FRANKFURT (Reuters) - Siemens will on Thursday start an energy project to convert wind power into hydrogen for re-use as a general fuel or in natural gas pipelines.

Siemens' electrolysis plant in Mainz is based on Proton Exchange Membrane (PEM) technology, which allows the capture and storage of electricity into hydrogen.

It said the plant can process up to 6 megawatts of electricity, making it the biggest PEM installation of its kind worldwide and able to supply 2,000 fuel cell cars.

PEM technology, which is capable of responding to fluctuations in power production within just milliseconds, has also been tested successfully by another Siemens partner, utility RWE.

The plant is a collaboration between Siemens, the Mainz energy utility, industrial gases company Linde, and the Rhein-Main University of Applied Sciences.

Energy storage is needed by power grids in Germany as they become increasingly vulnerable to gaps in output, as a result of the closure of nuclear reactors and increasing reliance on intermittent green energy such as wind or solar power.

Linde will be responsible for cleaning, compressing, storing and dispensing the hydrogen. It can be fed to industrial uses, or fill tanker lorries to go to hydrogen-based car filling stations, or into the gas pipeline network.

(Reporting by Vera Eckert, editing by William Hardy)