Using fuel cells in... renewable energy systems Background Generating renewable electricity is an important way to reduce carbon dioxide (CO2) emissions and many countries are installing wind and solar power plants to help meet targets for cutting CO2. One drawback of these energy sources is their variability: the wind tends to blow intermittently and solar power is only available during the daytime. Hence renewable power plants either have to be over-engineered to take account of this lower capacity factor, or they must be supported by spinning reserve power stations, typically fast-response open-cycle gas turbines – which goes against the environmental aims of the projects. Ideally, excess renewable energy generated during times of plenty can be stored for use during periods when sufficient electricity is not available. But storing this energy is a difficult task: batteries and similar technologies perform well over short timescales, but over periods of weeks or months a different approach is necessary. Energy storage in the form of hydrogen is one such possibility: excess electricity is fed into an electrolyser to split water into its constituent parts, oxygen and hydrogen. The hydrogen is then used in fuel cells to produce electricity when needed, releasing the stored energy back to the grid.
KEY BENEFITS This process allows excess energy produced in wind farms and solar power plants to be stored and used, instead of wasted. Increasing the utilisation of renewable power plants helps to maximise the return on investment and lower the cost of electricity. The need for spinning reserve is also reduced as these facilities now have stored energy which can be readily converted back to electricity when required. Hydrogen can also be produced in a number of ways from biomass, allowing for the integration of this energy source in a complete renewable energy system. The most efficient way to convert hydrogen back to electricity is via fuel cells. This is not confined to grid electricity: in certain cases the stored hydrogen can be diverted for sale as fuel to fuel cell electric vehicle owners.
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Technology Used In This Application Today Fuel cells and electrolysers are complementary technologies. An electrolyser cell is much like a fuel cell run in reverse, using electricity instead of producing it. Commercial electrolyser technology is widely available and includes both proton exchange membrane and alkaline electrolysers. As pure hydrogen is the fuel produced in this scenario, any type of fuel cell can be used to convert this into electricity in stationary power generation. For fuel cell vehicles, the technology of choice is proton exchange membrane fuel cells (PEMFC).
Wind-to-Hydrogen Hybrid Power Plant Berlin Brandenberg Airport, scheduled to open in late summer 2012, is to run a fleet of fuel cell vehicles using hydrogen produced in a nearby hybrid power plant. This plant converts excess wind energy into hydrogen and stores it for later use.
Solar-to-Hydrogen MYRTE (a French acronym for Renewable Hydrogen Mission for Integration into the Electric Grid) is located on the French island of Corsica, in the Mediterranean Sea, and combines solar power with electrolysers, hydrogen storage and fuel cells. The project is a partnership between CEA (the French Nuclear and Alternative Energies Commission), energy company AREVA and the University of Corsica. MYRTE contains a 560 kW photovoltaic power plant which has been connected to the Corsican electricity grid since December 2011. The system can provide electricity during the day but, using the electrolysers and AREVA’s hydrogen energy storage system, excess electricity can be stored and returned when required, for example during the night, using fuel cells. The aim of this initial project is to prove the concept, but there are plans to develop the system further with a second phase planned for 2013. This will see the inclusion of AREVA’s Greenergy Box, an integrated hydrogen energy system housing electrolysers, fuel cells, fuel storage and heat management systems inside a standard container.
The wind–hydrogen hybrid power plant, located in Prenzlau, Germany and opened on 25 October 2011, is primarily a 6 MW wind power plant. ENERTRAG AG has partnered with DB Energie GmbH, Vattenfall and TOTAL Germany on the project with DB Energie seeking to use the electricity to contribute to decarbonising the railway network. The rail network needs a predictable and reliable source of power and it is encouraging to see DB Energie recognising the solution a combination of hydrogen and renewables can offer. During periods of excess wind, the three wind energy plants (each of 2 MW nominal power) generate carbon dioxide-free hydrogen through a series of electrolysers. This stored energy, effectively acting as renewable base-load energy, can be used in a number of ways: • The hydrogen can be mixed with biogas and fed into cogeneration plants which produce electricity and heat. The electricity can then be fed back into the grid at times when little or no wind is available; the heat is fed into a district heating network, increasing the overall efficiency of the hybrid power plant. • The hydrogen is also used as a fuel by TOTAL hydrogen refuelling stations in Berlin and Hamburg which support fleets of fuel cell vehicles.
Images: Corsica (University of Illinois); Angela Merkel lays the first stone at the Hybrid Power Plant (Bundesregierung); representation of Berlin Brandenburg airport
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