Solar energy

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Energy gained from the sun, and can be collected by solar panels. This is a renewable resource. Plants and producers also collect solar energy. Solar energy, radiant light and heat from the sun, has been harnessed by humans since ancient times using a range of ever-evolving technologies. Solar radiation, along with secondary solar-powered resources such as wind and wave power, hydroelectricity and biomass, account for most of the available renewable energy on earth. Only a minuscule fraction of the available solar energy is used.Solar powered electrical generation relies on heat engines and photovoltaics. Solar energy's uses are limited only by human ingenuity. A partial list of solar applications includes space heating and cooling through solar architecture, potable water via distillation and disinfection, daylighting, solar hot water, solar cooking, and High temperature process heat for industrial purposes.To harvest the solar energy, the most common way is to use solar panels.

[edit] Sun energy

Sun Energy is based in Minnesota. Owner Jerry Lilyerd attended Iowa's first Annual Energy Fair in 1990 and found himself hooked for life. He then attended and helped at many subsequent energy fairs during the next 4 years. After realizing that his personal 'hobby' had him working full-time, Jerry founded Sun Energy in 1994. He had been installing solar and inverter systems, as well as working full-time at NSP (Northern States Power). Jerry reports that he has been booked solid ever since!

All of Sun Energy's business comes from referrals. Jerry and his experienced team of energy professionals handle small to large installations-commercial and residential.

[edit] Working system

A small electric fan moves air behind the metal roofing or siding, heating the air up to 150 degrees F.

Our systems deliver a controlled source of solar heated air to the building to reduce or eliminate conventional gas, electric, propane or oil energy use.

For detailed information on the how our systems work, please visit our page on solar heating technology.

We combine the best roofing system and simplest solar energy technology to save building owners the most on their energy and roofing expenses.

Energy needs: Typically the largest energy use and cost in building operations is for heating, whether it's for:

space heating

water heating

industrial process heating

equipment heating… or

drying

In the United States. the Largest Energy Demand is for HEAT

Roofing and siding needs: One of the largest single expenses during the life of a building is the replacement of a worn out roof. Most large buildings with flat roofs require re-roofing every 15-20 years. Some less expensive roofing systems require replacement or re-treatment even more often.

Studies show the most durable roof for these low-slope applications is a metal roof. It is the most cost-effective roof over the life of a building. Similarly, metal siding is the most cost-effective way to build permanent, weather tight walls.

Now, commercial, government, and institutional facilities have a new cost-effective way to meet their heating, roofing and siding needs with American Solar’s systems.

Our systems efficiently gather large amounts of solar energy by collecting solar heated air from long life metal roofs and siding. That simple source of solar energy, solar-heated air, is then be used in a variety of ways to reduce the heating needs from expensive energy sources.

Solar-heated air can save on heating costs from:

electric heat

natural gas heat

propane

heating oil

coal

and more

With these innovative technologies, customers can use solar energy in several new ways. Solar-heated air can improve indoor air quality, by bringing in fresh outside air that is preheated with low cost solar heat. Low cost solar heated air can also heat industrial and agricultural feedstock before they enter the expensive heating portion of the process.

With American Solar's air-heating systems, customers can more efficiently heat:

Air in buildings such as schools, factories, warehouses, offices, etc.

Hot water for commercial or industrial processes

Boiler intake air and make up feed water

Spray/bake paint booths

Industrial and agricultural feed stocks

Emergency generators to insure a ready-to-start environment

and more

Compared to today's high energy prices, solar thermal energy is relatively inexpensive. Facilities using expensive heating sources, such as electric resistant heating, reap the greatest saving with solar-air-heating systems. Customers with year round energy loads that that use high-cost heating sources benefit the most. However, even customers with only seasonal heating needs will reduce energy costs and see strong returns on their solar investment. Most building owners will reduce their energy costs with American Solar systems.

[edit] Applications of solar power

Depending upon your needs and where you live, there are a variety of solar power system that could work for you.

Solar Power Systems - Grid-Tied (On Grid)

Most people install grid-tied solar power systems -most often in cities, suburbs and industrial areas where access to utility-generated power is available. You can supplement your solar powered electricity with utility-generated energy if you use more electricity than the solar power system supplies. When your solar power system produces more electricity you need, you can sell the excess to the PG&E, who delivers the clean, renewable energy to other customers. Consequently, the good you do for the environment doesn't stop at your home or office. Even neighbors without solar power can draw upon the renewable energy of the sun - while you bank credit to offset the utility-generated power you use at night.

