Low-Cost Thermoacoustic Cogenerator for Use in Bio-Mass Burning Cook Stoves

Open Access
Author:
Montgomery, Paul James
Graduate Program:
Mechanical Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
None
Committee Members:
  • Steven Lurie Garrett, Thesis Advisor
  • Horacio Perez Blanco, Thesis Advisor
Keywords:
  • thermoacoustics
  • cook stoves
  • acoustics
  • soot
  • global warming
Abstract:
Indoor smoke produced while cooking using bio-mass fuel causes 1.6 million premature deaths every year; one million children die of pneumonia and 600,000 woman die prematurely of chronic obstructive pulmonary diseases like bronchitis and emphysema. This corresponds to 38 million disability-adjusted lost years in addition to needless pain and suffering from increased eye and respiratory illnesses. Worldwide, black carbon particulate matter (soot) is second only to carbon dioxide as an anthropogenic contributor to global warming in the atmosphere (direct radiative forcing) and also accelerates glacial and snowpack loss due to the melting caused by reduced albedo. In South-East Asia, soot can make a more significant contribution to global warming than carbon dioxide. With 2.53 billion humans using biomass as their primary cooking fuel, biomass-fired cook stoves account for 75% of that soot. 1.6 billion of these cook stove users live in villages that have no electricity. An improved cook stove addresses both of these problems, but because the atmospheric lifetimes of black carbon particles is only a week or two, instead of 100 years for carbon dioxide (CO2), 114 years for nitrous oxide (NO2), and 12 years for methane (CH4), atmospheric soot reduction is the only remediation activity that can produce immediate benefits. Recent research has shown that fan-enhanced convection in the biomass combustion chamber makes the most significant reduction in the products of incomplete combustion that degrades indoor air quality and also is most successful in reduction of soot production. This prototype will utilize a thermoacoustic engine to extract a small amount of heat, generate a standing sound wave thermoacoustically, and use that sound wave to drive a loudspeaker-like linear alternator to generate electricity. Since the fan typically consumes only one watt and a simple thermoacoustic generator could produce 5-10 watts of electrical power, the excess electrical power can be used to provide lighting using a high-efficiency LED lamp, charge a mobile phone or other small electronic device, or run a small appliance. Thermoacoustics seems like a potential candidate for low-cost electrical power generation because it is so simple and requires no moving parts other than the loudspeaker which must reciprocate to generate electrical power. The design and fabrication of a low-cost thermoacoustic power generator presented many challenges. The prototype described herein used air at atmospheric pressure as the thermodynamic working gas. This choice allowed the hot-duct of the resonator to be constructed from folded sheet steel producing a rectangular cross-section instead of the circular cross-section typical of vessels designed to contain gases at higher pressures. The rectangular cross-section meant that the ceramic “stack” could also have a large-aspect rectangular cross-section (i.e., very wide, but not too tall) so that the heat leaving the engine would not have to travel very far to reach the ambient temperature reservoir. This made the ambient heat exchanger simple but efficient. Cooling fins sold to cool computer chips were used to remove heat from the stack. The exhaust heat collected by the fins was measured by determining the rate at which ice melted in the “pool” above the ambient temperature portion of the resonator. By coating the hot-end of the ceramic stack with carbon black from a candle, about 20 watts of heat was deposited on the stack directly by electromagnetic radiation making a hot heat exchanger unnecessary. Experiments demonstrated that under similar conditions, the amplitude of the thermoacoustically-generated pressure increased for shorter stacks with larger ratios of pore size to thermal penetration depth in a resonator with smaller volume corresponding to larger gas stiffness. A loudspeaker was sealed to the ambient end of the resonator and utilized its moving mass, in conjunction with the gas stiffness in the duct, to create a Helmholtz-like resonator that was more compact than a standing-wave resonator operating at the same frequency. Although the overall efficiency of thermal to electrical power conversion was fairly poor in this prototype, and only 18-26 mW of useful electrical power was extracted. This low output level was due, in part, to the fairly large mechanical resistance of the commercial loudspeakers. This effort has proven that several of our innovations were workable and provides guidance for design of a second-generation device.