2011 Volume 33 Issue 4 Pages 169-178
In this study, laboratory experiments were conducted to evaluate the effect of filling materials on heat exchange performance of a vertical ground heat exchanger (GHE). The main objective of this study is to demonstrate that large grained gravel filling can enhance the heat exchange rate of vertical GHE in ground source heat pump systems. The filling materials used for the laboratory experiments were water, large or small gravel saturated with water and fine silica sand saturated with water. The grain sizes of large and small gravel are distributed between 10 to 20 mm and between 5 to 10 mm, respectively. The laboratory experiments were conducted under the condition of constant temperature at inlet of GHE. By considering heating and cooling situation, the temperatures of heat medium at GHE inlet were maintained smaller and larger than the initial temperature of the sand layer in which the GHE is installed. From the results of the laboratory experiments, the averaged heat exchange rate in the case of small gravel filling was 7.8 % larger than the one in the case of silica sand filling. On the other hand, the averaged heat exchange rate in the case of large gravel filling and water filling are 20 % and 31 % larger than the one in the case of silica sand filling, respectively. In addition, the borehole thermal resistance decreased with increasing heat exchange rate except the case of silica sand filling. The reason why the heat exchange rate was enhanced between the silica sand filling and the other fillings is interpreted as the developments of natural convection in the annular space of the heat exchange well. This natural convection also decreased the borehole thermal resistance in the case of water and gravel filling. The largest driving force of natural convection occurred in the case of water filling, the second-largest one occurred in the case of large gravel filling. The presence of natural convection is confirmed by investigating the relationship between averaged Rayleigh number and averaged Nusselt number in the annular space. The above results showed that for enhancing heat exchange rate of vertical GHE, the use of large grained gravel as a filling material is one of the prospective options.
