Analytical study of integrating downhole thermoelectric power generation with a coaxial borehole heat exchanger in geothermal wells.

Geothermal power generation employing Organic Rankine Cycle (ORC) technology is a widely acknowledged and conventional approach for harnessing geothermal energy. In an innovative advancement, we propose a novel design integrating downhole thermoelectric power generation with a coaxial borehole heat exchanger. This design aims to enhance the efficiency and sustainability of geothermal energy utilization. In this innovative design, the geothermal well is divided into two distinct sections: a power generation section and a heat exchanging section, achieved through the implementation of a packer positioned from the uppermost part of the targeted zone. The process involves the injection of cold fluid downhole via an insulated pipe. Subsequently, a portion of the injected fluid is directed to flow in reverse within the casing-tubing annulus above the packer, while another portion circulates into the casing-tubing annulus below the packer before ascending through the tubing. This dual flow mechanism establishes distinct cold and hot sources for the thermoelectric generator, a key feature facilitated by this innovative design. Analytical models detailing of downhole temperature distribution for thermoelectric power have been meticulously developed. A comprehensive case study, focusing on a geothermal well with 3000 m length of power generation section and 500 m heat exchanging section, has been conducted. The results indicate that a significant generating capacity could be achieved with a higher wellhead temperature, and the payback period under different carbon tax scenarios is about 6-8 year. Furthermore, the effects of injection rate, fluid diversion ratio, and casing-tubing configuration on power performance and thermal-electricity efficiency are also discussed. This method not only enables the concurrent harvesting of geothermal energy and power generation but also operates consistently throughout the year. The results thus emphasize the viability and economic feasibility of the proposed approach.