Techno-Economic Modeling and Optimization of Flexible Geothermal Operations Coupled with Energy Storage

Abstract

Dispatchable energy resources are key for a reliable power supply. Whereas in the past the development of fossil fuel resources has dominated the supply for dispatchable capacity, this trend is slowing down due to the growing climate change concerns and the transition to renewable resources. Geothermal energy has always been an economic resource for district heating and baseload power. With the decline of fossil fuel plants, geothermal facilities have been expanding beyond baseload to supply flexible heat and electricity. There has been a limited number of studies that investigate the feasibility of flexibly dispatching geothermal power to the wholesale power market in the absence of fixed-price power purchase agreements (PPAs). This study performed techno-economic modeling and optimization of flexible geothermal power dispatch when coupled with thermal energy or Lithium-ion battery storage. We developed a techno-economic model to optimize system design (e.g., power plant, thermal tank, and Lithium-ion battery storage capacities) and dispatch schedule (e.g., turbine bypass, and dis/charging thermal tank and Lithium-ion battery) with the goal of maximizing the project net present value (NPV). Costs included capital and operational expenditure for upstream reservoir development, power plant, and storage facilities, while revenue streams included wholesale power markets, capacity value, and renewable energy certificates (RECs). We posed the problem as a mixed integer, multi-objective setting and solved it using the Reference-Point Based Non-Dominated Sorting Genetic Algorithm (R-NSGA-II) which offers robust means of efficiently finding the global optima. We examined a 220 deg C, liquid-dominated geothermal resource developed with 6 producers and 6 injectors developed with a subcritical Organic Rankine Cycle power plant. We used a conceptual subsurface model to simulate temperature decline over time alongside correlations to estimate power plant efficiency under different operating conditions. Economics was based on 30-year forecasts of the California power markets. Results indicate that participating in the free market with a 66 MW power plant without added storage facilities results in profits equivalent to signing a PPA with 82.4 $/MWh fixed price. While adding thermal storage was found to be suboptimal, Lithium-ion battery storage results in a more profitable system equivalent to signing a PPA with nearly 105 $/MWh. This work shows that flexible geothermal dispatch is techno-economically feasible from the perspective of the developer. Flexible portfolios are also desired by system operators, especially in decarbonized grids. Additionally, our results indicate that geothermal systems could be developed economically for power generation even in the absence of PPAs.