Elsevier

Renewable Energy

Volume 143, December 2019, Pages 1162-1171
Renewable Energy

The optimization of geothermal extraction based on supercritical CO2 porous heat transfer model

https://doi.org/10.1016/j.renene.2019.05.090Get rights and content

Highlights

  • The correlation of thermal dispersion is obtained from the fitting process between the corresponding model and experiment.

  • The maximum geothermal extraction is found based on the optimal process.

  • The porous heat transfer model is developed to process the optimization for the evaluating the geothermal extraction.

Abstract

This study presents a process to find the maximum heat extraction in supercritical CO2 geothermal system. Little attention in the previous studies about the thermal dispersion receives. A porous heat transfer model and the corresponding porous experimental system are proposed to build the correlation of thermal dispersion. The correlation of thermal dispersion is obtained from the fitting process between the corresponding model and experiment. The maximum geothermal extraction is found based on the optimal process and porous heat transfer model considered thermal dispersion. The optimal operated pressure and mass flow rate based on supercritical CO2 flow are obtained. These results will result in the range of the realistic operation and reduce the cost of process in in-situ.

Introduction

The efficiency of geothermal system is an important issue for increasing the role of geothermal energy in the major renewable energy. Nowadays, there is no effective method to evaluate the efficiency of geothermal power plant but capacity test. This is due to the poor-known thermal-fluid information about the reservoir. Geothermal power is a kind of renewable energy, which exhausts less CO2 emission [1]. Owing to this reason, the researches of geothermal energy have prospered throughout recent years aiming to obtain the better low carbon power generation. The studies for comparison among kinds of energy are flushing in recent years. Li et al. [2] compare the geothermal energy with solar and wind power systems to prove the value of geothermal power.

The deep geothermal power technology common with enhanced geothermal system and closed-loop heat collection system (CEEG) are proposed to increase the efficiency of geothermal system. The principle of EGS is to inject the high pressure water for generating the multiple artificial fractures in the reservoir. Then, the working fluid flows through the reservoir to extract the heat and flows out through production well. Water is first applied on the EGS as the working fluid for its high latent heat, specific enthalpy and cheaper cost. However, there are also numerous of disadvantages that needs to be overcome, the main ones are mineralization, higher viscosity and density. These will result in the pipeline fouling and more pumping power. The preview study is to review 18 significant enhanced geothermal system (EGS) sites and technologies that have been applied in the European Union, Japan, South Korea, Australia and the USA by Lu [3]. A 3D thermal-hydraulic-mechanical coupling model with fractured media is proposed to obtain the geothermal heat production from EGS [4]. Asai et al. [5] propose that an efficient system to extract heat from an enhanced geothermal system requires proper understanding of the behavior of the reservoir over a long period and considers the key parameters such as well spacing, fracture spacing, well inclination angle, injection temperature and injection rate. In this view, this study proposes a thermal resistance concept to find the optimal operated parameters for extracting heat from EGS.

