A group of scientists from the Chiang Mai University in Thailand has designed a hybrid storage system based on a polymer electrolyte membrane fuel cell and a battery and has sought to identify its best DC coupling voltage.
Intended for use in solar PV applications, the system was built with a 4.8 kW hybrid inverter provided by Taiwan-based Voltronic Power Technology Corp, a 250 W PV unit consisting of three cells in series in four parallel connections, a lead-acid AGM battery 2 V from Chinese manufacturer MCA, an electrolyzer and a 3 kW polymer electrolyte membrane fuel cell from Taiwanese manufacturer M-Field.
Under this configuration, hydrogen is generated by tapping an AC 220 V power line to the electrolyzer and is then stored in a high-pressure tank. The electrolyzer can produce 500 L/h and a maximum pressure of 32 bar. The voltage output between the fuel cell and battery was linked through a DC bus bar. The battery has a storage capacity of 50 kWh and the capacity of the electrolyzer is 2,180 L at 30 bar
“The data were recorded in real time during the experiment,” the scientists specified. “This work assumes that the battery capacity is sufficiently large, where the hydrogen capacity is limited.” The turn-on voltage of fuel cell operation was set at 48 V and the turn-off voltages were set at 49, 50, 51 and 52 V.
The testing showed that, with a voltage difference of 4 V, only the batteries supplied power to the load until the voltage declined to 48 V. “After that, the fuel cell started to work and provided mainly power to the load until the hydrogen was used up,” the academics further explained. “There was no transition period where the fuel cell and batteries provided power simultaneously.” The total time duration of this range reached 795 minutes.
When the voltage difference was 3 V, the duration of the range was 645 minutes and when the difference was 2 V, the transition time between the batteries and fuel cell was longer at 920 minutes. The longest operating time of 1,350 minutes was achieved with a voltage difference of 1 V. The scientists explained, however, that the voltage profile dropped below that for the turn-on voltage of the fuel cell. “This indicates the malfunction of the controller and possible failure of the system,” they emphasized. “Hence, the optimum voltage difference during the on-off operation of the fuel cell was set at 2 V, where the storage performed with the longest time without damaging the system.”
The group also ascertained that the fuel cell's turn on-off with 49–51 V is the best range for optimizing its operation. In this way, the fuel cell and batteries facilitate each other and the overall system remains stable.
The system is described in the study “Optimal voltage of direct current coupling for a fuel cell–battery hybrid energy storage system based on solar energy,” published in Energy Reports.
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