Publications

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We built and tested a subterranean thermoelectric power source that converts diurnal heat flow through the upper soil layer into electricity. This paper describes the operation, design, and performance of the device. Key features of the power source include the use of bismuth-telluride thermopiles optimized for small ΔT and aerogel insulation to minimize thermal losses. The device weighs 0.24 kg and was designed with a flat form factor measuring 12 × 12 × 1.7 cm to facilitate modularity, packing, and assembly into larger arrays. One full year of field testing was performed between June 2009 and May 2010 in Albuquerque, New Mexico where the device generated an average power output of 1.1 mW. The season with the highest performance was spring (March–May) while the season of lowest performance was winter (November–January). During May 2010, the device generated an average power of 1.5 mW and a peak power of 9.8 mW at 9.3 V. Ten years of continuous operation at 1.1 mW would yield an energy density and specific energy of 1384 W h/L and 1430 W h/kg respectively, which is competitive with chemical batteries and is orders of magnitude greater than published subterranean and ambient thermoelectric harvesters. Numerical simulations show that performance is sensitive to the thermal properties of the soil and environmental conditions. This class of energy harvester may provide an option for supplemental power, or possibly primary power, for low power remote sensing applications.

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A straightforward and accurate method for measuring the laser beam diameter at focus is desired in order to develop fundamental understanding and for routine process control. These measurements are useful for laser materials processing by assuring laser performance consistency at the workpiece. By employing multiple-shot exposures on Kapton™ film, an unambiguous and precise measurement of the focused Nd:YAG laser beam diameter for spot welding lasers was obtained. A comparison of focused beam measurements produced with the Prometec laserscope and an ISO variable aperture method found that these two methods, which both measure the 86% energy contour, do closely agree. In contrast, Kapton film was found to measure the 99% beam energy contour and to diverge from measurements made with the other two methods. The divergence between Kapton and the other two methods was shown to be due to changes in the laser irradiance distribution that do not affect the location of the 99% energy contour. Since the 86% beam diameter was seen to not always be representative of the true beam diameter, the 99% Kapton film diameter can provide a more representative measurement of the focused laser for in-situ process control.

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The load relaxation behavior of small Elgiloy helical extension springs has been evaluated by a combined experimental and modeling approach. Isothermal, continuous heating, and interrupted heating relaxation tests of a specific spring design were conducted. Spring constants also were measured and compared with predictions using common spring formulas. For the constant heating rate relaxation tests, it was found that the springs retained their strength to higher temperatures at higher heating rates. A model, which describes the relaxation behavior, was developed and calibrated with the isothermal load relaxation tests. The model incorporates both time-independent deformation mechanisms, such as thermal expansion and shear modulus changes, as well as time-dependent mechanisms such as primary and steady state creep. The model was shown to accurately predict the load relaxation behavior for the continuous heating tests, as well as for a complex stepwise heating thermal cycle. The model can be used to determine the relaxation behavior for any arbitrary thermal cycle. An extension of the model to other spring designs is discussed.

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