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earthboundtech

Design, construction, analysis and promotion of solar cookers

EB Hybrid Parabolic (Hypar) Solar Cooker

Abstract

At earthboundtech.com we recognize the benefits of slow cooking, but one of our goals has been to develop solar cookers that cook more like the kitchen stove, because we believe they will be more widely accepted.  Large parabolic cookers satisfy this requirement, but may be very expensive and/or quite dangerous.  Also, for those who want to build their own parabolic cooker, the construction process may be challenging.  In the past several years, we have designed several double and triple cone cookers that approximate the performance of parabolic cookers.  However, if these cookers are too large, not all of the incoming sun's rays hit the cooking pot

 

The EB Hybrid Parabolic (Hypar) Solar Cooker overcomes this size limitation of double and triple cone cookers.  It is a new type of solar cooker that is easier and less labor intensive to build than most parabolic cookers.  The Hypar Cooker is primarily a multisided parabolic with the outermost portion of the parabola replaced with a conic section.  The addition of the conic section to the parabola allows the complete reflector to be cut from a single piece of material with adjacent petals firmly attached to each other by a continuous strip of the reflector material.  The Hypar cooker is safer than typical parabolic cookers because, with the multisided parabola, the focal point is spread over a focal area.  Also, the focal area is located inside the Hypar cooker making it safer than cookers that have the focal point located outside the cooker.

 

Introduction

The EB Hybrid Parabolic (Hypar) Solar Cooker is a new type of solar cooker.  It is primarily a multisided parabolic cooker with the outermost portion of the parabola replaced with a multisided conic section.  The addition of the conic section to the parabola allows the complete reflector to be cut from a single piece of material where adjacent petals are firmly attached to each other by a continuous strip of the reflector material.  Each petal of the reflector will have a curved section corresponding to the parabolic portion of the cooker and a trapezoidal section corresponding to the conical portion of the cooker.  The structural stability of the cooker is improved by having the edges of adjacent trapezoids formed as folds in the continuous piece of material.

 

Parvati and DATS double cone cookers have been available for do it yourselfers to build for some time.  They are relatively easy to build, cook quite well and have been built by many do it yourselfers.  However, since the cooking pot is located deep within these cookers, most of the sun's rays heat the top and sides of the pot.  When the EB double cone cookers such as the EB30D and EB800D were designed, the pot was moved out near the mouth of the cooker where more of the sun's rays heat the bottom, as well as the top and sides, of the pot making the heat transfer into the food more efficient.  However, if all of the sun's rays entering the mouth of the cooker are to be focused onto the pot, double cone cookers are limited in size.  Triple cone cookers such as the EB42T can be made larger than double cone cookers but still have size limitations.  Parabolic cookers do not have fundamental size limitations.  They can be made as large as desired and still focus all of the sun's rays onto the cooking pot.

 

A multisided parabolic cooker called the EB Parabolic previously had been designed at Earthboundtech.  Two 45 inch (1.14 meter) sixteen sided prototypes were constructed and tested.  They worked very well and one was used to cook a meal on the 2010 winter solstice in Massachusetts.  However, the cooker construction was difficult and very labor intensive and the resulting cooker was heavy and bulky.  Therefore, except for a brief introduction on the Earthboundtech website, the EB Parabolic has not been promoted.  It was clear, from the experience with the EB Parabolic, that there was a need for a parabolic type cooker that would be easier and less labor intensive for a do it yourselfer to build, while also being lighter and less expensive to build. 

 

Double and triple angle cone cookers such as the EB800D with the focal area near the mouth of the cooker had previously been designed and built at Earthboundtech.  An project to design a cooker that would cook much like the parabolic, but would be easier and less labor intensive to build, was started.  The EB Hypar Cooker is the result of that project.  It was developed to incorporate the best aspects of the cone type cookers and the EB Parabolic.   The EB Hypar cooker satisfies the need for a parabolic type cooker that is much easier and less labor intensive to build than the EB Parabolic.  It also is lighter and less expensive than the EB Parabolic.  Construction of the EB Hypar Cooker follows steps similar to those used in the construction of double angle cone cookers except the edges of the parabolic portion of each petal are curved lines rather than straight lines.

 

The outer portion of the EB Hypar Cooker is a multisided cone with 45 degrees between the axis of the cooker and each side of the cone.  This cone is similar to the cone that is used as the outer portion of the EB800D double angle cone cooker (   ).  The remainder of the cooker is a multisided parabolic.  The petals and the layout are described in more detail in a later section of this paper.

