The Performance of a Carbon-Dioxide Plume Geothermal Energy Storage System

Abstract

CO2-Plume Geothermal (CPG) is a system that can produce electricity from low-temperature heat from the subsurface of the earth, effectively combining geothermal energy and carbon capture and geologic storage; two technologies that have the potential to significantly reduce the amount of CO2 emitted into the atmosphere and limit the impacts of climate change. This system is different from other geothermal concepts as 1) the system uses CO2 as the heat extraction fluid in the subsurface reservoir, 2) the system does not rely shallow-natural hydrothermal locations or engineered (i.e. enhanced or fractured) reservoirs, instead using naturally permeably sedimentary basins, and 3) CPG systems utilize low-temperature resources which are currently undeveloped for geothermal energy. Therefore, CPG has significant potential to expand the geographic region where geothermal energy can operate, while providing an end used for captured CO2. This research demonstrates how the unique properties of the CPG system allow the system to be modified to operate as an energy storage system, which can increase the penetration of variable wind and solar resources on the grid, by using an additional shallow reservoir to separate the components that generate and consume power. To operate, the system generates power by extracting CO2 from the deeper-hotter reservoir and generates power in the turbine before the CO2 is slightly cooled and injected into the shallow reservoir, making use of the thermosiphon effect, where the thermal expansion of CO2 results in a density difference in each vertical well that can circulate CO2 without the need for pumps. To store power, the CO2 can be produced from the shallow reservoir, cooled and compressed, and then reinjected into the deep reservoir where it is heated. This research began by establishing the feasibility of the CPGES cycle for a single reservoir configuration and a mass flow rate near the optimum energy generation condition, demonstrating the effects of the intermittent injection and production of CO2 on the transient reservoir pressures and the power generated and consumed by the system over the first 10 years of operation (Chapter 2). The results demonstrated that the system was at a quasi-steady state condition at 10 years, and that the system could generate more energy to the grid than it consumed, providing both net energy generation of and energy storage. Using historical electrical price data, it was found that the CPGES system could use price arbitrage to be competitive with a CPG system, for the same geothermal heat extraction rate. Work was then expanded to illustrate how the CPGES system can operate over a range of time scales, with the cycle duration ranging from diurnal to seasonal (Chapter 3), and over a range of duty cycles (Chapter 5), demonstrating the versatility of this system. The CPGES system was compared to the CPG system for a range of geologic conditions, and it was determined that the trade-off of the flexible energy storage system was a reduction in the net energy generated per cycle (Chapter 4 & 5). However, these energy losses could be alleviated by operating the CPG and CPGES systems concurrently in the CPG+CPGES system. The addition of the second reservoir required for the energy storage operation increases the capital cost of the system, however, the increased cost of this flexible system could be alleviated by the value that the system adds to the grid as the amount of variable renewable energy increases (Chapter 5). Lastly, the effect of the co-production of water in solution with the CO2 is considered and found to increase the generation capacity of the CPG system, a result of the higher production temperature despite the reduced CO2 mass flow rate (Chapter 6). Overall, this research has demonstrated how the CPG system can be modified to operate as an energy storage system. The impact of this work is to establish the flexibility of the CPG technology and demonstrate that captured carbon can be used to increase the penetration of renewable energy technologies onto the grid, thereby further mitigating the emission of CO2 into the atmosphere. This will enable CPG to be integrated into future renewable energy portfolios.