CO 2 Thermosiphon for Competitive Geothermal Power Generation Aleks D. Atrens,* Hal Gurgenci,* and Victor Rudolph The UniVersity of Queensland, Queensland Geothermal Energy Centre of Excellence, School of Engineering, St Lucia, Queensland 4072, Australia ReceiVed July 28, 2008. ReVised Manuscript ReceiVed October 7, 2008 Engineered geothermal systems represent a significant unutilized energy source, with the potential to assist in meeting growing energy demands with clean, renewable energy. Traditional geothermal systems use water as the working fluid. An alternative working fluid is carbon dioxide which offers potential benefits including favorable thermodynamic and transport properties and the potential for sequestration. An important feature is that CO 2 does not dissolve mineral salts, and this will serve to reduce fouling and corrosion problems which afflict piping and surface equipment in conventional water cycles. Our modeling shows that a CO 2 -based power plant has net electricity production comparable to the traditional approach, but with a much simpler design, and demonstrates the comparative efficacy of CO 2 as a heat extraction and working fluid. While the economic viability of a CO 2 -based system remains to be proven, this analysis provides a starting point for more detailed thermodynamic and economic models of engineered geothermal systems power conversion utilizing CO 2 . 1. Introduction Population growth coupled with increasing economic devel- opment and prosperity are resulting in unprecedented demands for energy. The environmental stresses, most prominently climate change, associated with expanding energy supply using current (fossil fuel) strategies have become unsupportable. Alternative baseload power which is free of CO 2 emissions is the key to meeting this challenge. Engineered Geothermal Systems (EGS) have the potential to provide significant amounts of clean, baseload electricity. In Australia, there is an estimated recoverable heat resource of 22 000 EJ. 3 A conservative estimate for the US is 280 000 EJ. 1 This compares with an annual worldwide energy consumption of 477 EJ in 2005. 8 EGS seeks to utilize hot rock reservoirs deeper than traditional near-surface volcanic geothermal energy. In Australia, depths of 4000-5000 m are required to reach temperatures of approximately 250 °C in the most prospective areas. 3 These resources typically require engineering intervention to exploit the thermal energy stored in an EGS reservoir. One EGS approach is shown in Figure 1a. The geothermal heat is extracted from the reservoir using water flowing through a system of injection and production wells, spaced to ensure that the fluid reaches thermal equilibrium and adequate tem- peratures over the field lifetime. The hot water is the heat source for the electrical power generation system. To provide the necessary connection between injection and production wells and allow water to recirculate, it is generally necessary to fracture the reservoir rock. Fracturing is commonly also applied in oil and gas reservoirs to increase productivity. The energy in the hot water from the production well is typically converted to electrical power in a Rankine cycle using an organic working fluid such as isopentane. 4 A binary system is necessary because the salts dissolved in the water, most commonly carbonates and silicates, 9 lead to unacceptable scaling 10 and corrosion in the power conversion equipment 11 in a direct steam system. 2. Theoretical Basis This paper considers an alternative approach for EGS: an integrated CO 2 -based power plant, or “CO 2 thermosiphon”. As shown in Figure 1b, CO 2 is used as the working fluid, which extracts heat from the reservoir and drives the turbine. The CO 2 thermosiphon consists of an injection/production well system, the geothermal reservoir, a turbine, and a cooling system. Building on the concept as proposed in prior work, 5-7 this paper evaluates the design as an alternative to a water-based system. For the modeling, the power plant processes have been considered to be ideal: losses from irreversible processes such as wellbore friction, turbulence, and pressure drops in process * To whom correspondence should be addressed. E-mail: aleks.atrens@ uq.edu.au or h.gurgenci@uq.edu.au. (1) Tester, J. W. The Future of Geothermal Energy; Massachusetts Institute of Technology: Boston, 2006. (2) Tester, J. W.; Anderson, B. J.; Batchelor, A. S.; Blackwell, D. D.; DiPippo, R.; Drake, E. M.; Garnish, J.; Livesay, B.; Moore, M. C.; Nichols, K.; Petty, S.; Veatch, R. W.; Baria, R.; Augustine, C.; Murphy, E.; Negraru, P.; Richards, M. Philos. Trans. R. Soc. London, A 2007, 365, 1057–1094. (3) Burns, K. L.; Weber, C.; Perry, J.; Harrington, H. J. Proc. World Geotherm. Congr. 2000, 99–108. (4) DiPippo, R. Geothermal power plants: principles, applications and case studies; Elsevier: Oxford, 2005. (5) Brown, D. W. Proceedings of the Twenty-Fifth Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, CA, 2000, pp 233-238. (6) Gurgenci, H.; Rudolph, V.; Saha, T.; Lu, a. M. Proceedings of the Thirty-Third Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, CA, 2008, pp 283-289. (7) Pruess, K. Geothermics 2006, 35, 351–367. (8) Agency, I. E. World Energy Outlook 2007; International Energy Agency: Paris, 2007. (9) Andre, L.; Rabemanana, V.; Vuataz, F.-D. Geothermics 2006, 35, 507–531. (10) Moya, P.; DiPippo, R. Geothermics 2007, 36, 63–96. (11) Pa ´tzay, G.; Sta ´hl, G.; Ka ´rma ´n, F. H.; Ka ´lma ´n, E. Electrochim. Acta 1998, 43 (1-2), 137–147. (12) Span, R.; Wagner, W. J. Phys. Chem. Ref. Data 1996, 25 (6), 1509– 1596. Energy & Fuels 2009, 23, 553–557 553 10.1021/ef800601z CCC: $40.75  2009 American Chemical Society Published on Web 12/15/2008