Transcript U21_energy conference poster_CS_Nott - Score Stove

Investigation of thermo-acoustically Driven
Linear Alternator
C. M. Johnson, P. H. Riley and C. R. Saha
Introduction
Thermo-acoustic impedance ;
Equivalent input impedance;
Thermo-acoustic engine converts thermal energy into sound energy
by transferring heat between the working media (gas) and a porous
solid structure stack. This sound energy could be used to drive a linear
alternator to generate electricity and also power a thermo-acoustic
refrigerator.
• This paper presents a standing wave thermo-acoustic prototype which
has been built and tested with the linear alternator.
• A simplified theoretical model of a linear alternator driven by a
standing wave thermo-acoustic engine is introduced.
• The model is validated against experimental results obtained from a
prototype standing wave thermo-acoustic engine.
Thermo-acoustic linear Alternator
2
( Bl )
Z in  Z e  RL 
Z m  ZTA
Where,
CTA
1

V
2 2 duct
0c Sc
Fc
1
Z TA 

1
uc
jCTA 
RTA
2
 0cS c
RTA  
VR
Vduct is the total volume of the thermo-acoustic duct, Sc is the area of generator, VR
is the volume of the regenerator section and α is the propagation constant
Mechanical Quality factor ;
m0
Qms 
RTA
Rm 
2 2
1  0 
Where,
  RTACTA
The device consists of five basic elements such as a regenerator
(stack), hot (HHX) and cold heat exchanger (AHX) facing both ends of
the stack, stove fitted on the top of the hot tube and the alpine SPR-17S Model verification
electromagnetic loudspeaker acts as a linear alternator connected at the • The impedance response of the alternator was measured with and without heat to
back end of the AHX. An air cooled car radiator was used for AHX and
understand the thermo-acoustic strength.
an LPG burner was used for heat input into the engine.
• Thermo-acoustic effect increases the input impedance and shifts the resonant
frequency downwards.
269 mm
• The measured mechanical quality factor (Qms) for hot and cold cases are 4 and 3.5 .
20 40
30 62
117
• The measured results agrees well with the theoretical model.
175 mm
Conclusions
Stack
30
Circular cross section
Electrical circuit model of the thermo-acoustic
electromagnetic generator
Where, B is the constant flux density in the coil, l is the effective length of
the coil, u is the relative velocity between magnet and coil.
I
R0
RL
F/Bl
L0
Blu
u
m
20
Cold phase angle
20
Hot phase angle
15
0
10
-20
5
-40
0
-60
40
60
80
100
120
140
160
Frequency (Hz)
Measured hot and cold impedance response of Alternator with duct
30
Calculated [Z]
Measured [Z]
Calculated angle
Measured angle
25
20
15
60
40
20
0
10
-20
5
-40
0
20
Rm 1/k
40
55
90
125
-60
160
30
Calculated [Z]
Measured [Z]
Calculated angle
Measured angle
25
60
40
20
20
15
0
10
-20
5
-40
0
-60
20
50
80
110
Frequency (Hz)
Angle (degrees)
Bli  u( Z m  ZTA )
Hot impedance
20
Input impedance
(ohms)
• The thermo-acoustic generator consists of electrical, mechanical and
acoustic components. It is easier to put the acoustic and mechanical
components into a single electrical circuits using lumped electrical
components.
• Electromagnetic Linear Alternator can be represented as coil internal
resistance (R0), coil inductance (L0) associated with source voltage (V)
and the load resistance (RL).
• Acoustic components can be represented by an equivalent impedance
(ZTA ) and force source (FTA ) and mechanical components can be
represented by a second order mass (m), damper (Rm) and spring
constant (1/k) model.
Force on voice coil;
Generated Voltage;
25
Phase angle (degrees)
Cold impedance
Tested prototype of standing wave thermo-acoustic
electromagnetic generator
Blu  V  I ( Z e  RL )
60
Input impedance (ohms)
Square cross section
Linear alternator
Angle (degrees)
AHX
Input impedance (ohms)
Bounce HHX
volume
• A simple theoretical model of thermo-acoustically driven linear alternator has been
developed and verified with real device.
• Measured results shows that significant mechanical loss present in the system which
prevent the self-oscillation of the system.
140
Frequency (Hz)
BlI
ZTA
V
Equivalent electrical circuit of the thermo-acoustic
electromagnetic generator
FTA
Measured and calculated results
for cold Case
Measured and calculated results for
hot Case
Acknowledgement
The Score project www.score.uk.com is funded by EPSRC, the UK Engineering and
Physical Research Council. Thanks to the Score partners, Universities of Manchester,
QMUL, City London and the charity Practical Action.