Storing solar energy as hydrogen: Photovoltaic systems for plants

In Germany, an innovative storage power plant stores the energy produced by photovoltaic systems as hydrogen for seasonal storage, in addition to batteries for daily storage. By employing the PLC-based system, smaller companies can reduce their carbon footprint to zero.

By Heiko Tautor October 9, 2015

Photovoltaic solar power can be used effectively in plants by converting the harnessed energy to hydrogen for long-term, seasonal storage, beyond daily storage provided by batteries. A programmable logic controller (PLC) monitors chemical processes in a electrolyzer and fuel cell used in the power storage system.

A stereotype paints the people from Germany’s far north as moving at a more leisurely pace. When applied to Mossau Energy, a specialist in photovoltaic systems headquartered in the East Frisian city of Aurich, this cliché falls flat. "We’ve always been pioneers," states Helmut Janßen, Dipl.-Ing. The company has only 19 employees, yet they enjoy the luxury of an in-house research and development department. This has led to enormous advances in their systems. "We have developed the first marketable system that can store renewable energy for long periods of time," declares Janßen, who is in charge of its development. Internal control of the system is directed by an input/output (I/O) controller. 

Using solar energy in winter

Perhaps it’s due to the unobstructed vistas that the East Frisians enjoy. In a place where dykes are the only thing obstructing a view of the ocean, ideas for new energy technology have found especially fertile ground.

Company founder, Günter Mossau, brought photovoltaics to East Frisia in the mid-90s. According to Janßen, people sneered at the concept back then. After all, the north is not generally known for an excess of sunlight; wind energy is the more obvious choice. Yet, wind is also important for photovoltaic systems, since the better one is able to cool the temperature-sensitive collectors, the higher the electrical yield.

"You can recognize Mossau roofs from far away," confirmed Janßen. While other photovoltaic companies exploit every square centimeter on the roof for their collectors, the Mossau installers always leave gaps between the individual modules. This allows the East Frisian winds to pass between and around the arrays, cooling them in the process. "Mossau Roofs" yield around 10% more electricity from the sun than comparable installations.

However, it continued to annoy the detail-oriented Günter Mossau that the increases in efficiency were lost, fading away with the setting sun. The technologies for storing the solar energy he could produce were insufficient. At peak times, the photovoltaic arrays and wind farms located in Germany generate more energy than can be consumed. Yet storage facilities for holding this massive amount of electricity as a supply against slack times are basically nonexistent, which means that as soon as the sun sets and the wind dies, conventional power plants have to take over. On the road to renewable energy sources that are clean and independent from fossil fuels, the storage of electricity from volatile sources, like solar and wind, remains a core problem.

"Our idea was to solve the storage problem for home owners and small businesses," explained Janßen. "Our systems are supposed to store the electricity generated by the photovoltaic system in the summer, when it isn’t immediately needed, to make it available in the darker days of winter." 

Splitting water into oxygen and hydrogen

To achieve this, the systems designed by Mossau Energy rely on electrolysis, a method that is well-suited for storing the large amounts of electricity generated by wind and solar for longer periods of time. In this process, electricity is used to split water into its constituent parts, oxygen and hydrogen. The oxygen is released to the environment, while the hydrogen is stored in a tank. Experts call this "power to gas." In the winter, the electrolysis is reversed using fuel cells; the hydrogen and oxygen are fed back together and reacted. This generates electricity with water as a by-product.

Electrolyzers and fuel cells are hardly new. However, combining them into a marketable product that makes renewable energy available year round is novel. It took the company three years before completing a prototype in 2013. Mossau Energy promptly received the "Federal award for outstanding innovation in industry and trade" from the Federal Ministry for Economics and Labor for their system. One year later, they had developed the prototype into a compact, market-ready system.

It is hard to imagine from the outside that the blue control cabinet, around the size of an average adult male, houses this innovative, small storage power plant. Janßen said, "The system stores solar energy according to various priorities." Initially, the current consumption needs are satisfied. If more energy is produced than consumed, then the system fills a short-term buffer based on lithium-ion batteries. These are already sufficient to bridge one to two sunless days. Once the batteries are fully charged, the system converts the additional excess energy into hydrogen. By storing this hydrogen during the sunny months of summer, it is available for a fuel cell to use later. This allows the system owner to "squirrel away" enough green energy for the cold days of winter, just like a hamster stores food for later. 

Brains from Minden

The functional principle of the system may sound simple, but it is actually a very complex process. At every point in time, the system must know how much solar energy is being produced, how much of that energy is being directly consumed or supplied to the national grid, the charging status of the batteries, and the fill level of the hydrogen tank. In addition, the chemical processes in the electrolyzer and in the fuel cell must be constantly monitored.

The central control unit for the system is a PLC with additional I/O modules to record the data and control the currents. In addition, all data are stored on a memory card in the PLC. The fuel cell can be linked to the system’s main controller using a controller area network (CAN). All data important for system monitoring are exchanged using the CAN, including fuel cell data, output currents, voltage, temperature, operating hours, and the amount of energy fed into the grid.

According to Christian Wilken, the system programmer, even though the fuel cell doesn’t need to be controlled, it is highly advantageous that communication in both directions is possible through the CAN. Since the CAN can be used to read and write data, it is also possible to specify some values for the fuel cell as well as to read the data it produces. These can be, for example, performance targets or current limits. "Since the processes that run in the fuel cell are already quite complex, it is good that the CAN bus provides a standardized external interface," explained Wilken. "This provides numerous opportunities to visualize data and react at the correct time if the parameters develop differently than they should." In addition, the CAN interfaces offer the possibility of integrating the additional battery storage, including the battery management system. External communications are planned.

A touchscreen is incorporated into the door of the control cabinet, which can be used to call up current system information. "It took about six months to set up the entire system," said Wilken. Although Wilken had never worked in the IEC 61131-3 programming environment with the PLC, a two-day introduction was sufficient to allow him to set up the system. Questions were answered by the PLC and I/O vendor.

First reference projects realized

One system can cover annual consumption of between 25,000 kW and 50,000 kW hours. For higher consumers, the systems can be duplicated. Mossau Energy has been supplying its own needs with the system for quite some time. Another installation is located at Klar Folien, headquartered in Dernbach in Westerwald, a village in the Rhineland-Palatinate. There are currently plans to market the system globally.

"Small companies can reduce their carbon footprint to zero, completely divorce themselves from fossil fuels supplied via the electrical grid, solve an image problem, or provide proof of sustainability in their practices," said Janßen. Ideas from East Frisia make it possible.

– Heiko Tautor, Wago Kontakttechnik GmbH & Co. KG; contributed and translated by CAN in Automation (CiA). CiA is a CFE Media content partner. Edited by Joy Chang, digital project manager, CFE Media,

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