Solar energy

The total solar energy available to the earth is approximately 3850 zettajoules (ZJ) per year.

Solar power (also known as solar energy) is a source of power that uses energy from the sun. The term solar energy is used more specifically to describe the utilization of this energy through human endeavor. It is a renewable energy source that has been used in many traditional technologies for centuries. It is also in widespread use where other power supplies are absent, such as in remote locations and in space. The primary forms of solar energy are heat and light. Secondary forms and effects include photosynthesis, wind, the smell of a camals ass ]], the hydrologic cycle, fossil fuels and electricity.

Solar energy has a long history going back to prehistoric times. Primitive architects incorporated windows to provide light. The Greeks, Native Americans and Chinese warmed their buildings by orienting them toward the sun. Medieval European farmers used thermal mass and elaborate field orientation to increase crop yields.

In 1865, the French engineer Auguste Mouchout successfully powered a steam engine with sunlight. This is the first known example of a solar powered mechanical device. Over the next 50 years inventors such as John Ericsson, Charles Tellier, Henry E. Willsie, Aubrey Eneas and Frank Shuman developed solar powered mechanical devices which were used for irrigation, refrigeration and electrical generation.

In 1954, researchers at Bell Laboratories developed a solar cell capable of converting light into electricity via the photovoltaic effect. This breakthrough marked a fundamental change in how power is generated. Since then solar cells have progressed from early cells priced at $1500 per watt to modern cells which cost less than $3 per watt.

The utilization of solar energy spans from traditional technologies that provide food, heat and light to electricity which is uniquely modern. The diversity of form and long history of solar energy are manifest in a wide variety of applications. These include:

• Heat (hot water, building heat, cooking, process heat)
• Lighting (daylighting, hybrid lighting, daylight savings time)
• Electricity generation (photovoltaics, heat engines)
• Transportation (solar car, solar plane, solar boat)
• Desalination
• Photosynthesis by plants (wood, biofuel)

Energy from the Sun

Solar energy as it is dispersed on the planet and radiated back to space. Values are in PW =10¹⁵ W

Theoretical annual mean insolation, at the top of Earth's atmosphere (top) and at the surface on a horizontal square meter.

Map of global solar energy resources. The colours show the average available solar energy on the surface (as measured from 1991 to 1993). For comparison, the dark disks represent the land area required to supply the total primary energy demand using PVs with a conversion efficiency of 8%.

On a global scale solar radiation reaches the Earth's upper atmosphere at a rate of 174 PW. While traveling through the atmosphere, 6% of the incoming solar radiation (insolation) is reflected and 16% is absorbed. Average atmospheric conditions (clouds, dust, pollutants) further reduce insolation by 20% through reflection and 3% through absorption. The absorption of solar energy in the atmosphere is not a loss of available energy. This energy produces our climate through its capture within derivative effects such as the hydrologic cycle, wind and ocean currents.

Atmospheric conditions not only reduce the quantity of insolation reaching the Earth's surface but also affect the quality of insolation by diffusing incoming light and altering its spectrum. After passing through the Earth's atmosphere approximately half the insolation is in the visible electromagnetic spectrum with the other half mostly in the infrared and ultraviolet spectrum.

The flows and stores of solar energy are very large relative to human needs.

• The total solar energy available to the earth is approximately 3850 zettajoules (ZJ) per year.
• Oceans absorb approximately 285 ZJ of solar energy per year.
• Winds can theoretically supply 6 ZJ of energy per year.
• Biomass captures approximately 1.8 ZJ of solar energy per year.
• Worldwide energy consumption was .471 ZJ in 2004.

The first map shows the average global irradiance calculated from satellite data collected from 1991 to 1993. For example, in North America the average insolation at ground level over an entire year (including nights and periods of cloudy weather) lies between 125 and 375 W/m² (3 to 9 kWh/m²/day). This represents the available power, and not the delivered power. At present, photovoltaic panels typically convert about 15% of incident sunlight into electricity; therefore, a solar panel in the contiguous United States on average delivers 19 to 56 W/m² or 0.45 - 1.35 kWh/m²/day.

The dark disks in the second map on the right are an example of the land areas that, if covered with 8% efficient solar panels, would produce slightly more energy in the form of electricity than the total world primary energy supply in 2003. While average insolation and power offer insight into solar power's potential on a regional scale, locally relevant conditions are of primary importance to the potential of a specific site.

