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EB Suncooker

December 2010

 

The EB Suncooker is a powerful multi-sided double cone cooker that is relatively easy to build and use.  The cooking pot is located at the bottom of the cooker and, in that respect, is unlike most other EB cookers where the pot is located near the mouth of the cooker.  When the pot is located near the mouth of the cooker, a larger portion of the solar radiation hits the bottom and sides rather than the top and sides of the pot.  However, when the pot is located near the mouth of the cooker, the cooker requires a stand to support the pot and the cooker reflector.  Because the bottom pot EB Suncooker normally is used with the pot raised above one of the surfaces of the cooker, a stand to support the cooker reflector is not required.

IMG_0305.JPG

Figure 1:  EB Suncooker

 

The EB Suncooker is effectively a descendent of the Parvati Parvati Cooker which was developed by Ravindra Paradisi in India.  The Parvati Cooker is a very efficient cooker.  We designed the EB Suncooker to overcome some of our perceived limitations of the Parvati cooker.

 

  1. Improved Stability.  One limitation of the Parvati Cooker is that it is difficult to make the pot and the cooker stable.  The only flat surfaces for the cooker and pot to rest on are the narrow sections of the cooker reflector.  We made the cooker reflector more stable by replacing two of the narrow sections with one wider section as shown in Figure 1.  We obtained some of the dimensions of the wide section by designing a cooker reflector with half as many sides as the original cooker and using its dimensions as a starting point for the design of the wide side of our cooker.  Also, dimensions of the lower part of the wide section were adjusted for proper fit at the base of the cooker reflector.  The length of the lower section was made equal to that of the adjacent narrow sides.  The width of the lower section was made equal to the wide side of the center disk as shown in Figure 3.  As can be seen from Figures 4 and 5, the width of the wide side is approximately, but not exactly, twice that of the narrow sections.

2.     Larger size.  We believe that the 24 inch diameter of the Parvati is a bit small.  For example, when the diameter is increased to 30 inches, the effective area goes from 0.292 m2 to 0.456 m2.  Assuming that all of the reflected power is directed onto the pot, the cooking power is increased by 56%.  As the cooker diameter increases, the number of sides that are needed to focus most of the incoming solar radiation onto the pot also increases.  If the width of the top of each side (C1 in Figure 2) is larger than the typical dimensions of the pot, some of the solar radiation that is reflected from the outer part of the cooker will miss the pot.

  1. Cooker Materials. 
    1. We could have used corrugated cardboard but we elected to use plastic polypropylene flute board (Coroplast™).  The flute board is more durable than cardboard and does a better job of resisting the elements of weather.  However, cardboard could be used if plastic flute board is not available or is too expensive.
    2. We elected to use Mylar™ foil instead of aluminum foil as the reflective surface because it has better reflectivity and maintains its reflectivity better.  The 2 mil Mylar™ was fastened to the flute board with a spray adhesive (3M Super 77™).  An investigation of adhesives showed that the commonly used mixture of white glue and water was ineffective.
    3. We used paper fasteners to join the reflector edges together when assembling the cooker.
    4. Other reflector materials could be used.  For example, Bright aluminum sheeting could replace both the plastic flute board and the Mylar™ foil.

 

Build Your Own Cooker

Choose the size of the EB Suncooker and the materials you will use to build it.

We believe that normally the best size for the diameter of the EB Suncooker is in the 30 to 40 inch (76.2 to 101.6 cm) range.  An EB Suncooker with diameter much less than 30 inches may cook only marginally faster than some panel cookers that are easier to build and use.  However, the double angle cone shape is not adequate to focus all of the incoming solar radiation onto the pot if the diameter is too large.  Thus for very large cookers, the double cone probably should be replaced with a triple cone or parabolic shape.  For example, for our first prototypes and for a build your own cooker workshop in November 2010, we chose a diameter of 30 inches. We could then cut out two cookers, each with a tab, from a 4 by 8 foot sheet of material. 

