Energy storage devices for fluid power applications that are significantly more compact than existing ones will enable energy regeneration for many applications, including fluid power hybrid vehicles and construction equipment. The current approach to hydraulic energy storage makes use of a compressed gas enclosed in a closed chamber. As the system must contain the expanded gas and the hydraulic oil displaced, the optimal energy density occurs at a modest expansion ratio resulting in a small energy density. By allowing intake and exhaust of compressed and expanded air from and to the atmosphere, a potential order of magnitude increase in energy density is available in the new open accumulator approach. Potential methods for realizing the new configuration are described. Analysis and simulation case studies illustrate both the advantages and challenges of the new approach.