Heatpipes for geothermal heat extraction

Abstract

Large quantities of heat stored in geothermal aquifers can be of interest to satisfy above-ground heating demands. With the use of heatpipes, placed into the aquifers, heat may be passively extracted. Vertical two-phase thermosiphons are considered in this study. As a result of a local increase in the saturation temperature in liquid pools, heat can only be transferred in the falling liquid film. The aim of this work is to investigate film heat transfer and its limitations at varying boundary conditions in the evaporator of a vertical experimental setup. An initially homogenized film is studied in a separate film heating section, in which a specific distribution of controlled band heaters and temperature transmitters enables the sensing of local film heat removal behaviour in the heatpipe. With increasing degrees of wall superheating, the mean film heat removal rate has been found to increase. Locally, beyond some degree of superheating, the heat transfer rate stagnates with increasing Jakob numbers, possibly as a result of film dry-out. As a result of such local behaviour, the slope of the mean heat transfer rate curve decreases with increasing Jakob numbers. The onset of dry-out, characterized by an increase in the intermittency of the results, has been studied by varying the initial liquid flow rate to the heated section. Both the initial film flow rate and the degree of superheating have been found to be of significance for the onset of film dry-out. Film dry-out takes place at flow rates less than some critical flow rate. The local heat removal rate decreases significantly if the initial film flow rate is decreased beyond the critical flow rate. The critical flow rate has been found to increase with increasing rates of superheating. The significance of the saturation temperature on the film heat transfer rate has also been studied in this work. The film heat transfer rate has been found to decrease with decreasing saturation temperature, for all in this study considered degrees of superheating. The steepest decline in heat transfer rates with decreasing saturation temperature is found for the smallest degree of superheating in the analysis. At last, the possibility of film re-distribution following dry-out has been considered. Melamine foam homogenizer rings, inserted in the film flow path, have been found to decrease the degree of film thickness variation in initially inhomogeneous films to some extent. Homogenized films were found to be able to transfer a greater amount of heat than films that had not been re-distributed.