Solar ovens have been a great interest to our sponsor, Earth Trek Gear. Solar cooking’s appeal comes from being an environmentally friendly method of cooking. The lack of fuel means the oven can be used in environments where other methods are impractical. A solar oven is a convenient device for campers because of the lack of fuel.
Earth Trek Gear is a seller of hiking and camping gear. There was a previous design that was sponsored by Earth Trek Gear, but there were concerns that the previous design did not heat up quickly enough. This led to The Trekkies being asked to improve on the design with a slightly increased production cost.
The design for the solar oven design is a set of seven parabolic leaves of plastic covered in vinyl used to reflect the sun onto the pot. The leaves are manufactured through vacuum forming in order to easily create an accurately curved piece. The size of the combined leaves is an increase over previous design and allows for more heat to be reflected into the pot. The leaves are connected with bended flanges on the sides that are held together through pieces of edge trim.
The pot is covered by a turkey cooking bag in order to reduce the heat losses to the ambient air. The pot is raised off of the ground with a tripod stand in order to allow the focal point of the solar oven to be higher. The stand also allows for the mirror to be adjusted around it in order to achieve the highest rate of heat transfer to the cooking pot. The stand is made of three aluminum rods and is held together in the center intersection with a collar made from injection molding.
The assembly process for the final design was kept to a minimum number of procedures. The steps were broken down into two parts, the reflector and the tripod. The assembly of the reflector consisted of mating each of the panels together at the molded flanges on their sides with the strip of spring core edge trim. Assembly of the tripod was in three steps. The first step was mating the poles at the intersection point of them and the tripod collar with aluminum rivets. The second part was to permanently mate the bottom of the poles and the feet together using an adhesive. The third was inserting the trivet in to the open tops of the tripod. Once all of these were done the reflector and the tripod were then joined together by sliding the reflector over the trivet until it rested on the legs of the tripod.
The manufacturing process consists of only a few steps; thermo forming the reflector panels, applying the reflective material to the panels, 3D printing the smaller components, preparing the tripod poles, and constructing the trivet.
Before the vacuum forming could begin, a negative mold was created. The mold was made of wood and shaped using a CNC mill. The wood was then sanded and sealed to provide a smooth surface. The vacuum forming process was continued by heating the plastic until it was soft and pliable at around 320°F. It was then laid over the top of the negative mold with the vacuum introduced to conform it to the mold and finalize its shape. Once the panels cooled the reflective vinyl was added to them.
The vinyl comes with the adhesive pre-applied in order to remove a step from manufacturing. The panels are first cleaned and degreased with acetone and then lightly coated with the adhesion promoter. Once the reflective material had been roughly cut to its final size the backing was removed and the vinyl was lightly laid over the panel. Using a cotton rag to protect from scratches the vinyl was conformed to the curved panel.
The next step in manufacturing was the printing of the smaller components, which consisted of the alignment tool, the tripod collar, and the tripod feet. All of these components are designed using a 3D modeling software compatible to the printer being used. This process was meant for the prototype of the final design and for small scale manufacturing. When the product goes to large scale manufacturing, 3D printing can be substituted for injection molding. Preparing the tripod poles consisted of cutting them to length and drilling holes at the attachment point of the tripod collar. The trivet was an off-the-shelf stainless steel eight inch diameter trivet. The legs were heated and bent to the match the angle on the legs of the tripod. Keeping the manufacturing down to only five parts helped to keep the price of manufacturing down.
For testing a quart of water was heated in a small pot and covered with an oven bag. A metal pot was used and a digital thermometer was used to collect temperature data in an effort to reduce unneeded variation. A pot of water was placed in the focal point of the reflector dish and water temperature was observed at regular intervals. During final testing, the solar oven was not entirely rigid. The resulting light scatter caused a longer boiling time than was observed in the presentation. The oven took 90 min. to raise tap water (55°F) to boiling temperatures (200°F). However, during the presentation, with similar weather, tap water was boiled in 45 min. The faster boiling time is attributed to braces being glued to the panels; braces increased rigidity and reduced light scatter.
Proof of Concept
For a proof of concept the Trekkies created a version of the reflective metal surface. This was used in order to test the efficiency of the reflective Mylar on the aluminum. The test was to heat a pot filled with two cups of water. The temperature of the pot and surfaces of the metal were measured using an infrared temperature gun and recorded in ten minute intervals. This was initially tested by heat lamps in order to imitate the sunlight. This was necessary because of the sunlight available in the season. Unfortunately, the lamps were not strong enough to heat the oven a noticeable amount.
The oven was then tested outdoors using the minimal sunlight that was available. The pot was hung from a board and the metal sheet was angled largely in order to match the angle of the sun in winter. Despite the season, the sunlight was working better than the lamps. The water in the pot was raised from 40 Fahrenheit to 150 Fahrenheit in 2 hours. Taking the season of the year into account these results showed promise for tests in more a optimal time of year.