Power Electronics

Replacing Energy Storage with Carbon Aerogel Supercapacitors

Supercapacitors provide a highly rechargeable energy source.

Building control systems use electric motors to drive valves and dampers that control the flow of water and air in HVAC systems. Historically, ac motors have been used i n combination with mechanical springs to open and close the valves and dampers. This electromechanical approach requires the use of larger, higher power ac motors that operate in stall mode whenever the device is under operation, resulting in greater power consumption and increased maintenance on the ac motors and spring assemblies.

A new approach uses dc motor systems operate the devices in both directions. Without a mechanical spring assembly, energy is required to return the devices to their normal or safe positions. Carbon aerogel supercapacitors are used as energy storage devices to operate the devices in normal operation — and during power failures. The advantages to this approach include lower power consumption, higher reliability/lower maintenance, and lower overall cost.

Supercapacitors are electrochemical double layer capacitors designed and constructed similar to electrolytic capacitors but have orders of magnitude more energy density. Supercapacitors use high surface area carbon to accumulate charge as opposed to the relatively low surface area foils in electrolytic capacitors. The electrochemical double layer is formed at the interface of the solid carbon electrode and a liquid electrolyte.

The figure shows carbon aerogel supercapacitor construction. Carbon aerogel supercapacitors are unique because they use aerogel carbon as the active electrode material instead of activated carbon used by other supercapacitors. The carbon aerogel material is known for high purity, highly usable surface area and high electrical conductivity. The result is a very high capacity product that offers extremely low ESR, high energy density, very low leakage current, wide operating temperature range, and nearly infinite charge-discharge cycle life.

The basic radial-lead cylindrical product has an operating voltage rating of 2.5Vdc but can be combined in series or parallel configurations to accommodate higher voltage or lower resistance applications as needed (see photo, on page 33). These supercapacitor products are ideal for HVAC valve and damper actuation where there is a need for a short duration, highly rechargeable energy source.

Two examples illustrate the use of carbon aerogel supercapacitors in HVAC applications. One involves industrial dampers used in air-conditioning systems in high-rises, and commercial and industrial buildings. The old approach opens the dampers using ac motors which also load and maintain tension on springs. When the dampers need closed, power is removed and the springs bring the dampers to the closed position. This also occurs when power is lost as a fail-safe mechanism in case of fire to prevent smoke from traveling through the ductwork. The new approach converts the ac to dc and then provides the dc to actuate the damper and charge a bank of supercapacitors, which is in parallel with the power supply. When main power is lost, the energy stored in the supercapacitor bank is used to actuate the damper. This approach can be more expensive initially; however, elimination of maintenance of the ac motors and spring systems represents a savings over the life of the product.

A second example involves the actuation of zone valves in a residential hot water heating system. Again, the traditional system uses a control voltage, in this case from a thermostat, to actuate the valve and maintain a load on a spring when hot water is required. The thermostat control voltage is removed when heat is no longer needed and the spring returns the valve to the normally closed position. The new approach uses the control voltage to charge a capacitor and operate the valve. The supercapacitor provides the energy to close the valve, when the thermostat removes the control voltage or if power is lost to the heating system, enabling the valves to fail safe in the closed position.

The design approach to implement the use of supercapacitors in the above applications is relatively straightforward. Four key parameters determine the carbon aerogel supercapacitor requirements. Using the damper application as an example, the table lists the four parameters and their values.

The discharge time is the amount of time required to operate the motor and actuate the damper. The minimum working voltage is the voltage at which the motor will no longer operate. Using this information, you can estimate the value of the carbon aerogel supercapacitor required. This calculation equates the energy needed during the hold-up period to the energy decrease in the supercapacitor, starting at Vwv and ending at Vmin, where C is capacitance in Farads (F).

Energy needed during hold-up period: ½ ILoad (Vwv + Vmin)t (1)

Energy decrease in supercapacitor: ½ C(Vwv2 - Vmin2) (2)

Therefore, the minimum capacitance value (in Farads) that guarantees hold-up down to Vmin (neglecting the small voltage drop due to I2R) is:

Using the parameters in the above case, the capacitance required is calculated to be 0.56F. The working voltage of 14Vdc requires that a minimum of six of the 2.5Vdc capacitors be used in series. Working backward from 0.56F, the minimum individual capacitance required is (0.56F × 6) 3.36F. The user can then choose from the available standard capacitance values (e.g. 4.7F, 6.8F, 10F, etc.) after adding safety margin to offset minor degradation over the life of the product, operating temperature range, voltage variability, and other considerations related to the application.

For design assistance, a Carbon Aerogel Supercapacitor Calculator developed in Microsoft Excel® is available on Cooper Electronic Technologies' Web site: www.cooperet.com/documents/PowerStor_Aerogel_Capacitor_Calculator.xls.

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