What good is a PV panel at night? What about windmills when the wind is not blowing? These questions and others like them form the basis for one of the fundamental constraints facing the widespread use of renewable energy power systems: the intermittent nature of renewable energy resources. The solution to this constraint is to store the energy generated by these inexhaustible, yet intermittent, resources. But how?
Many storage media are employed, including batteries, flywheels, ultracapacitors, pumped-storage hydroelectric, and superconducting magnets. These media have had varying degrees of success, but all have significant disadvantages, such as self-discharge, high cost, and the use of toxic materials in their construction. At SERC we have focused our attention on an emerging, better, and more complete energy storage option: hydrogen.
SERC uses the electrolysis of water to store renewable energy resources as hydrogen.
Electrolysis is the use of electrical energy to produce a chemical change. In the renewable hydrogen cycle, electrical energy (from renewable resources) is used to break the bonds between the hydrogen and oxygen in the water, releasing them as elemental gases. Hydrogen is stored renewable energy.
An electrolyzer is a device that facilitates the electrolysis of water to produce hydrogen gas. Electrolyzers most commonly used today generate hydrogen at relatively low pressures (from nearly atmospheric pressure up to 200 pounds per square inch) and use a liquid alkaline electrolyte (KOH or NaOH). At these pressures, storage of large quantities of hydrogen requires extremely large storage vessels. One solution to this problem is to use a compressor to increase the hydrogen pressure. However, the energy investment required to pressurize hydrogen, as well as the maintenance of hydrogen compressors, makes this option infeasible for large-scale application of this technology. Furthermore, the operation of alkaline electrolyzers requires frequent maintenance that includes disposal and replacement of the highly caustic electrolyte.
A proton exchange membrane (PEM) electrolyzer can be designed to electrochemically generate hydrogen at pressures of 2000 psi or greater, thus eliminating the need for mechanical compression. The PEM electrolyzer uses a solid electrolyte membrane that can be expected to last the lifetime of the electrolyzer. No caustic alkaline or acidic fluid electrolyte is required. Additional advantages of PEM electrolysis over alkaline electrolysis include lower parasitic energy losses and higher purity hydrogen output. PEM electrolysis is potentially a simple, sustainable, and cost-effective technology for generating, compressing, and storing hydrogen.
With funding from the California Energy Commission’s Energy Innovations Small Grant (EISG) program, SERC designed, built, and tested a prototype high-pressure proton exchange membrane (PEM) electrolyzer. The electrolyzer successfully operated at pressures of up to 2,000 pounds per square inch. If commercialized, high pressure PEM electrolysis could eliminate the need for energy-intensive mechanical compression of hydrogen and increase the overall efficiency of hydrogen energy systems.
Read the final EISG PEM electrolysis report (PDF; 938K)