There is an opportunity to expand the baseload geothermal electricity generation capacity through the development of engineered geothermal systems (EGS). Carbon dioxide (CO(2)) could be used as an alternative to water to extract heat from these systems considering its advantages of ease of flow through the geothermal reservoir, strong innate buoyancy that permits the use of a thermosiphon rather than a pumped system over a large range of fluid flow rates, and lower dissolution of materials that lead to fouling. However, the thermodynamics of EGS using CO(2) to extract heat from subsurface rock masses is not well understood. Here we show that the wellbore frictional pressure losses are the dominant factor in CO(2)-based EGS. Wellbore friction is the major limiter on the amount of energy that can be extracted from the reservoir by CO(2), as measured by the exergy available at the surface. The result is that CO(2) is less effective at energy extraction than water under conditions similar to past EGS trials. Nevertheless, CO(2) can perform well in lower permeability reservoirs, or if the wellbore diameter is increased. Our results demonstrate that CO(2)-based EGS need to be designed with the use of CO(2) in mind. We suggest this work to be a starting point for analysis of the surface infrastructure and plant design and economics of CO(2)-based EGS. (C) 2010 Elsevier Ltd. All rights reserved.