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 gCO 2 /kWh). However, about 70% of geothermal systems are low enthalpy (<160°C), rendering them uneconomic to develop for electricity production. A solution for boosting conversion efficiency of these systems is hybridization with biomass fuel. In this work, we introduce and verify the concept of biomass hybridization combined with in-line dissolution and reinjection of biomass flue CO 2 . This subclass of bioenergy and carbon capture and storage (BECCS), termed geothermal-BECCS, has improved power production and negative emissions. This dual approach of using geothermal systems for power production and as carbon sinks has the potential to decarbonize energy systems in areas with suitable geothermal and bioenergy resources.  Here, we quantify the thermodynamic and sequestration performance of five geothermal-BECCS configurations. Up to 100% of flue gas is dissolved and reinjected with the spent geothermal fluid. On a per kg/s mass rate basis, flash and binary benchmark plants generated 13-14 kWe at a conversion efficiency of 2%. In comparison, two novel designs for a compound flash-binary plant and a distilled water Rankine cycle plant yielded, respectively, 80 kWe at 7.6% efficiency and 227 kWe at 14.7% efficiency. Corresponding sequestration rates were 504 and 578 tonnes of CO 2 per year, or emissions intensities of -711 gCO 2 /kWh and -292 gCO 2 /kWh. By simultaneously boosting low-emissions energy and sequestering bio-origin CO 2 , geothermal-BECCS promises to be an essential technology for meeting climate targets.

Keywords: geothermal, hybrid power, Biomass, dissolved CO2, BECCS, forestry waste, reinjection, renewable energy, negative emissions