Question

Electrochemical capacitor and battery are electrochemical energy storage system. The former has a very good power density (fast charging/discharging) but low energy density, while the latter has the opposite properties. Please design a device that can have good power density and energy density by combining the electrochemical capacitor and battery. Please provide an equivalent circuit (use -||- to represent capacitor and -©- to represent battery) to describe your design and how they work under charging/discharging conditions. You can use multiple battery and capacitors for your design and explain how it work. Do you think your design can be used to power an electric vehicle to compete with gasoline-based cars?

Answer 1

Figure 1

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Ragone plot for significant energy storage and conversion devices.

From the plot in Figure 1, it can be seen that supercapacitor technology can evidently bridge the gap between batteries and capacitors in terms of both power and energy densities. Furthermore, supercapacitors have longer cycle life than batteries because the chemical phase changes in the electrodes of a supercapacitor are much less than that in a battery during continuous charging/discharging (Yu, Davies, and Chen, 2012). These key attributes make supercapacitors more attractive and versatile as high powered energy storages. The US Department of Energy (DOE) has spotlighted batteries and supercapacitors as major future energy storage technologies (Goodenough, 2007).

Application Market and Economy of Electrochemical Supercapacitors

The earliest application of ESs was a backup power supply for electronics. On one hand, supercapacitors, capable of discharging large amounts of power in a matter of seconds, are ideal for supplying instant and uninterruptable backup power in response to energy surges or a shutdown. Batteries, on the other hand, are less‐than‐ideal for this type of application because they are more expensive and may induce uncontrollable temperature escalations (Yu, Chabot, and Zhang, 2013; Yu, Davies, and Chen, 2012).

Supercapacitors with its long cycle life and high power delivery are applicable to both consumer and military devices. Coleman's portable cordless screwdrivers, which are powered by supercapacitors, are currently listed on the market for home usage. This power tool is fully charged within 90 s for immediate use (Miller and Burke, 2008). In military applications, supercapacitors are generally implemented and used as alternative power for electronics in armed vehicles, black boxes on helicopters, and so on (Yu, Chabot, and Zhang, 2013).

Energy recovery in public transportations and hybrid electric vehicle (HEV) has further reenergized interest in supercapacitors. This is because the primary challenge for public transportation was harnessing the regenerative energy when braking for the frequent stops and reusing the stored energy when accelerating to depart. Supercapacitors are capable of storing instantaneous brake energy and discharging upon demand, which improves fuel efficiency. Moreover, current HEVs encounter temperature stability challenges where the charging/discharging mechanisms in HEVs generate undesirable heat from resistances and exothermic reactions. Possessing the ability to dissipate heat effectively, supercapacitors can be implemented into HEVs to manage the thermal issues, thus enabling them easier entry into the HEV market (Miller and Burke, 2008).