Projektinfo – Detailed information on energy research

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The Power-to-Gas plant was constructed adjacent to the Mainz exhibition grounds. The plant is directly connected to the medium-voltage network of the Mainz municipal utility company and to four neighbouring wind power plants supplying the majority of the electricity.
© Siemens AG, Erich Malter
Sector coupling
Projektinfo 05/2018

Three PEM electrolysers operating in electrolysis hall, capable of producing 1,000 standard cubic metres of hydrogen per hour at a maximum output of 6 MW.
© Siemens AG, Erich Malter

Three direct current stations each supply one PEM electrolyser.
© Siemens AG, Erich Malter
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Mainz Energy Park

Companies and scientists are investigating the storage of fluctuating wind energy in the form of hydrogen on a research plant located in the Mainz-Hechtsheim commercial zone. All key components from generation and storage to utilisation at an industrial scale are being looked at and interlinked with innovative technology components. The PtG plant (Power-to-Gas) at megawatt scale facilitates not only the chemical long-term storage of energy. It furthermore interconnects the electricity, heat and mobility sectors.

Energy transition envisages an enormous expansion in energy generation from wind power and photovoltaics. At the same time, the proportion of electrical energy for heating supply and mobility is climbing. If renewable energy sources increasingly supplant fossil fuels, power storage units will prove a viable element in adapting energy systems to cope with the changing requirements. For the long-term storage of electrical energy in particular, however, no large-scale technical solution has yet been adequately tested or made operationally available at reasonable cost. Hydrogen is a promising chemical energy medium and storage option. A wealth of experience has been attained in using the important base material of the industry and the associated technical components are commercially available. The researchers hope to demonstrate in the project how enhancing local load management can impact positively on grid stability and how the supply of wind power can respond with greater flexibility to demand and local grid congestion.

The Power-to-Gas plant was constructed adjacent to the Mainz exhibition grounds. The plant is directly connected to the medium-voltage network of the Mainz municipal utility company and to four neighbouring wind power plants supplying the majority of the electricity. The hydrogen produced in the energy park can be fed into the natural gas network of the Mainz municipal utility company. The mixture supplies the Mainz-Ebersheim district. Hydrogen is mixed with the natural gas at a rate of up to 10 per cent without any negative impact on the customer. Hydrogen fuel stations and industrial operations can also be supplied. A filling station is used in this respect to allow the filling of trailers in a fully automated process. With a maximum output of 6 MW and a rate of generation of 1,000 standard cubic metres of hydrogen per hour, the plant also constitutes a link between current smallscale (100 kW) and future large-scale (100 MW) electrolysis plants. The plant consequently is currently the largest of its kind. Commercial test operation of the plant followed the research phase from 2017. The energy park has since moved into regular operation.


Megawatt class PEM electrolysis

Three electrolysis units housed in a hall similar in height to a supermarket are the central components of energy conversion. As surplus electricity derived from wind and solar systems is extremely volatile, electrolysers must adapt dynamically to the fluctuating electricity  feed. Alkaline electrolysers tried and tested in largescale hydrogen production are comparatively inert and are suboptimal in their effect in partial and peak load operation. This is why designers opted for systems based on polymer electrolyte membranes (PEM). These pressure electrolysers are suitable for high currentdensities and can respond within seconds to large  fluctuations in electricity production. They require less maintenance, are reliable and do not require chemicals or foreign substances. The produced hydrogen is alreadyhighly pure even without costly post-purification and with the exception of moisture and traces of complementary gas oxygen contains no further impurities. At the outset of the project, plant capacities of PEM  electrolysers usually below 100 kW were typically recorded. They are consequently used where low hydrogen volumes are required or where maximum purity is a consideration. The research program intended, however, to test large and scalable megawatt class PEM electrolyser systems in practical application. The first three samples of a newly developed product series of project partner Siemens were successfully used to this end in the project. Each unit accomplishes 1.3 MW in continuous operation and can cope with load peaks of up to 2 MW. The high output pressure of the gas of 35 bar significantly reduces the outlay associated with redensification.

Water and direct current

PEM electrolysis places considerable demands on the water unit. Tap water used in this regard therefore passes through a four-step cleaning process in the water treatment system. The first step involves removing calcium and magnesium cations in a softening stage. Reverse osmosis then filters out further dissolved salts. Carbonic acid and oxygen are released in the subsequent membrane degasification. Once an electro-deioniser has removed residual salts in the final treatment stage, the result is highly pure water with a conductivity of less than 1 μS/cm. A buffer tank ensures a continual water supply for electrolysis.

The electrolysis process requires direct current of high current intensity as a secondary “raw material”. Three transformer stations connected to the medium-voltage network each supply up to 3,500 A with downstream rectifier. A multi-stage filter circuit system in this regard prevents negative repercussions on the grid. The available output also allows overload stations of electrolysis units to be tested.


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Project management
Stadtwerke Mainz AG

Setup and operation of the plant
Linde AG

Electrolysis units
Siemens AG, Energy

Scientific support
Hochschule RheinMain


Mainz Energy Park
Website of the municipal utilities of Mainz

Energy storage systems
Recent reports on research, development and demonstration of energy storage systems