In light of the shortage of resources and the increasing impact of CO2 on the environment, hybrid drive systems are increasingly used, for example, in automotive technology. The systems currently in use are mostly electromotive hybrids, in which electric energy obtained during braking operations is stored and the stored energy is converted again to drive energy, in order to assist the vehicle when in driving mode and, in particular, during accelerations. This system can reduce the drive capacity of the internal combustion engine functioning as the primary drive for comparable driving performances. A “down-sizing” of this kind not only results in a drop in consumption, but also allows the possibility of assigning particular vehicles to a more favorable emission class corresponding to a lower performance class. A significant disadvantage of electromotive hybrids, however, is the energy loss that occurs due to the steps of converting from mechanical energy to electric energy and back. The energy loss may amount to as much as 66%.
Due to the high energy density and the compact design of hydraulic systems, these goals may also be achieved by a hydraulic hybrid system. To provide additional drive torque for accelerations even at low speeds and starting from zero speed, and to boost the braking effect during braking operations, hydraulic energy is stored in such case in a hydraulic accumulator by a motor-pump unit, in order, when needed, to utilize the motor operation of the motor-pump unit for reconversion. Such a hydrostatic drive system including recovery of braking energy was previously disclosed by the applicant in WO 2011/116914.
However, that, due to the lower power loss in the hydraulic system as compared to electromotive hybrids, a very large surplus of energy is stored in the hydraulic accumulators. Hence, there is a demand on the part of the user to also harness this surplus energy for other purposes.