Solar Power Systems - Grid-Tied with battery backup

Solar energy panels combined with batteries and generators for grid-tie applications couple the clean, power supplied by solar panels with the assurance that you will have electricity even during power outages that last for extended periods. During the day, the solar panels generate electricity as needed and charges batteries. If more power is required, or the batteries begin to run low, the natural gas or propane generator kicks in to recharge the batteries. It automatically shuts off when the batteries are fully charged.

Solar Power Systems - Off-Grid

Standalone, or off-grid, solar power systems consist of solar panels and a battery bank. They are typically used in rural areas and regions where there is no access to the utility grid. They may also be appropriate where the grid is somewhat close to the site, but expensive to bring in - for example, across a neighbor's property. We have installed a number of systems with battery back-up where the grid is available but where the homeowner has experienced unreliable power in the past or believes that he/she will be subject to power outages in the future. We have seen a number of property owners install battery back-up system just for philosophical reasons, for the desire to be independent of the grid and the “gaming” to which utility companies and their power suppliers have subjected customers in the past.

It may cost you as high as $50 per foot to bring utility power to your property, after which you'll continue to pay for power forever. It's often less expensive to add a solar energy system from the start and be your own power company. You can add the solar power system cost to your mortgage, reducing the combined costs of mortgage and utility bills.

When your off-grid solar power system produces excess electricity during the day, it is used to charge the batteries. When the sun's not shining, electricity is drawn from the batteries to power the home or business. The advantage is greater independence for you. The disadvantage is greater complexity and cost.

Solar Power System - Direct DC

Simple, direct DC solar power systems produce energy where and when it's needed. Common uses include powering water pumps and fans. There is no complex wiring, so storage and control systems aren't required. Small systems are easy to transport and install.

Hybrid Power Systems

Hybrid power systems combine various sources of electrical generation, and are well suited for electrification. Solar and wind technologies are modular, and seasonal variations of sun and wind often complement each other.

Advantages of Solar Power

The first commercial use of photovoltaic cells nearly 50 years ago was powering communications satellite in near-earth orbit. Today, the declining cost and increasing efficiency of solar energy technology has given rise to practical applications on earth - from powering personal electronic devices, homes and factories to generating utility-scale power.

Solar energy provides a huge advantage for satellites because they can be launched into orbit without the added weight of a fuel supply. But the advantages on earth are even greater: Solar-generated energy provides abundant and pollution-free energy that's not dependent on fuel-delivery infrastructures, foreign relations or the price machinations of energy brokers and big business.

Moreover, solar power generation provides energy when and where you need it, and is highly scalable to match your electrical demand. Since solar energy cells have no moving parts, they are reliable and easy to maintain.

Agricultural Applications of Solar Energy

Solar energy can supply and/or supplement many farm energy requirements. The following is a brief discussion of a few applications of solar energy technologies in agriculture. For more information, you may wish to consult the publications listed below.

Crop and Grain Drying

Using the sun to dry crops and grain is one of the oldest and most widely used applications of solar energy. The simplest, and least expensive technique is to allow crops to dry naturally in the field, or to spread grain and fruit out in the sun after harvesting. The disadvantage of these methods is that the crops and grain are subject to damage by birds, rodents, wind, and rain, and contamination by windblown dust and dirt. More sophisticated solar dryers protect grain and fruit, reduce losses, dry faster and more uniformly, and produce a better quality product than open-air methods.

The basic components of a solar dryer are an enclosure or shed, screened drying trays or racks, and a solar collector. In hot, arid climates the collector may not even be necessary. The southern side of the enclosure itself can be glazed to allow sunlight to dry the material. The collector can be as simple as a glazed box with a dark colored interior to absorb the solar energy that heats air. The air heated in the solar collector moves, either by natural convection or forced by a fan, up through the material being dried. The size of the collector and rate of airflow depends on the amount of material being dried, the moisture content of the material, the humidity in the air, and the average amount of solar radiation available during the drying season.

There is a relatively small number of large solar crop dryers in the United States. This is because the cost of the solar collector can be high, and drying rates are not as controllable as they are with natural gas or propane powered dryers. Using the collector at other times of the year, such as for heating farm buildings, may make a solar dryer more cost-effective. It is possible to make small, very low cost dryers out of simple materials. These systems can be useful for drying vegetables and fruit for home use.

