The needs of efficiency improvement of the vehicle energy systems require to find innovative solutions during the design process, integrating all vehicle services and energy requirement on a vehicle system level. In this article the boundary of the energy system are extended to the powertrain and the cabin and the requirements for mobility and comfort are integrated. The energy balance of the internal combustion engine is done and discussed, according to its operating points. The energy requirement for comfort in the cabin is also determined, according to the seasonal requirement for heating or cooling. In this article an energy integration methodology, using process integration techniques is discussed and applied on the extended vehicle energy system. The minimal energy requirement is determined for different mobility and comfort situations. The energy recovery potential of an organic Rankine cycle, with sensitivity of different working fluids, is assessed. The energy integration methodology is applied on a hybrid electric vehicle energy system, and is studied for adapted dynamic profile, represented by characteristic clustered operating points. Multi-objective optimization is applied to define the optimal design of a hybrid electric powertrain and the optimal ICE size, from efficiency and cost point of view.