Two pilot ‘solid hydrogen’ power plants were both connected to the grid in southern China on Saturday, allowing variable wind and solar power to be stored in a solid for later use.

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Project developer China Southern Power Grid (CSPG) has not revealed exactly what solid substance is being used to store H2, merely stating that it works by attracting hydrogen “into metal voids through a chemical reaction between hydrogen gas and new alloy materials”.

“This hydrogen energy development project, organised and implemented by China Southern Power Grid Corporation, successfully solved the technical ‘bottleneck’ of storing hydrogen in solid form under normal temperature conditions,” the company says on its website.

Hydrogen is then released by raising the temperature, which can either be used to fuel vehicles or sent through a fuel cell to generate electricity for the grid.

“By integrating hydrogen power production, storage, generation and refueling, the power station can achieve conversion between green electricity and green hydrogen,” said Lei Jinyong, general manager of CSPG’s Hydrogen Energy Research Centre in Guangzhou, China’s fifth most popluous city.

“This can solve the problem of unreliability and seasonal fluctuations in new energy power generation.”

Wang Chengshan, an academic at the Chinese Academy of Engineering, added: “Solid-state hydrogen storage solves the problem of flexible conversion between ‘green electricity’ and ‘green hydrogen’.

“This major transformative technology is expected to become an important driving force supporting the evolution of the power system to an advanced form in the future.”

All the conventional forms of storing H2 come with their own challenges. Compressed hydrogen takes up a lot of space and required strong tanks; liquid H2 must be stored at cryogenic temperatures of minus 253°C and suffers hydrogen losses due to boil-off; ammonia is extremely toxic; methanol needs captured CO2 to be considered “green”; and liquid organic hydrogen carriers require high amounts of heat energy to release their stored H2.

Solid forms of hydrogen storage are said to have terrible round-trip efficiencies, meaning that much of the energy contained in the H2 is lost when being stored and released.

But the Chinese system has been described as “highly efficient and easy to use”.

The Nansha Smart Hydrogen Station in Guangzhou, one of the two new pilot plants, can store 200 cubic metres of hydrogen “with high density”, according to Zheng Xin, a senior CSPG researcher.

“The volume would be 20 times larger if we use the normal 3 MPa [30-bar] hydrogen tanks,” he explained.

A 700-bar hydrogen tank, like those used in fuel-cell vehicles, would be able to contain even more H2 by volume than the solid storage medium, but this would require added compression, which requires additional energy that would therefore reduce the round-trip efficiency.

The other new pilot plant, in the city of Kunming, southwest China, can supply 2.3MWh of electricity during peak hours and send power to the grid constantly for 23 hours.

“In the future, hydrogen power will account for an increasingly larger proportion in the power grid, while the proportion of green hydrogen will go up as well,” said Wang.

“The electro-hydrogen coupling technology will be one of the revolutionary technologies that drives the evolution of power system.”

Pan Fusheng, an academic at the Chinese Academy of Engineering, told a conference last September that magnesium-based alloys “have the highest hydrogen storage density among metal solid-state hydrogen storage materials” and can “fundamentally change the problems of low efficiency [and] high cost”.