Chapter 15: Nuclear Applications – Nuclear Heat for Hydrogen Production | A Book: Nuclear Energy

Nuclear heat for hydrogen production is another advanced application of nuclear energy.

The hydrogen economy is getting higher visibility and stronger political support in several parts of the world. In recent years the scope of the International Atomic Energy Agency (IAEA) program on non-electric applications of nuclear energy has been widened to include other more promising applications such as nuclear hydrogen production and high temperature process heat applications. Nuclear hydrogen production technologies have great potential and advantages over other sources that might be considered for a growing the hydrogen share in a future world energy economy. The selection of hydrogen technologies (to be coupled to nuclear power reactors) greatly depends on the type of the nuclear power plant itself. Some hydrogen production technologies, such as conventional electrolysis, require only electric power. Whereas others, such as thermochemical cycles, may require only process heat (which may be delivered at elevated temperature values) or hybrid technologies such as the high temperature steam electrolysis (HTSE) and hybrid thermochemical cycles, which require both heat and electricity.

Hydrogen is the most abundant element in the universe and the third most abundant on Earth. Hydrogen gas does not exist on the earth or in our atmosphere in significant quantities. Instead, it reacts quickly with other elements to form more stable compounds. Hydrogen compounds are abundant in water and fossil fuels; its supply is effectively limitless. Because pure hydrogen is not as readily available as fossil fuels, hydrogen is not considered to be a source of energy but an energy carrier. Like electricity, hydrogen is “Manufactured”. Energy carriers are a convenient medium to store, transport, and use energy. But the convenience comes at a price, that is, efficiency.

There are several methods for producing hydrogen. All involve splitting compounds that contain hydrogen and capturing the hydrogen gas that results. To split water directly with heat (Thermolysis) requires temperatures in excess of 2500°C for significant hydrogen generation. This method is not currently practical for industrial production, as those temperatures cannot be sustained. Instead, thermochemical cycles or electrical drivers allow the splitting to occur at lower temperatures.

Hydrogen has been researched as an energy transport medium since the 1960s. Two recent technological developments have piqued the interest in hydrogen:

Fuel cell technology to cleanly make electricity on location (or perhaps in vehicles); and
The direct use of hydrogen as a fuel, such as in rocket fuel or military aircraft.

Currently, hydrogen production is a major area of research throughout the world, especially in the US, Europe, and Japan. Burning hydrogen with oxygen, as is done in the space shuttle, creates no pollution. The only by product of that combustion is water. Burning hydrogen with air does form some pollutants, such as NOX, but in much smaller quantities than when burning fossil fuels. Therefore, there are significant potential environmental benefits to the use of hydrogen as an energy carrier.

Here is a graph which illustrates the process of using nuclear heat for hydrogen production:

A number of thermo-chemical water splitting cycles have been identified in recent years. These cycles essentially split water into hydrogen and oxygen through a series of heat-driven chemical reactions. Early progress — including bench-scale testing of the leading cycles best suited for the high temperature gas-cooled reactor — is under development in the U.S., Japan, France and other countries. In the thermo-chemical processes, only water, heat and electricity (as a utility) are needed to produce hydrogen and oxygen. Although many of these cycles have been identified, most of the current development work is focused on the sulfur-iodine (SI) process.

Use of the high-temperature nuclear reactor as the heat source would eliminate carbon dioxide emissions and result in efficiencies approaching 80 percent. Areas of commercial interest in hydrogen include: oil refining, ammonia manufacturing (fertilizer), and methanol production. Hydrogen can be combined with gasoline, ethanol, methanol, or natural gas to increase engine performance and reduce pollution. This increasing demand for hydrogen in the refining sector is driven by the need to produce cleaner transportation fuel for meeting environmental regulations. Hydrogen can be added in the refining process to create a cleaner-burning fuel.

A fuel cell is an electrochemical energy conversion device. A fuel cell converts the chemicals hydrogen and oxygen into water, and as a result, it produces electrical power efficiently, without producing any CO2. The by-products of an operating fuel cell are heat and water. In principle, a fuel cell operates like a battery. However, unlike a battery, a fuel cell does not run down or require recharging. With a fuel cell, chemicals constantly flow into the cell so it never goes dead – as long as there is a flow of chemicals into the cell, the electricity flows out of the cell. Most fuel cells in use today use hydrogen and oxygen as the chemicals.

