Theoretical and experimental studies were performed for a solar-ground source heat-pump (SGSHP) system with a vertical double-spiral coil (VDSC) ground heat-exchanger (GHX). The heating mode of the SGSHP system is alternated between a solar energy-source heat-pump (SSHP) and a ground-source heat-pump (GSHP) using a low-grade energy utilization system built by the authors. The measured performances were for the SSHP, GSHP and SGSHP systems, as well as the ground heat-exchanger (GHX). The heat absorption rate per unit of ground surface area occupied by the GHX is shown to be 18 times greater than a single-pipe horizontal heat-exchanger. The coefficient of performance (COP) of the system with GHX is a 21% improvement over the system with the single-pipe horizontal heat-exchanger. The underground two-dimensional symmetry temperature field of the GHX was simulated using the volume-control method. The analytical results are compared with the experimental data. The mathematical model presented herein may provide design guidance for the design of a GHX for GSHP systems. The heat-output performance of a solar collector which was used for a solar-ground source heat-pump with the heating mode, is also predicted. Valuable numerical results were obtained in the experimental study of the real SGSHP system. (C) 2003 Elsevier Ltd. All rights reserved.