Stagnation

Refers to any period when the heat transfer fluid is not circulating, even though the solar panel is receiving the suns energy.

In hot weather, a system can receive more heat from the sun than is needed. Usually, in these circumstances a system is allowed to ‘stagnate’. The stagnation temperature that the collector can reach if the fluid is not circulating is governed by the balance of heat gained from the sun and heat lost to the environment. In the case of LaZer2 collectors the temperature is 100 – 120°C.

When we refer to stagnation in the context of boil tolerance, we are assuming that the full temperature range is achievable as this has an effect on the fluid content in the panels. As the temperature passes the 135°C mark the fluid which is at the applied system pressure begins to evaporate. The small amount which is first to change state forces the rest of the fluid (which is still liquid) from the panels where it can no longer receive any energy input. The remaining moisture in the collectors continues to receive heat input. The collectors continue to rise in temperature as they are now no longer filled with fluid. The conditions inside the collectors begin to stabilize as they are now filled completely with water vapor at the same pressure as the liquid which fills the rest of the system. Any water vapor which exceeds the volume of the collectors at applied pressure will condense in adjoining, localized pipe work, as energy can be dispersed in these locations and no further energy can be transferred to it. A state of ‘stagnation’ is achieved when the input energy equals the energy being lost through all forms of possible energy transfer. At this point the pressure is stable and the panels are effectively drained back even though the circuit is a sealed system. As temperatures in the panel drop and input energy diminishes as daylight is lost, the amount of losses begin to exceed the input energy and the fluid re-enters the collector at the same time as the water vapor condenses. The actual pressure inside the system will in real terms fluctuate as the system goes into and out of stagnation, and during stagnation the expansion vessel must be able to allow for expansion of the fluid through the range of temperatures plus the sum of the cold fluid volume in the collectors and manifold: therefore it must be sized carefully for the system to work. Based on the proportion of expansion volume to system volume, the expansion vessel on a solar system will be considerably larger than other systems.