1

Thermodynamic principles of energy conversion

Md. Mizanur Rahman School of Mechanical Engineering Universiti Teknologi Malaysia

Introduction

• Mechanical and electrical power developed from the combustion of fossil fuels or the fission of nuclear fuel or renewable sources

• This released energy is never lost but is transformed into other forms

• This conservation of energy is explicitly expressed by the first law of thermodynamics

• The work of an engine cannot be the same as the energy available in the fuel sources

• Second law of thermodynamics is to meet to happen the process

2

3

Forms of energy

• Energy can exist in numerous forms such as thermal, mechanical, kinetic, potential, electric, magnetic, chemical, and nuclear, and their sum constitutes the total energy, E of a system.

• Mechanical Energy

• Kinetic energy, KE: The energy that a system possesses as

a result of its motion relative to some reference frame,

KE=1/2mV2.

• Potential energy, PE: The energy that a system possesses

as a result of its elevation in a gravitational field, PE=mgh

• For a moving body, E=PE+KE remains constant

4

Internal energy, U: The sum of all the

microscopic forms of energy.

• For gases, molecules are so widely

separated in space that they may be

considered to be moving

independently of each other, each

possessing a distinct total energy.

• In the case of liquids or solids, each

molecule is under the influence of

forces exerted by nearby molecules

• Changes in internal energy are

measurable by changes in

temperature, pressure, and density.

5

• Chemical energy: The internal energy associated with the atomic bonds in a molecule.

• Nuclear energy: The tremendous amount of energy associated with the strong bonds within the nucleus of the atom itself.

Internal = Sensible + Latent + Chemical + Nuclear

Thermal = Sensible + Latent

Energy form examples

• In a gasoline engine, the combustion of the fuel–air mixture involves U, PdV and Echem

• In a steam and gas turbine, only H and KE change

• In a nuclear power plant fuel rod, U and Enuc are involved

• In a magnetic cryogenic refrigerator, U and Emag are important.

6

7

Total energy

per unit mass

The various forms of energy that can be possessed by a material body can

be added together to define a total energy, to which we give the symbol E,

Work and Heat interactions

• Work Interaction

W = pΔV

Heat Interaction

Q = mCpΔT

8

9

THE FIRST LAW OF THERMODYNAMICS

• Energy can be neither created nor destroyed during a process; it can only change forms.

• The First Law: For all adiabatic processes between two specified states of a closed system, the net work done is the same regardless of the nature of the closed system and the details of the process.

• The change of energy in the system dE equals the heat dQ transferred to the system minus the work dW done by the system

In a cyclic process, Ei=Ef

I.e. Net heat and work are equal

Processes proceed in a certain direction and not in the reverse direction

A process must satisfy the first law to occur, however, satisfying the first law alone does not ensure that the process will actually take place.

10

In a process for which dQ = 0, which is called an adiabatic process, the entropy may

remain the same or increase but may never decrease. An adiabatic process for which

the entropy increases is an irreversible process.

The Carnot cycle is composed of four reversible

processes—two reversible isothermal and two

reversible adiabatic process

In a P-V diagram the area under the

process curve represents the boundary

work for quasi-equilibrium (internally

reversible) processes,

The area under curve 1-2-3 is the work

done by the gas during the expansion

The area under curve 3-4-1 is the work

done on the gas during the compression.

The area enclosed by the path of the cycle

(area 1-2-3-4-1) is the difference between

these two and represents the net work done

during the cycle. 12

Net work done during Carnot cycle

REFRIGERATORS AND HEAT PUMPS

Heat is transferred in the direction of decreasing temperature, that is, from

high-temperature mediums to low temperature ones.

The reverse process, however, cannot occur by itself.

The transfer of heat from a low-temperature medium to a high-temperature one

requires special devices called refrigerators and heat pump .

13

Coefficient of Performance

The COP of a refrigerator can be expressed as

14

15

Rankine Cycle: The Ideal Cycle for Vapour Power Cycles

Many of the impracticalities associated with

the Carnot cycle can be eliminated by

superheating the steam in the boiler and

condensing it completely in the condenser.

The cycle that results is the Rankine cycle,

which is the ideal cycle for vapor power

plants. The ideal Rankine cycle does not

involve any internal irreversibilities.

The simple ideal Rankine cycle

1-2 Isentropic expansion in a turbine

2-3 Constant pressure heat rejection in a condenser

3-4 Isentropic compression in a pump

4-5-1 Constant pressure heat addition in a boiler

16

The simple ideal Rankine cycle

17

Energy Analysis of Basic Rankine Cycle (ideal)

The steam flows round the cycle and each process is analyzed using steady

flow energy equation. Using energy balance for a steady flow system

For single stream (one-inlet-one-exit) systems, mass flow rate remains

constant.

