Time Well Served
By John SiegenthalerGreen Technology HVAC Systems Solar Residential Buildings Residential Design Siegenthaler
After 30 years it was time for a collector transplant.
My wife Joyce and I have lived with an array of solar thermal collectors on our house for the last 30 years. Those collectors came from Revere Solar & Architectural Products, a division of the now defunct Revere Copper and Brass, where I worked as an engineer from 1978 to 1981.
Figure 1 shows the collector array. Each of the six collectors measured 35 inches wide by 77 inches long. Those dimensions are one inch longer and wider than the glass used in standard sliding patio doors. Because it was mass produced, 34 in. by 76 in. tempered glass was available at lower cost, and thus became the benchmark around which the remainder of the Revere collectors were designed.
Other than size and insulation material, the Revere collectors are similar to current flat plate collectors. They had all copper absorber plates with a black chrome “selective surface” on the upper side. The absorber plates were mounted in an extruded aluminum housing, covered with low-iron tempered glass, and insulated with low-binder, high temperature fiberglass insulation.
Initially, these collectors had thermal performance indices similar to a modern flat plate collector (FR ta = 0.72, and FRUL = 0.83). However, I am convinced some of this initial performance was lost due to aging effects of the selective surface, insulation R-value, and perhaps even some changes in the transmissivity of the glazing.
Back in the 1970s, the engineering team at Revere estimated their collectors would have an average design life of 20 years. Apparently I got collectors destined for the right side of the statistical bell curve. They were still going at 30 years of age, albeit with some loss of performance and a few repairs of minor pin-hole leaks. The home’s shingle roof actually wore out before the collectors. We had to temporarily remove them to install the new shingles. I was concerned that “disturbing” them for this work might create more problems. That turned out not to be true. the collectors were reinstalled and continued to operate for several more years.
Still, at the 30 year mark I felt it was time for a solar “refresh” on our roof and a graceful retirement of the Revere collectors. At the risk of sounding sentimental, those aging collectors had done their job exceptionally well and removing them from the roof was bittersweet. Still, what lay ahead was very exciting.
The Next Act
It just so happened that a manufacturer was finished testing its new patent pending five-port flat plate collector at the same time I decided on the solar refresh at our house. This collector could be configured for either closed loop/antifreeze systems, or drainback systems. I have long been a fan of the latter and wanted to keep the system operating in its original drainback configuration.
This five-port drainback collector is designed with slightly sloping internal headers, that form a very shallow “V” shape. Both ends of the upper and lower headers terminate just outside the anodized aluminum enclosure with union-type piping connections. These are the connections one would use if connecting the collectors for a standard antifreeze-type system. The unique feature is found at the bottom centre of the collector. The low point of the bottom header is configured as a tee to allow all fluid to drain out of the absorber plate. Thus, when used in a drainback system, these collectors would have an external lower header in combination with an internal upper header. It is a simple idea that eliminates the need to side slope the collectors and is perfect for the retrofit situation I had.
Knowing my interest in these collectors, Bob Rohr, who is the inventor of this five-port collector, made me a great offer. He would personally come out and help me install the new collectors.
The first task was getting the mounting brackets and rails in place. Based on the mounting flange that runs around the lower perimeter of the collectors, we decided to use an aluminum framing system to support the array. The framing was secured to 1.5″ x 1.5″ x 0.25″ aluminum clips that I fabricated from angle stock. We decided to support these clips on top of small pieces of pressure treated lumber to help spread the load over the shingles. We snapped chalk lines to maintain alignment of the framing. One rail would be located about a foot below the upper edge of the collector and the other about one foot above the lower edge.
A doubled sided roof sealing tape was applied to the back side of each treated wood support. This tape has an amazing ability to stretch, as can be seen by how it coats the threads of the lag screws in Figure 2. It definitely beats dealing with asphalt roofing mastic on a warm sunny day.
The aluminum clips and their supporting blocks were lagged into pilot holes drilled directly into the upper chords of the roof trusses. These supports were spaced 48 inches apart. We used a small drill bit as a “probe” to find the centre of each truss and then sealed up the probe holes with a dab of silicone caulk. A cordless impact driver was ideal for driving the 3/8″ lags home (see Figure 3).
The aluminum frame was supplied in nine-foot lengths. Butt ends were joined using stainless steel splines supplied by the frame manufacturer and held in place with set screws. The splines fit snugly and allowed for accurately aligned and very strong joints that we staggered so they did not fall directly over the mounting clips. Spring-loaded stainless steel “T-bolts” with ¼” x 20 TPI threaded studs, also supplied by the frame manufacturer, provided the hold down for the collector mounting clips. After the framing was in place, we drilled several ¼-in. holes near the lower edge to drain out rainwater.
With the rails in place it was time to hoist the collectors. The 4-ft. by 8-ft. panels weigh about 90 pounds each. We set up an extension ladder as a slide, and slid each collector as far up as possible while standing on the deck below the roof. My wife Joyce, along with some help from friction, held each collector in position on the ladder while Bob and I got back up on the roof for the final lift. We discovered that leaving the cardboard packing on the ends of the collector provided protection while sliding them up the ladder.
The first collector was carefully positioned, levelled, and secured at all four locations where its edges crossed the aluminum frame rails as shown in Figure 4. The mounting clips coordinate with the collector edge flange and tightly grip the frame as the nut is tightened on the T-bolts inside the frame.
With the first collector in place, the others quickly followed. Each connected to its neighbour using the double nutted brass fittings. A watertight seal is created at each connection using a flat gasket that swells upon contact with water. The space between adjacent collectors is only three inches. Just enough for comfortable access to the mounting hardware.
The low point connection (e.g. the “5th port”) on each collector was adapted to ¾” copper tubing. We used a copper street elbow on the left collector and cut progressively longer tube stubs for the other four. This allowed the lower manifold to slope at ¼” per foot for proper drainage.
The lower header was insulated with ½” wall thickness foam rubber insulation. The end of the header passes directly through the siding at the right side of the recessed array. From this location we were able to maintain a minimum ¼” per foot slope back to the mechanical room.
The system is currently operating on the original differential temperature controller. The temperature sensor is inserted into the sensor well in one of the collectors.
With the piping and wiring completed, we flipped the system controller on, snugged up a couple of collector fittings, and cycled the array through a few fill/drain cycles. As has been true during the previous 30 years, gravity did not let us down. Th
e system filled and drained as expected.
Perhaps in another 30 years, the next generation of solar professionals will be changing out these collectors with the latest technology. Perhaps those new collectors will be constructed of the latest composite materials and placed by a solar-powered construction robot. I will probably be directing work from the ground at that point. Whatever the case, the sun will still be ready to serve up more Btus. <>
John Siegenthaler, P.E. is the author of Modern Hydronic Heating (the third edition of this book is now available). For reference information and software to assist in hydronic system design visit www.hydronicpros.com.