China Racing Ahead of U.S. in the Drive to Go Solar

WUXI, China — President Obama wants to make the United States “the world’s leading exporter of renewable energy,” but in his seven months in office, it is China that has stepped on the gas in an effort to become the dominant player in green energy — especially in solar power, and even in the United States.

Chinese companies have already played a leading role in pushing down the price of solar panels by almost half over the last year. Shi Zhengrong, the chief executive and founder of China’s biggest solar panel manufacturer, Suntech Power Holdings, said in an interview here that Suntech, to build market share, is selling solar panels on the American market for less than the cost of the materials, assembly and shipping.

graphic China’s Solar ShareFEB. 16, 2015
Shi Zhengrong of Suntech, which makes solar panels, said he had misunderstood a question concerning pricing of the panels.Chinese Solar Firm Revises Price StatementAUG. 26, 2009
Times Topic: Solar Energy
Backed by lavish government support, the Chinese are preparing to build plants to assemble their products in the United States to bypass protectionist legislation. As Japanese automakers did decades ago, Chinese solar companies are encouraging their United States executives to join industry trade groups to tamp down anti-Chinese sentiment before it takes root.

The Obama administration is determined to help the American industry. The energy and Treasury departments announced this month that they would give $2.3 billion in tax credits to clean energy equipment manufacturers. But even in the solar industry, many worry that Western companies may have fragile prospects when competing with Chinese companies that have cheap loans, electricity and labor, paying recent college graduates in engineering $7,000 a year.

“I don’t see Europe or the United States becoming major producers of solar products — they’ll be consumers,” said Thomas M. Zarrella, the chief executive of GT Solar International, a company in Merrimack, N.H., that sells specialized factory equipment to solar panel makers around the world.

Since March, Chinese governments at the national, provincial and even local level have been competing with one another to offer solar companies ever more generous subsidies, including free land, and cash for research and development. State-owned banks are flooding the industry with loans at considerably lower interest rates than available in Europe or the United States.


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Suntech, based here in Wuxi, is on track this year to pass Q-Cells of Germany, to become the world’s second-largest supplier of photovoltaic cells, which would put it behind only First Solar in Tempe, Ariz.

Hot on Suntech’s heels is a growing list of Chinese corporations backed by entrepreneurs, local governments and even the Chinese military, all seeking to capitalize on an industry deemed crucial by China’s top leadership.

Dr. Shi pointed out that other governments, including in the United States, also assist clean energy industries, including with factory construction incentives.

China’s commitment to solar energy is unlikely to make a difference soon to global warming. China’s energy consumption is growing faster than any other country’s, though the United States consumes more today. Beijing’s aim is to generate 20,000 megawatts of solar energy by 2020 — or less than half the capacity of coal-fired power plants that are built in China each year.


Solar energy remains far more expensive to generate than energy from coal, oil, natural gas or even wind. But in addition to heavy Chinese investment and low Chinese costs, the global economic downturn and a decline in European subsidies to buy panels have lowered prices.

The American economic stimulus plan requires any project receiving money to use steel and other construction materials, including solar panels, from countries that have signed the World Trade Organization’s agreement on free trade in government procurement. China has not.

In response to this, and to reduce shipping costs, Suntech plans to announce in the next month or two that it will build a solar panel assembly plant in the United States, said Steven Chan, its president for global sales and marketing.

“It’ll be to facilitate sales — ‘buy American’ and things like that,” Mr. Chan said, adding that the factory would have 75 to 150 workers and be located in Phoenix, or somewhere in Texas.

But 90 percent of the workers at the $30 million factory will be blue-collar laborers, welding together panels from solar wafers made in China, Dr. Shi said.

Yingli Solar, another large Chinese manufacturer, said on Thursday that it also had a “preliminary plan” to assemble panels in the United States.

Western rivals, meanwhile, are struggling. Q-Cells of Germany announced last week that it would lay off 500 of its 2,600 employees because of declining sales. It and two other German companies, Conergy and SolarWorld, are particularly indignant that German subsidies were the main source of demand for solar panels until recently.

“Politicians might ask whether this is still the right way to do this, German taxpayers paying for Asian products,” said Markus Wieser, a Q-Cells spokesman.

