Cu stp 04_fundamentals

SOLAR THERMAL POWER!GEEN 4830 – ECEN 5007!

Manuel A. Silva Pé[email protected]!

4. Fundamentals of solar thermal concentrating systems!

Solar Thermal Concentrating Systems

Systems that make use of solar energy by first concentrating solar radiation and then converting it to thermal energy

}  Uses: }  Electricity (Solar Thermal Power) }  Industrial Process Heat }  Absorption cooling }  Chemical processes }  …

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Solar energy

}  Abundant }  High-quality energy }  Variable (on time) }  Unevenly distributed (on space) }  Low density

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Why high temperature?

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The sun as a heat source

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Why concentrate solar radiation?

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Ideal concentrating system

}  The receiver (or absorber) converts concentrated solar radiation to thermal energy (heat)

}  An ideal receiver may be characterized as a blackbody, which has only radiative losses

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Geometrical concentration ratio

abs

C

AACg =

}  The geometrical concentration ratio, Cg, is defined as

Where Aabs is the receiver (or absorber) area and Ac is the collection area.

Absorp'on  area  

Concentrator  

Collec'on  area  

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Optical efficiency of the receiver

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Ideal concentrator

}  The maximum theoretical optical efficiency (when Tabs≥TSky) is the effective absorptivity of the receiver.

}  The higher the concentrated solar flux (C*I), the better the optical efficiency.

}  The higher the absorber temperature, the higher the radiative loss and, therefore, optical efficiency is lower.

}  The higher the effective emissivity, ε, the lower the optical efficiency.

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Global efficiency of the ideal concentrating system

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Ideal concentrating system

}  For each value of the geometrical concentration ratio, there is an optimum temperature.

}  The higher the geometrical concentration ratio, the higher the optimum temperature and the global efficiency.

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Concentration limits

SsennnDC

θ22

2

3max,′

=

}  The Sun is not a point light source. Seen From the Earth, is a disk of apparent diameter θS ≈ 32’.

}  The maximum concentration ratio is given by

Where n and n’ are the refractive indices of

the media that the light crosses before and after the reflection on the concentrator surface

32’

32’

Focus  

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Types of concentrating systems

}  Line focus (2D) }  Parabolic troughs; CLFR

}  Point focus (3D) }  Central receiver systems,

parabolic concentrators (dishes)

SDmáxC θ23, sin/1=

SDmáxC θsin/12, =

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Real concentrating systems

Theoretical 3D: < 46200

2D: < 215

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