Compare storage capacity in terms of the energy density (watts-hours per kg) bet

ID: 1715650 • Letter: C

Question

Compare storage capacity in terms of the energy density (watts-hours per kg) between traditional capacitors and batteries (lead-acid and lithium-ion). For a traditional capacitor with a breakdown voltage of 2V, using mica as a dielectric that can sustains electric field up to 2e8 V/m, ignore the mass of the plates as compared to that of the dielectric insert. Calculate what is the minimum spacing between the plates and what is the maximum energy density stored. In this context, what are supercapacitors? What can they do? What are the future developments?

Explanation / Answer

A Supercapacitor also known as ultracapacitor, is a high-capacity electrochemical capacitor with capacitance values much higher than other capacitors that bridge the gap between electrolytic capacitors and rechargeable batteries. They typically store 10 to 100 times more energy per unit volume or mass than electrolytic capacitors, can accept and deliver charge much faster than batteries, and tolerate many more charge and discharge cycles than rechargeable batteries. They are however 10 times larger than conventional batteries for a given charge.

Supercapacitors are used in applications requiring many rapid charge/discharge cycles rather than long term compact energy storage within cars, buses, trains, cranes and elevators, where they are used for regenerative braking, short-term energy storage or burst-mode power delivery. Smaller units are used as memory backup for static random-access memory (SRAM).

Future developments:

Supercapacitors which are typically composed of highly porous carbon that is impregnated with a liquid electrolyte are known for possessing an almost indefinite lifespan and the impressive ability to recharge extremely rapidly, in seconds even. But existing versions also possess a very low energy-storage-to-volume ratio in other words, a low energy density. Because of this low energy density 5-8 Watt-hours per liter in most supercapacitors they’re not practical for most purposes. They would either need to be extremely large or be recharged very, very often for most uses.

Function

Supercapacitor

Lithium-ion (general)

Charge time

Cycle life

Cell voltage

Specific energy (Wh/kg)

Specific power (W/kg)

Cost per Wh

Service life (industrial)

Charge temperature

Discharge temperature

1–10 seconds

1 million or 30,000h

2.3 to 2.75V

5 (typical)

Up to 10,000

$20 (typical)

10 to 15 years

–40 to 65°C (–40 to 149°F)

10–60 minutes

500 and higher

3.6 to 3.7V

100–200

1,000 to 3,000

$0.50-$1.00 (large system)

5 to 10 years

0 to 45°C (32°to 113°F)

–20 to 60°C (–4 to 140°F)

Supercapacitors have much lower energy density than lithium batteries. The energy storage (kWh) requirement

using supercapacitors is much smaller than using batteries in high power applications due to the much lower power

capability (kW/kg) of the batteries. This can have a large effect on the effective energy density of the energy storage

unit.