Fuel Cell Efficiency
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 under standard conditions is limited by the ratio of the standard Gibbs free energy to the standard enthalpy of the overall chemical reaction. The practical efficiency is often lower than this however.
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).
Fuel cells cannot store energy like a battery, but in some applications, such as stand-alone power plants based on discontinuous sources (solar, wind power), they are combined with electrolyzers and storage systems to form an energy storage system. The round-trip efficiency (electricity to hydrogen and back to electricity) of such plants is between 30 and 40%.
In "combined heat and power" applications, a fuel cell is placed in a location where heat is also needed. A lower fuel-to-electricity conversion efficiency is tolerated (typically 15-20%), because most of the energy not converted into electricity is utilized as heat. Some heat is lost with the exhaust gas just as in a normal furnace, so the combined heat and power efficiency is still lower than 100%, typically around 80%. In terms of exergy however, the process is inefficient, and one could do better by maximizing the electricity generated and then using the electricity to drive a heat pump. [edit]
Fuel cell applications
Fuel cells are very useful as power sources in remote locations, such as spacecraft, remote weather stations, large parks, rural locations, and in certain military applications. A fuel cell system running on hydrogen can be compact, lightweight and has no major moving parts.
A new application is combined heat and power (CHP) for family home, office buildings and factories. This type of system generates constant electric power (selling excess power back to the grid when it is not consumed), and at the same time produce hot air and water from the waste heat. Phosphoric-acid fuel cells (PAFC) comprise the largest segment of existing CHP products worldwide and can provide combined efficiencies close to 80% (45-50% electric + remainder as thermal). The largest manufacturer of PAFC fuel cells is UTC Power, a division of United Technologies Corporation. Molten-carbonate fuel cells have also been installed in these applications, and Solid-oxide fuel cell prototypes exist.
However, since electrolyzer systems do not store fuel in themselves, but rather rely on external storage units, they can be successfully applied in large-scale energy storage, rural areas being one example. In this application, batteries would have to be largely oversized to meet the storage demand, but fuel cells only need a larger storage unit (typically cheaper than an electrochemical device).
One such pilot program exists on Stuart Island off the State of Washington. There the Stuart Island Energy Initiative has built a complete system by which solar panels generate the current to run several electrolyzers whose hydrogen is stored in a 500 gallon tank at 150-200 PSI. The hydrogen is then used to run a 48V ReliOn hydrogen fuel cell that provides full electric back-up to the residential site on this off the grid island (see external link to SIEI.ORG).
Plug Power Inc. is a major player in the design, development and manufacture of PEM fuel cells for stationary applications, including products aimed at telecommunication, prime power, and combined heat and power (CHP) applications.
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