Solar heat collector variations, Part 2

A Folded Harp is a collector (or pair of collectors) that was designed with the supply and return Header a one end. The parallel risers make a U-turn inside the collectors, and the Header connections are grouped close together at one end of the collector frame. This can be a distinct advantage when installing a solar water heater when only a few collectors are required.  

A pair of collectors for a water heater can be arranged (usually on a roof) to allow for short supply and return pipes to connect to one end of the panels. This results in a better looking installation and less plumbing materials for collector installations requiring four panels or less. Stieble Eltron and Enerworks both have examples of Folded Harp collector designs intended for quick installation of solar water heaters.

The Folded Harp should not be miss-applied, however. Because of the U-turn plumbing, this type of collector configuration may not drain easily, and should not be used in Drainback installations. Also, since the internal piping does not provide a straight and simple flow path, it cannot be used with Thermosyphon self-cooling equipment that we use to prevent overheating (even during power failures) on most of our whole-building systems. 

When four of these collectors were included by the local installer in one of our standard installations, we used active heat dumping with steam back overheat protection as a last resort, which is all included as part of our standard control strategy.

Serpentine variation 

Some solar heat collectors do not use straight risers, or even U-shaped risers, but go even further by bending a single riser back and forth in a snake-like Serpentine configuration. When a single riser is made very long, and includes many sweeping U-turns inside the collector, the result is slower flow and hotter fluid. One good example of this is the Viessmann flat plate collectors such as the Vitosol 100 series. 

The high resistance to flow posed by a very long serpentine riser has been engineered by Viessmann to act like a flow restrictor that provides a self-balancing function. When multiple single-riser collectors are plugged together side by side, the flow through each riser is largely self-limiting, so the flow through all the risers in parallel is balanced. This allows the supply and return pipes to be connected to one end of a group of these collectors up to eight collectors in a group, while the flow through the eight risers remains balanced.

The end connections and self-balancing can potentially save time and materials. But, because the fluid temperatures are typically higher than a Harp collector, the need for overheat protection is important, especially in our climate in the desert southwest where overheat temperatures can be very high indeed. Since serpentine collectors are not compatible with Thermosyphon self-cooling methods, our local installers are in the habit of adding Photovoltaic (PV) circulator pumps to these collector banks and cooling them during sunny idle periods with solar powered pumps which remain functional during grid power failures. Steam back is usually included as the overheat protection of last resort.

Figure 87-4 shows a group of Viessmann serpentine panels installed on one of our Standard Combisystems in Santa Fe, N.M. If you look closely, you can see the both Header connections that are visible are plugged, and the supply and return pipes are both attached to hidden pipe connections on the far end, out of view. The PV panels power the glycol circulator pumps.

Evacuated tube single pipe header

Although the Evacuated Tube solar heat collector technology looks very different from the enduring flat plate collectors the principles are similar since each tube contains a tiny flat plate collector. There is a black absorber plate inside each tube, surrounded by a vacuum to provide superior thermal insulation for the plate. This tiny absorber plate is cooled by fluid that rises to the top of the tube when it is hot. The fluid is typically a phase-change refrigerant working within a heat pipe. It gives off its heat through a heat-exchanger node in the Header pipe at the top and then cools and sinks down by gravity to collect more heat from the black plate.  

There is a single Header pipe on top that acts as a manifold and heat-exchanger for all the collector tubes plugged into it. Liquid is pumped through the header pipe to carry away the heat as hot liquid (typically a glycol mixture). There is a limit to how many Evac. Tube collectors can be pumped in series, and so large groups of these collectors are usually broken up into banks of four or five in series, and these banks are connected in parallel so they don’t overheat one another. Follow the manufacturer’s instructions as to whether these can be used for Drainback, and what kind of overheat protection is required. Different brands of vacuum tubes have different specifications, so be sure to look at the fine print.

Figure 87-5 shows an example near Denver where Apricus collectors are included in a New Standard Solar Combisystem using pressurized glycol mix in the Headers. The local installer included PV (solar electric) over-heat protection pumps to guard against high temperature stagnation during daytime power failures.  

Final notes

These articles are targeted toward residential and small commercial buildings smaller than ten thousand square feet. The focus is on pressurized glycol/hydronic systems since these systems can be applied in a wide variety of building geometries and orientations with few limitations. Brand names, organizations, suppliers and manufacturers are mentioned in these articles only to provide examples for illustration and discussion and do not constitute any recommendation or endorsement. 

Bristol Stickney has been designing, manufacturing, repairing and installing solar hydronic heating systems for more than 30 years. He holds a Bachelor of Science in Mechanical Engineering and is a licensed Mechanical Contractor in New Mexico. He is the Chief Technical Officer for SolarLogic LLC in Santa Fe, N.M., where he is involved in development of solar heating control systems and design tools for solar heating professionals. Visit for more information.

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