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Chapter 4: 4.12 (page 129)

Explain why a rectangular P V cycle, as considered in Problems 1.34 and 4.1, cannot be used (in reverse) for refrigeration.


Short Answer

Expert verified

The temperature range at which working substance absorbs heat must be less than the temperature range at which working substance rejects heat. As the rectangular PV cycle doesn't show this property, therefore it cannot be used for refrigeration.

Step by step solution

01

Given Information

The rectangular PV cycle.

Explain why it cannot be used (in reverse) for refrigeration.

02

; Explanation

A square PV cycle is shown as below.

The PV diagram is a closed loop. The area of the diagram represents the amount of work done during the process.

The temperature range at which working substance absorbs heat must be less than the temperature range at which working substance rejects heat.

As the rectangular PV cycle doesn't show this property, therefore it cannot be used for refrigeration.

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Most popular questions from this chapter

It has been proposed to use the thermal gradient of the ocean to drive a heat engine. Suppose that at a certain location the water temperature is 22°Cat the ocean surface and 4°Cat the ocean floor.

(a) What is the maximum possible efficiency of an engine operating between these two temperatures?

(b) If the engine is to produce 1GWof electrical power, what minimum volume of water must be processed (to suck out the heat) in every second?

Calculate the efficiency of a Rankine cycle that is modified from the parameters used in the text in each of the following three ways (one at a time), and comment briefly on the results: (a) reduce the maximum temperature to 500°C; (b)reduce the maximum pressure to 100 bars; (c)reduce the minimum temperature to 10°C.

At a power plant that produces 1 GW109 watts) of electricity, the steam turbines take in steam at a temperature of 500o, and the waste heat is expelled into the environment at 20o
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(b) Suppose you develop a new material for making pipes and turbines, which allows the maximum steam temperature to be raised to 600o. Roughly how much money can you make in a year by installing your improved hardware, if you sell the additional electricity for 5 cents per kilowatt-hour? (Assume that the amount of fuel consumed at the plant is unchanged.)

In an absorption refrigerator, the energy driving the process is supplied not as work, but as heat from a gas flame. (Such refrigerators commonly use propane as fuel, and are used in locations where electricity is unavailable.* ) Let us define the following symbols, all taken to be positive by definition:
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Qr= waste heat expelled to room
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(a) Explain why the "coefficient of performance" (COP) for an absorption refrigerator should be defined as Qc / Qf.
(b) What relation among Qf, Qc, and Qr is implied by energy conservation alone? Will energy conservation permit the COP to be greater than 1 ?
(c) Use the second law of thermodynamics to derive an upper limit on the COP, in terms of the temperatures Tf, Tc, and Tr alone.

In table 4.1, why does the entropy of water increase with increasing temperature, while the entropy of steam decreases with increasing temperature?
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