SOLAR COOKING TECHNOLOGY

Why Solar Cookers are needed High cost or Unavailability of commercial fuels – Kerosene, Coal, Gas, Electricity Deforestation caused by Increasing Firewood Consumption Use of Dung and Agricultural Waste as Fuels Instead of for Soil Enrichment Diversion of Human Resources for Fuel Gathering

Heat losses during cooking The principle ways of cooking food are Boiling, Frying, Roasting, and Baking Heat losses during cooking Vaporization of water : 35 per cent Heating food to boiling : 20 per cent temperature Convection losses from vessel : 45 per cent Solar cooker should be designed to provide about 1.0 kW of energy which can be obtained with 2.0 sqm of collector area with 50 per cent efficiency.

Principle of Cooking The quantity of heat required for physical and chemical changes involved in cooking are small compared to the sensible heat of increasing food temperature and energy required for meeting heat losses that normally occur in cooking. Once the contents of the vessel have been sensibly heated up to the cooking temperature (100C), the speed of cooking is practically independent of heat rate as long as thermal losses are supplied. Differences in the time required to cook equal quantities of food in cookers of various heat supply capabilities are due mainly to different sensible heating up periods.

Reasons for the non-acceptance of the solar cookers Too expensive for individual family ownership Incompatible with traditional cooking practices too complicated to handle cooking can be done only in the direct sun can not cook at night can not cook in cloudy weather can not cook indoors danger of getting burned or eye damage are not locally available less durable; needs repair or replacement of parts which are not easily available The cooker needs frequent adjustment towards the sun and exposure of the cooking pot to the blowing dust and sand effected the food taste Easy availability of alternative cooking fuels like wood and fuel wood There is no provision of storing the heat therefore cooking of food was not possible where there are clouds or sun is not strong No proper education, training and involvement of women folk

Direct or focusing type solar cooker Types of Solar Cookers Direct or focusing type solar cooker In these cookers some kind of single or multifacet solar energy concentrator (parabolic, spherical, cylindrical, fresnel) is used which when directed towards the sun focus the solar radiation on a focal point or area where a cooking pot or frying pan is placed. In these cookers the convection heat loss from cooking vessel is large and the cooker utilizes only the direct solar radiation.

Direct or focusing type solar cooker Design variations Parabolic / spherical / cylindrical reflector Single mirror or mosaic type Diameter : 0.5 m – 8.0 m Focal length: 0.3 m – 5 m Manual / automatic tracking Temperature achieved: 150C - 450C User – single family / community

Direct (focusing) type solar cookers

Indirect or Box type Solar Cooker In these cookers an insulated hot box (square, rectangular, cylindrical) painted black from inside and insulated from all sides except window side which is double glazed is used. Single plane or multiple plane reflectors are used. Some times these are also known as oven type solar cookers. These can be electrical cum solar cookers and some cookers utilize a kind of latent heat storage material.

Indirect or Box type Solar Cooker Design variations Square / rectangular / oven Single, double or multiple plane reflectors Frequent tracking not required Temperature achieved : 100C - 250C User single family / community Electrical back up Latent heat storage material

Box type Solar Cookers

Advanced type Solar Cooker In these cookers, the problem of cooking outdoors is avoided to some extent. The cookers use either a flat plate collector, cylindrical (PTC) concentrator, or a multifacet or large parabolic (mosaic type) concentrator which collect or focuses the solar heat and transfers or reflect from a secondary reflector to the cooking vessel. The cooking in some cases can either be done with stored heat or the solar heat is directly transferred to the cooking vessel in the kitchen.

Advanced type Solar Cooker Design variations Cooking can be done indoors also Storage of heat is possible Frequent tracking required / not required Temperature achieved upto 350C User – single family / community Flat plate, cylindrical or large mosaic parabolic reflector

A Few Advanced Type Solar Cookers

Feasible Applications of Solar Box Cooker Solar Box Cookers have been found to be technically feasible for various applications of cooking such as rice, khichadi, pulao, basundi, biryani, kheer, dal, bread, biscuits, cakes, nankhatai, dhokla, fish, chicken, meat, egg etc.

