Part B6: Concentrating collectors

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Part B6 Concentrating collectors
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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)

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B6.1 Concentrating collectors
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B6.1 Concentrating collectorsAbel Pifres
Printing press (1882)
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B6.1 Concentrating collectors Concentration ratio

C Concentration ratio Aa Aperture area
(m2) Ar Receiver area (m2)
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B6.1 Concentrating collectors Concentration
ratio Ideal
Cideal Concentration ratio of an ideal
collector q Acceptance half angle
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B6.2 Concentrating collectors Concentration
ratio Parabolic collectors (imaging collectors)
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B6.2 Concentrating collectors Concentration
ratio Parabolic collectors (imaging collectors)

C Concentration ratio q Acceptance half
angle f rim angle
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B6.2 Concentrating collectors Concentration
ratio Parabolic collectors (imaging collectors)

C Concentration ratio q Acceptance half
angle f rim angle
When
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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)

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B6.3 Concentrating collectorsNon imaging
collectors Compound parabolic concentrator (CPC)
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B6.3 Concentrating collectors Non imaging
collectors Compound parabolic concentrator (CPC)
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B6.3 Concentrating collectors Non imaging
collectors Compound parabolic concentrator (CPC)
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B6.3 Concentrating collectorsNon imaging
collectorsCPC Variations
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B6.4 Concentrating collectors Alternative
concentrators
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B6.4 Concentrating collectors Alternative
concentrators
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B6.4 Concentrating collectors Alternative
concentrators
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B6.5 Concentrating collectors Plane reflectors
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B6.5 Concentrating collectors Plane reflectors
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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

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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

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B6.6 Concentrating collectorsTracking
Manual Seasonal adjustment for CPC
Tracking for parabolic trough (E-W)
f
Tracking for parabolic trough (N-S)
Two axis tracking
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B6.6 Concentrating collectorsTracking Moving
receiver
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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
Note
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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
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B6.6 Concentrating collectors Performance
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B6.7 Concentrating collectorsExamples

• 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

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B6.7 Concentrating collectorsExamples

• Power tower
• Solar 2 (USA - 1994)
• Generates 10MW (100MW peak)
• Heat gt 550ºC
• Uses melted rock salt for heat removal
• Solar Tres, a 15MW commercial system is planned
  for Spain (and has been for some time)

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B6.7 Concentrating collectorsExamples

• Power tower
• Odeillo (France - 1969)
• Built for materials testing
• Heat gt 3,800ºC (but over an area of 50 cm2

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B6.7 Concentrating collectorsExamples

• Solar cooker
• Haiti
• Uses Fresnal reflector

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Next Refrigeration and other applications