But organizing resistance to Chinese exports could be difficult, particularly as Chinese discounting makes green energy more affordable.

Even with Suntech acknowledging that it sells below the marginal cost of producing each additional solar panel — that is, the cost after administrative and development costs are subtracted — any antidumping case, in the United States, for example, would have to show that American companies were losing money as a result.

First Solar — the solar leader, in Tempe — using a different technology from many solar panel manufacturers, is actually profitable, while the new tax credits now becoming available may help other companies.

Even organizing a united American response to Chinese exports could be difficult. Suntech has encouraged executives at its United States operations to take the top posts at the two main American industry groups, partly to make sure that these groups do not rally opposition to imports, Dr. Shi said.

The efforts of Detroit automakers to win protection from Japanese competition in the 1980s were weakened by the presence of Honda in their main trade group; they expelled Honda in 1992.

Some analysts are less pessimistic about the prospects for solar panel manufacturers in the West. Joonki Song, a partner at Photon Consulting in Boston, said that while large Chinese solar panel manufacturers are gaining market share, smaller ones have been struggling.

Mr. Zarrella of GT Solar said that Western providers of factory equipment for solar panel manufacturers would remain competitive, and Dr. Shi said that German equipment providers “have made a lot of money, tons of money.”

The Chinese government is requiring that 80 percent of the equipment for China’s first municipal power plant to use solar energy, to be built in Dunhuang in northwestern China next year, be made in China.

source:http://www.nytimes.com/2009/08/25/business/energy-environment/25solar.html?_r=0

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Energy from moving water

Hydro-power generates electricity

Hydro-power is the largest renewable energy source for electricity generation in the United States. In 2014, hydropower accounted for about 6% of total U.S. electricity generation and 48% of generation from all renewable.

Because the source of hydroelectric power is water, hydroelectric power plants are usually located on or near a water source.

Hydro-power relies on the water cycle

Understanding the water cycle is important to understanding hydropower. There are three steps in the water cycle:

• Solar energy heats water on the surface, causing it to evaporate.
• Water vapor condenses into clouds and falls as precipitation (rain, snow, etc.).
• Water flows through rivers back into the oceans, where it can evaporate and begin the cycle over again.

Mechanical energy is harnessed from moving water

The amount of available energy in moving water is determined by the volume of flow and the change in elevation (or fall) from one point to another. Swiftly flowing water in a big river, like the Columbia River that forms the border between Oregon and Washington, carries a great deal of energy in its flow. Water descending rapidly from a high point, like Niagara Falls in New York, also has substantial energy in its flow.

At both Niagara Falls and the Columbia River, water flows through a pipe, or penstock, then pushes against and turns blades in a turbine to spin a generator to produce electricity. In a run-of-the-river system, the force of the current applies the needed pressure, while in a storage system, water is accumulated in reservoirs created by dams, then released as needed to generate electricity.

History of hydropower

Hydropower is one of the oldest sources of energy. Hydropower was used thousands of years ago to turn paddle wheels to help grind grain. The nation's first industrial use of hydropower to generate electricity occurred in 1880, when 16 brush-arc lamps were powered using a water turbine at the Wolverine Chair Factory in Grand Rapids, Michigan.

The first U.S. hydroelectric power plant opened on the Fox River near Appleton, Wisconsin, on September 30, 1882.

sources:http://www.eia.gov/energyexplained/index.cfm?page=hydropower_home

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Environmental Benefits of Solar Power

Dense forests, stunning mountain ranges, and sprawling plains characterize Colorado, a state known throughout the country for its scenic landscape and rugged environment. From the first pioneers who set their eyes on the Rocky Mountains to recent transplants new to the state, Colorado has always represented a vast wilderness to explore and enjoy.

If you’ve ever gazed upward from a hiking trail toward a looming snow-covered mountain peak, or stumbled across a field full of grazing deer, you understand the appeal of the state – and the need to protect those elements that many hold sacred.

Colorado’s natural beauty and environment relies on a delicate balance that is increasingly at risk of derailment due to continued use of fossil fuels, their contribution to global warming and the hazards associated with the extraction of these fuels.

