Power generation analysis of super-long gravity heat pipe geothermal systems

Abstract

Highly-efficient and environment-benign heat extraction and utilization technologies are key to large-scale geothermal energy deployment. In recent developments, the innovative super-long gravity heat pipe (SLGHP) geothermal power plant has revolutionized deep geothermal power generation by obviating the need of flash devices or heat exchangers in vapor generation for turbine propulsion. This advancement promises substantial structural simplification, a marked reduction in exergy losses, and significant cost savings. Nevertheless, a comprehensive understanding of the performance of SLGHP power generation systems is still lacking in published research. For a single-well geothermal system, the geothermal gradient assumes critical importance as it serves as a key indicator of resource intrinsic conditions, and the depth of the well emerges as an essential controllable variable in manual drilling. The present investigation seeks to elucidate the impact of these two key parameters on the performance of SLGHP power generation systems. For this purpose, a semi-empirical formula is analytically derived to describe the heat extraction process within the SLGHP system. This formula is integrated with a thermo-economic model to evaluate the performance of the SLGHP power generation system. It is found that adjusting the two parameters can yield simultaneous increases in the heat extraction rate and the energy efficiency, thereby resulting in a significant boost in power generation of SLGHP. Specially, the SLGHP system of a 4000 m deep well and with a 0.045 K/m geothermal gradient can output 35.95 kW electricity, while that of a 5000 m deep well and with a 0.05 K/m geothermal gradient achieves 86.39 kW electricity output. In practical terms, it is found that the optimal strategy for an SLGHP power plant involves drilling multiple wells in a reservoir with a higher geothermal gradient. However, when repurposing abandoned oil/gas wells for power generation, the preference is for deep wells.