1 Part B6: Concentrating collectors

2 B6.1Concentrating collectors Used to increase outlet temperature Increase in temperature due to reduction in collector surface area and commensurate reduction in heat loss ( A c U L ) 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 B6.1Concentrating collectors

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

5 B6.1Concentrating collectors Concentration ratio C = Concentration ratio A a = Aperture area (m 2 ) A r = Receiver area (m 2 )

6 B6.1Concentrating collectors Concentration ratio: Ideal C ideal = Concentration ratio of an ideal collector  = Acceptance half angle

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

8 C = Concentration ratio  = Acceptance half angle  = rim angle

9 B6.2Concentrating collectors Concentration ratio: Parabolic collectors (imaging collectors) C = Concentration ratio  = Acceptance half angle  = rim angle When

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

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

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

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

15 B6.4Concentrating collectors Alternative concentrators

16 B6.4Concentrating collectors Alternative concentrators

17 B6.4Concentrating collectors Alternative concentrators

18 B6.5Concentrating collectors Plane reflectors

19 B6.5Concentrating collectors Plane reflectors

20 B6.6Concentrating 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 B6.6Concentrating 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 B6.6Concentrating collectors Tracking Manual Seasonal adjustment for CPCTracking for parabolic trough (E-W) Tracking for parabolic trough (N-S)Two axis tracking 

23 B6.6Concentrating collectors Tracking: Moving receiver

24 B6.6Concentrating collectors Performance q = Energy per meter of collector (W m -2 )  = reflectivity of collector  = proportion of the beam intercepted by the receiver  = proportion of the beam reaching the reflector  = proportion absorbed by the receiver G T = Incident irradiance U L = Loss coefficient of the receiver T c = Cold water temperature T A = Ambient temperature C = Concentration ratio Note:

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

26 B6.6Concentrating collectors Performance

27 B6.7Concentrating collectors Examples Parabolic concentrator system Lutz (USA – ) 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 B6.7Concentrating collectors Examples Power tower Solar 2 (USA ) 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 B6.7Concentrating collectors Examples Power tower Odeillo (France ) Built for materials testing Heat > 3,800ºC (but over an area of 50 cm 2

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

31 Next – Refrigeration and other applications