Electric lift trucks have used lead-acid batteries for more than 80 years. Now that hydrogen fuel cells are commercially available as an alternative energy source, it is important to understand that retrofitting the lift trucks to use fuel cells can have an impact on the operation of a lift truck originally designed to use lead-acid batteries. The challenge today is designing fuel cell systems that emulate batteries to ensure lift truck performance is maintained, refueling time is reduced and thus, productivity increases. Lift truck and fuel cell manufacturers should collaborate to ensure optimal lift truck operation is achieved.
Lead-acid batteries have always played a critical role in electric lift truck design beyond just providing power. The placement of the forks, wheels, mast and operator platform must be considered in relation to the placement, size and weight of the battery. Traditionally, lift truck manufacturers have assumed that an electric lift truck design must include a battery that has a specific weight and provides a certain amount of energy to the truck.[pullquote]
However, hydrogen fuel cells are inherently lighter and not necessarily the same shape as lead-acid batteries, so they do not naturally fill the same space in lift trucks. To evaluate the operation of hydrogen fuel cells in electric lift trucks, The Raymond Corporation (Greene, N.Y., U.S.A.) began a multi-year study on the performance of hydrogen fuel cells in lift trucks in January 2007. Raymond’s research indicates that converting to hydrogen fuel cell-powered lift trucks will eventually impact lift truck design. Future lift trucks may have the fuel cells wholly incorporated into the design of the truck from the original concept, but today, fuel cells are being used as drop-in replacements for batteries. If these fuel cell units sufficiently emulate the lead-acid batteries they replace, then no lift truck modifications are necessary to accommodate the fuel cell.
The Industrial Truck Association (ITA) established the Energy Storage System (ESS) committee in 2008 to facilitate discussions between lift truck manufacturers and energy storage system manufacturers to ensure fuel cells or other energy storage systems — such as lithium-ion batteries or supercapacitors — meet lift truck requirements, especially in trucks that are being retrofitted to accommodate the energy storage system. One current initiative of this committee is considering recommendations that describe the minimum requirements and key characteristics of energy storage systems as they relate to a lift truck originally designed for use with a lead-acid battery. Future lift trucks may fully encompass the fuel cell in the design, but according to the preliminary work of the ESS committee, today’s lift trucks require the fuel cell to emulate batteries in five key areas for a successful retrofit:
- Center of gravity
- Power delivered
- Power absorbed
In today’s lift trucks, the hydrogen fuel cell needs to fit in the same space occupied by the battery. Defining the available space for the hydrogen fuel cell, which varies in different types of lift trucks, ensures the fuel cell will fit in a lift truck designed for a lead-acid battery. In some pallet trucks, for example, the fuel cell height can exceed the battery height because the battery compartment is not enclosed (see Figure 1). However, in a reach truck or counterbalanced truck, the fuel cell must fit within an enclosed battery compartment. For example, a 48-volt counterbalanced lift truck might have a battery compartment that is 38.7 inches long, 27.9 inches wide and 22.8 inches tall.
Every lift truck displays a specification tag or plate that defines the minimum and maximum battery weights. For example, minimum weight may be 2,600 lbs. and maximum weight may be 3,000 lbs. When hydrogen fuel cells are used as battery replacements, they must meet the same weight requirements listed on this spec tag. Meeting the weight requirement is important because it is a critical factor in ensuring the stability of the lift truck. Hydrogen fuel cells can be significantly lighter than lead-acid batteries, so fuel cell manufacturers typically add steel plates to their fuel cell units to make up for the weight required.
Center of gravity
In a lift truck powered by a lead-acid battery, the center of gravity of the battery is the geometric center of the battery box. In a fuel cell that’s being used as a battery replacement, the fuel cell unit incorporates many separate components, including a hydrogen tank, a fuel cell stack and steel counterweight added to meet the minimum weight requirements. Balancing the placement of these components is necessary to ensure the proper center of gravity. Lift truck manufacturers need to perform tests or analysis on each lift truck they intend to use or approve for use with a fuel cell to determine the exact tolerance for where the center of gravity can be located. The tolerance can be described as the radius of a cylindrical shape, with the top of the cylinder located at the volumetric center of the battery being replaced (see Figure 2). If the center of gravity does not fall within the tolerance indicated, the overall stability of the lift truck will be affected. Sample values for the center of gravity tolerance are included.
Different types of lift trucks are designed for different applications and capabilities, and varying amounts of power are required to accomplish these tasks. It takes more power to lift 4,000 pounds to 30 feet than it does to drive with no load on a level floor, and hydrogen fuel cells must provide the same amount of power as lead-acid batteries to accomplish these tasks. Specifying the current and voltage delivery requirements of the lift truck at various time durations ensures the lift truck receives the power from a hydrogen fuel cell that is necessary to maintain acceptable performance in various applications. The ESS committee has developed a chart that can be used to describe the power delivered requirement (see Figure 3).
When electric lift truck brakes are applied or forks are lowered, they can create regenerative energy. This energy must be absorbed by the energy source, and hydrogen fuel cells are not able to recapture energy like lead-acid batteries. Therefore, all hydrogen fuel cell units are actually hybrids and contain a lead-acid battery or supercapacitors. It is necessary to define how much regenerative energy each lift truck creates that needs to be absorbed by the battery or supercapacitors. As with power generated, lift truck manufacturers can specify the maximum and minimum current and voltage generated by the lift truck as it brakes from various speeds or lowers forks with different amounts of weight for different time durations. If the fuel cell is unable to absorb the energy generated, then a fault code would be generated that could potentially disable the lift truck. The ESS committee has developed a chart that can be used to specify power absorbed (see Figure 4).
The ITA’s ESS committee intends to develop these recommended practices for defining key criteria to facilitate discussions between lift truck and energy storage system manufacturers when retrofitting electric lift trucks to accommodate hydrogen fuel cells. These criteria can help ensure the energy storage system meets the minimum lift truck requirements and operates as an effective lead-acid battery replacement. By defining these criteria, lift truck manufacturers can make it easier for fuel cell manufacturers to design a power source that integrates with today’s lift trucks.
About the Authors
Steve Medwin is manager of systems and advanced engineering at The Raymond Corporation. Since 2004, he has been evaluating the application of fuel cell technology to the material handling business. Before joining Raymond, Medwin spent 20 years in research and development at DuPont. He holds nine U.S. patents and is a member of the UL 2267 Standards Technical Panel for Fuel Cell Power Systems for Industrial Electric Trucks and chairman of the new Energy Storage Systems committee of the ITA.
Medwin received his bachelor’s degree in mechanical engineering from Cornell University in Ithaca, N.Y., and his master’s degree in mechanical engineering from the University of Pennsylvania in Philadelphia. He also has completed the Executive MBA program at Binghamton University in Binghamton, N.Y.
Dave Norton is manager of corporate product engineering at The Raymond Corporation. He has been with the company for more than 20 years and has had responsibilities in design engineering, product engineering, program management and engineering management. Norton is a member of ANSI/ITSDF B56.1 and B56.5 and an alternate member of B56.9 and B56.11 safety standards for industrial trucks. He is Raymond’s representative on the ITA’s General Engineering Committee.
Norton received his bachelor’s degree in electrical engineering technology from Rochester Institute of Technology in Rochester, N.Y.