 

Computer Programs have been written that can generate the dimensions of any size Hypar cooker using either English or metric units.  The first prototype that was designed and constructed is a 42 in (107 cm) diameter sixteen sided cooker.  Figure 1 is a photograph of this prototype.  The second prototype was an all metal Hypar 42.  the first prototype was moved to a 'basic' stand and second prototype was mounted on the 'internal post' stand.

Figure 1:  EB Hypar Cooker

Design of the Reflector for the Hypar Cooker

This section describes some of the basic concepts used in the design of the Hypar Cooker Reflector.  Consider the profile of the cooker reflector shown in Figure 2.  The profile shown is for a Hypar 42 cooker with the cone portion shown in blue and the parabolic portion shown in red.  For the Hypar 42, the diameter at the mouth of the cooker, measured between opposite corners, is  42 in (106.7 cm).  The edge diameter measured between the centers of opposite sides is 41.193 in (104.63 cm.  The x coordinate of the point at the upper right corner of the profile (x1) is half the the edge diameter or 20.596 inches. The depth of the cone (conedepth) was chosen to be 5 in (12.7 cm).  Since the angle of the cone is 45 degrees,  The x coordinate of the point (x2,y2) at the junction between the cone and parabola will be x2=(x1 - 5) or 15.596 inches.  With x2 known, a parabola can be defined which passes through the point (x2,y2), and has focal point f at the center of the conic section.  Two simultaneous equations, that were solved both analytically and numerically, for f and y2 are:

 

f = y2  + conedepth/2             (1)  

 

y2 = x2^2 / 4* f                      (2)

 

 

The resulting focal length (f) and parabola depth (y2) are 9.148 in (23.236 cm) and 6.648 in (16.886 cm ) respectively  

Figure 2: Profile of EB Hypar Cooker

Build Your Own EB Hypar Cooker Reflector

1.  Choose the Material for Cooker Reflector

The question of what material to use in the construction of the cooker reflector is shared by almost all solar cookers.  Probably the best material is highly reflective metal such as brite anodized aluminum (similar to that used for the reflectors on the Global Sun Oven).  However, we have not been able to find a source for small quantities of this material at a reasonable cost.  Currently we are using mostly plastic flute board (Chloroplast® is one brand of plastic flute board) with an added layer of Mylar® or other reflective material such as self adhesive vinyl.

 

2. Make a full Size Pattern for one Petal

Since the cooker reflector has 16 or more identical petals (sides), we recommend making a full size pattern for one petal.  In the near future, we expect to have full size patterns available at nominal cost.  However, all of the information needed to make your own pattern is given here.  Ideally the pattern should be made of relatively thin material that holds its shape when tracing the pattern onto the cooker reflector.  We prefer transparent patterns cut from clear plastic, but it is not necessary for the pattern to be transparent

 

Figure 3 shows one petal for the EB Hypar Cooker Reflector.  The dimensions shown are for the 42 inch 16 sided Hypar 42 prototype.  The dimensions for larger prototypes using both English and Metric  units will be available later.  The trapezoid on the left end of Figure 3 will form one side of the outer conical portion of the cooker.  The middle portion of the petal with the red curved edges will form the parabolic portion of the same side.  The triangular portion at the right hand edge of the petal forms a tab that will be used to attach the petal to the center disk when the reflector is assembled

Figure 3:  One Petal for EB Hypar 42 Cooker

The dimensions needed to lay out your own full size pattern for a petal are given in Table 1.  The distances given in Table 1 are relative to an origin located at the center of the left edge of the petal shown on Figure 3.  All of the black lines on Figure 3 are straight lines and only the end points are specified.  It is important to maintain the proper curvature of the red lines in figure 3 for the parabolic portion of the cooker to have the correct shape.  Therefore, multiple points are specified along the red lines.  After transferring all of the specified points onto the pattern, connect the points with lines as shown in Figure 3.  You then can cut out the pattern if you plan to use the pattern by drawing lines around its outside edges.

Table 1: Dimensions of EB Hypar 42 Petal

3. Lay out and Cut out the Cooker Reflector

To cut the EB Hypar 42 cooker reflector from a single piece of material, you will need a piece of material that is 4 feet wide by about 5 feet long.  If you are using a smaller piece of material, we recommend making any necessary splices along the fold between petals.

