive solar homes also provide daylight all year and comfort during the cooling season through the use of nighttime ventilation.

To be successful, a passive solar home design must include some basic elements that work together:

Properly oriented windows. Typically, windows or other devices that collect solar energy should face within 30 degrees of true south and should not be shaded during the heating season by other buildings or trees from 9 a.m. to 3 p.m. each day. During the spring, fall, and cooling season, the windows should be shaded to avoid overheating.
Thermal mass. Thermal mass in a passive solar home -- commonly concrete, brick, stone, and tile -- absorbs heat from sunlight during the heating season and absorbs heat from warm air in the house during the cooling season. Other thermal mass materials such as water and phase change products are more efficient at storing heat, but masonry has the advantage of doing double duty as a structural and/or finish material. In well-insulated homes in moderate climates, the thermal mass inherent in home furnishings and drywall may be sufficient, eliminating the need for additional thermal storage materials.
Distribution mechanisms. Solar heat is transferred from where it is collected and stored to different areas of the house by conduction, convection, and radiation. In some homes, small fans and blowers help distribute heat. Conduction occurs when heat moves between two objects that are in direct contact with each other, such as when a sun-heated floor warms your bare feet. Convection is heat transfer through a fluid such as air or water, and passive solar homes often use convection to move air from warmer areas -- a sunspace, for example -- into the rest of the house. Radiation is what you feel when you stand next to a wood stove or a sunny window and feel its warmth on your skin. Darker colors absorb more heat than lighter colors, and are a better choice for thermal mass in passive solar homes.
Control strategies. Properly sized roof overhangs can provide shade to vertical south windows during summer months. Other control approaches include electronic sensing devices, such as a differential thermostat that signals a fan to turn on; operable vents and dampers that allow or restrict heat flow; low-emissivity blinds; operable insulating shutters; and awnings.

REFINING THE DESIGN

Although conceptually simple, a successful passive solar home requires that a number of details and variables come into balance. An experienced designer can use a computer model to simulate the details of a passive solar home in different configurations until the design fits the site as well as the owner’s budget, aesthetic preferences, and performance requirements.

Some of the elements the designer will consider include:

Insulation and air sealing
Window location, glazing type, and window shading
Thermal mass location and type.
Auxiliary heating and cooling systems.
The designer will apply these elements using passive solar design techniques that include direct gain, indirect gain, and isolated gain.

DIRECT GAIN

In a direct gain design, sunlight enters the house through south-facing windows and strikes masonry floors and/or walls, which absorb and store the solar heat. As the room cools during the night, the thermal mass releases heat into the house.

Some builders and homeowners use water-filled containers located inside the living space to absorb and store solar heat. Although water stores twice as much heat as masonry materials per cubic foot of volume, water thermal storage requires carefully designed structural support. An advantage of water thermal storage is that it can be installed in an existing home if the structure can support the weight.

INDIRECT GAIN (TROMBE WALL)

An indirect-gain passive solar home has its thermal storage between the south-facing windows and the living spaces. The most common indirect-gain approach is a Trombe wall.

The wall consists of an 8-inch to 16-inch thick masonry wall on the south side of a house. A single or double layer of glass mounted about one inch or less in front of the dark-colored wall absorbs solar heat, which is stored in the wall's mass. The heat migrates through the wall and radiates into the living space. Heat travels through a masonry wall at an average rate of one inch per hour, so the heat absorbed on the outside of an 8-inch thick concrete wall at noon will enter the interior living space around 8 p.m.

ISOLATED GAIN (SUNSPACES)

The most common isolated-gain passive solar home design is a sunspace that can be closed off from the house with doors, windows, and other operable openings. Also known as a sunroom, solar room, or solarium, a sunspace can be included in a new home design or added to an existing home.

Sunspaces should not be confused with greenhouses, which are designed to grow plants. Sunspaces serve three main functions -- they provide auxiliary heat, a sunny space to grow plants, and a pleasant living area. The design considerations for these three functions are very different, and accommodating all three functions requires compromises.

PASSIVE SOLAR HOME DESIGN FOR SUMMER COMFORT

Experienced passive solar home designers plan for summer comfort as well as winter heating.

In most climates, an overhang or other devices, such as awnings, shutters, and trellises will be necessary to block summer solar heat gain. Landscaping can also help keep your passive solar home comfortable during the cooling season

sources: http://energy.gov/energysaver/passive-solar-home-design

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Record-Breaking Solar Cell Points the Way to Cheaper Power

Panasonic reveals a design that surpasses a 20-year-old mark for solar cell efficiency.

For roughly two decades, the most efficient silicon solar cells in the world used a structure invented in Australia at the University of New South Wales. This week, in a packed conference room at the IEEE Photovoltaic Specialists Conference in Denver, Panasonic gave details for the first time about a new structure that allows silicon solar cells to surpass that efficiency, setting a new world record and possibly pointing the way to cheaper solar power that can compete widely with fossil fuels.


The result reflects a new surge forward for silicon solar cells, the type that account for almost all solar cells on the market. “Amazingly, the 20-year-old efficiency record was eclipsed at this conference by three companies, Panasonic, Sharp, and SunPower,” says Richard Swanson, cofounder and former president of SunPower (see “Three Questions with a Solar Pioneer”).

The new design combines elements of two of the most efficient types of solar cells available commercially: those produced by the solar company SunPower and an earlier design by Panasonic. SunPower’s cells are highly efficient in part because they do away with the front contacts that block some of the incoming sunlight. Both positive and negative contacts are on the back.

