Heat extraction analyses and energy consumption characteristics of novel designs of geothermal borehole heat exchangers with elliptic and oval double U-tube structures

•Heat extraction performance of BHE with various elliptic and oval shapes is studied.•A robust numerical model coupled with a finite line heat source model is developed.•Key parameters are identified for the optimal design of borehole heat exchangers.•Double U-tube oval BHE (type II) performs the highest heat extraction performance.•Energy consumption of the water pump of elliptic BHE (type II) is reduced by 36.9%.