 

Figure 4 shows the layout for the 16 sided EB 42 cooker reflector.  The green lines in Figure 4 show the edges of the 48 inch wide material from which the cooker reflector is to be cut out.  You will be cutting red lines and folding solid blue lines.  The layout may look complicated but is not too hard to construct if you proceed step by step.  The recommended steps for creating a layout similar to that shown in Figure 4 are:

 

1.  Locate a center point that is 29.5 inches from both the left end and the top of the material.  Clearly mark the location of this center point because you will be using it in later steps.

 

2.  Draw a circular arc using the center point established in step 1 with a 29.42 inch (about 29 7/16 inch) radius.  It is  important that this radius be as accurate as possible.  The arc should go from the bottom edge of the material all the way around and back to the bottom edge.  It is not essential but we found it useful to draw two additional arcs using the same center point with 22.27 inch (about 22 ¼ inch) and 7.72 inch (about 7.75 inch radii.  (Note: We found that, unless you have special tools, probably the best way to draw large circles is to use a long thin wood, metal, or plastic stick (A yardstick works well if you have one that you are willing to drill holes in).   Drive a small nail through the stick near one end.  Carefully measure the desired radius from the center of the nail and drill a small hole just large enough to fit the pen or pencil you plan to use do draw the circle.  Drive the nail into the material at the center point, insert a pen through the hole that you drilled and draw the circle as you swing the stick around the nail.)


3.  Starting at the lower left corner of the largest arc, use your full size pattern to mark the ends and center of each of the sixteen 8.2 inch chords around the arc.  The sixteenth chord should end short of the point where the right hand end of the arc intersects the edge of the material.  Use a straightedge to draw solid lines from the edge of each chord to the center of the arc.  Also draw dashed lines from the center of each chord to the center of the arc.

 

4.  Use a straightedge to draw solid lines from the end of each chord to the center of the arc.  Also, draw dashed lines from the center of each chord to the center of the arc.

 

5.  Use your full size pattern to draw the outline of each of the 16 petals.  When positioning the pattern, the corners should just touch the larger circular arc and the point of the tab on the small end of the pattern should be on the dashed line you drew in step 4.  Draw the lines that cross each petal between the trapezoidal portion and the curved portion, and the lines between the curved portion and the pointed tab.

 

6.  Assuming that your layout looks similar to Figure 4, you can now cut out the reflector.  How you cut the material depends upon the material you are using.  If the material is Chloroplast® or cardboard, it can be cut with a utility knife.

 a.  Cut around the outside of the reflector.  You can make this cut either along the outside circular arc or along the chords.  We prefer to cut along the circular arc.
 
b.  Before cutting the curved section of each petal, we recommend punching or drilling a small hole at the junction between the trapezoid portion and the curved portion of each petal.  If you drew the second smaller circular arc in step 2, the junction where the hole is to be punched is where the circular arc intersects the edge of the petal.  These holes positively locate the point where each cut ends.  Note: The smaller circular arc is not shown in Figure 4
c.  Cut along the curved sides and pointed tab of each petal.  Also, cut around the large tab at the edge of the reflector
 
d.  With the cutout complete, score and bend the material at all of the edges that will be folds when the cooker reflector is assembled (the solid blue lines in 
Figure 4).  We do this by placing a straightedge along the edge, scoring the edge with a relatively blunt instrument such as a screwdriver, and then bending the edge while holding the straightedge in place to help force the bend to be at the proper place.  This step insures that the bends will be at the proper place when the cooker is assembled.

Figure 4: Layout of EB Hypar 42 Cooker Reflector

4. Layout and Cutout the Center Disk

 

All of the petals have a tab that is attached to the center disk when the cooker reflector is assembled.  As shown in Figure 5, the center disk is not a true disk but a regular polygon with a side for each petal.  We chose a radius of three inches to the point of each side.  Lay out the disk by drawing a circle with 3 inch radius, then mark points every 22.5 degrees around the circle and draw chords between each pair of points.  Cut out the disk by cutting along the chords.

Figure 5:  The Center Disk

 

5. Attach the Reflective Material

If the material from which you cut the reflector in step 4 does not have a reflective surface, you will need to attach a layer of reflective material.  For our first prototype, we used Mylar® as the reflective layer.  The Mylar® was attached with spray adhesive.  Figure 6 shows the reflector with the Mylar attached.  The Mylar was cut about 2 inches larger than the reflector and folded over the outside edge as shown in Figure 7.  After attaching the Mylar® to the reflector, attach Mylar to one surface of the center disk.

