When coupled with renewable energy sources, water electrolysis produces hydrogen gas without carbon emissions or the need to process feedstocks containing carbon. Electrolyzers can be designed to operate with a variety water inputs (e.g. freshwater, saltwater, wastewater, etc.), or in a closed system mode with water recirculation from fuel cells or combustion processes. The electrolysis and power generation functions can also be combined within a single unit known as a regenerative (hydrogen) fuel cell.
Conversely, hydrogen can be produced from biomass and other feedstocks containing hydrocarbons. Biomass sources ideally provide a sustainable cycle when customized for the application, geographic location, and available resources. Steam methane reformation, coal gasification and other fossil fuel based processes can also be used to generate hydrogen, although the associated carbon byproducts must be dealt with.
Other advanced techniques for hydrogen production have been demonstrated including: radiolysis (from nuclear radiation), photobiological (from algae), photocatalytic (from solar), among others. Most of these technologies are early stage with limited commercial systems in place. However, they have the potential for improving efficiency, flexibility and sustainability in future systems.
Oxygen and Other Beneficial Byproducts
Electrolysis and related processes produce oxygen as a byproduct that can be used within the system application, or sold as a commodity output. Other beneficial byproducts are also produced depending on the water input. Saltwater electrolysis, for example, produces sodium hydroxide and chlorine that have variety of commercial uses. Urine electrolysis produces nitrogen that can be used or sold for plant fertilization.
Byproducts from hydrocarbons depends on the composition of the feedstocks and the processing methods used. In general, these byproducts may be less desirable from a sustainability standpoint due to the remaining carbon content. However, if properly sequestered and rendered into an economically viable form, these methods of hydrogen production can provide a transitional step toward the reduction of undesirable hydrocarbon combustion emissions.
Once the hydrogen is produced, it must be stored until needed. The U.S. Department of Energy uses the taxonomy shown below to categorize hydrogen storage methods as either physical-based or material-based.
Comparison of the density as a function of temperature and pressure for physical-based methods is shown below, and falls into one of three categories:
- Compressed gas is a high pressure, ambient temperature, and moderate density condition that is currently the most common hydrogen energy storage method. Typical storage pressures are 300 to 700 bar; requiring compressors, heat of compression cooling, and high strength storage tanks (e.g. composite overwrap stainless steel at the highest pressures).
- Cryo-compressed is high pressure, high density storage near the normal boiling point temperature of hydrogen (-253 C). Additional capabilities beyond compressed gas systems are required to establish and maintain the low temperature conditions (e.g. a method of cryocooling and high performance insulation).
- Liquid hydrogen is a low pressure, low temperature (-253 C) storage method that eliminates the need for compression, but adds liquefaction. Maintaining the desired thermodynamic conditions requires careful design of tankage and associated components to minimize environmental heat leak through piping and structural penetrations, and high performance insulation.
Material-based hydrogen storage systems, by contrast, use a variety of structural (e.g. nanopores) and chemical (e.g. hydrides) technologies to “trap” hydrogen. Techniques for later recovery of the hydrogen depends on the technology used - adding heat to metal hydrides, for example. Research focused on maximizing the storage capacity of material-based hydrogen storage is extensive and ongoing.
Matt Moran is a Managing Partner at Isotherm Energy and has been developing power, thermal, and fluid systems since 1982. He has a passion for the business and engineering of technology development and its integration into commercial products. Matt was the Sector Manager for Energy and Materials at NASA Glenn Research Center where he worked for over 30 years. He has also co-founded or been a key contributor to five technology based start-ups; and provided R&D and engineering consulting to many industrial, government and research organizations. More about Matt here…