**4. Conclusion**

needed.

**3.3. EGS using CO2 as the working fluid for green power generation and simultaneous**

It was reported (Pruess, 2006) that previous attempts to develop EGS in Japan, USA, Europe and Australia have all employedwater as a heat transmission fluid. Although, water has many properties that make it a favorable medium for this purpose, it also has serious short‐ comings. An unfavorable property of water is that it is astrong solvent for many rock miner‐ als, especially at elevated temperatures. In this case, injecting water at high pressure intohot rock fractures, as part of an EGS resource operation & utilization, results in strong dissolu‐ tion and precipitation effects that change fracture permeabilityand make it very difficult to operate an EGS reservoir in a stable manner. In 2000, Brown, D. (Pruess, 2006) proposed a novel EGS concept that would utilize supercritical CO2 instead of water as heat exchange (carrier) fluid, and would simultaneously achieve CO2 geologic sequestration as an addition‐ al benefit. There are only very few investigations that characterized the performance of CO2 as working fluid in EGS applications. For example, Pruess (Pruess, 2006) performed numeri‐ cal simulations and evaluated thermophysical properties in order to explore the heat trans‐ fer and fluid dynamics characteristics in an EGS reservoir that would be operated with CO2. It was found that CO2 is superior to water in its ability to exchange heat from hot fractured rock. Carbon dioxide also offers certain advantages with respect to wellbore hydraulics, in that its larger compressibility and expansivity as compared to water would increase buoyan‐ cy forces and would decrease the parasitic power consumption (thus reduce pumping cost) of the EGS fluid circulation system. This is because the larger expansivity of CO2 would gen‐ erate large density differences between the cold CO2 in the injection well and the hot CO2 in the production well, and therefore provide buoyancy force that would reduce the power consumption of the fluid circulation system. Another interesting feature of CO2 is that its lower viscosity, tend to yield larger flow velocities for a given pressure gradient. In addi‐ tion, CO2 would be much less effective as a solvent for rock minerals, which would reduce or eliminate scaling problems, such as silica dissolution and precipitation in water-based systems (Pruess, 2006). It was also reported (Pruess, 2006) that while the thermal and hy‐ draulic aspects of aCO2-based EGS system look promising, major uncertainties remain with regard to geochemical interactions betweenfluids and rocks. It was concluded in (Pruess, 2006) that an EGS system running on CO2 has sufficiently attractive features to warrant fur‐ therinvestigation. It was suggested that an EGS using CO2 as heat transport and exchange fluid could have favorable geochemical properties, as CO2 uptake and sequestration by rock

Supercritical CO2 can also be used as the working fluid of the power cycle before it is sent back to the EGS reservoir. For example, ina study by (Gurgenic et al., 2008), it was reported that there is a significant potential to use supercritical CO2 as working fluid in the power loop as illustrated (Gurgenic et al., 2008) in Figure 5. Significantly higher energy conversion efficiencies were predicted using a single-loop system with the CO2 being both the heat ex‐ change and the power cycle working fluid. It was reported (Gurgenic et al., 2008; Atrens et al., 2011) that the loops in either of the two cycles (i.e. subsurface loop and surface power loop) do not have to be closed. For example, if there is ready access to CO2 (e.g., at a geother‐

**carbon sequestration**

322 New Developments in Renewable Energy

minerals would be quite rapid.

