Supercaps in DC emergency power supplies: The reliable alternative to batteries for short-term power failures
Uninterruptible power supplies (UPS) with reliable energy storage devices are indispensable for bridging unstable supply networks and short-term power failures and for protecting sensitive devices and systems. Maintenance-free ultracapacitors, also known as Supercaps or supercapacitors, are particularly suitable for this purpose. These work according to the principle of the double-layer capacitor (EDLC - Electric Double-Layer Capacitor) and have a high power density. Depending on the load, they can realise bridging times up to the minute range.
Unlike batteries, Supercaps can be fully charged or discharged within a very short time. This makes them ideal for systems that require a quick response time, such as during short-term power outages or voltage fluctuations. In addition, Supercaps have a longer life than conventional batteries because they do not use chemical reactions and are therefore less susceptible to wear.
1. Highly efficient and fast-charging energy storage
Supercaps work on the principle of the double layer capacitor (EDLC)
Supercaps can absorb and release energy quickly compared to conventional batteries and have a longer life and more charge cycles. They also require less maintenance and have a higher temperature resistance, making them ideal for applications in demanding environments. Supercaps work on the principle of electrochemical double layer capacitors (EDLC). In this process, ions are stored and discharged at the interface between electrodes and electrolyte to store and release energy. Compared to conventional batteries, Supercaps have a very high charge and discharge efficiency due to this principle and can go through a large number of charge cycles, which increases their longevity and reliability as energy storage devices..
Capacitive energy storage
In principle, capacitors consist of two electrode surfaces facing each other at a small distance and a dielectric as a non-conductive insulation layer in between. If the electrodes are connected to a voltage source, they are - described in a simplified way - charged with opposite poles and generate an electric field due to the electric potential between the two electrode surfaces. If both electrode surfaces are fully charged positively or negatively, the current flow stops, i.e. the capacitor is charged and stores the electrical energy so that it can be withdrawn again by connecting a consumer circuit. The storage capacity, or capacity C for short, of a capacitor depends essentially on the surface size of the electrodes and their distance from each other. The dielectric properties are also included in the formula for calculating the capacitance of a capacitor in the form of the dielectric constant:
C [F] = ε • A / d C
C capacitance / ε dielectric constant / A area / d plate spacing
Double layer capacitors (EDLC) with particularly high power density
In the development of double-layer capacitors or Supercaps, these parameters have been decisively optimised in some places so that, compared to ceramic, tantalum or electrolytic capacitors, high capacities (up to several thousand farads) can be realised in a much smaller space: On the one hand, the electrodes are made of activated carbon, i.e. pure carbon with a particularly large surface area of up to 1000 square metres per gram. Secondly, the dielectric was replaced by an electrically conductive electrolyte and an ion-permeable separator. The illustration shows the basic structure of a double-layer capacitor. During the charging process, the negative anions move through the separator to the positive electrode, the positive cations move to the negative electrode. The Helmholtz double layers, which are only a few molecular layers thick, form at the two boundary layers between the carbon electrodes and the electrolyte. Due to the extremely small distance, electrical charge carrier layers with a particularly high power density are created, which behave like two capacitors of the same capacity that are connected in series via the electrolyte. The combination of a large electrode area and minimal distances at the boundary layers ultimately makes the double-layer capacitor a capacitance giant with compact dimensions.
2. Supercaps – maintenance-free and extremely durable
Long-term system availability with low total cost of ownership (TCO)Unlike batteries, which store energy via a chemical reaction, Supercaps are based on electrophysical principles and are charged and ready for use within a very short time, operate in a wide operating temperature range (-40 to +85°C) and impress with their high current-carrying capacity, power density and reliability. Due to their high cycle stability (>500,000 charge and discharge cycles), energy storage units with double-layer capacitors have a particularly long service life. For the supplied application, this means an increase in long-term availability with simultaneous minimisation of the maintenance effort. Even after reaching the EOL (End of Life), a double-layer capacitor is not defective, but only shows a predefined reduction in capacity and a higher ESR (equivalent series resistance).
