Research On Fuel Cell Power System

The environment concern is now the driving force for seeking new clean energy such as geothermal, photovoltaic, hydroelectric, wind generation, etc. Fuel cell is an electrochemistry device, it uses hydrogen, propane, natural gas, or other fuels to generate electricity without increasing pollution. The emission of fuel cell is only water, and the noise is very low. Therefore, it has been receiving more and more attention in distributed generation power system and electrical vehicles.This paper proposes a universal fuel cell power system structure. Based on the system specifications we design, a hybrid fuel cell power system structure is employed in this system. It consists of a fuel cell, an isolated uni-directional converter, a bi-directional converter, an inverter and a battery. The proposed system has several advantages as follows: 1) Fuel cell is the most expensive component in the system. Thanks to the introduction of battery, the power rating of fuel cell can be decreased, so the total cost of the system is reduced. 2) When the load varies, the fuel cell cannot response immediately, battery will provide or absorb the dynamic energy, so the dynamic characteristic of the system can be improved. Battery also powers the system during the system start process, which make the system be easy to cold start.3) Bi-directional converter can limit battery charge and discharge current and lead to a longer life of battery. 4) One side of bi-directional converter is connected to DC bus, whose voltage is steady, thus the inductor of bi-directional converter can be minimized, leading to a good dynamic characteristic.5) Power management can be employed to ensure that fuel cell and battery cooperate well and improve system efficiency.Zero-voltage-switching pulse-width-modulation three-level converter (ZVS PWM TL converter) is widely used in high voltage applications. But it has some shortages. This paper proposes a improved ZVS PWM TL converter, which is improved from the original ZVS PWM TL converter just by exchanging the position of the resonant inductor and the transformer, such that the transformer is connected with the lagging switches. The improved converter not only keeps the advantages of original converter, but also has several advantages over the original one, e.g., the clamping diodes conduct only once in a switching period, and the resonant inductor current is smaller in zero state, leading to a higher efficiency and reduced duty cycle loss. A blocking capacitor is usually introduced to the primary side to prevent the transformer from saturating, this paper analyzes the effects of the blocking capacitor in different positions, and a best scheme is determined. A 3kW prototype converter verifies the effectiveness of the improved converter and the best scheme for the blocking capacitor.The output voltage of fuel cell fluctuates with the load significantly, which is too wide to power the inverter. Therefore, a uni-directional DC-DC converter is needed to convert the variable output voltage of the fuel cell to a constant one. The DC-DC converter should be adaptive to the wide input voltage range. This paper proposes a novel hybrid full-bridge three-level LLC resonant converter. It integrates the advantages of the hybrid full-bridge three-level converter and the LLC resonant converter. It can operate under both three-level mode and two-level mode, so it is very suitable for wide input voltage range application, such as fuel cell power system. Compared with the traditional full-bridge converter, the input current ripple and output filter can be reduced. In addition, all the switches can realize zero-voltage- switching from zero to full load, and the switches of the three-level leg only sustain half of the input voltage. Moreover, the rectifier diodes can achieve zero-current- switching, and the voltage stress across them is the output voltage. A 200-400V input, 360V/4A output prototype is built to verify the operation principle of the proposed converter.Fuel cell has slow response and it is difficult to cold start. Therefore, a battery and a bi-directional converter are employed in the system. This paper proposes a new three-level Buck/Boost bi-directional converter, which is suitable for fuel cell power system. Compared with the traditional Buck/Boost bi-directional converter, the inductor also can be reduced significantly, so the dynamic response can be improved. The voltage stress on the switch of the proposed converter is just half of the voltage on the high voltage side. Therefore, it is very suitable for fuel cell power systems. This paper illustrates the operation principle and implementation of the control circuit. A 1kW prototype converter is built to verify the theoretical analysis. In the system, there are two power sources: fuel cell and battery. Therefore, it needs to manage two sources to ensure the system operate with high efficiency and high reliability. This paper proposes a power management scheme. The key point of this power management scheme is to control bi-directional operates under Buck, Boost or Shut-Down mode according to the conditions of fuel cell and battery. Cold start and overload processes are analyzed in detail.According to the theoretical analysis, a 1kW fuel cell power system is built in lab to verify the theoretical analysis. Experimental results illustrate that system operates well under steady, cold start, overload, load step up and down.