Types of technologies

Many types of technology have been developed to make use of solar radiation. Some classifications of solar technology are active, passive, direct and indirect.

• Active solar systems use electrical and mechanical components such as tracking mechanisms, pumps and fans to process sunlight into usable outputs such as heating, lighting or electricity.
• Passive solar systems use non-mechanical techniques of capturing, converting and distributing sunlight into usable outputs such as heating, lighting or ventilation. These techniques include selecting materials with favorable thermal properties, designing spaces that naturally circulate air and referencing the position of a building to the sun.
• Direct solar generally refers to technologies or effects that involve a single conversion of sunlight which results in a usable form of energy.
• Indirect solar generally refers to technologies or effects that involves multiple transformations of sunlight which result in a usable form of energy.

Architecture

The Zion National Park Visitor's Center incorporates several aspects of solar design.

Main article: Passive solar building design

Solar architecture is designed to use the sun as much as possible for temperature control, lighting and ventilation while minimizing negative effects such as overheating and glare. The basic elements of solar architecture are building orientation, proportion, thermal mass and window placement.

• A solar building's axis should run lengthwise east to west and the structure should be twice as long as wide.
• In the northern hemisphere, north facing windows should be minimized and south facing windows should be equal to 5-7% of the building's floor space. In the southern hemisphere, the south facing windows should be minimized and north facing windows should also be equal to 5-7% of the building's floor space.
• The thermal mass in the building should be sized to smooth out temperature swings.
• Spaces can be designed to naturally circulate air. Cooling elements such as a solar chimney can be incorporated to help with ventillation.

Lighting

Main articles: Daylighting and Light tube

Daylighting is a passive solar method of using natural light to provide illumination. Daylighting directly offsets energy use in electric lighting systems and indirectly offsets energy use through a reduction in cooling load. Although difficult to quantify, the use of natural light also offers physiological and psychological benefits compared to conventional lighting.

Daylighting features include building orientation, window orientation, exterior shading, sawtooth roofs, clerestory windows, light shelves, skylights and light tubes. These features may be incorporated in existing structures but are most effective when integrated in a solar design package which accounts for factors such as glare, heat gain, heat loss and time-of-use. Architectural trends increasingly recognize daylighting as a cornerstone of sustainable design.

Hybrid solar lighting (HSL) is an active solar method of using natural light to provide illumination. Hybrid solar lighting systems collect sunlight using focusing mirrors that track the sun. The collected light is transmitted via optical fibers into a building's interior to supplement conventional lighting.

Daylight saving time (DST) utilizes solar energy by matching available sunlight to the time of the day in which it is most useful.

Water Heating

Solar water heaters, on a rooftop in Jerusalem, Israel

Main articles: Solar hot water, Solar combisystem, and Solar pond

Solar hot water systems use sunlight to heat water. Commercial solar water heaters began appearing in the United States in the 1890s. These systems saw increasing use until the 1920s but were thereafter gradually replaced by relatively cheap and more reliable conventional heating fuels. The economic advantage of conventional heating fuels has varied over time resulting in periodic interest in solar hot water; however, solar hot water technologies have yet to show the sustained momentum they lost in the 1920s. That being said, the recent price spikes and erratic availability of conventional fuels is renewing interest in solar heating technologies within the US.

As of 2005, the total installed capacity of solar hot water systems is 88 GWth and growth is 14% per year. China is the world leader in the deployment of solar hot water systems with 80% of the market. Israel is the per capita leader in the use of solar hot water with 90% of homes using this technology.In the United States heating swimming pools is the most successful application of solar hot water.

On a technical level, solar water heating is highly efficient (up to 87%) and is particularly appropriate for low temperature (75-150F) applications such as domestic hot water, heating swimming pools and space heating.

The basic components of a solar water heating systems are solar thermal collectors, a storage tank and a circulation loop. The three basic classifications of solar water heaters are:

• Batch systems which consist of a tank that is directly heated by sunlight. These are the oldest and simplest solar water heater designs, however; the exposed tank can be vulnerable to cooldown.
• Active systems which use pumps to circulate water or a heat transfer fluid.
• Passive systems which circulate water or a heat transfer fluid by natural circulation. These are also called thermosiphon systems.

