In one type of TCS, heat is applied to decompose certain molecules. The reaction products are then separated, and mixed again when required, resulting in a release of energy. Some examples are the decomposition of potassium oxide (over a range of 300-800 degrees C, with a heat decomposition of 2.1 MJ/kg), lead oxide (300-350 degrees C, 0.26 MJ/kg) and calcium hydroxide (above 450 degrees C, where the reaction rates can be increased by adding zinc or aluminum). The photochemical decomposition of nitrosyl chloride can also be used and, since it needs photons to occur, works especially well when paired with solar energy.