Numerical study and economic analysis of gravity heat pipe hot dry rock geothermal system

doi:10.11949/0438-1157.20200639

Abstract

Abstract:

The use of ultra-long gravity heat pipes to extract thermal energy from hot dry rocks can avoid the problems of working fluid loss, corrosion and scaling, and difficulties in underground communication in enhanced geothermal systems. Moreover, compared with downhole heat exchanger (DHE) geothermal system, the evaporation and condensation process in heat pipe can increase the temperature difference between heat pipe and heat reservoir, probably leading to a much higher heat extraction rate. The present work develops a numerical model that couples the phase transition and flow process in heat pipe with the seepage flow and heat transfer process in fractured reservoir. The operation of a 4500 m heat pipe geothermal system is thus simulated and compared with single-well DHE geothermal system. The influence of nature fluid flow in fractured reservoir is particularly analyzed. Furthermore, the electricity-production cost of heat pipe geothermal system is calculated and compared with that of the DHE geothermal system and the traditional enhanced geothermal system (EGS). The results show that the capacity of heat pipe geothermal power plant is about 240 kW and the cost of electricity-production is 1.124 CNY/（kW·h）, which is almost the same as that for the EGS and much lower than that for the DHE. If abandoned oil and gas wells are used to construct a gravity heat pipe geothermal system, the cost of power generation is only 0.644 CNY/（kW·h）.

Key words: renewable energy, thermodynamics process, numerical study, hot dry rock, gravity heat pipe, electricity cost

摘要：

利用超长重力热管开采干热岩热能可以避免增强型地热系统存在的工质损失、腐蚀结垢和井下连通困难等问题；相比于单井井下换热器系统，热管管内液-气相变使得管壁与热储之间的传热温差增大，使用重力热管开采地热能有望获得更高的单井采热量。本文开发了能对重力热管内部过程与裂隙热储渗流过程进行耦合求解的数值模型，模拟了管长4500 m的重力热管地热系统运行过程，研究了裂隙热储中的流体自然对流对地热系统性能的影响，并与套管式井下换热器地热系统进行了对比研究。进一步分析比较了重力热管、套管井下换热器、增强型地热系统电站的发电成本。研究结果表明，重力热管地热系统单井发电量可达240 kW以上，发电成本约为1.124 CNY/（kW·h），与增强型地热系统电站发电成本接近，优于井下换热器式地热系统；如果利用废弃油气井建设重力热管地热系统，发电成本仅0.644 CNY/（kW·h）。

关键词: 再生能源, 热力学过程, 数值模拟, 干热岩, 重力热管, 成本电价

CLC Number:

HUANG Wenbo, CAO Wenjiong, LI Tingliang, JIANG Fangming. Numerical study and economic analysis of gravity heat pipe hot dry rock geothermal system[J]. CIESC Journal, 2021, 72(3): 1302-1313.

黄文博, 曹文炅, 李庭樑, 蒋方明. 干热岩热能重力热管采热系统数值模拟研究与经济性分析[J]. 化工学报, 2021, 72(3): 1302-1313.

Figures/Tables 9

Fig.1 Schematic diagram of heat pipe (a) and downhole heat exchanger (b) geothermal system

Fig.2 Geometric dimension and computational mesh

Fig.3 Comparison of heat extraction rate (a) and electric power production (b) between heat pipe and DHE geothermal system

Fig.4 Temperature field variation in the heat pipe and DHE geothermal systems with or without a fractured reservoir (axial to radial = 1∶10 for better view)

Fig.5 Seepage flow in the reservoir of heat pipe geothermal system at 30 a into the process

Fig.6 Heat flux through well surface (a) and temperature in well (b) along the well-depth direction for the heat pipe and DHE geothermal systems with a reservoir at 30 a into the process

Fig.7 Influence of reservoir permeability on the heat extraction performance of gravity heat pipe system

Table 1 Economic evaluation of the three geothermal power stations

采热方案	重力热管方案	套管方案	EGS方案
钻井费用/万元	1250.0/0^①	1250.0/0^①	10675.0^[8](钻井费用+压裂费用)
井内改造费用/万元	336.0	264.0
热储激发费用/万元	500.0	0
发电设备费用/万元	485.8	142.6	3000.0
总投资费用/万元	2571.8/1321.8^①	1646.6/406.6^①	13675.0
折现率/%	8	8	8
运行维护费用/（万元/年）	29.0	8.5	179.0
使用寿命/年	50	50	30
发电量/kW	242.9	63.3	1500^[35]
发电成本/(CNY/(kW·h))	1.124/0.644^①	2.580/0.753^①	1.061

Fig.8 Parameter sensitivity analysis of electricity-production cost

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