Solar cooker

solar cooker.

1. Welcome

2. Seminar on Solar cookers Presented by Phadtare prajakta dinkar Ph.D . Scholar

3. Content Introduction Cooking principle Classification Box type solar cooker Parabolic cooker Panel cooker Shefller dish Folding umbrella type solar cooker Cylindro parabolic type solar cooker Spiral reflector type solar cooker Vacum or evacuated type solar cooker Solar Steam Cooking System at Shri Saibaba Sansthan Trust, Shirdi , MS, India

4. Introduction Need of solar cooker Major portion of total energy consumed in cooking. Half the word’s population burn wood or dried dung to cook food. In village, 95% energy consumed for cooking. Source of fuel used for cooking, coal, kerosene, cooking gas, firewood, dung etc . Contd ….

5. The burning of wood as fuel leads to massive increases in deforestation and greenhouse gas emissions. People are exposed to indoor air pollution as a result of burning solid fuels for cooking and heating. Millions of people can’t find enough wood to cooking, so using solar cookers is a good idea. Hence need to harness the solar energy.

6. Cooking principle Solar cookers are passive solar devices. Sunlight is converted to heat energy which is retained for cooking. Solar cookers utilize the simple principles of reflection, concentration. The steps involved in the solar cooker are concentrating capturing and converting the solar energy. Clean cooking technology.

7. Concentrating sunlight: designed to achieve temperatures of 150°F (65°C) (baking temperatures) to 750°F (400°C) (grilling/searing temperatures) on a sunny day. Converting light energy to heat energy: Solar cookers concentrate sunlight onto a receiver such as a cooking pan. The interaction between the light energy and the receiver material converts light to heat. Trapping heat energy: It is important to reduce convection by isolating the air inside the cooker from the air outside the cooker.

8. Classification 1. Direct Type : Use some solar energy concentrator to focus sunlight onto an area . Eg : Parabolic solar cooker 2 . Indirect Type : A box covered with transparent material like glass. Employs greenhouse effect for cooking Eg : Solar box cooker 3. Advanced Type : The cookers use either a flat piece or focusing collector, which collect the solar heat and transfer this to the cooking vessel. Eg : Thermal storage solar cooker

9. Box type solar cooker

10. Working Consists of an insulated box with a glass or a plastic window . The window acts as a solar energy trap by exploiting the greenhouse effect. The solar rays penetrate through the glass covers and absorbed by a blackened metal tray kept inside the solar box. To maximize the heating effect , the walls and outer side of the pots should are painted black .

11. The upper cover of the cooker has two glass sheets in parallel and thus heat loss through re-radiation is minimized from the blackened surface. The loss due to convection is minimized by making the box air tight by providing a rubber strip all round between the upper lid and the box.

12. A dvantages There is no problem of charring of food and no over flowing . Orientation or sun tracking is not needed. No , attention needed during cooking as in other devices. No , fuel, maintenance or recurring cost. Simple to use and easy to manufacture. No pollution of utensils, house or atmosphere. Vitamins in the food are not destroyed and food cooked is nutritive and delicious with natural taste .

13. D isadvantages One has to cook according to the sunshine, the menu has to be pre-planned . One can not cook at short notice and food can not be cooked in night or over, cloudy days. It takes comparatively more time. Chapaties are not cooked because high temperature for baking required and also needs manipulation at the time of baking.

14. Parabolic cooker

15. Parabolic type solar cooker developed by National Physical Laboratory (NPL) of India at New Delhi as early as 1955 . Focus a lot of sun energy onto a very small space, using parabolic shapes. Reach temperature up to 450 ⁰C . Works on the principle that when a 3D parabola is aimed at sun, the rays are reflected on to the focus.

16. Consists of a large parabolic and cooking pot holder When the reflector surface is aimed at the sun, the rays falling on the parabolic surface converges to the focus of the parabola . The cooking pot is placed at the focus of the reflector The pot surface are blacked to improve the absorption. Working

17. Advantages: Cooks nearly as fast as a conventional oven. High temperatures of the order 450 °C of allow for food to be fried and grilled. Disadvantages: Costly and complicated to make and use have to turn frequently to follow the sun. Generally more expensive than panel and box cookers . Housewife has to cook the food out of doors in the sun hence it is not favoured.

18. Panel cooker

19. Cooking pot is enclosed by a panel of reflectors. Eight reflectors made of silvered glass mirrors, four of square shape (35x35x0.3) and four of tringular shape (35,25,0.2x0.3 ). Sunlight is reflected off of multiple panel onto a pot under a glass lid or in a bag. Can be built quickly and at low cost Many different varieties Popular with relief agencies

20. It incorporates elements of both parabolic and box solar cookers . The reflective panel directs sunlight onto a dark colored pot . The pot is enclosed in an insulting shell such as high temperature cooking bag or an inverted bowl. On very clear days, maximum plate temperature in the oven reaches to 350⁰C and 250 ⁰C in winter season. Practically all types of food preparations like cooking ,Roasting , Baking and Boiling can be done within 25 to 75 minutes under clear sky conditions. Working

21. Advantage Better performance than box cooker Disadvantage Relies more on reflected radiation

22. Scheffler dishes

23. A Scheffler reflector is a small lateral section of a paraboloid which concentrates sun’s radiation over a fixed focus. The collector of Scheffler Dish is an assembly of flat shaped solar grade glass mirrors or Aluminium mirror reflectors arranged on a structural steel framework. The receiver of scheffler dish is placed at the focus of the dish to capture the incident solar radiation and transfer it to the thermal medium. Tracking system enables the dish to be focused towards the sun to capture maximum possible direct radiation during the day .