Box type Solar Cooker

BOX SOLAR COOKER

Solar Box – type Cooker : Design Details Component Material Thickness / size Requirements / Remarks Outer Box Galvanished iron Aluminium FRP 0.48 mm thick (60 x 60 x 17 cm) 0.56 mm thick 2 mm thick Resistant to ultraviolet radiation and atmospheric variations Inner Cooking Box (50 x 50 x 10 cm) Painted dull black Should not touch outer body Insulation (Back and side) Glass fibres in the form of pads 5 cm or more thick k = 0.052 W/m K Free from resin binders Stable upto 250C Glazing (Double glass lid) Water white glass (Temperated / toughned) 3-4 mm thick 50 x 50 cm size spacing between sheets 1 cm Double glass system must be air tight Transmittance > 85% Reflectors (Mirrors) Silvered or Glass aluminized 4 mm thick 54 x 54 cm Reflectivity > 85% Scratch resistant Resistant to solar radiation and atmospheric variation Cooking Containers Aluminium alloy sheet Stainless Steel sheet 1.2 mm thick Two pots – dia 200 m Two pots – dia 150 mm Depth of pots – 67 mm Dull black painted stable upto 250C Very good adhesive characteristics

Solar Box-type Cooker : Cooking Time for Recipes It takes about 2 – 2.25 hours for cooking depending upon the kind of food and season. Different items like dal, rice, vegetables etc. are normally cooked simultaneously in separate containers. The time taken for cooking is less in summer than in winter. Based upon experience following are the results for cooking time for various recipes:

SK - Type Solar Cookers (SK-10, SK-12, SK-14, SK-98) SK – Solar Cooker uses parabolic reflector made of thin, hard aluminium sheets with protected, high reflecting surface mounted at a rigid basket structure. Reflector with short focal distance for safety reasons, long tracking intervals and high efficiency. Cooking pot in a standard 12 – litres pot of black enamelled steel with a diameter of 28 cm. Tracking is done by moving the whole cooker (azimuth) and by turning the reflector around the horizontal axis (elevation), adjustment of the reflector to the sun by use of a shadow indicator.

Technical Data (SK Type Solar Cooker) Reflector diameter : 140 cm Nominal effective power : 0.6 kW Pot capacity : 12 litres Pot diameter : 28 cm Max. temperature : 200C Capacity : Boils 48 litre of water in a day Tracking : Manual Cost : INR Rs. 6000/- Cooking Food : 10-15 people at a time

Seifert Parabolic Domestic Solar Cooker (SK 14)

Community Solar Cooker Components Primary Reflector Secondary Reflector Tracking Mechanism Technical details Primary reflector is 7 m2 parabolic dish fitted with a set of acrylic / glass mirrors. Primary reflector reflect and concentrate the sunlight falling on the dish onto the secondary reflector. Temperature of concentrated sunlight at focal point reaches to 350C Cook meals for 60-80 persons.

Technical Specifications of the Community Cooker at IIT Delhi Reflector shape& material Elliptic shape (a=3800 mm, b=2750 mm), acrylic mirror sheets (0.6 m x 0.6 m) Reflector area 7.3 m2 Secondary reflector Aluminium foil Focal length 3m (along the polar axis) Maximum height of system 4 m Maximum width of system 3 m Number of mirror facets 12 Material of constructions of mirror Acrylic mirror sheets Size of focal spot of main reflector at front aperture of chullah Shape-Elliptical, D1=30cm, D2=20cm Size of focal spot of after secondary reflector (at the bottom of the vessel) Circular shape, D=12cm Type of Tracking Device Semi-automatic Surface area of secondary reflector 0.36 m2 Reflectivity of mirrors of main concentrator 0.75 Reflectivity of secondary reflector 0.65

