# What is a Thermodynamic Cycle?

A system completes a

## thermodynamic cycle

when it undergoes two or more processes and the system returns to its initial state.
This lesson is only an introduction to thermodynamic cycles. You will be learning more about them in each of the following chapters.
In this lesson we will consider:
1. Power Cycles

2. Refrigeration Cycles

3. Heat Pump Cycles

Two ways to categorize thermodynamic cycles:
Open Cycles
Gas Cycles
Closed Cycles
Vapor Cycles
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### Ch 4, Lesson F, Page 1 - What is a Thermodynamic Cycle?

• You may recall the definition of a thermodynamic cycle from Chapter 1.
• Hopefully it is a bit more meaningful to you now.
• A system completes a thermodynamic cycle when it undergoes two or more processes and returns to its initial state.
• This may seem silly to you at first, but cycles are at the heart of many important applications of thermodynamics.
• Try the roll-overs while I tell you about some important applications of thermodynamic cycles.
• For example, the water and steam used in fossil fuel and nuclear power plants operates in a Power Cycle to produce work which is used to generate electrical power.
• The refrigerant used in the refrigerator in your kitchen flows though a series of devices that make up a Refrigeration Cycle.
• Household refrigerators and thermal power generation are both examples of closed, vapor cycles.
• They are closed cycles because the working fluid is reused.  It does not enter and leave the system
• These cycles are vapor cycles because both the steam in the power plant and the refrigerant in the refrigerator are condensed and vaporized during the cycle.
• An automobile engine and a jet airplane engine are examples of open, gas cycles.
• These engines are open because the working fluid, air, flows into the system and out of the system as the cycle proceeds.
• These engines are gas cycles because the air remains in the gas phase at all times.  It never condenses to the liquid phase.
• OK, now you have a general idea of what a thermodynamic cycle is and how it can be used industrially.  Now, let’s apply the 1st Law to a thermodynamic cycle.