For many applications, fuel cells are an increasingly attractive source of power. To improve the overall power system, they can usefully be combined with ultracapacitors, which help provide peak power to meet short-term burst power demands. Ultracapacitors' high power density, excellent efficiency and long operating life make them a good match to fuel cells.

This article examines how fuel cells and ultracapacitors can work together, using the example of electric fork lift trucks to demonstrate the potential benefits. Many of the principles discussed also apply to other applications that could use fuel cells as a power source, for example backup power.

Application requirements

The industrial material handling market is currently 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, has a negative effect on the productivity of the lift trucks. Charging lift truck 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 toxic chemicals used in these vehicles present environmental and health risks when the batteries are ultimately disposed of, as well as mess, odour and expense from the acid during operation.

Benefits of fuel cell / ultracapacitor solution

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 refuelling 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.

The fuel cell hybrid runs effectively silently compared to internal combustion engines. Operators also appreciate that there are no offensive odours or fumes from the fuel cell hybrid compared to those produced from batteries.

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 into the lift truck, to capture the energy involved in 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 the above 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 Corp. Both of these 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 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.

Adding ultracapacitors to the system enables the lift trucks to achieve a suitable dynamic response, so that the 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.