Solar water heaters are also classified by the type of circulation loop used to transmit and deliver heat. These can be direct or indirect.

• Direct solar hot water systems use a single loop to heat and deliver hot water.
• Indirect solar hot water systems use a primary loop to capture heat, a heat exchanger and a secondary loop to deliver hot water.

A solar pond is a pool of salt water that collects and stores solar energy. A basic solar pond has three layers of water:

1. A top layer with a low salt content.
2. An intermediate insulating layer with a medium salt content.
3. A bottom layer with a high salt content.

The salt gradient produces a density gradient that dampens convection currents which would otherwise transfer heat from the bottom to the surface and then to the air above. This allows the temperature in the bottom layer to approach 90 degrees Celsius. The heat trapped in the bottom layer can be used for heating buildings, industrial process heat and generating electricity. Representatives of this technology are the solar pond in Bhuj, Gujarat, India and another at the University of Texas El Paso.

Heating, Cooling and Ventilation

Main articles: Solar heating, Thermal mass, Trombe wall, and Solar chimney

• A thermal mass is a body that absorbs and holds heat. In the context of solar energy, it is a mass designed to store heat during sunny periods and release heat when sunlight levels are reduced or unavailable. A properly sized thermal mass will smooth out temperature swings and help keep rooms at a comfortable temperature throughout the day and night.

• A Trombe wall is a passive solar heating and ventilation system consisting of an air channel sandwiched between a window and a sun-facing thermal mass. Sunlight heats the air space during the day causing natural circulation through vents at the top and bottom of the wall and storing heat in the thermal mass. During the evening the Trombe wall radiates stored heat.

• A transpired collector is a perforated sun-facing wall. The wall absorbs sunlight and pre-heats air up 40F as it is drawn into the building's ventilation system. These systems are highly efficient (up to 80%) and can pay for themselves within 3-12 years in offset heating costs.

• Solar cooling can be achieved via absorption refrigeration cycles, desiccant cycles and solar mechanical processes. In 1878 the solar pioneer Auguste Mouchout was the first to make ice using a solar steam engine attached to a refrigeration device.

• A solar chimney (or thermal chimney) is a passive solar ventilation system composed of a black-painted vertical chimney. The chimney is designed to absorb sunlight and heat the air in the chimney. This creates an updraft of air which enhances the natural stack ventilation through a building. These systems have been in use since Romans times and are common in the Middle east.

Photovoltaics

Photovoltaic (PV) cells produce electricity directly from sunlight

File:Bp-solarmodul.JPG

Photovoltaic (PV) modules are composed of multiple PV cells. Two or more interconnected PV modules create an array.

Main article: Photovoltaics

Solar cells, also referred to as photovoltaic cells, are devices or banks of devices that use the photovoltaic effect of semiconductors to generate electricity directly from sunlight. Until recently, their use has been limited because of high manufacturing costs. One cost effective use has been in very low-power devices such as calculators with LCDs. Another use has been in remote applications such as roadside emergency telephones, remote sensing, cathodic protection of pipe lines, and limited "off grid" home power applications. A third use has been in powering orbiting satellites and spacecraft.

To take advantage of the incoming electromagnetic radiation from the sun, solar panels can be attached to each house or building. The panels should be mounted perpendicular to the arc of the sun to maximize usefulness. The easiest way to use this electricity is by connecting the solar panels to a grid tie inverter. However, these solar panels may also be used to charge batteries or other energy storage device. Solar panels produce more power during summer months because they receive more sunlight.

Total peak power of installed PV is around 6,000 MW as of the end of 2006. Installed PV is projected to increase to over 9,000 MW in 2007. This is only one part of solar-generated electric power.

Declining manufacturing costs (dropping at 3 to 5% a year in recent years) are expanding the range of cost-effective uses. The average lowest retail cost of a large photovoltaic array declined from $7.50 to $4 per watt between 1990 and 2005. With many jurisdictions now giving tax and rebate incentives, solar electric power can now pay for itself in five to ten years in many places. "Grid-connected" systems - those systems that use an inverter to connect to the utility grid instead of relying on batteries - now make up the largest part of the market.

In 2003, worldwide production of solar cells increased by 32%. Between 2000 and 2004, the increase in worldwide solar energy capacity was an annualized 60%. 2005 was expected to see large growth again, but shortages of refined silicon have been hampering production worldwide since late 2004. Analysts have predicted similar supply problems for 2006 and 2007.