Looking at the graph, there are six types of fuel cells, connected to two different types of applications – Stationary and Transport. Here is a brief description of each type of fuel cell:

Proton Exchange Membrane Fuel Cells (PEMFC): This type also known as Poly Electrolyte Membrane. These fuel cells deliver high power density and offer the advantages of low weight and volume compared to other fuel cells. PEMFC are particularly suited to powering passenger cars and buses due to their fast start-up time, favourable power density, and power-to-weight ratio;
Phosphoric Acid Fuel Cells (PAFC): Phosphoric acid fuel cells (PAFC) use phosphoric acid as an electrolyte and porous carbon electrodes containing a platinum catalyst. They were the first fuel cells ever used commercially and over 200 units are currently in use. Primarily used in stationary power applications, as well as for powering buses;
Direct Methanol Fuel Cells (DMFC): Most fuel cells are powered by hydrogen, which can be fed to the fuel cell system directly or can be generated within the fuel cell system by reforming hydrogen-rich fuels such as methanol, ethanol, and hydrocarbon fuels. DMFC, however, are powered by pure methanol. DMFC fuel cell technology is relatively new, compared to that of fuel cells powered by pure hydrogen, and research and development are roughly 3-4 years behind that of other fuel cell types;
Alkaline Fuel Cells (AFC): Alkaline fuel cells (AFC) were the first fuel cell technology ever developed and used in the United States’ space programme. They use a potassium hydroxide solution as the electrolyte and a variety of non-precious metals as a catalyst at the anode and cathode. AFC typically operate at between 100-250 °C, but recent versions operate at between 23-70 °C. AFC are high-performance devices that achieve an efficiency of 60 percent, but they are vulnerable to poisoning by even small amounts of carbon dioxide;
Molten Carbonate Fuel Cells (MCFC): Molten Carbonate Fuel Cells (MCFC) are being developed to be fuelled by natural gas. These fuel cells cannot be fuelled by pure hydrogen. MCFC use a molten-carbonate-salt electrolyte suspended in a porous, inert ceramic matrix. They do not need an external reformer, because they operate at high temperatures (>650 °C). In addition, they do not use precious-metal catalysts, further reducing their cost; and
Solid Oxide (SOFC): Solid Oxide Fuel Cells (SOFC) used a non-porous ceramic electrolyte and appeared to be the most promising technology for electricity generation. When combined with a gas turbine, SOFC, expected to achieve an electrical efficiency of 70 percent and up to 80-85 percent efficiency in cogeneration. High operating temperatures of 800-1000 °C mean precious-metal catalysts and external reformers are unnecessary, helping to reduce the cost of SOFC.

The applications have been divided into the following two categories:

1. STATIONARY APPLICATIONS:

In the energy field, most hydrogen is used through Fuel Cells (FCs). A fuel cell is an electrochemical device that combines hydrogen and oxygen to produce electricity, with water and heat as by-products. In its simplest form, a single fuel cell consists of two electrodes – an anode and a cathode – with an electrolyte between them. At the anode, hydrogen reacts with a catalyst, creating a positively charged ion and a negatively charged electron. The proton then passes through the electrolyte, while the electron travels through a circuit, creating a current. At the cathode, oxygen reacts with the ion and electron, forming water and useful heat.

Stationary applications are defined as:

Primary power and heat for homes and buildings;
Emergency power for critical lighting, generator or other uses when regular systems fail; and
Uninterrupted power supply (UPS) to provide instant protection from power outages.

2. TRANSPORT APPLICATIONS:

Hydrogen fuel is considered a good candidate to contribute to the decarbonization of the road transport sector if it is produced renewable energy sources through the process of electrolysis. In this case, the main advantages of fuel cell electric vehicles are the zero emission of carbon dioxide (CO2) and pollutants (the emission at the tailpipe is only water), and the higher efficiency of fuel cells compared with internal combustion engines. Passenger cars and urban buses, among other vehicles, as material handling equipment, etc., are good examples of the new technology ready for mass commercialization in the coming years.

The application options for hydrogen as a fuel for mobility can be differentiated firstly by the chemical form or bonding of hydrogen and secondly by the energy converter by means of which the energy stored in the hydrogen is made available.

In direct use, (pure) molecular hydrogen (H2) is used by the transportation means directly, i.e. without further conversion, as an energy source. In this case hydrogen can be used both in internal combustion engines and in fuel cells (fuel cell systems).
In indirect use, hydrogen is used to produce final energy sources or is converted by means of additional conversion steps into gaseous or liquid hydrogen-containing fuels. Such PtG (Power-to-Gas) and PtL (Power-to-Liquids) fuels can then in turn be used in heat engines. Use in fuel cells would also be possible (in some cases), using a reformer, but it is not economically viable.

