b) Serial hybrid
The figure indicates the structure of this concept. The characteristic feature of this concept lies in the serial positioning of its components and the double power conversion of the combustion engine. The disadvantage being that the efficiency of the electric machine appears as a product in the energy demand is offset against the advantages resulting from the mechanical decoupling of the combustion engine from the drive train and the existence of a short-term storage unit. The power output of a combustion engine is largely independent of the vehicle's power requirement. As a result, the combustion engine can be operated at steady-state and more or less stationary. By designing a combustion engine for a small operating area, an additional improvement of efficiency can be achieved.
The characteristics of the short-term storage unit essentially influences the advantages of this concept. In traditional batteries, the power density presents a problem (Table 3-1). The amount of instantaneous energy available is limited and the efficiency sharply drops at quick discharges. In other words, the battery would have to be designed rather large and bulky. This is the reason behind the use of electro-mechanical flywheels or super-condensers as a short-term storage units in alternative concepts.
In choosing a suitable operating strategy, attention has to be paid in order that, on the one hand, the combustion engine operates along a favorable characteristic, while on the other hand, as little energy as possible is stored in between, in order to minimize the associated losses.
Compared to the parallel concept, the advantage of the serial concept lies in its better emission behavior as a consequence of its more or less stationary engine operation, higher potential for regenerative braking when using flywheels or condensers, better driving comfort, and lower excess weight. However, the complex control strategy and short-term storage unit technology which has not yet matured, still poses problems.