Power Electronics
Thin Ultracapacitor Shrinks  Series-Connected Cell Modules

Thin Ultracapacitor Shrinks Series-Connected Cell Modules

Individual ultracapacitor cells have an operating voltage of about 2.7V, so modules containing multiple cells connected in series allow operation at higher voltages. Now, module size can shrink by using multiple Ioxus THiNCAP™ cells (Fig. 1) instead of using similarly rated cylindrical cells. These high power, flexible THiNCAP cells have a maximum capacitance of 1600F. A module with from 20 to 80 THiNCAP cells can operate from 54 to 162 volts and provide 20.8 to 72 F.

Compared with a THiNCAP cell, a cylindrical ultracapacitor cell occupies more space (Fig. 2). A cylindrical cell module with the same capacitance and number cells as the THiNCAP module described above would be about 17% larger. Table 1 compares the characteristics of individual ultracapacitor cells (see sidebar “Ultracaps”) using conventional cylindrical and THiNCAP types.

One key to the smaller size is Ioxus’ proprietary gas management system. Gas management is important because excessive applied voltage for a prolonged period can generate gas inside the ultracapacitor, which may cause leakage or rupture its safety vent. Ioxus’ thin pouch cell design also reduces cooling requirements because it provides double-sided cooling.

Electrodes from the THiNCAP cell are also easier to connect in a series than cylindrical cells. This allows more capacitance and power to fit in a smaller space while maintaining ultracapacitor performance and shrinking its size. THiNCAP cells stacked in series produce higher voltage iMOD™ modules and are said to provide the highest power form-factor in the industry. Its compact design is ideal for space-limited applications.

With a smaller footprint, the THiNCAP iMOD modules (see sidebar “THiNCAP iMOD Module Features”) allow flexible product development across a wide variety of applications. They are ideal for power conditioning and voltage sag compensation in short-term uninterruptible power supplies (UPS) where they protect against voltage disruptions and ensure a smooth power supply. Additionally, robotics benefit from the quick charge and long life of the THiNCAP iMOD modules, cutting charge times in half and increasing productivity and operating time while eliminating the need for battery replacement or charging.

For module cells in series, it is best to derate each cell’s rated voltages to reduce the impact of unbalanced cell voltages on system life. For series connected module cells ESR (equivalent series resistance) increases with the number of cells and capacitance decreases by the quotient of the number of cells:

ESRTOTAL = ESRCELL × Number of cells

CTOTAL = CCELL ÷ Number of cells)

You can connect cells in parallel if you need a capacitance that is larger than an available cell size. Cells of different sizes can be connected in parallel, as long as the same types of cells are used for each series connected unit to match capacitance and ESR. For identical cells connected in parallel, ESR decreases by the quotient number of cells and capacitance increases as a multiple of the number of cells.

ESRTOTAL = ESRCELL÷ Number of cells

CTOTAL = CCELL × Number of cells)

Cell Balancing

By using cell balancing in a multiple cell module you can reduce the variation in cell voltages resulting from an imbalance in leakage currents, capacitance, or power losses from ESR. In particular, use cell balancing for applications requiring long cell life. Voltage distribution of series-connected cells depends primarily on their capacitance, which is related to the standard capacitor equation:

Where:

I = Cell current in amperes (A)

C = Cell capacitance in Farads (F)

dV/dt = Rate of voltage change with time

Re-arranging this equation shows that voltage is inversely proportional to the capacitance:

From this equation, we see that the larger the capacitance, the lower the voltage variation for a given charge rate and time. Therefore, if we have a series stack of capacitors with a 20% capacitance variation, then initially we can get a similar variation in cell voltage. A more important source of voltage variation is leakage current. For a series of cells that remain on charge for an extended period cells with higher leakage currents will have a reduced voltage, which in turn will cause the remaining cells to increase in voltage. Over time, this reduces the life of some cells and creates premature failures.

Passive voltage balancing compensates for variations in leakage current by placing a bypass resistor in parallel with each cell, sized to dominate the individual cell leakage current. This effectively reduces the variation of voltage among the cells. For example, if the cells have an average leakage current of 10mA ±3mA, a resistor that will bypass 100mA may be an appropriate choice. The average leakage current will now be 110mA, ±4mA. Adding this resistor decreased leakage current variation from 30% to 3.6%.

If all the parallel resistances are the same, the cells with higher voltages should discharge through the parallel resistance at a higher rate than the cells with lower voltages. This helps distribute the total stack voltage evenly across the entire series of capacitors. A typical recommendation is to use a resistor that bypasses 10x the leakage current of a cell. Higher ratios can be used for faster balancing.

