Hydraulic accumulators (HAs) have been used successfully in regenerative braking systems by companies such as Ford and Eaton Corp. to increase fuel efficiency of heavy vehicles by as much as 25–35%. However, the relatively low gravimetric and volumetric energy densities of conventional HAs prohibit their use in average-sized passenger vehicles. In an attempt to address this problem, an elastomer will be used to construct a HA that will use strain as the primary energy storing mechanism. By using strain in the composition material, as opposed to compression of a precharged gas, this accumulator should virtually eliminate heat losses due to extended holding times. Because its gravimetric and volumetric elastic energy storage density values are among the highest of any material, polyurethane was the elastomer chosen as the constituent material. Using a curable type of polyurethane, an α-prototype is currently being manufactured to provide empirical data for validation.