Ultracapacitors and Utility Vehicles

There are many different types of utility vehicles, but they all have one thing in common–a need for inexpensive motive power.

Sep 1st, 2010
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There are many different types of utility vehicles, but they all have one thing in common–a need for inexpensive motive power. Regardless of the type of power a vehicle requires, ultracapacitors can play a role in making it more fuel- and power-efficient. The ability of any fleet manager to shave fuel costs, by any means, is seen in a positive light. One technique includes the reduction of idle time to route planning, which minimizes fuel requirements. Ultracapacitors provide another tool for utility fleet managers to reduce costs by increasing fuel efficiency.

Ultracapacitors, perhaps the most exciting energy storage devices in the marketplace, first were produced commercially in the late 1970s and used for computer memory backup. New materials have led to several iterations of devices that are considerably better than those introduced in the 1970s. Today, ultracapacitors range in size from a fraction of a Farad up to a single cell with a capacitance of 5,000F or more. The high capacitance of these devices allows them to store enormous amounts of energy. A fully charged, 1,000F ultracapacitor will store approximately 3,000 joules, which can be delivered to a load at an exponential rate; only the internal resistance of the capacitor and the load resistance limit the delivery rate. The ability of capacitors to deliver power rapidly is referred to as high-power density. In addition to high-power density, ultracapacitors are characterized by rapid charging rates, a virtually unlimited number of charge-discharge cycles, no maintenance requirements and an ability to operate without degradation from -35C to +65C. A 5,000F cell is shown in Figure 1.

Figure 1. 5,000F Ultracapacitor

In low-power applications, such as short-duration LED lighting, it is possible to replace a battery with an ultracapacitor–it is this feature that makes the ultracapacitor so exciting. The distinction between ultracapacitors and batteries has been blurred in the past 10 years, and ultracapacitor manufacturers are doing their best to blur the distinction even further by investigating new materials and structures that will increase the energy density of ultracapacitors to that of many battery types. A 10-fold increase in energy density, for instance, would bring the ultracapacitor to an energy density comparable to lead-acid storage batteries. This is presently not the case, but many companies are investigating methods to increase energy densities by an order of magnitude or more. If successful, this would lead to the wholesale replacement of these battery types because of the superior durability and rapid charge rate associated with ultracapacitors.

Ultracapacitors used in parallel with batteries form a robust power source that has both high energy and power density. Batteries provide the energy and ultracapacitors supply the high power required for a sudden load. Capacitors derive their high-power density from their low internal resistance, referred to as effective series resistance (ESR), and from a capacitor's energy stored in an electric field. Batteries, by contrast, store electrical energy as chemical potential energy that can be extracted over a long period of time at a rate that is limited by the reaction rate of its chemistry. Batteries typically have a much higher internal resistance than capacitors, and, when large currents are demanded, internal heat is produced within the battery that in turn will shorten the device's effective service life by limiting the number of charge-discharge cycles it can tolerate before failure. If a parallel configuration is used, the battery will supply a moderate, steady current over a long period of time, and the capacitor will provide the power required (high current) under heavy load. This allows the battery to avoid high-current drain. This type of combination not only lengthens the useful life of the battery, but also allows more useful energy to be withdrawn before battery charging is necessary. Parallel configurations of batteries and ultracapacitors can be built economically for 12V to 96V supplies, making them ideal for forklifts and automotive applications, to name a few uses. The systems can be as simple as a parallel electrical connection between the storage battery and the ultracapacitor bank, or much more sophisticated–employing programmable computer control.

Ioxus has pioneered the design of a programmable controller that can be used with a lead acid storage bank. The controller senses sudden loads and switches the ultracapacitor bank online, limiting the current draw from the battery packs. When the load decreases back to its steady state, the controller diverts enough energy from the battery bank to keep the ultracapacitor bank fully charged for the next load event. These systems prolong useful battery life, extend the operation cycle of the power supply between required charging cycles and are cost-effective because the expensive battery packs often are reduced in size and last longer.

Figure 2. Transit Bus ESS Module 165F at 48V

Ultracapacitors are also useful in recapturing energy that would otherwise become waste heat. Regenerative braking systems offer an excellent example of such energy harvesting. Automobile braking systems use friction to slow the vehicle, which results in the generation of heat that is not used for any useful purpose. Employing ultracapacitors and a motor-generator set creates the heart of a regenerative braking system. Rather than simply generating heat using conventional brakes, a generator is driven by the car's rotating axle. If the generator is freewheeling (under no load), it will simply rotate. When braking is required, however, a generator is switched on, slowing the vehicle by converting the car's kinetic energy to electrical energy stored in the capacitor bank. The energy stored in the capacitors then can be used to drive an electric motor that is used to assist the engine. Such systems do not eliminate the need for conventional brakes, but they are cost-effective and make the car much more efficient to operate. Such systems are routinely designed into hybrid vehicles.

An energy storage system (ESS), which is essentially a regenerative braking system, has been designed for mass transit buses. The purpose of the system is to harvest energy that then can be used to help power the bus as previously described by harvesting the energy used in braking. The system will use a specially designed motor-generator set and an ultracapacitor bank that stores the energy from braking and releases the energy upon acceleration of the bus. The ESS will consist of a bank of capacitor modules like the one in Figure 2. The module shown is approximately 17-by-7-by-7 inches and weighs 30 pounds.

System testing will begin in the fall of 2010, and it is estimated that the system will result in an improved fuel economy of 25 percent and a payback period of fewer than five years with no subsidies. These systems are efficient, cost-effective and require little or no maintenance.

Because of the advances in ultracapacitor technology, most of which are material-related, and the intense efforts being made by manufactures to increase the energy density of the devices, there is reason to believe some battery classes might be replaced by ultracapacitors–a genuinely exciting possibility because of the robust properties of ultracapacitors as compared to batteries. Regardless of this possibility, there are solutions available today using ultracapacitors for improved fuel efficiency. The solutions available today apply to any utility vehicle–regardless of size or fuel used. The described systems allow for improved battery performance and energy-harvesting scenarios, both of which will improve overall cost-effectiveness for utility fleet operators.

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