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2.3: Energy Efficiency and Energy Content of Different Fuels

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    Remember that energy is the “ability or capacity to do work,” where “work” is defined as a change in energy. Energy is measured in a variety of units including calories, kilowatt hours (kWh), British Thermal Units (Btu). Power is measured in watts (or joules per second) and is the rate at which energy is generated or used.

    Fuels are converted into useful energy according to their heat content. The heat content of a fuel is normally measured in Btus. One Btu is the amount of energy required to raise the temperature of one pound of water (about a pint) one degree Fahrenheit. The relative efficiency of a fuel or technology is summarized in the “heat rate” – the amount of fuel required to generate one kilowatt-hour (kWh). As might be expected, not all fuels are created equal. Comparison of fuels is possible by converting them into useful energy according to their heat content as measured in Btus.

    Equation 1: Heat rate = Btu/kWh

    Under ideal conditions, in which the energy input is equal to the energy output, the heat rate is equal to 3,412 Btu/kWh. But not all energy-conversion processes are ideally efficient. So Equation 1 must be modified to account for these inefficiencies.

    Equation 2: Heat rate = (ideal heat rate)/efficiency = (3412 Btu/kWh)/efficiency

    Table 1 summarizes the average heat content of several familiar fuels. These numbers are averages because variations in the quality can affect the efficiency with which these fuels can be converted into useful energy.

    Table 1: Heat (energy) contents of fuels
    Fuel Amount Heat Content (Btus)
    Oil 1 barrel* 5,800,000 (138,095/gal)
    Natural gas 1 cubic foot 1,026
    Coal** 1 ton 20,700,000
    Gasoline 1 gallon 124,000
    Electricity*** 1 kWh 3,412
    Diesel or heating oil 1 gallon 139,000
    White oak (20% moisture content) fn 1 cord 27,000,000

    Credit: Source: U.S. Energy Information Administration

    * 1 barrel of oil = 42 gallons.

    ** Heat content of coal is an average of bituminous coals. Lignite (brown coal) will have a lower heat content. Anthracite will have a higher heat content.

    *** The number for electricity assumes perfectly efficient conversion of fuel into electricity.

    2.3: Energy Efficiency and Energy Content of Different Fuels is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Marcellus Matters via source content that was edited to conform to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.