# Thermodynamics

## Availablity and Irreversibilty

### Introduction

The second law of the thermodynamics states that complete conversion of heat into work is not possible in cotinous process. The most efficient cycle to produce work has been proven to be reversible power cycle which is known as Carnot cyle. However even in Carnot cyle the efficiency of the conversion can never be unity. Thus, to establish comparison of the work-energy conversion in actual proesses, the maixum theoretical work obtainable with respect to some data must be determined.

Accroding to the second law of thermodynamics, many processes occur spontaneously in one direction only and thus they are irreversible under a given set of conditions. Work obtained in an irreversible process will always be less than that of a reversible process. This difference is termed as irreversibility. The difference between the reversible work and the actual work for a given change of state of a system is called irreversibility.

### Systems undergoing a steady state process

If the state and flow rate of the stream leaving and entering a system does not change with time, it is called steady state system. Another way to define steady state is a state of a process during which a fluid flows through a control volume steadily.

A steady state flow process requires conditions at all points in an apparatus remain constant as time changes. Principle of conservation of engery states that rate at which energy entering the system will be equal to the rate at wich energy flows out of the system. When a system is experiencing steady state, the system is considered to be stable.

A flow process should satisfy the following conditions, to a steady flow process.

1. The mass and energy content of the control volume remains constant with time.
2. The state and energy of the fluid at inlet, at the exit and at every point within the control volume are time independent.
3. The rate of energy transfer in the form of work and heat across the control surface is constant with time.

### Control volume undergoing steady state process

A volume in space of special interset for particular analysis is called Control volume. The surface of the control volume is referred as a control surface and is a closed surface. The surface is defined with relative to a coordinate system that may be fixed, moving or rotating. Mass, heat and work can cross the control surface and mass and properties can change with time within the control volume.

### Availablity

Enegry content of a system can be devided into two parts. The first part is available energy, which under ideal conditions may be completely converted into work. The second part is unavailable energy which is usually rejected as wate. The available and unavailable part of energy content of a system depends on the ambient conditions also. The maximum useful work that can be obtained from the system such that the system comes to a dead state, while exchanging heat only with the surroundings, is known as availability of the system.

### Second law effiency

According to the second law of thermodanymics, due to the increase in entropy, heat cannot be converted to work without creating some waste heat. There due important efficiency equations with respect to this concept:
**Carnot Efficiency**
Carnot efficiency is the theoretical maximum efficiency that a heat engine can achieve operating between hot and cold reservoirs with temperatures TH and TL, respectively:

**Second Law Efficiency**
Second Law efficiency is a measure of how much of the theoretical maximum (Carnot) you achieve, or in other words, a comparison of the system’s thermal efficiency to the maximum possible efficiency. The Second Law efficiency will always be between the Carnot and First Law efficiencies.