In some applications, however, the additional leakage current produced by passive balancing is not acceptable. You can remedy this by active voltage balancing that forces the voltage at the nodes of each series-connected ultracapacitor to be the same as a fixed reference voltage. Active voltage balancing circuits typically draw lower current in steady state and only require larger currents if the capacitor voltage is out of balance. Thus, active voltage balancing is ideal for applications where the ultracapacitor is frequently charged and discharged, as well for applications where the ultracapacitor is powered by a finite energy source, such as a battery.

Wind System Application

Ultracapacitors have been used for pitch control and emergency power in wind systems. Compared to batteries, ultracapacitors maintain higher performance in cold temperatures and have a longer cycle/calendar life while generally requiring no maintenance. Fig. 4 shows a pitch control system employing ultracapacitors) for energy storage.

Capacitors could be placed at other schematic points in the system depending on specific design requirements. A switch-mode power supply serves as the charging circuit for the energy storage component. The energy storage subsequently powers the motor controllers, which controls the pitch motor. A complete pitch control system requires one of these circuits for each turbine blade.

To maximize power generation efficiency pitch control systems dynamically adjust blade position relative to wind speed as well as minimize the effects of tower shadow. Also, pitch control is a necessary safety feature if wind speeds go too high or the wind turbine’s grid connection is lost. For either of these problems, pitch control adjusts the blade position to neutral, which acts as a brake for the wind turbine system.

Although the power supply may be located as part of the rotating assembly or stationary inside the hub, the energy storage is often located in the rotating assembly. Lightweight ultracapacitors make them a top choice for energy storage. Heavier batteries require more significant structure to support them in rotation. and need insulation to stave off cold effects .

In cold weather, the higher power capability of ultracapacitors translates to faster response time for similarly designed systems. Other requirements include venting to remove hydrogen gas build up from cycling the batteries, as well as protection from moisture and management systems. By contrast, batteries require moisture protection, inexpensive balancing circuits, and a designed charge voltage that maximizes their life.


Ultracaps

Electric double-layer capacitors (EDLCs), also known as ultracapacitors or supercapacitors, are electrochemical capacitors with an energy density hundreds of times greater than conventional electrolytic capacitors. An EDLC has a capacitance of several farads, more than twice that of an electrolytic capacitor with the same physical size.

You can model an ultracapacitor as an ideal capacitor (C) with an equivalent series resistance (ESR) and an equivalent parallel resistance (EPR), as shown in Fig. 3. ESR includes resistance of the electrode material, the current collectors, the terminals, etc. EPR, which depends on temperature, self-discharges the capacitor over time. ESR and EPR vary from cell to cell.

A small amount of self-discharge, called leakage current, can vary slightly because of variations in ultracapacitor materials and manufacturing. Over time, as the cells are at a high state of charge, the small variations in leakage currents may cause cell voltages to spread apart. Cells with lower leakage current will increase in voltage and cells with high leakage current will decrease in voltage. Leakage current increases with voltage, so cell voltages eventually stop spreading apart when the individual leakage currents become equal. Occasionally there can be enough of a spread so that cells at higher voltages degrade prematurely when a series-connected cell bank is at a nominal voltage.

Due to their low internal resistance ultracapacitors exhibit low heat generation. And, the cooler they operate the longer their reliable service life. Therefore, natural air convection should provide adequate cooling for most applications.

Ultracapacitor thermal resistance, RTH, is usually determined assuming free convection at ambient (25°C). Data sheet RTH is useful for determining the operating limits for the ultracapacitors. For a module containing multiple ultracapacitors you can calculate the temperature rise using the RTH value at any current and duty cycle. Temperature rise above ambient is:

ΔT = D×RTH×I2×RESR

Where:
D = Duty cycle
I = current AC or DC (A)
RTH = Thermal resistance (C/W)
RESR = Equivalent series resistance (Ω)

This temperature rise above ambient should remain below the specified maximum operating temperature for the module. If forced cooling methods are employed, it is possible to operate the units at higher currents or duty cycles.


THiNCAP iMOD Module Features

  • Quick charge and long life increase productivity and reduce operation costs
  • Complete system is available, including DC-DC converter, to fit a range of applications
  • Maintenance-free to reduce total cost of ownership
  • Compliant with Restriction of Hazardous Substances (RoHS)
  • Low ESR for higher efficiency and power capabilities
  • Front terminal products that replace or enhance rack-mount battery systems
  • Compatible with common rack enclosures, including 19-inch, 23-inch wide and greater than 20-inches in depth
  • Highest possible energy content per shelf for a capacitor-based system with up to 40 kW (10s) per 4U rack slot; provides up to 100% more energy than the competition

Related Articles:

Ultracapacitor Technology Powers Electronic Circuits

Ultracapacitors Boost Performance and Lower Cost

Supercapacitors Enhance LDO Efficiency- Part 1: Low Noise Linear Power Supplies

Supercapacitors Enhance LDO Efficiency- Part 2: Implementation

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