## 5 Future Prospects and Challenges  
The energy regeneration and conversion technologies based on mechanical-electric-hydraulic hybrid energy storage systems in vehicles are used in a wide scope of vehicles, from passenger to commercial vehicles, and applied in a variety of scenarios with or without a road. They also have a number of features that distinguish them from a typical vehicle, particularly in their hydraulic hybrid configuration, characteristics of each component, weight range, corresponding control strategy, etc. These differences and characteristics make energy regeneration and conversion technologies based on mechanical-electric-hydraulic hybrid energy storage systems in vehicles challenging.  
1. In view of the general principle and operation of hydraulic energy regeneration and conversion, the intrinsic drive of the dual carbon scheme and electric vehicle technology calls for hydraulic hybrid vehicles to move towards electrification and intelligence. Moreover, innovations in hydraulic-powered energy recovery systems and components will be an important direction for future efforts [96; 112; 113].
2. For the different kinds of hydraulic vehicles, current hydraulic hybrid vehicle research is still primarily based on a gasoline or diesel engine as the central power source and the hydraulic accumulator only as the supplemental energy, which only reduces the emission of detrimental gases but does not satisfy the requirements of zero carbon vehicles. However, considering that there is no perfect powertrain, it is not wise to simply choose only one power source. Obviously, different hydraulic vehicles need to be selected due to dual carbon schemes and scenarios. A small-size or load vehicle can use an FC or battery electric system, while a large-size or load vehicle can equip the hydraulic vehicle with an engine, fuel cell or hybrid of them, which will provide a trade-off between the dynamics and economy of the vehicle.  
Buses, delivery vehicles and refuse trucks are widely researched in the urban area, on which hydraulic vehicle research is mainly concentrated. However, this research rarely considers municipal construction vehicles, sanitation vehicles, urban logistics vehicles, urban mini-buses, etc. Therefore, a hydraulic hybrid vehicle of more types of urban commercial vehicles is worthy of our attention.  
Accordingly, we believe that mechanical-electric-hydraulic hybrid energy storage systems based on a wide-range-capacity battery, a hydraulic and pneumatic accumulator, a fuel cell or a combination of them may be the best choice for the future primary propulsion mechanism for vehicles, with the choice of power form ultimately dependent on the combination between its characteristics and the application scenario [139; 140].
3. As to energy management techniques, the energy management method is a primary part of mechanical-electric-hydraulic hybrid energy storage system research. Recently, the energy-saving design for hydraulic hybrid vehicles has mainly concentrated on the rule-based control strategy in the application. Although it is relatively easier and more straightforward to control, more energy-saving developments call for more advanced and closer-to-reality control methods. Therefore, the application and research of the intelligent energy control strategy will be the focus of the next phase of hydraulic hybrid vehicle research.