If kinetic and potential energy are negligible, the energy equation becomes

18

Types of Gas Turbine Cycles

There are two types of gas turbine cycle; Brayton/Joule

cycle and Atkinson cycle

Brayton Cycle

Heat added and rejected is at constant pressure

19

Brayton Cycle: Ideal Cycle for Gas Turbine Cycle

Gas turbines usually operate on an open cycle.

Air at ambient conditions is drawn into the compressor, where its temperature

and pressure are raised. The high pressure air proceeds into the combustion

chamber, where the fuel is burned at constant pressure.

The high-temperature gases then

enter the turbine where they expand

to atmospheric pressure while

producing power output.

Some of the output power is used

to drive the compressor.

The exhaust gases leaving the

turbine are thrown out (not re-

circulated), causing the cycle to be

classified as an open cycle.

20

The open gas-turbine cycle can

be modelled as a closed cycle,

using the air-standard

assumptions

The compression and expansion

processes remain the same, but

the combustion process is

replaced by a constant-pressure

heat addition process from an

external source.

The exhaust process is replaced

by a constant-pressure heat

rejection process to the ambient

air.

Brayton Cycle - Closed Cycle Model

21

The ideal cycle that the working fluid

undergoes in the closed loop is the Brayton

cycle. It is made up of four internally reversible

processes:

1-2 Isentropic compression;

2-3 Constant-pressure heat addition;

3-4 Isentropic expansion;

4-1 Constant-pressure heat rejection.

The T-s and P-v diagrams of an ideal Brayton

cycle are shown beside.

Note: All four processes of the Brayton cycle

are executed in steady-flow devices thus, they

should be analyzed as steady-flow

processes.

The Brayton Cycle Process

22

Otto and Diesel Cycle

23

The ideal Otto and Diesel cycles are not totally reversible because they involve heat

transfer through a finite temperature difference

The irreversibility renders the thermal efficiency of these cycles less than that of

a Carnot engine operating within the same limits of temperature.

The Stirling cycle has two isentropic processes featured in the Carnot cycle

1-2 Isothermal heat addition (expansion)

2-3 Isochoric heat removal (constant volume)

3-4 Isothermal heat removal (compression)

4-1 Isochoric heat addition (constant volume)

Stirling cycle

24

1-2 Isothermal heat addition (expansion)

2-3 Isobaric heat removal

3-4 Isothermal heat removal (compression)

4-1 Isobaric heat addition

The Ericsson cycle is often compared with the Stirling cycle, since the engine

designs based on these respective cycles are both external combustion

engines with regenerators.

The most well-known ideal cycle is the Carnot cycle, although a useful Carnot

engine is not known to have been invented. The theoretical efficiencies for both,

Ericsson and Stirling cycles acting in the same limits are equal to the Carnot

Efficiency for same limits.

Ericsson Cycle

25

FUEL CELLS

• We see several different systems for converting the energy

of fuel to mechanical energy by utilizing direct combustion of

the fuel with air, each based upon an equivalent

thermodynamic cycle.

• In these systems, a steady flow of fuel and air is supplied to

the “heat engine,” within which the fuel is burned, giving rise

to a stream of combustion products that are vented to the

atmosphere.

• The thermal efficiency of these cycles, which is the ratio of

the mechanical work produced to the heating value of the

fuel, is usually in the range of 25% to 50%.

26

FUEL CELLS

• This efficiency is limited by the combustion

properties of the fuel and mechanical limitations of

the various engines.

• Is there a more efficient way to convert fuel energy

to work? The second law of thermodynamics

places an upper limit on the amount of work that

can be generated in an exothermic chemical

reaction, such as that involved in oxidizing a fuel in

air.

27

• A fuel cell is an electrochemical device that converts

chemical energy from a fuel into electrical energy without

any moving parts

• Fuel cells are operationally equivalent to a battery, but the

reactants or fuel in a fuel cell can be replaced unlike a

standard disposable or rechargeable battery

28

29

Group discussion

Discuss with your friends about the following issues (5 minutes)

1. An modern engine can have more efficiency than a Carnot engine operating between same temperature limits?

2. Entropy generation can be avoided?

3. A fuel cell can have more efficiency than a Carnot efficiency?

4. How can we achieve close to 100% efficiency from fossil fuels to final energy consumption?

5. Heat and work, which is more valuable to you?

30