Colorado and Global Warming

Burning fossil fuels gives off a number of emissions that increase the threat of global warming. In Colorado, warming temperatures could drastically change the environment in the state as we see it today. Some potential impacts include:

• Decreased soil moisture and crop production
• Earlier snowmelt and shorter ski season
• Decrease in stream flow and water reserves
• Increased threat of pine beetle outbreak
• More frequent heat waves
• Changes in animal wildlife and activity

Source: Environmental Protection Agency

Currently, 90% of electricity in Colorado is produced from burning fossil fuels like coal, oil, and natural gas. When burned, these fuels emit greenhouse gases, such as CO2 and methane, which not only worsen air quality, but also add to global warming’s impact on the environment.

In Colorado, this could mean lighter snow falls, increased threats of droughts, and higher risks of forest fires – damaging many of the natural features that attract residents to the state in the first place.

However, our reliance on fossil fuels for energy doesn’t need to be permanent. In fact, the Colorado solar industry is experiencing historic gains that could reduce many of the negative environmental impacts that years of fossil fuel use have caused.

In 2012 alone, more than 49,000 Colorado residents installed solar panels on their homes, generating 69.9 megawatts of power. If more people continued to switch to solar power, it could mean cleaner air, more abundant and usable water, and the protection of Colorado’s scenic environment.

Cleaner air for Colorado

All electricity-generating technologies emit pollutants and emissions during their life cycles, but solar and other renewable energy sources are unique in that they emit very little.

According to the Union of Concerned Scientists, the only emissions associated with solar technologies occur during their construction – and those levels are still quite low compared to fossil fuels. During the life cycle of a solar panel, it will only emit 0.07 pounds of CO2 per kWh, compared to coal at 2.10 pounds and natural gas at 1.12 pounds per kWh.

When higher levels of CO2 are expelled into the atmosphere, it warms temperatures, which in turn increases the effects of air pollution from fossil fuel-fired power generation, such as smog.

The Environmental Protection Agency notes that breathing in smog can adversely affect the health of people and wildlife, potentially leading to:

• Irritated respiratory systems
• Reduced lung function
• Increased risk of asthma
• Damaged cell lining in the lungs

By installing more solar panels in Colorado, many of these risks can be greatly reduced. The organizations Vote Solar and Environment Colorado estimate that if Colorado generated 1,000 megawatts of electricity through solar, the combined emissions reduction could equate to taking nearly 670,000 cars off the road by 2030 – representing 30 million tons of carbon dioxide and 766,000 pounds of nitrous oxide.

Consider that in Colorado, there are 13 coal-fired power plants that emit 47 million tons of CO2 per year, in addition to thousands of natural gas wells. Reducing our reliance on these energy sources by generating higher levels solar electricity would help protect Colorado’s air quality and reduce the effects of global warming – which in turn would strengthen the state’s most precious resource: water.

Protecting our water supplies

Outside of the state’s tourism industry and aesthetic appeal, Colorado’s annual snow fall serves a very important purpose: supplying people and wildlife with the water they need to live.

However, temperatures in Colorado are slowly rising due to fossil fuel use, affecting the levels of snowpack that the state relies on for their water supplies. Early in April 2013, for example, water content in the South Platte River Basin, the principal source of water for eastern Colorado, was 29% below normal.

Given how precious Colorado’s water supply is to the wellbeing of the state, one would imagine its protection would be of the utmost importance. However, drilling from the hydraulic fracturing, or fracking, industry uses vast amounts of water to uncover natural gas wells in the ground.

Fracking and Colorado’s Water Supply

Despite the industry’s assertion that burning natural gas is a much more positive practice compared to burning coal, fracking still has harmful effects on Colorado’s environment. Consider the following:

• Water used annually in fracking could serve between 44,200 and 79,000 Colorado households.
• Water used in fracking is 100% consumptive and cannot be returned to streams.
• Fracking’s use of water equals one to two-thirds of total public supply and domestic water use.

Source: “Fracking our Future” Western Resource Advocates

Fracking projects have increased throughout Colorado rapidly as more natural gas reserves have been uncovered. Since 2010 for instance, more than 1,300 fracking wells have been permitted in Weld County, CO.

Now consider that, according to “For Farms in the West, Oil Wells are Thirsty Rivals,” published in The New York Times, a single fracking well can require five million gallons of water. For a state that’s already rated the seventh driest in the country, Colorado can’t afford to continually provide water to industries that produce dirty energy.

