Abstract

The current commercial hydrogen production has a significant carbon footprint. Now, projects co-funded by the U.S. Department of Energy and commercial nuclear utilities with operating nuclear power facilities aim to change that by exploiting the capabilities of nuclear power plants. This article delves into four projects aimed at demonstrating technology to make hydrogen from water on an industrial scale using energy from an operating commercial nuclear power plant.

Article

The 900-MW Davis-Besse Nuclear Power Station on Lake Erie in Ohio will host a demonstration of nuclear-powered hydrogen production.

Photo: Bill Rayburn, Davis-Besse Nuclear Power Station

The 900-MW Davis-Besse Nuclear Power Station on Lake Erie in Ohio will host a demonstration of nuclear-powered hydrogen production.

Photo: Bill Rayburn, Davis-Besse Nuclear Power Station

Hydrogen is the most abundant element in the universe, making up 74 percent of ordinary matter. Fittingly, it has myriad industrial uses.

“You use hydrogen in the production of steel and petroleum refining. You use hydrogen in the support of vehicles that burn hydrogen in fuel cells. You use it for the production of ammonia fertilizers,” said Bruce Hallbert, director of the U.S. Department of Energy’s Light Water Reactor Sustainability Program Technical Integration Office at Idaho National Laboratory (INL) in Idaho Falls. “So, there’s a very large demand.”

Significant as it is now, however, the opportunity presented by hydrogen could be much larger in the future.

“Hydrogen has the highest energy content by weight of all known fuels, but because hydrogen is a small molecule, its energy density is about 50 percent less than methane,” Hallbert said. However, new methods to compress or liquefy hydrogen are being developed with lightweight, high-pressure tanks to make it possible to efficiently deliver this clean source of energy to filling stations and other specialty users.

Right now, there are thousands of hydrogen fuel cells in commercial vehicles, forklifts, and backup power units throughout the United States. To get an idea of the potential demand for hydrogen fuel, consider that car and truck sales in the U.S. typically run about 16 million to 17 million vehicles annually. If a significant fraction of those were hydrogen powered, then the total demand for the fuel would be more than double what it is now.

But for that future shift to happen, many parts of infrastructure must be put into place, starting with the fuel itself. Virtually all the hydrogen available on Earth is bound to other elements in molecules. Extracting pure hydrogen from those compounds involves energy-intensive processes with significant carbon footprints.

Now, the U.S. Department of Energy (DOE) and four commercial utilities are co-founding projects aimed at reducing the cost and impact of hydrogen production. The utilities are leading projects over the next few years to demonstrate technology to make hydrogen from water on an industrial scale using energy from operating commercial nuclear power plants. INL is also participating, having worked on the concept for years under previous and current DOE national programs, including the Next Generation Nuclear Plant project, the Crosscutting Technology Development Integrated Energy Systems program, and the Light Water Reactor Sustainability (LWRS) program.

If successful, these projects could lead to a day when hydrogen is produced economically and with no carbon emis-sions—and nuclear utilities have a new and reliable revenue stream.

Getting H2 to You

In recent years, between 60 million and 70 million metric tons of hydrogen is produced worldwide, with a sixth of that in the United States. Hydrogen is manufactured at industrial scale using steam reforming of a carbon-based feedstock, most commonly natural gas. The chemical reaction between a methane molecule and a water molecule produces three molecules of hydrogen and one of carbon monoxide.

CH4 + H2O⇌CO + 3H2

This reaction is strongly endothermic and so requires substantial heat. When done on an industrial scale, the process takes place in a reformer vessel where the feedstock and high-pressure steam are brought together at temperatures around 900 °C (1650 °F) in the presence of a nickel catalyst. To maximize production, the catalyst is shaped so that it has a high surface-area-to-volume ratio.

The carbon monoxide generated by the high-temperature process also can react with water, producing one molecule of hydrogen and one of carbon dioxide.

CO + H2O⇌CO2+H2

Ultimately, the steam reforming process results in the generation of carbon dioxide at a ratio of one CO2 per four H2 molecules. By weight, this means 5.5 tons of carbon dioxide are produced per ton of hydrogen. The ratio can be even higher, depending on the source of the energy for the high temperature steam; some reports put the weight ratio as high as 9:1. Since this carbon dioxide is generally vented into the atmosphere, hydrogen produced via steam reforming cannot be considered free of carbon emissions.

