3.4　Supercapacitors and Ultracapacitors

Text（课文）

Capacitors are devices that store energy by the separation of equal positive and negative electrostatic charges.The basic structure of a capacitor consists of two conductors，known as plates，separated by a dielectric，which is an insulator.The power densities of conventional capacitors are extremely high（1，012 W/m3），but the energy density is very low［50（W·h）/m3］.

These conventional capacitors are commonly known as“electrolytic capacitors.”They are widely used in electrical circuits as intermediate energy storage elements for time constants that are of a completely different domain and are of much smaller order compared to the energy storage devices that are to serve as the primary energy sources for EVs.

Supercapacitors contain an electrolyte that enables the storage of electrostatic charge in the form of ions，in addition to conventional energy storage in electrostatic charges，like in an electrolytic capacitor.The internal functions in a supercapacitor do not involve electrochemical reaction.The electrodes in supercapacitors are made of porous carbon with high internal surface area to help absorb the ions and provide a much higher charge density than is possible in a conventional capacitor.The ions move much more slowly than electrons，enabling a much longer time constant for charging and discharging compared to electrolytic capacitors.

Power density and energy density of supercapacitors and ultracapacitors are of the order of 106 W/m3 and 104（W·h）/m3，respectively.Energy density is much lower compared to those of batteries［5 to 25×104（W·h）/m3］，but the discharge times are much faster（1 to 10 s compared to 5×103 s of batteries），and the cycle life is much more（105 compared to 100 to 1，000 of batteries）.

Current research and development aim to create ultracapacitors with capabilities in the vicinity of 4，000 W/kg and 15（W·h）/kg.The possibility of using supercapacitors and ultracapacitors as primary energy sources is quite far reaching，although it is likely that these can be improved to provide sufficient energy storage in HEVs.On the other hand，supercapacitors and ultracapacitors with high specific power are suitable as an intermediate energy transfer device in conjunction with batteries or fuel cells in EVs and HEVs to provide sudden transient power demand，such as during acceleration and hill climbing.The devices can also be used efficiently to capture recovered energy during regenerative braking.

New words and expressions（单词和短语）

1.New words（单词）

2.Expressions（短语）

Notes to the text（难点解析）

★1.The possibility of using supercapacitors and ultracapacitors as primary energy sources is quite far reaching，although it is likely that these can be improved to provide sufficient energy storage in HEVs.尽管超级电容器能够为混合动力汽车提供充足的能源存储，但超级电容器成为汽车主能源的可能性还很小。

Exercises（练习）

◆1.Translate the following passages（expressions）into Chinese（英译汉）

（1）Capacitors are devices that store energy by the separation of equal positive and negative electrostatic charges.

（2）energy density

（3）electrical circuits

◆2.Translate the following passages（expressions）into English（汉译英）

（1）时间常数

（2）主要能源

（3）在电动汽车或者混合动力汽车上，将超级电容器同蓄电池或者燃料电池结合使用，可满足提供瞬时功率需求。

Reading material（阅读材料）

Because of the frequent stop-and-go operation of EVs and HEVs，the discharging and charging profile of the energy storage is highly varied.The average power required from the energy storage is much lower than the peak power for acceleration and hill climbing in a relatively short duration.The ratio of peak power to average power can reach over 10∶1.In HEV design，the peak power capacity of the energy storage is more important than its energy capacity，and usually constrains its size reduction.Based on present battery technology，battery design has to carry out the trade-off among specific energy，specific power，and cycle life.The difficulty in simultaneously obtaining high values of specific energy，specific power，and cycle life has led to some suggestions that the energy storage system of EV and HEV should be a hybridization of an energy source and a power source.The energy source，mainly batteries and fuel cells，has high specific energy，whereas the power source has high specific power.Power sources can be recharged from the energy source during less demanding driving or regenerative braking.The power source that has received wide attention is the ultracapacitor.