GREEN SYSTEMS | SOLAR SOLUTIONS | CONTINUED FROM PAGE 67
Simple controls A single hydronic circulator pump provides flow through
the collectors and into the floors only whenever solar heat is available and the heat is needed in the house. The original Grundfos circulator was kept in service and used for both the solar and the boiler. The solar heat is controlled by a single differential thermostat that allows flow through the solar collectors only when they are hot, by activating a
Figure 33-1
rises up and out the tee at the top. This thermosyphoning continues as long as the sun
provides heat, or until the circulation pump turns back on. When the pump is on, the cooling loop snaps shut, using a passive check valve (located near the bottom inlet tee) that closes in response to the relatively high flow and pressure provided by the pump. The vertical ball-check valve shown in the photo
(Fig.33-2, lower right) is the original thermosyphon check valve installed in 1987 and still in operation today. It is the legendary “boogie” valve, manufactured by Zomeworks Corporation, but it is no longer widely available. As an alternative in this plumbing configuration today, an ordinary 1” brass swing check valve (with a vertical flap) could be substituted if mounted horizontally and tilted slightly downward toward the bottom tee.
20/20 hindsight It is always valuable to learn from the past, unless you
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motorized valve. In this simple design, when solar heat is not needed in the house, the flow to the collectors simply stops, even when they are exposed to full sun. When we designed this system, we knew that we had to
think of a way to dissipate the unwanted solar heat from these collectors safely and reliably to prevent them from generating steam and causing high temperature damage to the glycol. Our solution was borrowed from the world of passive solar design and based on cooling by natural convection. A thermosyphon cooling loop was added to the collectors so that they could cool themselves by natural convection when the pump-flow stops.
Thermosyphon cooling with fin tubes Thermosyphoning can be defined as the movement of
fluid around a plumbing loop, driven only by a temperature difference across the loop (fluid “pumped” only by heat). Even today there are solar water heaters for sale that collect solar heat without electricity and without a pump, simply by thermosyphoning. The same principle can be used to dissipate solar heat. Hot fluid is less dense than cool fluid, so when it is contained in a loop, the cool fluid tends to
“fall downhill” and the hot fluid tends to “float uphill.” See Figure 33-2 for photos of the cooling
loop installed on this job. (Remember that this equipment has been out in the weather for almost 25 years, so it is not showroom new.) When the pumped circulation in the collector stops, the hot fluid in the collector tends to rise to the top header. A tee connection at the outlet pipe (Fig.33-2, upper left) connects to cooling fins mounted on the back of the collector (Fig.33-2, center) that contain cool antifreeze, which is heavier than the hot antifreeze and tends to “fall” downhill. A tee connection at the collector inlet allows the cold fluid to slide downhill into the bottom of the collector as hot fluid
enjoy reinventing the wheel every few years. In this installation, we found some things that worked great and some things that could have been better. Generally speaking, it seems that much of our success here can be attributed to controlling and limiting high temperatures. Good Stuff: The thermal mixing worked to keep the plastic tubing
safe from solar overheating. The use of all-copper tubing near the collectors was a
good idea. Keep plastic away from solar collectors. The thermosyphon cooling loop kept the glycol in
service for a very long time. Direct floor heating with solar glycol works well even
without heat exchangers and even without water storage tanks. The large volume of glycol required to fill the floor
tubing seems to contribute to the longevity of the glycol. The original Grundfos circulator was still there and still
running. Do Over: The thermosyphon cooling would probably work better
with larger diameter tubing. Upsize the ¾-inch supplies and tees to 1". The thermosyphon cooling would work even better with
Figure 33-2
phc april 2011
www.phcnews.com
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