CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority based on Japanese Patent Application No. 2020-026058 filed on Feb. 19, 2020, and the entire disclosure thereof is incorporated in the present application by reference.

BACKGROUND

Field

The present disclosure relates to the art of a fuel cell system.

Related Art

In the known art of a conventional fuel cell system, in cases when making a fuel cell generate power at a low temperature such as below freezing point, the fuel cell system drives a cooling medium circulation pump configured to circulate a cooling medium in the fuel cell according to the temperature increase speed of the fuel cell (Japanese Patent Application Laid-open Publication No. 2010-277704).

In general, when the temperature of the cooling medium is low, for example, −30° C. (Celsius), the cooling medium has high viscosity. In cases where a cooling medium circulation pump is driven to circulate such a cooling medium with high viscosity existing in a cooling medium circulation path, the cooling medium need not be mixed evenly, and may have increased temperature distribution in the cooling medium circulation path. In cases where a cooling medium with significant temperature distribution is circulated, the temperature increase speed of the fuel cell fluctuates frequently. This significantly changes the rotational speed of the cooling medium circulation pump, and thus significantly changes the flow rate of the cooling medium circulating in the cooling medium circulation path. As a result, the temperature of the fuel cell may drop significantly, and the water generated in the fuel cell may be frozen. If the water generated in the fuel cell is frozen, oxidizing gas is not smoothly supplied to a cathode, with the result that that pumping hydrogen may be generated due to the shortage of oxidizing gas. The pumping hydrogen is hydrogen generated at the cathode by the recombination of electrons and hydrogen ions conducted from an anode due to the shortage of oxygen at the cathode.

SUMMARY

The present disclosure in one aspect provides a fuel cell system. The fuel cell system includes a fuel cell stack, a cooling medium circulation path allowing a cooling medium to flow into the fuel cell stack, a cooling medium circulation pump disposed on the cooling medium circulation path and configured to regulate the circulation flow rate of the cooling medium, a stack temperature acquisition unit configured to acquire the temperature of the fuel cell stack, and a control unit configured to control operation of the cooling medium circulation pump by use of an acquisition temperature acquired by the stack temperature acquisition unit. The control unit executes warm-up operation processing of the fuel cell stack to raise the temperature of the fuel cell stack by use of heat generated by the fuel cell stack when a predetermined condition is satisfied. The control unit drives the cooling medium circulation pump according to the acquisition temperature in the warm-up operation processing. After a first time point when a change in the acquisition temperature turns from downward to upward after the change in the acquisition temperature turns from upward to downward for the first time after the start of the warm-up operation processing, the control unit sets a decrease speed in cases of decreasing the rotational speed of the cooling medium circulation pump to a smaller value than a value set before the first time point.