Principle of working of Scheffler Community Solar Cooker

World's Largest Solar Steam Cooking System at Tirupati, Andhra Pradesh Location Installed at the temple town of Tirumala, Andhra Pradesh with nearly 50 percent funding from MNES System Employs automatic tracking solar dish concentrators to convert water In to high pressure steam which Is used for cooking purpose* In the community kitchen. Technical Details Solar dish concentrators (106 Nos) with total reflector area of about 1000m2. Modular in nature and consists of several units (parallel & series) connected to central pipe-line system. Each dish consists of scheffler mirrors with an aperture area of 9.4 sq.m. Generates 4,000 kg of steam per day at 180°C and 10 Kg/cm2. Cook meals for around 15,000 persons per day. The cooker saves about 1,20,000 litres of diesel per year. The total cost of the system Is about Rs. 110 lakh. Implementing Agency Ministry of Non-Conventional Energy Sources (MNES).

WORLD’S LARGEST SOLAR STEAM COOKING SYSTEM AT TIRUPATI

SOLAR ENERGY CENTRE (SEC) TEST PROCEDURE Two types of tests are carried out in India on all solar thermal devices including solar cookers. These are performance test and degradation test. Before conducting these tests, a third test namely the physical verification test is also done which is performed just after the cooker is received for getting it tested. Under this test, it is verified whether all the components have been supplied as per the specifications of MNRE/SEC or not. In case the specifications are not followed, the manufactured/nodal agency/marketing agency is informed about the deficiency alongwith the first performance test report and asked to follow the specifications.

SOLAR ENERGY CENTRE (SEC) TEST PROCEDURE (contd.) (a) PERFORMANCE TEST Two types of tests are carried to evaluate the performance of the cooker. These are: I) No Load Test - The cooker is kept in the sun with empty cooking pots inside and its temperature profile along with solar insolation and ambient temperature is recorded. Simultaneously, temperature profile is also recorded for cooker fabricated at SEC as per the specifications. If the profile of the cooker under test is not less than 5°C of the cooker fabricated at SEC, the cooker under test is passed. For a solar insolation of 600 Watts/m2, the temperature should not be less than 120°C, provided that the SEC cooker is able to attain 125°C under the same conditions. II) Load Test - 2 kg of water is distributed in four cooking pots and kept inside the cooker. The temperature profile along with solar insolation and ambient temperature is recorded for both the cookers. The maximum temperature in this case should not be less than 100°C for a solar insolation of 600 Watts/m2. Temperature profile of the cooker under test should not be less than 5° of that for the cooker fabricated at SEC.

SOLAR ENERGY CENTRE (SEC) TEST PROCEDURE (contd.) (b) DEGREDATION TEST: A minimum of 30 clear sunny days are required to complete the degradation test. The cooker is kept in the sun with empty cooking pots inside and degradation, if any, of various components is studied carefully. There should not be any degassing from insulating material, sealing gaskets, paints etc. collection of water vapours within the space between the glasses of the lid, the change in colour of the cooking pots, tray, cooker box etc., observed and deposition of water vapours or any other material on the glass lid (both inside and outside) on exposing the cooker to outside atmospheric conditions in the sun is not permitted. Cookers showing such degradation are not allowed to be sold under the subsidy scheme.

SOLAR ENERGY CENTRE (SEC) TEST PROCEDURE (contd.) NOTE: All the manufacturers / suppliers are required to give a guarantee card along with instruction manual. The guarantee card should read as: The solar cooker manufactured and supplied by M/s ......... ..... ..... ...... ... ....................... (complete address) is guaranteed for a period of two years against any manufacturing defect, use of inferior quality materials and poor worksmanship including improper silvering/aluminising of reflecting surface etc. The breakage of glass components is, however, not covered under this guarantee. Name of the manufacturer................... Full Address and......................... Telephone No. .................................. Name of purchaser .............................. Full Address and ................................ Telephone No. ................................... Cooker No. ....................................... Date of Sale .................................... Signatures: (1) Supplier (2) Purchaser

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