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Comparing Energy Sources

Comparing Energy Sources. Law of Conservation of Energy. Energy does not appear from nothing or disappear to nothing; It just changes form. “No such thing as a free lunch”. Must get the energy from existing source, convert to more desirable form or use. Takes other energy to do this.

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Comparing Energy Sources

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  1. Comparing Energy Sources

  2. Law of Conservation of Energy Energy does not appear from nothing or disappear to nothing; It just changes form. “No such thing as a free lunch”. Must get the energy from existing source, convert to more desirable form or use. Takes other energy to do this. Transfers of energy in form, use or in space are never 100% efficient. Some waste always occurs. So, input of energy always less than useful output. If system appears to produce “free” energy, look again

  3. Comparing Energy: Units There are many equivalent unit of energy (and some that are not). Need to keep them straight. Equivalent energy units: Btu, joule, calorie, watthour. Can be used to compare sources of potential energy such as coal, oil, or wind turbine. http://www.eia.doe.gov/basics/conversion_basics.html Weight, volume, energy density, pollution, and cost matter when comparing energy sources. Btu most commonly used if comparing industrial heat sources; kilowatt hour (kWh) if comparing electricity production

  4. Efficiency In the real world, gas or coal do not convert directly (100% efficiency) to electricity (kWh). Must first convert to heat, which animates a medium (water to steam), which drives turbine blades on the electricity generator. Energy, usually in the form of heat, is lost at each stage. Such power plants 30-40% efficient. Efficiency = (useful energy or work output / total energy input) x 100% . If comparing different electricity production methods, efficiency factor is important.

  5. Units of Energy (cont) Again, equivalent energy units: Btu, joule, calorie, watthour (more commonly kilowatt hour) Watt (or kilowatt) is unit of power rating, NOT energy. Must add time period in which power is produced or used to get energy produced or used. A light bulb rated for 100 watts, used over period of 10 hours, would use one kilowatt hour of electricity. If bulb used for 10,000 hours, would use 1 megawatt hour. A generator rated at one megawatt, used to produce electricity over a one hour period, would produce one megawatt hour. NOT one megawatt PER hour!!! How many hours in a year? Can a generator be powered 24/365?

  6. Sample Conversions If could convert efficiently, how many kWh in a barrel of oil? 1 barrel (42 gallons) of crude oil = 5,800,000 Btu 1 kilowatthour of electricity = 3,412 Btu So: (1 kWh/3,412 Btu) / (1brl oil/5,800,000 Btu) Or 1700 kWh / brl oil Compare a gallon of gasoline with gal. of propane 1 gallon of gasoline = 124,000 Btu 1 gallon of propane = 91,000 Btu So (1gal gas/124,000 Btu) / (1 gal prop/91,000 Btu) Or 0.73 gallons of gas per gallon of propane

  7. An Efficiency Conversion If a diesel fired electrical generating plant is 40% efficient, how many kWh per gallon of fuel would be produced? 1 gallon of diesel fuel = 139,000 Btu 1 kilowatthour of electricity = 3,412 Btu Efficiency factor for the plant is 0.4 So: ((1 kWh/3,412 Btu) / (1gal diesel/139,000 Btu)) x 0.4 Or 40.7 kWh / gallon of diesel fuel in this plant.

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