Clean Energy 101: The Colors of Hydrogen
What is hydrogen and how does it fit into a decarbonized future?
Hydrogen has an essential role to play in the global effort to decarbonize the economy. How big a piece in the puzzle it is and where it best fits are two questions top of mind for business leaders and policymakers. To help inform this discussion, RMI will be publishing a new article series to help dispel misinformation about hydrogen. To start us off, this article explains what hydrogen is and where it can fit in the energy system.
Hydrogen is the most abundant chemical element in the universe. You may remember from grade-school science that it is the first element in the periodic table — the lightest, consisting of one proton and one electron. Hydrogen is highly reactive and a potent energy carrier.
Hydrogen is an essential element for life on earth. It makes up all living things — including plants, animals, and you and me — and is also found in molecules like water (it is the H in H2O) that enable life to exist and in fossil fuels like natural gas and coal. However, a significant amount of energy is needed to break down a molecule to get pure hydrogen, and that energy must be supplied either by heat or electricity.
The Colors of Hydrogen
Hydrogen as a fuel (H2) is popularly classified into different colors depending on the initial molecule being broken down, the energy source used to take hydrogen from it, and the byproducts of the chemical reaction. Regardless of its classification, all hydrogen has the same chemical properties and can be used in the same way.
The most carbon-intensive and darkest-color coded hydrogen pathways involve fossil fuel energy sources, and result in carbon dioxide (CO2) and carbon monoxide (CO) byproducts. Black, brown, and grey hydrogen are produced from breaking down coal or natural gas via heat-powered processes. The CO2 and CO byproducts are usually released directly into the atmosphere as greenhouse gas emissions. Today, 95 percent of hydrogen produced in the United States is black, brown, or grey hydrogen. For this reason, hydrogen production contributes to 2.2 percent of global emissions
So how could hydrogen possibly help reduce greenhouse gas emissions?
Industry has looked to so-called blue and turquoise hydrogen as a way to continue to use natural gas as a feedstock while producing less carbon dioxide. Blue hydrogen is produced in a process similar to grey hydrogen with the addition of capturing and storing the CO2 byproducts.
These technologies — commonly grouped together under the term carbon capture, utilization, and sequestration or CCUS — are today still largely unproven at high capture rates and not yet cost-effective, but could offer a decarbonization pathway in the future if technology performance and upstream emissions abatement improve dramatically.
Turquoise hydrogen is also produced using natural gas as a feedstock. However, in contrast to blue hydrogen there is no byproduct carbon dioxide, but rather solid carbon. This solid carbon can be disposed of or reused as plastic, construction material, or tires.
We also must consider the upstream emissions of any fossil-based hydrogen — emissions from the extraction and production of the fossil-fuel input. These emissions could significantly affect the lifecycle carbon footprint of these fuels, given the particularly potent climate effect of methane leakage associated with oil, gas, and coal production.
The most promising game changer for the energy transition is hydrogen made from water and electricity, using a centuries-old technology called an electrolyzer, which applies electrical current to split water molecules into oxygen and hydrogen. If the electricity used in this process is from a renewable source — solar, wind, geothermal, etc. — it is called green hydrogen. The inputs (water), the power source (renewable-powered electricity), and the byproduct (oxygen) of this process are all virtually carbon free.
Pink hydrogen is also emerging as another low-carbon variant. In this process, nuclear energy is used to power the electrolytic conversion of water into oxygen and hydrogen. Proponents see this path as a promising option, when electricity demand is low, to use spare nuclear capacity to produce and store hydrogen for periods of high energy demand.
Hydrogen is used today to refine oil products — its reactivity is leveraged to bond with other elements and decontaminate the final product — and as a chemical feedstock to produce ammonia for fertilizer and other key chemicals.
Where would hydrogen fit into the modern energy system?
Just as carbon is the building block of the fossil fuel economy, hydrogen will be the building block of the clean energy economy in which renewable electricity, hydrogen (both gaseous and liquid), ammonia, and other synthetic fuels will dominate to produce, store, and move clean energy. Hydrogen’s flexibility as a zero-carbon fuel, clean energy carrier, and bridge to clean electricity make it the missing piece for a fully decarbonized economy.
Hydrogen provides a way to decarbonize industrial processes that rely on certain chemical feedstocks or agents. Hydrogen can be used in steel and aluminum production — where its chemical reactivity also helps reduce ore into more useable forms — instead of the currently used coking coal or natural gas. In these industrial processes, hydrogen will be consumed directly as a chemical feedstock in the production process or burned to provide the necessary heat for operation.
Hydrogen can also be used for energy storage, potentially playing a role in managing a renewable-powered electrical grid, storing excess power generated on sunny, windy days for later use. Here hydrogen is used in a fuel cell, a technology that uses hydrogen to produce electricity, heat, and water. Fuel cells enable clean energy molecules (i.e., hydrogen) to be turned back to electricity.
Additionally, hydrogen can be used as a fuel to power mobility applications — including maritime shipping, aviation, and heavy-duty trucking.
Exactly how hydrogen is used in each of these mobility applications will differ depending on fuel weight or volume thresholds for each transportation mode. Sometimes pure hydrogen will be put through a fuel cell or burned directly in an engine similar to those seen today in gas-powered cars and boats. However, given that there is no carbon in the fuel, no carbon dioxide can be formed. Other times, hydrogen will be used as the raw material to create other clean energy molecules such as synthetic fuels or ammonia, creating an energy-dense chemical and fuel that is more easily transported. Alternatively, hydrogen can be used to provide “clean electricity on the go,” providing a way to power electric motors with less reliance on heavy and large batteries, given that hydrogen can be used in a fuel cell to produce electricity.
Hydrogen can expand the set of locations where clean energy can be supplied and diversify the ways in which we move energy around. Hydrogen, or its derivatives like green ammonia, can be transported via pipeline or ship. In a low-carbon energy economy, moving energy in these ways could be a crucial way for countries with high renewable energy capacity, like those in North Africa, to transform electricity into fuel for export to large markets like Europe.
A report by the Energy Transitions Commission calls hydrogen “the second vector” for decarbonization after direct electrification, projecting it will meet 13 percent of final energy demand in 2050, or 20 percent if hydrogen-based fuels are included. The long-term market for hydrogen will be huge, potentially $2.5 trillion or more. The announced projects for the next five years alone will require tens of billions of dollars in investments in electrolyzers and renewable power generation.
Given its centrality in the energy transition, it is time to demystify hydrogen. Next in our hydrogen series, we tackle the question of hydrogen’s real emissions benefits.