Upload
manuel-silva
View
672
Download
1
Tags:
Embed Size (px)
Citation preview
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 } …
07/07/11 1 GEEN 4830 – ECEN 5007
Solar energy
} Abundant } High-quality energy } Variable (on time) } Unevenly distributed (on space) } Low density
07/07/11 2 GEEN 4830 – ECEN 5007
Why high temperature?
07/07/11 3 GEEN 4830 – ECEN 5007
The sun as a heat source
07/07/11 4 GEEN 4830 – ECEN 5007
Why concentrate solar radiation?
07/07/11 5 GEEN 4830 – ECEN 5007
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
07/07/11 6 GEEN 4830 – ECEN 5007
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
07/07/11 7 GEEN 4830 – ECEN 5007
Optical efficiency of the receiver
07/07/11 8 GEEN 4830 – ECEN 5007
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.
07/07/11 9 GEEN 4830 – ECEN 5007
Global efficiency of the ideal concentrating system
07/07/11 10 GEEN 4830 – ECEN 5007
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.
07/07/11 11 GEEN 4830 – ECEN 5007
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
07/07/11 12 GEEN 4830 – ECEN 5007
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, =
07/07/11 13 GEEN 4830 – ECEN 5007
Real concentrating systems
Theoretical 3D: < 46200
2D: < 215
07/07/11 14 GEEN 4830 – ECEN 5007