A heating or cooling system based on solid materials capable of absorbing or releasing heat during a phase change is provided. The system comprises solid members that absorb or release heat during a phase change, locking members to mechanically lock in a desired phase, and a heat exchange medium.
STATEMENT REGARDING RIGHTS TO INVENTION MADE UNDER FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT
 This invention was made with Government support under Contract DE-AC05-76RL01830 awarded by the U.S. Department of Energy. The Government has certain rights in the invention.