Partial Stagnation in Direct Flow Vacuum Tube Collectors: Conditions for Occurrence, Risks and Consequences
Abstract
Partial stagnation means that the heat transfer fluid evaporates inside a solar thermal collector field although the pump is operating and the collector field outlet temperature is significantly below the evaporation temperature. This happens at a pronounced non-uniform temperature distribution in combination with a low mass flow rate and/or a high temperature level. ISFH investigated the conditions for the appearance of partial stagnation at a direct-flow vacuum tube collector group. Experiments were conducted to identify further criteria that promote partial stagnation: a low system pressure, an unequal flow distribution and a high gas content of the fluid. Measurements show no efficiency reduction during partial stagnation at a horizontal collector slope. With several collector groups connected in parallel, partial stagnation of high intensity could pass into full stagnation in some of the collector groups. This would lead to a significant efficiency reduction of the collector field. However, partial stagnation leads to an unstable operation and a high load of the collector fluid and should therefore be avoided in any case. Possible measures are a more equal flow distribution obtained by the collector and collector field design as well as the removal of air and/or solid particles. In addition the system pressure level may be increased or the gas content solved in the fluid may be reduced.