Optimum cell voltage extends Supercap life
Although the temperature resistance and service life of double-layer capacitors is particularly high compared to other energy storage devices, their capacitance (C) and equivalent series resistance (ESR) change over time. The end of the service life of a supercap is reached when the capacitance drops to 70% of its original value or the internal resistance doubles. The effective service life depends crucially on the ambient temperature, cell voltage and charge/discharge currents. Minus temperatures do not pose too many problems for Supercaps, unlike standard lithium-ion batteries, although the internal resistance increases at low temperatures due to the reduced ion mobility in the electrolyte, but this is quickly compensated for by the resulting heat development in the supercap. However, high temperatures have a negative effect on service life.
In addition, the chosen cell voltage is also an important factor. The figure shows the direct relationship between temperature and service life at different cell voltages of a Supercap with a nominal cell voltage of 3.0 volts and a capacitance of 100 F. For our Supercap energy storage modules, a balanced solution with a reduced cell voltage of 2.6 volts (nominal 3.0 volts) per supercap was chosen to ensure long-term operation in the defined operating temperature range of -20°C to +70°C.
Since the amount of stored energy in the capacitor increases or decreases by the square of the cell voltage (W = 0.5 * C * U2), it was important to carefully weigh the reduction in cell voltage during development. Moreover, the usable amount of energy is further reduced by the fact that Supercaps in practice are only discharged to a minimum voltage Umin of about 1.0 V, since already when the capacitor nominal voltage Umax drops to half its value, around 75% of the stored energy has been released. Therefore, the effective amount of energy
Weffective = 0.5 * C * (U2max - U2min)
is available for the application. A deep discharge below Umin is therefore not technically sensible, although a complete discharge would not damage the Supercap. Completely discharged Supercaps are particularly advantageous for longer storage and safe transport.
Cell balancing for optimum capacity and service life
In supercap systems, different cell voltages and cell capacities can occur, which can lead to an imbalance between the cells. This would not mean a reduction in performance, but would also lead to a shortened lifetime of the Supercaps. Therefore, cell balancing is a crucial function that compensates for the imbalance between the cells and ensures that the cells operate at a similar voltage and capacity level. Active or passive cell balancing improves the performance of the Supercap system and increases the lifetime of the Supercaps. Overall, cell balancing is an important factor in the performance and lifetime of Supercap-based energy storage systems.
3. High reliability for demanding applications
Supercap energy storage with wide operating temperature range
Unlike conventional battery technologies, which normally have to be operated within a narrow temperature range, our Supercap energy storage solutions can be used in a wide operating temperature range from -20° to +70°C. This enables 24/7 continuous use in environments with extreme temperatures, such as those found in industrial or outdoor environments. In addition, the wide operating temperature range reduces the need for costly cooling and heating systems, which lowers the overall cost of the system and increases efficiency.
Maintenance-free DC UPS solutions with Supercaps - Made in Germany
Our UPS and energy storage solutions with Supercaps (EDLC) are not only environmentally friendly and sustainable, but can also make more economic sense than batteries due to their particularly long service life and freedom from maintenance. Supercap energy storage systems are reliable and have a long service life. They are also less prone to failure due to high or low temperatures, making them ideal for applications in harsh environments.
Compared to conventional lead-acid batteries, Supercap energy storage systems are significantly more efficient and durable. Under comparable operating conditions, Supercaps have up to ten times longer life and offer higher current carrying capacity, power density and reliability. Supercaps can go through many thousands of charge and discharge cycles without degrading their performance. This makes them an ideal choice for applications where long life and reliability are required. Supercaps also offer a higher current carrying capacity than lead-acid batteries.
Supercaps are thus a promising alternative to traditional batteries as energy storage in DC UPS systems for demanding applications with short and medium bridging times. They convince with short charging times and high power density, are reliable and particularly durable.