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Chapter 19: Problem 65

Refrigerators remain among the greatest consumers of electrical energy in most homes, although mandated efficiency standards have decreased their energy consumption by some \(80 \%\) in the past four decades. In the course of a day, one kitchen refrigerator removes \(30 \mathrm{MJ}\) of energy from its contents, in the process consuming \(10 \mathrm{MJ}\) of electrical energy. The electricity comes from a \(40 \%\) efficient coal-fired power plant. The electrical energy a. is used to run the light bulb inside the refrigerator. b. wouldn't be necessary if the refrigerator had enough insulation. c. retains its high-quality status after the refrigerator has used it. d. ends up as waste heat rejected to the kitchen environment.

Short Answer

Expert verified
The correct answer is option 'd'. The electrical energy ends up as waste heat rejected to the kitchen environment.

Step by step solution

01

Analyze the options

First, consider each option individually.\n\na. The electricity is probably not just used to run the light bulb inside the refrigerator. This part is only a tiny fraction of the power consumption.\n\nb. This statement is not entirely true because a fridge needs energy to operate no matter how much insulation it has. The insulation only reduces the rate of heat transfer, not eliminating it.\n\n c. Quality of energy is a concept related to how useful an energy can be. Electrical energy is known as high-quality energy because firstly, it is easily harnessed to do work, and second, it could be converted with high efficiency. The statement, however, does not stand, because transforming electricity into coolness (removing heat) in the fridge results in a decrease in energy quality.\n\n d. 'Waste heat' is accurately defined as heat energy that has been converted from the original electrical energy but isn't going to be used.
02

Choose the correct answer

After analyzing each option, it is clear that option 'd' seems to describe the process correctly. The electrical energy consumed by the refrigerator ends up as waste heat rejected to the kitchen environment because once fridge uses electrical energy, it is transformed into derivate mechanical energy of the compressor and refrigerator, and also the generated heat, which is released into the kitchen environment.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Electrical Energy Efficiency
Refrigerators today are far more efficient than those built decades ago. Thanks to regulatory standards and technological advancements, energy consumption has decreased significantly. Energy efficiency in this context means using less energy to do the same amount of work. In the case of refrigerators, that work is keeping food items cool.
  • Modern refrigerators consume around 80% less energy than older models.
  • This efficiency is not just good for the environment but also reduces electricity bills for homeowners.
With efficient designs and improved compressors, refrigerators now require less energy input for the same cooling output.
Consequently, this efficiency reduces the environmental impact of energy consumption, particularly when the energy is generated from coal-fired power plants. However, the fundamental operation of a refrigerator still requires a certain amount of energy input. This is due in part to unavoidable energy losses during energy transformation processes.
Energy Transformation
In refrigerators, electrical energy undergoes a fascinating transformation. The journey begins with high-quality electrical energy being input into the system, where it is used to power the refrigerator's compressor. The compressor then transforms this electrical energy into mechanical energy.
The compressor's primary job is to pressurize the refrigerant, which absorbs heat from inside the fridge. However, energy transformation is not entirely efficient. According to the second law of thermodynamics, some energy is always lost in conversion. Here, the lost energy manifests as waste heat, which is expelled into the surrounding environment.
  • The energy transformation process is what enables refrigerators to remove heat from their interiors.
  • This process also highlights why energy efficiency is crucial, as less efficient models waste more energy as heat.
It's critical to understand that even the most efficient refrigerators cannot transform all input electrical energy into usable cooling power.
Insulation Impact on Refrigerators
Insulation plays a crucial role in the efficiency of a refrigerator. It serves as a barrier that minimizes heat transfer from the outside to the inside of the fridge. Good insulation reduces the amount of work the compressor needs to perform. This, in turn, means less energy is needed to maintain the desired temperature. The right kind of insulation can significantly enhance a refrigerator's performance. However, it's not a complete solution. Even with ideal insulation, a refrigerator will still need electrical energy to function due to heat transfer when the door is opened and other unavoidable factors.
  • Insulation reduces the energy required but cannot eliminate the need for energy input.
  • Continuous advancements in insulation technology contribute to the overall decrease in energy consumption over time.
Thus, while insulation impacts energy usage, it cannot entirely negate the energy needs of a refrigerator.
Waste Heat Management
Waste heat management refers to how the heat produced by the refrigeration process is handled. Once electrical energy is used, a significant portion is transformed into heat. This heat becomes 'waste' because it cannot be used for further productive purposes in the refrigerator. Managing this waste heat is crucial for maintaining the efficiency and longevity of the appliance.
  • This excess heat is typically released into the kitchen environment, contributing to overall room temperature.
  • Some modern refrigeration systems incorporate heat recovery systems to utilize this waste heat beneficially.
Understanding and improving waste heat management can help enhance the energy efficiency of appliances and reduce their environmental impact.
Efficient waste heat management systems can lead to additional energy savings, increasing the overall effectiveness of a refrigerator.

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Most popular questions from this chapter

A 5.0 -mol sample of an ideal diatomic gas is at 1.0 atm pressure and 300 K. Find the entropy change if the gas is heated to \(500 \mathrm{K}\) (a) at constant volume, (b) at constant pressure, and (c) adiabatically.

A power plant extracts energy from steam at \(250^{\circ} \mathrm{C}\) and delivers 800 MW of electric power. It discharges waste heat to a river at \(30^{\circ} \mathrm{C} .\) The plant's overall efficiency is \(28 \% .\) (a) How does this efficiency compare with the maximum possible at these temperatures? (b) Find the rate of waste-heat discharge to the river. (c) How many houses, each requiring \(18 \mathrm{kW}\) of heating power, could be heated with the waste heat from this plant?

The molar specific heat at constant pressure for a certain gas is given by \(C_{p}=a+b T+c T^{2},\) where \(a=33.6 \mathrm{J} / \mathrm{mol} \cdot \mathrm{K}\), \(b=2.93 \times 10^{-3} \mathrm{J} / \mathrm{mol} \cdot \mathrm{K}^{2},\) and \(c=2.13 \times 10^{-5} \mathrm{J} / \mathrm{mol} \cdot \mathrm{K}^{3} .\) Find the entropy change when 2 moles of this gas are heated from \(20^{\circ} \mathrm{C}\) to \(200^{\circ} \mathrm{C}\).

In an alternative universe, you've got the impossible: an infinite heat reservoir, containing infinite energy at temperature \(T_{\mathrm{h}} .\) But you've only got a finite cool reservoir, with initial temperature \(T_{\mathrm{c} 0}\) and heat capacity \(C .\) Find an expression for the maximum work you can extract if you operate an engine between these two reservoirs.

It costs \(\$ 180\) to heat a house with electricity in a typical winter month. (Electric heat simply converts all the incoming electrical energy to heat.) What would the monthly heating bill be after switching to an electrically powered heat-pump system with \(\mathrm{COP}=3.1 ?\)
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