Fresnel Solar Cooker Design

By Ed Norman C.U.S.O. 1980

Introduction

In areas where sunshine is plentiful and conventional fuels are expensive, like the coast or the sierra of Peru, the solar cooker is an ideal complement to a regular stove. It is cheap, easy to use and requires no fuel. The cooker works by concentrating the power of the sun onto a small area in which a pot or other implement is placed. Under strong sunlight, a litre of tap water can be brought to a rolling boil in about 10 minutes.

The solar cooker consists of 3 main parts. A parabolic reflector serves to concentrate more than one square metre of sunlight into an area about 17 cm in diameter. The control arm allows the reflector to be set facing the sun and holds the pot at the focal point regardless of the reflector tilt angle. The stand holds the other two components together and allows the cooker to be rotated to follow the sun as it moves across the sky.

Materials

The cooker can be made by anyone with experience in simple carpentry and access to basic hand tools. The following is a list of materials needed:

  • One half of a sheet of 'masonite' (saw dust pressboard) 4'x4'
  • 2 metres x 25" of aluminized mylar (metalized plastic)
  • 3 metres x 1"x6" and 12 metres x 1"x2" dry cedar (or other lightweight, non-warping wood), smooth on both sides.
  • Aluminium pot (About 2 litre capacity) blackened on outside, with lid.
  • 115 cm x " aluminium tube.
  • 4 hinges and 4 small casters (with screws)
  • 12x2" and 24x" flat-heat wood screws.
  • Assortment of common nails (1", 1", 2")
  • Wood glue.
  • Contact cement.

In Peru the cost of these materials is about $25 (in 1980)

Reflector Preparation

Cut the plastic in half and layout the two pieces to cover the 4'x4' masonite sheet. Mark the four circles shown in the figure and cut out six semicircular pieces using scissors. Roll up and save each piece. Next, mark the masonite sheet as shown. Starting with the outermost radius cut out the three rings and the gap in each ring. Also make the short radial cuts in the outermost ring. (These act to equalise the curvature when the ring is bent to form a conic section.) Also cut out the three 'joiners' as shown. A hacksaw blade is good for cutting masonite. Finally, smooth all edges with a file.

By bringing together the edges of the gap in each rig, a conic section is formed. The 'joiners' are used to hold the edges together. Glue each joiner to one side of the gap (rough surfaces together). When dry, glue the other side of the gap and joiner, bend the ring to match up the two gap edges and clamp between two blocks of wood until dry as shown. As elsewhere, this construction is easier to do with two people. Glue all three rings this way.

Reflector Support

From the 1"x6" wood stock, cut two reflector supports as shown below (only half is shown; they are symmetric about the centre line). Draw a straight reference line and layout all measurements (in centimetres) from it. On one end of one support, cut the faces that contact the rings an extra 0.3cm deep (dashed lines) to accommodate the extra thickness of the joiners. This will be the top of the reflector. In each support, cut a notch (one on the front and the other on the back) in the centre to allow the two supports to be joined in a cross as explained in the following section.

Reflector Assembly

A) Join the two reflector supports in a cross as shown in the following detail: First glue the auxiliary pieces on each support. When dry, glue the two supports together and hold with screws, making sure that the supports are at right-angles B) Position the outermost ring (#1) on the crossed support structure, with the joiner centred holes. Remove the ring and drill and countersink the holes in the masonite to fit the " flathead screws. Replace the ring in its position, drill guide holes and fasten the ring to the support. Be careful not to twist or warp the rings away from its natural shape. No glue is needed. Make sure that each screw head is flush with the surface. Clean any dust off the aluminized mylar and the front surface of the ring. Spread a thin coat of contact cement on half the rings and one side of the mylar semi-circle. Let the glue dry to the touch. Carefully position the middle of the semi-circle above the ring (easier with two people) then press the mylar onto the ring, working from the middle out to each end. A soft cloth may be used to press out wrinkles and bubbles. Small ripples in the plastic do not matter. Repeat with the other half of the ring. Trim off any excess plastic with a sharp knife. Repeat part B) for the remaining two rings to complete the reflector. Try not to get contact cement on the front surface of the mylar.

Stand and Control Arm

Using the 1"x2" wood stock, construct the stand and control arm of the solar cooker as shown in the following two views. Glue and nail all joints to achieve a strong structure. Mount the four hinges and four castors as shown. Use scrap masonite or plywood for the angle bracket backing. Drill a hole in the end of the control arm to fit the aluminium tube. The angle bracket is shown in detail on the following page.

Angle Bracket

From the remaining 1"x6" wood, cut out the angle bracket as shown. Adjust the dimensions 'x' and 'y' to fit the control arm as shown on the previous page. Drill 27 pin holes 1cm apart starting at the right hand end a shown. Make the holes slightly bigger than a 3" nail, which will be the pin. Construct a tube socket on the end of the bracket to support the aluminium tube. One possibility is shown here. Make sure that the socket will not interfere with the pin latch in the last hole. Mount the angle bracket on the control arm using glue and screws to achieve a solid joint. The bracket should be perpendicular to the plane of the control arm.

Pin Latch

Cut out the pin latch from a piece of tin can as shown. Bend along the dashed lines and nail to the top end of the reflector support. Position the latch to allow the support arm to rest on the angle bracket while the pin (3" nail) goes through both holes in the latch and the last hole in the angle bracket at the same time.

Pot Holder

Flatten the end of the aluminium tube as shown and cut a slot for the pot handle 112.7cm from the other end of the tube.

Final Assembly

Place reflector on the stand and adjust its lateral position so that the pin latch (nailed to reflector support) fits over the angle bracket. Drill guide holes and screw hinges to reflector. Insert aluminium tube in its socket and your solar cooker is finished.

Use of the Cooker

To use the solar cooker, simply hang the pot on the end of the aluminium tube, rotate the stand and adjust the tilt angle of the reflector until the pot's shadow falls in the centre of the reflector. When properly adjusted, there should be no glare from the reflector and the pot handle should not become too hot to hold. For longs cooking times, the cooker will have to be adjusted every ten minutes or so to follow the sun. If a lower heat is required, the cooker may be rotated to move the pot out of the focus or part of the reflector can be covered. When not in use, the cooker should be stored inside, out of the sun or covered with a waterproof cover.

Maintenance

The solar cooker should not be mishandled, overloaded (>5kg) and never left out in the rain or allowed to get wet. Otherwise, the only maintenance it should need is cleaning of the reflector surface. Since the aluminized plastic attracts dust, it should be lightly wiped periodically with a soft, dry cloth, A few grease spots will not seriously affect the performance. If too much dirt and grease accumulates, it can be cleaned off gently with detergent and warm water, followed by drying with a soft towel.

Photos

For better resolution photos please click First Photo, Second Photo or Third Photo.

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Contact Ed Norman

Peru Children's Trust

Last updated on 25th January 2001.