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Electrical efficiency(system) = HHV of H 2 produced Electricity used + Heat su pplied Recommendations The following quote is drawn from a 2002 publication on hydrogen by the Bellona Foundation, of Oslo, Norway. “When calculating the efficiency in a fuel cell, the lower heating value is used. In the electrolysis process, the high heating value is used.” The full report is available at http://bellona.org/filearchive/fil_Hydrogen_6-2002.pdf, and it basically sums up the European point of view. In the United States, however, the situation is more equivocal. Developers of natural gas fuel cells do use LHV. Gas turbine manufacturers and manufacturers of IC engine generator sets, however, use HHV to calculate electrical efficiency. These groups complain that comparisons of fuel cells with heat engines are unfair because of the different basis used for the efficiency calcu- lations. In the United States, natural gas is sold by the therm (1 therm = 100,000 Btu), but it is measured by the cubic foot. The conversion from cubic feet to therms used by a gas supplier in billing customers uses either the measured gas composition and the sum of the HHV of each fractional component, or the actual calorimeter measurements of the HHV of the gas supplied to the customer. In either case, the customer pays based on the HHV of the gas used. It therefore makes sense that calculations of system efficiency for all electric generators should use the HHV of the fuel. Electrolyzer manufacturers appear to have standardized on kWh/Nm3 or kWh/kg as a measure of system efficiency, which sidesteps the LHV versus HHV controversy. As noted above, Europeans prefer HHV for calculating electrolyzer efficiency on the basis of heating value. As stated, splitting a mole of liquid water to produce a mole of hydrogen at 25°C requires 285.8 kJ of energy—237.2 kJ as electricity and 48.6 kJ as heat. It then follows that the ratio of reversible free energy potential (1.229 V) over the thermoneutral voltage (1.481 V) is 83%. This represents the highest efficiency attainable when using the LHV to determine stack voltage efficiency. Likewise, the same can be said for electrolyzer system efficiency calculations. Therefore, it is worth stating that the highest attainable efficiency is 83% when referencing electrolyzer system and stack efficiencies to the LHV. The HHV easily is converted into more common forms of the higher heating value. 285,840 J ∗ 1mol H 2 ∗ 1,000g = 141,799,781 J = 141.8 MJ mol 2.0158g 1kg kg kg 141.8MJ∗1watt−sec∗ 1kW ∗ 1h =39.4kWh kg J 1,000w 3,600sec kg The U.S. Department of Energy (DOE) Fuel Cell Technologies Program Multi-Year Plan [1] includes targets for distributed water electrolysis and for central wind water electrolysis using three measures of efficiency: HHV, LHV, and kWh/kg. It also sets goals [2]. In all other hydrogen-production schemes, however—for example in natural gas reforming and biomass gasification production scenarios—“energy efficiency is defined as the energy in the hydrogen 8PDF Image | Hydrogen Production: Fundamentals
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