What are the applications of ultracapacitors?

Ultracapacitors have very large power delivery capability, several times that of battery. Even though their energy storage is much smaller than batteries, this unmatched capacity to deliver high power, that too instantly, is a major benefit, which has introduced ultracapacitors in several niche fields. They get charged within second to minutes (batteries take hours), and can be fully vacated of their stored energy because of high current discharge capacity. They are used extensively today to increase capabilities of systems, which were not possible earlier. Not only that, but ultracapacitors have even started replacing batteries in several low energy applications.

Battery Backup

The single most discussed application of ultracapacitors relates to its use as battery backup.  Consider the simplest circuit as follows.

The left figure is a normal circuit where a load is served by a battery. In case of sudden changes in load, an inrush current of transient nature is drawn from battery.  This heavy current cannot be supplied properly by the battery, since it has to catch up with corresponding rate of chemical reaction within the battery. This delay of few milliseconds in response, and failure to meet the sudden demand of current by load causes a deep drop in battery terminal voltage (battery EMF) for a short time, which then recovers to normal within a short time. However, during this period, the battery voltage can go dangerously low, below the minimum permissible voltage across terminals. The sudden burst in chemical reactions beyond safe limits, plus associated heat, causes some degradation of battery with every such event. Hence the battery life is seriously affected.

Ignition coil circuit of an IC Engine vehicle has the same set up, with ignition coil as load, with a lead acid battery. A 12 V battery has a minimum permissible safe voltage level of 10.8 V. It has been observed that during ignition spark, the battery voltage goes down as low as 5.8-6.0 Volt, and it does not recover for 100 milliseconds. Car SLI battery life goes down to as low as 200-300 cycles, as against normal expectation of 700-1000 cycles under normal conditions.

Now consider an ultracapacitor connected across battery as in the figure on right. In this circuit, in case of sudden load demand, the ultracapacitor, being a capacitor, discharges suddenly into the load, and battery gets time to respond if needed. The dip in voltage across battery is now very small. It has been observed that voltage during this surge event goes down only by small degree, and remains above 10.8 V all the time (it may not go below 11 V any time). The battery voltage is always within limits, and no fluctuations in battery voltage with load conditions. The steady voltage at terminals, no sudden or fast chemical reactions lead to increased life of battery by four to five times.

1] Vehicle Applications

Ultracapacitors serve several functions in vehicles.

• Regenerative braking
• Electric / Hybrid Electric Vehicles
• Cold Start / battery backup
• Start – stop system
• Power windows
• Power steering
• Air conditioning
• Accessories like horn, radio etc.

In all these functions, ultracapacitors take over functions of battery at local level and relieve battery of transient loads, thereby extending battery life, as also ensuring effective fast response.

Considering that about 50-60 million vehicles are produced worldwide annually, and possibility of above functions in vehicles, it is apparent that there is very large market potential for ultracapacitors in vehicles. Apart from this, ultracapacitors are also finding place in vehicle battery charging systems.

2] Power Grids

Power systems use ultracapacitors for a number of purposes.

• Short term energy storage for few seconds to minutes.
• Surge and impulse conditions, where they save the system from sudden surge / impulse loads.
• Meeting Peak power demand
• Capture and store excess power during transient state and lean period.
• Help in power quality issues.

3] Renewable energy short term storage

Wind and solar power are subject to time of day, and local varying conditions like short lull in wind, passing cloud shadow etc. Such short time conditions can be taken care by ultracapacitor systems, which keep supply steady during these periods. They store energy produced during peak periods, and small energy harvesters can rely on ultracapacitors alone, without the need for battery.

4] Military applications

Military must get reliable energy storage, available in all weather conditions, including extreme low temperatures. Batteries cannot work at these temperatures, and cost is often secondary consideration, first requirement being availability of light and functioning of all systems reliably. Military vehicles use their ability to start IC engines at these temperatures.

5] Electronic Industry

Development of smart phones with modern features use ultracapacitors in large numbers. They are used in flash lights, memory and battery backup purposes. Electronic toys are driven by ultracapacitors with their quick charge feature. Torches, hand tools like portable drill machines use them, which does away with battery, and can be charged almost instantly when needed.

Each of these applications needs separate treatment for detailed functions and benefits of ultracapacitors. Full details are available in my book ‘Ultracapacitors – Future of Energy Storage’ a McGraw Hill Publications.