A compressed air energy storage device, which is able to store electricity and release it when needed, thus improving energy efficiency and reducing waste, is in the final stages of testing before it enters operation in Zhangjiakou, Hebei province.

Once it is in use, which is expected by the end of the year, the device will be the first of its kind with a power generation capacity of up to 100 megawatts, according to the Institute of Engineering Thermophysics at the Chinese Academy of Sciences, which is providing technical support.

It is located in a cloud computing industrial park in Zhangbei county.

"Zhangbei is rich in new energy power generation resources, including wind and solar power, which provides a firm foundation for the project," said Ji Lyu, director of the industrialization office for energy storage at the institute's R&D center.

According to Ji, the device is able to store otherwise unstable electricity with an efficiency rate as high as about 70 percent.

He said the device compresses air using surplus power from the grid generated during off-peak hours and stores the air in a reservoir under high pressure. When power consumption peaks, the air is released to drive generators to create electricity. "The process does not involve chemical reactions and therefore is environmentally friendly."

Like a giant power pack, the device is able to store some 400,000 kilowatt-hours in four to six hours, which is enough to meet the daily needs of between 40,000 and 60,000 households, Ji said.

Zhangjiakou is located about 160 kilometers northwest of Beijing and by the end of last year boasted an installed renewable energy capacity of 23.47 million kilowatts, which accounted for 32.5 percent of its energy consumption, according to a report by Xinhua News Agency.

The city is a national-level renewable energy demonstration zone, and the project will be one of its landmarks.

"Energy storage is key to improving the grid's ability to adjust and improve its overall efficiency," said Chen Haisheng, a researcher at the institute.

Zhangbei Giant Energy Co invested 707 million yuan ($111 million) in the project, which completed construction at the end of last year. The device was connected to the grid earlier this year, and is currently undergoing comprehensive testing.

Testing was finished on the expansion machines, a type of power generation device, last month, and tests on the heat reservoir were completed in June.

"During the 168-hour test period, the reservoir operated smoothly and its performance indicators met our design requirements," said Li Wen, a senior engineer at the institute.