RENEWABLE energy is often generated in places far from the cities and industrial centres that consume it. To boost renewables and drive down carbon-dioxide emissions, a way must be found to send energy over long distances efficiently. The technology already exists. Most electricity is transmitted today as alternating current (AC). But transmission over long distances requires very high voltages, which can be tricky for AC systems. Ultra-high-voltage direct-current (UHVDC) connectors are better suited to such spans. They also make the grid more stable by balancing supply. The same UHVDC links that send power from distant hydroelectric plants, say, can be run in reverse when their output is not needed, pumping water back above the turbines.

One country has grasped the potential of these high-capacity links—driven by geography. Three-quarters of China’s coal is in the far north and north-west of the country. Four-fifths of its hydroelectric power is in the south-west. Most of the country’s people, though, are in the east, 2,000km or more from these sources of energy. State Grid, the state-owned electricity utility, is halfway through a plan to spend $88bn on UHVDC lines between 2009 and 2020. The first 800,000-volt line from Xiangjiaba dam, in Yunnan province, to Shanghai was completed in 2010. It has a capacity of 6,400MW, equivalent to the average power consumption of Romania. The largest connector under construction, the Changji-Guquan link, will carry 12,000MW (half the average power use of Spain) over 3,400km, from Xinjiang, in the far north-west, to Anhui province in the east. This journey is so long that it requires 1.1m volts to push the current to its destination. China has its foot on the gas.