Fracking not only reduces the overall water supply, but could also damage the quality of it. Fracking relies on a chemical cocktail of water, acids, and detergents that are terribly harmful should they seep into drinking water.

Unlike fracking, generating solar energy doesn’t require the use of a limited, precious resource, and won’t pollute it either. If Colorado residents installed a combined 1,000 MW of solar power by 2030, it could save 6.8 billion gallons of water over the life of the panels, assuming solar displaces fracking projects.

More solar panels on Colorado roofs means not wasting water on a finite fuel like natural gas. Furthermore, offsetting greenhouse gases with solar power generation could help reduce the warmer temperatures in the state, leading to higher levels of snowpack once again.

Installing solar panels at your home or business

Increasing the number of solar panels in Colorado isn’t just a smart personal decision, but a responsible one in the face of challenges posed by global warming. Rising temperatures, worsening air quality, and a dwindling water supply can be reduced and even reversed if more Colorado residents abandoned fossil fuels and switched to solar energy.

sources;http://www.colorado-solar.org/environmental-benefits-of-solar-in-colorado.php

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User's Guide to Solar Power

While many people are interested in using sustainable energy sources, the terminology can make the process of “going solar” seem more difficult than it really is.

Before contacting solar installers in your area, learn more about how solar power works, and what to look for when installing panels at your home or business.

Important terms you may come across in research

• Direct Current (DC) – Energy captured by solar panels that can be stored.
• Alternating Current (AC) – Electricity that flows directly from the energy source into your wall socket. DC energy is converted to AC energy through an inverter, allowing you to use it at home.
• Inverter – A device that converts DC energy to AC energy. The AC is fed into your home from the inverter, much like it would from an electrical provider.
• Photovoltaics – Materials that convert sunlight into electrical energy.
• Photovoltaic Cells – Electricity producing devices consisting of silicon-based semiconductor materials
• Photovoltaic Modules – Many photovoltaic cells assembled together, creating panels or modules
• Photovoltaic Arrays – Many photovoltaic modules assembled together
• Renewable Energy Certificate – A kind of currency used to show that energy you generate comes from renewable sources.
• Solar Radiation – The general term for electromagnetic radiation emitted from the sun. This radiation can be captured and harnessed into heat and electricity with photovoltaic cells.
• Charge Controller – A device that limits the amount of energy that is added to a battery, preventing overcharging that decreases the life of a battery.

How does a solar cell work?

You’re already set on solar, but naturally have some questions. How does all of this stuff work anyway?

Solar panels are actually made up of smaller devices known as photovoltaic cells, or solar cells. Solar cells consist of two layers of silicon sandwiched between two metal strips that conduct the flow of electricity into the house.

Silicon by itself is not a good conductor of electricity, so each layer is “doped” with two separate chemicals. The top layer of silicon receives phosphorous, while the bottom layer is doped with boron.

With the addition of the chemicals, the top layer gains more electrons, while the bottom layer receives less. Once sunlight hits the solar cell, the electrons in both layers become excited, and are “knocked loose,” flowing freely in each layer. In this state of excitement, some of the electrons hit the metal strips on the outside of the cell, generating electricity.

This electricity transfers from the metal strips to wires that eventually wind their way into a home. With dozens of cells working in conjunction as panels, homeowners generate the electricity they need to turn on lights, watch television, and use computers.

In the next section, we’ll explore how the electricity moves from panels, through several other devices, and into the home as useable energy.

How does a solar panel create energy for my home?

Creating lasting, renewable energy for your home isn’t as simple as slapping a solar panel on your roof and waiting for the meter to run backward. Instead, solar installers create solar electric systems that include several pieces of equipment that allow energy to safely flow to the proper areas.

There are several different types of solar electric systems, but many typically include the following devices:

• Solar panels
• An inverter
• Wiring
• A support structure

Grid-Connected Systems

These solar electric systems are the most common, allowing you to supplement energy from the utility company with energy from your solar panels. With this system, you’ll power your house with solar energy during the daytime as you generate it, while using power from the utility company at night or during cloudy periods.

