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Frontiers in Energy

ISSN 2095-1701

ISSN 2095-1698(Online)

CN 11-6017/TK

Postal Subscription Code 80-972

2018 Impact Factor: 1.701

Front. Energy    2022, Vol. 16 Issue (2) : 246-262    https://doi.org/10.1007/s11708-021-0749-9
RESEARCH ARTICLE
Thermo-economic analysis of a direct supercritical CO2 electric power generation system using geothermal heat
Xingchao WANG1, Chunjian PAN2(), Carlos E. ROMERO2, Zongliang QIAO3, Arindam BANERJEE4, Carlos RUBIO-MAYA5, Lehua PAN6
1. Department of Mechanical Engineering, Colorado School of Mines, Golden, CO 80401, USA; National Renewable Energy Laboratory (NREL), Golden, CO 80401, USA
2. Energy Research Center, Lehigh University, Bethlehem, PA 18015, USA
3. Key Laboratory of Energy Thermal Conversion and Control of the Ministry of Education, Southeast University, Nanjing 210096, China
4. Department of Mechanical Engineering and Mechanics, Lehigh University, Bethlehem, PA 18015, USA
5. Mechanical Engineering, Universidad Michoacán de San Nicolas de Hidalgo, Morelia, Michoacán C.P. 58030, Mexico
6. Energy Geosciences Division, Lawrence Berkeley National Laboratory, University of California, Berkeley, CA 94720, USA
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Abstract

A comprehensive thermo-economic model combining a geothermal heat mining system and a direct supercritical CO2 turbine expansion electric power generation system was proposed in this paper. Assisted by this integrated model, thermo-economic and optimization analyses for the key design parameters of the whole system including the geothermal well pattern and operational conditions were performed to obtain a minimal levelized cost of electricity (LCOE). Specifically, in geothermal heat extraction simulation, an integrated wellbore-reservoir system model (T2Well/ECO2N) was used to generate a database for creating a fast, predictive, and compatible geothermal heat mining model by employing a response surface methodology. A parametric study was conducted to demonstrate the impact of turbine discharge pressure, injection and production well distance, CO2 injection flowrate, CO2 injection temperature, and monitored production well bottom pressure on LCOE, system thermal efficiency, and capital cost. It was found that for a 100 MWe power plant, a minimal LCOE of $0.177/kWh was achieved for a 20-year steady operation without considering CO2 sequestration credit. In addition, when CO2 sequestration credit is $1.00/t, an LCOE breakeven point compared to a conventional geothermal power plant is achieved and a breakpoint for generating electric power generation at no cost was achieved for a sequestration credit of $2.05/t.

Keywords geothermal heat mining      supercritical CO2      power generation      thermo-economic analysis      optimization     
Corresponding Author(s): Chunjian PAN   
About author:

Tongcan Cui and Yizhe Hou contributed equally to this work.

Online First Date: 13 July 2021    Issue Date: 25 May 2022
 Cite this article:   
Xingchao WANG,Chunjian PAN,Carlos E. ROMERO, et al. Thermo-economic analysis of a direct supercritical CO2 electric power generation system using geothermal heat[J]. Front. Energy, 2022, 16(2): 246-262.
 URL:  
https://academic.hep.com.cn/fie/EN/10.1007/s11708-021-0749-9
https://academic.hep.com.cn/fie/EN/Y2022/V16/I2/246
Fig.1  Direct turbine expansion system with CO2 injection and production geothermal loop (adapted with permission from Ref. [20]).
Fig.2  Sketch of well pattern and CO2 stream lines in T2Well/ECO2N modeling domain.
Fig.3  Predicted results by T2Well/ECO2N over 30 years under the base case conditions (adapted with permission from Ref. [21]).
Fig.4  Effects on production well head pressure, injection well head pressure, production well head temperature, and production CO2 flowrate.
Fig.5  Effect of well distance and CO2 injection flowrate (one-quarter) on output parameters.
Fig.6  Schematic of direct sCO2 expansion system (adapted with permission from Ref. [21]).
Fig.7  T-s diagram of direct sCO2 turbine expansion for the base case condition (adapted with permission from Ref. [21]).
Fig.8  Sketch of a well field with well-sets and power plant.
Parameter Value
Loan period n/a 20
Capacity factor Fcapacity 0.75
Geothermal well drilling successful rate S/% 85
Annual interest rate i/% 4
Taxes and insurance factor Finsurance&tax/% 1.5
Counted CO2 sequestration credit b co2/($·t–1) 0–5
Tab.1  Parameter values for cost estimation and optimization analysis
Design parameter Value
Power plant capacity/MWe 100
Turbine isentropic efficiency/% 88
CO2 compressor isentropic efficiency/% 85
Designed CO2 pre-cooler outlet temperature/°C 31.5
Cooling water inlet temperature/°C 20
Heat exchanger effectiveness ε 0.85
Counted CO2 credit/($·t–1) 0
Tab.2  Parameter values for base case LCOE optimization analysis
Fig.9  Flowchart of LCOE optimization analysis process.
Parameter Value
Minimal LCOE/($·kWh–1) 0.177
Overall cycle thermal efficiency ηth/% 15.13
Optimal well distance R/m 500
Optimal m ˙CO2,inj per well-set/(kg·s–1) 120
Optimal TCO2,inj/°C 30
Optimal Pbottom,prod/MPa 30
Optimal Pt,dis,opt /MPa 8.07
Number of geothermal well-sets needed 20
Plant capital cost/($1M) 264.25
Geothermal well cost/($1M) 260.01
Tab.3  Results of base case LCOE optimization analysis
Fig.10  LCOE and capital cost versus power plant capacity.
Fig.11  LCOE break points considering CO2 sequestration credit.
Fig.12  LCOE, system thermal efficiency, and net power output per well-set versus turbine discharge pressure.
Fig.13  Effects of design parameters and operating conditions on LCOE, system thermal efficiency, capital costs, well head pressure difference, and produced sCO2 flowrate per well-set.
R The radial distance from the injection well to production well/m
D Diameter of well/m
m ˙ Mass flowrate/(kg·s–1)
T Temperature/°C
P Pressure/MPa
h Specific enthalpy/(kJ·kg–1)
s Isentropic process
Q Thermal Energy/kWth
v Velocity/(m·s–1)
z Well depth/m
S Well drilling successful rate/%
C Cost/M$
Fcapacity Capacity factor
Finsurance&taxes Taxes and insurance factor
i Annual interest rate/%
n Loan period/a
bCO2 CO2 sequestration credit/($·t–1)
V Volume flowrate//(L·min–1)
Nwellset Number of well-set required
W Power plant capacity/MWe
e Error in RSM design
r2 Coefficient of determination
t Turbine
comp Compressor
inj Injection well
prod Production well
reinj Re-injection well
dis Discharge
opt Optimal value
th Thermal
α RSM regression coefficient
ε Heat exchanger effectiveness
ρ Density//(kg·m–3)
η Efficiency
sCO2 Supercritical carbon dioxide
RSM Response surface methodology
LCOE Levelized cost of electricity
O&M Operation and maintenance
HX Heat exchanger
APF Annual payment factor
M$ Million US Dollars
  
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