 

You should pick one of the cookers from Figures 4 or 5.  Each of these figures has all of the dimensions needed for the reflector, along with some other parameters, for four different cookers.  The 24 inch diameter cooker in Figure 4 was included for comparison with Parvati Cookers Parvati, but we recommend selecting one of the larger cookers.  The metric cookers in figure 5 are not exactly the same size as the cookers that use English units in Figure 4 because the diameters of the metric cookers were chosen to be whole numbers in metric units.  The row in Figures 4 and 5 that gives the area of the mouth of the cooker in square meters is useful in comparing the cooking power of the cookers.  Since the direct component of solar radiation is often assumed to be in the range of 700 to 1000 w/m2, a row is included in Figures 4 and 5 where the solar intensity is assumed to be 700 w/m2.  The last two rows are useful in comparing the cooking power of the cookers.  However keep in mind that the calculated power is input power and that the power hitting the pot will be somewhat less.

 

Make a full size pattern for one of the narrow sides.

Since your cooker will have at least 10 narrow sides, we recommend making a full size pattern of one side.  The pattern can be made of almost any convenient material.  Figure 2 shows the layout for one side.  We chose to use the same labels that were used on the Parvati website to make comparison easier.  The actual dimensions are obtained from either Figure 4 or 5.  We recommend laying out the side by constructing the dashed red lines first and then adding the outside lines.  Be especially careful with the C1 and B1 dimensions because small errors will be multiplied as multiple sides are laid out.  Since only one wide side is needed, we laid out the wide side directly on the cooker rather than making a full size pattern.  When making patterns, we usually make the outside dimensions slightly smaller to account for the width of the pencil or marker that is used to trace the pattern onto the cooker.  In Figure 2, the left end labeled A is a tab used to attach the reflector to the center disk.

SuncookerSection.jpg

Figure 2:  Drawing of Single Side of EB Suncooker

EBSucookerBase.jpg

Figure 3:  Drawing of Bottom Center Disk

Figure 4:  Dimensions of EB Suncooker (English Units)
All dimensions are in inches unless otherwise noted.  
Cooker Diameter 24   30   35   40  
  Narrow Wide Narrow Wide Narrow Wide Narrow Wide
sides 10 1 10 1 14 1 14 1
C 8 8 10 10 11.67 11.67 13.33 13.33
B 6.25 6.25 7.81 7.81 9.11 9.11 10.42 10.42
A 1.58 1.58 1.58 1.58 1.58 1.58 1.58 1.58
C1 6.27 12.42 7.83 15.53 6.86 13.66 7.84 15.61
B1 4.18 8.28 5.22 10.35 4.57 9.10 5.23 10.40
A1 1.88 3.62 2.34 4.53 2.05 4.02 2.34 4.60
D1 0.88 2.62 1.34 3.53 1.05 3.02 1.34 3.60
C2 7.93 7.73 9.91 9.66 11.61 11.44 13.27 13.08
B2 6.14 5.80 7.68 7.25 9.03 8.75 10.32 10.00
 A2 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5
C1/2 3.13 6.21 3.92 7.76 3.43 6.83 3.92 7.80
B1/2 2.09 4.14 2.61 5.18 2.29 4.55 2.61 5.20
A1/2 0.94 1.81 1.17 2.26 1.03 2.01 1.17 2.30
D1/2 0.44 1.31 0.67 1.76 0.53 1.51 0.67 1.80
Radii for Semicircular Layout of Cooker Reflector                
First (Largest) Radius  24   30   35   40  
Second Radius 16 20   23.33   26.67  
Third Radius 9.75 12.19   14.22   16.25  
Fourth Radius 8.25 10.69   12.72   14.75  
Center Disk Radius 3.62 4.53   5.26   6.01  
Other Cooker Parameters                
Cooker Depth 11.35 11.35 14.18 14.18 16.55 16.55 18.91 18.91
Diameter cm 60.96 60.96 76.2 76.2 88.9 88.9 101.6 101.6
Cooker Opening Area sq meter 0.29 0.29 0.46 0.46 0.62 0.62 0.81 0.81
If the direct component of solar intensity is assumed to be 700 Watts per sq meter:                
Cooker Input Power (Watts) 204.3   319.23   434.5   567.51  
Power Ratio 1.00   1.56   2.13   2.78  

