Page Content (OCR)
05/01/2006 10:29 AM AINAINIG SVUIULIVIT 1uu ° . VUVIIIIIICILIAVINGSCAIUll cell kW 80°C System: 62% PEM FC - Proton 0,1 to 70-200 Cell: 50-70 exchange Polymermembrane 500 °C % System: Commercial/Research membrane fuel cell kW , 30-50 % DBFC - Direct borohydride fuel alkaline solution: NaOH 70 °C Research cell FAFG - Formic acid Formic acid 90-120 Research fuel cell Cc DMFC - Direct mW to 90-120 Cell: 20-30 Polymermembrane Commercial/Research methanol fuel cell 100 kw °C % DEFC - Direct- ethanol fuel cell Research ; Cell: 55 % PAFC - Phosphoric- : . Up to ° : . acid fuel cell Phosphoric acid 10 MW 200 °C em 40 Commercial/Research fo} MCFC - Molten 100 Cell: 55 % carbonate fuel cell Molten Alkaline-Carbonate MW 650 °C em 47 Commercial/Research fo} PCFC - Protonic : ceramic 700 °C Research ceramic fuel cell Up to go9_ Cell: 60-65 Oxide Ceramic Electrolyte 100 4000 °C % System: Commercial/Research MW 55-60 % SOFC - Solid oxide fuel cell Efficiency Fuel cells are not constrained by the maximum Carnot cycle efficiency as combustion engines are because they do not operate with a thermal cycle. Consequently, they can have very high efficiencies in converting chemical energy to electrical energy. The efficiency of a fuel cell, 1, under standard conditions is limited by the ratio of the change in standard Gibbs free energy, \G°, to the change in standard enthalpy of the overall chemical reaction, AH?- The practical efficiency is often lower than this however. AG?° 1 AH A fuel cell typically converts the chemical energy of its fuel into electricity with an efficiency of about 50%. The efficiency is however very dependent on the current through the fuel cell: the more current drawn, the lower the efficiency. For a hydrogen cell the efficiency (actual power / theoretical power) is equal to cell voltage divided by 1.23 volts, at a temperature of 25°C. This voltage depends on the fuel used, quality and temperature of the cell. A cell running at 0.6V has an efficiency of about 50%, meaning that 50% of the energy content of the hydrogen is converted into electrical energy. An electrolyzer and fuel cell would return less than 50 percent of the input energy (this is known as round- trip efficiency), while a much cheaper lead-acid battery might return about 90 percent. It is also important to take losses due to production, transportation and storage into account. Fuel cell vehicles running on compressed hydrogen may have a power plant to wheel efficiency of 22% if the hydrogen is stored as high-pressure gas, and 17% if it is stored as liquid hydrogen (efficiency of Hydrogen Fuel Cell, Diesel-SOFC-Hybrid and Battery Electric Vehicles). Page 4 of 8 Fuel cell - Wikipedia, the free encyclopedia http://en.wikipedia.org/wiki/Fuel_cell