An Introduction to Solar Thermal by hilen

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									An Introduction to Solar Thermal Electric Power

image courtesy of John Perlin/Ken Butti , Solar Photo Archives

Ausra, Inc. 2585 East Bayshore Rd. Palo Alto, CA 94303 phone: 650.424.9300 fax: 650.494.3893 url: www.ausra.com

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Ausra, Inc. builds power plants which generate electricity by driving steam turbines with sunshine. Solar concentrators boil water with focused sunlight, generating high-pressure steam which drives conventional turbine generators. Thermal energy storage systems can allow solar electric power to be generated on demand, day and night. This document provides an introduction and overview of the technology behind solar thermal power. The sun is humanity’s oldest energy source, and scientists and engineers have long sought to harness the power of sunlight for a wide range of heating, lighting, and industrial tasks. Every child knows that focused sunlight is hot enough to set things afire; engineers and scientists know that every square meter of the Earth receives 1 kilowatt of thermal energy when the sun is overhead. Gathering and converting this energy into usable form has been explored since burning mirrors were first used in China at about 700 BC for ignition of firewood.

Leonardo da Vinci’s notebooks contain designs for solar concentrators, and intensive experimentation took place during the 18th and 19th centuries towards building practical engines powered by the sun.

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A 55-kilowatt solar powered pumping station was brought online in Egypt in 1913.

image courtesy of John Perlin/Ken Butti , Solar Photo Archives

All solar thermal systems capture the energy of the sun by absorbing light as heat. Solar thermal power systems focus sunlight, usually with mirrors, to heat a fluid to high temperatures and drive an engine. This approach stands in contrast to photovoltaic solar power systems, in which light interacts with special materials directly to separate charges and generate electricity. Photovoltaic power enjoys many advantages, such as unattended operation and small-scale feasibility, but remains significantly more expensive as a source of large-scale power than solar thermal technologies. The modern era of large scale solar power generation was born in California’s Mojave Desert in the 1980s, when Luz Industries built a total of 354 MW of Solar Electric Generating System, or SEGS, power plants. The SEGS plants use long parabolic mirrors with pipes at the focus point, where circulating oil is heated to 700 F (350 C). The oil is pumped through heat exchangers which boil water to make high-pressure steam, which drives turbine generators to make electric power. For many years the SEGS plants produced the majority of the world’s solar electric power.

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Subsequent research in solar power generation has reduced the cost of parabolic trough power systems, and has developed several optical systems other than the parabolic trough. Recently, Abengoa has placed in service the PS-10 power tower system, which employs a field of reflectors which move in two dimensions to focus light at the top of a tower, where a boiler is located which absorbs the light and generates steam. Such systems have been in development since the 1960’s.

A large pressure tank, or “steam accumulator”, stores energy as pressurized hot water and allows the plant to continue generation in cloudy conditions for up to an hour.

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This ability to store energy as heat makes solar thermal electric power particularly valuable, because energy can be stored when the sun is shining and released for electricity generation when the power is needed most. Often peak electricity demand extends well into the evening on hot summer days; solar thermal electric power is uniquely able to deliver zero-carbon electric power to meet these demands. Ausra’s CLFR technology builds on the experience with troughs and towers. Ausra’s core technology, the Compact Linear Fresnel Reflector (CLFR) solar collector and steam generation system, was originally conceived in the early 1990s by Ausra’s founders in Australia. The CLFR system retains a key advantage of troughs – fewer foundations and positioning motors per square meter of mirror – and a key advantage of the PS-10 tower system – direct steam generation and energy storage. Compared to trough systems, the CLFR system reduces costs by replacing special heat-curved reflectors with standard flat glass, and keeps all mirrors close to the ground, lowering wind loads and steel usage. Ausra's solar thermal power plants gather the sun’s energy as heat, and Ausra is developing low cost storage systems which can store enough heat to run the power plant for many hours. Going beyond the PS10 1-hour steam accumulator concept, Ausra's plants will gather enough energy during daylight hours to generate power as needed for up to 20 hours. By storing energy as heat during the day, a power plant can continue to produce electricity during dark or cloudy periods. This flexibility makes Ausra's power plants an important potential contributor to peak, shoulder and base electricity loads.

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A CLFR collector gathers solar energy by reflecting and concentrating sunlight to roughly 30 times the intensity of sunshine at Earth’s surface. Mirrors focus on an elevated absorber in which water is heated and boiled by the focused sunlight. Ausra’s CLFR design keeps all the mirror glass low, out of high winds and within easy range for maintenance and cleaning. Innovative space-frame semi-monocoque construction keeps Ausra’s reflector units light and low cost. The mirror glass itself contributes to the structure, further reducing the total weight and total cost of steel. Computer systems manage the mirror positions, tracking the motion of the sun throughout the day to maintain the focus point on the absorber. At night and during stormy weather, the reflector units invert, exposing steel to the sky for maximal resistance to weather events such as ice, hail and high winds. A CLFR Power Plant An Ausra CLFR solar thermal power consists of multiple solar collector lines which feed saturated steam to thermal storage and then to the turbine block. Ausra’s solar power plants use a simple Rankine cycle system for power generation from the steam collected by the solar field. Pipes in the absorber carry water which boils and can reach over 545 degrees F (285 C) at about 70 times atmospheric pressure. This highpressure steam drives a steam turbine generator, then is recondensed to water and used over and over. This power system is common to conventional types of power plants; what is different is that sunlight, not burning fuel or splitting atoms, produces the heat to boil the water. Ausra’s power plant designs use very similar steam conditions as used in the PS-10 station and in typical nuclear power plants. This operating point provides cost savings and efficiencies in the solar collectors and thermal energy storage systems.

