"Solar Funnel Cooker" / solar cooking research at BYU


Steven E. Jones
Department of Physics and Astronomy
Brigham Young University
Provo, UT 84602 USA

A funnel-shaped solar cooker has been developed to concentrate the energy brought by sunlight into a cooking pot or jar, then trap the heat using the greenhouse effect. A simple system allows pressure-cooking to increase the cooking rate while releasing steam. In this way, solar fusion energy can be used for cooking and pasteurizing water. The Solar Funnel will be particularly useful in sunshine-rich countries where the vast resource of the sun's fusion energy is appreciated.


The Parabolic Concentrator
An early type of solar cooker uses a parabolic reflector as shown in Figure 1 to concentrate sunlight to a point. This parabolic concentrator was first tried in the 1860's but is difficult and expensive to build and can only cook small quantities of food. There is also a significant safety problem associated with concentrating sunlight to a point, where damage to eyesight can occur quickly. In Bolivia, fires have accidentally been started when such parabolas have been stored near wooden structures.

---->fig 1
At BYU, we have built an inflatable parabolic cooker about 1.2 meters in diameter in order to make an inexpensive cooker of this type. We using aluminized plastic sheeting on one side and clear plastic sheeting on the other, sandwiched between two pieces of plywood with a 1.2-m circular hole cut into each. A tube was inserted between the plastic sheets to permit inflation. When inflated, a reasonably good and inexpensive parabolic reflector is formed. Sunlight on January 11, 1996 in Provo, Utah was focussed to a spot about 6 cm in diameter at a focal length of about 2 m. The concentrated sunlight ignited paper after a few seconds in the spot. Durability, portability and safety remain as deterrents to widespread use of this type of cooker.

I have also conceived of replacing the curved parabolic reflecting mirror with a grooved but flat Fresnel parabolic reflector, as shown in Figure 2. This reflector is effectively a series of concentric reflecting conical sections of short-height, nested on a plastic plate. The resulting reflector is a flat reflector that is designed to concentrate sunlight to a diameter of about 10 cm rather than a point for safety reasons (and to avoid the likelihood of burning food). A wire mesh is mounted above the reflector so as to identify the location of the hot- spot, and a frying pan or cooking pot would be placed on this mesh-grill. A 1.1-meter diameter Fresnel reflector of this type would deliver about 500 Watts on a frying pan, at mid-day near the equator. The Fresnel Fryer is under development at BYU.

--> fig 2
The Solar Box Cooker

The solar box cooker uses a glass plate to admit sunlight while keeping infrared radiation ("heat") effectively trapped inside the box. A pot or jar is placed inside the box, preferrably a dark color to absorb the sunlight. The cooking pot radiates heat mostly as infrared radiation while the glass or plastic plate on top of the box inhibits cooling by the outside air. The solar box oven thus formed dates from the 1760's, but most designs have been slow-cooking and expensive.

In the mid-1970's, Barbara Kerr in the U.S. developed a solar oven that can be made from cardboard and aluminum foil along with a glass or plastic window. An adjustable reflecting plate above the glass plate concentrates additional sunlight into the oven (see Figure 3a). This type of box oven is promoted by "Solar Cookers International" ( SCI, formerly Solar Box Cookers International), a nonprofit organization with headquarters in Sacramento, California. It has found acceptance in third-world countries throughout the world. A portable SCI solar box cooker is being sent to Bolivia in May 1996 for field-tests on the Altiplano, where sunlight is abundant but fuel wood is extremely scarce. A modification shown in Figure 3b permits additional concentration of sunlight by adding three more reflecting panels.

-->figs 3a, 3b
Reflective Cookers/ The Solar Funnel

A weakness of the solar box cooker is that sunlight is admitted only through the top window, while heat escapes from all sides of the box. Good insulation is therefore required along the sides and bottom of the box, and typically two or three nested boxes (often with crumpled paper stuffed between boxes) are used to improve the insulation. Cooking is also slow, typically requiring rotation of the box to follow the sun over a 1 - 3 hour period.

While studying the solar box cookers in Figure 3, I realized that one could place a clear glass jar (or oven-safe plastic bag) at the bottom of the reflectors, which would substitute for the box altogether, and allow sunlight to enter from all sides except just the bottom, which is insulated. A black pot or jar inside the clear jar or plastic bag would serve as the cooking vessel.

By using a 60-degree cone-shaped reflecting funnel, sunlight would be concentrated along the axis of the funnel at the bottom, where the cooking pot or jar would be placed, as shown in Figure 4. Sticks or rope or cloth should be placed under the cooking jar (inside the plastic bag) to provide insulation. Safety comes from the fact that the sunlight is concentrated along a line deep inside the funnel where the eyes cannot go without blocking the sun. If you put your arm up the bottom of the funnel, it feels warm in a hurry; but it won't burn you. This is much safer than concentrating sunlight to a point above a parabolic collector, for example. The Solar Funnel was born.

