Seite - 64 - in Hybrid Electric Vehicles

higher energy efficiency, lower running cost can be achieved than conventional internal com- bustion-engine vehicles. Recently, value-added utilization of electric vehicles also has been proposed and developed including the ancillary services for the electrical grid and electricity support to certain energy management system [2–5]. Therefore, the economic performance of the electric vehicles can be significantly improved. Some literatures have proposed and described well the grid integration, especially the introduc- tion of renewable energy, and electric vehicles [6]. The fluctuating renewable energy sources, such as wind and solar, require a fast-response energy buffer to cover their intermittency as well as and to store the surplus electricity due to higher supply side than demand side. Electric vehicles are considered as the appropriate resource to balance and store these kinds of renew- able energy sources [7]. The battery owned by the electric vehicles can absorb and release the electricity from and to the electrical grid, respectively, to balance the electrical grid promptly. In general, there are four types of electric vehicles currently running and developed: (i) con- ventional hybrid electric vehicle (HEV), (ii) plug-in hybrid electric vehicle (PHEV), (iii) battery electric vehicle (BEV) and (iv) fuel-cell electric vehicle (FCEV). HEV combines electric motor and internal combustion engine; hence, it is also fitted with a battery to power the motor as well as store the electricity. The energy to power the motor comes from the engine and regen- erative breaking. However, recently, many HEVs have been redeveloped and shifted to PHEV due to the excellent characteristics and higher flexibility of PHEV than HEV. Like HEV, PHEV also owns electric motor and internal combustion engine. According to IEEE standards, PHEV is HEV having following additional specifications: bat- tery storage of larger than 4 kWh, charging system from external energy source and capabil- ity to run longer than 16 km [8]. Furthermore, BEV is generally defined as the vehicle driven solely by electric motors and the source of electricity is stored and converted from chemi- cal energy in the battery. Therefore, BEV relies on external charging and its driving range depends strongly on its battery capacity. As the battery capacity of BEV is significantly larger than HEV and PHEV, battery makes up a substantial cost of BEV. Advanced development of battery and decrease of its price is highly expected in the near future; hence, more massive deployment of PHEVs and BEVs can be realized. On the other hand, FCEV uses only electric motor like BEV. However, it utilizes hydrogen as the main fuel that is stored in the tank. The oxidation of hydrogen produces electricity to power the electric motor and if there is any surplus it is stored in the battery. In practice, as the hydrogen refuelling can be performed in a very short time, almost similar to one of the gasoline refuelling, FCEV basically facilitates no charging from the external charger. Although it varies, the battery capacity of PHEV is generally larger than HEV. According to survey conducted by Union of Concerned Scientists (UCS), about 50% of drivers in US drive less than 60 km on weekdays [9]. Therefore, many available PHEVs can hold for a weekday commuting without additional charging outside. In addition, although its battery capacity is lower than BEV, PHEV has higher flexibility on driving range as the power can be supplied by the engine once the battery capacity drops to certain low value. Both PHEV and BEV are believed will dominate the share of vehicles in the future. In addition, according to Electric Power Research Institute (EPRI), around 62% of vehicles will encompass of PHEVs [10]. Hybrid Electric Vehicles64