In the new design, Panasonic used a similar approach to get rid of the front contacts and eliminate that shading. The main difference is that Panasonic applies this design to its own high-efficiency cell structure, which addresses another major problem with conventional silicon solar cells. Imperfections at or near the surface of the crystalline silicon wafers used in the cells can trap electrons, decreasing current and voltage. Panasonic prevents this by applying thin films of silicon to the front and back of the silicon wafer.


The new cell converts 25.6 percent of the energy in sunlight into electricity, edging past the long-standing record of 25 percent. Such small improvements in efficiency can have a big impact on total power output from a solar cell, but the main reason the advance is important is that it shows the potential of a new way to design solar cells, which could lead to larger improvements in the future.

Although both aspects of the new design are used in commercial solar cells, it’s not clear yet that the structure that combines them can be manufactured at competitive prices. Martin Green, the professor at the University of New South Wales whose cells had held the previous record, says one drawback of all the record-setting designs is their use of high-quality silicon crystal, which is expensive

thanks:http://www.technologyreview.com/news/528351/record-breaking-solar-cell-points-the-way-to-cheaper-power/

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How do solar cells work?

Solar (or photovoltaic) cells convert the sun’s energy into electricity. Whether they’re adorning your calculator or orbiting our planet on satellites, they rely on the the photoelectric effect: the ability of matter to emit electrons when a light is shone on it.

Silicon is what is known as a semi-conductor, meaning that it shares some of the properties of metals and some of those of an electrical insulator, making it a key ingredient in solar cells. Let’s take a closer look at what happens when the sun shines onto a solar cell.

Sunlight is composed of miniscule particles called photons, which radiate from the sun. As these hit the silicon atoms of the solar cell, they transfer their energy to loose electrons, knocking them clean off the atoms. The photons could be compared to the white ball in a game of pool, which passes on its energy to the coloured balls it strikes.

Freeing up electrons is however only half the work of a solar cell: it then needs to herd these stray electrons into an electric current. This involves creating an electrical imbalance within the cell, which acts a bit like a slope down which the electrons will flow in the same direction.

Creating this imbalance is made possible by the internal organisation of silicon. Silicon atoms are arranged together in a tightly bound structure. By squeezing small quantities of other elements into this structure, two different types of silicon are created: n-type, which has spare electrons, and p-type, which is missing electrons, leaving ‘holes’ in their place.

When these two materials are placed side by side inside a solar cell, the n-type silicon’s spare electrons jump over to fill the gaps in the p-type silicon. This means that the n-type silicon becomes positively charged, and the p-type silicon is negatively charged, creating an electric field across the cell. Because silicon is a semi-conductor, it can act like an insulator, maintaining this imbalance.

As the photons smash the electrons off the silicon atoms, this field drives them along in an orderly manner, providing the electric current to power calculators, satellites and everything in between.

HOW PHOTOVOLTAIC CELLS WORK - PHOTOVOLTAIC CELL OVERVIEW:

Photovoltaic Cells (Solar Cells), How They Work

 

The photovoltaic cell (PV cell) offers a limitless and environmentally friendly source of electricity. Also called a solar cell, the photovoltaic cell is able to create electricity directly from photons. A photon can be thought of as a packet of light and the energy of a photon is proportional to the wavelength of light.

Photovoltaic Cell Structure:

A. Encapsulate - The encapsulate, made of glass or other clear material such clear plastic, seals the photovoltaic cell from the external environment.

B. Contact Grid- The contact grid is made of a good conductor, such as a metal, and it serves as a collector of electrons.

C. The Antireflective Coating (AR Coating)- Through a combination of a favorable refractive index, and thickness, this layer serves to guide light into the photovoltaic cell. Without this layer, much of the light would bounce off the surface of the cell.

D. N-Type Silicon - N-type silicon is created by doping (contaminating) the silicon with compounds that contain one morevalence electrons* than silicon does, such as with either phosphorus or arsenic. Since only four electrons are required to bond with the four adjacent silicon atoms, the fifth valence electron is available for conduction.

E. P-Type Silicon- P-type silicon is created by doping with compounds containing one less valence electrons* than Si does, such as with boron. When silicon (four valence electrons) is doped with atoms that have one less valence electrons (three valence electrons), only three electrons are available for bonding with four adjacent silicon atoms, therefore an incomplete bond (hole) exists which can attract an electron from a nearby atom. Filling one hole creates another hole in a different Si atom. This movement of holes is available for conduction.

F. Back Contact - The back contact of a photovoltaic cell is made out of metal that covers the entire back surface and acts as a conductor.

Photon's Path Through the Photovoltaic Cell:

After a photon makes its way through the encapsulate it encounters the antireflective layer. The antireflective layer channels the photon into the lower layers of the photovoltaic cell. Click on the following link if you would like to learn about our novel room temperature wet chemical growth antireflective layer (RTWCG - AR).

Once the photon passes the antireflective layer, it will either hit the silicon surface of the photovoltaic cell or the contact grid metallization. The metallization, being opaque, lowers the number of photons reaching the Si surface. The contact grid must be large enough to collect electrons yet cover as little of the photovoltaic cell's surface, allowing more photons to penetrate.

A Photon causes the Photoelectric Effect*.

The photon's energy transfers to the valence electron of an atom in the n-type silicon layer. That energy allows the valence electron to escape its orbit leaving behind a hole. In the n-type silicon layer, the free electrons are called majority carriers whereas the holes are called minority carriers. As the term "carrier" implies, both are able to move throughout the silicon layer, and so are said to be mobile. Inversely, in the p-type Si layer, electrons are termed minority carriers and holes are termed majority carriers, and of course are also mobile.

The p-n junction.

