Investigation of buoyancy effects on heat transfer between a vertical borehole heat exchanger and the ground

• The groundwater effect on vertical borehole heat exchanger is investigated.
• The model can be applied with or without groundwater at the borehole location.
• The convection heat transfer from the ground surface is considered.
• Comparing with conduction only models, the borehole length is longer by 50%.

In this paper, the heat transfer from a vertical borehole heat exchanger in the presence of groundwater is investigated using a general convection–conduction heat transfer model. The model can be used for estimating the required length of the borehole for a given constant heat load in locations with or without groundwater. All thermal resistances involved in the heat transfer process between the borehole heat exchanger and the surrounding ground are taken into consideration. In addition, the effect of heat transfer from the ground surface to the ambient is investigated. A comparative case study is presented to illustrate the differences in estimating the length of a vertical borehole heat exchanger based on the proposed model in comparison with other methods that are based on conduction only mode of heat transfer. It is found that the natural convection heat transfer arises from the presence of groundwater increases the heat flux from the borehole surface. Consequently, the required borehole length decreases by about 50% at soil permeability of 5 × 10−9 m2. In addition, a convective type ground surface reduces the length by about 10% compared with an adiabatic surface assumption.