|U.S. at the Ground Level in Solar Water Heating
In 2008 (the last year for which data is available) U.S. solar-thermal collector manufacturers sold 30,000 units for residential hot-water heating, according to Alanod -Westlake Metals’ vice president Frank Lee. Typically at 64 sq. ft. (or 6 sq. m) per installation, that equates to about 180,000 sq. m.
Worldwide, solar installations for hot-water heating totaled 37 million sq. m., “so we’re at the ground level here in the United States,” Lee says. “The solar-thermal process is five-times more efficient than photovoltaic production. There’s a lot of marketing to be done, but if the incentives remain in place and productivity at the manufacturers continues to improve to bring costs down, industry growth should be significant for a long time to come.”
|The laser pulses at 10 Hz with beam-on time of 1.2-1.8 msec., creating 1-mm-long stitch welds on 2-mm centers
to join strip to tube.
Tube runs through a straightener as the strip feeds over the top of the tube and through a punching die that punches a rectangularpilot hole. Next, as the tube and strip come together at the laser-welding booth, guide rolls center the copper tube underneath the strip; downward roll pressure keeps a tight weld joint at the tangent point. Laser welding occurs at both sides of the tube simultaneously.
The welded material then feeds through a roll-profiler that imparts a gentle corrugation pattern to the strip to add rigidity; a protective film on the top of the strip maintains the critical surface integrity during processing. Finally, a cutoff die, signaled by presence sensing of the punched pilot hole, cuts the absorber fins to length, anywhere from 4 to 10 ft.
Laser Stitch-Welding Sheet to Tube
At the heart of the system is a 500-W pulsed-Nd:Yag solid-state laser (an FLS N-Series model from Lasag Industrial Lasers, Buffalo Grove, IL) outfitted with a beam splitter to deliver a beam to each side of the weld joint. Fiberoptic cables deliver the beams to the welding heads fixtured on each side of and underneath the feeding strip and tube.
Weld-joint position tolerance is ±3 mm. Cameras installed on each side help line operators set up the line and ensure good fitup. During production, the laser pulses to create 1-mm-long stitch welds on 2-mm centers.
Each hour or so—at the beginning and end of each shift and every time it changes coils—operators cut off samples from the production line to test weld strength in a tensile-test setup.
“The industry standard for weld strength is 100 N/cm,” says Lee. “We’re getting 180 to 200 N/cm or more.” MF
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