The use of super-critical carbon dioxide as the working fluid in enhanced geothermal systems (EGSs): a review study

B. L. Avanthi Isaka, P. G. Ranjith, T. D. Rathnaweera

Research output: Contribution to journalReview ArticleResearchpeer-review

34 Citations (Scopus)

Abstract

Naturally-occurring high-temperature geothermal energy resources contribute significantly to the world's emerging energy demand by allowing renewable and clean power generation, and the technology of enhanced geothermal systems (EGSs) continues to progress at deeper levels, providing the ability to extract heat stored in deep underground at 2–5 km depths, which would have previously been considered impractical. Water-based EGSs are fairly well established, and a number of research studies have been carried out on the performance of water as the working fluid in geothermal reservoirs. Besides, the applicability of super-critical carbon dioxide (ScCO2) as the working fluid in EGSs by replacing water was recently investigated by research studies due to a number of advantages accompanied in CO2 as the working fluid in EGSs. The aim of this review study is to comprehensively investigate the applicability of ScCO2 as a working fluid in EGSs by summarising the recent research studies conducted on the operation of CO2-based geothermal systems. The study is further intended to highlight the advantages of ScCO2 in the EGS environment compared to conventional water-based methods. Of the advantages, a reduction of the external power requirement for the pumping of ScCO2 due to enhanced injectivity and flowability is the most attractive. In terms of heat extraction, higher mass flowrates can be achieved using ScCO2 as a working fluid, depending on the density-to-viscosity ratio, and the ratio is doubled with ScCO2 compared to water in the temperature range of 200–250 °C. Moreover, an accelerated energy recovery can be achieved through the higher thermal extraction rates of ScCO2, which are 50% greater than for water. Importantly, ScCO2 has the ability of inducing an interconnected fracture network with multiple secondary branches under comparatively low breakdown pressures compared to water. ScCO2 being a linear molecule with lower viscosity has the percolation ability into microcracks of the rock matrix, and thereby, the pressurisation of these microcracks induce multiple flow paths long the rock matrix by interconnecting the microcracks. In addition, the use of ScCO2 eliminates the problems associated with silica dissolution followed by precipitation, takes place in surface piping, heat exchangers and other surface equipment, because dry ScCO2 is less reactive in hot-dry reservoirs. However, in water-based EGSs, enhanced reactivity of water with the increase of pH accelerates the dissolution and precipitation of minerals, and is a critical issue in water-based EGSs. The enhanced reactivity of ScCO2 in the presence of pore fluid may also affect the long-term integrity of EGSs, because it alters the internal pore structure, due to the dissolution and precipitation of primary and secondary rock minerals, resulting in altered porosity and permeability, which ultimately affect the energy extraction process.

Original languageEnglish
Article number100547
Number of pages18
JournalSustainable Energy Technologies and Assessments
Volume36
DOIs
Publication statusPublished - Dec 2019

Keywords

  • Enhanced geothermal systems (EGSs)
  • Geothermal energy
  • Heat extraction
  • Mineral interaction
  • Supercritical carbon dioxide (ScCO)

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