# Metal vs Cardboard: which liner for a solar oven would allow it to reach a higher temperature, and why?

Lets's say I make two solar ovens. They each have a set of reflectors angled to direct sunlight into a box topped with glass, and all other sides of the box are covered with insulation that can stand temperatures of ~300o F.

One oven's box is made of cardboard, and the other is metal. Both box interiors are coated with the same thin black paint. (The paint is thin because it is only here to change the albedo, and not the other material properties). For the sake of discussion, the sides can be 1 cm thick. Other than the lining, the ovens are identical, and are put out in the full sun with exactly the same conditions.

Which one reaches the hotter maximum/equilibrium temperature, and why?

(Or do they reach the same temperature?) The internal air temp. is being measured.

Intuition tells me it is the metal one. After all, a sheet of metal lying out in the sun is much hotter than a sheet of cardboard. However, I want the science principles:

The incoming sunlight must be either absorbed or reflected. Does the black coating control this entirely?

In equilibrium, the energy being absorbed by the box' sides must be equal to that being lost by the box' sides. If the thin black coating entirely controls the absorption, then both metal and cardboard transfer the same total energy to their surroundings. The only difference can be in the type of heat transfer. I believe the percentage of heat transer that is in the form of conduction is higher for metal than for cardboard. Air absorbs all of the heat from conduction, but radiation only imparts some of its energy to the air it passes through. Is this significant in determining the equilibrium air temperature? Is my first assumption incorrect?

• Both boxes will reach the same internal temperature because they both form what's called a "black body cavity". As the other answer noted, however, metal is less insulating and will therefore dissipate more heat away from the cavity. So you want to insulate the cavity in both cases. Aug 17, 2020 at 17:06