Steam reforming accounts for 95 percent of current industrial hydrogen production, but there is an alternative way to make hydrogen: electrolysis. A standard chemistry demonstration of this process passes a current through water, separating it into hydrogen, which collects at one electrode, and oxygen, which gathers at the other. The reaction—in which two water molecules produce two hydrogen molecules and one oxygen molecule—is strongly endothermic, requiring quite a bit of energy to complete.

With nuclear-powered hydrogen, heat from reactors will be used either to run steam turbines for electric generation or to produce hydrogen for industrial purposes.

Photo: Idaho National Laboratory

With nuclear-powered hydrogen, heat from reactors will be used either to run steam turbines for electric generation or to produce hydrogen for industrial purposes.

Photo: Idaho National Laboratory

The great advantage of electrolysis is that no carbon-bearing feedstock is involved. Also, if the electricity source itself is free of carbon emissions, then the carbon footprint of the process is essentially zero.

“Industrial scale electrolysis is predominantly performed at low-temperature using an alkaline electrolyte to optimize the cell conductivity,” said Richard Boardman, the technology development lead for integrated energy systems at INL. (Another low-temperature electrolysis process uses a polymer exchange membrane [PEM, sometimes called a proton exchange membrane] to separate hydrogen from water molecules.)

Today, commercial electrolysis is accomplished at low temperature, which often means cells are operated at 90 °C or less. But in some settings the electrolysis process is much more efficient when carried out at higher temperature, even as high as 800 °C with superheated steam.

Improving Efficiency

Low temperature alkaline electrolysis takes a minimum of about 52 kWh of electricity to make a kilogram of hydrogen, according to Boardman. Using a PEM-based approach cuts that figure to around 45 kWh per kg. (The energy content of 1 kg of hydrogen is around 33 kWh.)

In contrast, using high temperature steam reduces the amount of electricity required substantially.

“With high temperature steam electrolysis, the amount of electricity used can be reduced to around 35 kWh per kg—and even lower than that by providing high temperature heat to the electrolysis cell to compensate for the endothermic splitting of H2O. With heat recuperation and cell heating, it could be possible to reduce the electricity used down to 30 kWh,” Boardman said.

The roughly 30 percent increased efficiency is critical to lowering the cost of hydrogen produced by electrolysis. Some three quarters of that cost comes from the expense of the electricity consumed. INL calculations show that at current wholesale prices of electricity around $30 per MWe, high-temperature electrolysis may cut the cost of hydrogen production to less than $2 a kilogram—a DOE goal.

In this schematic of a high-temperature nuclear hydrolysis plant, coolant from the reactor is sent to a heat exchanger to superheat steam.

Photo: Idaho National Laboratory

In this schematic of a high-temperature nuclear hydrolysis plant, coolant from the reactor is sent to a heat exchanger to superheat steam.

Photo: Idaho National Laboratory

Thermodynamics lies behind the efficiency improvement. It takes less energy to convert steam into hydrogen and oxygen because the water is already in a gaseous state. In addition, at a superheated state, the bonds of the oxygen and hydrogen atoms are easier to break. Because the steam is already at a higher state of energy, it requires less electricity to finish splitting the steam molecules.

The efficiency of transforming steam energy to electricity in a traditional large nuclear power plant is, at best, about 34 percent, Boardman said. So, avoiding that step and using steam heat in a more direct fashion improves the overall electrolysis system performance significantly. The result is about a 40 percent increase in the amount of hydrogen that can be produced, according to calculations performed at INL using the power from actual nuclear power plants.

In practice, such a high-temperature electrolysis plant would involve a heat transfer loop sitting between the nuclear power plant and the electrolysis unit. The loop would be filled with a working fluid that would transport heat out of the plant without compromising the safety or performance of the plant’s existing systems.

Some design choices help achieve the results modeled by LWRS Program researchers. For instance, Boardman noted that nuclear plants produce steam at about 300 °C. With the intermediate heat transfer loop, the quality of the steam that is produced for electrolysis is around 250 °C.

The question that is often raised is how much extra energy does it take to raise the temperature of this steam up to the electrolysis temperature of 800 °C? Boardman explained how that can be done with very little extra, or topping, heat.

“We use the heat from the hydrogen and the oxygen that’s coming out of that cell at 800 °C and send it through a heat exchanger to raise that incoming steam all the way to, maybe, 775 °C. This means very little electrical topping heating is needed to get the last little bump from 775 °C up to 800 °C. And that’s only one percent of the overall total energy,” Boardman said.

He added, however, that this makes the system more complex: It now includes another heat loop and additional heat exchangers.

