Reflective Solar Box Cooker

N K Srinivasan

Introduction

This article describes a solar box cooker design with reflective surfaces or panels, reducing the insulation and providing heating from the bottom of cooking pots in an easy manner. Another feature is that the cooker uses only one corrugated cardboard box.

The solar box cookers are, conventionally, an insulated black box, with a glazing of glass plate or plastic sheets.

The design described here ,called Reflective Box Cooker or RBC for short, is thus an improvement over the conventional box cooker, pioneered and promoted extensively by Barbara Kerr and her associates through Solar Cooker International [SCI],located at Sacramento, California.

The present design can also be considered as a hybrid design, using box cooker plus panel cooker. Panel cookers were advanced by Roger Bernard from Lyon, France; Panel cookers of several designs have been pioneered by SCI, the most popular being ‘CooKit” cardboard ,foldable model which had found extensive use in refugee camps and elsewhere in African nations.

I have built a small cooker of this RBC design, approximately 10” x 10” x 7” ,with a sloping front window or glazing, tapering to 3” height front wall. The detailed description is given later.

I have tested this cooker extensively in Cupertino, Santa Clara County, in California under California sun.

My motivation factors for this design are as follows:1 A solar box cooker with single [corrugated] cardboard box, with minimal insulation. The cooker becomes compact and has less weight and volume.2 The cooker can be kept in balcony or patio or even front porch.3 There is considerable heating of cooking pot from the bottom, by a special design of the stand for the cooking pot or pots.4 No need to tilt the box for orientation.5 [ Also avoid the danger of looking at bright spots in a concentrating type cookers.]

The RBC design is quite similar to the box cookers popularized by Richard Wareham and his organization: SunStove. But this is different is many respects: use of single back reflector; Richard Wareham does not use a back reflector to simplify the cooker; the stand for heating the bottom of the cooking pot and minimum of insulation between the box and side panels and use of a single cardboard box with back door.

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RBC description

1 A cardboard box [10” x 10” x7” ] was taken and a sloping front face was made by cutting a window opening of approximately 9” x 9” square cut out. The side walls were made sloping i.e., trapezium, with 3 in for the front edge.The cut out frame was pasted with glue to the side wall…Now we have a sloping top face , slant angle approximately 20 degrees..The opening is pasted with glazing [glass or plastic] For plastic sheet, I have used polythene sheets ,two layers.[This glazing will be of limited durability ,but can be replaced by other ‘green-house’ sheets.]

2 I lined all the inner surfaces of the box with heavy-duty aluminum foil {Reynolds wrap or similar foil], including the bottom surface.

3 Four reflecting panels with cardboard, pasted with aluminum foil, bright side up were made.The dimensions correspond to front wall [10”x4”], back wall [10”x8”] and two side walls of trapezium shape [4”x10” plus a triangle with sides 10” x 8” x sloping hypotenuse.]

4 The four panels are kept inside, with a slope towards the center, with duct tape and glue. Before placing the four panels, I stuff some crumpled newspaper pieces between the four panels and the inner walls as additional insulation. This is the only insulation used.The side panels can be kept slanting to focus the sun’s rays towards the cooking pot.

5 The back wall has a cut out of 4” x 4” , made into a flap to provide a back door for putting the pots inside.[The size of the door can be larger too.] The flap is kept such that air tight closing is possible , with additional closing flap over the door. You can make a hinged door too .

6 The cooking pot is kept on a special stand. The stand is made of a cardboard or metal plate with a central hole of 4 inches dia. The plate sits on four thin legs of two inches height….An alternative design for this stand is as follows:Use a drip pan , available in supermarkets/hardware stores with kitchen appliances, which are used for electric stoves. The drip pan has a central hole and a side opening [rectangular] .The drip pan/stand is painted dull matt black. The drip pan is kept inverted inside the box, over the reflective bottom sheet, with central hole on the top. The side opening is positioned facing the sun, facing the front wall inside the box.This design feature reflects the sunlight from the bottom reflecting sheet onto the bottom of the cooking pot, through the central hole.

7 Keep the cooking pot over the circular hole on the stand or drip pan.8 Position the cooker to face the sun.

Performance

1 I have used this cooker to cook 100 to 200 grams of rice or lentils/beans and vegetables , kept in a tight plastic ,microwave proof , container [‘Rubber-maid “ brand].

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2 I have used an oven thermometer kept near the cooking pot .{ “Good Cook” brand]

3 On a bright day, the temperature shoots up to 210 deg F in about 30 minutes. The maximum ,after steady state temp, is 250 deg F [about 120 deg C] between 10 and 12 AM .4 The cooking time for rice is about 90 minutes. For potatoes, two hours.

Note: 1The performance could vary a lot , depending on the solar insolation, the latitude, cloud cover and prevailing wind conditions. It is less sensitive to ambient temperature.2 The performance is largely a matter of how well air gaps are sealed, particularly around the glazing window and the back door.3 Vegetables , including root veg like potatoes, carrots and beans can be cooked in about two hours.4 The performance is almost the same in the afternoons between 1PM and 3 PM or later.

Some suggestions

The user of this design by me given above can modify several things: 1. the sloping angle of the galzing,2. add more insulation with corrugated cardboard pieces at the side wall,front wall and back wall and bottom area. Keep a few sheets of newspaper at the bottom—the easy way to improve the insulation at the bottom.3 Use enameled cook ware or other cook ware to suit their needs.

Disclaimer: The author has used some brand names only for illustrative purpose.He does not endorse any product through this article.

Contact: You can contact me at: nksrinivasan@hotmail.comVisit also my website: www.solardevicesforthirdworld.webs.com

About the AuthorThe author is a retired scientist/research engineer and obtained his Ph D from School of Engineering and Applied Science,Columbia University,New York, way back in 1972. He lives in Palo Alto, spends some time in Cupertino and also in Bangalore [Bengaluru], India.

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