Carbon negative geothermal: Theoretical efficiency and sequestration potential of geothermal-BECCS energy cycles

Abstract

Geothermal systems are an attractive option for baseload electricity generation with low emissions intensity (average 122 gCO₂/kWh). However, about 70% of geothermal systems are low or medium enthalpy (<160°C), which often renders them uneconomic to develop for electricity production. A solution to increase both power production and utilization efficiency of these systems is hybridization with a biomass fuel source. In this work, we introduce and verify the concept of biomass hybridization combined with in-line dissolution and reinjection of biomass flue CO₂. This subclass of bioenergy and carbon capture and storage (BECCS), termed geothermal-BECCS, has improved power production and negative CO₂ emissions. This dual approach of using geothermal systems for power production and as carbon sinks can be a potential decarbonisation tool in areas with suitable geothermal and bioenergy resources.

Here, we quantify the thermodynamic and sequestration performance of four geothermal-BECCS configurations. Up to 100% of flue gas is dissolved and reinjected with the spent geofluid. Scaled to a 1 kg/s geofluid production rate, flash and binary benchmark plants generated 32 and 43 kWe at efficiencies of 6 and 8%, respectively. In comparison, four geothermal-BECCS designs yielded 64 kWe at 9% efficiency (flash plant), 76 kWe at 9% efficiency (ORC binary plant), 62 kWe at 7% efficiency (compound flash-binary plant), and 589 kWe at 20% efficiency (bioenergy based geothermal-preheat plant). Annual biogenic CO₂ sequestration rates ranged from 217 to 675 tonnes per kg/s with emissions intensities from -131 to -922 gCO₂/kWh. By simultaneously boosting low-emissions energy and sequestering biogenic CO₂, geothermal-BECCS promises to be an essential technology for meeting climate targets.