This article focuses on the issues and challenges facing the widespread adoption of supercapacitors in electronic devices and equipment.
Are Supercapacitors Feasible for Large-Scale Power Applications?
Current supercapacitor technology faces several limitations that surround their use in electronic applications. Let's take a look at some of the factors that will help answer the above question.
When weighing up whether to bring large-scale implementation to supercapacitors, an essential factor is their energy density (aka specific energy). The energy density is the amount of power (or charge) capable of being stored per unit mass of the capacitive material, measured in watt-hours per kilogram (Wh/kg). This area is one where supercapacitors lag behind batteries: the former has energy densities of about 1 to 30Wh/kg, compared to what is between 100 to 200Wh/kg in the latter’s case.
In supercapacitors, charges accumulate in the dielectric layer between the carbon electrodes and the electrolyte. Using an electric double-layer (EDLC) construction and advanced materials, manufacturers have designed supercapacitors with up to 500 farads of capacitance.
Their energy density, however, pales in comparison to batteries. Individual supercapacitor cells deliver up to 3V dc power and will require a series combination to achieve higher voltage levels. On the other hand, batteries can deliver 12 to 24 volts per unit, so having supercapacitors with battery-like voltage levels would require manufacturers to design much larger supercapacitors.
Eaton's XV large-cell supercapacitor. Image Credit: Eaton.
Another apparent distinction between supercapacitors and batteries is the disparity in their discharge times. The discharge time refers to the rate of power consumption from an energy source. Supercapacitors’ discharge times are in the ballpark of 30 to 60 seconds, making them unsuitable for devices and equipment that need sustained power delivery. Additionally, supercapacitors have high self-discharge rates, losing up to 20% of total charge per day.
Cost of Production
Supercapacitors are relatively more expensive to produce than batteries, owing to the materials of construction. The most prominent type of supercapacitor, namely the said EDLC setup, uses activated carbon or graphene as the chief capacitive material. This can be costly to produce: the high cost of supercapacitor manufacturing also translates into higher costs for end users. The average cost per KWh for lithium-ion batteries is around $250 to $1,000 for larger systems, while supercapacitors cost about $10,000 per KWh.
Eaton's XLM supercapacitor module. Image Credit: Eaton.
Benefits Over Batteries
In spite of the above limitations, supercapacitors possess many advantages over batteries. Firstly, they are highly durable, supporting millions of charge-discharge cycles with minimal degradation.
Supercapacitors are more cost-effective for end users in the long run. Unlike batteries, whose lifetimes average about 3 to 4 years with regular use, supercapacitors can last up to several decades and require little or no maintenance. Also, supercapacitors charge significantly faster than batteries. A supercapacitor can charge up fully within 10 seconds, while batteries can take up to 90 minutes.
Lithium-ion (Li-ion), lithium-polymer (Li-Po),nickel cadmium (NiCd), and other battery types are prone to overheating during use and storage, which considerably degrades the internal components. Some underlying causes are their high internal resistance, high storage temperature, and self-discharge tendencies.
Supercapacitors, on the other hand, have low equivalent series resistance and are suitable for use in a broad range of operating temperatures. For instance, Eaton's XT family of large-cell supercapacitors can withstand operating temperatures anywhere from −40 °C to 85 °C—temperatures at which standard batteries could start to fail.
How Supercapacitors Are Used
Unlike secondary batteries, supercapacitors do not deliver stored energy to loads for a long time. Instead, they release large bursts of high-density power for a short duration (typically within a few minutes). The following subsections show some useful applications of supercapacitors.
Supercapacitors can entirely substitute batteries for regenerative braking in electric and hybrid-electric vehicles. Regenerative braking systems use electric motors that intermittently reverse in direction. When running in the reverse direction, the electric motor acts as a generator, converting mechanical energy into electrical energy.
Supercapacitors capture the generated power and release it to accelerate the car again after stopping. This arrangement lowers fuel consumption and lessens wear and tear on the conventional braking system.
Supercapacitors provide a reliable alternative to batteries in industry 4.0 applications, such as manufacturing plants and warehousing facilities. For example, supercapacitors can power electric pallet movers used to pick and place objects in a warehouse, as well as other electric propulsion systems. Moreover, supercapacitors have a wide range of operating temperatures, making them well-suited for harsh industrial environments.
Medical Devices and Equipment
Life-critical electronic devices at healthcare facilities (such as pacemakers with insulin pumps, MRI (medical resonance imaging) machines, and computed tomography (or CT) scanners, require a constant power supply to sustain life.
Note that in the United States, the National Fire Protection Association (NFPA) specifies in its NFPA 99 standard a ten-second requirement following a power outage to bring auxiliary power online to support healthcare systems. Supercapacitors, accordingly, can bridge the gap up to 15 seconds before backup generators come online. In hospitals and other large buildings, supercapacitor modules can provide ride-through power to prevent a total loss of energy in life-sustaining equipment.
An array of supercapacitors and supercapacitor modules from Maxwell Technologies. Image Credit: Maxwell.
A Promising Future Ahead for Supercapacitors
Despite the issues facing large-scale implementation, supercapacitors remain attractive energy storage solutions in terms of high-power density, speed of charging, and longevity. Moreover, supercapacitors comprise materials that ensure compliance with environmental regulations.
While batteries are still the preferred energy storage solutions in today's world, supercapacitors are rapidly evolving to meet power demands in a host of applications. With proprietary materials and improvements in EDLC technology, leading manufacturers are ramping up the amount of charge that these devices are capable of storing.