Space and Water Heating

Livestock and dairy operations often have substantial air and water heating requirements. Modern pig and poultry farms raise animals in enclosed buildings, where it is necessary to carefully control temperature and air quality to maximize the health and growth of the animals. These facilities need to replace the indoor air regularly to remove moisture, toxic gases, odors, and dust. Heating this air, when necessary, requires large amounts of energy. With proper planning and design, solar air/space heaters can be incorporated into farm buildings to preheat incoming fresh air. These systems can also induce or increase natural ventilation levels during summer months. Solar water heating systems can provide low to medium temperature hot water for pen cleaning.

Commercial dairy farms use large amounts of energy to heat water to clean equipment, as well as to warm and stimulate cows’ udders. Heating water and cooling milk can account for up to 40% of the energy used on a dairy farm. Solar water heating systems may be used to supply all or part of these hot water requirements.

Greenhouse Heating

Another agricultural application of solar energy is greenhouse heating. Commercial greenhouses typically rely on the sun to supply their lighting needs, but are not designed to use the sun for heating. They rely on gas or oil heaters to maintain the temperatures necessary to grow plants in the colder months. Solar greenhouses, however, are designed to utilize solar energy for both heating and lighting. A solar greenhouse has thermal mass to collect and store solar heat energy, and insulation to retain this heat for use during the night and on cloudy days. A solar greenhouse is oriented to maximize southern glazing exposure. Its northern side has little or no glazing, and is well insulated. To reduce heat loss, the glazing itself is also more efficient than single-pane glass, and various products are available ranging from double pane to "cellular" glazing. A solar greenhouse reduces the need for fossil fuels for heating. A gas or oil heater may serve as a back-up heater, or to increase carbon dioxide levels to induce higher plant growth.

Remote Electricity Supply

Solar electric, or photovoltaic (PV), systems convert sunlight directly to electricity. They can power an electrical appliance directly, or store solar energy in a battery. A "remote" location can be several miles or as little as 50 feet (15 meters) from a power source. PV systems may be much cheaper than installing power lines and step down transformers in applications such as electrical fencing, lighting, and water pumping.

Water Pumping

Photovoltaic (PV) water pumping systems may be the most cost-effective water pumping option in locations where there is no existing power line. When properly sized and installed, PV water pumps are very reliable and require little maintenance. The size and cost of a PV water pumping system depends on the local solar resource, the pumping depth, water demand, and system purchase and installation costs. Although today’s prices for PV panels make most crop irrigation systems too expensive, PV systems are very cost effective for remote livestock water supply, pond aeration, and small irrigation systems.

Agricultural technology is changing rapidly. Farm machinery, farm buildings, and production facilities are constantly being improved. You should consider these factors when purchasing and installing a solar system. Payback periods may be shortened by the multiple use of a solar system, such as for space heating and crop drying.

Architecture applications

Architecture is one field that can use solar energy to its advantage. Architects are charged with designing plans for residential and commercial buildings. Part of designing buildings is integrated energy systems into the plans. These systems can include heating systems, cooling systems, and ventilation systems. Architects also need to account for the equipment needed to produce solar energy when they are designing building plans. Solar cells, solar panels, and other equipment needed to convert the sun’s energy into solar energy needed to be integrated into the overall design of the home to ensure that they can function properly.

Solar Heating for Schools

Solar power systems are very economical for one-story and two-story school buildings that:

Have a high heating energy cost and/or

Need a new roof

American Solar can provide a school with a new solar roof, which the school can expense as an operating cost as opposed to a capital expense. We provide the roof for free and bill annually for the cost of the heat delivered and to re-coop the financing and installation charges. Our offerings bring great value to school systems that:

Face diminished budgets due to lower revenue from real estate taxes as property values decrease

Confront increased operating expenses as sources of heat increase in cost

Value indoor air quality

Need a significant amount of the heat during day-time hours

Need several new roofs throughout a school system, which can lower acquisition costs

We can provide the solar roof at no up- front cost to the school system. A Solar Energy Investment Trust (SEIT) will own the roof. The SEIT will invoice the school annually for the heat delivered by the roof and the amortized installation costs. The SEIT will take the tax benefits, which the school cannot. The transfer of the tax benefit makes the free roof delivery possible.

