Low-cost optimization of geothermal heating system with thermal energy storage for an office building

•The geothermal heating system including thermal energy storage is optimized by two-stage approach.•The introduction of thermal energy storage increases the capital investment of the geothermal system.•The time-of-use tariffs is conducive to the economic operation of the geothermal system.•A reduction occurs in the comprehensive cost of the optimized geothermal system.•The optimal operation strategy and equipment selection parameters are determined.