Originally Posted by Bolster
Question for Quake (or whomever): If your solar oven isn't full, does it make sense to put in some mass to fill the empty space, or no? For example, if you had room left over, imagine you put a brick in the oven beside your food, would that help retain heat? (Sort of like how we keep our freezers full of ice, if possible?) Or would you be wasting energy trying to heat the brick, that could be cooking your food?
Just the opposite; pretty much as RedHaze says. Mass is the enemy. A brick will hold btu's better than empty space will, but it also 'consumes' or absorbs btu's in the process of heating up; btu's that would have otherwise gone toward something productive (cooking).
That's the exact mistake I made with my second home-built solar cooker. It was better insulated, had a better glass-to-space ratio, more reflected light into it, and better angling of the glass. It absorbed btu's better, but there was just so much of it that those btu's got spread out over a lot more mass, making it less hot than the lighter, simpler ones I've made. The added mass does kind of seem like a good idea at first glance, but think of your normal kitchen oven, cooking a 15-pound turkey; and adding a dozen bricks in the oven at the same time. It will definitely make the oven retain heat longer once turned off, but it will also mean it either takes more time to cook the turkey since the heating element now has to cook the turkey AND the bricks. Only way to compensate for the bricks and cook in the same time frame is to turn up the oven to a higher setting; and with a solar cooker, the sun is the heating element, so there's just no 'turning up the setting'.
Mass is the enemy in a solar cooker.
All that said, I have seen some that were made pretty massive, out of mud, adobe, etc. But they were near equatorial in location (maximizing the sun's input), and didn't get as hot as a lightweight box cooker either; used mostly in very remote areas for low-temp, slow cooking of things like beans, stews & such.