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

Abstract

Co-axial closed-loop geothermal heating systems coupling abandoned oil/gas wells face more grievous corrosion issues than the shallow ground source heat pumps due to the higher temperature and pressure conditions. Instead of water, using CO2 as a working fluid would extract more power and eliminate the corrosion issue; however, these advantages might be offset by the energy and capital cost of high operating pressure. To better guide the working fluid selection for the co-axial closed-loop geothermal heating systems, the effects of using CO2 as a working fluid on the system thermo-economic performance as well as the effects of geothermal gradients and tube diameters are evaluated. We first established a 2D thermodynamic model to simulate the heat extraction from a vertical co-axial closed loop geothermal well and the heat conduction in host rocks and then applied the key parameters into an economic model for the calculation of thermo-economic indicators. It is found that the wellbore set with smaller tube diameters can profit the thermo-economic performance of the co-axial closed-loop geothermal heating system by substantially lowering the total annual cost, promoting the energy efficiency, and slowing down the growth of cost and the descent of energy efficiency within the lifetime of the system. Besides, CO2 will be a better working fluid for small-scale co-axial closed-loop geothermal heating systems leading to lower cost per net power compared to water, and it also can slow down the descent of energy efficiency year by year.