Geothermal power currently provides only a tiny fraction of the nation’s electricity. But as states ramp up their transitions to renewable electricity, some leaders see a big role for geothermal as a stable, renewable power source. Used in the United States since 1960, geothermal plants pipe steam or hot water from deep wells to power turbines that produce electricity. Harnessing underground heat is more expensive than developing wind or solar energy, but experts say the dependable output from sources like geothermal is critical to shore up the grid at times where the sun isn’t shining or the …