Solar Power Plants

Solar Two power tower surrounded by a field of heliostats.

Arrays of parabolic troughs

Parabolic dishes use Stirling engines for power conversion.

Main articles: Solar thermal energy, Parabolic trough, Solar power tower, Solar updraft tower, and Energy tower (downdraft)

Solar power plants use a variety of methods to collect sunlight and convert this energy into electricity, distill water or provide heat for industrial processes. Concentrating solar thermal power plants are the most common form of solar power plant.

Concentrating Solar Thermal (CST) systems use lenses or mirrors and tracking systems to focus a large area of sunlight into a small beam. CST technologies require direct insolation to perform properly. This requirement makes them inappropriate for significantly overcast locations.

The three basic CST technologies are the solar trough, solar power tower and parabolic dish. Each technology is capable of producing high temperatures and correspondingly high thermodynamic efficiencies but they vary in the way they track the sun and focus light.

• Line focus/Single-axis
  • A solar trough consists of a linear parabolic reflector which concentrates light on a receiver positioned along the reflector's focal line. These systems use single-axis tracking to follow the sun. A working fluid (oil, water) flows through the receiver and is heated up to 400 °C before transferring its heat to a distillation or power generation system. Trough systems are the most developed CST technology. The Solar Electric Generating System (SEGS) plants in California and Plataforma Solar de Almería's SSPS-DCS plant in Spain are representatives of this technology.
• Point focus/Dual-axis
  • A power tower consists of an array of flat reflectors (heliostats) which concentrate light on a central receiver located on a tower. These systems use dual-axis tracking to follow the sun. A working fluid (air, water, molten salt) flows through the receiver where it is heated up to 1000 °C before transferring its heat to a power generation or energy storage system. Power towers are less advanced than trough systems but they offer higher efficiency and energy storage capability. The Solar Two in Daggett, California and the Planta Solar 10 (PS10) in Sanlucar la Mayor, Spain are representatives of this technology.
  • A parabolic dish or dish/engine system consists of a stand-alone parabolic reflector which concentrates light on a receiver positioned at the reflector's focal point. These systems use dual-axis tracking to follow the sun. A working fluid (hydrogen, helium, air, water) flows through the receiver where it is heated up to 1500 °C before transferring its heat to a sterling engine for power generation. Parabolic dish systems display the highest solar-to-electric efficiency among CST technologies and their modular nature offers scalability. The Stirling Energy Systems (SES) and Science Applications International Corporation (SAIC) dishes at UNLV and the Big Dish in Canberra, Australia are representatives of this technology.

A solar updraft tower (also known as a solar chimney, but this term is avoided by many proponents due to its association with fossil fuels) is a relatively low-tech solar thermal power plant where air passes under a very large agricultural glass house (between 2 and 8 km in diameter), is heated by the sun and channeled upwards towards a convection tower. It then rises naturally and is used to drive turbines, which generate electricity.

An energy tower is an alternative proposal to the solar updraft tower. It is driven by spraying water at the top of the tower, evaporation of water causes a downdraft by cooling the air thereby increasing its density, driving wind turbines at the bottom of the tower. It requires a hot arid climate and large quantities of water (seawater may be used) but does not require the large glass house of the solar updraft tower.

Cooking

Solar Cookers use sunshine as a source of heat for cooking as an alternative to fire.

Main article: Solar cooker

A solar box cooker traps the sun's energy in an insulated box; such boxes have been successfully used for cooking, pasteurization and fruit canning. Solar cooking is helping many developing countries, both reducing the demands for local firewood and maintaining a cleaner breathing environment for the community.

The first known western solar oven is attributed to Horace de Saussure in 1767, which impressed Sir John Herschel enough to build one for cooking meals on his astronomical expedition to the Cape of Good Hope in Africa in 1830. Today, there are many different designs in use around the world.

Solar chemical

Solar chemical is any process that harnesses solar energy by absorbing sunlight and using it to drive an endothermic or photoelectrochemical chemical reaction. Prototypes, but no large-scale systems, have been constructed.

