System Design and Control Strategy of Fuel Cell/Lithium-ion Battery Hybrid Electric Vehicles

Abstract

Economic and/or practical points to be addressed As kind of critical materials handling vehicle, forklift has been widely used in various places, such as warehouses, logistics centers, airport, and distribution centers etc. However, the conventional forklifts are majorly powered by fossil fuel or lead-acid batteries that still causing serious environmental problems. Hydrogen fuel cell powered forklifts will provide notable advantages, because of hydrogen is a zero-emission fuel, it only produce electricity and water through chemical reaction that will improve the safety of the working environment. The lithium battery can provide auxiliary power supply for accelerating, climbing, lifting, and the forklift can still move to the hydrogen refueling equipment when the hydrogen fuel runs out. Besides the application in forklifts, the hydrogen fuel cell electric commercial vehicles will take very short time (i.e. 3 minutes) for refueling that will greatly reduce the downtime to help improve operation efficiency. Methodology There are different kinds of hydrogen fuel cells, including Alkali Fuel Cells (AFC), Molten Carbonate Fuel Cells (MCFC), Phosphoric Acid Fuel Cells (PAFC), Proton Exchange Membrane Fuel Cells (PEMFC) etc. Each type of fuel cells has different characteristic suits for different conditions or applications. The PEMFC can be constructed as fuel cell stack, designed for power output up to 250kw, operated at low enough temperature from 50 ℃ to 100 ℃ that most suitable for the application such as cars, buses, forklifts and trains etc. At current development stage, at a nominal driving speed (less than 30 mph), the power efficiency of a fuel cell electric drive using direct hydrogen is two times higher than that of a conventional internal combustion engine, so it is especially suitable for low driving speed application like forklifts. Main scientific and technical The hydrogen fuel cell/lithium battery hybrid system will comprise the hydrogen fuel cell stack and lithium battery pack. The hydrogen fuel cell stack has a high energy density and is suitable to be the primary energy source for forklift driving and static loading. The lithium battery pack has a high power density and is suitable as an auxiliary power source for power assisting of dynamic loading like forklift startup, accelerating, climbing and lifting. If the required power exceeds the output power of the fuel cell, the lithium battery will provide auxiliary power for the extra demand power of the forklift operation. If the required power is lower than the fuel cell output power, the fuel cell will provide power supply for the lithium battery. Achieved results We propose to develop the system design and control strategy for the hydrogen fuel cell/lithium battery hybrid electric commercial vehicles. The hybrid electric commercial vehicles will comprise two kinds of energy source, including hydrogen fuel cell stack and auxiliary lithium battery pack. Two DC/DC converters, a DC/AC converter and BMS for the battery pack will be designed to provide energy management strategy for the hybrid system. It would be a good reference for the future development for other applications.