This invention relates generally to fuel cells and more specifically to the efficiency of fuel cell systems.
Fuel cells provide clean, direct current (DC) electricity. Fuel cells convert reactants, namely fuel and oxidant (air or oxygen), to generate electric power and reaction products. A typical fuel cell power source can be constructed from a stack of cells coupled in series as shown in FIG. 1. Fuel cells exhibit a substantially linear decreasing output voltage as the power is increased as represented by FIG. 2 graph 200.
A typical direct methanol fuel cell (DMFC) has an open cell voltage of approximately 0.7 volts, with the cell voltage dropping to approximately 0.25 volts near peak power. The operating voltage for portable devices is typically in the range of 1.5-15 volts. For example, a two-way radio battery may require 10 volts to charge properly. A DC/DC boost converter can be used to provide a 10 V output to such devices. The total cell voltage going to the input of the regulator cannot exceed the output voltage of the regulator. The total number of cells is determined by calculating the regulator output by the open cell voltage (in this example 10V/0.7V would be 14 cells). When the system operates at near peak power, the cell voltage is thus 14xc3x970.25 volts=3.5 volts. It is known that the efficiency of the regulator is dependent on the input voltage to the converter. Generally, closer input voltage to output voltage provides higher efficiency. As an example, some DC/DC converters are rated to have 87% efficiency for converting from 5 volts to 10 volts but only 67% from 2 volts. A fuel cell configuration that increases overall system efficiency is highly desirable. For example, a fuel cell structure that has an open cell voltage of 10 volts that is capable of providing a higher operating voltage near peak power would increase overall efficiency.
Accordingly, it would be highly desirable to have a fuel cell configuration that provides improved efficiency at higher power levels.