Services for Federal, State and Local Government

American Solar simplifies solar for Federal, state, and local government facilities by providing comprehensive solar energy services. We handle all aspects of the solar project, from audits, through design and construction.

Our management team has experience with a broad range of contract vehicles. We have significant experience in delivering solar heating projects under energy savings performance contracts (ESPC's) and Utility Energy Service Contracts (UESC's). We have teamed with several Energy Service Companies (ESCO's) and utilities to provide cost-effective projects with excellent paybacks on very large contracts.

Our Solar re roofing system also competes well for DOD Energy Conservation Investment program (ECIP) funding. Recent projects have shown life cycle savings to investment ratios of 2 to 4 and simple payback periods of 4-8 years.

Our senior staff has years of experience in design, construction, and construction management of solar heating projects for government facilities. We have installed solar heating and heat recovery systems for: offices, laboratories, boiler rooms, animal buildings, warehouse and maintenance depots, postal facilities, modular buildings, housing, aircraft hangars, agricultural buildings, fire stations, and industrial equipment heating.

American Solar’s government clients include:

National Institutes of Health

US Army

US Air Force

US Department of Defense HQ (the Pentagon)

US Food and Drug Administration

US Department of Agriculture

US Geological Survey

State of Virginia

Dulles International Airport

Projects for Commercial Buildings

Industrial use is another one of the many applications of solar energy. Most industries operate in environments where traditional power supplies are readily available. However, there are some industries that have operations in areas where there is no access to a power grid or energy is not readily available in some form. Solar energy is very beneficial in this instance because it can be generated in remote areas at very little cost. Solar energy is often used to power stations used for telephones, microwaves, and radio. Transportation is another industry that uses the applications of solar energy. Solar energy can be used to power lighthouse signals, traffic signals, and the warning lights found on aircraft.

Since 2001, American Solar has developed, designed, and installed some of the most advanced solar heating systems in the United States. Our advanced designs use the conventional walls and roofs of buildings as solar air heating surfaces.

In many cases, our projects use the existing metal wall and roof systems as part of the building's new solar heating system. Our systems capture solar heat from the building's walls and roofs and deliver the heated air for space heating, water heating, equipment heating, ... even solar air conditioning ...saving energy and money for the customers.

Federal Tax Incentives Enhance the Economics of American Solar's Commercial Projects:

Current Federal law provides a 30% tax deduction on the installed cost of commercially owned solar energy systems.

Commercial systems can also take advantage of a 5-year accelerated depreciation deductions, instead of the 39 years required for a typical building.

86% of that depreciation deduction occurs in the first three years, compared to 7% with a typical nonsolar wall or roof.

For a company with a typical tax rate, that means a 48% reduction in installed costs after taxes, in 3 years, and 51% in 6 years.

Our projects deliver continuous low-cost heat

An American Solar project delivers heat energy at a value of up to $4/per square ft/per year depending on the customer’s current heating energy costs and use.

As conventional energy costs rise, the solar energy delivered from walls and roofs becomes more valuable. Building integrated walls and roofs deliver lower cost heat than conventional fossil fuels or other solar technologies. Energy cost savings combined with tax savings from solar walls and roofs gives customers very high returns. These savings are especially great compared to separately purchasing conventional wall and roof systems and decades of heating fuel purchases.

Our commercial and industrial clients have included:

Archstone Smith

Avon

McGraw-Hill

Novartis

Residential Areas

Recent years have seen rapid growth in the number of installations of PV on to buildings that are connected to the electricity grid. This area of demand has been stimulated in part by government subsidy programmes (especially Japan and Germany) and by green pricing policies of utilities or electricity service providers (e.g. in Switzerland and the USA). The central driving force though comes from the desire of individuals or companies to obtain their electricity from a clean, non-polluting, renewable source for which they are prepared to pay a small premium.

In these grid-connected systems, a PV System supplies electricity to the building and any day-time excess may be exported to the grid. Batteries are not required because the grid supplies any extra demand. However, if you want to be independent of the grid supply you will need battery storage to provide power outside daylight hours.

Solar PV modules can be retrofitted on to a pitched roof above the existing roof-tiles, or the tiles replaced by specially designed PV roof-tiles or roof-tiling systems. If you are planning to put a PV system on to a building and have it connected to the grid supply there are likely to be local regulations that need to be met, and permission required from your utility or electricity service provider. The level of credit for any exported electricity will vary depending on local schemes in place.

