(149c) An Integrated Model to Compare Net Electricity Generation for CO2- and Water-Based Geothermal Systems

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.