Ultracapacitors Offer Improved Energy Storage Options
To the Transportation Market

Chad Hall, Chief Operating Officer
Ioxus

The efforts to create better and more efficient power sources for the automotive industry began more years ago than most people might assume.  In fact, automotive engineers, driven by the benefits of utilizing energy as efficiently as possible, have been trying to solve this problem for a long time.  They began by reducing the gross weight of an automobile by using lighter metals when possible, such as replacing copper core radiators with aluminum core radiators. These early efforts proved valuable, but were just the beginning of better energy efficiency in automotive design. The demand for better and more efficient power sources in the automotive industry has had a noticeable and beneficial effect on energy storage in transportation.

A little history is helpful as we consider how the industry got to this moment of innovation in battery and capacitor technology and electronic power supply design.  As computer control mechanisms and sensors became more sophisticated, engineers developed fuel injected systems that increased engine performance and fuel efficiency. The early efforts to make more fuel-efficient automobiles focused on how to squeeze more useful energy out of the internal combustion engine. Meanwhile, alternate power sources were largely ignored due to their expense. One of the more obvious alternative power sources is the battery, but the size and weight of batteries limited innovation, particularly in terms of energy-efficient vehicles. The next best approach was a hybrid power plant, which uses a small internal combustion engine (ICE), to work in concert with a battery pack. Hybrid power plants have come a long way in the last five years, but they still leave a great deal to be desired.

The internal combustion engine has remained dominant in the automotive industry because it has high energy density (gasoline) and high power density controlled by the rate of fuel ignition. This combination of energy and power density does not exist for batteries or fuel cells. To make the point even more clear, gasoline has an energy density on the order of 45 MJ/kg [1] while batteries are, in most instances, more than an order of magnitude less having energy densities on the order of a fraction to a few MJ/kg [2]. Therefore, it is not surprising that the latest entries into the energy efficient power plant still have an ICE component. In order to make batteries and fuel cells more attractive, their overall performance must be increased.

What is an Ultracapacitor, and Why Should the Transportation Industry Care?
The introduction of the first ultracapacitors by NEC in 1978 ushered in a new generation of capacitors characterized by energy storage capacities on the order of a million times greater than conventional capacitors. For example, a large electrolytic filter capacitor in a power supply might be 5,000 micro Farads, as compared to a 5,000 Farad ultracapacitor device that is readily available today. The ultracapacitor market was slow to gain traction because the devices were initially expensive.  The first applications of ultracapacitors supplied backup power for volatile memory and internal clocks in computers, VCRs and other electronic devices. Today, ultracapacitors are finding their way into hybrid cars and public transportation buses.

Ultracapacitors, like all capacitors, have a high power density. What differentiates ultracapacitors from their traditional counterparts, electrolytic capacitors, is their high energy density, allowing them to store a vast amount of energy in a small package. This energy storage is possible because of nanoscale carbons, with extremely high surface areas. For example, to create a 1 Farad capacitor with traditional methods, 100m2 of surface area was required, making a capacitor of 5,000 Farads impractical, if not impossible. In an ultracapacitor, materials with surface areas of up to 2,000m2 are used, thus reducing the physical size, allowing for a capacitance of 5,000 Farads to fit within the size of a brick. The capacitors with which most design engineers are familiar have short time constants, meaning their voltage cycles quickly, whereas ultracapacitor arrays have time constants of the order of tens of seconds to minutes. The large capacitance and extremely low frequency time constants allow ultracapacitors to be used in applications that have not been practical or economical for other types of capacitors.  Using such supplies in concert with power electronic techniques make the design and cost of power conditioning equipment available to most volume users of electrical energy.

Since ultracapacitors have a much lower internal resistance and much faster charge rate than batteries, they can make a battery-powered system run much more efficiently.  An array of ultracapacitor cells in series coupled to a load in parallel with a storage battery creates a hybrid power source with higher power and energy density than either device in a stand-alone configuration. By gradually taking on a load, batteries are insulated from high current drains that cause thermal, chemical and mechanical stresses.  By reducing current spikes, the internal temperature of batteries is decreased substantially, extending the life of the batteries significantly. Additionally, there are times when a battery simply cannot deliver the current needed for an application, and in particular when one needs to continually charge and discharge, such as is needed for load leveling. Conversely, there are times when an ultracapacitor alone will not work, in particular when there is a need for extended discharge for minutes or hours.

The use of ultracapacitors often requires sophisticated control circuits and power electronic circuits for an efficient and economical application. One of the most important applications of ultracapacitors is their ability to increase power density of an energy source. Any time a battery is used to supply power to a variable load, an ultracapacitor can be used to supplement the primary energy source by increasing its power density. Batteries in general provide electrical energy by virtue of a chemical reaction, and the rate of reaction limits the rate at which energy can be delivered. When a battery is subjected to a variable load, the current draw can become enormous, causing internal heating that will shorten the longevity of the battery; however, when an ultracapacitor is introduced into the equation, this negative effect can be significantly mitigated.

Pairing Ultracapacitors and Batteries to Build “Smart” Supply
Pairing a capacitor with a battery will improve the power density of hybrid supply, which has the added advantage of allowing the battery to operate without seeing large current spikes that would be present in the absence of the capacitor. The ability to prevent the battery from experiencing these current spikes under load allows the battery to have a longer, more effective life. A typical starter battery, for example, will degrade quickly if it is required to supply high current for any length of time. So-called deep cycle batteries are designed specifically to supply higher currents, but even such batteries with their thicker lead plates are not immune from damage due to repeated deep cycling. A parallel configuration of a battery with an ultracapacitor can dramatically reduce the deep cycling of the battery under heavy load conditions and thus extend the life of the hybrid power supply as well as providing a more efficient supply.

In most instances, though, it is necessary to construct a “smart” supply; one must do more than just connect a battery in parallel with an ultracapacitor and hope for the best [3].The typical ultracapacitor has a voltage rating of only 2.5 to 2.7 volts, and for higher voltage applications, the capacitors must be configured in series strings for higher voltage stand offs. For example, an automotive application consistent with a nominal 12-volt system would require six ultracapacitors in series for a 15-volt stand off, which is necessary since voltages at that level are used for charging the battery, and it also provides design margin. As voltage requirements rise, a series configuration may not be the most economical approach. In some instances it makes sense to use a DC-to-DC converter, taking advantage of the boost characteristics of a switch mode power converter. In addition to the use of the many topologies available for power conversion using switch mode circuitry, a microprocessor controller may be necessary. In a hybrid power source, for example, it is often desirable to disengage the ultracapacitor bank from the main power buss, or it may be necessary to monitor voltage levels on the buss and be able to disengage the capacitor bank in the event of a surge voltage on the buss to prevent damage to the capacitor bank.

Designing integrated systems based on ultracapacitors solves many of the energy storage dilemmas the transportation industry faces today. Switch mode devices and smart controllers effectively extend the usefulness of ultracapacitors far beyond what they once offered.  Many engineers might find themselves surprised by what today’s techniques deliver: microcomputer-based control circuitry, switch mode circuitry for DC-DC or DC-AC, overall required capacitance and voltage standoff requirements.  In fact, the options for energy storage in transportation have never been better.

Chad Hall is the chief operating officer of Ioxus, Inc.  Previously, he spent 14 years with Ioxus’ parent company, Custom Electronics, Inc. (CEI).  His extensive mechanical engineering and business experience helped establish Ioxus from funding to factory to launch. 

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