The key to capacity: capacitors for energy storage
01 September 2019
In energy storage applications, capacitors are the ‘unsung heroes’ of the field. These two-terminal passive electrical components store energy & discharge when needed – often times as a critically important ‘back-up’ power source.
This article was originally featured in the September 2019 issue of EPDT magazine [read the digital issue]. Sign up to receive your own copy each month.
Capacitors are more part of our daily lives than we may realise. Something as simple as an alarm clock, for instance, keeps a charged capacitor on hand in case there is a power failure. If the power source goes out, the capacitor discharges – sending current through the clock circuit to ensure it is kept running. Here, Helmut Hartl, Head of Research & Development for automotive applications at glass-to-metal sealing specialist, SCHOTT explores the increasing demand for capacitors, their vital role across a range of applications, and how glass-to-aluminum-seal (GTAS) technology supports long, reliable service lives for these electronic components.
As platforms for their effective use continue to evolve and develop, new capacitor types are entering the marketplace on a regular basis, to the point that supercapacitors – also known as electric double-layer capacitors (EDLC) or ultracapacitors – are now available for larger-scale usage. Electric vehicles, such as EVs, HEVs and E-Buses, rely on these components as they have far greater charge storage areas than standard capacitors, and high power applications and renewable energy applications also utilise the technology. Other example application areas include the defence, energy and aerospace sectors, as well as a variety of industrial applications.
Applications for capacitors and supercapacitors
In the United States and beyond, increased demand for capacitors and supercapacitors has a number of drivers across a wide range of applications. Influences such as the Internet of Things (IoT) and the rise of smart devices are a prime example – these have led to demand for powerful lightweight components, requiring little or no maintenance, with the ability to operate in harsh environments across a wide range of operating temperatures, high-power density, and high reliability.
The automotive sector is a key market for capacitors and supercapacitors, as there are a large number of functions where their capabilities can be effectively deployed. Start/Stop functions and power steering make use of capacitors, whereas hybrid electric drives require the increased power capability of a supercapacitor. As electric vehicles continue to develop and enter the mainstream automobile market, these components will experience further increased demand. Technological advances in the future could even result in supercapacitors replacing lithium-ion batteries as a power source and delivering a comparable driving range to that of gasoline and even diesel-powered vehicles.
The rail industry has also started to realise the potential of supercapacitor technology. Manufactured in Zaragoza, Spain by rail company, CAF, the Urbos 3 Tram uses a series of supercapacitors situated in the upper part of the carriages to allow braking energy to be recovered – enabling 35% electricity savings. The supercapacitors can be charged at tram stops without the need for overhead cables, and can also run between certain stops without the need to be connected to a cable at all.
In the renewable energy sector, supercapacitors are already prominently found in wind turbines, among other applications. The turbine’s blades must be regularly adjusted to ensure the turbine is operating at optimal levels in line with wind conditions and to avoid mechanical stress. To achieve this, a pitch control system is used to make those adjustments when required. Should the power supply fail, supercapacitors are able to engage and deliver enough of a burst of power to return the blade pitch to a safe position to shut a turbine down, which is not always possible with battery power.
Supercapacitors also have the potential to play a key role in the future of energy storage, in both photovoltaic (solar) and wave-powered applications, as they develop. Both photovoltaic and wave technologies experience fluctuations in terms of output power. Supercapacitors can mitigate the effects of this: helping to deliver a more constant level of power through a process referred to as smoothing.
Both solar and wave-powered applications produce alternating current (AC) voltage, which must be converted to direct current (DC) via what is called a bridge rectifier. However, if this power were to be applied without the use of a capacitor, a ripple would remain in the supply as the voltage oscillates between 0V and maximum voltage. If applied to a light bulb, for example, the bulb would flash on and off. The capacitor ensures there is no break in the supply through constantly storing energy when output voltage increases and discharging that energy when it decreases. Execution of this process supports greater longevity of specialised, costly components in these applications, ultimately making them more viable as clean energy resources.
The challenges of aluminium electrolyte capacitors
Aluminium electrolyte capacitors and supercapacitors have a weakness: over the duration of their usage lifetime, they are prone to electrolyte dry-out – often experienced as a result of imperfect terminal seals. Small amounts of moisture can penetrate an imperfect seal, which can cause gases to build up inside the capacitor over time. Electrolyte dry-out causes continuous deterioration in performance levels and is often counteracted by oversizing or using two capacitors where one could be sufficient without the presence of this chronic issue. The slow and continuous evaporation of electrolytes can lead to a capacity loss of up to 20%, which is significant in terms of efficiency.
Polymer seals, often used to seal capacitor terminals, are a weak link in the capacitor structure and can be faulted for both damaging moisture intrusion and loss of electrolytes through evaporation. Polymers, as with all organic materials, are prone to aging and become brittle over time, subsequently losing gas tightness. As humidity intrusion occurs through a faulty seal, moisture is able to penetrate the weakened seal, while electrolytes are able to evaporate, ultimately leading to significant losses in capacity and lifespan.
Hermetic, gas-tight sealing: the answer for efficient, long-lasting capacitors
The remedy for electrolyte leakage lies in the capacitor lids. By replacing seals made from organic compounds, such as polymers, with a specialty glass seal, capacitor terminals can be hermetically sealed into the aluminium lids. This protects the capacitor from moisture intrusion via the pin sealing and eliminates the issue of electrolyte dry-out. As glass is an inorganic material, glass seals maintain permanent leak-tightness as they do not age or wear out over time. Glass-sealed lids can be customised to suit a wide range of applications for both small and large can types – including radial type, axial type, snap-in, supercapacitors and electric double layer capacitors. Glass-to-aluminium sealing (GTAS®) is a new technology specifically developed by glass expert, SCHOTT for capacitors (Figure 1) and batteries with high energy density. The methodology is based on the company’s expertise in specialty glass and glass-to-metal sealing dating back to 1939.
Utilising glass-to-aluminium seals (GTAS) has wide ranging peripheral benefits. By design, glass-to-aluminium seals available on the market today can provide high temperature resistance ranging from -40°C to +150°C, allowing them to be utilised in a vast array of applications. The leak-tight nature of the seals also means that similar or smaller-size units with higher capacitance can be designed. The reliable leak-tightness of the non-aging glass seal not only allows for the extension of shelf life of a product, it increases the service life of the product itself – even in harsh environment conditions.
Glass-to-aluminium sealing adds tangible value, with greater reliability and longevity
New designs of aluminium electrolyte capacitors, including cutting-edge supercapacitors, are currently in development for modern and emerging applications. Possibilities include electric vehicles, high-power applications, heavy industry and renewable energy, among others. These applications pose new technical requirements on the capacitor, demanding long-lasting high performance. These innovation demands are where GTAS affords an opportunity for improvement: leak-tight capacitor lids can support both small and large capacitor designs to fulfil such requirements.
GTAS Capacitor Lids can offer up to a 20% reduction in capacitor package volume, a 60% reduction in capacitance loss and a 50% improvement of internal resistance. This, coupled with the components’ ability to operate across a broad range of extreme temperatures, equates to an increase in product longevity of anywhere from 10-15 years, compared to that of organic-sealed counterparts.
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