Engineered geothermal systems (EGS) present an opportunity to expand the implementation of geothermal power. There has been some interest in the possible application of CO2 as a heat extraction fluid in these systems instead of water, due to a variety of inherent benefits, including being a non-polar fluid, low viscosity at the temperatures and pressures within the reservoir and a large buoyancy effect. The buoyancy effect in particular indicates the potential for a simple surface plant design, or “CO2 thermosiphon”. Balancing these advantages, CO2 has some drawbacks, leading to lower thermodynamic performance in the overall system. This is mostly due to a combination of lower heat capacity, and large frictional losses due to gas-phase flows in the production wellbore. Here we explore some of the thermodynamics underlying this characteristic, and explore some scenarios that could be expected to offset it. Ultimately, economic analysis will be necessary to determine in what cases CO2 would be applicable as a geothermal heat extraction fluid. This should give some indication as to which geothermal resources may benefit from the usage of CO2.

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