Energy travels from your panels through wiring as Direct Current (DC) energy. This energy is best stored in batteries, and must be turned into Alternating Current (AC) energy before you can use it in your home.

This conversion occurs in the Inverter, which will allow you to use the energy you’ve produced in outlets throughout your home. The inverter is a critical component of any solar system, taking the formally unusable DC energy and converting it into AC energy.

This AC energy is then wired through your electricity meter, where it is treated as if it came from any typical energy source. The major difference is that if you generate more energy than you’re using, you can actually watch your meter run backwards, as you contribute to the energy grid. By contributing to the energy grid, it’s actually possible to gain money back from the utility company each month through net metering programs.

Solar Electric System with Battery Backup

Instead of relying on utility companies during cloudy days or dark nights, people with solar batteries can store unused solar energy over a period of time. With a battery attached to their solar system, people can last longer without using grid-based power sources.

Systems that have batteries typically involve the same devices as normal grid-connected systems, with the addition of a battery and charge controller. Charge controllers ensure your battery does not become overcharged, which can degrade the battery over time and lead to inefficiencies.

With the addition of a battery, you have the ability to live completely off the grid in some cases. Living completely off the grid requires many batteries, and not many homeowners choose this option. However, many do choose to retain their grid connection even with batteries, giving them a backup in case power is low.

What do I need to consider when buying solar panels?

Most solar panel installers do not manufacture their own panels, but offer several choices to consumers based on their specific needs. When you contact an installer, they’ll ask you about your energy needs and schedule an at-home consultation, allowing them to gauge your eligibility.

Those buying solar panels should pay attention to some of the following factors:

• The Type of Solar Cell
• Solar Efficiency rating
• Roof Orientation and Shading

Types of Solar Cells

Not all solar panels are created equal. Actually, there are several different kinds you’ll be able to choose from when negotiating with installers. Depending on what your installer offers, you’ll have access to solar panels made from three main types of solar cells:

• Monocrystalline Silicon Cells
• Polycrystalline Silicon Cells
• Thin Film Cells

Monocrystalline silicon cells are the most efficient kinds of solar cells, but also the most expensive. Monocrystalline cells are created from one single silicon crystal, and are usually cut into small wafers, then assembled into panels.

Polycrystalline silicon cells are the most common cells used in solar panels. Poly cells are less efficient than mono cells, but are also less expensive. Where the monocrystalline cell is cut from one large silicon crystal, polycrystalline cells are made up of many tiny crystals. You can identify polycrystalline panels from the geometric shapes within the cells.

Thin film panels are formed by coating a silicon mixture onto a flexible material, sacrificing efficiency for cost. While these panels are the cheapest to purchase, they require nearly twice as much space as poly and monocrystalline panels to produce the same amount of energy.

Solar Efficiency Rating

Years of technological research have created more effective solar cells, allowing us to convert more of the sun’s energy into electricity. A panel’s Solar Efficiency Rating refers to what percentage of solar energy absorbed is able to be converted into electricity (the previously mentioned process where electrons bounce between the silicon layers until some hit the metal strips).

Most residential solar panels typically offer between a 13% to 15% efficiency rating, but some companies have achieved higher. Most recently, SunPower Corp, a manufacturer of high efficiency solar cells, achieved an efficiency rating of 24.2%.

A panel’s efficiency can be impacted by a number of outside factors, including dust, dirt, and anything that could block light from hitting the solar cells. While solar panels generally require little maintenance, those are some of the first things you should check for if you notice a drop in efficiency.

Roof Orientation and Shade

In Colorado (and the rest of North America) the sun follows a southern path. This means that solar panels work best on south-facing roofs, with some systems losing efficiency even when facing southeast or southwest.

Aspects like roof tilt may require you to buy different mounting systems that allow you to position your panels in more efficient directions. The most common mounting systems for solar panels include ground mounts and roof mounts.

When even one solar cell on your panel is exposed to shade, it can lead to an up to 50% reduction in efficiency in that cell and surrounding cells in the panel. This means that wherever you install your solar panels, you must be aware of any trees, chimneys, or parts of the roof that could block sunlight during the day.

page source:http://www.colorado-solar.org/solar-power-guide.php

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Wind Power-wind energy

Wind Power

Wind power is the conversion of wind energy into useful form, such as electricity, using wind turbines and constitutes approximately 1% of global electricity output. Learn more.

