Hydraulic accumulators are energy storage devices commonly used to provide supplementary fluid power and absorb shock.  One particularly interesting recent application of these devices is regenerative braking.  Although a theoretically appealing concept, hydraulic regenerative braking (HRB) is difficult to implement due to some major inherent limitations and non-ideal properties of conventional accumulators. 
Gas extendable membrane accumulators (or gas bladder hydraulic accumulators) and piston accumulators with a gas pre-charge (PAGPs) use gas for energy storage.  In these accumulators, a gas, separated by a bladder or a piston, occupies a certain volume of a container which is otherwise filled with a fluid, typically hydraulic fluid.  As fluid is forced into this container, the gas inside the separated volume is compressed and energy is stored in this compressed gas.  Such accumulators are subject to two serious drawbacks: 1) inefficiency due to heat losses, and 2) gas diffusion through the extendable member into the hydraulic fluid.  The drawback of inefficiency via heat loss is addressable, but the gas diffusion issues gives rise to high maintenance costs associated with “bleeding” the gas out of the fluid often. 
With regard to inefficiency, if the energy stored in the compressed gas of such an accumulator is not retrieved soon, the heat flow from the gas to its immediate surrounding results in much less energy being retrieved.  It has been shown that with as little as 50 seconds passing between gas compression and expansion, a piston-type gas accumulator's efficiency can fall to about 60%.  Pourmovahed, A., Baum, S. A., Fronczak, F. J., and Beachley, N. H., 1988. “ Experimental Evaluation of Hydraulic Accumulator Efficiency With and Without Elastomeric Foam”.  Journal of Propulsion and Power, 4(2), March-April, pp.  188.  Since a vehicle remains immobile at a stop light for such a length of time or longer, this makes gas extendable member and piston accumulators with a gas pre-charge not ideal for HRB applications.  Several methods to mitigate these heat losses have been proposed.  For piston accumulators with a gas pre-charge, one method involves placing an elastomeric foam into the gas enclosure.  This foam serves the purpose of absorbing the generated heat during gas compression that would otherwise be transferred to the walls of the gas enclosure, and ultimately lost.  The foam is capable of collecting a large amount of this generated heat and returning it to the gas when the latter expands.  According to Pourmovahed, “the insertion of an appropriate amount of elastomeric foam into the gas enclosure . . . [ can] virtually eliminate thermal loss”.  Pourmovahed, A., Baum, S. A., Fronczak, F. J., and Beachley, N. H., 1988. “ Experimental Evaluation of Hydraulic Accumulator Efficiency With and Without Elastomeric Foam”.  Journal of Propulsion and Power, 4(2), March-April, pp.  188.  Incorporation of elastomeric foam has shown how gas-charged accumulator efficiency can be improved, however, this modification still does not solve the other problems associated with existing accumulators. 
With regard to gas diffusion, the problem persists despite developments in the material used for the extendable member that separates the gas and fluid due to the fact that diffusion can be reduced but not eliminated completely.  This is the case due to the pressure gradient across the bladder from the gas to the fluid.  This gradient is zero when the accumulator is in static equilibrium.  However, when the accumulator is discharging for example, there is a gradient of higher pressure on the gas side to lower pressure on the fluid side that drives the gas through the bladder and into the fluid.  Possible solutions to this problem are to either ensure that the gas is always at a lower pressure than the fluid, or to eliminate the existence of gas altogether.