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Solar Solutions Bristol Stickney, chief technical director, SolarLogic LLC, Santa Fe, N.M.


Bristol’s Six Principles for Good Solar Hydronic Design Overheat-cooling with thermosyphon loop


A


few months ago, I went on a service call to evalu- ate an old solar heating system at a house near Santa Fe. As we approached the house in our ser-


vice truck, I was struck with a strong feeling of deja vu. As it turns out, I had designed this solar heating retrofit myself back in 1987 and had worked as one of the installers that put it together that summer. If this heating system had been an automobile, it would


have been designated a “classic car” and given special license plates. It is nearly 25 years old and still runs. In fact, the original owner still depends on it to control his heating bills. A new backup boiler had just been installed, along with some other new heating components, and the owner just wanted to upgrade the controls to handle the new equipment. The system is presently back in service. Before the solar retrofit in 1987, we walked into a house


where construction had recently been completed, includ- ing hydronic radiant heat downstairs with plastic tubing embedding in concrete mass floors. A conventional “hot water” electric boiler provided heat to the floors, using a single Grundfos circulator (UP26-96) and some zone valves. The warm mass floors added up to just under 2,000 square feet. The retrofit included six 4 5 8 collectors mounted on


the ground next to the house. The south-facing slope of the terrain allowed us to screen the collectors from view with existing trees. See Figure 33-1 for a view of the collectors as they look today. They appear in the trees to the left of the wooden “coyote” fence just below the house.


Single-minded design goal This solar heating retrofit was not intended to be a com- bisystem. The original intention for these solar collectors was simple: to provide baseline winter heat for the mass floors. The owner wanted enough solar heat to temper the warm floors without using the electric boiler all the time and also wanted to divert unused solar heat to a spa when the heating season was over. The owner also wanted the retrofit to be as simple as possible, using the minimum equipment and simple controls. We accomplished this by attaching the boiler, heat tub-


ing and solar collectors together in a “solar direct” config- uration, where a glycol mixture is circulated through all the heating equipment. The glycol is contained in a closed system under normal hydronic boiler pressure (typically greater than12 psi cold and less than 25 psi hot) and cir- culated directly from the collectors to the boiler and into the tubing in the floors. There are no heat exchangers in the system other than the plastic tubing in the concrete. The plastic is protected from solar overheating by using a thermal mixing valve.


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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 con- vection. 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 tempera- ture 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


Continued on page 28 April 2011


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 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


Figure 33-1


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