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Chapter 6: Problem 31

A heat pump is a device that absorbs energy from the cold outdoor air and transfers it to the warmer indoors. Is this a violation of the second law of thermodynamics? Explain.

Short Answer

Expert verified
Answer: No, a heat pump does not violate the second law of thermodynamics because it requires work (energy input) to operate, ensuring that the total entropy of the universe increases in accordance with the second law.

Step by step solution

01

Understand the heat pump operation

A heat pump is designed to transfer heat from a colder region to a warmer region. It does this by using a refrigeration cycle, where refrigerant absorbs heat from the cold environment (evaporating in the process) and releases it in the warmer environment (condensing in the process). The refrigeration cycle also involves a compressor and an expander, which require work to operate.
02

The Second Law of Thermodynamics

The second law of thermodynamics states that natural processes increase the total entropy (disorder) of the universe. In other words, it says that heat cannot spontaneously flow from a colder body to a warmer body without any input of work or energy. If heat was able to move from cold to hot spontaneously, the entropy of the universe would decrease, which violates the second law.
03

Heat Pump and the Second Law

In the case of a heat pump, heat is being transferred from the cold outdoor air to the warm indoor air, which seems to be a violation of the second law. However, remember that a heat pump requires work to operate (the compressor and expander), which essentially is an input of energy into the system. This work input compensates for the entropy decrease associated with transferring heat from cold to hot, ensuring that the total entropy of the universe increases and the second law is upheld.
04

Conclusion

A heat pump, which transfers heat from cold outdoor air to warmer indoor air, does not violate the second law of thermodynamics because it requires work (energy input) to operate. This work input ensures that the total entropy of the universe increases, in accordance with the second law.

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Most popular questions from this chapter

Show that processes that use work for mixing are irreversible by considering an adiabatic system whose contents are stirred by turning a paddle wheel inside the system (e.g., stirring a cake mix with an electric mixer).

Devise a Carnot heat engine using steady-flow components, and describe how the Carnot cycle is executed in that engine. What happens when the directions of heat and work interactions are reversed?

Using EES (or other) software, determine the maximum work that can be extracted from a pond containing \(10^{5} \mathrm{kg}\) of water at \(350 \mathrm{K}\) when the temperature of the surroundings is \(300 \mathrm{K}\). Notice that the temperature of water in the pond will be gradually decreasing as energy is extracted from it; therefore, the efficiency of the engine will be decreasing. Use temperature intervals of \((a) 5 \mathrm{K},(b) 2 \mathrm{K}\) and \((c) 1 \mathrm{K}\) until the pond temperature drops to \(300 \mathrm{K}\). Also solve this problem exactly by integration and compare the results.

A heat pump is absorbing heat from the cold outdoors at \(5^{\circ} \mathrm{C}\) and supplying heat to a house at \(25^{\circ} \mathrm{C}\) at a rate of \(18,000 \mathrm{kJ} / \mathrm{h}\). If the power consumed by the heat pump is \(1.9 \mathrm{kW},\) the coefficient of performance of the heat pump is \((a) 1.3\) (b) 2.6 \((c) 3.0\) \((d) 3.8\) \((e) 13.9\)

A promising method of power generation involves collecting and storing solar energy in large artificial lakes a few meters deep, called solar ponds. Solar energy is absorbed by all parts of the pond, and the water temperature rises everywhere. The top part of the pond, however, loses to the atmosphere much of the heat it absorbs, and as a result, its temperature drops. This cool water serves as insulation for the bottom part of the pond and helps trap the energy there. Usually, salt is planted at the bottom of the pond to prevent the rise of this hot water to the top. A power plant that uses an organic fluid, such as alcohol, as the working fluid can be operated between the top and the bottom portions of the pond. If the water temperature is \(35^{\circ} \mathrm{C}\) near the surface and \(80^{\circ} \mathrm{C}\) near the bottom of the pond, determine the maximum thermal efficiency that this power plant can have. Is it realistic to use 35 and \(80^{\circ} \mathrm{C}\) for temperatures in the calculations? Explain.
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