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Descriptive and Background Information

4/27/2007 10:56 AM

Introduction

The Earthbound (EB) Cooker is a new solar panel cooker.  Some design features are similar to those used in other cookers, but the overall design is different from any existing cooker that the authors are aware of. The basic shape is that of a single cone, a double cone or a triple cone with the cooking pot located near the mouth of the cooker.  Locating the cooking pot near the mouth of the cooker allows the cooker to heat the bottom and sides of the cooking pot.

 

In a solar panel cooker, the pot is the target for the reflected solar power.  The EB cooker is designed to focus all of the reflected energy onto a spherical target.  Usually the pot is cylindrical and the cooker is rotated about the pot as the sun angle changes.  The aspect ratio of the cylindrical pot changes as the sun elevation angle changes.  Therefore, the spherical target was chosen for use in the design process because it’s aspect ratio does not change as the sun angle changes.  All of the reflected power will hit the cooking pot for all sun angles with the cooker pointed directly at the sun, providing that the smallest dimension of the pot is equal to or greater than the diameter of the target sphere.  Usually, one should choose a cooking pot with smallest dimension larger than the diameter of the target sphere to insure that all of the reflected energy will hit the pot even when the cooker is pointed a few degrees from the sun.

A cross section of the solar cooker is used in the design of the cooker.  In cross section, the target appears as a circle.  Figure 1 shows cross section views of single cone, double cone, and triple cone EB cookers.  For example, consider the single cone cooker (Figure 1a).  The sides are at an angle of 45 degrees.  The profile of the cooker consists of three segments, a left side, a base, and a right side.  The double cone cooker consists of five segments, and the triple cone cooker consists of seven segments.  If the cooker diameter (D) is known, the target diameter (d) can be determined by dividing D by 3 for the single cone cooker, by 5 for the double cone cooker, and by 7 for the triple cone cooker. 

pfigure_1cropped.jpg

Figure 1. Cookers and Pots

 

    Single Cone              Double cone                 Triple Cone

Cone diameter              D=3d                         D=5d                                    D=7d

Cone area/target Area      9                                  25                                        49

Energy Concentration       9                                  25                                       49

 

Earthboundtech has chosen to initially promote double and triple cone cookers.  However, the single cone cooker provides a concentration ratio of 9, which is larger than that from many other panel cookers.  The EB Ring Cooker is a variation on the single cone cooker.  In the EB Ring Cooker, the concentration ratio can be increased above 9 by considering the cone as a ring and increasing the ring diameter leaving a gap between the pot and the ring.  The EB Ring Cooker will be developed and tested later.  The extension to a 4 level cone is not considered as part of this work.

 

Earthbound Solar Cookers are designated as: EB followed by the cooker cone diameter in inches and by a D or T indicating a double or triple cone.  For example, the EB-42T has a diameter of 42 inches and is a tripe cone design while the EB-30D has a diameter of 30 inches and is a double cone design. 

Comparison With Other Cookers

The DATS1 and Parvati2 cookers are double cone cookers with the cooking pot located at the base of the inner cone.  When designing the earthbound cooker, the pot was moved from the base of the inner cone and located such that the center of the pot is at the center of the outer cone level.  This was done to focus more of the power from the sun on the bottom and sides of the pot rather than the top and sides of the pot.

 

Parabolic cookers also can be designed to heat the bottom and sides of the cooking pot.  Any of the many support structures that have been developed for parabolic cookers can be used with EB Cookers.  However, EB Cookers have some important advantages relative to parabolic cookers.

  1. The EB Cookers are safer than parabolic cookers because they do not have the dangerous point focus that the parabolic cookers have.
  2. The EB Cookers are easier to build than parabolic cookers because the only curves that need to be cut for the EB Cooker are circles.  When building parabolic cookers, one must work with parabolic curves.
  3. The EB Cooker may distribute the heat more evenly across the pot then parabolic cookers.

Power vs. Cooker Diameter

When building a cooker, one of the first decisions is how large it should be.   Larger cookers will cook faster, but smaller cookers usually are less expensive to build and are easier to transport and store.  We usually favor larger cookers because we believe that solar cookers will be more widely accepted if their performance is closer to that of the typical kitchen stove.  However, if you don’t mind waiting longer for the food to cook, food cooked in slow cookers can be especially delicious.

 

Oven temperatures are usually considered slow below 300 degrees F, moderate at 350 degrees F,  hot at 400 degrees F and very hot at 450 degrees F. which follows the temperature scale on a Good Cook™ Oven Thermometer . Most non-parabolic solar panel cookers fall into the slow category while EB Cookers and parabolic cookers are in the moderate to fast category.  Both EB Cookers and parabolic cookers can be in the fast category if they are large enough.

 

A spreadsheet was developed to show how much solar power circular aperture cookers intercept.  When using the spreadsheet, one has to assume a value for the sun’s intensity (Solar Insolation).  Two examples are shown here.  The first example uses 1000 W/m2 and the second uses a more conservative 700 W/m2.