The region in the photovoltaic cell where the n-type and p-type silicon layers meet is called the p-n junction. As you may have already guessed, the p-type Si layer contains more positive charges, called holes, and the n-type Si layer contains more negative charges, or electrons. When p-type and n-type materials are placed in contact with each other, current will flow readily in one direction (forward biased) but not in the other (reverse biased).

An interesting interaction occurs at the p-n junction of a darkened photovoltaic cell. Extra valence electrons in the n-type layer move into the p-type layer filling the holes in the p-type layer forming what is called a depletion zone. The depletion zone does not contain any mobile positive or negative charges. Moreover, this zone keeps other charges from the p and n-type layers from moving across it.

So, to recap, a region depleted of carriers is left around the p-n junction, and a small electrical imbalance exists inside the photovoltaic cell. This electrical imbalance amounts to about 0.6 to 0.7 volts. So due to the p-n junction, a built in electric 

P = V × I

When photons hit the photovoltaic cell, freed electrons (-) attempt to unite with holes on the p-type silicon layer. The p-n junction, a one-way road, only allows the electrons to move in one direction. If we provide an external conductive path, electrons will flow through this path to their original (p-type) side to unite with holes.

The electron flow provides the current ( I ), and the photovoltaic cell's electric field causes a voltage ( V ). With both current and voltage, we have power ( P ), which is just the product of the two. Therefore, when an external load (such as an electric bulb) is connected between the front and back contacts, electricity flows in the photovoltaic cell, working for us along the way.

Sources:http://www.physics.org/article-questions.asp?id=51

Credit: http://specmat.com/photovoltaic%20cell%20specmat.html

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Solar Cell Structure

A solar cell is an electronic device which directly converts sunlight into electricity. Light shining on the solar cell produces both a current and a voltage to generate electric power. This process requires firstly, a material in which the absorption of light raises an electron to a higher energy state, and secondly, the movement of this higher energy electron from the solar cell into an external circuit. The electron then dissipates its energy in the external circuit and returns to the solar cell. A variety of materials and processes can potentially satisfy the requirements for photovoltaic energy conversion, but in practice nearly all photovoltaic energy conversion uses semiconductor materials in the form of a p-n junction.

The basic steps in the operation of a solar cell are:

=>the generation of light-generated carriers;

=>the collection of the light-generated carries to generate a current;

=>the generation of a large voltage across the solar cell; and

=>the dissipation of power in the load and in parasitic resistances.

Another Structure of a solar cells - function and working principle

Solar cells are structured in layers with different functions. The working principle is the same as in semiconductors. 

The main part of a silicon (Si) solar cell generating solar power is formed by two differently doped (n- and p-) silicon layers. A physical barrier is created between them along the p-/n- junction, with electrons and holes diffusing into regions of lower concentration.

This depleted region or space charge region can only be overcome with the help of photons i.e. sunlight.

To be able to channel electrones and holes and generate electric power, metal contacts need to be printed onto the front and rear side. Generally, a full aluminium or silver layer is screenprinted onto the rear. A thin grid forms the front contact keeping the impact on light entering the silicon cells as low as possible.

To reduce light reflection, a thin film of silicon nitride or titanium dioxide is coated onto the surface.

credit:http://www.pveducation.org/pvcdrom/solar-cell-operation/solar-cell-structure

    AND http://www.renewable-energy-concepts.com/solarenergy/solar-technology/si-solar-cell-structure-function.html

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Solar cell

A solar cell (or a "photovoltaic" cell) is a device that converts photons from the sun (solar light) into electricity.
In general, a solar cell that includes both solar and nonsolar sources of light (such as photons from incandescent bulbs) is termed a photovoltaic cell.

Fundamentally, the device needs to fulfill only two functions: photogeneration of charge carriers (electrons and holes) in a light-absorbing material, and separation of the charge carriers to a conductive contact that will transmit the electricity.

This conversion is called the photovoltaic effect, and the field of research related to solar cells is known as photovoltaics.

Solar cells have many applications.

Historically solar cells have been used in situations where electrical power from the grid is unavailable, such as in remote area power systems, Earth orbiting satellites, consumer systems, e.g. handheld calculators or wrist watches, remote radio-telephones and water pumping applications.

Solar cells are regarded as one of the key technologies towards a sustainable energy supply.

A new twist on an old solar cell design sends light ricocheting through layers of microscopic spheres, increasing its electricity-generating potential by 26 percent.

By engineering alternating layers of nanometer and micrometer particles, a team of engineers from the University of Minnesota has improved the efficiency of a type of solar cell by as much as 26 percent. These cells, known as dye-sensitized solar cells (DSSC), are made of titanium dioxide (TiO2), a photosensitive material that is less expensive than the more traditional silicon solar cells, which are rapidly approaching the theoretical limit of their efficiency. Current DSSC designs, however, are only about 10 percent efficient.

One reason for this low efficiency is that light from the infrared portion of the spectrum is not easily absorbed in the solar cell. The new layered design, as described in the AIP's Journal of Renewable and Sustainable Energy, increases the path of the light through the solar cell and converts more of the electromagnetic spectrum into electricity. The cells consist of micrometer-scale spheres with nanometer pores sandwiched between layers of nanoscale particles. The spheres, which are made of TiO2, act like tightly packed bumpers on a pinball machine, causing photons to bounce around before eventually making their way through the cell.

