A new study shows that the United States could shutter coal-fired power plants by 2030, maintain a steady power supply—and save billions of dollars. The country has more than enough clean renewable energy capacity to replace the coal-fired power stations. The challenge: How to move electricity generated by the sun or wind over long distances without losing too much of it in the process. The study offers an existing solution: High-voltage direct current can carry power across long distances more efficiently than alternating current, the standard power transmission mode in the US. Utilities could add direct-current infrastructure to alternating-current transmission lines over the next 15 years as part of planned updates and upgrades without breaking the bank, according to the study.