One approach has been to use conventional solar thermal collectors to drive chemical dissociation reactions. Ammonia can be separated into nitrogen and hydrogen at high temperature and with the aid of a catalyst, stored indefinitely, then recombined later to release the heat stored. A prototype system was constructed at the Australian National University.

Another approach is to use focused sunlight to provide the energy needed to split water via photoelectrolysis into its constituent hydrogen and oxygen in the presence of a metallic catalyst such as zinc.. Other research in this area has focused on semiconductors, and on the use of examined transition metal compounds, in particular titanium, niobium and tantalum oxides . Unfortunately, these materials exhibit very low efficiencies, because they require ultraviolet light to drive the photoelectrolysis of water. Current materials also require an electrical voltage bias for the hydrogen and oxygen gas to evolve from the surface, another disadvantage. Current research is focusing on the development of materials capable of the same water splitting reaction using lower energy visible light.

Solar thermal energy also has the potential to be used directly to drive chemical processes that require significant amounts of process heat, including at high temperatures that can be otherwise quite hard to attain.

Solar Vehicles

The solar powered car The Nuna 3 built by the Dutch Nuna team

Helios UAV in flight

Main articles: Helios Prototype, Electric boat, and Solar balloon

Development of a practical solar powered car has been an engineering goal for 20 years. The center of this development is the World Solar Challenge, a biannual solar powered car race over 3021 km (1877mi) through central Australia from Darwin to Adelaide. The race's stated objective is to promote research into solar-powered cars. Teams from universities and enterprises participate. In 1987 when it was founded, the winner's average speed was 67 km/h (42 mph). By the 2005 race this had increased to an average speed of greater than 100 km/h (62 mph), even though the cars were faced with the 110 km/h (68 mph) South Australia speed limit.

Helios, named after the Greek sun god of the same name, was a prototype solar powered unmanned aircraft. AeroVironment, Inc. developed the vehicle under NASA's Environmental Research Aircraft and Sensor Technology (ERAST) program.

On 13 August, 2001, it set an unofficial world record for sustained altitude by a winged aircraft. It sustained flight at above 96,000 feet (29,250 m) for forty minutes, and at one time it flew as high as 96,863 feet (29,524 m). Later, in June 2003, the prototype broke up and fell into the Pacific Ocean about ten miles (16 km) west of the Hawaiian Island Kauai.

Helios is a forerunner of what some call artificial "atmospheric satellites". NASA claimed such atmospheric satellites might someday replace conventional artificial satellites.

The first practical solar boat was probably constructed in 1975 in England (see Electrical Review Vol 201 No 7 12 August 1977). By 1995 passenger boats began appearing and are now used extensively.[8] Solar powered boats have advanced sufficiently to cross the Atlantic. The first crossing crossing of the Atlantic Ocean was achieved in the winter of 2006/2007 by the solar catamaran sun21.[9] With the present state of technology, many believe the time is right for the increased use of solar boats.

A solar balloon is a black balloon that is filled with air. As sunlight shines on the balloon the air inside is heated. As the air is heated it expands reducing the density of the air inside the balloon relative the the air surrounding the balloon. As such, the balloon functions like a hot air balloon. Usage at the moment is restricted to the toy market, although it has been proposed that it be used in the investigation of planet Mars. Some solar balloons are large enough for human flight.

Solar desalination

Main article: desalination

This technique uses solar energy to evaporate sea water. The humid air is then condensed and desalinated water is collected.

Advantages

US annual average solar energy received by a latitude tilt photovoltaic cell.

• The 89 petawatts of sunlight reaching the earth's surface is plentiful - almost 6,000 times more - compared to the 15 terawatts of average power consumed by humans. Additionally, solar electric generation has the highest power density (global mean of 170 W/m²) among renewable energies.
• Solar power is pollution free during use. Production end wastes and emissions are manageable using existing pollution controls. End-of-use recycling technologies are under development.
• Facilities can operate with little maintenance or intervention after initial setup.
• Solar electric generation is economically competitive where grid connection or fuel transport is difficult, costly or impossible. Examples include satellites, island communities, remote locations and ocean vessels.
• When grid-connected, solar electric generation can displace the highest cost electricity during times of peak demand (in most climatic regions), can reduce grid loading, and can eliminate the need for local battery power for use in times of darkness and high local demand; such application is encouraged by net metering. Time-of-use net metering can be highly favorable to small photovoltaic systems.
• Grid-connected solar electricity can be used locally thus reducing transmission/distribution losses (transmission losses were approximately 7.2% in 1995).
• Once the initial capital cost of building a solar power plant has been spent, operating costs are low compared to existing power technologies.