Each time the photon interacts with one of the spheres, a small charge is produced. The interfaces between the layers also help enhance the efficiency by acting like mirrors and keeping the light inside the solar cell where it can be converted to electricity. This strategy to increase light-harvesting efficiency can be easily integrated into current commercial DSSCs.


credit for:http://www.sciencedaily.com/releases/2011/07/110729175554.htm

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solar energy and electricity analysis by IER

The power of the sun’s radiant energy is what makes life on earth possible. Efforts to harness it in concentrated form and direct it to man’s ends have long been a human pursuit. The current state of technology generally uses two approaches: solar thermal collectors and the more complex design and manufacture of photovoltaic cells.

The collection of thermal radiation from the sun is relatively easy, and involves the use of a fluid passing through a heat sink exposed to sunlight. The circulated fluid can be used as a heat source, or if concentrated, be used to turn a wheel or turbine to generate electricity. Photovoltaic cells derive electrical current by using sophisticated semiconductors to convert photons into electrons.

Solar energy has played a minor role in the US since the advent of industrialization although prior to that, the photosynthetic conversion of solar energy into plant life was the basis for dominant biomass energy sources—wood for heating and fodder for horse-powered transportation.

Today, solar energy provides four-tenths of 1 percent of the total energy consumed in the United States.[1] While the amount of utility-scale solar electricity capacity in the US has increased in recent years—rising from 334 megawatts in 1997 to 6,623 megawatts in 2013, it still only accounts for 0.4% of net utility-scale electricity generated in the United States – the least among the renewable sources of hydroelectric, biomass, wind and solar. However, if rooftop solar panels and other solar lighting in the residential and commercial sectors were included in the generation statistics, total solar generated electricity in the US could represent a larger, but still small share.[2]

Efforts to expand solar capacity face several challenges. One of the most significant impediments to solar power, like wind power, involves the availability of its source. Solar radiation is rarely constant and varies with changing atmospheric conditions (clouds and dust), and the changing position of the Earth relative to the sun (day and night). Solar energy is also relatively weak because it must first pass through the atmosphere, which protects the Earth from the sun’s intensity. As such, the intermittent and variable manner in which solar energy arrives on the Earth’s surface means it creates reliability problems. This necessitates some form of back-up energy system to be installed for when the sun is not shining or the weather is adverse.

Though solar technologies are improving, meeting current US electricity needs with today’s photovoltaic technology would require about 10,000 square miles of solar panels—an area the size of New Hampshire and Rhode Island combined. Moreover, if photovoltaic power is established in those areas of the country like the desert southwest where sunshine is abundant, consideration must be made for transmission lines as well as the “line loss” that accompanies electrical transmission over great distances.

As electricity is transmitted along power lines, a substantial amount of the produced power is lost, and the longer the transit, the more the loss. Although solar energy is fueled freely by the sun, the cost of the technology relative to the amount of energy produced makes solar significantly more expensive than other more widely used energy sources. Further, often the costs of requiring back-up energy are not generally included in the assumed production costs of solar energy, making comparisons of true production costs with other energy sources even more difficult.

Solar energy has proven well-suited to a number of site-specific applications where a remote location can be serviced by electricity generated through photovoltaics and consumed without any line loss. Because it also saves on the costs of building transmission lines to supply power, the economics of this type of solar energy improve greatly. On a small scale, many Americans are familiar with this concept because of solar powered yard lighting that similarly obviates the need for electrical wiring.

For the foreseeable future, solar energy is likely to make up a very small part of our overall energy mix because its costs and reliability place it at a disadvantage to other forms of electrical generation. However, it may gain favor in isolated applications for certain uses, and its use has been growing as government mandates compel consumers to use more renewable forms of energy without regard to cost.

All credit for: http://instituteforenergyresearch.org/topics/encyclopedia/solar/

[1] IER uses the same definitions as the U.S. Energy Information Administration when calculating energy consumption and electricity generation by source.

[2] The Energy Information Administration is planning to update the solar generation data to include other sources of generation besides utility-scale generation. When EIA publishes these data, IER will update its information.

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Advantages and 5 Disadvantages of Solar Energy

Solar energy is derived from the sun’s radiation. The sun is a powerful energy source. The energy that it provides to the Earth for one hour, could meet the global energy needs for one year. We are able to harness only 0.001 percent of that energy. Below you can read about some advantages and disadvantages of solar energy.

Advantages of Solar Energy
1. Renewable Energy Source
Solar energy is a truly renewable energy source. It can be harnessed in all areas of the world and is available everyday. We cannot run out of solar energy, unlike some of the other sources of energy. Solar energy will be accessible as long as we have the sun, therefore sunlight will be available to us for at least 5 billion years, when according to scientists the sun is going to die.

2. Reduces Electricity Bills
Since you will be meeting some of your energy needs with the electricity your solar system has generated, your energy bills will drop. How much you save on your bill will be dependent on the size of the solar system and your electricity or heat usage. Moreover, not only will you be saving on the electricity bill, but if you generate more electricity than you use, the surplus will be exported back to the grid and you will receive bonus payments for that amount (considering that your solar panel system is connected to the grid). Savings can further grow if you sell excess electricity at high rates during the day and then buy electricity from the grid during the evening when the rates are lower.

3. Diverse Applications

Solar energy can be used for diverse purposes. You can generate electricity (photovoltaics) or heat (solar thermal). Solar energy can be used to produce electricity in areas without access to the energy grid, to distill water in regions with limited clean water supplies and to power satellites in space. Solar energy can also be integrated in the materials used for buildings. Not long ago Sharp introduced transparent solar energy windows.

