Hydraulic accumulators are hydraulic components on which the hydraulic fluid can perform work by compressing the gas contained in the accumulator. As soon as the compressed gas relaxes, the hydraulic accumulator performs work on the hydraulic fluid. Albeit somewhat imprecise, hydraulic accumulators are therefore energy storage systems. In commercial vehicles, hydraulic accumulators are used in hydro-pneumatic suspension struts for vibration isolation. The compression and expansion described at the beginning takes place periodically. As with any spring-mass system, the lower the spring stiffness, the better the isolation for a given mass. However, the spring stiffness decreases with the construction volume of the hydraulic accumulator.

At the Chair of Fluid Systems, therefore, the idea arose to incorporate highly porous adsorbents such as activated carbon, to reduce the volume of the accumulator without changing its stiffness. In pressure-dependent adsorption, gas molecules adhere to the surface of the adsorbents and thus no longer contribute to the system pressure. The stiffness of the hydraulic accumulators decreases. However, heat is released during adsorption, which partially cancels out the effect of stiffness reduction, especially in the case of rapid excitation and thus isentropic change of state. In this work, the question is answered as to how large the potential is for design space reduction by adsorbents in hydraulic accumulators. Related to this is the question of how to adequately model hydraulic accumulators with adsorbents. From the axioms for mass and energy as well as material equations, ordinary differential equations were derived which describe hydraulic accumulators with adsorbents. The solution in frequency space shows that from the numerous mass and heat transport phenomena in the hydraulic accumulator and the adsorbent bead, the heat transfer to the environment, the heat capacity of the adsorbents and the adsorption equilibrium play the most important role. Comparison of the model with measurements on a prototypical hydraulic accumulator with adsorbents reveals differences between the model and reality that can be attributed to the formation of dynamic temperature boundary layers. This approach leads to a second model with partial differential equations, which qualitatively agrees with the measurements. Both models developed in this work should be understood as upper and lower bounds on the stiffness and hence design space reductions possible with adsorbents.