In a known fuel cell system employing phosphoric acid electrolyte, cooler plates interposed between groups of fuel cells have a simple serpentine cooler flow path and utilize water coolant. Liquid water enters the cooler plates and a two-phase, water/steam mixture exits the cooler plates. A small fraction of the heat removal is due to increasing the sensible heat of the water as it increases to its boiling temperature, and a major fraction of heat removal is due to the latent heat of evaporation of liquid water to steam. U.S. Pat. No. 3,969,145 describes such a coolant system.
As is known, the pressure drop across a typical cooler is diatonic if the inlet pressure is such that at some coolant flow rate and resulting pressure drop and temperature rise vapor is produced. Because the pressure drop characteristic is diatonic there are three flow rates that give the same pressure drop, one that is low flow with high vapor content, one that is moderate flow with lower vapor content, and one that is high flow with no vapor content. This means that in a typical coolant manifold design that attempts to minimize the pressure variation in the inlet and exit manifolds, the flow to any of more than 30 cooler plates could be any one of the three coolant flow rates that give the desired pressure difference between the coolant manifolds. Typical cooler plates want to run at a moderate flow rate that gives a low quality (low vapor content) two phase flow. The potential variation in flow rates between more than 30 cooler plates would result in temperature variations, and therefore performance variations which are undesirable.
A seemingly simple solution to this problem is the use of flow control devices, which basically raise the total pressure across each cooler plate so as to provide a pressure/flow characteristic which is monotonic. This may be done with flow restriction orifices as in U.S. Pat. No. 4,923,767; flow control devices which comprise tortuous flow path coolant tubes are disclosed in U.S. Pat. No. 4,574,112.
The cooler to cooler flow distribution problems associated with generating a vapor phase in the coolers can be avoided if the coolers were run in a single phase mode, in this case all liquid flow. This is because in the range of flows associated with all liquid flow the pressure drop characteristic is monotonic. The advantages of a single phase coolant, however, are offset by the need to provide steam for fuel processing (such as a conventional reformer), in order to generate fuel for the fuel cell stack.
The benefit of single phase flow versus two phase flow can be seen in the example illustrated in FIG. 1. The coolant flow ranges from a low of 80 pounds per hour (pph) (36 kgm/hr) to a high of 350 pph (158 kgm/hr) for the two phase flow case without flow control devices. Fifty percent of the coolant flows through seven of the thirty-three coolers. The cells adjacent to low flow coolers will have a higher than desired temperature as a consequence of the low coolant flow. This will lead to a more rapid performance decay, a higher acid loss rate and a reduced cell stack life.
The coolant flow ranges from a low of 150 pph (66 kgm/hr) to a high of 210 pph (95 kgm/hr) for the two phase flow case with flow control devices. This improved flow uniformity results in an increased life at the expense of installing flow control devices on each cooler.
However, the flow distribution for a single phase coolant is very uniform at about 190 pph (86 kgm/hr) without the added cost of complicated flow restriction devices.
The flow control devices increase the cost of fuel cell stacks and present a reliability problem, due to the potential for the distribution devices to become plugged after extended operation.