24. Consists of heliostat and secondary reflector . Heliostat concentrates the beam on to the secondary reflector which focuses it on to the bottom of pot. When not cooking the energy can be used for heating water or can be stored. C ommon applications where Scheffler steam systems are used are: Boiler feed water preheating Oil heating for cooking or industrial applications Steam cooking

25. Advantages It is a renewable energy. The solar cooker requires neither fuel. It preserves more of the natural nutrients of the foods by cooking at slower and lower temperatures. S aves a lot of firewood. Can be used in areas where fuel and firewood are not available.

26. It is not continuous. It cannot be used during rainy season or cloudy conditions . Performance could be affected by strong winds Time required is higher than conventional cooking methods . Disadvantages

27. Folding umbrella type solar cooker

28. This is a lightweight portable parabolic cooker, based on an umbrella . The model can be used year-round, but in winter, it is necessary to cover the receptor with a high density polythene bag (HDPE). A Polypropolyne bag can be used following the same cooking instructions as for other classic cookers such as cookit . An umbrella (preferably a non-folding model in order to keep the cooker in good condition)Reflective sheeting ( aluminium foil or similar) Metal support for the cooking pot

29. Cylindro -Parabolic type solar cooker

30. The cylindro -parabolic concentrator focused the rays into an insulated. C ylindrical box in which two or more cooking vessels could be accommodated. Bowman , who built and tested the design, encountered several difficulties (Bowman, Blatt 1978), and he tried to improve upon the design. This resulted in a series of new concepts.almost similar to Prata’s design but it has only a single cylindro -parabolic swinging reflector.

31. Spiral reflector type solar cooker

32. Design begins with a computer program that prints a spiral pattern. This pattern is transferred onto a flat material already covered with a reflective medium, then cut out on a band saw. By lining up mounting points (also printed by the program) along a straight frame member, the proper twist is given to each segment of the spiral to concentrate sunlight at the selected focal point. The computer program can vary the focal length of the spiral or even alter the design to focus light behind the reflector.

33. Vacuum or evacuated tube solar cooker

34. The design is a simple flat plate collector housed in an evacuated glass tube. The tubes are made from a type of glass called Borosilicate, which is resistant to thermal shock. Borosilicate glass has the characteristic of being very strong and also has excellent light transparency . It consists of two concentric glass tubes with vacuum in between. The outer tube is transparent while the inner is coated with Aluminium nitride for better absorption.

35. The evacuated glass tube tube receives the solar rays that pass through and is absorbed by the inner lining.

36. The combination of the highly efficient absorber coating and the vacuum insulation means that the coating can be well over 200 o C . Due to the presence of vacuum, the heat losses will be negligible. A reflector is provided for concentrating sunlight onto the tubes. A tray is provided inside the glass tube for cooking purposes.

37. Solar Steam Cooking System at Shri Saibaba Sansthan Trust, Shirdi , MS, India The Sai Baba temple complex at Shirdi , Maharashtra’s Ahmednagar district, has installed one of the world’s largest solar cooking system based on schfeller dishes .

38. A Scheffler type concentrating solar steam cooking system was commissioned at Shri Saibaba Sansthan , Shirdi on 30th July, 2009. It cooks food for about 3000 devotees. The 73 nos. of solar Scheffler concentrators raise the water temperature to 550oC to 650oC and convert it into steam for cooking purposes. This system is integrated with the existing boiler to ensure continued cooking even at night and during rainy or cloudy weather.

39. The solar cooking system installed at Shirdi follows the thermosyphon principle and so does not need electrical power or pump. 73 parabolic concentrators / dishes placed on the terrace of Sai Prasad Building No.2. They reflect and concentrate the solar rays on the 40 receivers placed in focus. Water coming from the steam headers placed above the header centers is received from bottom of the receiver, gets heated up to 550 C due to concentration of solar rays on the receivers and get pushed up via top pipe of receiver into the header.

40. The principle of anything that gets heated is pushed up is called thermosyphon principle. The advantage of thermosyphon principle is no pumping (thus no electricity) is needed to create circulation since the heated water is pushed into the header and water from the same headers come into the receivers for heating. The cycle continues till it reaches 100 0C and gets converted into steam. Only once during the day i.e. in the early morning the dishes have to be turned manually onto the morning position, subsequently the automatic tracking takes over.

