Efficiency of refrigerator unit

In summary: ZN: In summary, you built a refrigerator which can freeze water at 273K, by relying on nothing but your physical abilities, a dixie cup, duct tape, and a piece of string. The refrigerator has a maximum possible efficiency, and the Latent Heat of Fusion of water is 3.33×105 J/kg. The change in entropy in the water per second is 77 J/K. The heat flow ( Q_hot/time ) into the hot well per second is 273 J/s. The power required to drive the refrigerator in Watts is W.
  • #1
confused1
21
0
Suppose that you want to freeze 1 kg of water for a party, and only have 5 minutes to do it. Relying on nothing but your keen physical abilities, a dixie cup, duct tape and a piece of string, you manage to build a refrigerator unit which can do the job.
The temperature inside the refrigeration unit is 273 K, and the temperature outside is 350 K. Since you are a brilliant UW student, assume that your refrigerator has the maximum possible efficiency. The Latent Heat of Fusion of water is 3.33×105 J/kg. Assume that the initial temperature of the water is 273 K.

a. What is the change in entropy in the water per second in J/K s?

HELP: Change in entropy = Change in energy/ Absolute Temperature.

b. What is the heat flow ( Q_hot/time ) into the hot well per second in J/s?

HELP: Use the entropy flux calculated in part b, 350 K, and use

c. How much POWER must you provide to drive your refrigerator in Watts? (That is, work done per second?)

HELP: Q(hot) = Q(cold) + Work. Remember, power is work/time.
Entropy = Energy transfer / Temperature.
 
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  • #2
Its 77 degrees C outside ;) woooo, party in hell?

You really need to try to do these on your own and then ask questions. Looks like your just posting questions and hoping we'll answer them for you.
 
  • #3
confused1 said:
Suppose that you want to freeze 1 kg of water for a party, and only have 5 minutes to do it. Relying on nothing but your keen physical abilities, a dixie cup, duct tape and a piece of string, you manage to build a refrigerator unit which can do the job.
The temperature inside the refrigeration unit is 273 K, and the temperature outside is 350 K. Since you are a brilliant UW student, assume that your refrigerator has the maximum possible efficiency. The Latent Heat of Fusion of water is 3.33×105 J/kg. Assume that the initial temperature of the water is 273 K.

a. What is the change in entropy in the water per second in J/K s?

HELP: Change in entropy = Change in energy/ Absolute Temperature.
What about time?

[tex]\frac{ds}{dt} = \frac{\frac{\Delta Q}{T_c}}{\Delta t}[/tex]

where [itex]\Delta Q = 3.33e5 J[/itex] and [itex]\Delta t = 300 sec[/tex]

b. What is the heat flow ( Q_hot/time ) into the hot well per second in J/s?

HELP: Use the entropy flux calculated in part b, 350 K, and use
For a Carnot cycle,

[tex]\frac{Q_c}{Q_h} = \frac{T_c}{T_h}[/tex]

c. How much POWER must you provide to drive your refrigerator in Watts? (That is, work done per second?)

HELP: Q(hot) = Q(cold) + Work. Remember, power is work/time.
Entropy = Energy transfer / Temperature.

[tex]CP = Q_c/W = \frac{1}{\frac{T_h}{T_c} - 1} [/tex]

so work out W from Q_c found in 1.

By the way, the problem facts are quite impossible. If you place water at 273K in a reservoir at 273K there will be no freezing at all, let alone in 5 minutes.

AM
 

Related to Efficiency of refrigerator unit

1. How is efficiency measured for a refrigerator unit?

The efficiency of a refrigerator unit is typically measured by its energy consumption, which is expressed in kilowatt-hours (kWh). The lower the energy consumption, the more efficient the refrigerator unit is considered to be.

2. What factors affect the efficiency of a refrigerator unit?

The efficiency of a refrigerator unit can be affected by several factors, including the age and condition of the unit, the type of compressor it uses, the insulation of the unit, and the surrounding temperature. Regular maintenance and proper usage can also impact the efficiency of a refrigerator unit.

3. What is the average efficiency of a refrigerator unit?

The average efficiency of a refrigerator unit can vary depending on the size, brand, and model. However, most modern refrigerator units have an energy efficiency rating of at least 3 stars, which means they consume 20% less energy than a 1-star rated unit.

4. How can I improve the efficiency of my refrigerator unit?

To improve the efficiency of your refrigerator unit, you can try adjusting the temperature settings to the recommended levels, keeping the door seals clean and tight, and avoiding overfilling the unit. You can also consider upgrading to a newer, more energy-efficient model.

5. Why is it important to consider the efficiency of a refrigerator unit?

The efficiency of a refrigerator unit is important for various reasons. It can help reduce your energy consumption and electricity bills, as well as contribute to environmental sustainability. A more efficient refrigerator unit can also help preserve food for longer periods, saving you money in the long run.

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