In Germany two demonstration power-to-gas (P2G) plants designed to store excess electricity generated by renewable sources have begun operation. The amount of electricity generated each year by renewables is rising, but the intermittency of some of these sources, such as wind and solar, poses challenges for the grid. Banking excess electricity to feed into the grid at a future point, when it is needed, can be achieved using various storage technologies such as batteries. However, P2G plants open up the possibility of using this excess energy in different ways. In Germany, which has the largest installed capacity of wind and solar, several demonstration P2G plants are being evaluated for their smart grid potential.
P2G plants use electrolysis to split water into hydrogen and oxygen using electrical energy. In January 2013 German utility RWE Power began testing a proton exchange membrane (PEM) electrolyser for the storage of renewable electricity in a facility at its coal innovation centre in Niederaussem, Germany. The electrolyser has a nominal capacity of 100 kW. But it also has a peak capacity of 300 kW for overloading, for limited periods of time, to absorb the fluctuations of renewable energy plants that can go from producing very little or no electricity at all, to ramping acutely.
Siemens product manager within the company’s hydrogen solutions business Andreas Reiner explains: ‘The PEM electrolyser manages to be both secure but also flexible, which is important when intermittent renewable energy sources are plugged into the system.’
The PEM separates the areas in which oxygen and hydrogen emerge. At the front and back of the membrane metal electrodes are connected to the positive and negative poles of the voltage source. The membrane is made from a polymer foil able to provide ionic conductivity while keeping the oxygen and hydrogen gases separate.
Fast response times, in milliseconds, are achieved by combining the properties of the PEM electrolyser with Siemens’ industrial control technology. The system will be tested from January to October 2013. The PEM module will be evaluated for its ability to function as the amount of power is ramped up and at partial load, to see the effect of frequent load changes on the functioning of the electrolysis system and on the quality of the hydrogen obtained.
Reiner says: ‘The project has already carried out lab tests of the PEM system using a real wind profile. But, at RWE’s Niederaussem facility it will be demonstrated in a real operating environment. Over the next several months the whole system will also be tested to see how it performs in real working conditions.’
250 kW demonstrator
Compared with PEM systems, pressurised alkaline electrolysers represent a very mature technology that is the current standard for large-scale electrolysis. It is this technology that is the core of a P2G demonstration plant that launched in December 2012. The 250 kW plant has been developed by German Center for Solar Energy and Hydrogen Research (ZSW) with partners Fraunhofer IWES and Solarfuel, which intends to commercialise the technology. It expands upon an earlier smaller 25 kW system.
The plant is designed to respond to the fluctuating and intermittent load profiles of wind and solar using pressurised alkaline electrolysis, able to produce hydrogen up to 11bar. The advantage is that it uses a commercially available and proven technology. The plant’s performance will be evaluated during 2013.
Applications for hydrogen
In the next three to five years, P2G plants, based on ZSW’s technology, will be scaled up from the 2-20 MW range. Solarfuel is already constructing a 6 MW power-to-gas plant for automaker Audi in Werlte, Lower Saxony. The knowledge gained from ZSW’s 250 kW research plant will be incorporated into Audi’s facility, which should be operational later this year. Power from four 3.6 MW offshore wind turbines will be used to produce fuel for 1,500 turbo-compressed natural gas (TCNG) Audi A3 vehicles for a year. Audi plans to begin serial production in 2014.
The gas grid could provide storage applications for solar and wind power. Once excess electricity generated by renewables is turned to hydrogen via electrolysis, it can then be converted into methane gas with carbon dioxide. This synthetic gas can be fed into the gas grid, whereas only a small amount of hydrogen – up to 3% – can be fed into the grid infrastructure due to gas regulations.
The PEM demonstrator supplied by Siemens for RWE’s facility is part of the €18 million CO2RRECT (CO2-Reaction using Regenerative Energies and Catalytic Technologies) project, which is supported by Germany’s Federal Ministry of Education and Research (BMBF).
CO2RRECT is investigating different ways that hydrogen can be deployed. For instance, some of it can be used with carbon dioxide from coal plants flue gas to produce methane, in the adjacent catalyst facility. Hydrogen can also be stored in the form of natural gas and, when required, turned into electricity or made available to the heating market.
Alternatively, hydrogen could be used for making further materials, such as methanol, for the production of chemicals. Together with carbon dioxide, hydrogen can be converted into chemical intermediates such as formic acid or carbon monoxide. From carbon monoxide it is possible to produce isocyanate, a building block in the production of polyurethane, a widely manufactured plastic. To establish how carbon dioxide, a waste greenhouse gas, could, in future, be converted into a raw material for chemicals production is part of wider R&D efforts by Bayer and its partners. The CO2RRECT project enables Siemens to demonstrate the potential of PEM electrolyser technology in a practical application.
PEM v alkaline
Despite it being a less mature technology, there are several benefits of PEM technology over classical alkaline electrolyser devices. These include the absence of corrosive electrolytes, good chemical and mechanical stability, high protonic conductivity and high gas-impermeability. PEM electrolysers achieve excellent gas separation for high quality hydrogen production, high current density at higher efficiency. The reduced number of moving parts in PEM electrolyser devices allows for easier maintenance. PEM systems can also achieve an excellent partial-load range and respond rapidly to fluctuating power inputs.
In countries that are banking on renewables, especially wind, for large-scale electricity generation P2G plants could be an important future storage asset. The technology also benefits Germany because it has extensive natural gas storage reservoirs. E.ON is among the first utilities to invest in a pilot-scale P2G plant for a renewables application. Last year the company chose Hydronics, a global supplier of hydrogen generation equipment, to build a 2 MW facility in Falkenhagen, which will use its HyStat alkaline electrolyser.
The plant will bank excess power that is generated by wind farms, producing about 360m³ of hydrogen an hour. The hydrogen will be fed into the natural gas pipeline at around 2% by volume, at a maximum operating pressure of 55bar, effectively storing and transporting surplus renewable energy.
‘THE PROJECT HAS ALREADY CARRIED OUT LAB TESTS OF THE PEM SYSTEM USING A REAL WIND PROFILE. BUT, AT RWE’S NIEDERAUSSEM FACILITY IT WILL BE DEMONSTRATED IN A REAL OPERATING ENVIRONMENT.’
The pilot includes the engineering, construction, commissioning and start-up of a containerised 2 MW electrolyser and compression plant. In addition the project will provide a power substation, metering station, hydrogen pipeline and natural gas grid access station. AEG Power Solutions is supplying rectifiers for the plant.
German independent power producer (IPP) Enertrag is also a P2G pioneer, having partnered with Swedish utility Vattenfall, Total and Deutsche Bahn on a 6 MW hybrid power station in Prenzlau, Germany. After converting excess wind energy to hydrogen, the plant uses the hydrogen and biogas to generate heat and power. An alkaline electrolyser is used in the plant, which has been operational since 2011.
By the latter part of this decade P2G could start to establish itself as a flexible storage technology in power grids as more electricity is produced from renewable sources. Collaborative efforts by partners within the CO2RRECT project and those undertaken by ZSW, Fraunhofer IWES and Solarfuel are taking this promising technology and adapting it for the demands of renewable generation.
Between them, these initiatives are opening up new opportunities both for mature and new, advanced electrolyser technologies. However, there are still many technical and regulatory challenges involved in the setting up and operation of such storage plants that early adopters, like E.ON, are starting to address.