Performance enhancement, economic analysis, and futuristic insight of single-well medium-deep and deep geothermal systems

Abstract

Geothermal energy is clean and renewable, derived from the heat stored within accessible depths of the Earth’s crust. The adoption of a single-well system for medium-deep and deep geothermal energy extraction has attracted significant interest from the scientific and industrial communities because it effectively circumvents issues such as downhole inter-well connections and induced seismicity. However, the low heat transfer capacity in geothermal formations limits the heat extraction performance of single-well systems and hinders their commercial deployment. This review covers various enhancement concepts for optimizing the heat transfer within single-well systems, emphasizing critical parameters such as heat transfer area, heat transfer coefficient, and temperature difference. Additionally, it presents the thermo-economic evaluation of different configurations of single-well borehole heat exchangers and super-long gravity heat pipes (SLGHPs). The SLHGP, utilizing phase-change heat transfer, is recognized as a highly effective and continuously productive technology, capable of extracting over 1 MW of heat. Its pumpless operation and ease of installation in abandoned wells make it cost-effective, offering a promising economic advantage over traditional geothermal systems. It also highlights the challenges and potential research opportunities that can help identify gaps in research to enhance the performance of single-well geothermal systems.

Graphical abstract

Keywords

borehole heat exchanger / coaxial downhole heat exchangers / geothermal energy exploitation / single-well system / super-long gravity heat pipe

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Acknowledgements

This work was supported by the National Key Research and Development Program of China (Grant Nos. 2021YFB1507300 and 2021YFB1507302), the National Natural Science Foundation of China (Grant Nos. 52206126 and 52206287), the Jiangyin Science and Technology Innovation Special Fund, China (Grant No. JY0604A021015210001PB), and the Jiangsu Provincial Carbon Peak Carbon Neutralization Technology Innovation Special Fund, China (Grant No. BE2022012).

Competing Interests

The authors declare that they have no competing interests.

Notations

CDHE	Coaxial downhole heat exchanger
DHE	Downhole heat exchangers
EGS	Enhanced geothermal system
HDR	Hot dry rock
NPV	Net present value
SLGHP	Super-long gravity heat pipe
CCAP	Capital investment/$
Ccost(t)	Costs in the t-th years/(million $)
Cex	Costs of exploration for geothermal resources/(million $)
Cwell	Cost of well drilling and completion in the vertical segment/($·m–1)
Cfuel(t)	Fuel cost in the t-th years/(million $)
Cplant	Installation cost of the plant per kilowatt/($·kW–1)
CM	Maintenance cost/$
Cma	Material costs/(million $)
CO	Operating cost/$
c	Specific heat/(J·(kg·K)–1)
Csc	Surface construction costs/(million $)
E	Electricity generated/kW
Fw	Drilling and completion costs/yuan
Fm	Maintenance cost/yuan
Fo	Operation cost/yuan
Fp	Plant construction cost/yuan
Fe	Total revenue/yuan
H	Vertical length of the horizontal well/m
Iincome	Incomes in the t-th years/(million $)
k	Cost per unit power, million/($· kW–1)
L	Horizontal length of the horizontal well/m
Lh	Horizontal length of geothermal well/m
Lcoe	Levelized cost of energy
Lv	Vertical length of geothermal well/m
M	Initial total cost/$
Mwell	Initial cost of a horizontal well including drilling and completion/$
Mm	Maintenance cost/$
Mplant	Power generation equipment purchase and installation cost/$
N	Pump power consumption/W
Welec	Power generation rate/kW
Me	Total heat output/J
n	Time duration of the geothermal exploitation/years
Pprice−e	Price of the electricity/($·kWh–1)
Q	Heat transfer rate/(W·(m2·K)–1)
Qpump	Total power consumed in water injection and circulation/kWh
Qpow	Total electric power generation/kWh
r	Discount rate/%
Sp	Electric power cost of pump operation/(yuan·kWh–1)
Se2	Price of geothermal secondary utilization/(yuan·kWh–1)
Se1	Price of geothermal power/(yuan·kWh–1)
t	Annual operation time/year
t0	Annual production time/s
T	Temperature/K
Tres	Temperature in the reservoir/K
Tt	Temperature in the tube/K
Wp	Pump load/W
Zoverall	Overall thermal resistance/(K·W–1)
ρ	Density/(kg·m–3)
ηp	Pump efficiency/%
λ	Thermal conductivity/(W·(m·K )–1)

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