Patent ID: 7553575

Claim:
A fuel cell power plant, comprising: a fuel cell stack wherein plural fuel cells are stacked in series, each of the fuel cells comprising a membrane electrolyte, an electrode in contact with the membrane electrolyte, and gas passages formed facing the electrode, the power plant generating power by a chemical reaction of a gas flowing in the gas passages at the electrode; a first manifold communicating with an end of each of the gas passages, the first manifold comprising a water-absorbing material facing a flow of the gas; and a second manifold communicating with the other end of each of the gas passages, the second manifold comprising a water-absorbing material facing a flow of the gas; and a gas supply mechanism for selectively changing over between a first gas supply direction wherein the gas is made to flow from the first manifold to the second manifold via the gas passages, and a second gas supply direction wherein the gas is made to flow from the second manifold to the first manifold via the gas passages, so as to supply the gas to the fuel cells; a first hygrometer installed upstream of the water-absorbing material of the first manifold with respect to the first gas supply direction; a second hygrometer installed downstream of the water-absorbing material of the second manifold with respect to the first gas supply direction; a third hygrometer installed in a communicating part between one of the gas passages and the first manifold; a fourth hygrometer installed in a communicating part between the one of the gas passages and the second manifold; and a programmable controller programmed to: calculate a difference between a humidity detected by the first hygrometer and a humidity detected by the third hygrometer as a first parameter; calculate a difference between a humidity detected by the second hygrometer and a humidity detected by the fourth hygrometer as a second parameter; and control the gas supply mechanism to change over the gas supply direction, when the first gas supply direction is applied, to the second gas supply direction at the earlier of a timing when the first parameter starts to decrease after increasing, and a timing when the second parameter becomes less than a lower limiting value, and when the second gas supply direction is applied, to the first gas supply direction at the earlier of a timing when the second parameter starts to decrease after increasing, and a timing when the first parameter becomes less than the lower limiting value.