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3.3.2. Supercapacitors An alternative energy storage device for hybrid power trains could be super-capacitors, which are designed to achieve fast-charging devices of intermediate specific energy [64]. A super- capacitor [65, 66] has the advantage of a fast charge-discharge capacity, allowing a higher regenerative braking energy and supplying power for larger acceleration [67] and can be clas- sified as a double-layer capacitor or a pseudo-capacitor according to the charge storage mode. However, the main drawback of a super-capacitor is that it has low energy density, which leads to a limited energy capacity. 3.3.3. Hydraulic accumulator The hydraulic storage approach converts the recoverable energy into hydraulic form inside an accumulator and then releases it by using secondary components or auxiliary cylinders [68–70]. Compared with an electric hybrid system composed of a battery or super-capacitor, a hydraulic accumulator device has an advantage in power density over an electric system. Moreover, hydraulic accumulator energy recovery systems are ideal for cases of frequent and short start-stop cycles [71, 72]. However, the application of such systems in work vehicles still presents several defects: The impact of the limited energy density is a design trade-off between the energy storage capacity and volume or weight [73]. 3.3.4. Flywheel energy storage system The flywheel energy storage system (FESS) has improved considerably in recent years because of the development of lightweight carbon fiber materials. This system has become one of the most common mechanical energy storage systems for hybrid vehicles [74, 75]. When in charge Figure 5. Energy storage for hybrid construction and agricultural machinery. Hybrid Electric Vehicles10