Developing kilometers-long gravity heat pipe for geothermal energy exploitation

Abstract

At medium-deep and deep parts of the Earth's crust lay vast and so far mostly untapped reserves of geothermal energy. This energy source has the potential to meet humanity's needs for energy for thousands of years. However, the large-scale deployment has been hindered by a lack of effective and dependable exploitation technologies. Gravity heat pipes have recently garnered significant attention due to their exceptional heat transfer capability and their ability to harness geothermal heat without water-mining. Nevertheless, it is a persistent challenge to extend gravity heat pipes to depths that are sufficient to exploit deep geothermal energy. In this study, we introduce an innovative ladder-structure inner pipe design to overcome this shortcoming. This design incorporates a series of stepped ladders and small condensate guiding pipes, allowing for the development of super-long gravity heat pipes (SLGHPs). Combined with an optimal working fluid, this novel structure enables kilometers-long gravity heat pipes. As a direct outcome of this innovation, a 4149-meter-long SLGHP was constructed and installed in a geothermal well using ammonia as the working fluid. The SLGHP geothermal system demonstrated the ability of continuous heat output exceeding 1 MW, with a heat flux across the radial section of the SLGHP reaching 4 × 10⁷ W m⁻². Furthermore, an ammonia vapor-driven power generator was developed and integrated with the SLGHP system. During a 72-hour test, this generator successfully produced electricity at a steady rate of approximately 7 kW. The breakthrough design of the SLGHPs holds the potential to unlock Earth's deep geothermal energy reserves, providing a sustainable and reliable energy source for generations to come.