"China will need to add a substantial amount of coal-fired power capacity by 2020 in line with its expanding economy, and the idea is to bring some of the capacity earlier than necessary in order to facilitate the wind-power transmission," said Shi Pengfei, vice president of the Chinese Wind Power Association.

Largely due to its reliance on coal, China is the world's biggest emitter of greenhouse gases in absolute terms. Last year, the country accounted for more than 85% of global growth in coal demand, according to BP PLC's statistical review of world energy.

Facing pressure from abroad over the pace of China's emissions growth, President Hu Jintao used a speech to the United Nations last Tuesday to stress his country's commitment to tackling climate change. He said China will lower energy intensity as the country grows, while raising output of renewable energy and nuclear power. China aims to cut carbon dioxide emissions per unit of gross domestic product by a "notable margin" by 2020, Mr. Hu said, without setting a concrete cap.

The city of Jiuquan, in the flat and arid northwestern province of Gansu, shows the complexities that crop up when implementing such plans. The city is meant to showcase the strides China is making in renewable energy. Wind turbines with a combined capacity of 12.7 gigawatts are due to be installed there by 2015—more than the country's present nuclear-power capacity.

But the Jiuquan government wants to build 9.2 gigawatts of new coal-fired generating capacity as well, for use when the winds aren't favorable. That's equivalent to the entire generating capacity of Hungary.

Construction of these thermal power plants is pending approval by Beijing, an official with the Energy Department under the Jiuquan Development and Reform Bureau said Tuesday.

The heavy reliance on coal-fired power plants to add to the power supply from large wind farms in order to meet minimum power demand is essential to grid safety, said Mr. Shi of the Chinese Wind Power Association.

To be sure, any kilowatt hour of wind power consumed by end users ultimately replaces a kilowatt hour of electricity generated by other, possibly dirty, sources such as coal, and the huge power supply expected from the new wind farms represents a major stride in China's clean energy push.

In addition to Jiuquan, there are plans for six other wind farms in China with a capacity of more than 10 gigawatts each, mostly in sparsely populated inland regions such as wind-swept Inner Mongolia and Xinjiang.

Several gigawatts of new thermal power capacity will need to be built at these sites as well, Mr. Shi said.

China has plenty of windswept plains and sun-baked deserts like the Gobi which can host turbines or solar panels, but these are often far from cities and existing infrastructure for shipping power. Sebastian Meyer, director of research and advisory services with clean-energy consultancy Azure International, says China needs a more modern and flexible grid if it wants to raise the share of renewable power in its energy mix.

So-called smart-grid technology aims to modernize the power sector by overlaying digital communications onto the grid, enabling utilities to manage supply more efficiently and compensate for any variance. But while the U.S. and many countries in Europe are lining up spending to exploit the technology, China is lagging behind.