6_concentrating [Compatibility Mode] [Repaired].ppt
- 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)
- 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
- 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