Most uninterruptible power supply (UPS) applications in telecommunications and telematics at present are based on battery technology. It is even currently used to buffer peak loads. In such applications the battery offers advantages thanks to its relatively high energy density, maturity, and years of familiarity in use. So the architectures are designed specially to match the properties of the available battery technology.
For almost 20 years now, ultracapacitors have been successfully used as peak and standby power solutions in numerous applications, including wind turbines, mobile base stations, industrial robots, and a variety of other electronic devices and industrial machines. In recent years, due to the rising demands in a wide range of high-volume applications, they have developed into energy storage devices which - in terms of both cost and performance - are also an interesting alternative for use in large and small UPS systems as standby power supply for data centers, hospitals and high-tech manufacturing facilities. Typical system sizes start at a few kilowatts, and can easily be expanded to megawattscale systems with modular solutions. In those solutions, single ultracapacitor cells are linked to form modules or even systems.
Perfect for peak power and short-term backup
All of these applications - regardless of the required power class - demand power quickly in order to either provide the required peak power or to bridge the mostly very short outages that occur nowadays. In the event of a prolonged power outage, the plant or system is set to a safe state (graceful powerdown). Peak power and bridging times typically range from a few milliseconds to 20 seconds. Ultracapacitors are ideal for these applications in particular. They are able to release, and recover, most of their stored energy within seconds (or fractions of a second), countless times, without being damaged. They are designed for a service life of ten years and more in such applications. They are also easy to handle. No maintenance or servicing is required, and they are very simple to monitor. The voltage curve can be used to easily evaluate and monitor the health status of the ultracapacitor. Lead-acid batteries - the predominant method of energy storage at present - have a short life even under ideal conditions, and unexpected failures occur due to their electrochemical composition. They are much more c o m p l e x and costly to monitor and health-check than ultracapacitors. Additionally, they are difficult to manufacture in an ecologically sustainable way. Ultracapacitors are different: their properties are based on an activated carbon material with an extremely large electrical surface area. The material is used as an electrode, and an electrolyte impregnated in the cells ensures the necessary exchange of charge.
Ultracapacitor design
Depending on the state of charge, the ions of the electrolyte accumulate on the activated carbon (carbon electrode) at a distance in the nanometer range. Since the capacitance is directly proportional to the surface area, and indirectly proportional to the charge gap, ultracapacitors can store several hundred times more energy than conventional capacitors. The charging and discharging process takes place electrostatically, without chemical reactions as happen in batteries. Ultracapacitors can therefore capture and release the stored energy much faster, and with no degradation. This makes them ideal for applications with high power output and energy requirements and large numbers of cycles. While batteries can store up to 20 times the energy, ultracapacitors offer up to 20 times the power density of batteries thanks to their very fast charge/discharge characteristics.
Batteries provide high energy availability - ultracapacitors ensure high power output
If high energy availability is a prerequisite, a battery storage system is the first choice - despite its known weaknesses, and regardless of the battery technology it employs. However, combinations of batteries and ultracapacitors are increasingly being used. It is important to note that the two storage technologies have different potential characteristics: batteries store and supply their energy via redox reactions (i.e. Faraday or mass transfer processes), and so maintain a virtually constant potential until the action mass is consumed. With ultracapacitors, on the other hand, the voltage changes with the stored charge.
Dream-team battery and ultracapacitor
For applications in telecommunications and telematics, however, benefits can be gained with a direct parallel combination. If, for example, individual lithium cells (~4V) are connected in parallel via two series-connected ultracapacitors (~2.5V), the ultracapacitor delivers a large part of the peak power required during transmission due to its very low internal resistance. The lithium-ion cell provides all the reserve and standby power. The combination leads to a significant improvement in operating time. Similar examples can be found in the power supplies of telecommunications base stations. They require local energy storage in case of voltage dips in the supply line and outages lasting from milliseconds to several seconds.
The active parallel combination requires a power processor that is as efficient as possible and a bidirectional DC/DC converter that is comfortable handling wide voltage fluctuations at the input and allows immediate power reversal without loss of control. With their very low internal resistance, today's ultracapacitors offer the possibility of achieving the efficiency rate of over 90% demanded for an efficient system (ultracapacitor plus DC/DC converter). Cost must of course also be taken into account. Nevertheless, an overall costbenefit analysis reveals the advantages of a combined solution. Lifetime tests have shown that combining with ultracapacitors can massively extend the service life of battery storage systems, and significantly enhance power availability. Many companies all over the world are currently focused on combination solutions, and have developed the necessary power electronics.
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