OPTIMAL FUNCTIONING PARAMETERS FOR A STIRLING ENGINE

HEATER

1 Universit de Valenciennes et du Hainaut-Cambrsis, ENSIAME, TEMPO (EA 4542) - DF2T Valenciennes France

2 GEPEA (UMR 6144), cole des Mines de Nantes, DSEE, 4 rue Alfred Kastler - BP20722 , 44307 Nantes Cedex 03 France

3 Universit de Monastir, Laboratoire LESTE, ENIM, Monastir - Tunisia

* E-mail : Fethi.Aloui@univ-valenciennes.fr

R. GHEITH 2,3, * F. ALOUI 1,2 and S. BEN NASRALLAH 3

10th EUROPEAN CONFERENCE

ON COAL RESEARCH AND ITS

APPLICATIONS: 10th ECCRIA

VI. Conclusions

II. Experimental device

III. Some experimental results

I. Introduction Objectives of the study

OUTLINE

Robert Stirling

Invention

Stirling engine (1816)

- A quiet engine (no vibrations because no internal combustion),

- Ecological engine (closed gas circuit),

- Any heating source can be used to heat the working fluid,

- Its efficiency is more important than that of an internal

combustion engine (between 30 and 40%)

This Stirling engine is:

1

The different Stirling engine configurations

Alpha type Heating

Cooling

Regenerator

Heating Cooling

Beta type

Regenerator

Gamma type

Heating

Cooling

Regenerator

2

The Stirling Cycle

1 2 : Isochoric heating 2 3 : Isothermal expansion

3 4 : Isochoric cooling 4 1 : Isothermal compression

Q340

V1

1

d

a b

c

3

Example of some applications using a Stirling engine 4

Stirling engine - generator Stirling engine dish Space domain (salellites) Submarine domain

Exchanger

To heating system

Electricity

Electricity

Stirling engine

Mean burner Gas valves

Natural

gas

From heating system

Micro -cogenerator system Liquefaction of gases (receipt machines)

GLOBAL THERMODYNAMIC MODELS

Isothermal model

Adiabatic model

Quasi-steady model

- Real gas assumption

- Thermal losses

- Mechanical losses Adiabatic model +

Compression

Space

Expansion Space

Regenerator

Cooler

Heater

l

D k

r h

5

- Studying heat transfers in:

- The regenerator (porous medium),

- The expansion room (hot source),

- The compression room (cold source).

- Seeing the effect of porous medium on the

efficiency, the heating and cooling temperatures.

Optimization of the heat transfer inside the Stirling engine in

order to increase its global efficiency, by:

Objectives of the study:

6

7

Hot source Tch Cold source Tfr Q1 < 0 Q2 > 0 Driven

machine

W < 0

2

2 2 2

' '. .

' 'glob mca th cal

QW W W

Q W Q Q

The amount of heat

supplied by the

electrical heating

(electrical resistance)

The amount of

heat actually

received by the

air

The

indicated

work

Mechanical work

effectively

recovered on the

brake shaft

2 'Q U I dt 2Q W'W

cal th mca

Operation of Stirling engine

Operation of Stirling engine

First law of thermodynamics on a cycle 0QQWU 21

Second law of thermodynamics on a cycle 0ST

Q

T

QS c

2

2

1

1

Efficiency 1Q

W

1

Heating source S1

at T1

Cold source S2

at T2

Stirling

Machine

W < 0

Q1 > 0 Q2 < 0

8

Compression Space

Expansion Space

Crank-Rod System

Regenerator

Cooler

Heater

Heating system

The Gamma type Stirling engine

9

PDet.

TE-input.

TE-input

PComp.

Tcold

Water input

Water output

Regenerator

Heating system

Expanxion Space

Compression space

TR2

TR4 TR3

TR1

TR6

TR8 TR7

TR5

Thot

Porous media

8 thermocouples 4 in each side

TR1

TR2

TR3

TR4

TR5 TR6

TR7

TR8

Cold working fluid

Hot working fluid

Hot working fluid

Porous media (Rgnrateur)

Cold working fluid

Dissipation energy system

Oscillant plate

Transmission belt

Alternateur

Force transducer

crank angle transducer

10

p

4

jjpjpj

jC

TVT

x

T

Cxt

p

C

1

x

TV

t

T

.

.''

..

air

air

11

Composed of: - 20 curved pipes (internal diameter: 1cm)

- 20 tubes (length 0.50m each one)

- 3 thermocouples located: inside the heater, outside the heat

and in heating system.

The heater exchange

13

PExpansion

TW-entrance

TW-Exit

PComp.

TCold

Water input

Water exit

Regeneratorr

Heating

system

Expansion space

Compression space

TR2

TR4 TR3

TR1

TR6

TR8

TR7

TR5

THot

Matrix

(regenerator)

1st series of 4

thermocouples 2nd series of 4

thermocouples Regenerator

Series of 8

thermocouples

Pourous media

(regenerator)

TR5

TR6

TR7

TR8

TR1

TR2

TR3

Cold working fluid

Hot working fluid

TR4

Cold working fluid

Hot working fluid

The heater exchange

14

Two thermocouples are located at the input and the

output of cooling water circuit

Composed of 225 strips

(or fins) in the inner

cylinder

For increasing the heat

transfer exchange

between the working

fluid and water (cold

source)

II. Experimental

The cooler exchanger (cold source)

15

The regenerator (porous medium)

16

Porous media

8 thermocouples (4 in each side)

TR1

TR2

TR3

TR4

TR5

TR6

TR7

TR8

Cold working fluid

Hot working fluid Hot working fluid

Porous medium (Regenerator)

Cold working fluid

Materials with porosity of 90%

Proprieties \ Material Stainless Steel (304L) Copper Aluminium Monel 400

Density (kg.m-3) 7,850 8,920 2,700 8,800

Specific heat Cp (J.kg-1.K-1) 477 385 902 430

Thermal conductivity (W.m-1.K-1) 26 390 237 21.7

Melting point ( C) 1530 1084 660 1300

The properties of used regenerator materials for a temperature of 300 C are:

Copper

Aluminum Monel

Stainless Steel

p

4

j

jpjpj

jC

TVT

x

T

Cxt

p

C

1

x

TV

t

T

.