CO2 is a possible working fluid for heat extraction of EGS. CO2-EGS is first proposed by Brown [6]; which leads the research of EGS proliferated recently. There are many benefits in the geothermal system, such as lower supercritical temperature and lower density of carbon dioxide, it results in thermal siphon for lower pumping power and higher efficiency of heat extraction. For the special chemical properties of CO2, a lot of topics are discussed for well-understanding of CO2-EGS. Zhang et al. [7] compared the thermodynamic performance between CO2-EGS and water-EGS systems. The sequestration of CO2 is discussed by Blum et al. [8]; and Fouillac et al. [9]. Rosenbauer et al. [10] present the CO2 sequestration in deep-saline aquifers in advance. Ueda et al. [11] and Wan et al. [12] discuss the interaction of fluid and rock for more detailed fluid phenomena. Pruess and Azaroual [13] publishes a series of studies of CO2-EGS, such as heat transmission, sequestration of carbon [14], and production behavior [15]. Several issues have been discussed to understand the availability of the CO2-EGS, including the CO2 mineralization, such as CO2 injection in granite and sandstone [16], CO2 flow in low permeability reservoirs [17]. Experiment of extracting heat of CO2 is studied such as the CO2 flow in the vertical tube [18], and miniature tube [19], convective heat in channel [20]. Pruess [21] build the numerical model TOUGH for the multiphase flow in permeable media in 2004 [22]. In addition, Xu follows the Pruess’s research to develop the advanced TOUGH and processed a series of numerical modeling about fluid-rock interaction [23], the effects of pH solution [24] and brine [25]. Spycher and Pruess (2010) discussed the effects of CO2-brine mixtures by TOUGH in advance [26]. Fard et al. [27] observed that the pressure drop and heat transfer performance of CO2 geothermal siphon can be superior to those of water-based systems. It can demonstrate that the performance of CO2 is better than water in low permeability reservoirs. A two-dimensional numerical model through the straight granite fracture was developed by Bai et al. [28] to investigate the flow and heat transfer characteristics of supercritical carbon dioxide. A THM model of CO2-EGS is proposed by Wang et al. [29]. A virtual full-scale CO2-EGS model is assumed based on the Darcy model. Samin et al. Samin et al. [30] propose an optimization approach to improve long-term performance of EGS. The finite element method (COMSOL) connects with the optimal tool of Matlab is used. It focuses the simulation of reservoir conditions but water. In the full-scale EGS, the fractural network is usually used for understanding the heat extraction but detailed phenomena of heat and mass transfer. Qu et al. [31] discusses the full-scale CO2-EGS based on the Darcy model in COMSOL and empirical equation of CO2 property with different fractural network. Guo et al. [32] compare the performance of water-EGS and CO2-EGS based on the similar method of Qu et al. [31]. Shi et al. [33,34] apply the fractured network in the study of multilateral well EGS and discuss the complex hydraulic nature fracture in advance (2019). The THM model with fractural network is also presented by the team of Ma and Wang, such as Wang et al. [35] consider the carbon sequestration in EGS with complex discrete fracture networks and the similar work in CO2-EGS with the complex fractural networks is presented by Chen et al. [36].

However, the above approaches have some limits in this application, for example, the absence of experimental system for thermal dispersion consideration in the reservoir. Lots of researches attend the CO2-EGS but few experiment of reservoir based on the optimal heat extraction. The reservoir of Brinkman model conjugated with heat transfer model is proposed by our team and fitted the thermal dispersion by experiment in two kinds of mass flow rate under various pressure [37]. Based on the fitted reservoir model, the optimal results by simplified conjugated gradient method build in package are presented. This study presented the optimal heat extraction with different pressure and mass flow rate under empty tube, and two kinds of porosity.

Section snippets

Methodology

This study will build the experiment and simulation of the supercritical CO2 system. The thermal dispersivity of this numerical model of supercritical CO2 on the reservoir will be obtain from the fitting of the experimental results. The different operated conditions of geothermal energy are studied such as pressure, mass flow rate and porosity. In addition, the optimization of this model will process to obtain the optimal operated condition of EGS.

Optimal results and discussion

The optimization of heat extraction under the varied pressure and mass flow rate can be obtained as the fitted simulated model is built.

The numerical design approach is developed by combining a direct problem solver, COMSOL code, with an optimal method (the simplified conjugate gradient method, SCGM). The COMSOL package is used as the subroutine to solve the temperature profiles associated with the variation of the operated conditions during the iterative optimal process. The SCGM method is

Conclusion

This study aims to find the optimal geothermal extraction of supercritical CO2 in the geothermal reservoir. Through the optimization, it is shown that the suitable operating pressure is necessary to obtain for the highest capability of heat extraction. It can be observed that the best heat extraction is close 9–10 MPa. The extreme variation is observed from 7 MPa to 9 MPa and 12.5 MPa decreases to 10 MPa. It results from the complex thermal characteristics of supercritical CO2. Therefore, the

Acknowledgments

The financial support provided to this study by the Ministry of Science and Technology of the Republic of China under Contract No. MOST 106-3113-M-024 -001 is gratefully acknowledged.

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