A process for fabricating a carbon composite structure that is lightweight, structurally sound, and characterized by high heat capacity. A carbon structure is devised with cavities therein receiving a phase change medium. The phase change medium demonstrates both high energy absorption capacity and high thermal conductivity and is formed from a carbon fiber to establish a high porosity medium having a large volume fraction. The surface energy of the carbon fibers is enhanced in various ways as by deposition of a carbide former, a metallurgical coating or a precursor liquid or by electroplating or etching the surfaces of the carbon fibers. The enhanced surface energy allows for the retention of phase change material.