Solid oxide fuel cells (“SOFCs” below) are fuel cells which operate at relatively high temperatures in which, using an oxide ion-conducting solid electrolyte as electrolyte, with electrodes attached to both sides thereof, fuel gas is supplied to one side thereof and an oxidizer (air, oxygen, or the like) is supplied to the other side thereof.
In such SOFCs, steam or CO2 is produced by the reaction between oxygen ions passed through an oxide ion-conducting solid electrolyte and fuel, thereby generating electrical and thermal energy. The electrical energy is removed from the SOFC, where it is used for various electrical purposes. On the other hand, thermal energy is used to raise the temperature of the fuel, the reformer, the water, the oxidant, and the like.
JP 2010-92836 (Patent Document 1) sets forth a fuel cell device. This fuel cell device is a solid oxide fuel cell of the type which changes generated power in response to power demand; operation is disclosed in which a fuel utilization rate is reduced more which it operates in the low load range than when it operates in the range where the power generation load is high. That is, in Patent Document 1, a proportion of fuel used for power generation with respect to the entire supplied fuel is reduced when generated power is in a low state, but on the other hand, fuel used to heat the fuel cell module and not used to generate electricity is not greatly reduced, and a large fraction of the fuel is used to heat the fuel cell module so that the fuel cell module is operated in a thermally independent manner at a temperature at which power generation can occur is maintained.
Specifically, when the module operates in the range where the generated power is low, heat generated in the fuel cell unit in association with electrical generation declines. As a result, the temperature inside the fuel cell module tends to decline. Therefore, if the fuel utilization rate is maintained at a certain level even when the module operates in the range where the power generation is low, a decline of the temperature inside the fuel cell module is induced, and it becomes difficult to maintain the temperature at which power can be generated. Therefore, fuel used to heat the fuel cell module is increased in order to operate in a thermally independent manner, even sacrificing the fuel utilization rate.
In the fuel cell device set forth in JP 2010-92836, in order to resolve these problems the fuel utilization rate is reduced when the module operates in the low load range where electrical generation is small, thereby preventing an excessive temperature drop in the fuel cell module while stably maintaining a certain high temperature state.
JP 2010-205670 (Patent Document 2), meanwhile, sets forth a method for operating a fuel cell system and fuel cell. In this fuel cell, a value is acquired which represents s sum of the fuel cell electrical loads, and the fuel utilization rate is controlled based on the acquired value. Control of the fuel utilization rate is performed by estimating the fuel cell temperature based on the value representing a sum of the fuel cell electrical loads. Then, the fuel utilization rate is controlled based on the estimated temperature. The fuel cell can therefore be operated in a thermally independent manner without the use of a temperature sensor. Also, when the value representing a sum of the electrical loads is equal to or greater than a predetermined value, a correction mode corrects the fuel utilization rate to a value equal to or greater than a reference value at which the fuel cell can be thermally independently operated. In such cases, because the temperature of the fuel cell has already been high, there is surplus heat in the fuel cell, and thermally independent operation is maintained even if the fuel utilization rate is corrected to a value equal to or greater than the reference value at which thermally independent operation is possible. System efficiency of the fuel cell system is by this means improved.    Patent Document 1 JP 2010-92836    Patent Document 2 JP 2010-205670