Smooth Black Stone

We live between shadow and light. We live in the shadows of our walls, and we fill our rooms with light through windows.

Windows can also be used to gather and store energy from the Sun.

Gather three smooth black stones, as like to each other as possible. Take your time with this - it is important!

Outdoors, put your stones on a board, or a piece of cardboard. Leave one uncovered. Cover one with an upside-down jar or clear drinking glass.
 

For the third stone we can construct a simple test house with a 1-litre and a 2-litre milk or juice containers, kleenex (or shredded paper towel or dry sawdust or cotton batten), some plastic wrap, and an elastic band. Cut the top from each carton, about 10 cm (~ 4”) up on from the bottom on the large one, and about 8 cm (~ 3 -1/8”) up on the smaller one.

Lightly scrumple up some kleenexes and put a few in the bottom of the bigger box, then put the little box inside and fill in the spaces between the walls with more scrumpled kleenexes (or other insulating material). Cut a piece of plastic wrap to fit over the opening and down the sides a bit. Put your third stone into the “house” and cover the opening with the plastic wrap, using the elastic band to hold it.
 
 

With your back to the Sun, figure out how to orient your “house” so that Sun-light shines into the box and onto the stone. (You may want to prop up the “house” so that its “window” is facing directly toward the Sun).

Make sure all three stones are in sunlight. A slight breeze may be blowing. Consider that the glass and the “house” protect those two stones from the cooling breezes. Wait for about 1/2 hour.

 Return. Check the temperature of the exposed stone, by picking it up in your hand and feeling it, and compare that to the temperature of the stone that was covered by glass alone. Then open up the test unit, and feel the warmth of the third stone.

Here, solar energy warms the rock. The warmed air in the room is protected and does not blow away. Insulation in the walls keeps heat from radiating away quickly.  Meanwhile, more and more solar energy is coming in through the window.
You might want to experiment with different insulating materials, and double glazing (by covering the interior box with a plastic wrap first), and with reflectors. Speaking of reflectors, what might be one way to keep heat in at night?