.''

..

4 matrices, with differents constituting materials, were used as Stirling engine regenerator:

- Stainless Steel matrix

- Copper matrix

- Aluminum matrix

- Monel matrix

Properties of the differents used matrices (regenerators)

Copper

Aluminum Monel

Stainless Steel

17

4 matrices, with differents constituting materials, were used as Stirling engine regenerator:

- Stainless Steel matrix

- Copper matrix

- Aluminum matrix

- Monel matrix

Properties of the differents used matrices (regenerators)

Materials with porosity of 90%

Proprieties \ Material Stainless Steel (304L) Copper Aluminium Monel 400

Density (kg.m-3) 7,850 8,920 2,700 8,800

Specific heat Cp (J.kg-1.K-1) 477 385 902 430

Thermal conductivity (W.m-1.K-1) 26 390 237 21.7

Thermal Diffusivity a (m/s) 6.94 . 10-6 1.14 10-4 9.73 10-5 5.73 10-6

The properties of used regenerator materials for a temperature of 300 C are:

Copper

Aluminum Monel

Stainless Steel

4

j

jjj

j

T..a'V'T

x

T.a

xt

pa

x

TV

t

T

18

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16136.08

136.1

136.12

136.14

136.16

136.18

136.2

136.22

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16243.95

244

244.05

244.1

244.15

244.2

244.25

244.3

Acquisition time [s]

TR1

TR4

TR1-4

mean TR1

mean TR4

Working fluid temperature evolution vs. acquisition time for a copper matrix

The temperature of

the working fluid in

the regenerator

increases in the

first half cycle until

a maximum value

(TR1 and TR4)

The temperature

decreases during the

second half cycle

until a temperature

which is close to that

of the cold source

The Stirling engine regenerator has two roles:

- Accumulating heat 1 and 4)

- Forming a thermal barrier between both heat and cold sources 1-4)

Temperature evolution in each kind of matrix regenerator)

Pourous media

(regenerator)

TR5

TR6

TR7

TR8

TR1

TR2

TR3

Cold working fluid

Hot working fluid

TR4

Cold working fluid

Hot working fluid

19

The regenerator formed of the material Monel 400 is the best heat accumulator.

The stainless steel represents the highest temperature gradient, and the Aluminum

the smallest temperature gradient.

Temperature evolution in each kind of matrix regenerator)

Working fluid gradient temperature between the thermocouples TR1 and TR4

Best Heat

accumulator

Highest

temperature

gradient

Working fluid gradient temperatures given by the thermocouples TR1 during one cycle

20

100

150

200

250

300

Bra

ke

po

we

r [W

]

TH

= 300C

TH

= 400C

TH

= 500C

CopperStainlessSteel

Aluminum Monel

The Aluminum

regenerator

presents the

worst brake

power

regardless the

heating

temperature

The Stainless steel

has the best brake

power regardless

the heating

temperature

The brake power increases with the heating temperature, but at different

levels of each kind of regenerator.

Influence of each type of matrix on the engine performance versus heating temperature

Brake power = 281 W TH = 500C

21

For all experimented regenerators, the brake power increases with the

initial charge pressure but at different levels.

100

150

200

250

300

350

Bra

ke

po

we

r [W

]

Pi = 3 bar

Pi = 5 bar

Pi = 8 bar

Stanless

Steel

Copper Aluminum Monel

The Aluminum

regenerator presents the

worst brake power

regardless the initial

charge pressure

The Stainless Steel

regenerator has the

best brake power

Influence of each type of matrix on the engine performance versus charge pressure

22

The regenerator thermal efficiency is calculated as the ratio of real heat

transferred through the regenerator by the ideal heat, which must be transferred

through the regenerator.

The copper presents the best regenerator thermal efficiency and the

Monel 400 presents the worst regenerator thermal efficiency.

Regenerator thermal efficiencies

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.160.25

0.3

0.35

0.4

0.45

0.5

0.55

0.6

0.65

Acquisition time fo one cycle [s]

Effic

ien

cie

s [%

]

Eff

Monel

EffCuivre

EffInox

Eff-Alum

Copper

Monel

Steel

Allum

23

Regenerator matrixes after about 15 hours of use

Copper Stainless Steel

Monel Aluminum

The copper oxidizes quickly because of the working fluid (air) which contains about 21%

of oxygen. This material oxidation changes the physical properties of the copper, and

then leads to bad heat exchanges inside the regenerator.

The stainless steel and the Aluminum, used as regenerators, have good

thermal efficiencies (about 44%).

These two materials do not present a problem of oxidation, but the use of

regenerator in Aluminum is limited by its melting temperature.

Regenerator thermal efficiencies

24

Four regenerator matrices were tested on a Gamma Stirling engine :

Stainless steel, Copper, Aluminum and Monel 400

- The Monel matrix presents the best thermal sponge, and the Stainless Steel

represents the best thermal barrier between hot and cold heat sources.

- The copper regenerator has the best thermal efficiency, but its oxidation

decreases extremely the brake power of the Stirling engine.

- Using an Experimental Design Approach (OCC or DOE), the impotant

parameters of the Stirling engine can be optimised to obtain a good

efficiency of this machine.

25