4. Low Maintenance Costs

Solar energy systems generally don’t require a lot of maintenance. You only need to keep them relatively clean, so cleaning them a couple of times per year will do the job. Most reliable solar panel manufacturers give 20-25 years warranty. Also, as there are no moving parts, there is no wear and tear. The inverter is usually the only part that needs to changed after 5-10 years because it is continuously working to convert solar energy into electricity (solar PV) and heat (solar thermal). So, after covering the initial cost of the solar system, you can expect very little spending on maintenance and repair work.

5. Technology Development

Technology in the solar power industry is constantly advancing and improvements will intensify in the future. Innovations in quantum physics and nanotechnology can potentially increase the effectiveness of solar panels and double, or even triple, the electrical input of the solar power systems.

Disadvantages of Solar Energy

1. Cost

The initial cost for purchasing a solar system is fairly high. Although the UK government has introduced some schemes for encouraging the adoption of renewable energy sources, for example the the Feed-in Tariff, you still have to cover the upfront costs. This includes paying for solar panels, inverter, batteries, wiring and for the installation. Nevertheless, solar technologies are constantly developing, so it is safe to assume that prices will go down in the future.

2. Weather Dependent

Although solar energy can still be collected during cloudy and rainy days, the efficiency of the solar system drops. Solar panels are dependent on sunlight to effectively gather solar energy. Therefore, a few cloudy, rainy days can have a noticeable effect on the energy system. You should also take into account that solar energy cannot be collected during the night.

3. Solar Energy Storage Is Expensive

Solar energy has to be used right away, or it can be stored in large batteries. These batteries, used in off-the-grid solar systems, can be charged during the day so that the energy is used at night. This is good solution for using solar energy all day long but it is also quite expensive. In most cases it is smarter to just use solar energy during the day and take energy from the grid during the night (you can only do this if your system is connected to the grid). Luckily our energy demand is usually higher during the day so we can meet most of it with solar energy.

4. Uses a Lot of Space

The more electricity you want to produce, the more solar panels you will need, because you want to collect as much sunlight as possible. Solar panels require a lot of space and some roofs are not big enough to fit the number of solar panels that you would like to have. An alternative is to install some of the panels in your yard but they need to have access to sunlight. Anyways, If you don’t have the space for all the panels that you wanted, you can just get a fewer and they will still be satisfying some of your energy needs.

5. Associated with Pollution

Although pollution related to solar energy systems is far less compared to other sources of energy, solar energy can be associated with pollution. Transportation and installation of solar systems have been associated with the emission of greenhouse gases. There are also some toxic materials and hazardous products used during the manufacturing process of solar photovoltaics, which can indirectly affect the environment. Nevertheless, solar energy pollutes far less than the other alternative energy sources.

sources:http://www.greenmatch.co.uk/blog/2014/08/5-advantages-and-5-disadvantages-of-solar-energy

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SOLARCITY'S ROOFTOP PANELS ARE ABOUT TO GET WAY, WAY BETTER

SOLAR CITY HAS A BIG ANNOUNCEMENT THAT MAY HELP RESHAPE THE ECONOMICS OF RENEWABLES.

Solar City, the solar energy company that Elon Musk helped found, has been on a tear over the past several years, adding thousands of employees, expanding across the country, and building what will be North America’s largest solar panel manufacturing facility in Buffalo, New York. Now it has a new claim to fame: maker of the world’s most efficient solar panels.

Later today at an event in New York City, SolarCity founder and CEO Lyndon Rive will announce that his company’s forthcoming rooftop panels will be the most efficient in the world, with a module efficiency of just over 22%. (The measurement refers to how much energy the panels generate. Industrial-scale panels can have much higher efficiencies, but the result is a breakthrough for a product that is small enough and affordable enough to go on your roof.) Calling the new super-efficient panels the "holy grail" of home solar energy, Rive says that they’ll help reduce costs while making renewable energy work for an ever-growing percentage of Americans. "This opens up the market," says the 38-year-old South Africa native.

BURNING MAN BEGINNINGS

SolarCity, which is worth roughly $4 billion and has market share among residential customers that is nearly three times its closest competitor, has come a long way from its startup days. In 2004, Musk, Rive’s cousin and now SolarCity’s chairman, pitched him and his brother Peter on the idea of starting a solar energy company while they were driving to the Burning Man festival. They launched the company two years later as a panel installer, using off-the-shelf mounting hardware and made-in-China solar panels, and focusing on making the process of buying a home solar system as easy and as affordable as possible. SolarCity employees wore spiffy green uniforms and used advanced software to help customers figure out if going solar made sense.

Rive says that the approach was met with skepticism; the idea of employing a large staff of installers—high tech construction workers, essentially—seemed to many venture capitalists expensive and unworkable. "They said you’re better off outsourcing the installation and have others do the work for you," says Rive. "We found that it’s better to do it yourself. You can maintain the customer experience and reduce your cost."

As SolarCity grew the company began taken over functions it had previously outsourced, offering financing for the panels so that customers could opt to pay a monthly fee rather than paying thousands of dollars outright, and then manufacturing its own mounting hardware. The moves helped bring costs down, and also helped SolarCity broaden its appeal, but Rive says it became increasingly clear that the company needed to make its own panels.

THE NEXT STEP

Though prices for solar panels have fallen—from $4 per watt in 2008 to around 65 cents today—prices for better, more efficient panels that produce more energy, have remained stubbornly high. "Nobody was making high-efficiency low-cost modules," says Rive, who considers this the "holy grail" for pushing solar adoption, because the more energy a home’s solar panels generate the more affordable solar power becomes. "If no one going to do it, you have to do it yourself."

