Figure 1 shows a photograph of a 10-inch diameter EverGen PowerStrap. The unit was designed for outdoor use, 120°C operation in a vertical orientation for an industrial exhaust pipe. Twelve identical TEG and heat sink assembly sections were spaced evenly around the perimeter of the strap base. Each thermoelectric generator module was held in compression between the heat sink and the strap base with two stainless steel bolts fitted with insulating phenolic washers. Thermal pads on both sides of the TEG were used to improve thermal resistance at the hot (strap) and cold (heat sink) interfaces. The clamping strap base was divided into three identical sections that form a compression fit around the pipe when bolted together. A series of lab tests were conducted with this design that mimicked varying operating conditions throughout the year. The test assembly was made from a section of 10” diameter steel pipe that was capped at one end and filled with oil. Submersible heaters, attached to an electronic temperature controller, were used to control test assembly wall temperature. The PowerStrap was clamped to the exterior of the test assembly, with non-setting thermal mastic applied between the pipe wall and the strap base to aid in heat transfer. During testing, ambient temperature around the test assembly was altered to reflect seasonal changes.
Both natural convection and forced convection up to 6.5 mph were studied. Omega OM-420 data acquisition equipment was used to collect temperature, voltage and current measurements during testing. Figure 4 is an expanded view sketch that depicts thermocouple placement on the test assembly. Readings were collected and recorded in two second intervals.
Figure 4: Expanded view sketch of the thermocouple placement on the test assembly.
The results of this testing, compared against model predictions, are shown in Figure 5
for two different pipe temperatures covering a wide range in ambient conditions. From the data, it is obvious that the EverGen PowerStrap performance is maximized when ambient temperatures are the coldest. This is to be expected since thermoelectric efficiency is greater for larger temperature differentials. In real world operation, this means that the PowerStrap performance will be maximized during the colder months of the year. Such performance makes this product a natural complement to solar cells, which usually perform poorly during the winter months. Another key point is that there is significant performance increase, by as much as 40 percent, when typical outdoor wind conditions are accounted for. For applications requiring higher power levels, multiple units can be employed. The data also shows that the model predictions close well with experimental data. By expanding the model to include different pipe temperatures and diameters, performance under different operating scenarios can be predicted.
Figure 5: The EverGen PowerStrap test results.
About the author
Josh Moczygemba is Power Generation Product Engineering Manager at Marlow Industries - www.marlow.com
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Courtesy of EETimes Europe
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