ENHANCED GEOTHERMAL SYSTEMS WITH CARBON DIOXIDE AS THE HEAT TRANSMISSION FLUID – A GAME-CHANGING ALTERNATIVE FOR PRODUCING RENEWABLE ENERGY WITH SIMULTANEOUS STORAGE OF CARBON

The resource base for geothermal energy is enormous, yet the geothermal contribution to primary energy consumption in the U.S. is only about 0.3 %. The reason for this limited role is that geothermal production is currently limited to hydrothermal systems, in which fluid circulation is facilitated by well-connected networks of natural fractures. Efforts have been made in several countries to tap into the much larger resource base of hot rock formations with low natural permeability, using hydraulic stimulation and recovering energy by circulating water through a system of injection and production boreholes. These efforts have met with limited success, due to several reasons including (1) inevitable water losses are costly, (2) the pumping system for maintaining fluid circulation consumes a substantial fraction of produced power, and (3) water is a powerful solvent for many rock minerals, giving rise to chemical dissolution-precipitation cycles with detrimental effects on fluid circulation and heat extraction.

This paper presents an initial evaluation of a novel concept for enhanced geothermal systems (EGS) in which CO2 (carbon dioxide) instead of water is used as the heat transmission fluid. A comparison of thermophysical and chemical properties suggests that CO2 offers considerable advantages while avoiding the main problems of water-based systems. Numerical simulations indicate that heat extraction rates would be approximately 50 % larger for CO2 than for water-based systems. CO2 also has distinct advantages for wellbore hydraulics. Another favorable aspect is that fluid losses in a CO2-based EGS system would offer geologic storage of CO2 that could provide an additional revenue stream. Our preliminary economic analysis indicates that, depending on the value of CO2 sequestration credits, operating EGS with CO2 can be cost-competitive with water-based systems.

This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Geothermal Technologies, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.