The industrial materialhandling market is cur- rently dominated by electric lift trucks powered by lead-acid batteries. However, the limitations inherent in battery-based operations, such as the loss of charge as shifts progress, have a negative effect on the productivity of such lift trucks: charging their batteries during a shift leads to considerable downtime. With these battery-powered lift trucks, there is usually overhead expense for a number of factors related to the limitations of batteries: additional battery packs are required for each lift truck, and there are associated personnel costs directly associated with battery maintenance. There are also the costs of the battery-recharging infrastructure, the chemical-disposal facilities required, and the related costs associated with lost facility space. In addition, the toxic chemicals used in these vehicles present environmental and health risks when the batteries are ultimately disposed of, as well as dirt and smell from the acid during operation. The conventional lead-acid-battery power train can be replaced with a fuel-cell electric hybrid power pack. This is a clean power solution that offers increased productivity as refueling is faster, power is consistent and abundant, and run time is extended. The fuel cell/ultracapacitor power pack also benefits from reduced maintenance and reduced infrastructure costs. Moreover, the fuel-cell hybrid runs silently compared to internal combustion engines. Operators also appreciate that there are no offensive smells or fumes coming from the fuel-cell hybrid.

Design considerations

Typically, the power output from a fuel cell registers some lag time at start-up. This lag can range from a few seconds to a minute, depending on the application. Another important design requirement is management of the power coming from the power system during periods of peak load without incurring loss of power to other electrical functions of the lift truck. This means that the fuel-cell power module's configuration needs to be compensated with additional surge capability in order to prevent a dip in voltage. To create the most fuel efficient design, regenerative braking capability can be added to the lift truck, to capture the energy released by the vehicle braking and store it for power and acceleration. Finally, the power solution for a fork-lift truck should be compact and relatively lightweight.

Implementation

To meet these design requirements, ultracapacitors can be used to provide the necessary peak power and regeneration capabilities. Two of the early adopters and leaders in the area are General Hydrogen and Hydrogenics. Both companies have developed products to replace the lead-acid batteries in electric fork lifts with a combination of fuel cell and ultracapacitors. One example is a compact hybrid power system developed by Hydrogenics in partnership with Maxwell Technologies. This system consists of a HyPM 10 Fuel-cell power module for base-load requirements combined with a bank of Maxwell's Boostcap ultracapacitors for energy storage (Figure 1). Adding ultracapacitors to the system enables the lift trucks to achieve a suitable dynamic response, so that they start without delay regardless of temperature, load or other variables. The ultracapacitors store electrical energy when the lift truck is not working at full capacity, and can then deliver this energy rapidly to handle transients and peak-power demands. They also store captured braking energy. As compared to batteries, ultracapacitors deliver up to ten times the power, last up to ten times as long and operate more reliably in high- and low-temperature conditions. They also require far less maintenance and reduce the environmental issues associated with battery disposal. Other key components that complete the HyPM fuel-cell power pack include hydrogen storage, power electronics and controls, and thermal management. Hydrogenics integrated this hybrid power system into two Hyster Class lift trucks in the original battery compartment of the lift truck. To complete the fuel-cell-lift-truck solution, Hydrogenics also developed an indoor hydrogen refueling station, based on the companies HySTAT-P Hydrogen Station technology. Maxwell's ultracapacitors have also been used in General Hydrogen's hydrogen fuel-cell-based power systems for electric fork lifts, to enhance performance and energy management in its Hydricity Pacack technology. Each Hydricity system incorporates 60 Boostcap MC2600 2,600-farad ultracapacitor cells. The Hydricity power systems are an economical "drop-in" replacement today for lead-acid batteries in Class I electric fork lifts, which triple the runtime of these vehicles.

Conclusion

The key benefits of this hybrid system design are instantaneous response, resulting in improved system performance; increased fuel efficiency, thanks to regenerative braking; flexibility to handle long-duration transients and load peaks; and reduced system cost, thanks to a smaller fuel-cell footprint. To meet industry-specific needs for improved productivity and system operations, the fuel-cell ultracapacitor design provides abundant clean power with fast refuelling. Unlike battery electric lift trucks, which require changing, watering and recycling, fuel-cell lift trucks can operate for 12 hours before refuelling, which on average takes only 2 minutes from empty to full. Operators can refuel their own lift trucks, eliminating the need for a "refuelling shift". Lead-acid batteries not only are environmentally hazardous, but also expensive to maintain and less productive as the fuel-cell/ultracapacitor solution, which does not need multiple battery modules and charging stations. Figure 3 shows typical productivity gains, which equates to about 8% increase in run time. This increase equates to a saving of about $1450 (1100 Euros) per month per truck, or $17,400 (13,400 Euros) per forklift per year. For a warehouse with several hundred forklifts, this saving amounts to well over one million Euros.