Refers to a type of solar heating system which makes use of phase changes in a chemical storage medium. At its melting point, any chemical compound must absorb a quantity of heat in order to change phase from solid to liquid, and at its boiling point, it must absorb an additional quantity of heat to change to a gas. Conversely, a compound releases a quantity of heat when condensing or freezing. Therefore, a chemical warmed by solar energy through a phase change releases far more energy when it cools than, for example, water heated from a cool liquid to a warm liquid state. For this reason, solar heating systems that employ phase-changing chemicals can store more energy in a compact space than a water-based system.