That extra complexity is justified because the cost of energy is the largest expense in making hydrogen and the additional equipment is not expensive. The reduction in power used means high temperature electrolysis production of hydrogen can be cost competitive with that from steam reforming plants.

Improving Economics

Of course, the power for electrolysis can come from any source, including carbon-free solar or wind in addition to nuclear power plants. No matter what source, though, electrolysis benefits from lower electricity costs, and the price of producing power carbon free continues to fall. The International Renewable Energy Agency states that renewables are now a less expensive source of electricity than fossil fuels.

Renewable energy is hampered by variability and inter-mittency. At times, renewable electricity production can fall to zero, while at other times the combination of wind and solar can produce power greater than the total electric demand. Hydrogen production offers a way to soak up extra generating capacity when it is present, thereby improving the economics of power production. In this, nuclear power plants have an advantage because they produce both electricity and steam. Thus, low- or high-temperature electrolysis facilities could be located adjacent to or near a nuclear power plant, with the connection made in such a way as to ensure safety of the plant and its operation. In practice, changes would have to be made to plant command and control systems as well as to plant simulators to accommodate this new function. This work is well underway, with national labs collaborating on needed tasks with industry partners, according to Hallbert.

Taking such a complete and thorough approach to operations and safety would allow hydrogen to be made during those times when excess capacity is available.

A collaboration between DOE, national labs, and industry is now preparing pilot projects designed to demonstrate the viability of hydrogen production via nuclear-powered electrolysis on an industrial scale.

It begins with a two-year project from Energy Harbor, based in Akron, Ohio, which will use a low-temperature electrolysis unit at a nuclear power plant to produce commercial quantities of hydrogen for public transportation fleets, steelmaking, and other applications. Raymond Lieb, senior vice president of fleet engineering, signaled eagerness to begin in a press statement.

Facilities like this one currently steam reform methane to produce hydrogen. The process results in waste carbon dioxide that is vented.

Photo: Linde

Facilities like this one currently steam reform methane to produce hydrogen. The process results in waste carbon dioxide that is vented.

Photo: Linde

Idaho National Laboratory was established in 1949 to conduct atomic research. Over the decades, many prototype nuclear reactors have been tested at the lab.

Photo: Idaho National Laboratory

Idaho National Laboratory was established in 1949 to conduct atomic research. Over the decades, many prototype nuclear reactors have been tested at the lab.

Photo: Idaho National Laboratory

“This is a great opportunity to show that hydrogen can be effectively generated in a carbon-free and safe manner,” he said of the project, which is planned to run through 2021.

Xcel Energy of Minneapolis, which is pursuing 100 percent carbon-free electricity production by 2050, is participating in current research activities to demonstrate hydrogen production and to investigate future markets for hydrogen production from their nuclear units. The utility sees a role for hydrogen production in this future.

“When we have surplus energy on the electrical grid, instead of ramping down a nuclear unit, we could shift that energy to hydrogen production. That would result in a higher capacity factor and would maximize the utilization of carbon-free power sources,” said Tim O’Connor, chief generation officer for Xcel Energy, in the news release about the project.

The third partner in the research is Arizona Public Service, which is studying ways that its nuclear plant near Phoenix may employ advanced hydrogen production technologies, such as high temperature electrolysis.

“This project allows us to explore a new form of energy storage while continuing to provide customers what they want—clean, affordable and reliable electricity,” said Bob Bement, the former APS executive vice president and chief nuclear officer, in a 2019 release announcing the project. (Bement recently retired from APS.)

Finally, Exelon Corporation, based in Chicago, has also received a DOE-cost-shared project to demonstrate hydrogen production using the power from a nuclear power plant. The Exelon project will be similar to the project at the Energy Harbor nuclear plant, but will address different hydrogen markets around the demonstration site. One aspect of the Exelon demonstration project will be an evaluation of how a low-temperature PEM electrolyzer can be ramped up and down to help firm the grid through demand response.

INL’s Hallbert noted that success in these demonstrations will confirm that commercial nuclear utilities can produce essential products like hydrogen without carbon emissions. Doing so, he said, could enable steel producers to reduce their carbon footprint by as much as 15 percent and help fertilizer manufacturers substantially cut carbon emissions.

“As you start to look into the future, it’s very feasible to scale up these types of technologies and produce significant quantities of hydrogen,” Hallbert said. “Commercial nuclear power plants could start to address some large portion of that demand for hydrogen.”