 

For our next prototype, we plan to try using self adhesive reflective material instead of the Mylar and spray adhesive.  The self adhesive material will be a little more expensive than the Mylar and spray adhesive, but it will be easier and less messy to use and we think that we may be able to apply it with less wrinkles.

Figure 6: Reflector with Mylar Surface Coating

 Figure 7: Back of reflector

 

6. Assemble the Cooker Reflector

Start the assembly process by bending the reflector shown in Figure 6 and attaching the tab shown on the left edge on figure 6 to the opposite edge of the outer ring using paper fasteners..  Then attach the tab on the inner edge of each of the petal to one of the edges of the center disk with a paper fastener.  We found that the assembly process is much easier if two people work together.

Support Stands for the Hypar Cooker

The EB Hypar Cooker requires a support stand to hold the cooker reflector and the cooking pot.  We used two different stands for our first two Hypar 42 cooker prototypes.  We also have used other stands with earlier 42 inch cookers which could be adapted to the Hypar 42.  Each type of stand has its own advantages and disadvantages.  I will describe each of the stands separately.  The stands are still a work in progress.

Basic Hypar42 Stand

We like the stand that we are calling the basic stand because it is easy and inexpensive to build.  It also provides very stable support for the cooker and pot.  This stand does not have a rotating mechanism. Therefore, the cooker is rotated by simply rotating the stand.   This stand is a larger version of the stand that is described in detail on the page "Stand for Parvati".  Therefore, not all details will be repeated here.

Figure 8:  Basic Stand with Hypar 42 Cooker

Figure 9:  Basic Stand from Rear with Hypar 42

Figure 8 shows a front view of the basic stand with a prototype EB Hypar Cooker mounted on it.  Figure 9 is a rear view of the same stand and cooker,

Figure 10:  Basic Stand With Parts Numbered


Table 2:  Parts for Basic Stand (Dimensions Inches}

Figure 11:  Part 7,  Brace Used Between Parts 1 and 3
Figure 12:  Part 5, Pot and Cooker Reflector Support

Figure 10 is a drawing of the basic stand with the parts numbered for easy identification.  Table 2 gives details about the parts identified in Figure 10.


Parts 1-4 and 7 are made of wood.  Almost any scrap wood that you have available will do.  For the first prototype, Part 1, and the two part 2's were cut from a piece of 2 by 3.  Part 4, the two part 3's, and the two part 7's were cut from 1.5" by 0.75" wood.  The part 7 braces were not used on the smaller Parvati stand, and are not shown on figure 8, but were added to the Hypar 42 stand.  They are attached diagonally at the intersection between parts 1 and 2.  they are partially visible in figures 8 and 9.  The braces used on the first prototype and seen on figures 8 and 9 seemed a bit short so the length was increased in table 2.


Part 5 supports the pot as well as the cooker reflector.  Figure 12 shows the finished part 5.  It is fabricated from a 3/4" wide by 3/16" thick aluminum or steel bar.  If a vice is available, it is not too difficult to make the four 90 degree bends shown on figure 12  .Part 6 is attached perpendicular to part 5 as shown on figure 10.


When attaching part 5 to the top of posts (3),  we recommend that it be placed near the front of the top of the posts rather than in the center as shown in figure 10.


Pipe Bushings: not shown on figure 10 but can be seen on figure 8.  We placed short pieces (3 to 4 inches long) of 3/4" internal diameter pipe over the bar (part 5) where it passes through the cooker reflector.  We had some scrap 3/4" copper pipe so we used it, but any 3/4" id pipe will do.  Also, while not absolutely necessary, we found that the cooker reflector keeps its shape better over time if some stops are are placed through the pipe on the inside edge of the reflector.  We drilled a hole through the pipe and bar and placed a bolt or cotter pin through the hole to keep the reflector from moving inward over time.



Figure 13: Sun Altitude Adjusting Mechanism.

Sun Altitude Adjuster:  The altitude adjusting mechanism is not shown on figure 10 or included in table 2.  This mechanism probably can be seen best in figure 13, although it also is visible in figures 8 and 9.  It consists of two L shaped brackets, an adjusting rod, and a clamp.  We made our 4" L brackets from scrap aluminum we had in the workshop, but 4" corner braces would work just as well.  As can be seen in figure 13, one L bracket is attached to the center of the rear bar (part 4).  The other one is attached on the center line of the bottom petal of the reflector about 11" from the outer edge of the reflector.  For the adjusting rod, we used a 28" long piece of 3/4" by 1/4" wood, but any piece of thin wood or metal would do.  The 28" length should work for most people, but if you live in an area where the sun may be directly overhead, it probably should be 2 to 3" longer.  One end of the adjusting rod should be loosely attached to the L bracket on the reflector with a bolt.  When the altitude is adjusted for the proper sun altitude, the other end of the adjusting rod is clamped to the rear L bracket.

