Like Goldilocks, those of us who have been following the electronics design industry for long enough get a sense of when something seems just right. In this case it isn’t porridge that is neither too hot nor too cold, but a component solution that seems just right for some particular applications.
So, with that in mind and under the category “look what we have here” I recently came across David Evans’ slide presentation used in his recent keynote address at CARTS 2013 in Houston, TX on March 26. The talk was titled “Hybrid Capacitors in Energy Applications”. Evans is president and founder of Evans Capacitor and inventor of hybrid electrolytic-electrochemical capacitor technology.
During his keynote the technical aspects of hybrid caps were discussed in great detail. I will offer up the main points here. This is probably also a good time to point out that hybrid caps are available in different flavors from different suppliers and following a look at the Evans product I will also talk about hybrids that are half supercapacitor and half lithium-ion battery from Cornell Dubilier Electronics and Ioxus.
First, however, a bit of background.
A battery has a pair of electrodes and can store energy and provide a sustained current flow by means of a chemical reaction. The chemical process involves the transfer of electrons between an electrode and an electrolyte. The electrolyte imposes restrictions on the process: conductivity decreases with temperature, for example, and the potential of electrolyte breakdown limits operating voltage. Repeating charge cycles induced changes to the electrodes that are not totally reversible. So usually the life of a battery is measured in hundreds or thousands of cycles.
Capacitors store charge physically by means of two metal plates arranged in parallel. Non-conducting insulation between the plates prevents a current from jumping directly between them; as long as the two plates remain electrically isolated the system can be harnessed to produce a useful electric current. As a physical storage devices caps can quickly absorb and release a lot of charge and since electric current causes no physical change in the conductor (ideally, at least) the cycle life of electrostatic capacitors is virtually unlimited. Electronic conduction is fast and independent of temperature. On the other hand, a capacitor’s charge density is not comparable to that of a battery; a supercapacitor, for instance, stores about 1/10th the energy for the same physical space as a chemical battery.
As one might expect Evans’s CARTS presentation focused on his company’s tantalum hybrid capacitors, which take advantage of the best features of electrolytic and electrochemical capacitors. They combine a tantalum pentoxide (Ta2O5) coated electrostatic anode and a ruthenium oxide (RuO2)= based cathode from an electrochemical capacitor with a compatible electrolyte. Using the electrolytic capacitor enables relatively high working voltages to be reached (without exceeding the breakdown potential of the aqueous electrolyte). The working electrolyte is 38% solution of sulfuric acid in water.
Tantalum hybrid capacitor properties are said to be comparable to those of aluminum electrolytic capacitors of the same capacitance and voltage rating, though their size is considerably smaller. Because the cathode requires little volume, available space can be used to increase the size of the anode. The hybrid capacitor has about 4X higher energy density compared to an equivalent electrolytic capacitor, according to Evans who noted that wet tantalum hybrid capacitors suitable for high temperatures have been developed and the company’s life test results performed for 1000 hours at 200°C and 50% rated voltage predicted life of greater than 2000 hours.
The Evans hybrid capacitor is an attractive alternative to aluminum electrolytic capacitors in applications where energy density is important. According to the company its hybrids produce up to 2 joules/cc. and 0.5 joules/g and are included in many avionic and aircraft systems (including in the Boeing 787) to provide power back up and interruption prevention. Other applications are in large phased array radar systems, laser targeting, communications modules, controls, cockpit displays, fire control and other systems. .
Cornell Dubilier’s Type CDHC hybrid capacitors are half supercapacitor and half lithium-ion battery. These hybrids are said to store more than twice the energy of typical supercapacitors. Although, as noted earlier, hybrid capacitors have more power than lithium-ion batteries but less energy storage capability, they also feature a much higher cycle life capability (though less than supercapacitors, which have a cycle life capability of a million cycles or more compared to a battery cycle life of around 1,000 cycles and more than 20,000 cycles for hybrid capacitors).
While supercapacitors have a usual working voltage range of 1.3 to 2.7 V Cornell Dubilier’s hybrid capacitors operate from 1.0 to 2.3 V. The company points out that the higher energy of hybrid supercapacitors make them especially suitable for use in emergency pulse applications, e.g., operation of electric doors and windows. Shelf life is 1000h without voltage at +60 °C. These snap mount hybrid capacitors are designed for board mountable power backup applications including solar lighting, LED lighting, and portable devices.
Ioxus Hybrid Capacitors also are a combination of supercapacitor and lithium ion battery with a cycle life of more than 20,000 charge/discharge cycles. Ioxus’ snap-in hybrid capacitors store 85 to 115% more energy than a conventional supercapacitor. They can be charged from 1.0 VDC up to 2.3 VDC. These hybrid cells offer a maximum power of up to 5kW/kg, compared with up to 3kW/kg for conventional power batteries.
Among the many applications for this technology is powering LED devices, which are much more energy efficient than their incandescent predecessors. For example, the hybrid capacitor can benefit new LED-based flashlights, which are brighter, last much longer and consume less than one-third the power and energy of comparable halogen products. Device manufacturers can now design hybrid capacitors permanently into LED flashlights because capacitor cycle life generally matches or exceeds device life. Compared with a nickel cadmium or lithium-ion battery, hybrid capacitors provide more than 20 times the cycle life and 60 times faster recharge rates. In addition to no longer needing to replace batteries, users can quickly recharge devices before use, unlike battery-operated products that usually take hours to recharge.