This is due to balanced ‘electrochemical’ reduction and oxidation processes occurring inside the brick at the two faces. As long as electrodes at these faces are at different temperatures, the electrochemical reactions occur and electricity is generated. The compounds inside are not consumed, do not run out and can never be overcharged. As long as there is a temperature difference there can be electricity. For example, if a house or shelter’s outside wall is sunny and hot, but the interior shaded and cool, electricity can be produced by the wall.