Experimental Internal Stand

The stand that we used with our first prototype EB Hypar cooker still is in the experimental stage.  It works, but it has some features that we will be perfecting and reporting on in the near future.  This stand can be seen in Figure 1.  The base is similar to that used with the PVC stand and consists of two crossed pieces of 2 by 4 with a vertical piece of 3/4 inch iron pipe attached with a flange.  A post made from 1 inch OD square tube fits over the vertical pipe.  The square tube extends through a slot in the bottom of the cooker reflector.  The supports that hold the reflector and cooking pot are attached to the upper end of the square tube.  The mechanism for adjusting the vertical position of the reflector works but we think that it would be difficult for some do it yourselfers to build.  Therefore, we still are working on this mechanism.

 


 

Sawhorse Type Stand

The first Stand we built was an A Frame sawhorse style structure.  This frame is shown in Figure 8 with an EB42T Cooker Reflector.  Figure 8 also shows the pot and pot holder. This frame consists of the two A Frame ends with reinforcing strips between the legs.  The pot holder is made from 3/8 inch diameter rebar and bent to form a circle around the pot.  The ends of the pot holder attach to the A frame.

 

The sawhorse type stand probably is the most stable stand we have made.  However, to rotate the cooker as the sun moves, one has to physically turn the whole stand.  Also, it is heavy and takes up a lot of room in storage.

Figure 8:  EB42T Cooker With Sawhorse Stand and Pot Support

 

Lightweight PVC Pipe Stand

The Sawhorse type frame was very strong and stable even on windy days.  However it proved to be cumbersome to move from place to place and to store.  Therefore we replaced it with a lightweight portable frame similar to the one that had been used with the EB800D.  For a more detailed description, see the section on the EB800D.  This frame, while not as stable as the sawhorse stand in windy conditions, is quite stable if the crossed 2 by 4 base is large enough.  Figure 9 shows the PVC stand and a newly designed potholder before the reflector was added.  (Note: Figure 9 shows the stand used for the EB 800D which is smaller than the EB 42T stand but otherwise identical and was included because it shows an alternate potholder structure.)   A piece of 1 inch iron pipe is mounted vertically to the 2 by 4 base with a flange.  The 1¼ inch PVC pipe fits over the iron pipe and rotates around it.  A set screw is used to prevent the stand from rotating after it is set in the correct direction.

This stand is lightweight but proved to be a bit weak when used to support an EB 42T.  Over time, the sides started to droop a bit and braces were added to stiffen the structure.  However, the EB Hypar cooker, when constructed using plastic flute board, is much lighter than the EB 42T thus this stand may prove adequate to support it.  Also, we are considering making this structure from other types of pipe.

Figure 9:  PVC Pipe Stand with Potholder

Conclusion and Future Plans

The EB Hypar cooker has been designed and a 42 inch sixteen sided prototype has been constructed.  Several meals have been cooked in the prototype.  It cooks very well and is lightweight, and easy to move and store.  We believe that most do it yourselfers should find it relatively easy to build.  To make it easier for them, we plan to make a full size pattern for one petal available at a nominal cost.

Some plans for future include:o edit text

The EB Hypar cooker has been designed and a 42 inch sixteen sided prototype has been constructed.  Several meals have been cooked in the prototype.  It cooks very well and is lightweight, and easy to move and store.  We believe that most do it yourselfers should find it relatively easy to build.  To make it easier for them, we plan to make a full size pattern for one petal available at a nominal cost.

Some plans for future include:

1.  Conduct some instrumented tests on the 42 inch sixteen sided prototype.  These tests will involve putting a known amount of water in the pot and using our data acquisition system to record the temperature of the water as the sun heats it.

2.  Continue perfecting the support stands.

3.  Build another 42 inch sixteen sided prototype.  We plan to make a video of this construction process.

4.  Calculate the required dimensions and then construct a larger prototype.  It probably will have a 1.2 meter diameter.  Computer programs have been written that will calculate all dimensions needed for any size prototype using either English or Metric units.

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