Figure 5:  Dimensions of EB Suncooker (Metric Units)
All dimensions are in centimeters unless otherwise noted.  
CookerDiameter 70   80   90   100  
  Narrow Wide Narrow Wide Narrow Wide Narrow Wide
sides 10 1 10 1 14 1 14 1
C 23.33 23.33 26.67 26.67 30 30 33.33 33.33
B 18.23 18.23 20.83 20.83 23.44 23.44 26.04 26.04
A 4.27 4.27 4.27 4.27 4.27 4.27 4.27 4.27
C1 18.27 36.23 20.88 41.41 17.64 35.12 19.60 39.02
B1 12.18 24.16 13.92 27.61 11.76 23.41 13.07 26.01
A1 5.47 10.57 6.25 12.08 5.27 10.34 5.86 11.49
D1 2.47 7.57 3.25 9.08 2.27 7.34 2.86 8.49
C2 23.13 22.54 26.44 25.76 29.86 29.42 33.17 32.69
B2 17.92 16.92 20.48 19.33 23.21 22.51 25.79 25.01
 A2 4 4 4 4 4 4 4 4
C1/2 9.14 18.12 10.44 20.71 8.82 17.56 9.80 19.51
B1/2 6.09 12.08 6.96 13.80 5.88 11.71 6.53 13.01
A1/2 2.74 5.28 3.13 6.04 2.64 5.17 2.93 5.75
D1/2 1.24 3.78 1.63 4.54 1.14 3.67 1.43 4.25
Radii for Semicircular Layout of Cooker Reflector                
First (Largest) Radius  70   80   90   100  
Second Radius 46.67   53.33   60.00   66.67  
Third Radius 28.44   32.50   36.56   40.63  
Fourth Radius 24.44   28.50   32.56   36.63  
Center Disk Radius 10.57   12.08   13.52   15.02  
Other Cooker Parameters                
Cooker Depth 33.10   37.83   42.55   47.28  
Cooker Diameter (inches) 27.56   31.50   35.43   39.37  
Cooker Opening Area sq meter 0.38   0.50   0.64   0.79  
If the direct component of solar intensity is assumed to be 700 Watts per sq meter:                
Cooker Input Power (Watts) 269.39   351.86   445.32   549.78  
Power Ratio 1.00   1.31   1.65   2.04  

 

Lay out the cooker reflector and the center disk.

The next step is to lay out the complete cooker on the material you are using to build the reflector for your cooker.  If you are making several cookers, we recommend making a full size pattern for the reflector.  For our cooker building workshop, we made a full size cardboard pattern for the cooker reflector (not including a tab), and then used the pattern to draw the cookers on the flute board.  Figure 6 shows our full size cardboard pattern being transferred to a sheet of flute board.  The cardboard pattern shown in Figure 6 is an example of the result of the layout process presented here.  The layout process described assumes the cooker is being constructed from a single piece of material.  However, if a large piece of material is not available, the cooker can be constructed from smaller pieces and joined together at any seam between two sides using tabs to join sections together with paper fasteners.  To make a tab, an approximately 2 inch wide piece of development material is added to one of the surfaces to be joined. 

 

To lay out a cooker reflector:

1.     On the cooker material, draw four concentric semicircles using the radii given in Figure 4 or 5 for your cooker.  

 

Our favorite method of drawing large semicircles is to find a small thin strip of wood that is at least a bit longer than the radius of the largest semicircle.  Drive a small nail through the stick near one end.  Drill a small hole at a point that is at a distance from the nail equal to the desired radius.  The hole should be just large enough to accommodate the point of the pencil or marker that you will use to draw the semicircle.  If you do not have any other thin strip of wood available, a wooden yard stick or meter stick can be used if you have one that you are willing to drill small holes in.  Drive the small nail through the cooker material at the point where you want to locate the center of the semicircle and draw the semicircle.

SucookerLayout.JPG

Figure 6:  Full Size Cardboard Pattern Being Transferred to Flute Board

 

1.     Before you draw in all of the sides of the cooker around the semicircle, it is useful to perform a test for accuracy.  Start at one side of the largest semicircle, use the wide end of your full size side pattern and make light pencil marks all the way around the largest semicircle.  If the last mark is approximately on the other edge of the semicircle, continue to the next step.  If not, adjust the spacing between marks a bit and try again.

2.     Draw dashed lines between the largest and next largest semicircle from each of the marks you made around the largest semicircle in the previous step toward the center of the semicircle.

