The world is transitioning away from fossil fuel towards renewable sources of energy, and hydrogen is a promising alternative. It is plentiful – the most abundant element in the universe; and versatile – it can be used both as a fuel and as a storage medium. Hydrogen is not a one-size-fits-all solution, as it has significant drawbacks, but one of the applications where it might develop a useful niche is in transportation. More precisely, it can be used as a fuel for light and heavy vehicles. Japan, Germany, and the United States are some of the countries that already have a network of hydrogen fueling stations, and a key component on them is hoses. They are the final link between the dispenser and the end user and need to withstand very challenging conditions. To understand this transition it is beneficial to learn more about hydrogen as a source of renewable energy and the role hoses play in conveying it for the automotive industry.
By Davi Correia, Senior Mechanical Engineer
Hydrogen (H) is the first element of the periodic table. If one remembers their chemistry lessons, they know that the chemical elements are arranged in order of increasing atomic number; and that the atomic number is somewhat related to the size of an atom. As hydrogen is the first element in the periodic table (atomic number 1), it comes as no surprise that it is a very small element as well as the lightest. As a consequence, hydrogen can leak easily.
Hydrogen is the simplest of all elements, and can be pictured as a single electron orbiting a single proton. As this arrangement is highly reactive, hydrogen atoms naturally combine into molecular pairs (H2, known as molecular hydrogen). At ambient temperature and pressure, H2 it is a colorless, odorless, tasteless, non-toxic, and highly flammable gas. However, even as a molecule, hydrogen is still highly reactive, forming covalent compounds with most non-metallic elements. As a result, most of the hydrogen on earth is found in more complex forms such as water or organic compounds.
For automotive applications being small and light means that high pressures are required to achieve useful energy densities.
There is a diverse range of markets for hydrogen applications. Some of the industries using it include: oil refining, ammonia and fertilizers production, metals production, methanol production, food processing, and electronics. Now, if hydrogen is not found freely on earth, where does it come from?
In order to be consumed in these industrial applications, hydrogen needs to be removed from some other molecule. Today, the two most common feedstocks for hydrogen production are natural gas and water; and the related extraction processes are natural gas steam reforming and water electrolysis, respectively. “With respect to the energy required, it is easy to remove hydrogen from compounds that are at a higher energy state, such as fossil fuels. This process releases energy, reducing the amount of process energy required. It takes more energy to extract hydrogen from compounds that are at a lower energy state, such as water, as energy has to be added to the process.”1 In a nutshell, obtaining hydrogen from natural gas is cheaper, but the process results in Greenhouse Gas (GHG) emissions; removing it from water results in zero emissions, but it is more expensive and not very energy efficient. Even if the electricity required for the process comes from renewable sources, such as wind or solar, there is still the need to use more energy than what you can obtain with the hydrogen itself. For this reason, some people regard hydrogen production from water as a waste of electricity.
Hydrogen as a Fuel
Hydrogen as a fuel is not something new. “The National Aeronautics and Space Administration (NASA) began using liquid hydrogen in the 1950s as a rocket fuel, and NASA was one of the first to use hydrogen fuel cells to power the electrical systems on spacecraft.” More recently, concerns over climate change have led hydrogen to be touted as a most attractive alternative to less extraterrestrial applications.
Transportation is responsible for more than a quarter of the GHG emissions in the United States, see Figure 1. The build-up of such gases acts as a sort of blanket around the planet and the resulting heat accumulation is associated with disruptive and unpredictable climate events. One of the ways to reduce GHG emissions from transportation is increasing the number of Zero Emission Vehicles (ZEVs). “There are two types of ZEVs, Battery Electric Vehicles (BEVs) and Fuel Cell Electric Vehicles (FCEVs). Commercial Success of BEVs has been challenging thus far also due to limited range of very long charging duration.”4 FCEVs using H2 infrastructure can be consistently fueled in a safe manner, are fast (under 5 minutes) and result in a range similar to conventional vehicles (300+ mile range). In comparison, charging modern BEVs can take anywhere from an hour to 12 hours for a full charge, depending on how full the car’s battery is and the type of charging station.
