Introduction to the special issue on geothermal energy

Global energy systems are transforming to meet the dual imperatives of decarbonization and rising energy demand.Geothermal energy is uniquely positioned within this transition as a renewable, dispatchable, and low-carbon resource capable of supplying baseload electricity and direct-use heat.Its high-capacity factor and minimal greenhouse gas emissions make it a strong complement or replacement to intermittent renewables such as wind and solar (Tester et al., 2006).Policymakers and investors are recognizing the value of geothermal energy, which has instigated renewed global exploration and development interest and investment (International Energy Agency, 2021; US Department of Energy, 2023).Worldwide installed geothermal electricity capacity reached approximately 16 GWe in 2023, with major producers including the United States, Indonesia, the Philippines, Turkey, New Zealand, and Kenya (International Renewable Energy Agency, 2024).Installed geothermal electricity capacity in the United States was 3.9 GWe, which represents ?0.4% of total national electricity generation, with all that generation centered in a few states in the Western United States (Robertson-Tait et al., 2023).Direct-use geothermal, spanning applications such district heating, greenhouse agriculture, aquaculture, and industrial processes, accounts for more than 100 GWth globally, with particularly high adoption in China, Turkey, Iceland, and parts of Europe (Lund and Boyd, 2016).Despite this progress, global geothermal potential remains vastly underutilized: currently tapped hydrothermal resources represent only a small fraction of the viable heat in the upper crust (Tester et al., 2006;Augustine, 2016).Next-generation geothermal technologies aim to overcome the geographic and geological limitations of conventional hydrothermal systems.Enhanced geothermal systems (EGS), closedloop heat exchangers, superhot rock development, and subsurface thermal energy storage are extending potential deployment beyond tectonically active areas (US Department of Energy, 2022).These approaches seek to engineer permeability or utilize conductive heat in deep, hot crystalline rock or sedimentary