Because hydrogen typically does not exist freely in nature and is produced from other sources of energy, it is known as an energy carrier. It is a clean-burning fuel, and when combined with oxygen in a fuel cell, hydrogen produces heat and electricity with only water vapor as a by-product.
Hydrogen can be made directly from fossil fuels or biomass, or it can be produced by passing electricity through water, breaking the water into its constituent components of hydrogen and oxygen. Some envision a future “hydrogen economy,” where hydrogen is produced from a variety of energy sources, stored for later use, piped to where it is needed, and then converted cleanly into heat and electricity.
Most hydrogen production today is by steam reforming natural gas. But natural gas is already a good fuel and one that is rapidly becoming scarcer and more expensive. It is also a fossil fuel, so the carbon dioxide released in the reformation process adds to the greenhouse effect. Hydrogen has very high energy for its weight, but very low energy for its volume, so new technology is needed to store and transport it. And fuel cell technology is still in early development, needing improvements in efficiency and durability.
Electrolysis
Renewable energy sources such as photovoltaics, wind, biomass, hydro, and geothermal can provide clean and sustainable electricity for our nation. However, renewable energy sources are naturally variable, requiring energy storage or a hybrid system to accommodate daily and seasonal changes. One solution is to produce hydrogen through the electrolysis—splitting with an electric current—of water and to use that hydrogen in a fuel cell to produce electricity during times of low power production or peak demand, or to use the hydrogen in fuel cell vehicles.
Researchers at NREL’s Energy Systems Integration Facility and Hydrogen Infrastructure Testing and Research Facility are examining the issues related to using renewable energy sources for producing hydrogen via the electrolysis of water. NREL tests integrated electrolysis systems and investigates design options to lower capital costs and enhance performance.
Photoelectrochemical Water Splitting
The cleanest way to produce hydrogen is by using sunlight to directly split water into hydrogen and oxygen. Multijunction cell technology developed by the photovoltaic industry is being used for photoelectrochemical (PEC) light harvesting systems that generate sufficient voltage to split water and are stable in a water/electrolyte environment. The NREL-developed PEC system produces hydrogen from sunlight without the expense and complication of electrolyzers, at a solar-to-hydrogen conversion efficiency of 12.4% lower heating value using captured light. Research is underway to identify more efficient, lower cost materials and systems that are durable and stable against corrosion in an aqueous environment.
Biological Hydrogen
NREL scientists are developing pretreatment technologies to convert lignocellulosic biomass into sugar-rich feedstocks that can be directly fermented to produce hydrogen, ethanol, and high-value chemicals. Researchers are also working to identify a consortium of Clostridium that can directly ferment hemicellulose to hydrogen. Other research areas involve bio-prospecting efficient cellulolytic microbes, such as Clostridium thermocellum, that can ferment crystalline cellulose directly to hydrogen to lower feedstock costs. Once a model cellulolytic bacterium is identified, its potential for genetic manipulations, including sensitivity to antibiotics and ease of genetic transformation, will be determined. NREL’s future fermentation projects will focus on developing strategies to generate mutants that are blocked selectively from producing waste acids and solvents to maximize hydrogen yield.