An investigation of a ground thermal energy storage system, which includes storage units containing phase-change materials (PCM), is presented. This study is related to a large-diameter helical heat exchanger, which is placed vertically in the ground. The PCM storage units under consideration have a cylindrical shell shape and are located inside and/or outside the helix. A modified numerical scheme for the solution of heal transfer in the ground in the PCM units, and within the heat exchanger pipe, is presented. The theoretical results show that the thermal diffusivity of the PCM dominates the thermal performance of the system. Incorporation of PCM storage units containing paraffin wax results in a reduction of the thermal efficiency in comparison with a system not containing these units. However, incorporation of PCM having the same thermal diffusivity as of the soil results in a significant improvement of the thermal performance.