Enhancing Geothermal Binary Orc Plants Through Wet-to-Dry Expansion

Abstract

The International Energy Agency estimates that by 2050 geothermal power could provide 15% of global electricity demand, yet at the moment it only meets 1%. Among several challenges, high upfront capital costs limit current implementation and thus technological innovation is necessary to help reduce the cost of electricity. Against this backdrop, this study investigates a new type of binary organic Rankine cycle (ORC) system, referred to as the wet-to-dry cycle, for geothermal applications; this cycle exploits the properties of certain organic fluids, such as siloxanes, to achieve two-phase expansion with high-efficiency turbomachinery. Moving beyond earlier work, this paper presents a practical wet-to-dry cycle that includes an atomisation pump and mixing chamber that should enable control of the two-phase conditions at the inlet to the turbine. Thermodynamic cycle optimisation of this practical wet-to-dry cycle is conducted, and performance is benchmarked against optimised recuperated and dual-pressure ORC systems. The results reveal that under the most promising operating conditions the wet-to-dry cycle can improve power output from a given geothermal reservoir by up to 23% and 15% compared to recuperated and dual-pressure ORC systems. However, this is at the expense of larger heat exchanger UA requirements, which are up to 93% larger in the most extreme case, and a larger turbine. In the final section of this paper, a screening process is adopted to identify natural refrigerants that could be suitable for wet-to-dry expansion. The results identify potential hydrocarbons, that despite having lower molecular complexity than siloxanes, could facilitate wet-to-dry expansion. Ultimately, the results from this study provide the first comprehensive assessment of the wet-to-dry cycle for geothermal applications, demonstrating how power output from a given geothermal reservoir can be increased, and help to direct future research efforts.

Keywords: geothermal power, binary cycles, organic Rankine cycle, ORC, two-phase expansion, wet-to-dry expansion