This document discusses solar thermal concentrating systems which collect solar radiation and convert it to thermal energy. It describes how these systems work, including defining the geometrical concentration ratio and discussing optical efficiency. The document outlines the ideal concentrating system and how increasing the concentration ratio leads to higher optimum temperatures and global efficiency. It also discusses concentration limits based on the sun being a disk rather than a point source, and types of concentrating systems like parabolic troughs and central receivers.

1. SOLAR THERMAL POWER!
GEEN 4830 – ECEN 5007!
4. Fundamentals of solar thermal concentrating systems!
Manuel A. Silva Pérez
!
silva@esi.us.es !

2. 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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3. Solar energy
} Abundant
} High-quality energy
} Variable (on time)
} Unevenly distributed (on space)
} Low density
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4. Why high temperature?
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5. The sun as a heat source
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6. Why concentrate solar radiation?
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7. 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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8. Geometrical concentration ratio
} The geometrical
concentration
ratio, Cg, is
defined as
A Concentrator
Cg = C
Aabs
Where Aabs is the
receiver (or
Collec'on
absorber) area area
Absorp'on
and Ac is the area
collection area.
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9. Optical efficiency of the receiver
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10. 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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11. Global efficiency of the ideal concentrating
system
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12. 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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13. Concentration limits
} 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 32’
n′2
C max,3D = 2 2
n sen θ S 32’
Focus
Where n and n’ are the refractive indices of
the media that the light crosses before
and after the reflection on the
concentrator surface
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14. Types of concentrating systems
} Line focus (2D)
} Parabolic troughs; CLFR
Cmáx,2 D = 1/ sin θ S
} Point focus (3D)
} Central receiver systems,
parabolic concentrators
(dishes)
Cmáx ,3D = 1 / sin θ S 2
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15. Real concentrating systems
Theoretical
3D: < 46200
2D: < 215
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