Rural vacation homes

Most people don’t have any problem getting electricity and other forms of energy into their primary homes. However, people who own vacation homes in rural areas may have a difficult time accessing energy. This is another one of the applications of solar energy. Rural vacation homes such as cottages and cabins may not have access to traditional power grids. If this is the case, these vacation homes can be designed to use solar energy so that vacationers can enjoy all the creature comforts of a hidden getaway. This can make it easier to keep food and living areas sanitary, wash hands to reduce the spread of germs, and heat and cool vacation homes.

Water heating and pumping

Heating water using electricity can be an expensive undertaking. Even the most energy-efficient gas and electricity-powered water heaters can cost homeowners a lot of money. As these traditional water heaters age, their efficiency decreases and homeowners see even more costs adding up. Using applications of solar energy can help people to reduce their water heating and pumping costs. In a solar water system, solar thermal collectors are used to heat water and then solar energy propels the water through the system. This is much more efficient and has fewer harmful effects than using gas or electricity-powered hot water systems.

The applications of solar energy can be used in almost all industries as well as in residential settings. Solar energy can reduce harmful pollutants in the air and water, decrease the monthly costs of energy for residential consumers, and reduce the number of health issues experienced by people with allergies and other medical conditions. More resources need to be put into researching and funding the applications of solar energy so that everyone can benefit from this clean, renewable energy source.

Cooking

Solar cookers use sunlight for cooking, drying and pasteurization. They can be grouped into three broad categories: box cookers, panel cookers and reflector cookers.The simplest solar cooker is the box cooker first built by Horace de Saussure in 1767.A basic box cooker consists of an insulated container with a transparent lid. It can be used effectively with partially overcast skies and will typically reach temperatures of 90–150 °C.Panel cookers use a reflective panel to direct sunlight onto an insulated container and reach temperatures comparable to box cookers. Reflector cookers use various concentrating geometries (dish, trough, Fresnel mirrors) to focus light on a cooking container. These cookers reach temperatures of 315 °C and above but require direct light to function properly and must be repositioned to track the Sun.

The solar bowl is a concentrating technology employed by the Solar Kitchen in Auroville, Pondicherry, India, where a stationary spherical reflector focuses light along a line perpendicular to the sphere's interior surface, and a computer control system moves the receiver to intersect this line. Steam is produced in the receiver at temperatures reaching 150 °C and then used for process heat in the kitchen.

A reflector developed by Wolfgang Scheffler in 1986 is used in many solar kitchens. Scheffler reflectors are flexible parabolic dishes that combine aspects of trough and power tower concentrators. Polar tracking is used to follow the Sun's daily course and the curvature of the reflector is adjusted for seasonal variations in the incident angle of sunlight. These reflectors can reach temperatures of 450–650 °C and have a fixed focal point, which simplifies cooking.The world's largest Scheffler reflector system in Abu Road, Rajasthan, India is capable of cooking up to 35,000 meals a day.As of 2008, over 2,000 large Scheffler cookers had been built worldwide.

Solar vehicles

Development of a solar powered car has been an engineering goal since the 1980s. The World Solar Challenge is a biannual solar-powered car race, where teams from universities and enterprises compete over 3,021 kilometres (1,877 mi) across central Australia from Darwin to Adelaide. In 1987, when it was founded, the winner's average speed was 67 kilometres per hour (42 mph) and by 2007 the winner's average speed had improved to 90.87 kilometres per hour (56.46 mph).The North American Solar Challenge and the planned South African Solar Challenge are comparable competitions that reflect an international interest in the engineering and development of solar powered vehicles.

Some vehicles use solar panels for auxiliary power, such as for air conditioning, to keep the interior cool, thus reducing fuel consumption.

In 1975, the first practical solar boat was constructed in England.By 1995, passenger boats incorporating PV panels began appearing and are now used extensively.In 1996, Kenichi Horie made the first solar powered crossing of the Pacific Ocean, and the sun21 catamaran made the first solar powered crossing of the Atlantic Ocean in the winter of 2006–2007.There are plans to circumnavigate the globe in 2010.

Holiday Properties

Photovoltaic Systems are frequently the preferred solution for holiday homes, which have no access to the electricity grid. These solar systems are configured to meet power needs less expensively than extending the grid to reach your location. Remote homes in sunny locations can obtain reliable electricity to meet basic needs for lighting (using energy efficient fluorescent lamps) and radio or television with a simple PV system comprising a PV panel, a rechargeable battery to store the energy captured during daylight hours, a regulator (or charge controller) and the necessary wiring and switches. Such systems are often called "solar home systems" or SHS for short.