3.     Start at each edge of the semicircle and use your full size pattern to draw in the sides of the cooker.  Skip the two center sections which will become the wide side.

4.     Lay out the wide side using wide side dimensions from Figure 4 or 5.  Use the dashed line at the center of the semicircle as the centerline of the wide side.  When finished erase or cross out the dashed line down the center of the wide side because it is not to be used as a fold line as the other dashed lines are.

5.     Use a straightedge and draw in all of the dashed A1 and B1 lines on the cooker layout.

6.     If there is enough material, we recommend adding about a 2 inch wide tab along the C edge on one side of the cooker reflector for use when assembling the cooker.

7.     Lay out the center disk on an unused portion of the cooker material.  Figure 3 shows the center disk for a 30 inch diameter cooker.  Start by drawing a circle with radius equal to the center disk radius from Figure 4 or 5.  Inside this circle, inscribe either a 12 or 16 sided polygon depending upon which cooker you are building.  For a 12 side polygon the angles between lines to the vertices are 30 degrees and for a 16 sides polygon these angles are 22.5 degrees.  Pick one vertex and draw a line between the two adjacent vertices to form the wide side.  As a check, measure the width of the wide side on the center disk and see if it matches the A1 dimension for the wide side of your cooker shown in Figure 4 or 5.

 

Cut out the cooker and the center disk.

If you are using plastic flute board or cardboard for your cooker, we recommend laying the material on a flat surface and cutting out the cooker and the center disk with a utility knife.  You may want to put scrap material under the material to protect the surface.  We recommend punching or drilling a small hole at each of the intersections of B and C lines before cutting out the cooker.  After the cooker has been cut out, the dashed lines where the cooker will be folded need to be scored.  Use a straightedge as a guide and score the lines with a blunt instrument.  Be careful not to cut into the material under the lines.  Just create a crease where there will be a fold.  Bend the cooker on each of the scored lines.  This is best done by holding a straightedge along the length of the line and bending the material toward the straightedge.

 

Glue the reflective material to the cooker and the center disk.

We found that it is possible to avoid wrinkles when putting Mylar™ on plastic flute board by being careful.  Place the reflector on an old curtain or sheet and spray the flute board with adhesive.  Nest move the reflector with the adhesive on it and place the reflector on a clean dry surface.  Cut a piece of Mylar™ large enough to cover the reflector plus four inches.  Hold the Mylar™ slightly above the cooker and starting in the middle carefully press the Mylar™ against the cooker as shown in Figure 6.  Trim the Mylar™ about 1.5 to 2 inches along the outside edges of the cooker.  Where the edges are curved, make perpendicular cuts in the overhanging Mylar™ every few inches along the edges.  Spray some adhesive onto the back of the cooker and/or the overhanging Mylar™ and then fold the Mylar™ over the edge onto the back of the cooker. 

 

Cover one side of the center disk with Mylar™.

AttachingMylar.JPG

Figure 6:  Attaching Mylar™ to the Flute Board

 

Assemble the cooker.

We used paper fasteners to hold the cooker together.  Fold the two edges of the semicircular cooker together with the tab overlapping the opposite edge on the outside of the cooker.  Punch a small hole through the cooker and the tab and insert a paper fastener through the hole from inside the cooker and open the ends of the fastener on the back.  Repeat with about three more fasteners through the cooker and the tab.  

 

Place the center disk inside the cooker reflector.  Then fold the bottom tabs over the center disk and attach each tab with a paper fastener inserted from the inside of the cooker.  See Figure 7.  This step can be a bit tricky and is best done with two people.  We usually attach the tab on the wide side first and then continue with the other sides.

This completes the cooker reflector.

AssemblingCooker.JPG

Figure 7:  Assembling the Bottom of the Cooker

 

Use the Cooker

Some kind of greenhouse container such as a cooking bag or glass container surrounding the cooking pot will improve the performance of the cooker, especially with the smaller cookers.  It is important that the cooking pot be a dark color, preferably flat black.  The center of the pot should be located approximately on the axis of the cooker.  The axis of the cooker is a line perpendicular to the center of the center disk.

 

The details of how best to hold the pot in the correct position and tilt the cooker for different sun angles is still a work in progress.  We will add more on this as the details are worked out.

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