Solar radiational energy, after being absorbed by a dark pot in a solar oven, is transmitted by conduction and convection to cook food.
INSULATION, and REFLECTIVE RADIATION BARRIERS
Some materials do not conduct heat well, and are known as thermal insulators. For example, because of the trapped dead air spaces in it, wool does not conduct heat energy nearly as well as a solid metal. So heat energy can be trapped behind wool, which is what is happening when we wear wool clothing. Eventually heat energy will pass through wool, but much more slowly than if the wool insulation wasn’t in place. Insulation doesn’t allow heat energy to travel through it easily; it slows heat transfer down.
What prevents easy heat conduction in wool is all the tiny air spaces. Air doesn’t conduct heat well, and the smallness of the air spaces prevent convection currents from being set up. This same idea is used in fibreglass insulation, blown cellulose, and styrafoam.
Want to keep heat in for a longer while? Use lots of insulation.
It’s not that the insulation is inherently warm, and shares its warmth with the interior object – the coat doesn’t make you hot. It’s that the insulation keeps the heat trapped longer – your own body heat can’t escape quickly. Eventually, if no more energy is added (as heat energy is to your coat by your body metabolizing food), no matter how well one insulates, an object hotter than its surroundings will share its heat with its surroundings, and will eventually wind up at the same temperature. It’s just that insulation slows that dispersion down…a lot.
The same is true for keeping heat out…which is why containers for keeping food cool are insulated. One wonders why they aren’t called out-sulated (but perhaps that’s reserved for the reflective outer coating on the better ones, which bounces heat radiation away). Warmer surroundings attempt to share their heat with the colder food. Got to keep that heat energy out if you want the food to stay cool. Insulation works as well here as it does at keeping heat in…it slows down the transfer of heat energy.
Food coolers and thermoses will keep even more heat energy out (or in) if they are shiny outside. Why?
If you are out camping, try wrapping the food cooler in a wool blanket, and see if the ice lasts longer. Then wrap the whole combination of food cooler and wool blanket in one of those shiny emergency “space blankets” ($2 at camping supply stores). See how much longer again the ice lasts.
While using solar energy to create electricity is about 12 -15 % efficient, direct absorption of solar energy to heat water or air can be up to 75% efficient.
Can we do both? Heat water AND make electricity with solar energy? The recent advances in nano-technology are allowing new materials to exist that can be insulative and yet conduct electricity at the same time. Nanosolar Technology, the 3rd way to produce electricity from solar…a good physics tour.
Onward to Solar Energy Uses to see solar heat in action!