Fuel cells (see Figure 2) are more suited for automotive applications than burning H2 directly (as in a rocket). This is because, although hydrogen does not contain carbon, its combustion with air produces oxides of nitrogen, which contribute to the formation of smog and acid rain. Fuel cells operate by converting the chemical energy in hydrogen to electricity, with pure water and heat as the only by-products. As hydrogen and oxygen react across an electrochemical cell, “energy is released as a combination of low-voltage DC electrical energy and heat. The electrical energy can be used to do useful work directly while the heat is either wasted or used for other purposes.”
Hoses for Hydrogen Fueling Stations
From a distance, a hydrogen fueling station look very similar to a conventional one, see Figure 3; there is a dispenser and a flexible hose assembly that needs to be connected to the vehicle’s tank. However, upon closer inspection, there is a noticeable difference between the hoses.
A hose assembly for hydrogen delivery resembles more of a fire hose than a conventional gasoline fueling one. Hoses for hydrogen are thicker, heavier, and use a nozzle with a locking mechanism; the reason for this is pressure.
Hoses for gasoline or diesel delivery are rated for a maximum of 10 bar. When attached to the tank, they convey a liquid in a very similar way to how one would pour a glass of water from a jar – if distracted, it is easy to overflow and cause a spillage. In contrast, hydrogen hoses operate with a gas and can handle a much higher pressures; 350 bar for buses (and other heavy-duty applications) and up to 700 bar for light vehicles. In order to accommodate these pressures, the hose requires extensive reinforcement layers; making it heavier and thicker, see Figure 4.
The reason for a locking mechanism on the nozzle lies with the fact that the hydrogen being delivered is a gas, see Figure 5. The dispenser has the gas at a certain pressure (700 or 350 bar), which is then equalized with the pressure in the tank. As gas leaking from the connection wastes hydrogen, and also poses a safety risk the connection between the hose and the tank has to be secure. It works like this: a Toyota Mirai, for example, has a tank capable of holding approximately 5 kg of H2 at 700 bar; but if the tank is half-full, and you connect a 350 bar hose, you will not be able to fill up the rest. As the pressures in the dispenser and in the tank will be the same, no hydrogen will flow.10 In this situation, the tank you connect to would need to have a minimum of 700 bar to fill the car tank.
Material selection for the inner tube poses some interesting challenges for the designer. Hydrogen molecules are very small and prone to leak easily through cracks, poor connections, or even some solid materials, as opposed to other common gases at equivalent pressures. Although hydrogen is generally non-corrosive and does not react with the materials used for hoses, at certain temperature and pressure conditions it can diffuse into a metal lattice causing a phenomenon known as ‘hydrogen embrittlement;’ this ultimately causes the steel reinforcement to fail.
Fuel cells are also very sensitive to contaminants and the inner tube material should not be a source of them. “Contaminants derived from fuel cell system component/ structural materials, lubricants, greases, adhesives, sealants, and hoses have been shown to affect the performance and durability of fuel cell systems.”
To ensure the integrity of the hose, hoses for hydrogen delivery should be assembled by the manufacturer and pressure tested to 1.5 times the rated pressure.12 They should be documented with serial number, test pressure, test date, and first date of service. It is highly advised that hydrogen hoses assemblies should also, “be discarded after 12 months of use or immediately after any mechanical abuse such as a breakaway incident.”
The general public will be responsible for handling hoses and nozzles during refueling. Considering the pressures involved and the characteristics of the gas, it comes as no surprise that hydrogen stations are subject to strict regulation and standards. Some of them are generic (such as Local and state fire and building codes), but there are already some very specific.
- NFPA 2 – Hydrogen Technologies Code
- ANSI/CSA HGV 4.2-2013 – Hoses for Compressed Hydrogen Fuel Stations, Dispensers and Vehicle Fuel Systems
- ISO 19880-5:2019 Gaseous hydrogen — Fueling stations — Part 5: Dispenser hoses and hose assemblies
Moving forward, it will be important to understand the unique requirements of hydrogen hoses and how to safely and efficiently maintain and operate them.