The oncoming cold spell will add to the country's energy shortage, as coal-transport harbors in northern China may freeze.

A cold wave from the northwest is expected to head east and drop temperatures 8-12 degrees Celsius, and possibly even 18 degrees Celsius, the Central Meteorological Observatory said.

"The cold wave will reach Bohai Bay around January 20," said Zhang Mingying, a weather expert at the Beijing Meteorological Bureau. "The temperature there might stay at minus-10 for a long while."About 40 percent of Bohai Bay, mostly in the north part of the bay, is already frozen due to the last cold wave that began at the start of the year. The thickest ice on the bay is 1 meter deep, the worst freezing in 30 years, according to the State Oceanic Administration.

Zhang said that the freezing may extend to the south part of the bay, where there are major coal-transport harbors, such as Qinhuangdao, Tianjin and Tangshan.

A majority of the coal transported from northern China to southern China is transported through these harbors, said Li Chaolin, an energy consultant with Beijing-based Anbound consulting.

About 70 percent of the country's coal is used to generate electricity. Coal shortages have caused blackouts in the middle and southern provinces, with the situation in Chongqing and Hubei Province the most severe.

"Three out of seven days, we had blackouts in the office," said Yuan Ming, an office worker in China's Southwest Chongqing Municipality. "In the other four days, we only had electricity part of the day."To ensure residential power demand, the city has limited electricity supply to factories and companies, especially large consumers of power like chemical and cement plants.

"The wide temperature drop will surely drive the energy use for heating, so the energy supply gap could get wider," Li said.

Many power plants only have coal reserves sufficient for less than a week, and some have only a three-day supply in reserve. In Beijing and the northeastern provinces, natural gas is also running low, Li said.

"Since the first round of the cold wave in November, the natural gas shortage has appeared in Beijing and Jilin," said a manager of a mainland-based subsidiary of Hong Kong gas pipeline operator Towngas, who declined to be named.

Even in Shanxi Province, which has about one fourth of the country's coal reserves, power supply to some factories was limited this month, the Shanghai-based Xinmin Evening News reported.