Flat-plate collectors using gravity heat pipes with temperature limitation prevent the development of steam in the solar circuit. This lowers costs for solar plants significantly and reduces maintenance costs. (Graphic: KBB)
It is a fact that every solar thermal system can go into stagnation at any time. The culprit could be just a broken pump. But with properly designed safety measures and system pressure stagnation is not a problem. Nevertheless, the high temperatures reached in the newest collectors can put hi stresses on components such as the heat transfer fluid, insulation, and pipes. which is why the industry has continued to look for new, elegant concepts for controlling stagnation. A proven process is the drain-back method, which Vaillant introduced this year in Germany under the name "return-line-controlled solar system". At night the heat transfer fluid drains into the return-line tank and the collectors contain nothing but air. "As soon as the sun comes up, the pump switches on and fills the collectors with the solar medium," explains Vaillant product manager Arne Finger. "Once the tank is full, the pump simply shuts itself off, and the solar liquid flows back into the return-line tank."
For companies such as Vaillant that also supply their products to HVAC installers that do not deal with solar technology on a daily basis, installing the systems cannot be difficult, which the company believes it has achieved with its return-line-controlled solar systems. Nevertheless, a new mindset is needed. Up to now installing perfectly level pipes was seen as a must in heating installation. However, the pipes in return-line-controlled solar-thermal systems require a gradient of at least 4%.
Paradigma has found another way to solve the stagnation problem. The German company has long used only pure water as a heat transfer fluid in its vacuum-tube collectors because it is not damaged by turning into steam. "All of the active elements, such as pumps and valves, are in the solar return line. This part of the system also contains a zone valve that closes completely during stagnation because when that occurs the system should drain through the solar supply line. This protects all of the active components from overheating," says Wilfried Griesshaber, the head of the project management department at Paradigma. In addition, this prevents the forced flow of the fluid through an overheated collector. Paradigma generally positions the diaphragm expansion tank on the heating side, down-stream from the heat exchanger in the hot water tank. "The only component approved for installation in the solar supply line is a heat and steam-resistant drain and filling cock between the collector and the hot water tank," says Griesshaber.