The Hot-Line Solar Collector

Iowa entrepreneur invents a fixed-position, concentrating solar collector. Learn how it works and how to build your own. Including photos, diagrams and detailed instructions.
By the MOTHER EARTH NEWS editors
May/June 1976
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Hot-Line modules furnish most of the heat used by this Iowa firm.

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Like many backyard inventors, Dan Lightfoot of Sigourney, Iowa has been interested in alternate energy concepts for some time — about 22 years, as a matter of fact. Unlike most of his peers, however, Dan has managed to make a major breakthrough in his field — one which even has the "experts" shaking their heads in disbelief.

That breakthrough is the Hot-Line solar collector you see pictured in the Image Gallery. At first glance, the Hot-Line module looks just about like a conventional flat-plate collector. What makes Lightfoot's panel highly unconventional is that it [1] contains a specially curved reflector which acts to concentrate incoming sunlight on a wedge-shaped absorption tube, [2] operates with an efficiency far surpassing that of any "normal" flat-plate solar panel, and [3] actually "tracks" the sun through a 50 degree vertical arc — and through 150 degrees in the east/west plane — without moving!

Impossible? That's what a University of Iowa physics professor said when Dan Lightfoot first explained the design to him. Rest assured, though, the device does work, and rather well, at that.

So well, in fact, that Lightfoot (after selling exclusive manufacturing rights on his invention to the Iowa City-based NRG Corporation) has recently been able to quit his job with a mining firm and devote full time to energy research. Lightfoot now heads up an outfit called Aerco, which is short for Alternate Energy Resources Company.

Dan Lightfoot came upon the idea for the Hot-Line collector quite by accident a decade ago. It seems Dan had been observing a sheet of aluminum that was resting up against his garage wall and noticed how the sun's reflection from that curved sheet formed a bright spot on an adjoining wall. Moreover, he noticed that the bright spot stayed in roughly the same place throughout the day, despite the sun's constant movement.

This got Dan to thinking, and to experimenting. With the aid of a small sheet of aluminum, a few scraps of wood and a handful of bolts and clamps, Lightfoot found (by trial and error) that he could curve the metal in such a way that it would focus light in a line — a line that, furthermore, moved only a small distance in or out from the metal as the jury-rigged reflector was tilted through various angles to the sun.

At this point, Dan knew that if he could just bend a long sheet of reflective material to the same curvature, lay a channel along the focal plane of the reflector thus created, and run air or water through that channel, he'd have what no one had developed before: a fixed-position, concentrating solar collector. (Focusing collectors are nothing new, of course, but they all have one drawback: in order to work, the reflector must face squarely into the sun at all times. This usually calls, in turn, for a costly and complex motorized gimbal mounting, to allow tracking of the sun. In contrast, Lightfoot's collector can focus light all day long while remaining stationary — a "major breakthrough" indeed!)

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.)

Thus, it's easy to believe Dan Lightfoot when he confidently states that just one of his 16-square-foot Hot-Line panels is sufficient to provide total daytime heating for one room in an average house. "Some rooms heat up more than others," Lightfoot explains, "depending on the degree of insulation, volume of air to be heated, and so on. However, you might like to know that one fellow in Kaloma, Iowa heats a room with a single Hot-Line module, and that room doesn't have any insulation or even any wallboard! Still, the 140-degree air coming out of that collector warms the place right up."

So far, Aerco and NRG have installed a total of 18 Hot-line units in Iowa, all of them air-heating (as opposed to water-heating) models. That is, in each of these installations a blower is used to force room air through the collector's absorption tube and straight back again into the room, without "tapping into" a heat storage reservoir at any point. (A water system employing heat storage is, however, under development at present and will be on the market soon.)

How does Dan Lightfoot like spending all of his time (and most of his cash reserves) on energy research, after giving decades of his adult life to a career in the coal-mining industry? "I'm happier doing what I'm doing now — trying to wake people up to the possibilities of alternative energy sources — than I've ever been," he proclaims. "I wouldn't trade the satisfaction I'm receiving now for anything in the world!"

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Post a comment below.


Mark Michaels
12/24/2008 3:39:43 PM
Great idea! Where can I get these materials? I have a great idea of how to apply this. thanks.

Peter Cross
12/3/2008 9:24:03 PM
I have worked on a simpler very inexpensive design for a parabolic trough, and its production approach could be used for the reflector in this one. (I see even molding the Styrofoam as a production expense one should work around if possible--that much foam is not needed--and I am looking for a durable reflecting medium more along the line of metalized polyester--although it might want to be in strips installed with a narrow central adhesive bead lengthwise, to prevent thermal distortions--or stretched between small springs lengthwise.) Thus far I've developed models of simple molds for the basic structure of a parabolic trough, basically out of cardboard, Styrofoam and light drywall compound, and then bent / glued two skins of some kind of paper--manila folder stock, etc.--over this mold with a layer of the CORRUGATED style of paper honeycomb in between. This type of honeycomb will mold around a curve, whereas the diamond-shape or "Hex-cell" honeycomb will not. Once you have a good mold, the honeycomb material is cheap, one can use any quality of skin one wants, from very durable fiberglass (already in thin sheet or laid up within the process) on down, and anyone in a neighborhood / community can ante up for the honeycomb, and pick whatever quality skin they can afford and figure out a way to protect for as long as they want to protect it for, while jointly working on alternative reflective surfaces, pointing mechanisms, and so on. Some long, narrow format of photocells has to be found if you are into direct electricity, and you would want to heat water via a tubular heatsink running behind them. This approach to a collector trough can be done at any level of precision (pretty crude is good for photo electrics, otherwise you risk burning them up!), and can be scaled up quite simply to, potentially, some huge, lightweight- cement- skinned units that are rotated on a floating lid which sits on top of a large storage tank. In a dry area that&

11/19/2007 10:48:59 PM
Hmmm, someone comes up with a way to make solar energy more efficient, and they disappear... Its the government Mulder. Trust no one.

11/6/2007 10:58:17 AM
I too have had no luck finding more information on this however if if I should I will most certainly post it here and hope you will do the same. Here's to us finding some answers.

9/26/2007 10:04:50 AM
Hi Chatur. I'm in the same boat as well. I've tried to find more information on this design with no luck. Maybe you could do a patent search to find who owns the rights to the design. The NRG corp doesn't seem to be around any longer either, nor does Dan Lightfoot's Aerco company. I tried using a people finder to locate any 'D' Lightfoot in the general area of Sigourney, Iowa but nothing looked promising. I even tried looking up the "Gordon Russell Enterprises" pictured in one of the article's photos. Another approach might be to find out who authored the article and talk to him/her. If you find anything let me know. Thanks, Todd B.

6/1/2007 6:56:41 AM
The article on Hotline Solar collector is very interesting. I have been trying to get more information on this type of collector but in vain. There is no trace of either NRG Corp. or Aerco, Iowa, all net searches lead only to this very article. What I am keen to know is what happened to this design, why has it not been commercially successful. If the design is truly very efficient it should have proliferated as space constraints are a really the bane of most Solar energy based energy solutions. The most imteresting feature of this design was its ability to track the sun while itself being stationery. Can you throw some light on this topic.I know the design is very old but all flat plate solar collectors still sport the same "old" designs except using modern manufacturing process. Eagerly awaiting your reply. Thank you. Chatur Das

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