The need for a complete energy storage solution is becoming more acute where fields of wind turbines are already generating gigawatts of electricity, often with a significant mismatch in grid power demand.
It’s a well-established problem for the industry, and there are a number of energy management and storage systems in the pipeline today, but few offer a complete solution allowing wind and solar energy to be plugged into the grid seamlessly.
Water electrolysis technology is the most flexible and tenable solution to store renewable energy on a large, long-term scale. Using excess renewable electricity the Proton Exchange Membrane (PEM) electrolyzer splits water into its constituent parts, hydrogen and oxygen, that can be stored in common tanks. Hydrogen is a flexible energy medium and the technology to produce the gas is working today. Our challenge is to scale-up hydrogen generators to meet the demand of the growing renewable power industry.
How a proton exchange membrane splits water into its constituent parts.
Hydrogen gas has the largest energy content of any fuel, making it a very good ‘vehicle’ for holding and distributing energy. With the ability to hold 120MJ/kg, a relatively small amount of hydrogen is needed to store significant amounts of energy. The stable chemistry of hydrogen also means you can store energy longer than any other medium.
A mapped comparison of alternative energy storage methods, in terms of capacity against discharge time.
The Choice for Germany’s Growing Infrastructure
Over the last few years, hydrogen is being taken more seriously by the power industry, as a solution to making renewable energy grid ready. One of the biggest benefits of hydrogen energy storage is that it’s scalable. A 2-MW hydrogen electrolyzer is the size of a shipping container and can be easily installed next to a field of wind turbines or a distribution substation. We have our megawatt prototype modules being tested in our manufacturing facility today.
In Germany in particular, hydrogen energy storage is already beginning to be implemented into its growing renewable energy network. Hydrogen is being pumped into its grid, and sub-megawatt sized electrolyzers are already storing excess wind energy as hydrogen gas.
As German Trade and Investment, an arm of the German Federal Ministry for Economic Affairs and Energy said: “Storage technology which allows electricity to be held in reserve in the megawatt range, represents a system solution to the problem of surplus energy reserves. The German government supports green hydrogen and power-to-gas projects through a number of funding instruments, initiatives and programs.”
Tried and Tested American Technology
In the United States, hydrogen storage technology is not being implemented in commercial markets, but increasing research, development and innovation is being done here by U.S. companies and U.S. institutions. Hydrogen energy storage technology has been tested in a simulated grid for years by the U.S. Department of Energy. Developed in a partnership with Xcel Energy, the National Renewable Energy Laboratory's (NREL) Wind-to-Hydrogen Project serves as a working model of such a scenario. Housed at the National Wind Technology Center near Boulder, Colorado, this demonstration project integrates wind turbines and photovoltaic arrays with electrolyzer systems to produce hydrogen.
Kevin Harrison, senior engineer at NREL said: “I think hydrogen as an energy storage medium has many benefits, one being that it offers modularity to expand. You can store hydrogen in large tanks and underground caverns to time-shift energy for days and weeks. It would be difficult to extend batteries, for example, into long-duration, time-shifting markets.”
The ‘Wind2H2’ project uses electricity from wind turbines to produce and store hydrogen at the NREL's national wind technology center in Boulder, CO.
Not Wasting a Watt of Clean, Free Energy
A field of large turbines may produce multi-gigawatt-hours of electricity each week. Utility providers do not want to waste any of this free, clean energy. The initial generators we’ll send into the field will be able to handle multi-megawatt power input instantaneously (currently 2MWs).
We’re working on the trade-off between the device being capable of handling an over-capacity beyond the rated input power (e.g. 2MW), and the subsequent additional cost associated with the balance of plant needed to facilitate an over-capacity capability. Even with an electrolyzer stack capable of operating beyond its rated capacity, the electrolyzer system requires over-sizing of subsystems such as the power supply and thermal management that will add cost and complexity. Today’s discussions with European utility providers are assessing these scale issues.
Making a Large-scale Generator Cost-Effective
PEM electrolyzers are comprised of cell stacks that are made up of a number of cells assembled together. The number of stacks and the number of cells per stack can impact cost and efficiency, and the rate of hydrogen production (i.e., current density) applied to each of the cells within the stack.
Proton Onsite’s M-series generator will be able to produce enough hydrogen to store all the energy produced by a wind turbine in a single day. Shipping will begin in 2015.
The M-Series will be made up of a number of PEM electrolyzer stacks.
Large-scale PEM electrolysis development is leveraging parallel developments in the PEM fuel cell industry, evaluating new membrane technologies that increase conductivity and reduce the thickness. Using new polymer chemistries, we’ve been able to identify a fluorinated membrane material that maximizes conductivity at the desired thickness.
Electrolyzers currently use platinum group metals (PGMs) as catalysts, which are expensive. Future multi-MW electrolyzer designs will incorporate new catalyst materials and advanced electrode structures. Through collaborations with organizations such as 3M and the Brookhaven National Laboratory, we have been working to significantly reduce the amount of precious metal catalyst without sacrificing cell performance. Using nanostructures and core cell catalysts, we have been able to reduce the amount of precious metal in a working catalyst by more than 50 percent.
Thanks to our overall efforts to reduce expensive materials and precious metals in our electrolyzers, we’ve been able to reduce the cost of a single PEM stack by 40 percent over the last five years.
We must continue to advance electrolysis technology to be able to store more energy cost efficiently. The market broadly, and utility providers specifically have set the capital cost of storing a kW of renewable electricity between $1,000-$1,500 per kW. The range is dictated by ancillary equipment that may be needed at a specific installation. As we scale up our electrolysis products and develop more efficient electrolyzers, we’ll achieve the capital cost benchmarks.
Multiple Uses in a Renewable Energy Infrastructure
The hydrogen produced from electrolysis can be easily stored using existing technology, either as a gas under high pressure, a liquid at very low temperature, or adsorbed by or chemically bonded to hydride complexes. Smaller amounts of hydrogen can be stored in above ground tanks or bottles under pressures up to 900 bar. For larger amounts of hydrogen, underground piping systems or even salt caverns with several 100,000 m3 volumes can be used.
The versatility of stored hydrogen gas means it can be placed at the center of new renewable energy infrastructure development. The renewably produced, stored gas (energy), could be used in large-scale fuel cells to produce electricity on-site, or it can utilize existing infrastructure, such as natural gas pipelines or other pipelines.
As Harrison noted: "Hydrogen gas gives you opportunities to do a lot of other things. You can turn it back into electricity for the grid, you can produce ammonia and you can use it to fill fuel cell electric vehicles (as we do here at the NREL). We're even working to demonstrate the process of combining the renewable hydrogen we produce with carbon dioxide to produce synthetic natural gas. This can be piped straight into existing natural gas infrastructure."
Hydrogen gas is a complete storage solution that would allow wind and solar energy to power our lives seamlessly.