Disadvantages

• Solar electricity can currently be more expensive than electricity generated by other sources.
• Solar heat and electricity are not available at night and may be unavailable due to weather conditions; therefore, a storage or complementary power system is required for most applications.
• Limited power density: Average daily insolation in the contiguous U.S. is 3-7 kW·h/m² (see picture)
• Solar cells produce DC which must be converted to AC (using a grid tie inverter) when used in currently existing distribution grids. This incurs an energy loss of 4-12%.

Energy storage

Main article: Grid energy storage

For a stand-alone system, some means must be employed to store the collected energy for use during hours of darkness or cloud cover. The following list includes both mature and immature techniques:

A Solar powered garden lamp

• Using traditional batteries
• Thermal mass
• Pumped-storage hydroelectricity
• Flow batteries
• Molten salt
• Cryogenic liquid air or nitrogen
• Compressed air in cylinders and in caverns
• Flywheel energy storage
• Hydrogen produced by electrolysis
• Hydraulic accumulator
• Superconducting magnetic energy storages
• Vegetable oil economy

Storage always has an extra stage of energy conversion, with consequent energy losses, increasing the total capital costs. One way around this is to export excess power to the power grid, drawing it back when needed. This appears to use the power grid as a battery but in fact is relying on conventional energy production through the grid during the night. However, since the grid always has a positive outflow, the result is exactly the same.

Electric power costs are highly dependent on the consumption per time of day, since plants must be built for peak power (not average power). Expensive gas-fired "peaking generators" must be used when base capacity is insufficient. Fortunately for solar, solar capacity parallels energy demand; since much of the electricity is for removing heat produced by too much solar energy (using conditioners). This is less true in the winter when the peak energy use is in the early evening when food is being prepared and lighting, heating, and entertainment equipment loads are higher. Winter heating loads can be time shifted by storing thermal energy in bulk materials such as rock, water, or thermal phase transition materials such as glauber's salt or wax, provided solar illumination is sufficient. Wind power complements solar power since it can produce energy when there is no sunlight but this advantage is highly dependant upon local and seasonal wind availability.

Deployment of solar power

Main article: Deployment of solar power to energy grids

"The Stone Age did not end for a lack of stones, and the oil age will end not for a lack of oil." — Sheik Yamani, Saudi oil minister, 1973

"We stopped using stone because bronze and iron were superior materials, and likewise we will stop using oil when other energy technologies provide superior benefits." — Bjørn Lomborg, The Skeptical Environmentalist (New York: Cambridge University Press, 2001), p. 120

Deployment of solar power depends largely upon local conditions and requirements. All industrialised nations share a need for electricity and it is believed that solar power will increasingly be used as an option for electricity supply. The Very Large Scale Photovoltaic Power Generation (VLS-PV) proposal argues that "PV systems could generate many times the current primary global energy supply". To compensate for night time energy demands they would need to be complemented with pumped storage.

Solar power by country

See the articles for individual countries listed at Category:Solar power by country

See also

• 1973 energy crisis
• Absorption (electromagnetic radiation)
• COMES, French Solar Energy Authority
• Deployment of solar power to energy grids
• Energy crisis
• Energy development
• Energy storage
• Energy: world resources and consumption
• European Union Climate Change Programme
• Future energy development
• Green electricity
• Global dimming
• List of conservation topics
• Microgeneration
• Photoelectric effect
• Photovoltaics
• Renewable energy
• Solar air conditioning
• Solar balloon
• Solar car
• Solar cell
• Solar heating
• Solar gain
• Solar ponds
• Solar power plants in the Mojave Desert
• Solar power satellite
• Solar power tower
• Solar radiation
• Solar thermal energy
• Solar tracker
• Solar updraft tower
• Sustainable design
• Timeline of solar energy
• Trans-Mediterranean Renewable Energy Cooperation (TREC)

External links

• White Paper Discussing the use of Carbon Finance to Develop Solar Power.
• SCIENTIFIC CALCULATION OF SUN EXPOSURE.
• Solar Power-Renewable energy resource.

• American Solar Energy Society

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