WSEAS TRANSACTIONS on SYSTEMS
2 HEV with flywheel system
2.1 Comparison between energy storage systems
HEV taking ICE as the main power source usually adopts chemical battery as assistant power. Compared with ultra-capacitor and chemical batteries such as lead-acid battery or nickel metal hydride (NIMH) battery, FESS has advantages of the longest use-life, the highest energy storage density, the shortest charging time and also the easiest maintenance. Moreover it has the least demand for working environment and will cause no pollution. The Comparisons between them are shown in Table 1. Table 1 Comparison between chemical battery, Ultra-capacitors and FESS 
Storage Mechanism Use-life
Temperature Range Environmental Concerns Relative mass (Equivalent energy) Practical time To hold a charge
3~5 years Limited
The key point of energy storage with flywheel is to reduce the loss of mechanical energy, namely the loss of kinetic energy that consists of air friction resistance and rotary resistance. According to different means for the reduction of energy loss, FESS can be divided into low-speed flywheel system and high-speed flywheel system. The first reduces air friction by
increasing the is to reduce environment
mass of flywheel while the second the air pressure of operating
Jianhui He, Guoqiang Ao, Jinsheng Guo, Ziqiang Chen, Lin Yang
flywheel system benefits from the use of high strength compound material (carbonic fibre). It has characteristics of small mass, low volume and high speed. Thus it is fit for vehicle using.
The high-speed flywheel system consists mainly of a flywheel, a motor and a generator. It is connected with exterior electrical systems through input or output electronic equipments and the power transported from exterior systems is converted from electric energy into mechanical energy by raising rotary speed of flywheel. When it is needed to output power, mechanical energy is converted back into electric energy through generator and meantime the rotary speed of flywheel is reduced . The motor and generator are usually integrated together and magnetic suspension bearings are adopted by flywheel system, through the support of which the flywheel is fixed in a vacuum container (Fig.2).
Excessive high cost is the main cause preventing FESS from popularizing. Its cost is about 1.5 times that of ultra-capacitor and is about 6 ~ 7 times that of chemical battery. But the use-life of flywheel system is 10 times or more than that of chemical battery and needs no recalling. Moreover the cost for installation and maintenance is also much lower than chemical battery and ultra-capacitor. So if the use-life of each vehicle were accounted for ten years, the annual expenditure consumed on FESS would be slightly higher than that consumed on chemical battery (Fig.1). Furthermore, the FESS will have greater potential of developing after industrialization.
Fig.1 Cost comparison between chemical battery and flywheel system
Issue 5, Volume 8, May 2009