Last year, the company bought Silevo, a solar panel startup, for a reported $200 million and began planning its factory in western New York. The result of those moves are the new panels that Rive is unveiling today, which will produce at least 30% more power, while reducing the cost of installing solar panels on one’s roof by between 20 and 30 cents per watt. It doesn’t sound like a lot of money, but given that Solar City pays just $2.90 to install a watt of capacity, it’s a big savings.

The improved cost structure should help SolarCity bring renewables further into the mainstream. Just 2% of the total energy produced in the U.S. comes from the sun, but, as Rive points out, solar energy accounts for a large share, 35%, of new energy production—ahead of natural gas, wind power, and everything other power source. 

Thanks for:http://www.fastcompany.com/3051844/tech-forecast/solar-panels-are-about-to-get-way-way-better

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Cost of Solar Power

hello dear viewer, Today i am going to write about the cost of solar.

On average the total cost of solar installation can be between $15,000 to $29,000 for average sized systems sized between 4kW and 8kW.2

Sunrun solar lets you get started for as little as $0 down and helps you lower your electric bill. If you want to own your system, we do that too. If you choose to buy your system, the graph shows an average breakdown of costs of a residential solar installation.

Equipment costs: Solar panels, inverter, mounting hardware and wiring

Installation and permits: Installation, supply chain, permitting and interconnection

Sales and operational: Monitoring and maintenance costs, repairs, additional operational and overhead

Solar vs. Your Utility Company:


Did you know that solar power already costs less than grid power in 10 states? That translates into big savings on your utility bill. Here’s why it’s happening:

1. Despite temporary drops in the price of oil and natural gas, utility companies continue to increase electric rates.3 Over time, fossil fuels will continue to be limited, further driving the costs up.
2. The cost of solar panels is falling, down almost 100 percent since 1977 (see chart) and more than 50% since 2007 alone.4
3. Installation costs are falling as well, as more solar providers enter the market to meet growing demand.


Incentives can help you save:

Federal, state and local incentives push costs down even further. Currently, homeowners can take advantage of a 30 percent federal tax credit now through 2016. Even if the credit falls to 10 percent with no additional rebates after 2016 as some predict, solar comes out ahead of grid electricity in most of the U.S. as shown in this map.5

So why wait? Solar is ready now, and your savings start immediately.


See why Sun run is the trusted choice?

Sunrun is one of the leading companies for home solar. We take care of everything from paperwork to installation so you don’t have to do a thing.

Even better, you can save 20% on your electric bill when you go solar with Sunrun.5 Plus, with our 100% guarantee your system will perform as expected, or we’ll cut you a check for the difference. We've got you covered on everything from maintenance to monitoring and repairs – at Sunrun, we always have your back.

Just see How much does going solar with Sunrun cost?

All credit for:http://www.sunrun.com/solar-lease/cost-of-solar

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Solar Technology-greenTechnology

Solar Energy: Past, Present and Future

The sun is the largest energy source in the solar system. In fact, more energy from the sun hits the earth in one hour than the entire world uses in one year! And every living thing on earth, even microbes deep in the blackest reaches of the ocean, depend on the sun for life. Solar energy technology harnesses the power of the sun for human use, but we’ve only just begun to tap its full potential

History of Humans and Solar Energy

Humans have been tinkering with solar energy since the dawn of time. Ancient civilizations learned how to use building techniques to store the sun’s energy during the day to keep their homes warm at night. They even used glass and mirrors to light fires. It wasn’t until the 1950s when technology was developed to convert the sun’s energy into electricity using photovoltaic cells, or what we call today, solar panels. One of the first uses of a solar panel was on the Vanguard I space satellite launched in 1958. Since then, innovative uses of solar have been invented to not only generate energy for homes and buildings, but move people in solar cars, boats, and even airplanes.

Types of Current Solar Technology

Passive Solar: This doesn’t involve the use of mechanical and electrical devices. Windows, walls, and floors collect, store, and distribute solar energy in the form of heat in the winter and reject solar heat in the summer. More and more homes are being built to utilize or deflect this type of solar energy.

Solar Thermal: Technology for harnessing the sun’s heat. One use is to heat water on small or large scale.
Solar Photovoltaic: Technology for producing electricity from the sun using solar cells, typically encased in panels.
Concentrated Solar: Technology for producing electricity from the sun using mirrors (heliostats) to concentrate a large area of solar thermal energy onto a small area. Electrical power is produced when the concentrated light is converted to heat, which drives a heat engine (usually a steam turbine) connected to an electrical power generator.

A Solar-Powered Future

Innovation in solar technology continues to improve efficiency, size and cost, making it more pervasive throughout society. The trend is leaning toward incorporating solar into more buildings beyond panels placed upon the roof. Cool applications include: solar shingles, solar film, solar roadways, and solar windows.

Other innovations being explored are: the solar orb, solar cars (commercially available), solar balloons, nanowires, and working with the infrared spectrum. As the manager of the Green Mountain Energy Sun Club, I’m excited about these advances in solar technology and the growing part this pollution-free resource will provide in our lives. A solar future is closer than you may think!

credit for:https://www.greenmountainenergy.com/2014/06/solar-energy-past-present-future/

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BENEFITS OF SOLAR ENERGY

What is solar energy?

Solar energy is energy which is created from sunlight, or heat from the sun.
Solar power is captured when energy from the sun is converted into electricity or used to heat air, water, or other fluids.

There are currently two main types of solar energy technologies:

solar thermal:
these systems convert sunlight into thermal energy (heat). Most solar thermal systems use solar energy for space heating or to heat water (such as in a solar hot water system). However this heat energy can be used to drive a refrigeration cycle to provide for solar based cooling. The heat can also be used to make steam, which can then be used to generate electricity using steam turbines. It is considered more efficient to build solar thermal electricity generators at large scale, typically in the tens to hundreds of megawatts
solar photovoltaic (PV):
the conversion of sunlight directly into electricity using photovoltaic cells. PV systems can be installed on rooftops, integrated into building designs and vehicles, or scaled up to megawatt scale power plants.