               \         /
                 \     /
--> fig 4          \o/
--> Start by cutting a 1-m diameter half-circle from cardboard (or thick plastic). Cut a concentric half-circle about 1-ft in diameter inside this. Glue (white glue + water 1:1 works well, just as Joseph Radabaugh said) Al-foil or emergency blanket to inside of cardboard. Then bring the two sides together to form a cone; hold with tape or 3 nuts and bolts. The cone does not get hot. Back to text.

The first Solar Funnels were made (at my home) cheaply from cardboard cut into a half-circle then covered with aluminum foil. The opening angle PHI of the funnel is related to the fraction F of a flat circle from which the cone is later formed as follows: sin (PHI/2) = F. White glue diluted about 1:1 with water was used to apply the aluminum foil to the cardboard.

--> For a half-circle, F = 1/2, so the opening angle of the funnel = 60 degrees. Our experiments show this is about right; 45 degrees is better if you want to keep moving the funnel to follow the sun. 60 degrees works for about 1.5 hrs, just point the funnel a little ahead along the sun's path.

We are exploring ways to make the solar funnel using aluminized mylar, laminated to another piece of mylar if necessary to make a self-supporting funnel. High-reflectivity is of course crucial to accelerate the cooking process. The funnel can be placed in a shallow hole in the ground to prevent winds from upsetting the cooker.

-->We have made a few of these mylar-funnels, and they are neat, I think-- light, roll-able, easy to form into a funnel. I plan to make a bunch from Al-ized mylar, soon. Costed these at about $2.50 each when made in large quantities. Cheap enough?

I have learned that SCI now sells a "Solar CooKit" which is somewhat similar in concept to the solar funnel described above. Figure 5 shows the geometry of this cooker, with five straight sides, which while clearly not as optimal as the funnel shape for concentrating sunlight will nevertheless be very useful. Two of these CooKits will also be taken to Bolivia in May for testing along with Solar Funnels and a solar box cooker.

-->These are now on their way to the Altiplano for field testing by the Benson Institute of BYU, along with a solar box cooker, and 2 solar funnels.


In sun-rich tropical countries like Bolivia, the sun's intensity on the ground is approximately 500 watts/m2 at noon. (In southwestern U.S., the figure is about 300 W/m2 at noon.) By assembling a funnel with a 60-degree opening angle to collect sunlight along an axis near the bottom of the cone, with a 0.5- meter opening at the top (chosen for ease of use and portability), we estimate heating of about 300 watts even after accounting for reflection and insulation losses. This should permit cooking in an hour or two, with perhaps one re-alignment of the funnel to follow the sun.

-->Check my numbers? Thanks.

In Provo, Utah, on January 11, 1996, the outdoor temperature was 7.6 C (46 F) at 12:30 p.m. The angle of the sun above the horizon was about 30 degrees (measured using a ruler casting a minimum shadow on the ground). Conditions were mostly sunny, with high clouds. A small can of baked beans (label was red and black mostly) was placed on cardboard (insulator) inside an oven- safe plastic bag and this was placed at the bottom of a solar funnel (laid on one side due to the very low angle of the sun) for 1.25 hours. A platinum-resistance thermometer was used to measure temperatures. At 13:45, the temperature on the can top was 97.1 C (207 F), to my surprise. The can was opened (with release of some steam) and the temperature at the top of the beans was 68.9 C. The beans were then stirred thoroughly, and the temperature in the beans was 61 C, showing that the beans had been warmed through rather evenly. I enjoyed a hot can of beans for lunch, heated using the sun's fusion energy. -->This was my first effort with the solar funnel.

On January 12, water was heated in a jar which had been painted black on the outside then placed inside a larger clear jar. The water reached boiling temperature in about one hour despite winter conditions in Utah. The metal lid on top of the tightly- closed blackened jar reached 109.2 C. Subsequent experiments in Utah demonstrated that the solar funnel always heated water significantly faster than either the "solar cookit" or the solar box cooker. Tests in Bolivia will follow.

We found that steam often builds up on the inside of the plastic bag or clear jar which is used to provide a "greenhouse effect" for the inner blackened jar. This steam tends to reduce the heating by the sun. So we developed a simple tube-on-disc that replaces the jar lid and allows steam to be carried out of the clear housing (Figure 6). In addition, we designed a simple cap to fit on top of this tube with a rim to permit circular weights to be added to the cap. This simple system allows pressure to build up in the jar to increase cooking rates. Every 10 C rise in temperature doubles the cooking speed. This pressure-cooking scheme should be particularly useful at high altitudes, such as the Altiplano in Bolivia, where cooking by boiling is retarded by the lower boiling temperature due to lower atmospheric pressure at the high altitude.
        / | \  = metal cap fits over tube; has rim to hold 
          |       heavy washers to allow pressure build-up
          |  -- plastic bag tied here; steam escapes tube
 _________|___________ = disc, replaces lid in Mason / Kerr jar
 --                 -- = rubber ring goes under disc; seals
Jar, painted black on outside, goes here, inside plastic bag.

In conclusion, it is possible that the people in sun-rich countries such as Bolivia may be the first to tap into useful fusion energy (from the sun) on a broad scale, effectively leap- frogging past other nations.

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