A review on heat transfer and energy conversion in the enhanced geothermal systems with water/CO 2 as working fluid

Summary Enhanced geothermal systems (EGS) technology is regarded as the future of geothermal energy and could become a major energy source in the future. However, EGS technology is not well developed. The challenges of EGS technology development, such as the small heat‐recovery factors and water limitations, need to be addressed. A comprehensive understanding of heat transfer in the complex subsurface structures is crucial for energy management in the reservoir, and an accurate heat transfer model of the reservoir is a key tool to optimize heat extraction to improve the heat‐recovery factor. The use of CO 2 as the EGS working fluid is regarded as an emerging technology to address water limitations. The analytical solutions, lab‐scale experiments, field‐scale modeling methods of water/CO 2 fluid flow and heat transfer in the reservoir, and a thermodynamic analysis of the energy conversion system at the surface are reviewed in this paper. Based on the existing literature, future research should focus on improving our knowledge and optimizing the heat transfer performance in the reservoir. Moreover, the emerging design of a heat exchange network in the reservoir and CO 2 ‐EGS could result from technology development. Copyright © 2015 John Wiley & Sons, Ltd.