Analysis of the effects of different types of loads on a Thermo-Acoustic Engine

Chitta Saha, Paul Riley and Mark Johnson

Presentation Outline

- Construction of the tested Thermo-acoustic Engine (TAE)

- Design issues of the low cost Alternator

- Different electrical loads with the TAE

- Power analysis for different load conditions

- Measured results

- Conclusions

Propane Burner TAE

• TAE consists of - Stainless steel bulge (HHX) - 30 layers stainless steel wire mesh regenerator ( 95 µm, 250 µm) - Car radiator (AHX)

• 5.5 kW propane burner, 4 inch pipe and B & C 6PS38 speaker.

• Each engine could be connected in series/parallel or independently.

Radiator

Hot buffer

tube

BulgeInsulation

Requirements of LA for SCORE project

• Alternator design : low cost ( £4/unit ) high efficiency and resonant frequency operation.

• Ultimate goals - Supply 12 V lead acid battery. - Generate 150 W dc power

• Small magnet constrains : (BL)2/Rc

• Meet the output power and cost : frequency & displacement

Limitations of Commercial low cost loudspeakers

• High suspension loss and limited mechanical stability.

• Operate over a large frequency range, LA needs to operate a fixed frequency.

• Lower efficiency and larger weight.

Cone

Voice Coil

Yoke pole pieces

Front suspension

Rear suspension

Magnet

Vent holes

Schematic of a loudspeaker type alternator

SCORE Alternator : Halbach array

• Alternator can be constructed without back iron material, no yoke piece is required.

• Smaller pumping loss due to large hole.

• High Efficiency and high air-gap reversal flux density.

Load power and efficiency with battery circuit for 1 mm gap between coils

0

130

260

390

520

650

0 0.2 0.4 0.6 0.8 1

Ratio : Vbattery/Vp

Av

era

ge

loa

d p

ow

er

(W)

0

20

40

60

80

100E

lec

tric

al e

ffic

ien

cy

(%

)

Power : 9 mm coilPower: 7.5 mm coilPower : 6.5 mm coilPower : 5 mm coilPower : 2 mm coilEfficiency : 9 mm coil

Performance of Alternator with Battery

M

L

N

W

Double coil case 2 mm height 10 coils case

p

L

p

L

p

L

p

L

c

Leout V

V

V

V

V

V

V

V

R

VP 1

222

, cos12

• Battery with rectifier circuit :

• Electrical efficiency for dual coils : 80 % for 125 W when Vbattery/Vp = 0.73, 76 % for 150 W when Vbattery/Vp=0.7

• Max. power : Vbattery/Vp = 0.39

Tested prototype : Halbach array

0

1

2

3

4

5

6

7

8

0 1 2 3

Displacement (mm)

Op

en C

ircu

it p

eak

volt

age

(V)

Measured

Simulated

0

15

30

45

60

0 7 14 21 28 35Load resistance (ohm)

Eff

icie

ncy

(%

)

Acoustic-electrical efficiency

Calculated acoustic-electrical efficiency

• Measured and simulated voltages agree well.

• Discrepancy between measured and calculated efficiency appears due to cracking in the suspension.

Alternator power analysis

RL

Rc

+

Sin

D1

D4

D3

D2

+

Sin

Rc

+-C RL

rmsrms

T

Lc

pps IVdt

RR

tVtV

TP

)

sin)(sin(

2 2/

0

L

LLrmsL R

VRIP

2

22

- Resistive load, source power and load power :

-Battery load can be considered as a RC load when C becomes very large. - Source power and load power for battery rectifier circuit

avgbatterydcLL IVIVP **

))](sin2

1(2

[1 1

22

p

LL

Lp

cavg V

VV

VV

RI

frmsrms

LpL

p

Lp

cs

IV

VVV

V

VV

RP

])(sin1

2

1([

1 2212

Measured results

• Pressure and temperature has been measured using NI DAQ module.• Voltage and power has been measured using PPA2530.• Electrical power is almost proportionally varied with the square pressure.

