Half battery, half capacitor, supercapacitors are all the rage for energy storage. Here’s what makes them so interesting.
This article is part of The engineer’s complete guide to capacitors. If you’re unsure of what type of capacitor is best for your circuit, read How to choose the right capacitor for any application.
What is a supercapacitor?
Supercapacitors, also called ultra capacitors or double layer capacitors, are specially designed capacitors that possess very large values of capacitance—as high as 12,000 F. They can be recharged very quickly and are used primarily for energy storage.
How do supercapacitors work?
Supercapacitors combine the electrostatic principles associated with capacitors and the electrochemical nature of batteries. Consequently, supercapacitors use two mechanisms to store electrical energy: double electrostatic capacitance and pseudocapacitance. Pseudocapacitance is electrochemical, like the inner workings of a battery.
The maximum supercapacitor cell voltage ranges from 2.5 to 2.7 V. While higher voltages are possible, they come at the cost of a reduced service life. The usual approach is to place cells in series to achieve higher voltages (up to 15 V), but that increases the series equivalent resistance and reduces the total equivalent capacitance.
A supercapacitor with constant-current charging produces a linear rise in voltage. The charge time is very short and takes seconds to complete compared to a lithium-ion battery charge time of perhaps hours. The discharge of a supercapacitor shows a rapid reduction in voltage. The voltage can be held constant by using a buck-boost DC to DC converter regulator. However, this raises costs and reduces efficiency.
Applications of supercapacitors
The rapid charging and discharging of supercapacitors is reflected in their specific power, a parameter with units of watts per gram (W/g). Power is the rate of receiving or delivering energy (p = dW/dt). The specific power of supercapacitors far exceeds that of the lithium-ion battery. Since supercapacitors charge and discharge so quickly, they are excellent candidates for energy storage during regenerative braking of hybrid and electric vehicles. Supercapacitors are also being applied to large-scale energy storage in renewable energy applications.
Even where they seem to be a promising option, supercapacitors may not be a fit for your circuit. Take the following example.
A small (0.31 Ω) resistor used in an aerospace application is to undergo a life test. The requirement is to apply a 2000 ± 100 ADC current pulse of 80 ± 10 ms repeatedly every five minutes. The voltage across the resistor must be 720 VDC nominally. Rather than use a large (and expensive) DC power supply, a capacitor bank is required. The proposed basic test scheme is depicted in the image below.
The DC power supply is set to 720 VDC. Its current limit is adjusted to 3.5 ADC. This means that it will act like a constant-current source. Consequently, the capacitor bank will be charged to 720 VDC linearly. If the DC power supply is switched off, its voltage will be less than the voltage across the capacitor bank. The protective diode prevents DC power supply damage from being back fed. A timer circuit and IGBT gate driver (not shown) sets the five minute interval and triggers the IGBT switch. The desired current pulse is shown in the image below.
The 47000 µF capacitor bank provides energy storage. An energy storage application and a large capacitance value suggests supercapacitors should be investigated, but because the voltage is so large, series-parallel combinations are necessary. In this case, the resulting equivalent series resistance (ESR) values of supercapacitors in series are too large compared to aluminum electrolytic capacitors, and so the latter are a better option.
Alternatives to supercapacitors
Supercapacitors are an evolving technology. Given their unique capabilities, the only alternative is waiting for the next best version.
