Review: The development of supercap technology

Supercap technology, also known as Electric Double Layer Capacitors (EDLC), has made remarkable progress in recent years. Super­caps are also known as EDLCs, ULTRACAPs, super­capacitors, double­layer­condensers or "supercap" for short. These are conden­sers with an electro­lyte and carbon electrodes as the electro­con­ductive layer. Compared to conven­tional condensers, they have a signi­ficantly larger surface area, which leads to a higher capacity. The technical and physical principles are very well described in the literature (see Wikipedia). The variety of terms for this type of capacitor is further extended by manu­facturers' own terms:

• Goldcap™  [Panasonic]
• Boostcap™  [Maxwell]
• Greencap™  [Samwha]
• PURIXEL™  [Pureechem] for supercap cells
• PURETRON™ [Pureechem] for supercap modules

It all started in Germany with NESSCAP

For CAPCOMP, the story in Germany started in 2005 when we signed a contract with the South Korean company NESSCAP, which emerged from the DAEWOO Group. At that time, the idea was still to use EDLCs as the primary energy storage device in cars. It was hoped that the development of the technology would progress in a similar way as semi-conductor technology had done for years. Unfortunately, this remained a dream.

Electro chemistry and the associated physical principles quickly revealed limits. At the beginning of the 2000s, there were various prototype developments in the USA for use in electrically operated vehicles. Tests at the University of Vermont showed that battery-powered vehicles supported by ULTRACAPs achieved about 35% - 40% more range, depending on the driving cycle.

No less interesting were the results con­firming an increase in battery life where power peaks were absorbed by ultra­capacitors.

In addition to high expectations on the part of the auto­motive industry, there was also scepticism, as the AECQ200 test results did not yet provide enough data from which binding technical speci­fications could be derived. As distributors from the beginning, we and our competitors had no choice but to define criteria based on those of electrolyte capacitors and to test according to them.

Times of complex tests under extreme conditions

Dozens of test series were set up for the most diverse appli­cations. Even the beha­viour of the ultracaps in a vacuum and at extreme minus tempera­tures was examined. The cells were frozen to -60°C, thawed, tested - and passed! No negative change in the technical properties. Just like practical tests in vehicles tested at 3500 m above sea level at -25°, like the appli­cation in a car after it had been standing at the air­port in the sun in Jeddah, Saudi Arabia, for 5 days. In addition, acce­leration tests, vibration tests, fire load and emission tests, as well as dura­bility tests under a wide range of condi­tions provided valuable insights. For example, single-cycle tests to assess the cells under a specific load behav­iour took half a year to produce initial results. These tests were then conti­nued for another 6 months to obtain a reliable database.

MAXWELL, NESSCAP and PANASONIC made the start

It was the companies MAXWELL, NESSCAP and PANASONIC, as the main players in ultracap develop­ment, who were instru­mental in driving the accep­tance of this new tech­nology. Other manu­facturers followed and built on this expe­rience. In principle, the require­ments of the market have not changed since the first hour.

From the prismatic cell to the high-voltage ultracap module

At the beginning (Vermont tests), prismatic cells with up to 5000F and 2.7 volts were still used. In the early phase, this techno­logy was consi­dered a promising solution, but then quickly showed physical weak­nesses, especially in heat distri­bution. In manu­facturing tech­nology, it turned out to be too complex and costly for large-scale production.

Great progress in module development

With the develop­ment of the 60mm cylin­drical, axial 3000F/2.7V cell, it became possible to develop high-voltage cells and high-energy modules. These cells could store and return large amounts of power quickly, with high relia­bility and cycle stability of over one million cycles. The horizontal heat distri­bution and much better thermal resis­tance than previous models allowed for more efficient thermal mana­gement. Different models of modules were created for a wide range of applications.

Recuperation solution for trucks

After some time, the 125V/62F module became an industry standard in buses and trains in several variants. For recu­peration, i.e. energy recovery from braking energy, the modules proved to be the ideal solution; even better than for passenger cars. Not only because of the more relaxed situation with regard to instal­lation space, but above all because of the better cost/benefit ratio.

Shorter payback times for recuperation technology in trucks

Commercial vehicles are usually operated much more intensively than passen­ger cars. The invest­ment in an ULTRACAP recu­peration system could there­fore pay for itself much more quickly. In city buses and trams, the modules were already widely used about 15 years ago and are only now reaching their service life limit. Several 125V/62F modules were connected in series/parallel. They were usually mounted on the roof or in the under­body of the vehicle. The balancing of the modules within them­selves and among each other was done via "Smart Balancing". Data was sent via CAN-BUS inter­faces to central ECUs for further evaluation and control. The smaller 48V/165F modules, also based on the 3000F/2.7 volt cells, are still an industry standard today.

48V/165F ultracap modules in driverless transport systems

They are used extensively in Auto­mated Guided Vehicles (AGVs), but are suitable for all uni­versal energy storage systems. They are easy to confi­gure in series-parallel circuits and also easy to handle due to their relatively low weight of approx. 15kg. In addition, there are now a large number of vari­ants in a wide range of volt­ages and capa­cities, which are used depen­ding on the application.

The next step in development: 3 volt cells

In the meantime, 3V cells in the identical form factor as the cells of the 2.7V tech­nology are increas­ingly gaining accep­tance. These are still a little more expen­sive than the older types, but it is a matter of time before only 3V techno­logy dominates. Today, almost 20 years after the early walking attempts with elec­trically assisted drive systems based on ultracap, we are entering a new phase. Fully electric vehicles with lithium-ion batteries have already been the new standard for passenger cars for several years.

Fuel cell & ultracap: dream combination of tomorrow?

In commercial vehicles, there are already the first vehicles with this combi­nation. But here, too, the discussion about whether hydro­gen tech­nology in combi­nation with fuel cells and ULTRACAPs will be used on a large scale as a fast high-perfor­mance energy storage system is not over. The first commercial vehicles are already being tested. There are now new hybrid tech­nologies that allow capacities of up to 100.000 farads. It remains exciting!