An Integrated Model to Compare Net Electricity Generation for CO2- and Water-Based Geothermal Systems

Monday, November 8, 2010: 4:05 PM
Deer Valley I (Marriott Downtown)
Vikas Agarwal, Department of Chemical Engineering, West Virginia University, Morgantown, WV and Brian J. Anderson, Chemical Engineering, West Virginia University, Morgantown, WV

Utilization of supercritical CO2 as a geothermal fluid instead of water has been proposed by Brown [1] (2000) and its advantages have been discussed. This work concentrates to assess the net electricity which could be generated by using supercritical CO2 as a geothermal working fluid and compares it with water under same reservoir conditions of temperature and pressure. This procedure provides a method of direct comparison of water and CO2 as geothermal working fluids, in terms of net electricity generation over project lifetime.

An integrated model has been developed to determine net electricity generation for CO2- and water-based geothermal reservoirs. This model consists of a wellbore model, reservoir simulation and surface plant simulation. To determine the bottomhole pressure and temperature of the geothermal fluid (either water or CO2) in the injection well, a wellbore model was developed using fluid-phase thermodynamic equations of state, fluid dynamics, and heat transfer models. A computer program was developed that solves for the temperature and pressure of the working fluid (either water or CO2) down the wellbore by simultaneously solving for the fluid thermophysical properties, heat transfer, and frictional losses.

For the reservoir simulation the TOUGH2 code has been to model the temperature and pressure characteristics of the working fluid in the reservoir. The EOS1 module of TOUGH2 code has been used for the water system and the EOS2 module of TOUGH2 code has been employed for the CO2 case.

The surface plant is simulated using CHEMCAD to determine net electricity generated. A binary organic Rankine cycle is simulated. The calculated net electricity generated for the optimized water and CO2 systems are compared over the working life time of the reservoir.

References

[1] Brown, D. A Hot Dry Rock geothermal energy concept utilizing supercritical CO2 instead of water. In: Proceedings of the Twenty-Fifth workshop on Geothermal Reservoir Engineering, Stanford University, Pages 233-238,  2000.


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