Loads effect on a Thermo-Acoustic Engine

Load Condition TAE parameters Alternator

HHX (oC) HHX- AHX (oC) Pressure (mBar)

Ac voltage (Vrms)

Ac power (W)

Total power (VA)

Idc (A)(Into 12V lead acid battery)

Load power* (W)

12 V battery + capacitor +

rectifier

413 347 38 13.12 8.96 11.35 0.43 5.59

Capacitor + 30 Ω resistance +

rectifier

411 346 39 13.57 8.89 11.36 0.49 6.85

20 Ω resistance 403 343 37 13.2 8.65 8.65 - 8.65

• Bridge rectifier required a fixed load resistance to generate the same amount of real power with battery.

• No effect on pressure and temperature when the real power is constant.

• Load power is less than generated power due to losses in the rectifier.

Conclusions

• The construction of dual loop 30 layers stainless steel regenerator SCORE TAE is introduced.

• Design issues of SCORE LA and advantages of double Halbach array are discussed.

• Voltage/power measurement issues of the alternator with linear and non-linear load with the full wave rectifier circuit are discussed.

• Variations of the measured pressure and temperature of the engine as well as electrical power are shown.

• Measured results show, no effect on the pressure and temperature with the changing the load condition.

Acknowledgment

The Score project www.score.uk.com is funded by EPSRC, the UK Engineering and Physical Research Council.

Specification of the regeneratorWire diameter 95 umWire spacing 250 umVolumetric Porosity: σ 0.783Solid fraction: (1-σ) 0.217Hydraulic radius 86 umRegenerator width 155 mmRegenerator length 180 mmRegenerator thickness 9 mm

𝑄𝑡ℎ𝑒𝑟𝑚𝑎𝑙 ℎ𝑒𝑎𝑡 = 𝑄𝑏𝑢𝑟𝑛𝑒𝑟 − 𝑄𝑝𝑎𝑛1+𝑝𝑎𝑛2 − 𝑄𝑐ℎ𝑖𝑚𝑛𝑒𝑦 − 𝑄ℎ𝑜𝑢𝑠𝑖𝑛𝑔 𝑊𝑎𝑐𝑜𝑢𝑠𝑡𝑖𝑐 (ℎ𝑒𝑎𝑡) = 𝑄𝑡ℎ𝑒𝑟𝑚𝑎𝑙 ℎ𝑒𝑎𝑡 − 𝑄𝑟𝑒𝑗𝑒𝑐𝑡𝑒𝑑 ℎ𝑒𝑎𝑡 − 𝑄𝑇𝐵𝑇

𝜂𝑆𝑡𝑜𝑣𝑒 = 𝑄𝑝𝑎𝑛 1+𝑝𝑎𝑛 2𝑄𝑏𝑢𝑟𝑛𝑒𝑟 × 100%

𝜂𝑇𝐴𝐸(𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐𝑎𝑙 ) = 𝑊𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐𝑖𝑡𝑦𝑊𝑎𝑐𝑜𝑢𝑠𝑡𝑖𝑐 (ℎ𝑒𝑎𝑡 ) × 100%

𝜂𝑇𝐴𝐸(ℎ𝑒𝑎𝑡) = 𝑊𝑎𝑐𝑜𝑢𝑠𝑡𝑖𝑐 (ℎ𝑒𝑎𝑡 ) 𝑄𝑡ℎ𝑒𝑟𝑚𝑎𝑙 ℎ𝑒𝑎𝑡 × 100%

General power/losses summary of the system

Burner (net power) 4657.71 W Rejected Heat1807.39

WEngine housing losses

519.24 W Acoustic power (Heat) 460.65 W

Heat to the Pans 1170.69 W Electricity power 15 WChimney losses 430.00 W Stove efficiency 25.13%

Heat to the TAE 2537.78 WTAE Efficiency (Heat to Acoustic power(Heat))

18.15%

TBT losses 269.74 WTAE and Generator Efficiency (Acoustic power (Heat) to Electrical power)

3.2%