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The annual fuel utilization efficiency (AFUE; pronounced 'A'-'Few' or 'A'-'F'-'U'-'E') is a thermal efficiency measure of space-heating furnaces and boilers.The AFUE differs from the true 'thermal efficiency' in that it is not a steady-state, peak measure of conversion efficiency, but instead attempts to represent the actual, season-long, average efficiency of that piece of equipment ...
For energy-conversion heating devices their peak steady-state thermal efficiency is often stated, e.g., 'this furnace is 90% efficient', but a more detailed measure of seasonal energy effectiveness is the annual fuel use efficiency (AFUE).
Direct method of boiler efficiency test is more usable or more common. Boiler efficiency = power out / power in = Q × (Hg − Hf) / (q × GCV) × 100%. where Q, rate of steam flow in kg/h Hg, enthalpy of saturated steam in kcal/kg Hf, enthalpy of feed water in kcal/kg q, rate of fuel use in kg/h
The heating value of a fuel can be calculated with the results of ultimate analysis of fuel. From analysis, percentages of the combustibles in the fuel (carbon, hydrogen, sulfur) are known. Since the heat of combustion of these elements is known, the heating value can be calculated using Dulong's Formula:
Energy conversion efficiency depends on the usefulness of the output. All or part of the heat produced from burning a fuel may become rejected waste heat if, for example, work is the desired output from a thermodynamic cycle.
The efficiency of a plant is the percentage of the total energy content of a power plant's fuel that is converted into electricity. The remaining energy is usually lost to the environment as heat unless it is used for district heating. Rating efficiency is complicated by the fact that there are two different ways to measure the fuel energy ...
Heat engine efficiency can be based on the fuel Higher Heating Value (HHV), including latent heat of vaporisation that would be recuperated in condensing boilers, or the Lower Heating Value (LHV), excluding it. The HHV of methane is 55.50 MJ/kg (23,860 BTU/lb), compared to a 50.00 MJ/kg (21,500 BTU/lb) LHV: a 11% increase.
Note that a Carnot engine is the most efficient heat engine possible, but not the most efficient device for creating work. Fuel cells, for instance, can theoretically reach much higher efficiencies than a Carnot engine; their energy source is not thermal energy and so their exergy efficiency does not compare them to a Carnot engine. [1] [2]