An increasing concern of environmental issues of emissions & pollution, in particular global warmingand the constraints on consuming conventional energy sources has recently result‐ ed in extensive research into innovative renewable and green technologies of generating electrical power. One of these innovative emerging technologies includes renewable lowtemperature (low-enthalpy) geothermal energy source for clean electrical power generation. This promising technology offers potential applications in generation of electric power which can be produced using the vast renewable low-temperature geothermal energy re‐ sources available worldwide.In this chapter, the concept of ORC binary technologyfor pow‐ er generation using low-temperature geothermal heat source was introduced and its potential applications and limitations for small-scale geothermal power generation and its relevant environmental and economic considerations were presented and discussed. Also, recent developments of ORC-based low-temperature geothermal power generation with their significant and relevant applications were presented and discussed. A number of suc‐ cessful ORC binary plants were installed in different locations (e.g. remote and rural sites) worldwide which demonstrated the ability of this promising alternative and green technolo‐ gy to utilize renewable low-temperature geothermal energy sources for generating electrici‐ ty. Also, several patents were reported on the application of this innovative technology. Geothermal ORC power generation plants are normally constructed and installed in small modular power generation units. These units can then be linked up to create power plants with larger power production rates. Their cost depends on a number of factors, but mainly on the temperature of the geothermal fluid produced, which influences the size of the ORC turbine, heat exchangers and cooling system. Currently, ORC power cycles exhibit great flexibility, high safety (installations are perfectly tight), and low maintenance when coupled with low-enthalpy geothermal heat sources. The future use of low-temperature geothermal energy resources for generating electricity would very much depend on further overcoming technical barriers both in utilization and production, and its economic viability compared to other conventional and renewable energy sources used for power production. Another emerging "dual-benefit" technology is EGS using CO2 as the working fluid for combined clean power generation and geologic CO2 sequestration. CO2 is of interest as a geothermal working fluid mainly because it transfers geothermal heat more efficiently than water. While power can be produced more efficiently using this technology, there is an additional benefit CCS for reducing GHG emissions. The second part of the chapter presented the mer‐ its and fundamental aspects of CO2-based EGS technology.In 2000, Brown, D. (Pruess, 2006) proposed a novel EGS concept that would utilize supercritical CO2 instead of water as a more efficient heat exchange (carrier) fluid (due to its favorable properties over water), and would simultaneously achieve CO2 geologic sequestration as an additional benefit.It was found that CO2 is superior to water in its ability to exchange heat from EGS hot fractured rock and reduce hydraulic power consumption for fluid injection and circulation in the EGS reservoir. It was concluded that an EGS system running on CO2 has sufficiently attractive features to warrant furtherinvestigation.It was also concluded that EGS for power genera‐ tion is still relatively a novel technology and remains to be proved on a large scale and that further research is needed for additional exploration oftechnological and economic aspects regarding the opportunities and challenges for CO2–based EGS technology for combined carbon sequestration and power generation.

**Acknowledgements**

**Author details**

Address all correspondence to:

Basel I. Ismail\*

**References**

da

The author of this chapter would like to acknowledge the funding contribution by Goldcorp Canada Ltd.-Musselwhite Gold Mine that mainly supported the collaborative geothermal energy & heat pump (GHP) technology research project (author was the PI of the project) at their site in Northern Ontario; a contracted research project with Lakehead University (2007-09). Acknowledgement also goes to Natural Sciences and Engineering Research Coun‐ cil of Canada (NSERC) for the Discovery Grant (Individual) funding that was provided to the author's research in the area of clean energy technologies related to CO2 membrane gas

ORC-Based Geothermal Power Generation and CO2-Based EGS for Combined Green Power Generation and CO2

Sequestration

325

http://dx.doi.org/10.5772/52063

Department of Mechanical Engineering, Lakehead University, Thunder Bay, Ontario, Cana‐

[1] Atrens, A. D., Gurgenic, H., Rudolph, V., & (2011, . (2011). Economic optimization of

[2] Azim, M. R., Amin, M. S., & Shoeb, A. (2010). Prospects of enhanced geothermal sys‐

[3] Cengel, Y. A., & Boles, M. A. (2008). Thermodynamics: an engineering approach (6th

[4] Chandrasekharam, D., & Bundschuh, J. (2008). Low-enthalpy geothermal resources for power generation. CRC Press Taylor & Francis Group,, 978-0-41540-168-5, New

[5] Cui, J., Zhao, J., Dai, C., & Yang, B. (2009). Exergetic performance investigation of medium-low enthalpy geothermal power generation. *IEEE Computer Society,*,

[6] Dickson, M. H., Fanelli, M., & (2005, . (2005). Geothermal energy: utilization and technology,. Earthscan, an imprint of James & James (Science Publishers) Ltd. in As‐ sociation with the International Institute for Environment and Development,,

a CO2-based EGS plant. *Energy & Fuels,*, 25, 3765-3775, ACS Publications.

tem in baseload power generation. *IEEE,*, 10, 176-180, 0000-9781.

ed.),. McGraw-Hill press,, 978-0-07352-921-4, New York.

York., 10.1201/9780203894552.

636-639, 0000-9780.

1-84407-184-7.

separation from industrial flue gases for GHG emissions reduction.
