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4 THERMODYNAMIC CYCLES

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Abstract

Energy sources such as coal, natural gas, or petroleum cannot be used directly to
perform a work. These sources are burned to generate heat which is then converted to
mechanical or electrical energy. These processes are governed by thermodynamic
cycles and the efficiency of the overall process depends mainly on the choice or
efficiency of the cycle. A number of thermodynamic cycles using various working
fluids have been suggested. The description and analysis of these cycles are presented
in this chapter.

Introduction

The energy resources available in nature such as coal, natural gas, petroleum, and
uranium cannot be used directly for most of the applications. These resources have
to be transformed into a useful form such as heat or electricity before their use.
For example, coal must first be burned or combusted to generate heat which is
then used to produce steam for space heating or electricity generation. Similarly,
crude petroleum must first be refined to obtain gasoline or diesel.

Carnot Cycle

The Carnot cycle was first studied by Nicolas Léonard Sadi Carnot in the 1820s.
A Carnot cycle acting as a heat engine is shown in Fig. 4.1 on a temperature
(T)-entropy (s) diagram. The cycle takes place between a hot reservoir at temperature
TH and a cold reservoir at temperature TC. The Carnot cycle consists of the
following steps:
Reversible isothermal expansion of the gas at the “hot” temperature, TH
(Isothermal heat addition): During this step (1 to 2 in Fig. 4.1) the expanding gas
causes the piston to do work on the surroundings. The gas expansion is propelled
by absorption of heat from the high temperature reservoir.
Reversible adiabatic expansion of the gas: For this step (2 to 3 in Fig. 4.1) it is
assumed that the piston and cylinder are thermally insulated, so that no heat is
gained or lost. The gas continues to expand, doing work on the surroundings. The
gas expansion causes it to cool to the “cold” temperature.