41. Schematic view of system installed at Sai Baba Sansthan`s Prasadalaya , Shirdi

42. Design Specification of box type solar cooker Outer box, (600 X 600 X 200 mm) Inner cooker box, aluminium sheet of 1 mm, (460 X 460 mm top face and 300 X 300 mm bottom faces with face depth of 150 mm   Glazing (double), ordinary glass sheets of 4 mm, insulation- fibre glass, Reflectors (mirrors), silvered glass mirror of 2 mm, 600 X 600 mm Cooking pot, 300 X 300 mm, with dull black paint and aperture area of 0.6 m  0.6 m.

43. Design consideration Concentration ratio (CR) Overall instantaneous insolation (I T ) Collector tilt First figure of merit (F 1 ) Second figure of merit (F 2 ) Internal cooking power (P) Standard cooking power (P s ) Temperature difference (T d ) Aperture area

44. Structure of box type cooker

45. Performance Testing I. Concentration ratio (CR) = Where, A t = total collector area A rc = area of the receiver / absorber surface II . Overall Instataneous Insolation (I T ) = I b + I d + I r Where, I b = Beam radiation I d = Diffused radiation I r = Beam insolation reflected on the reflector surface

46. III. First figure of merit (F 1 ) = where, F 1 = first figure of merit Km 2 w 1 ή = optical efficiency (%) U L = Overall heat loss T p = Absorber plate temperature (℃) T a = ambient temperature (℃) I s = Insulation (Wm -2)

47. IV. Second figure of Merit (F2)= Where, M= mass of water (kg) C= Specific Heat (J/Kg ℃ V . Interval Cooking Power (P) = VI . Standard cooking power(P s ) = VII . Temperature Difference (T d ) = T w - T a

48. Design of concentrator cooker 1.Required Power for Cooking ( Qr ): Q r = Where, Qr = Required power for cooking (kcal) M = Mass of water and rice (Kg) Cp = Specific heat capacity (Kcal/Kg K) Temperature difference ( Tf -Ti)(k)  

49. 2. Selection of parabolic aperture and focal length: Q d = Q f * τ * α * ϕ Where, Q d = Energy delivered by paraboloid ( kcal ) Q f = Average energy falling at Dapoli (kcal) τ = Specular reflectance of reflector α = Absorptance of absorber(greater than 0.98) ϕ = Intercept factor silver/glass mirrors can have a reflectance of 0.94 and aluminum reflecting surfaces have a reflectance of about 0.86. 3 . Aperture diameter calculation: Q r = Q d * Where, Q r = Energy required for cooking (kcal) D = Aperture diameter of paraboloid (m)  

50. 4 . Conventional losses: Q conv = h c * A( T r - T a ) Where, Q conv = Heat transfer rate by convection(W) h c = Convective heat transfer coeff .(W/m 2 K) A = Area of receiver( m 2 ) Tr = Surface temperature of receiver(K) Ta = Temperature of the air(K) 5 . Radiation losses: Q rad = A* σ * ( ) 4 Where, σ = Stefan-Boltzmann constant (5.670 × 10 -8  W/m 2   K 4 ) Tr = Surface temperature of receiver (K)  

51. 6 . Focal length calculation: 4f Z = Where, X = X co- ordinate of parabola Y = co- ordinate of parabola f = Focal length of parabola (m) Z = Depth of paraboloid (m) 7 . Rim angle of the paraboloid : tan ϕ = Where, d = Diameter of dish h = Height of dish  

52. 8 . Surface area of paraboloid : A = Where, A = Surface area of paraboloid ( m 2 ) f = Focal length(m) 9 . Concentration ratio: C = Where, Aa = Area of aperture( m 2 ) Ar = Area of receiver( m 2 )  

53. 10. Efficiency of solar concentrating collector: η = Where, Twf = Final temperature of water( °C ) Twi = Initial temperature of water( °C ) Mw = Mass of water(litter) Cw = Specific heat of water(KJ/Kg K) 11. Thermal losses factor: F ʹ U L = Where, F ʹ U L = Thermal losses factor Mw = Mass of water(litter) Cw = Specific heat of water(KJ/Kg K) Apot = Area of pot ( m 2 ) = Sensible cooling constant  

54. 12. Optical efficiency factor: F’η = Where, τ = Duration of interval(eg.10 min or 600s ) 13. Thermal efficiency: Thermal efficiency( η t ) = Where, η t = T hermal efficiency of parabolic cooker (%)  

55. Components a nd Material selection Reflecting surface: made of single/ multiple reflectors Concentrating Reflector: Anodized aluminum sheet/ glass mirrors/ aluminum foil/ any other better and durable material with protective layers of coating on back surface and sides. Supporting frame of the dish: made of MS rings supported by MS strips Bowl stand: mild steel Cooking pot: ISI mark pressure cooker of 2 to 5 lit capacity Tracking mechanism: Manual or automatic

56. Thank you…….