Large storage capacity
To witness an electrical storm – one of nature’s more spectacular shows – is to witness first hand both the phenomenon of capacitance in action and its power. Capacitance is the ability of a material or device to store an electrical charge and, as lightning strikes show clearly, these types of system are capable of storing significant quantities of power – and delivering it extremely rapidly.
As is often the case, humanity’s efforts are way behind those of Mother Nature, but the property of capacitance has nonetheless been exploited for use in numerous applications over the years. However, only within the last 30 years or so have capacitors moved on from being limited predominantly to basic parallel plate types with either a dry or wet electrolyte separating the charge-carrying elements. Capacitance is measured in F (farads), the SI (international system of units) derived unit of electrical capacitance standardized by the IEC and named after the scientist Michael Faraday. These types of capacitor have a low storage capacity typically measured in pico, nano or micro farads. Applications include signal and power filtering and buffering.
In the late 1950s, General Electric began experimenting with double-layer capacitors, which led to the development of a primitive supercapacitor. With no commercial applications, the breakthrough was not vigorously pursued until advances in materials sciences and manufacturing improved performance and reduced costs to enable such devices to begin emerging on a commercial basis in the 1970s.
Rated in farads, supercapacitors are capable of storing tens of thousands of times more power than their conventional electrolytic cousins.
Materials advances, better performance
Although capacitors store electrical energy, this energy is stored electrostatically on the surface of the material rather than chemically as is the case with batteries. The key to capacitor performance is therefore a large surface area which is available to carry charge. Uniquely in supercapacitors, the electrostatic charge is stored in an electrochemical double layer and that is far thinner than can be achieved with any dielectric. Capacitance is therefore boosted by both this and the exceptional area offered by advanced carbons.
Supercapacitors – most commonly EDLCs (Electrochemical Double Layer Capacitors) – are based on high performance materials that allow a very high power density (W/kg). EDLCs feature electrodes comprised of multiple stacked layers of nonreactive highly porous carbon and thus have an enormous surface area. Graphene – a layer of carbon one atom thick – in particular is attracting considerable attention in this field and is expected to appear in commercially available products within the next decade. Researchers are also investigating carbon aerogels and nano tubes for use in supercapacitors.
An example of these materials is a family of ultracapacitors revealed in June 2012 with devices offering from 2,47 to 12,53 kW. According to the manufacturer, the cells delivered a 300% increase in power and 200% increase in energy in comparison with commercially available products as a result of their carbide-derived carbon material. This allowed raising their energy density to 10 Wh/kg and their power density to more than 40 kW/kg.
In June 2014, another manufacturer revealed the latest addition to its series of ultracapacitors a new 2,85 V, 3400 F ultracapacitor in an industry-standard 60 mm cylindrical form. Supercapacitors exhibit very favourable characteristics in terms of power density and also have the ability to be charged and discharged countless times without any degradation in performance. This is in stark contrast to chemical batteries which have a defined life span in terms of cycling.
In addition, supercapacitors can be charged and discharged in a matter of seconds and function well over a broad temperature range. They are also resistant to shock and vibration. However, they have a low energy density (ranging from around 1 Wh/kg to 30 Wh/kg), particularly when compared with Li-ion (lithium-ion) batteries, which have about five times the energy density. Another disadvantage in comparison with chemical batteries is the discharge curve, which sees the output voltage drop as the capacitor is discharged.
Although costs are falling rapidly, materials costs for supercapacitors are still relatively high due to the increased difficulty in creating advanced materials like graphene. Nonetheless, their characteristics make supercapacitors ideal for applications requiring frequent charge and discharge cycles at high power but of short duration.
A growing world of applications
Inevitably there are any number of applications that may benefit from the use of supercapacitors, but there are a number which stand out, particularly within the transport sector. For example, vehicle braking occurs over timescales measured in seconds – a duration not compatible with regenerative systems using chemical batteries which can take hours to charge. In contrast supercapacitors can capture and store energy produced by regenerative braking, before releasing it quickly for the maximum power demands of acceleration. As a result they are increasingly being found in cars, trams and buses, for example in start-stop technology for automobiles. A number of carmakers already offer vehicles with this feature. Some estimates suggest that in this role supercapacitors could deliver energy savings of perhaps 15% – 25%.
Other applications are found in cordless power tools, computers and consumer electronics. Suitable examples include delivering the power pulses required to focus camera lenses or sending bursts of information over wireless systems.
Supercapacitors are also emerging as a direct replacement for batteries in heavy goods vehicles where their low temperature performance – which far exceeds that of lead-acid batteries – is seeing them used for cold weather starting. Another example comes from the renewable energy sector where supercapacitors can provide energy storage for renewable energy installations and increased grid stability. They are also found in wind turbine blade pitch systems, particularly offshore where their long life and reliability is a key advantage over battery technologies.
While at face value it may appear that supercapacitors are competing head-to-head with batteries, in particular with Li-ion technology, a more likely scenario is a complementary development. Under this scenario in transport for example batteries would deliver durable power for range while supercapacitors would provide high power for acceleration. This reduces the requirement for batteries, thereby cutting weight – a critical factor in vehicle performance. IEC TC 40: Capacitors and resistors for electronic equipment, has already published International Standards for EDLCs, and has now earmarked these and hybrid EDLCs, which combine a capacitor and a battery, as being in need of appropriate standardization.
Dr Peter Harrop, chairman of research firm IDTechEx, argues that supercapacitors represent a rapidly emerging multi-billion dollar market. The company notes that there are around 200 major manufacturers of electric motors for traction and more than 100 battery suppliers for this market. However, this compares with around 50 or so major supercapacitor manufacturers. “Supercapacitors are improving much faster than are batteries,” Harrop tells e-tech. He explains that while the cost of lithium batteries has fallen by around 40% over the last 10 years, the cost of supercapacitors has fallen to less than a 100th of their initial cost over the same period.
Harrop acknowledges that the impact of supercapacitors on battery sales is still limited but that the gap in sales is narrowing, albeit from a very small base. He notes that the largest reported supercapacitor transaction has increased by a factor of more than 10 within the last year.
In particular, Harrop points to the potential emergence of supercapacitors integrated into a variety of structures – anything from electronics cases to buildings – or even into clothes. “The buzz word is structural electronics, structural supercapacitors,” he says, adding: “There could be standards there for structural supercapacitors that are being developed in the lab.” Ultimately, supercapacitors offer environmentally-friendly energy conservation. In a complementary role with other energy storage technologies, the supercapacitor is charging ahead.