The Hot-Line Solar Collector

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So, in his spare time, Dan built models and refined them over a period of years until he felt he had optimized all aspects of the unorthodox solar panel's design. The end result was the Hot-Line solar collector.

Measuring 26 1/2-by-96-by-10 inches, the production version of the Hot-Line module has a surface area of 16.1 square feet. Nestled in the unit's sturdy aluminum frame or chassis (see Fig. 1 above) is a pre-shaped block of styrofoam which serves to [1] insulate the structure and [2] hold the reflector — a sheet of mirror-finish aluminum — in the proper curvature. Laid along the deepest part of the reflector's curve is the triangular-section aluminum absorption tube. (The wedge shape is necessary in order to accommodate the lengthening and shortening of focal lengths as light enters the panel at different angles.) Finally, covering the surface of the panel is a sheet of Kalwall Sun-Lite fiberglass glazing, which Lightfoot claims "deflects very little incoming sunlight, even at oblique angles."

How does the Hot-Line collector work? As you can see in Fig. 2 (see Image Gallery), when sunlight enters the panel at a nearly perpendicular angle (such as would occur around sunrise) the reflector focuses incoming rays of light either on the outermost edge of the absorption tube or the part closest to the reflector itself, depending on whether the incident light enters the collector's upper portion or lower portion. As Ole Sol rises higher in the sky, the focal line, or "hot line," hitting the top surface of the absorption wedge moves closer to the reflector — while the hot line on the aluminum extrusion's underside moves away from the reflector sheeting. Not until the sun is more than 60 degrees above the horizon, or better than 75 degrees displaced to the east or west, does the incoming light fail to focus or become "blanked out" by the chassis itself.

What this means in the case of a vertically mounted, south-facing unit is: the Hot-Line collector [A] starts working as soon as the morning sun peeks over the horizon, [B] continues to concentrate light energy all day until the sun rises more than 60 degrees (that is, a good deal more than its maximum winter height) above the horizon, and [C] does so at continuously high efficiency. (At the moment, Dan Lightfoot is not advertising any particular set of numbers when it comes to the efficiency of his collector, but preliminary tests by others in the field point to heat-recovery percentages in the very high 80s or low 90s — which is about double the efficiency of most commercial flat-plate collectors.)

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