This instructable describes how we built a solar cooker. The basic principle is to focus light using a Fresnel Lens. We managed to reach temperatures around 350ºF and to fry eggs and melt cheese and chocolate.
We carried out this project during the last 2 days of school and we were a team of 12 students and 3 teachers at a school in Bermuda. Our challenge was to create a solar cooker made entirely out of recycled materials. To achieve this I gathered materials for about 30 days. These were discarded materials from the school, from the road-side or from the local dump. A few days before the start we also received a generous donation from a local Solar Panel company (BeSolar Bermuda). They gifted some of their precious waste! :) Aluminium bar cuttings, plastic panels with reflective metal coatings and some wood and insulation left overs. One company's rubbish another team's treasure!
Our goal was as much to experiment and learn as it was too cook delicious food (like fruit-chocolate fondue or grilled cheese sandwiches!).
Step 1: Gathering the Materials
Our scrap heap had a bit of everything:
- cardboard, plywood, metal, chicken wire, lampshades,
- somewhat spherical metal parts
- a Pallet box with wheels (from another instructable)
- Tin foil
- reflective tape, duck-tape, nails, screws, etc
- Perspex sheets
- Glass sheets (TO BE HANDLED WITH EXTREME CARE!)
The main tools we used were:
- Hack Saw
- Wood Saw
- Exacto Knifes
- Hammers & Pliers
Safety was mainly about:
- Cuts (gloves)
- Sharps (gloves)
- Eye protection from high intensity light (sunglasses & welding goggles)
- Burns (High temperature mitts)
Using our materials, we first created a couple of solar oven prototypes (of a more traditional kind). These were a box oven and a parabolic cooker made with an umbrella (see pictures). These prototypes demonstrated the principles of solar cooking ... but didn't reach temperatures beyond ~80ºC (~175ºF).
The next step involved finding our pièce de résistance! The Fresnel Lens!
Step 2: Finding a Fresnel Lens
To build powerful parabolic reflectors one needs a very precise geometry, no wrinkles on the reflector and direct sunlight. To build powerful box cookers one needs good materials and good insulation (Paying very close attention to thermal leaks). Instead, we decided to go with a completely different design: Concentrating sunlight with lenses.
The light from the sun carries energy. At the top of the atmosphere, each square meter receives around 1300W of power. Even on a clear day, the air in the atmosphere absorbs a good part of it, so at sea level we get around 900W/m^2 [this is called solar irradiance]. That's like having a microwave running at full power on each square meter of land and sea. However, this energy is very diffuse. So to burn or cook you need to concentrate those 900W on a smaller area.
For this we decided to use a Fresnel Lens. It's just like a normal lens, but with a clever design that saves space and weight. Large Fresnel lenses are used in lighthouses, like the one in the picture. But a smaller and lighter version can also be found inside back projection TVs. These are some old TVs that are hard to find today, but now and again people will discard them for the newer and slimmer LED ones.
The local recycling and trash-burning plant kindly agreed to show us their installation and to let us try to reuse some of their electronics. Fortunately we found a rear projection TV that very day!
As seen in the pictures, the lens is a rectangular plastic layer where the image appears. We simply unscrewed the frame which held it on the TV and took it apart. The screen is made of 3 layers. Two appear to be linear lenses (focus light onto a line) and one a spot lens (focuses onto a point). We picked the middle layer which contained the spot lens (which focused light onto a point the size of a coin).
Step 3: Framing the Lens
The lens is made from a thin plastic surface with concentric circles etched onto it:
It's pretty flimsy so we needed to build a frame for it (to keep it straight and make sure the focal point formed around a single point. When the lens bends the lens loses focus.).
We did some brainstorming and drew some ideas. Our first frame to hold the lens was made of chipboard but we soon discovered that it was not strong enough. We decided to build it out of aluminium instead (they were sawn to size and held together with angle brackets).
Our first design idea was to concentrate the light, bounce it off a mirror and make it focus underneath the pan. So we set out the test it's effectiveness.
Step 4: Testing the Solar Cooker
We tried our first idea which was to bounce the sun off a mirror and have it focus under the pan. This proved to be very slow and ineffective.
The main reasons were:
- the imperfect reflectivity of the mirror,
- the constant need to adjust the angle
- the time it would take to heat the pan before starting to cook.
We subsequently decided to fry an egg by focusing the light on it directly (holding the lens with our hands). This worked much better and the egg was done after about 15 minutes (even with a somewhat hazy sky). Given this experience we decided to try cooking on the box by focusing the light directly on the food.
Step 5: Final Setup and Cooking
The lens will easily make cardboard and wood catch on fire. And the same applies to sandwiches and cheese as we soon found out! So we learnt that instead of focusing on the food, we simply had to make the light converge on a larger area. Usually the size of the pan or slightly smaller. With this, and a little bit of moving the pan around we managed to cook an egg pretty quickly and easily. We also made some chocolate fondue with fruit! yum!
Some of the challenges are adjusting the tilt and attitude of the lens and preventing it from moving when it's windy. Overall, it was great fun to build something entirely out of recycled materials and to be able to cook some food so quickly.