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Ausra’s CLFR technology is now moving from the prototype stage into commercial deployment. Ausra has a CLFR project in commissioning and testing which feeds solargenerated steam into a coal-fired power station in New South Wales, Australia. The company shortly expects to announce significant projects in North America and Europe.

Solar Resource and Land Usage Solar thermal electric power plants need direct sunshine to make power. Suitable locations exist in most of the United States. The best locations are those with highest daily sunshine; outstanding sites exist through the Southwest. Electric power generation in the US today emits approximately 3 billion tons of carbon dioxide (CO2) every year. Solar thermal power plants can augment and replace our fossil fired power plants and lead to a “zero carbon” grid within a few decades. Using Ausra’s CLFR technology, power plants occupying a total area of land 92 miles on a side could provide all US electric power – the entire US grid – day and night. This amount of land is readily available without significant impact; it corresponds to less than 10% of the Federal land in the state of Nevada.

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Similar outstanding solar resources are available in southern Europe, north Africa, and many other regions of the world. With current technology, under 3% the land area of Morocco could power the entire European grid. Europe’s electric power system currently emits roughly 2 billion tons of CO2 annually; as solar thermal electric power plants are phased in these emissions can be phased out.

China’s explosive growth in electric power generation, now dominated by the construction of coal-fired power plants which can accelerate and deepen the climate crisis, can be redirected to CSP technologies with very positive environmental and economic impacts. Like the USA, China easily has the desert solar resource to power its rapidly expanding economy. India also has a similar capability.

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Solar Thermal Electric Power Economics, Emissions, and Employment Impacts The U.S. need for electric power generation is growing rapidly. The Energy Information Administration projects ongoing growth in annual electrical energy use through 2025.

This growth is occurring at a moment in time when scientists have recognized that preserving a climate on Earth similar to today’s requires dramatic reductions in total emissions of greenhouse gases by mid-century. Our current electric power system emits over 40% of total human-caused emissions, and is the fastest growing source of emissions.

The business-as-usual scenario for power generation results in total atmospheric concentrations of CO2 far beyond the “point of no return” as described by James Hansen,
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NASA’s chief climate scientist, who refers to our current situation as one of “imminent peril” and calls for 80% reductions in total emissions by 2050. It is obvious that technologies which can deliver reliable power day and night are urgently needed to augment and supplant our current reliance on fossil-fired electric power generation. It would be preferable if these could be implemented without harming the overall economy or significantly increasing the cost of power. Current renewable, zero-carbon sources of electric power include geothermal, photovoltaic solar, and wind generation. Of these, geothermal has limited total resource – the known locations for geothermal generation can provide only a fraction of American power needs. Wind and PV solar systems have large resources, but only operate roughly 20 to 30% of the time during sunny or windy hours, and the hours of generation are not controlled by utilities – the sources are not “dispatchable”, but generate on an asavailable basis. Solar thermal electric power is uniquely suited to meet America’s and the larger world’s electric power needs. Solar thermal electric output peaks when the grid need for power peaks, and the potential/projected low cost of thermal energy storage means that solar plants can be built to be “dispatchable”, gathering energy during daylight hours and releasing it during times of peak demand. The figure below, taken from [Mills and Morgan 2007], shows the seasonal correlation of a solar thermal electric power plant and California’s grid demands.

The three SM lines in the graph above indicate “Solar Multiple” figures, the ratio of the size of the solar field to the peak input of the turbine; larger solar multiples provide more hours of generation per year. The SM3 case is very close to the needs of the California electricity grid (dotted line). What about the rest of the nation? An additional calculation suggests that CLFR plants in states like California or Texas could supply over 90% of the annual national US grid electricity load through nationwide HVDC lines. Other renewable sources such as hydroelectricity and wind could supply the remainder.

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Using reasonable financial assumptions, the table above shows that Ausra’s current CLFR solar array technology combined with 700MWe turbines and 16 hours of thermal energy storage could deliver energy at costs that are directly competitive with new gas- fired generation facilities and much cheaper than coal-fired plants with sequestration. With still larger turbines and lower cost finance (available after sufficient demonstration), Ausra believes that its technology will be competitive against new pulverized coal plant without sequestration. Solar thermal electric plants are permanently hedged against fuel cost and emissions cost variations, and can provide a stable, safe, long-lasting energy supply without emissions. Several studies have examined the local economic impact of solar thermal electric power projects. The construction and maintenance of large solar collector fields, which contribute to the tax base, replaces untaxed fuel payments. Black & Veatch’s 2006 study on solar thermal power in California found that solar thermal electric power projects create roughly twice the construction jobs and twice the permanent jobs of fossil-fired power stations, and more than four times the in-state retained earnings. Similar findings have been reported in studies of solar thermal electric power technology for New Mexico and Nevada in the past few years. Solar thermal electric power projects in several locations have created alliances among environmental, labor, government, and industrial groups which all share benefits from such projects. Solar thermal electric power with CLFR collectors provides a practical, scalable solution to one of the greatest challenges of our times: moving quickly to a low-carbon industrial infrastructure. Solar power harnesses the sun’s thermonuclear fusion without nuclear materials on Earth. It can provide reliable, night and day electric power at market prices without carbon emissions, and has availability that closely matches human energy requirements by hour and by season. It uses less land than coal mining and transport. It is quick to implement. It is available widely around the planet, not just in a few countries. It has enormous primary energy resource which is inexhaustible over time. At Ausra, we’re ready to deliver.

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