The invention relates to a fluid power accumulator in which the fluid undergoes a state change as the system is pressurized to store energy. A state change can be a phase change, a chemical reaction, or a combination of these. Generally the state change results from the interaction of a compressible fluid contained in the accumulator with another substance, which can be a fluid or a solid. Preferably, the state change includes the physical adsorption of a fluid by a solid adsorbant. The invention can improve the energy storage density of a fluid power accumulator, allow a given energy storage density to be achieved at a lower maximum pressure, facilitate heat transfer and storage within an accumulator, and/or improve accumulator efficiency by storing energy in a form other than thermal energy, such as in the form of chemical energy.