How is solar energy used in Australia?

Australia has the highest average solar radiation per square metre of any continent in the world.

More than 2 million Australian households now have solar hot water systems or solar photovoltaic (PV) systems on their rooftop. Deployment of megawatt-scale solar electricity generation systems is still at an early stage of development in Australia.

The increased deployment of solar energy generation depends critically on the commercialisation of large-scale solar energy technologies.

Benefits of Solar Energy

Creating electricity from sunlight instead of fossil fuels avoids the emission of greenhouse gases and other air pollutants and stimulates the economy.

Environmental

100% pollution-free: Solar power is one of the cleanest sources of energy because it does not emit any pollution when it is produced or consumed, so it can help avoid the carbon dioxide (CO2) and other air pollutant emissions associated with conventional electricity generation
Renewable: It’s inexhaustible so it will never run out, unlike limited fossil fuel sources
Limited land impact: Solar doesn’t require fossil fuel extraction, which damages the land

Economic

Energy independence: Producing renewable energy at home supports a homegrown energy source, helping secure America’s energy future

Demand creates supply: As more people install solar on their homes and businesses, the greater the demand will be for additional home solar systems and larger-scale solar farms

Green job growth: Increased support for renewable energy development creates more employment opportunities in the green job sector, which helps to stimulate our economy

Financial return: A solar energy system can instantly reduce your electric bill and provide a long-term fixed energy rate for the life of your system (20-25 years), which means cost savings now and protection against unpredictable electric prices in the future
Plus, solar panels can increase a home’s resale value—one study found that solar systems added on average $5.50 per watt to a home’s value above the cost of a comparable, non-solar home.

sources: http://arena.gov.au/about-renewable-energy/solar-energy/
           https://www.greenmountainenergy.com/why-green/renewable-energy-101/solar-energy-101-benefits/

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Alternative Energy(solar and wind energy)

Everyday, the world produces carbon dioxide that is released to the earth’s atmosphere and which will still be there in one hundred years time.

This increased content of Carbon Dioxide increases the warmth of our planet and is the main cause of the so called “Global Warming Effect”. One answer to global warming is to replace and retrofit current technologies with alternatives that have comparable or better performance, but do not emit carbon dioxide.

We call this Alternate energy.

By 2050, one-third of the world's energy will need to come from solar, wind, and other renewable resources. Who says? British Petroleum and Royal Dutch Shell, two of the world's largest oil companies. Climate change, population growth, and fossil fuel depletion mean that renewables will need to play a bigger role in the future than they do today.

Alternative energy refers to energy sources that have no undesired consequences such for example fossil fuels or nuclear energy. Alternative energy sources are renewable and are thought to be "free" energy sources. They all have lower carbon emissions, compared to conventional energy sources. These include Biomass Energy, Wind Energy, Solar Energy, Geothermal Energy, Hydroelectric Energy sources. Combined with the use of recycling, the use of clean alternative energies such as the home use of solar power systems will help ensure man's survival into the 21st century and beyond.

Solar. This form of energy relies on the nuclear fusion power from the core of the Sun. This energy can be collected and converted in a few different ways. The range is from solar water heating with solar collectors or attic cooling with solar attic fans for domestic use to the complex technologies of direct conversion of sunlight to electrical energy using mirrors and boilers or photovoltaic cells. Unfortunately these are currently insufficient to fully power our modern society.

Wind Power. The movement of the atmosphere is driven by differences of temperature at the Earth's surface due to varying temperatures of the Earth's surface when lit by sunlight. Wind energy can be used to pump water or generate electricity, but requires extensive areal coverage to produce significant amounts of energy.

Hydroelectric energy. This form uses the gravitational potential of elevated water that was lifted from the oceans by sunlight. It is not strictly speaking renewable since all reservoirs eventually fill up and require very expensive excavation to become useful again. At this time, most of the available locations for hydroelectric dams are already used in the developed world.

Biomass is the term for energy from plants. Energy in this form is very commonly used throughout the world. Unfortunately the most popular is the burning of trees for cooking and warmth. This process releases copious amounts of carbon dioxide gases into the atmosphere and is a major contributor to unhealthy air in many areas. Some of the more modern forms of biomass energy are methane generation and production of alcohol for automobile fuel and fueling electric power plants.

Hydrogen and fuel cells. These are also not strictly renewable energy resources but are very abundant in availability and are very low in pollution when utilized. Hydrogen can be burned as a fuel, typically in a vehicle, with only water as the combustion product. This clean burning fuel can mean a significant reduction of pollution in cities. Or the hydrogen can be used in fuel cells, which are similar to batteries, to power an electric motor. In either case significant production of hydrogen requires abundant power. Due to the need for energy to produce the initial hydrogen gas, the result is the relocation of pollution from the cities to the power plants. There are several promising methods to produce hydrogen, such as solar power, that may alter this picture drastically.

Geothermal power. Energy left over from the original accretion of the planet and augmented by heat from radioactive decay seeps out slowly everywhere, everyday. In certain areas the geothermal gradient (increase in temperature with depth) is high enough to exploit to generate electricity. This possibility is limited to a few locations on Earth and many technical problems exist that limit its utility. Another form of geothermal energy is Earth energy, a result of the heat storage in the Earth's surface. Soil everywhere tends to stay at a relatively constant temperature, the yearly average, and can be used with heat pumps to heat a building in winter and cool a building in summer. This form of energy can lessen the need for other power to maintain comfortable temperatures in buildings, but cannot be used to produce electricity.

