There are two primary ways to convert sunlight into electrical energy: thermodynamic and photovoltaic methods. The thermodynamic approach utilizes the conventional approach of converting solar energy into electricity in thermal power stations, where the heat released by burning fuel is replaced with concentrated solar power.
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Long parabolic (in their cross-section) heliostats with tubes for heating the transfer medium |
Depending on their structural features, solar thermal power stations can be categorized into three main types:
- solar power towers with a central receiver steam generator concentrating the solar light reflected by heliostats;
- parabolic troughs, where along the focal lines of parabolic collectors there are tube receivers with a heat transfer fluid;
- dish systems, which are made up of a parabolic reflector and a receiver located at the reflector’s focal point.
Solar Power Towers
Solar power towers contain five basic elements: an optical system, a control system for heliostats and the station as a whole, a steam generator, a tower supporting a solar collector, and a system responsible for energy conversion. The latter is made up of heat exchangers, energy accumulators, and a power conversion unit.
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Solar power tower diagram |
Since such a power station uses direct sunlight, concentrating solar systems continuously track solar activity, with each heliostat operating independently of each other.
The temperature achieved atop a tower with the help of concentrating elements is 300 to 1500°С. A single module provides a capacity of up to 200 MW, which is explained by losses during energy transfer from the furthermost concentrators to the top of the tower.
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Solar thermal power plant “Solar Two” |
The global experience in utilizing solar towers has proved the feasibility and effectiveness of these projects. Some of the main disadvantages of such power stations are their high cost and the large area they occupy. For instance, a 100 MW solar power tower occupies an area of 200 hectares.
One of the first large-scale solar-thermal projects, the power station “Solar Two” was in operation from 1981 to 1999 in the Mojave Desert (California, USA); its output was over 10 MW. The power station’s solar tower was surrounded with 1926 heliostats, a total area of 83000 m 2. It is interesting to note that the sunlight heated not water, but a heat transfer medium made up of sodium and potassium nitrates. In its turn, the medium caused the water producing steam for the turbines to boil. In 1999 the station was rebuilt into a huge Cherenkov telescope with the purpose of studying the effect of gamma rays on the atmosphere.
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Solar power plant in Seville (PS1), Spain |
The light reflected from large mirrors is so bright that it illuminates the particles of dust and moisture in the air; due to this we can see the beams surrounding the stunning white tower.
A more recent solar tower project was launched on March 30th, 2007 in Sanlucar la mayor near Seville, Spain. A beautiful concrete tower 115 m high and 624 heliostats, an area of 120 m 2 each, supply steam to the power conversion unit with a capacity of 11 MW. This is enough to provide electricity to 5000 houses, saving 18000 tons of carbon emissions per year.
A similar station, but with a higher capacity of 20 MW, was later added to this solar power station. The larger tower is surrounded with 1255 heliostats; its launch reduced carbon emissions to the atmosphere by 54000 tons a year and gave electricity to 18000 houses. The total capacity of this solar power plant is 300 MW, which is enough to meet the electricity demands of a city the size of Seville.
Parabolic Troughs
Parabolic troughs use parabolic reflectors, which concentrate beams of sunlight on the receiver tubes located at the focal point of the structure and contain liquid heat transfer medium. The medium heats u[ to about 400°С and goes through a series of heat exchangers. The resulting steam powers a typical turbine generator, which task is to generate electrical energy.
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Parabolic trough diagram |
Parabolic troughs are used throughout the world; their popularity being due to a simple system ensuring tracking of the Sun’s position and higher cost-efficiency compared to the other types of solar power plants. The cost of parabolic troughs is close to the cost of nuclear power stations.
Dish Systems
Dish-type power stations use parabolic dishes, which concentrate solar energy on the receiver located at the focal point of each dish.
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Dish system |
The working liquid in the receiver heats up to 1000°С and its energy is used for generating electricity either in a Stirling engine or in a Brayton cycle engine unit. Such installations also have systems that track the Sun. The maximum diameter of the dish is 20 m, which ensures a capacity of 60-75 kW. The cost of a dish-type power station can be lower than that of tower or parabolic designs.
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Dish system |
The solar power station of the company Solucar in Sanlucar la mayor tests various innovative technologies. For example, it combines parabolic concentrators with Stirling engines and extra-long parabolic (in their cross-section) reflectors with tubes for heating the heat transfer medium.
Naturally, solar power stations are more efficient in regions with a high level of sun radiation and low nebulosity. Their efficiency ratio can reach up to 20%, and capacity - 100 MW.
The next article is going to discuss photoelectric conversion of solar radiation.
The next article is going to discuss photoelectric conversion of solar radiation.
Source (Russian language): http://energetika.in.ua
Alexei Tiatiushkin
Marketing manager
KharkovEnergoPribor Ltd.
marketing@keppowertesting.uk
http://news.chivindo.com/370/types-and-design-of-solar-power-plants-solar-power-industry.html
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