Before the last server crash, I had several pics posted, but canít find them now Ė Iíll see if I can dig them up from photobucket.
Never used the parabolic or Fresnel approach, but weíve used a couple box-style cookers for 6-8 years now. Havenít lately as weíve been silly busy with stuff, but it does work surprisingly well. Made our first one out of wood and it worked great until my then-young son left it open in the rain for two days and it molded irrecoverably before I found it.
First one, made from wood scraps, a mirror, and plain styrofoam insulation. On a cloud-free day it would usually reach 370-380 degrees and more than once broke 400:
Used a table-leg bracket and eye-bolts for the mirror brace and adjustment:
Closed. Prop & all fit down inside for storage:
Donít have pics of the second one, but when I tried to make better I ended up making it worse. Made it larger and heavier, angled the top to face the sun more directly, and insulated it much thicker with cellulose blow-in type insulation. Turned out to be too much mass and it never got above 200 degrees. Lesson learned Ė mass is the enemy. Obvious once I thought about it, but I just thought about it too late.
Third one, I made out of an old fire-alarm control backbox. Sealed up the conduit entry holes with sheet metal and epoxy, insulated it with two layers of ductboard, and added the mirror & prop similar to the first one. Actually, the mirror is
the mirror from the first one; not an exact fit for this new one, but close enough that I used it. This one reaches 300 pretty regularly, but has never broke 375 afaik. The construction makes it much more durable than the first two wood ones, but more mass to try & heat up and makes it heavier to carry around as well.