This document discusses concentrating solar collectors that increase outlet temperature by reducing collector surface area and heat loss. It describes various types of concentrating collectors including parabolic collectors, compound parabolic concentrators (CPCs), and plane reflectors. It provides equations for calculating concentration ratios of different collectors and discusses factors that affect collector performance such as reflectivity, intercepted beam proportion, and heat loss coefficients. Examples of large-scale concentrating solar power plants and a solar cooker are also mentioned.

1. 1
Part B6: Concentrating collectors

2. 2
B6.1 Concentrating collectors
• Used to increase outlet temperature
• Increase in temperature due to
reduction in collector surface area and
commensurate reduction in heat loss
(AcUL)
• Image from the sun is a finite size not
a point
• subtends an angle of about 5º
• image will be a disc (3d) or a rectangle (2d)

3. 3
B6.1 Concentrating collectors

4. 4
B6.1 Concentrating collectors
Abel Pifre’s Printing press (1882)

5. 5
a
r
A
C
A

B6.1 Concentrating collectors
Concentration ratio
C = Concentration ratio
Aa = Aperture area (m2)
Ar = Receiver area (m2)

6. 6
B6.1 Concentrating collectors
Concentration ratio: Ideal
2
1
sin
230
ideal d
R
C
r 
 

2
3 2 2
1
sin
50,000
ideal d
R
C
r 
 

Cideal = Concentration ratio of an ideal collector
 = Acceptance half angle

7. 7
B6.2 Concentrating collectors
Concentration ratio: Parabolic collectors
(imaging collectors)

8. 8
B6.2 Concentrating collectors
Concentration ratio: Parabolic collectors
(imaging collectors)
C = Concentration ratio
 = Acceptance half angle
f = rim angle
 
2
sin cos
1
sin
d
C
f f 


 
 
2 2
3 2
sin cos
sin
d
C
f f 




9. 9
B6.2 Concentrating collectors
Concentration ratio: Parabolic collectors
(imaging collectors)
C = Concentration ratio
 = Acceptance half angle
f = rim angle
max2
1 3
2sin 2
110
d
C

 

max3 2
1 1 3
2sin 4
4sin
13,000
d
C


  

1
2 2

f 
 
 
 
 
When

10. 10
B6.3 Concentrating collectors
Non imaging collectors: Acceptance angle
• Total angle the sun can move through
without its image missing the receiver
– An ideal collector with an acceptance angle
of 60º will have a concentration ratio of 2
• Ideal concentrators are non imaging
– Most common form is the Compound
parabolic concentrator (CPC)
– CPCs can collect diffuse radiation with a
proportion of 1/C (C of 1.5 will pick up 2/3
of diffuse radiation)

11. 11
B6.3 Concentrating collectors
Non imaging collectors: Compound
parabolic concentrator (CPC)

12. 12
B6.3 Concentrating collectors
Non imaging collectors: Compound
parabolic concentrator (CPC)

13. 13
B6.3 Concentrating collectors
Non imaging collectors: Compound
parabolic concentrator (CPC)

14. 14
B6.3 Concentrating collectors
Non imaging collectors:CPC: Variations

15. 15
B6.4 Concentrating collectors
Alternative concentrators

16. 16
B6.4 Concentrating collectors
Alternative concentrators

17. 17
B6.4 Concentrating collectors
Alternative concentrators

18. 18
B6.5 Concentrating collectors
Plane reflectors

19. 19
B6.5 Concentrating collectors
Plane reflectors

20. 20
B6.6 Concentrating collectors
Receivers
• 2d systems should use selective
surface as radiator radiates to the sky
• 3d systems can use any black surface
as C is very high so heat loss is small

21. 21
B6.6 Concentrating collectors
Tracking
• 3d systems must track on 2 axes
• 2d systems should track in one
direction
– North-South aligned collectors have (fairly)
even output throughout the day but must
track the sun from East to west but be
careful about shading
– East-West aligned collectors need not track
over the day but may need seasonal
adjustment and will have a strong variation
in output throughout the day
• C-3 – adjust four times/year
• C-1.5 no adjustment necessary

22. 22
B6.6 Concentrating collectors
Tracking
Manual Seasonal adjustment for CPC Tracking for parabolic trough (E-W)
Tracking for parabolic trough (N-S) Two axis tracking
f

23. 23
B6.6 Concentrating collectors
Tracking: Moving receiver

24. 24
B6.6 Concentrating collectors
Performance
q = Energy per meter of collector
(W m-2)
r = reflectivity of collector
g = proportion of the beam
intercepted by the receiver
t = proportion of the beam
reaching the reflector
a = proportion absorbed by the
receiver
GT = Incident irradiance
UL = Loss coefficient of the
receiver
Tc = Cold water temperature
TA = Ambient temperature
C = Concentration ratio
 
L c A
T
U T T
q G
C
rgta

 
rgta ta

Note:
q

25. 25
B6.6 Concentrating collectors
Reflectivity for mirror surfaces
Material r
Silver 0.93-0.95
Back silvered low iron glass 0.88
Back aluminiumised glass 0.76-0.80
Plated silver 0.96
Aluminium sheet 0.82
Aluminiumised PTFE 0.77
Silvered PTFE 0.86

26. 26
B6.6 Concentrating collectors
Performance

27. 27
B6.7 Concentrating collectors
Examples
Parabolic concentrator system
Lutz (USA – 1984 - 1990)
• Several sites between 13 and
80MW and covers an area of
460 ha
• Collectors generate 360ºC
• Uses synthetic oil for heat
removal to a conventional
turbine
• Cost effective against fossil fuel
(9 US cents/kWh in 1991)
• 22% peak and 14% average
efficiency

28. 28
B6.7 Concentrating collectors
Examples
Power tower
Solar 2 (USA - 1994)
• Generates – 10MW
(100MW peak)
• Heat > 550ºC
• Uses melted rock salt for
heat removal
• “Solar Tres”, a 15MW
commercial system is
planned for Spain (and
has been for some time)

29. 29
B6.7 Concentrating collectors
Examples
Power tower
Odeillo (France - 1969)
• Built for materials
testing
• Heat > 3,800ºC
(but over an area of 50
cm2

30. 30
B6.7 Concentrating collectors
Examples
Solar cooker
Haiti
• Uses Fresnal reflector

31. 31
Next – Refrigeration and other applications