The size of the PV module and battery is designed to provide enough power and storage to meet peak energy requirements - this is called "sizing" the system. . All systems with battery storage should use solar batteries of the right design and specification for the system, car batteries will not do! Also, high quality compact fluorescent lamps are available with good lifetimes; poor quality lamps will blacken quickly and their light output will drop off. The quality of the regulator is a key factor in the reliability of the overall system. The PV modules are usually mounted on a pole, or on the roof.

In this application, 50 to 100W modules would probably be required although smaller panels of 10 to 15W might provide sufficient power for the most basic single lamp system.

[edit] Market of solar energy

Even in a worldwide recession, the global market for photovoltaic solar energy surged in 2009, adding 6.4 gigawatts of capacity, according to recently figures released by the European Photovoltaic Industry Association.

Solar: Rapidly Growing Markets

Solar technologies are likely to follow similar market penetration commercialization paths to that of other high technology products such as the fax machine, the cellular telephone and the personal computer.

There are now more than 10,000 homes in the United States that are entirely powered by solar energy.

Over 60% of solar technology sales are exports. Sales are primarily to developing countries, which comprise over one-third of the earth's population.

Photovoltaic (PV) Technology

Solar energy is harnessed in many ways. Photovoltaics (PV) converts sunlight directly to electricity and are used to power everything from roadside phones to suburban homes. Solar energy is used to heat pools and to produce hot water, saving consumers millions of dollars annually. Large scale solar thermal power plants generate enough clean electricity to power thousands of homes day and night.

Worldwide sales of photovoltaic products totaled over 90 megawatts in 1996, up 14% from 1995. US firms have a 43% share of this $1 billion global market.

The annual US Market for PV on buildings could top $2.5 billion in the next 10 years with Japanese and European markets having similar potential.

The potential U.S. market for photovoltaics is nearly 9,000 megawatts. At a low estimate of $3 per watt, this translates into a $27 billion market in the US alone.

The fastest growing markets for photovoltaic systems include the utility, rural electrification, telecommunications, and military sectors. These market sectors are expected to require more than 1,200 megawatts of PV by the year 2000.

Solar Thermal Energy

The U.S. spends more than $13 billion-a-year on energy for home water heating. That is the equivalent of 11.4 barrels of oil per household, more than the amount of oil (in the form of gasoline) burned by a medium-sized automobile driven 12,000 miles.

There are more than one million solar hot water heating installations in the U.S.

More than 300,000 solar pool heating systems have been installed in the United States, providing efficient heating to residential and commercial swimming pools around the country.

Over 700 megawatts of solar thermal electric systems should be deployed by the year 2003 in the U.S. and internationally. The market for these systems should exceed 5,000 megawatts by 2010, enough to serve the residential needs of 7 million people (or a state larger than Georgia) which will save the energy equivalent of 46 million barrels of oil per year.

[edit] Influence for economics

Beginning with the surge in coal use which accompanied the Industrial Revolution, energy consumption has steadily transitioned from wood and biomass to fossil fuels. The early development of solar technologies starting in the 1860s was driven by an expectation that coal would soon become scarce. However development of solar technologies stagnated in the early 20th century in the face of the increasing availability, economy, and utility of coal and petroleum.

The 1973 oil embargo and 1979 energy crisis caused a reorganization of energy policies around the world and brought renewed attention to developing solar technologies.Deployment strategies focused on incentive programs such as the Federal Photovoltaic Utilization Program in the US and the Sunshine Program in Japan. Other efforts included the formation of research facilities in the US (SERI, now NREL), Japan (NEDO), and Germany (Fraunhofer Institute for Solar Energy Systems ISE).

Commercial solar water heaters began appearing in the United States in the 1890s.These systems saw increasing use until the 1920s but were gradually replaced by cheaper and more reliable heating fuels.As with photovoltaics, solar water heating attracted renewed attention as a result of the oil crises in the 1970s but interest subsided in the 1980s due to falling petroleum prices. Development in the solar water heating sector progressed steadily throughout the 1990s and growth rates have averaged 20% per year since 1999.Although generally underestimated, solar water heating is by far the most widely deployed solar technology with an estimated capacity of 154 GW as of 2007.