Other forms of energy. Energy from tides, the oceans and hot hydrogen fusion are other forms that can be used to generate electricity. Each of these is discussed in some detail with the final result being that each suffers from one or another significant drawback and cannot be relied upon at this time to solve the upcoming energy crunch.

Solar Power
From an environmental perspective, solar power is the best thing going. A 1.5 kilowatt PV system will keep more than 110,000 pounds of carbon dioxide, the chief greenhouse gas, out of the atmosphere over the next 25 years. The same solar system will also prevent the need to burn 60,000 pounds of coal. With solar, there's no acid rain, no urban smog, no pollution of any kind.

Mankind has been crazy to have not bothered to harness the sun's energy until now. Think about this. Go outside on a sunny day. The light falling on your face left the Sun just 8 minutes go. In that 8 minutes it traveled 93 million miles. Those photons are hauling and when they strike your PV module you can convert that motion to electricity. As technology, photovoltaics are not as glitzy as that new sport utility vehicle the television tells us to crave. But in many ways PV is a much more elegant and sophisticated technology.

Whether it be for your business or for your home, why not invest in Solar Panels.Today's solar panels are bombproof and often come with a 25 year warranty or more. Your solar panels may outlive you. They are also modular—you can start with a small system and expand it over time. Solar panels are light (weighing about 20 pounds), so if you move you can take the system with you.

Grid interactive systems and net metering

Some utilities object to net metering. Usually the issue isn't money, but control. They don't want your juice on their wires or they don't want to set a precedent that could come back to haunt them. There are some distributed generation technologies coming down the pike that utilities definitely won't want to net meter, including fuel cells and 50 kw microturbines the size of beer kegs. However in the USA and Australia electricity suppliers are becomg more supportive of solar enegy buy back schemes.Also busineses can now take advantage of different suppliers of both gas and electricity and shop for the most economical.

Solar advocates delight in bashing utilities. But for all its faults, the industry has strung an amazing amount of wire. Rarely is an American or an Australian, or a European more than 50 feet from an electrical outlet. It's an everyday miracle we take for granted. From an engineering perspective, the grid is a tremendous resource. A grid-tied PV system will be more efficient, arguably greener, and certainly cheaper than a backwoods one. More efficient because the inverter can track the modules "maximum power curve" rather than the lower voltage needed to recharge batteries. Arguably greener because you don't need batteries, which contain caustic chemicals, emit sulfurous gases, and eventually wear out. And much cheaper because, with the grid as backup, you don't have to buy batteries, charge controller, control panel or generator.Right there, you've knocked up to $5,000 off a typical stand-alone system. Getting the price down is critical, because no one on the grid needs PV, at least not in the same way an off-grid homeowner needs it. We've already got juice. It may be from a nuke, it may be from a coal plant, it may be hydro (or "embodied salmon"), but it's there. To sell grid-connected PV systems you've got to get the price down and then help prospective customers understand that solar is to coal as a croissant is to a Twinkie. On a gut level, many people already grasp the key difference between fossil fuels and renewable energy. One is stealing from our kids, the other isn't.

The current cost of solar panels means that grid-interactive systems do not pay for themselves in terms of the cost saving when compared with electricity from the grid. In spite of this, many people with grid connected houses are choosing to install grid-interactive solar systems, as they do not create any greenhouse gases when generating electricity, unlike coal-fired power plants. Numerous studies have demonstrated that the equivalent amount of electricity used to make a solar panel is generated by the panel within the first two years of operation, hence a solar panel will repay its greenhouse gas "debt" within this time

Wind Power
Societies have taken advantage of wind power for thousands of years. The first known use was in 5000 BC when people used sails to navigate the Nile River. Persians had already been using windmills for 400 years by 900 AD in order to pump water and grind grain. Windmills may have even been developed in China before 1 AD, but the earliest written documentation comes from 1219. Cretans were using "literally hundreds of sail-rotor windmills [to] pump water for crops and livestock."

Today, people are realizing that wind power "is one of the most promising new energy sources" that can serve as an alternative to fossil fuel-generated electricity. The cost of wind has dropped by 15% with each doubling of installed capacity worldwide, and capacity has doubled three times during the 1990s and 2000's.As of 1999, global wind energy capacity topped 10,000 megawatts, which is approximately 16 billion kilowatt-hours of electricity. That's enough to serve over 5 cities the size of Miami, according to the American Wind Energy Association. Five Miamis may not seem significant, but if we make the predicted strides in the near future, wind power could be one of our main sources of electricity.

Though wind energy is now more affordable, more available, and pollution-free, it does have some drawbacks. Wind power suffers from the same lack of energy density as direct solar radiation. The fact that it is a "very diffuse source" means that "large numbers of wind generators (and thus large land areas) are required to produce useful amounts of heat or electricity." But wind turbines cannot be erected everywhere simply because many places are not windy enough for suitable power generation. When an appropriate place is found, building and maintaining a wind farm can be costly. It "is a highly capital-intensive technology." If the interest rates charged for manufacturing equipment and constructing a plant are high, then a consumer will have to pay more for that energy. "One study found that if wind plants were financed on the same terms as gas plants, their cost would drop by nearly 40%." Fortunately, the more facilities built, the cheaper wind energy is.

But there is increasing energy being put in finding many other alternative sources of power and making them viable, such as geothermal and wave energy and biomass.

credit:http://www.altenergy.org/

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