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Work, Power, energy: Appliances (refrigerators, freezer etc)

  1. May 15, 2016 #1
    1. The problem statement, all variables and given/known data
    Fill in the empty columns of the following table:

    upload_2016-5-15_16-5-42.png
    2. Relevant equations
    y = ΔE x c
    P = ΔE/Δt
    ΔE = Y/ Δt
    3. The attempt at a solution

    Part d) Yearly energy cost:
    Use Y=ΔE x c

    Refrigerator, top freezer:
    Y = (448 kWh/yr) ($ 0.06)
    Y = $ 26. 88/yr

    Refrigerator, bottom freezer:
    Y = (393 kWh/yr) ($ 0.06)
    Y = $ 23.58/yr

    Upright freezer:
    Y = (420 kWh/yr) ($ 0.06)
    Y= $ 25.20/yr

    Chest freezer:
    Y = (362 kWh/yr) ($ 0.06)
    Y = $21.72/yr

    Top-loading clothes washer:
    Y = (301 kWh/yr) ($ 0.06)
    Y = $ 18.06 / yr

    Front-loading clothes washer:
    Y =(153 kWh/yr) ($ 0.06)
    Y = $ 9.18/yr

    Part e) Energy used in one week:
    Use ΔE = Y/Δt

    Refrigerator, top freezer:
    ΔE = 448 kWh /yr / 52 weeks
    = 8.62 kWh/week

    Refrigerator, bottom freezer:
    ΔE = 393 kWh/yr / 52 weeks
    = 7.56 kWh/week

    Upright freezer:
    ΔE = 420 kWh/yr / 52 weeks
    = 8.08 kWh/week

    Chest freezer:
    ΔE = 362 kWh/yr / 52 weeks
    = 6.96 kWh/ week

    Top- loading clothes washer:
    ΔE = 301 kWh/yr / 52 weeks
    = 5.79 kWh/ week

    Front-loading clothes washer:
    ΔE = 153 kWh/yr / 52 weeks
    = 2.9 kWh/week

    Part f) Estimate of hours in use per week:

    Okay, so for all the refrigerators and freezers, I estimated the hours of use to be 168 hours, since I assumed these appliances are being used all day for the entire week. ( I am not sure if that is correct)

    For the washers, I estimated the hours per week to be 5 hours.

    Can someone please tell me if my estimate is correct, or do I need to put a different amount of hours for each appliance?

    Here is part g) using my estimate of hours per week from part f)
    Use P = ΔE/ Δt

    Refrigerator, top freezer:
    P = 8.62 kWh/ week / 168 hrs/week
    = 0.05 kW or 50 W

    Refrigerator, bottom freezer:
    P = 7.56 kWh /week /168 hrs/week
    = 0.045 kW or 45 W

    Upright freezer:
    P = 8.08 kWh/week / 168 hrs/week
    = 0.048 kW or 48 W

    Chest freezer:
    P = 6.69 kWh/week / 168 hrs/week
    = 0.041 kW or 41 W

    Top-loading clothes washer :
    P = 5.79 kWh /week / 5 hrs/week
    = 1.158 kW or 1158 W

    Front-loading clothes washer:
    P = 2.9 kWh /week / 5 hrs/ week
    = 0.58 kW or 580 W

    Is my process and answer correct?
     
  2. jcsd
  3. May 16, 2016 #2
    Any help would be appreciated :biggrin:
     
  4. May 16, 2016 #3

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    Should be fine if you have to guess values.

    The approach is fine everywhere, didn't check all the numbers, computers can do that better.

    Real freezers tend to switch between cooling and staying passive frequently, but the duty cycle will depend on the specific product.
     
  5. May 18, 2016 #4
    If I changed the hours of use per week to between 80 and 100 hours, would that make sense?

    Like this:

    Top freezer: 95 hrs
    bottom freezer: 85 hrs
    upright freezer: 90hrs
    chest freezer: 80 hrs

    I am estimating the hours, so I can choose any values right?
     
  6. May 19, 2016 #5

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    I'm not sure if that is a better answer.
     
  7. May 24, 2016 #6
    Sorry for the late reply.
     
  8. May 24, 2016 #7
    Okay, so should I just use my original estimated hours?
     
  9. May 24, 2016 #8

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    I would use those.
     
  10. May 24, 2016 #9
    Okay, but does it make sense that the power for the freezers are so low while the power for the washers are high?
     
  11. May 24, 2016 #10

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    Why do you think this would be an issue?
     
  12. May 24, 2016 #11
    Well, I just thought that since the freezers are being used more hours per week the power would be higher.. but I guess that is not what they are asking of me in this question.
     
  13. May 25, 2016 #12
    Anyway, thanks for the help! :biggrin:
     
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