canivan's blog

Low Cost Solar Thermal Roof

Harry E Thompson, a North Carolina engineer invented, a low cost solar heating system during the latter part of the 20th Century. This system allows water to flow freely from the top of a corrugated steel roof into a gutter that channels solar heated water into a large, stone covered storage tank in the basement. His invention, as you may have guessed is known as the “Trickle Down Solar Roof”. Many engineers were skeptical about the concept since they expected the heat loss through the glazing material would be unacceptable, but Harry’s total solar thermal roof has been successfully applied to numerous dwellings all over the world. As a matter of fact Trickle Down Solar Roofs are still heating a number of homes along the US/Canadian border.
Since the trickle down solar roof takes advantage of the entire surface area of a roof the cost per square foot of collector surface has been greatly reduced, but a do-it-yourself project of this magnitude requires special skills and the glazing supports that are not commonly available. Another disadvantage of the Harry Thompson system has to do with the difficulty of installing and troubleshooting. A modified, modular version of Harry’s original design has been specially created for DIYers. It’s called Modified Trickle Down solar heating. MTDs are made from commonly available parts and the mounting supports are integrated into the design to facilitate installation. A steep pitched roof is not a good place to build a collector array, but a garage workshop can provide an ideal space for MTD construction.
I began developing the basic MTD concept in 2007 with the help of Richard Heiliger. Our goal was to make Harry’s design modular, and also improve the heat collection efficiency by using an inner film to prevent moisture on the TDM from condensing on the glazing. I have estimated that the heat collection efficiency is improved from 30% to 50% by the addition of an inner film. Richard and I have tried a number of inner films and we both now agree that Mylar is more suitable than polypropylene even though a thin sheet of polypropylene clings better to the sheet of polyester felt that carries the water. If the temperature of polypropylene is kept under 180* F it will last a long time but it will degrade at higher temperatures. Even though 3 mill Mylar is 10x more expensive than polypropylene it is worth the investment. The problem in past has been that Rolls of Mylar are 48” wide and the ideal width of an inner film is 30” so the waste of materials was intolerable. Just recently I’ve discovered that two sheets of overlapping 30”x48” work better than one 30”x96”sheet. This has to do with the way plastics expand and contract as temperatures change. The thing that’s truly magical about this latest MTD improvement has to do with the rubber bumpers connected to the cross supports that hold the overlapping Mylar in place. The bumpers hold the Mylar in place without restricting the flow of water through the polyester felt and this reduces the expansion bubbles as well as supply additional support for the Suntuf glazing.

These concept, although simple, are difficult to explain without the help of an MTD Steel Stud Starter Kit. All the materials you’ll need to make the parts for the collector are commonly available but you’re first collector assembly will be difficult if not impossible without a kit. Don’t waste your time repeating the same mistakes Richard and I made when we first began developing this idea. It may be fun reinventing the wheel but it’s more fun putting together a practical project in a reasonable period of time.

MTD Improvement

What you’re looking at is a piece of heater hose with a protruding screw. Perhaps not the most earth shattering piece of hardware that you’ve ever seen but it does solve a lot of technical problems associated with previous MTD Steel Stud Slot collector designs. Since 2007 I have been modifying the Modified Trickle Down collector to keep the cost down and the quality up. Both Richard Heiliger and I share the same basic design concepts, but the details of our designs vary in the way we construct the TDM or trickle down mat. Richard uses a 2x8 corrugated steel roof section as the waterproof underlayment and I use overlapping sheets of Mylar. Both methods work fine and you may discover additional methods that work as well, but both Richard and I are satisfied with our present designs.

My method involves layering in the way shingles are layered. The Trickle Down Mat is basically a waterproof, layered envelope that encloses a felt like material used to disperse water as a uniform film. Layering allows for expansion and contraction without diminishing the waterproofing ability of the Mylar. At the junction where the Mylar sheets meet rubber bumpers, are attached to a cross supports to hold the Mylar down and prevent the Suntuf glazing from sagging. The MTD collector is what I call a superstructure in which all the parts work together and support each other. The backing of the collector is also the insulation, side strips of insulation hold the backing in place, a cross support holds the side supports in place and rubber bumpers attached to the cross support hold the Mylar in place. The gutter support, collector support and bottom mounting support are all tied together to facilitate the mounting as well as the installation of the gutter.

MTD collectors are designed to be part of an array and a typical array would consist of between eight or sixteen collectors, pressed together. If you are just now becoming familiar with the MTD concept you are urged to focus on building one collector right before jumping into the assembly of multiple collectors. This is where the MTD kit can help you get off to a good start. The hand on experience of building your first collector pays for itself in a short time. As you know the surface area of collectors determines the amount of heat that may be collected. A set of sixteen 12-foot long collectors would occupy a surface area of about 380 sq ft and harvest the fuel oil extractable heat equivalent found in about 500 gallons of fuel oil per year in a typical USA mainland area. This may not seem like much as long as the price of fuel oil stays below $3 per gallon but we all know cheap oil won’t be available much longer.