| Abstract |
In this work, Direct Air Capture (DAC) of CO2 is coupled directly with a sedimentary geothermal system with the goal of creating a stand-alone, carbon-negative CO2 capture system. An isobutane Organic Rankine Cycle (ORC) is used to generate electricity from a 2.5 km deep, 50 mD porous-media reservoir with a temperature ranging from 90°C to 140°C. The heat required by the DAC is extracted directly from the produced geothermal stream. A 1 km2 inverted 5-spot reservoir configuration is used. Four different system configurations are tested, including different ORC configurations and the use of an external electrical supply. We find that Direct Air Capture and a geothermal cycle can be coupled in a stand-alone power-island to capture up to 0.04 MtCO2 per year with a 140°C reservoir and the given 1 km2 reservoir configuration. When 12 MWe of external power is supplied, possibly from a nearby wind turbine farm or photovoltaic park, the same combined system can capture up to 0.18 MtCO2 per year. If a thirty year system lifetime is assumed, the systems can capture from 1.2 MtCO2 to 5.4 MtCO2 over the system lifetime. Also, as the 5-spot reservoir system is designed to be up-scaled by tiling additional 5-spots together into larger configurations without thermal or pressure interference, we considered coupling a 5 km x 5 km geothermal reservoir with Direct Air CO2 Capture as well. When using this 25 km2 reservoir, the stand-alone power island system captures 1.04 MtCO2 per year while the grid-connected system captures 4.63 MtCO2 per year using 282 MWe of externally supplied electricity. Additionally, we find that there is a temperature optimum for the geothermal resource at which the ORC and steam generator both provide the required proportions of heat and electricity to the DAC. At higher resource temperatures, additional electricity is needed for the DAC, either from a parallel ORC or from an external source. At lower resource temperatures, necessary additional heat for the DAC is provided via an isobutane Heat Pump (HP). With the current assumption of an adsorption-based DAC process whose heat demand is provided through saturated steam at atmospheric pressure, a temperature of 110°C is most ideally suited. Lastly, we find that most resource temperatures have an electric opportunity cost—the forgone electricity per tonne CO2—of approximately 700 kWe-h per tonne CO2. |