The expanding market for wind power

Wind turbineMan has been capturing wind energy for millennia and it is now seen as an increasingly mainstream source of power.

It is forecast that, worldwide, the market for wind energy will continue its recent trend of growing massively (by 30-40% per annum). While much attention has been paid to the fact that China brings a new coal-fired power station on-line every ten days, less well known is the country’s interest in wind power: China has doubled its capacity every year since 2004 and is expected to be the world’s biggest wind power market by 2010.

How Wind Energy Works

Wind energy can be harvested in many different ways: with large scale wind farms featuring giant turbines, on or off shore; smaller embedded systems built in to office blocks and modern houses and domestic scale turbines (also known as ‘microwind’), often mounted to chimneys.

Regardless of the size of the turbine, the operating principles are the same: blades are angled into the prevailing wind at different degrees according to wind strength. When it reaches around 4 meters per second (mps), 30% less than the UK local average, the turbines start generating electricity.

They operate most efficiently in winds of around 15mps and have a variety of features built-in, such as variable blade pitch or designed-in ‘passive stall’ which ensure the system is not overloaded by gusts or storms where speeds may be far higher.

Criticisms of wind power

Perhaps surprisingly for an inexhaustible, clean energy source, wind power has been subject to intense criticism, principally for its relatively low overall availability (turbines only work to maximum effect when the wind speed is neither too low nor too high) and therefore cannot be relied on as a sole energy source.

Increasing the efficiency of wind power

Turbine efficiency is increasing year on year, in line with new investment and research. While there is some threat to bird life, especially if turbines are located in migration paths (as has been the case in the early days of the industry), this is no greater and probably less than that posed by any tall building and power cables – both of which are far more prevalent.

Case study: Building wind farms


Vestas has installed over 35,000 wind turbines and were responsible for the turbines at the UK’s first and largest offshore wind power plant at North Hoyle in Wales.


page source:http://www.imeche.org/knowledge/themes/energy/energy-supply/renewable-energy/

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Learning on Renewable energy

The United States currently relies heavily on coal, oil, and natural gas for its energy. Fossil fuels are nonrenewable, that is, they draw on finite resources that will eventually dwindle, becoming too expensive or too environmentally damaging to retrieve. In contrast, renewable energy resources—such as wind and solar energy—are constantly replenished and will never run out.

Solar

Most renewable energy comes either directly or indirectly from the sun. Sunlight, or solar energy, can be used directly for heating and lighting homes and other buildings, for generating electricity, and for hot water heating, solar cooling, and a variety of commercial and industrial uses.

Wind
The sun's heat also drives the winds, whose energy is captured with wind turbines. The Earth's rotation also contributes to the winds, particularly through the Coriolis effect.

Biomass
Along with the rain and snow, sunlight causes plants to grow. The organic matter that makes up those plants is known as biomass. Biomass can be used to produce electricity, transportation fuels, or chemicals. The use of biomass for any of these purposes is called biomass energy.

Geothermal

Not all renewable energy resources come from the sun. Geothermal energy taps the Earth's internal heat for a variety of uses, including electric power production and the heating and cooling of buildings.

Hydrogen

Hydrogen also can be found in many organic compounds, as well as water. It's the most abundant element on the Earth. But it doesn't occur naturally as a gas. It's always combined with other elements, such as with oxygen to make water. Once separated from another element, hydrogen can be burned as a fuel or converted into electricity. Because energy is always needed to produce hydrogen, hydrogen is not in itself an energy source, but rather a way to store and transport energy, so it is often referred to as an energy carrier.

Ocean

The ocean can produce thermal energy from the sun's heat and mechanical energy from the tides and waves. Tides are driven by the gravitational pull of the moon and sun upon the Earth, while waves are driven by winds blowing over the ocean's surface. See the U.S. Department of Energy's Energy Savers for basic information on ocean energy.

Hydropower

Flowing water creates energy that can be captured and turned into electricity. This is called hydroelectric power or hydropower. For more information on hydroelectric power, see the hydropower basics from the U.S. Department of Energy's Water Power Program.


source:http://www.nrel.gov/learning/

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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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