ecen 5007 lecture 7

Central Receiver Systems - CRS

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Solar Thermal Power Plant. Basic configuration

Beam irradiance

Concentrator

Receiver

Thermal Storage

Concentrated irradiance Electricity

Power conversion system

Thermal energy

Boiler Fossil fuel Biomass

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Central Receiver System

http://www1.eere.energy.gov/solar/power_towers.html

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CRS (Gema Solar, Molten salts)

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CRS (Phoebus, Open air volumetric receiver)

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Collector System (concentrator)

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Heliostat field }  The concentrator is the “heliostat field”, a Fresnel

concentrator }  Main elements

}  Heliostat }  Control System

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The heliostat

}  “an instrument consisting of a mirror mounted on an axis moved by clockwork by which a sunbeam is steadily reflected in one direction”

}  Basic components }  Reflecting surface }  Structure and tracking mechanism }  Control system

}  Typology: }  Glass - metal }  Stretched membrane

}  Size: 1 m2 to 150 m2

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Heliostat

Back support structure

Elevation drive

Azimuth drive

Reflecting surface

Reflected ray

Incident ray

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Burning mirror, Hoesen (18th century)

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Glass – metal heliostat

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Heliostat

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Glass – metal heliostat

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Glass – metal heliostats

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Stretched membrane heliostats

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Stretched membrane heliostats

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Stretched membrane heliostats

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Heliostat field layout

}  Surround field }  North (south) field

}  Secondary concentration }  Secondary concentrator optics tower (SCOT)

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North field

0

200

400

600

800

1000

-600 -400 -200 0 200 400 600

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Surrounding field

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Geometrical performance of heliostats

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Cosine factor

Yearly average cosine factor for a north heliostat field

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Reflectivity

}  Reflectivity of a new, clean mirror ≈ 0.90 ÷ 0.94

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Shading and blocking

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Shading & blocking

Shading

Blocking

Air transmittance

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Air transmittance

Mirror quality }  Heliostat facets are spherically curved

}  For large focal distances, a parabolic surface can be approximated by an spherical surface of radius r = 2f (f: focal distance).

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Mirror quality

2222cspSD σσσσ ++=

Ideal spherical curvature

Spherical curvature, with waviness

2222cspSD σσσσ ++=

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Distortion

}  Spherical reflectors generate distortion of the image }  Dependent on time (relative position sun-heliostat)

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Summer solstice, noon

Summer solstice, 7:30 a.m. Summer solstice, 7:30 p.m.

Spillage

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Receiver system }  Function }  Components }  Types of receivers }  Working fluids

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CRS – Sistema Receptor

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Tipos de receptores

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Cavity receivers

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Cavity receivers

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Receptor - Fluidos de trabajo

}  Características térmicas apropiadas: capacidad térmica, entalpía de cambio de fase, estado a temperatura ambiente

}  No corrosivo, tóxico o inflamable }  Abundante, barato, tecnología de manejo asequible... }  No existe el fluido perfecto: hay que llegar a

compromisos

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Molten salt cavity receiver

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External, cylindrical receiver

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External, cylindrical receiver

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External, cylindrical receiver

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Tubular receiver (boiler)

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Heat transfer characteristics of tubular and volumetric receivers

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Volumetric receivers

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Volumetric receivers

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Detail of a volumetric absorber (wire mesh)

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Detail of a volumetric absorber (ceramic cups)

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Working fluids

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Working fluids for CRS }  Water / steam

}  Saturated steam }  Superheated steam

}  Molten salts }  Air

}  Pressurized }  Atmospheric

}  Sodium }  Thermal oils

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Comparison of HTF’s

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Power conversion system }  Rankine cycle (steam turbine) }  Brayton cycle (gas turbine) }  Combined cycle (gas turbine + steam turbine) }  Stirling engines }  (MHD)

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CRS: pros and cons. }  Pros:

}  Ability to achieve high temperatures }  Wide industrial base for most components }  Multiple technological options }  Technologically proven }  Multiple thermal energy storage options }  High potential for improved effciency or cost reduction

}  Cons.: }  Complexity }  Short commercial record }  Best technology still undefined

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Energy Balance of Central Receiver Systems

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High concentration concepts }  Secondary concentration

}  Increase flux density on the absorber }  Reduce requirements for primary concentrator (heliostats)

}  Secondary Concentrator Optics Tower }  The receiver can be placed at the ground level

}  Solar Furnaces }  Very high concentration ratios }  Combine a field of flat heliostats and a parabolic concentrator }  Not for electricity generation

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Secondary concentrators }  REFOS Project (DLR)

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SCOT (Secondary Concentrator Optics Tower)

}  Weizmann Institute (Israel)

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SCOT (Secondary Concentrator Optics Tower)

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Beam Down

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Solar Furnaces

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Solar Furnaces

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