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Homework Help: Energy from Waste

  1. Nov 12, 2013 #1
    1. The problem statement, all variables and given/known data
    If all the solid waste of 230 million tonnes were burned in waste-to-energy power plants that are 20% efficient,

    a) how many kilowatt-hours of electrical energy could be produced in a year? Compare this quantity with the total U.S. electrical energy production of approximately 3.8 trillion kWh annually.

    b) what would be the equivalent average electric power output from all these waste burning power plants? how many 1-GW coal-fired plants could they displace

    2. Relevant equations

    20 MJ/KG from waste

    3. The attempt at a solution
    Part A)

    230 million tonnes=230 billion kg

    230kg x 20 MJ/KG= 4.6x10^12 MJ

    4.6x10^12 MJ/3600 seconds per hour = 1.28x10^9 MWh

    2.56 x 10^11 kWh/3.8 trillion kWh = 6.7 %

    Part B I'm not sure where to start. The back of the book says the answer is 29 Gw and 29 coal fired plants but I don't know how they got to this answer. I tried to convert the 2.56x10^11 Kwh to GW and got 256000 GWH but according to the book, that's not correct.
  2. jcsd
  3. Nov 12, 2013 #2


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    Everything looks good to this point.

    What happened to the 20% efficiency factor?

    It's not clear from the problem statement over what period the waste is burned. It would be easier to find out how many joules of energy are contained in the total annual U.S. electrical energy production figure of 3.8 trillion KW-h.

    How many KW-h or joules can a 1 GW coal plant produce in 1 year of operation? Compare this figure to the amount of energy produced by burning the waste.
  4. Nov 12, 2013 #3

    1 GW is equivalent to 1,000,000 KW. Is this the same as Kwh or do I need to multiply by the number of hours? In which case a year of production would have 8760 hours times 1,000,000 equals 8,760,000,000 kWh. Is this correct?

    Compared to the 256,000,000,000 kwh from burning waste, the 1GW power plant produces more.
  5. Nov 12, 2013 #4


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    Is 1 GW equal to 1 million KW-h? Why don't you look at the definitions of the units?

    You still haven't answered my question about what happened to the 20% efficiency of the waste heat plants.
  6. Nov 12, 2013 #5
    No, 1 GW is equal to 1 million KW. I'm trying to figure out how to convert KW to KW-hours.

    And sorry, I missed that. I didn't add in the 20% efficiency. Would I multiply the 2.56 x 10^11 kWh by 0.2 to factor in the efficiency and get 5.12x10^10
  7. Nov 12, 2013 #6

    I missed a step when I typed this up.

    (2.3E11 KG)(20 MJ/KG) = 4.6E12/3600seconds= 1.28E9 MWh * (.2 efficiency)= 2.56E8 Mwh *1000= 2.56E11/3.8E12= 6.7%

    As far as part B I'm still confused. I need to convert the 2.56E11 to GW?
  8. Nov 12, 2013 #7


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    If 1 KW = 1000 J/s, then how many J are in 1 KW-h? Pay careful attention to the units.
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