Direct measurement and analysis of thermosiphon flow

The thermosiphon flow rate has been measured directly by adapting a simple and well-known laboratory technique, a constant level device, to a solar collector operating in the thermosiphon mode. The measurements were performed as a function of thermosiphon head and inlet temperature. The thermosiphon flow rate data were correlated both with the temperature change across the solar collector (ΔT) and the global insolation rate. The minimum ΔT threshold value, necessary to initiate thermosiphon flow in the morning was determined as a function of thermosiphon head and the corresponding thermosiphon pressure heads were observed to be invariable. The thermosiphon flow data were utilized to construct a standard efficiency test curve, thus showing that this technique can be applied for testing collectors in the thermosiphon mode. The instantaneous collector efficiency was also determined as a function of time of day.