Comparative thermo-economic analysis of co-axial closed-loop geothermal systems using CO2 and water as working fluids

•Thermo-economic evaluation is performed for co-axial geothermal heating systems.•Decreasing tube diameters dramatically reduces capital cost per net power (CPP).•Smaller tube diameters slow the growth of CPP caused by heat depletion in host rocks.•CO2 leads to lower CPP than water when the heat duty smaller than a critical value.•In systems without pumps, CO2 produces two times power than water by thermosiphon.