2.1 Aviation:

In civil aviation, hydrogen-powered fuel cells are regarded as potential energy providers for aircraft as they have been in space travel for some time now. Thus, fuel cell modules can supply electricity to the aircraft electrical system as emergency generator sets or as an auxiliary power unit. More advanced concepts include starting of the main engine and the nose wheel drive for airfield movements by commercial aircraft.

2.2 Maritime Applications:

As in aviation, fuel cells are currently being tested as energy providers for the on-board power supply. The use of hydrogen-powered fuel cells for ship propulsion, by contrast, is still at an early design or trial phase – with applications in smaller passenger ships, ferries or recreational craft. The low- and high-temperature fuel cell (PEMFC) and the solid oxide fuel cell (SOFC) are seen as the most promising fuel cell types for nautical applications (EMSA 2017). As yet, however, no fuel cells have been scaled for and used on large merchant vessels.

2.3 Trains:

In electric locomotives, motive power is supplied via stationary current conductors (overhead lines, conductor rails) and current collectors on the vehicles. However, for technical, economic or other reasons, not every railway line can be electrified. Especially on lines with a low transport volume, the high up-front investment that is needed for electrification of the lines cannot always be justified. Moreover, overhead lines cannot be used for shunting if cranes are also in use for moving transport goods. In subsurface mining, by contrast, traction vehicles have to operate without air pollutants.

Rail vehicles that use hydrogen as an energy store and energy source can offer an additional alternative. Fuel cell-powered rail vehicles combine the advantage of pollutant-free operation with the advantage of low infrastructure costs, comparable with those for diesel operation.

2.4 Material Handling Vehicles:

Fuel cell industrial trucks, like forklifts or towing trucks (airports) are especially suitable for indoor operation, because they produce no local pollutant emissions and only low noise emissions. Fuel cell vehicles have advantages over battery-operated industrial trucks in terms of refueling. Instead of having to replace the battery, the trucks can be refueled within two to three minutes.

They take up less space and are cheaper to maintain and repair. Fuel cell industrial trucks allow for uninterrupted use and are therefore particularly suitable for multi-shift fleet operation in material handling (FCTO 2014b). In the case of larger industrial truck fleets in multi-shift operation, (moderate) cost reductions can be achieved in comparison to battery technology, and productivity in material handling can also be increased.

2.5 Buses:

In terms of road transport, buses in the public transport network are the most thoroughly tested area of application for hydrogen and fuel cells. Since the early 1990s, several hundred buses have been and are being operated with hydrogen worldwide – predominantly in North America, Europe and increasingly also in Asia.

Although hydrogen was initially still used in buses with internal combustion engines, bus developers are now concentrating almost entirely on fuel cell electric buses (FCEB). The use of small FCEB fleets is being promoted in urban areas as a way of contributing to technological development and to clean air policy.

Fuel cell buses have now reached a high level of technical maturity, although they are not yet in series production. Owing to the small numbers, until now they have still been much more expensive, at around 1 million EUR, than standard diesel buses, which cost in the region of 250,000 EUR. The maintenance costs have also been significantly reduced and the reliable operating times increased (Hua et al. 2013).

2.6 Passenger Cars:

Along with battery electric vehicles, hydrogen-powered fuel cell passenger cars are the only zero-emission alternative drive option for motorized private transport. The first fuel cell passenger cars were tested back in the 1960s as demonstration projects. A new boost to fuel cell development came in the 1990s. In most cases the fuel cell test vehicles were converted cars that had originally been fitted with an internal combustion engine. At the time, however, the early test models were still not competitive, either technically or economically. In addition, up until about 10 years ago petrol engine prototypes were still being tested with hydrogen as an alternative energy and low-emission fuel. These were vehicles with modified bivalent engines, which could run on both petrol and hydrogen. Owing to the fuel, hydrogen-powered internal combustion engines not only achieve somewhat higher efficiencies than in petrol operation, they also emit much lower levels of pollutants.

The fuel cell stacks in the latest fuel cell models have an output of 100 kW or more. As compared with battery electric cars they have a greater range – of around 400 to 500 kilometers today – with a lower vehicle weight and much shorter refueling times of three to five minutes. They usually carry 4 to 7 kg of hydrogen on board, stored in pressure tanks at 700 bar.

Resources:

This chapter was published on “Inuitech – Intuitech Technologies for Sustainability”
on February 6, 2012; and
This chapter was updated on 20 June 2020

Chapter 16 …