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Hydrogen is the most abundant element in the universe. However, naturally occurring atomic hydrogen is rare on earth since it readily combines with other elements to form molecules such as water, methane (natural gas) and methanol. Hydrogen is “produced” by breaking the chemical bonds in the molecules that form these substances. Today, most hydrogen is made from natural gas, some from electrolysis of water and some from bio-methane. Since Hydrogen can be made from many different sources, every region of the world has the potential to produce its own fuel, which ultimately benefits the environment and the local economy.
Historically, NASA has been the primary user of hydrogen resources for its space program—it fueled the shuttles using liquid hydrogen and employs backup hydrogen fuel cells for electricity. In recent years, the focus has turned to Fuel Cell Electric Vehicles (FCEVs), which have lower green house gas emissions than their gasoline counterparts. Hydrogen also has the potential to be used as stationary power (for buildings), backup power, storage of energy harvested through wind and solar processes, and as battery-like portable power (most commonly used in forklifts today).
Hydrogen can be produced from diverse, domestic resources. Currently, most hydrogen is produced from fossil fuels, specifically natural gas. Electricity—from the grid or even from renewable sources such as wind, solar, geothermal, or biomass—is also currently used to produce hydrogen. In the long term, solar energy and biomass can be used to directly generate hydrogen.
Fossil fuels can be reformed to release the hydrogen from their hydrocarbon molecules and are the source of most of the hydrogen currently produced in the United States. Combining these processes with carbon capture, utilization, and storage will reduce carbon dioxide emissions in the future. Natural gas reforming is an advanced and mature hydrogen production process that builds upon the existing natural gas infrastructure. Today, 95% of the hydrogen produced in the United States is made by natural gas reforming in large centralized plants. This is an important pathway for short-term hydrogen production.
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Sunlight can directly, or indirectly, used to provide the energy to produce hydrogen. This resource is abundant, but it is diffuse and only available for a portion of the day.
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Biomass is an abundant renewable resource that can be produced domestically, and it can be converted to hydrogen and other products through a number of methods. Because growing biomass removes carbon dioxide from the atmosphere, the net carbon emissions of these methods can be small.
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Wind is an abundant, but variable, resource used to generate renewable electricity. Wind-generated electricity can power water electrolysis which produces hydrogen, which can then be used to fuel vehicles or stored and then used in fuel cells to generate electricity during times of the day when the wind isn't providing much power.
Electricity can be used to split water into hydrogen and oxygen through electrolysis. This technology is well developed and available commercially. Systems that can efficiently use renewable power for this process—for example, wind, geothermal, or solar—are being developed.
Power to Gas
Power-to-Gas is an innovative energy conversion and storage solution that uses electrolysis. It integrates renewable sources of power generation, converts surplus electricity into hydrogen or renewable gas, and leverages upon the attributes of the existing natural gas infrastructure.
Power-to-Gas is a highly effective way of integrating renewable energy. It can provide a rapid, dynamic response to the Independent Grid Operator’s signal to adjust to the variations in renewable generation output. The siting of a Power-to-Gas facility is not restricted to any geologic formation as it can be deployed wherever the power and gas grids intersect. It is a scalable technology.
Power-to-Gas provides an unparalleled energy storage capacity in the TWh range—seasonal storage capability. It can charge for several days, or even consecutive weeks, without needing to discharge the stored energy.
Unlike other energy storage technologies, Power-to-Gas provides the means to both store and transport energy. By storing hydrogen or substitute natural gas in the existing natural gas pipeline network and associated underground storage facilities, the stored energy can be discharged where and when it is needed most. This results in a higher overall integrated system efficiency.
Hydrogen can be stored in a variety of ways, but for hydrogen to be a competitive fuel for vehicles, the hydrogen vehicle must be able to travel a comparable distance to conventional hydrocarbon-fueled vehicles.
Hydrogen can be physically stored as either a gas or a liquid. Storage as a gas typically requires high-pressure tanks (5000–10,000 psi tank pressure). Storage of hydrogen as a liquid requires cryogenic temperatures because the boiling point of hydrogen at atmospheric pressure is -252.8°C.
Hydrogen can also be stored on the surfaces of solids (by adsorption) or within solids (by absorption). In adsorption, hydrogen is attached to the surface of a material either as hydrogen molecules or as hydrogen atoms. In absorption, hydrogen is dissociated into H-atoms, and then the hydrogen atoms are incorporated into the solid lattice framework.
Hydrogen storage in solids may make it possible to store larger quantities of hydrogen in smaller volumes at low pressure and at temperatures close to room temperature. It is also possible to achieve volumetric storage densities greater than liquid hydrogen because the hydrogen molecule is dissociated into atomic hydrogen within the metal hydride lattice structure.
Finally, hydrogen can be stored through the reaction of hydrogen-containing materials with water (or other compounds such as alcohols). In this case, the hydrogen is effectively stored in both the material and in the water. The term "chemical hydrogen storage" or chemical hydrides are used to describe this form of hydrogen storage. It is also possible to store hydrogen in the chemical structures of liquids and solids.
Current on-board hydrogen storage approaches involve compressed hydrogen gas tanks, liquid hydrogen tanks, cryogenic compressed hydrogen, metal hydrides, high-surface-area adsorbents, and chemical hydrogen storage materials. Storage as a gas or liquid or storage in metal hydrides or high-surface-area adsorbents constitute "reversible" on-board hydrogen storage systems because hydrogen regeneration or refill can take place on-board the vehicle. For chemical hydrogen storage approaches, hydrogen regeneration is not possible on-board the vehicle; and thus, these spent materials must be removed from the vehicle and regenerated off-board.
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A viable hydrogen infrastructure requires that hydrogen be able to be delivered from where it's produced to the point of end-use, such as a dispenser at a refueling station or stationary power site. Infrastructure includes the pipelines, trucks, storage facilities, compressors, and dispensers involved in the process of delivering fuel.
Delivery technology for hydrogen infrastructure is currently available commercially, and several U.S. companies deliver bulk hydrogen today. Some of the infrastructure is already in place because hydrogen has long been used in industrial applications, but it is not sufficient to support widespread consumer use of hydrogen as an energy carrier.
Hydrogen is not just the smallest element on earth, it's also the lightest—as a point of comparison, the mass one gallon of gasoline is approximately 2.75 kg where one gallon of hydrogen has a mass of 0.00075 kg (at 1 atm pressure and 0°C). In order to transport large amounts of hydrogen it must be either pressurized and delivered as a compressed gas, or liquefied.
Where the hydrogen is produced can have a big impact on the cost and best method of delivery. For example, a large, centrally located hydrogen production facility can produce hydrogen at a lower cost because it is producing more, but it costs more to deliver the hydrogen because the points of use will be further away. In comparison, distributed production facilities produce hydrogen on site so delivery costs are relatively low, but the cost to produce the hydrogen is likely to be higher because production volumes are smaller.
Today, hydrogen is transported from the point of production to the point of use via pipeline, over the road in cryogenic liquid tanker trucks or gaseous tube trailers, or by rail or barge. Hydrogen used in portable or stationary applications can be delivered by truck to a storage facility or in cylinders, similar to the propane used for gas grills, or in cartridges that would resemble a battery. Hydrogen used in FCEVs is dispensed very much the way gasoline is. Drivers pull into a filling station, connect the dispenser to the vehicle, fill, disconnect, pay, and then drive away with a full tank. Refueling an FCEV takes approximately the same amount of time as refueling a gasoline powered car—3–5 minutes.
Hydrogen delivery and storage technologies: