Seite - 47 - in Our Car as Power Plant

47 As the main difference among fuel cell types is the elec- trolyte, fuel cells are classified by the type of electrolyte they use, followed by the difference in startup time ranging from 1 sec to 10 min. Electrons are drawn from the anode to the cathode through an external circuit, producing direct current (DC) electricity. In addition to electricity, fuel cells produce de-mineralized water and heat as a ‘waste’ product (22,23). Fuel cells come in a variety of sizes. Individual fuel cells produce relatively small electrical potentials, about 0.7 volts. So cells are "stacked", or placed in series, to increase the voltage and meet application's requirements (24). The energy efficiency of a fuel cell is generally between 40 and 60% at maximum capacity. Interesting is that the effi- ciency goes up when fuel cells are operated in partial load. In power plants this is normally the other way around: efficiency goes down when operating in partial load.