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 Bulge Insulation

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 /R c Meet the output power and cost : frequency & d isplacement

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.

Performance of Alternator with Battery M L N W Double coil case 2 mm height 10 coils case Battery with rectifier circuit : Electrical efficiency for dual coils : 80 % for 125 W when V battery /V p = 0.73, 76 % for 150 W when V battery /V p =0.7 Max. power : V battery /V p = 0.39

Tested prototype : Halbach array Measured and simulated voltages agree well. Discrepancy between measured and calculated efficiency appears due to cracking in the suspension.

Alternator power analysis - 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

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 ConditionTAE parametersAlternator HHX ( o C)HHX- AHX ( o C) Pressure (mBar) Ac voltage (V rms ) Ac power (W) Total power (VA) Idc (A) (Into 12V lead acid battery) Load power* (W) 12 V battery + capacitor + rectifier Capacitor + 30 resistance + rectifier resistance 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 is funded by EPSRC, the UK Engineering and Physical Research Council.

Specification of the regenerator Wire diameter95 um Wire spacing250 um Volumetric Porosity: σ0.783 Solid fraction: (1-σ)0.217 Hydraulic radius86 um Regenerator width155 mm Regenerator length180 mm Regenerator thickness9 mm

General power/losses summary of the system Burner (net power) WRejected Heat W Engine housing losses WAcoustic power (Heat) W Heat to the Pans WElectricity power15 W Chimney losses WStove efficiency25.13% Heat to the TAE W TAE Efficiency (Heat to Acoustic power(Heat)) 18.15% TBT losses W TAE and Generator Efficiency (Acoustic power (Heat) to Electrical power) 3.2%