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The use of solar thermal energy for the generation of electric power has been experimented and demonstrated on the basis of various technologies for the collection and concentration of solar radiation: linear parabolic collector systems, tower systems for the generation of energy to be input into a grid, and point-shaped concentration systems, especially suited for "on-site" power generation.
The most developed technology at present is based on linear parabolic collectors (SEGS, Solar Electric Generating Systems). Nine large-scale power plants of this type, with over 350 MWe of total nominal power, are in operation at Kramer Junction, California, in the Mojave desert since the 1980’s.
The collector pipe, located on the focal line of the mirrors, consists of two concentric cylinders separated by vacuum for thermal insulation. The external glass cylinder has a diameter of 11,5 cm, and functions as protective casing. It is joined to the internal steel cylinder by metal bellows. This has a diameter of 7 cm and is the pipe for absorbing solar energy. The heat transfer fluid passes through it. A spectrally selective compound developed by ENEA coast the external surface of the steel pipe and ensures maximum absorption of the solar spectrum and minimum infra-red emission from the hot tube, enabling the plant`s operational high temperature (550 degrees C) to be achieved.
The heat transfer fluid used at the Kramer Junction plant is an inflammable und toxic mineral oil.
For this reason, the ENEA project preferred to apply a molten salt eutectic mixture, 60 % NaNO3 – 40% KNO3.
Their temperature varies from 290 to 550 degrees C when the solar field is operating.
In such plants, made up of rows of collectors interconnected in a grid covering several hundred metres, solar rays are concentrated on Heat Collecting Elements (HCE) placed along the focal point of each row of collectors. A heat carrying fluid, typically a mineral oil, pumped through the HCE, transports the heat to a power station located at the centre of the facility. The heat thus generated is used to produce steam that then drives an electricity turbo-generation unit. Typical operating temperature is about 390°C.
Tower systems were developed with a view to eliminating these shortcomings. In a tower system, mirrors that follow the movement of the sun on a dual axis, known as heliostats, reflect and concentrate the solar rays onto a receiver mounted on a tower located at the centre of the plant. The solar energy is collected by the receiver at the top of the tower, that heats a fluid, such as salt mixture, that apart from transporting the heat, can also store thermal energy. The heat stored in the molten salts generates steam at 540°C, that is used to drive an electricity turbo-generator.
The main advantages of solar tower plants over the previous technology, include:
Safety of the heat carrying liquid, that is a sodium or potassium nitrate, an natural fertiliser, that is neither inflammable nor toxic.
Improvement in the yield of the thermodynamic cycle, as a result of a rise in the operating temperature from 390°C to 565°C.
The possibility of using stored heat to counter daily variations in solar intensity. This leads to important advantages in terms of continuity of the turbine-alternator unit (that can be dimensioned to mean power and not necessarily to peak power, about 3 times installed power), resulting in reduced costs.
Reduction in the area unit costs of the mirrors as a result of the new technology based on honeycomb materials, that are lighter, stronger and cheaper than the simple sheet glass used in SEGS.
The system designed by ENEA combines the solutions used in linear parabolic collector systems and solar towers, by introducing a series of profound innovations that overcome the critical points of both.
The system is based on a technologically adapted version of the linear parabolic geometry of SEGS, that allows the use of molten salts as an energy carrier, thus providing for the higher temperatures typical of solar tower plants.
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