Solar Power Intercepted by Circular Aperture Cookers
(Independent input variables are in bold font)
Example 2
Solar Insolation 1000 W/m2 92.90 W/ft2 0.645 W/in2 317.02 Btu/(ft2*hr)
Ambient Temperature 68 Deg F 20 Deg C
Diameter (in) 12 18 24 30 36 42 48 54
Diameter (cm) 30.5 45.7 61.0 76.2 91.4 106.7 121.9 137.2
Area (in2) 113.1 254.5 452.4 706.9 1017.9 1385.4 1809.6 2290.2
Area (m2) 0.1 0.2 0.3 0.5 0.7 0.9 1.2 1.5
Watts 73.0 164.2 291.9 456.0 656.7 893.8 1167.5 1477.6
Btu/hr 249.0 560.2 995.9 1556.2 2240.9 3050.1 3983.7 5041.9
Btu/min 4.1 9.3 16.6 25.9 37.3 50.8 66.4 84.0
Minutes to boil the amount of water below (British units), assuming no losses in the system;
One cup 17.4 7.7 4.3 2.8 1.9 1.4 1.1 0.9
One pint 34.7 15.4 8.7 5.6 3.9 2.8 2.2 1.7
One quart 69.4 30.8 17.4 11.1 7.7 5.7 4.3 3.4
Minutes to boil the amount of water below (metric units), assuming no losses in the system.
1/4 liter 19.1 8.5 4.8 3.1 2.1 1.6 1.2 0.9
1/2 liter 38.3 17.0 9.6 6.1 4.3 3.1 2.4 1.9
One liter 76.5 34.0 19.1 12.2 8.5 6.2 4.8 3.8

Example 2
Solar Insolation* 700 W/m2 65.03 W/ft2 0.452 W/in2 221.91 Btu/(ft2*hr)
Ambient Temperature* 68 Deg F 20 Deg C
Diameter (in)* 12 18 24 30 36 42 48 54
Diameter (cm) 30.5 45.7 61.0 76.2 91.4 106.7 121.9 137.2
Area (in2) 113.1 254.5 452.4 706.9 1017.9 1385.4 1809.6 2290.2
Area (m2) 0.1 0.2 0.3 0.5 0.7 0.9 1.2 1.5
Watts 51.1 114.9 204.3 319.2 459.7 625.7 817.2 1034.3
Btu/hr 174.3 392.1 697.2 1089.3 1568.6 2135.0 2788.6 3529.3
Btu/min 2.9 6.5 11.6 18.2 26.1 35.6 46.5 58.8
Minutes to boil the amount of water below (British units), assuming no losses in the system;
One cup 24.8 11.0 6.2 4.0 2.8 2.0 1.5 1.2
One pint 49.6 22.0 12.4 7.9 5.5 4.0 3.1 2.4
One quart 99.1 44.1 24.8 15.9 11.0 8.1 6.2 4.9
Minutes to boil the amount of water below (metric units), assuming no losses in the system.
1/4 liter 27.3 12.1 6.8 4.4 3.0 2.2 1.7 1.3
1/2 liter 54.6 24.3 13.7 8.7 6.1 4.5 3.4 2.7
One liter 109.3 48.6 27.3 17.5 12.1 8.9 6.8 5.4

Note: In practice the time required to boil water will be much larger than that predicted in the table.  This is because of losses in the system which are not included in the tableSome losses that will increase the actual time required to boil water are:

1.  Reflective losses at the surface of the reflector.  Usually the reflectance of the reflective material is between 0.8 and 1.0.

2.  Transmission loss from the surface of the pot to the water.

3.  Hear loss from the pot into the amibent air.  Surrounding the pot with a greenhouse container such as a cooking bag or glass container will reduce the heat loss but not eliminate it.  This heat loss is proportional to the temperature difference between the pot temperature and the ambient air.

The smallest cookers may never boil water.  The temperature will increase until an equilibrium is reached where the heat folw into the water is equal to the heat loss.

Target Size vs. Cooker Diameter

Table for double and triple cone.  Pot size=cooker diameter divided by 7 for triple cone and 5 for double cone.

 

Double cone cooker  Pot size vs Cooker Diameter

 

 

 

 

 

 

 

Diameter in

24

30

36

42

 

Pot size in

4.8

6

7.2

8.4

 

 

Triple cone Cooker Target Size vs. Cooker Diameter

 

 

 

 

 

 

 

 

 

 

 

 

Diameter

24

30

36

42

48

54

 

Target Size

3.43

4.29

5.14

6.00

6.86

7.71

 

Double Cone Design

Double cone cookers are adequate for smaller size cookers.  Our first double cone prototype was an EB‑30D.  Its design will be described in a separate web page.

Triple Cone Design

We recommend the triple cone design for all except the smallest EB Cookers.  The part dimensions for triple cone cookers for cooker diameters of 24, 30, 36, 42, 48, and 54 inches have been calculated and are included on the EB42T web page

 

 

A comparison of EB30D and EB42T cross sections is shown in Appendix I.

Appendix A  Cooker Geometry

A comparison for the cross section profile of the EB30D and EB42T is shown in the Figure  A1

EB42T30DComaprison.jpg