As Doug said, solar cookers can be effective and energy-saving.
Here are instructions from wikiHow how to build these.
Method 1: Lightweight Solar Oven
- Place a cardboard box inside a larger cardboard box and fill the gap with shredded newspaper, which will act as an insulator.
- Line the inside of the smaller box with black construction paper, to absorb heat.
- Cover each piece of cardboard with reflective material such as foil (secure it with rubber cement or tape).
- Attach each reflector to the top of one side of the box (use glue or staple).
- Prop each reflector up at around a 45 degree angle.
Position the oven in full sun, place food in the smaller box, and wait for it to cook.
Method 2: Heavy Duty Solar Oven
- Cut a large metal drum in half vertically with a jigsaw (e.g. oil drum).
- Clean the inside of the half-drum thoroughly with a de-greasing soap (use a scrub brush).
- Size and cut three pieces of sheet metal to contour to the insides of the half-drum. You need one large rectangle for the curved interior and two half-circles for the end pieces.
- Attach the sheet metal to the inside of the drum.
- Paint the inside of the oven with a reflective paint rated for barbecues.
- Create a continuous metal lip around three of the four top edges of the oven. This will hold the glass top (which you will slide in and out through the fourth, open side) in place.
- Flip the half-drum over and apply spray-on insulation to the outside walls.
- Attach a base to the bottom of the oven (ex. a piece of wood, a square aluminium frame with wheels, etc.).
- Drill drainage holes in the bottom of the oven. This will allow liquefied steam that drips down the sides to escape the oven.
- Slide a custom-sized sheet of tempered glass into the metal lip.
- Insert a magnetic thermometer.
- Place a thin aluminium grill along the bottom (optional).
Test your oven’s heat capacity on a sunny day. Though you can reasonably expect a max heat of between 250 and 350oF (90 and 175oC) depends on the size, materials, insulation and weather.
Method 3: Solar Veggie Steamer
- Use 2 cardboard boxes with 1 inch (2.5 cm) different dimensions, 5 cardboard panels, one vastly larger than the rest, Styrofoam, transparent wrap, aluminium foil, black craft foam, black Tupperware (with lid), water, your favourite veggie, glue, and 5 sturdy sticks.
- Place the large box onto the large cardboard panel, glue in place. Place the smaller box inside the larger box, glue in place. Trim off any difference in height.
- Line the empty space in between the 2 boxes with Styrofoam (don't use glue). Line the inside of the small box with 2 or 3 layers of the black craft foam, then glue together. Completely line 4 cardboard panels with the foil and glue the foil to the cardboard. Try to avoid creasing the foil.
- Glue the panels at a 45 degree angle to the box.
- Cut a rectangular hole just large enough to slip the Tupperware through on one side of the cooker.
Take the transparent wrap and stretch it over the entire mouth of the solar cooker.