Investigation of geothermal power plant performance using sequestered carbon dioxide as a heat transfer or working fluid.

Abstract

This study investigates the potential for combining carbon dioxide (CO2) sequestration with geothermal power production in areas with low geothermal resource temperatures. Using sequestered CO2 as the working fluid or heat transfer fluid, power production of a Direct Single-Loop System and Binary Organic Rankine Cycle (ORC) System were simulated using Engineering Equation Solver (EES) and the ASHRAE reference state for thermophysical properties. The two power plants were simulated under a variety of operating conditions, with the main variables being the reservoir temperature (100ºC-150ºC), the mass flow rate of the CO2 (70kg/s, 90kg/s, 120kg/s, 140kg/s), the reservoir depth (2.5km, 3.1km, 3.6km), and the condensing temperature. The condensing temperature was taken as the monthly average ambient wet-bulb temperature for the summer months of April through September and dry-bulb temperature for the remaining months of the year at Minneapolis, Minnesota. Results showed that using CO2 for geothermal power production is a viable possibility. Power production from the simulations matched or surpassed the power production of currently installed low-temperature water-based geothermal plants. A second-law analysis showed that for the Direct Single-Loop System the largest considerations needed to be put into turbine design, while for the Binary ORC System the CO2 heat exchanger should receive the most attention. In addition to this, replacing the expansion valve in the Binary ORC System with a supplemental power-generating device, such as a screw or scroll expander, has the potential for additional power production.